New England Orogen 2010
NEO 2010 Conference Proceedings i
NEO 2010
New England Orogen 2010 Proceedings of a conference held at the University of New England, Armidale, New South Wales, Australia, November 2010 Edited by Solomon Buckman School of Earth & Environmental Sciences, University of Wollongong, NSW, Australia.
Cover Illustration: This is painting five of a series of five paintings by Dr. John H. Jackson (known as “The Rock Doctor”) highlighting the geological evolution of “The Green Cauldron” landscapes that are located in South East Queensland and North East New South Wales.
Published by the University of New England
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Forward This volume records the proceedings of a symposium held at the University of New England from 16-19th November 2010. It has been over a decade since the last New England conference and this conference follows on a tradition of New England geology symposiums held in 1982, 1988, 1993 and 1999. This conference covered four important themes including: I) Tectonics; II) Alumni; III) Granites; and IV) Mineralisation. This proceedings volume honours the lifelong works of Professor Peter Flood, Professor Bruce Chappell and Professor Paul Ashley for their contributions to tectonics, granites and mineralization respectively in the New England Orogen. The committee members comprise: Solomon Buckman, Peter Flood, Phil Blevin, Bruce Chappell, Jonathan Aitchison, Paul Ashley, John Patterson and Mel Jones. Extended abstracts are presented in alphabetical order according to first author.
Copyright is retained by the first or sole author, who grants right of first publication to the NEO 2010 Conference Proceedings. Permission is granted to distribute this article for non-profit, educational purposes if it is copied in its entirety and the Conference Proceedings is credited. ISBN 978 1 921597 24 4
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Sponsors The following companies and organisations have kindly sponsored this event from which the proceeds will go towards undergraduate geology scholarships.
Platinum Sponsors Industry and Investment, NSW - Geological Survey, NSW Zeolite Australia
Gold Sponsors University of New England UNE Foundation AusIMM Hunter Branch Whitehaven Coal Mining Pty Ltd
Silver Sponsors Elementos Limited Malachite Resources
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Table of Contents PREFACE Solomon Buckman
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“MIGRATING IN TIME” J.H. Jackson
1
ACCRETIONARY COMPLEXES IN EASTERN AUSTRALIA AND NEW ZEALAND: MATCHING THEIR SEDIMENT SOURCES AND DESTINATIONS
C.J. Adams
5
COMPARATIVE ANATOMY OF OROGENIC SYSTEMS: COMPARING THE HIMALAYA-TIBET WITH NEW ENGLAND J.C. Aitchison
12
MORPHO-TECTONIC UNITS OF THE ZAGROS OROGENIC BELT, NE IRAQ: A MODERN ANALOGUE FOR SUBDUCTION ACCRETION PROCESSES
S.A. Ali, B.G. Jones, S. Buckman, S.A. Ismail and K.J. Aswad
16
ENVIRONMENTAL GEOCHEMICAL LEGACY OF ANTIMONY AND ARSENIC DISPERSION IN THE MACLEAY RIVER CATCHMENT, NEW ENGLAND OROGEN P.M. Ashley, P. Lockwood, M. Tighe and S. Wilson
23
EXPLORATION AND OPPORTUNITIES IN THE NEW ENGLAND OROGEN, NSW R.G. Barnes
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THE GEOLOGY AND MINERALISATION OF THE NEW ENGLAND OROGEN IN QUEENSLAND P.R. Blake and I.W. Withnall
31
IGNEOUS METALLOGENIC CONTRASTS BETWEEN THE NORTHERN AND SOUTHERN NEW ENGLAND OROGEN, EASTERN AUSTRALIA P.L. Blevin
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OLD FRIENDS IN A WHOLE NEW LIGHT: A NEW CHRONOLOGY FOR THE IGNEOUS METALLOGENY OF THE SOUTHERN NEW ENGLAND OROGEN, NEW SOUTH WALES P.L. Blevin
45
THE NEW ENGLAND OROGEN – BEGINNINGS OF GEOLOGICAL KNOWLEDGE D. Branagan
49
POTENTIAL EXPLORATION USES OF HIGH RESOLUTION GEOPHYSICAL DATA IN THE SOUTHERN NEW ENGLAND OROGEN, NSW R.E. Brown
55
MID PERMIAN TO MID TRIASSIC DEVELOPMENT OF THE SOUTHERN NEW ENGLAND OROGEN J. Brownlow
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TIMS U-PB AND SHRIMP U-PB ZIRCON DATING OF THE DUNDEE RHYODACITE, NORTHERN NEW ENGLAND, NSW J. Brownlow and A. Cross
69
NEW ENGLAND BATHOLITH: UNRAVELLING THE COMPOSITIONAL DIVERSITY C.J. Bryant and B.W. Chappell
75
HAWKWOOK IRON-TITANIUM-VANADIUM PROJECT SOUTHEAST QUEENSLAND P. Buckley and P.M. Ashley
81
CU MINERALISATION ASSOCIATED WITH INTRUSIVE PHASES OF THE GAMILAROI AND WERAERAI TERRANES AT BARRY STATION, SOUTHERN NEW ENGLAND OROGEN S. Buckman, T. Line, J.C. Aitchison and A. Nutman
88
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SILICA-CARBONATE (LISTWANITES) RELATED GOLD MINERALISATION ASSOCIATED WITH EPITHERMAL ALTERATION OF SERPENTINITE BODIES
S. Buckman and P.M. Ashley
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VALUE OF ACCRETED OCEANIC ISLANDS IN DECIPHERING THE CONVERGENT MARGIN EVOLUTION OF EASTERN AUSTRALIA D.M. Buchs, P.G. Flood, R.J. Arculus and J.C. Aitchison
107
GEOCHEMISTRY AND ISOTOPE SYSTEMATICS OF CARBONIFEROUS TO TRIASSIC FELSIC MAGMATISM IN NORTHEASTERN AUSTRALIA – PUTTING THE NEW ENGLAND OROGEN IN ITS PLACE D.C. Champion, R.J. Bultitude and P. L. Blevin
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A CHEMICAL DATABASE FOR THE NEW ENGLAND BATHOLITH B.W. Chappell
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THE WONGWIBINDA COMPLEX: A HTLP METAMORPHIC TERRAIN S.J. Craven
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USING VOLCANIC FACIES ANALYSIS TO UNLOCK THE MINERAL POTENTIAL OF THE DRAKE GOLD FIELD G. Cumming and G. Lowe
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OPHIOLITE ORIGIN AND OBDUCTION: ARC-CONTINENT COLLISION: AN INTEGRATED MODEL J. Dewey
134
CONRAD SILVER PROJECT M. Donnelly
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SOIL GEOCHEMISTRY AND PATHFINDER ELEMENT DISTRIBUTION ASSOCIATED WITH THE HILLGROVE ANTIMONY-GOLD-TUNGSTEN DEPOSIT, NEW ENGLAND OROGEN, NSW. R. Ellsmore, S. Buckman and C. Simpson
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THE DOONBA DUNITE DEPOSIT, BARRABA, NSW P. English and P.M. Ashley
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INTERPRETING NEW ENGLAND SUBDUCTION COMPLEX ROCKS USING DEEP-SEA DRILLING RESULTS FROM THE NANKAI TROUGH (OFFSHORE SOUTHWEST JAPAN) C.L. Fergusson
150
UNDERSTANDING NEW ENGLAND GEOLOGY IN THE CONTEXT OF CHANGING PARADIGMS: 1850 – 2010 P.G. Flood
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ARCHITECTURE AND TECTONICS OF THE TAMWORTH BELT AND ITS FRONTAL THRUST SYSTEM, SOUTHERN NEW ENGLAND OROGEN, NEW SOUTH WALES R. A. Glen and J. Roberts
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THE PROSPECTIVITY OF BUILDING STONE IN NEW ENGLAND H.-D. Hensel
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THE WEBB’S SILVER DEPOSIT D. Hobby
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A PETROLOGICAL, MINERALOGICAL AND GEOCHEMICAL ANALYSIS OF LISTWAENITE ALTERATION AT SPRING CREEK BINGARA IN THE GREAT SERPENTINITE BELT, NEW ENGLAND, NSW E. Holcroft, S. Buckman and I. Neuss
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THE WHITEWASH MOLYBDENUM - COPPER PROJECT AT RAWBELLE, SETTING AND PROGRESS TO DATE M.E. I’Ons and J.L. Goody
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ZIRCON U-PB AND O ISOTOPIC EVIDENCE FOR THE AGE AND SOURCE OF THE S-TYPE BUNDARRA SUPERSUITE GRANITES, SOUTHERN NEO H.Jeon, I.S. Williams and B.W. Chappell
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GEOCHEMICAL AND GEOCHRONOLOGICAL EVOLUTION OF THE TAMWORTH BELT, SOUTHERN NEW ENGLAND OROGEN
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R.J. Korsch, P.A. Cawood and A.A. Nemchin
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GENESIS AND AGE OF MAGMAS OF THE HILLGROVE BATHOLITH, SOUTHERN NEW ENGLAND OROGEN B. Landenberger, S. McKibbin and B. Collins
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A CHAPTER IN THE OROGENIC HISTORY OF AN ACCRETIONARY OROGEN: THE EARLY PERMIAN TRANSITION IN THE SOUTHERN NEW ENGLAND FOLD BELT E. Leitch, P.A. Cawood, R. Merle and A. Nemchin
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EMPLACEMENT AND DEFORMATION RECORDED IN THE HASTINGS BLOCK - CONSTRAINTS FROM SERPENTINITE BODIES AND STRUCTURES WITHIN AND ADJACENT TO THE BLOCK
P.G. Lennox and R. Offler
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STRUCTURAL OBSERVATIONS FROM THE HINGE OF TEXAS OROCLINE P. Li and G. Rosenbaum
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MINING INTO THE NEXT CENTURY: ENVIRONMENTAL ADVANCES, OPPORTUNITIES AND CHALLENGES B.G. Lottermoser
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ENVIRONMENTAL GEOCHEMICAL LEGACIES OF ABANDONED METALLIFEROUS MINE SITES, NEW ENGLAND OROGEN B.G. Lottermoser and P.M. Ashley
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GEOCHEMICAL, ISOTOPIC AND PETROGRAPHIC CONSTRAINTS ON THE ORIGIN AND DEVELOPMENT OF THE BARRINGTON TOPS BATHOLITH I. Meek and B. Landenberger
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YOUNG TRANSPRESSIVE POSITIVE FLOWER STRUCTURE ALONG THE ZAGROS COLLISION ZONE, NAHAVAND AREA, WEST IRAN Mohajjel, M. and Behyari, M.
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DEVONIAN VOLCANICS IN THE NEW ENGLAND OROGEN: TECTONIC SETTING AND POLARITY R. Offler and C. Murray
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GEOCHEMICAL AND MINERALOGICAL ZONATION OF THE CONRAD SILVER DEPOSIT D. Ogdon-Nolan and K. Maher
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ANALOGUES TO MINERAL SEQUESTRATION OF CO2: SOURCES OF CARBON IN MAGNESITE OF ATTUNGA MAGNESITE QUARRY, NSW, AUSTRALIA, A STABLE ISOTOPE STUDY H.C. Oskierski, J.G. Bailey, E.M. Kennedy and B.Z. Dlugogorski
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DISCONTINUOUS OR SLOW EXHUMATION AFTER SUBDUCTION - EVIDENCE FROM HIGH-PRESSURE ROCKS IN THE PEEL MANNING FAULT SYSTEM G. Phillips
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UNRAVELLING THE NEW ENGLAND OROCLINE: GEOLOGICAL AND PALEOMAGNETIC CONSTRAINTS S. Pisarevsky, P. Cawood and E. Leitch
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CORRELATING ORDOVICIAN ELEMENTS OF THE LACHLAN AND NEW ENGLAND OROGENS, EASTERN AUSTRALIA C.D. Quinn and I.G. Percival
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RECENT ADVANCES IN UNDERSTANDING THE WILD CATTLE CREEK ANTIMONY DEPOSIT DORRIGO, NSW G. Rabone
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THE MINERALISATION AND CHARACTERISATION OF THE WILSONS CREEK AU DEPOSIT, WEST OF URALLA, NSW, AUSTRALIA, AS AN INTRUSION-RELATED GOLD DEPOSIT R. A. Robertson
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STRUCTURE OF THE NEW ENGLAND OROCLINES G. Rosenbaum
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THE PETROGENESIS OF LATE PLEISTOCENE BARAMBAH BASALTS FROM SOUTHEAST QUEENSLAND, AUSTRALIA
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R.V.R. Talusani and P.M. Ashley
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THE NEW ENGLAND BATHOLITH: INTERPRETATIONS FROM ZIRCON HF ISOTOPIC RATIOS S. E. Shaw, R. H. Flood and N. J. Pearson
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GOLD, ANTIMONY & ARSENIC TRENDS IN THE SYNDICATE DEPOSIT, HILLGROVE C. Simpson
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MAINLAND SOUTHEAST ASIA - EXPLORATION AND METALLOGENESIS S. Smith
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EARLY CRETACEOUS MAGMATISM IN NEW ZEALAND AND QUEENSLAND: INTRA-PLATE OR INTRA-ARC ORIGIN? A. Tulloch, J. Ramezani, K. Faure and A. Allibone
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CONTRASTING METAMORPHISM AND DEFORMATION OF MAFIC AND FELSIC ROCKS IN A LONG-LIVED MAGMA TRANSFER ZONE, STEWART ISLAND, NEW ZEALAND R. H. Vernon, W. J. Collins, N.D.J. Cook
337
DEFINITION OF A NEW MEMBER OF THE URALLA SUPERSUITE, NEW ENGLAND BATHOLITH: THE NEWHOLME MONZOGRANITE AND ITS DISTINCTION FROM THE MOUNT DUVAL MONZOGRANITE N.Vickery, P.L. Blevin, B. Brown, R. Rutten and P.M. Ashley
343
ATTUNGA SKARN PROJECT: LATEST EXPLORATION FINDINGS AND INTERPRETATIONS N.Vickery, J. Leigh, M. Oates
351
GEOTOURISM IN THE NEW ENGLAND REGION N.Vickery and B. Brown
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GOLD DISCOVERY IN THE GREAT SERPENTINITE BELT A.H. White
363
MARONGHI CREEK BEDS, YARRAMAN BLOCK, SE QLD: PETROGRAPHY, PROVENANCE AND MICROSTRUCTURE
E.C. Willey
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POKOLBIN INLIERS - A RESTRAINING BEND POSITIVE FLOWER: IMPLICATIONS FOR TIMING OF THE HUNTERBOWEN OROGENY E.C. Willey
376
ENHANCED GEOTHERMAL SYSTEMS – CONTRIBUTION OF GRANITES TO THE NEW ENERGY ECONOMY D. Wyborn
384
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Preface Solomon Buckman School of Earth & Environmental Sciences, University of Wollongong, NSW, Australia
The New England Orogen The New England Orogen (NEO) developed along the eastern margin of Gondwana as a result of convergent margin tectonic processes in the Paleozoic to early Mesozoic. It is Australia’s youngest orogen and extends approximately 1300 km along eastern Australia from Bowen (20˚ S) to Newcastle (33˚ S). The eastern limit is not known as it is masked by the Pacific Ocean, but part of the orogen may occur on the western Lord Howe Rise, separated from Gondwana by rifting in the Tasman Sea so it is with great interest that we have contributions in this volume from across the Tasman by Chris Adams investigating potential sediment sources for Permian to Cretaceous tectonostratigraphic terranes in New Zealand. Similar detrital zircon provenance studies are presented by Korsch et al. (this volume) for accretionary complex rocks of the NEO. The exact timing of initiation of the NEO and its relationship to the older Lachlan Orogen to the west is a matter of widespread debate as is the nature of termination of convergent tectonics in the Permian-Triassic. Each major tectonic event has the potential to concentrate minerals in the crust and this conference proceedings presents a wealth of new data that will help to more accurately refine the tectonic evolution and metallogenesis of the New England Orogen.
Tectonics Numerous tectonic models have been proposed for the development of the NEO since the original planetable mapping carried out by Benson (1911-1918). In the past, interpretations of the development of the NEO have been constrained by various models favoured at the time of study (Flood, 1988) highlighting the importance of making analogies to modern, better-understood convergent margin systems to better explain the distribution of rocks within the NEO. Early attempts to explain the geology of the NEO were based on the geosynclinal theory. Schuchert (1916) first referred to the rocks of eastern Australia as the Tasman Geosyncline. Voisey (1959) applied the eugeosynclinal model, as described by Kay (1950), to explain the abnormally thick sedimentary sequences and suggested localised compressive forces were responsible for the folding and faulting. This model involved the almost continuous deposition of sediments since Precambrian times. Deep burial increased temperatures and pressures at depth resulting in regional metamorphism and emplacement of plutonic rocks. The PMFS was recognised as separating a eugeosynclinal central complex from the gently deformed miogeosynclinal western belt of folds and thrusts (Voisey, 1959; Leitch, 1969; Price, 1972). The relationship between plate tectonics and geosynclines was recognised and described by Dewey and Bird (1970). By then it was widely accepted that orogenies and fold and thrust belts, as well as the linked foreland basins were associated with convergent plate margins where subduction of oceanic floor was the dominant process. Wrench fault tectonics accompanies compressional regimes where the direction of movement of the down-going slab is oblique to the trench (Flood, 1988). Packham and Leitch (1974) reviewed the role of plate tectonics in the interpretation of the Tasman Orogenic Zone. Subsequently the next fifteen years were dominated by publications based on this theory (Scheibner, 1973; Harrington, 1974; Leitch, 1974, 1975, 1980; Day et al. 1978; Crook, 1980; Evans & Roberts, 1980; Leitch & Cawood, 1980; Cawood, 1982a, b, c, 1983,1984; Fergusson, 1984a, b, 1985; Fergusson & Flood, 1984; Harrington & Korsch, 1985a, b; Korsch & Harrington, 1981, 1987; Degeling et al. 1986; Murray et al. 1987; Roberts & Engel, 1980, 1987; Korsch et al. 1990). The long-lived, west dipping subduction zone proposed by Leitch (1975) encapsulated the general consensus for the development of the NEO in a postgeosynclinal, plate tectonics paradigm. It involves westward-dipping subduction zone and the development of a continental island arc from latest Silurian to Middle Devonian. Subduction ceased temporarily in the late Middle Devonian but recommenced in the Late Devonian and continued throughout the Carboniferous producing an accretionary complex. By the Early Permian subduction was either oblique or changed to a ix
transform margin. New, rapidly filled basins formed as a result of strike-slip motion or extension. At the end of the Permian, activity in the volcanic chain ceased as did subduction, and a major orogeny occurred throughout the fold belt. Most areas underwent low-grade metamorphism. Blake and Murchey (1988) proposed a comparative approach to explain the New England geology with the application of a Californian model to the NEO. More recently attention has been focused on the methodology of terrane analysis (Cawood & Leitch, 1985; Scheibner, 1985, 1989; Flood & Aitchison, 1988; Aitchison et al. 1992; Flood & Aitchison, 1993a & b). The terrane concept cautions that any diverse assemblages of lithologies within a composite orogenic system, cannot be assumed to have a direct inherent affiliation (Howell, 1995). This is illustrated by the discovery of a 530 Ma age zircons within plagiogranites within small slivers of ophiolite along the Peel-Manning Fault System (PMFS) by Aitchison and Ireland (1995). Traditionally it was thought that lithologies became progressively younger towards the east. These Cambrian ophiolites separating unrelated Devonian strata suggest an alternative model is required to explain the Cambrian to Devonian lithologies within the NEO. The eastern margin of Gondwana changed from a convergent margin setting to either a highly oblique convergent margin or a transform margin setting during the latest Carboniferous to Early Permian. The mechanisms for this are not fully understood but for what ever the reason, a major strike-slip fault system the PMFS, developed within the NEO, extending from Warialda in the north to Port Macquarie in the south. South of Nundle the PMFS bends towards the east resulting in en echelon faulting as opposed to an almost continuous straight fault north of Nundle. Ophiolitic blocks within a serpentinite-matrix were emplaced along the PMFS as a series of cold intrusions or diapirs during the latest Carboniferous to Early Permian. The Cambrian age of the ophiolite blocks (Aitchison et al. 1990) and the Ordovician blueschist and eclogite blocks within the mélange indicates that older subduction-related material has been emplaced rapidly during this event in the Early Permian. It has been speculated that the ophiolite blocks are derived from the Lachlan Fold Belt (Crawford, 1993) and emplaced into the NEO along a low angle decollement (Korsch et al. 1993). A big question that emerged in the 1980’s was the structural development of mega-scale structural features such as the Texas, Manning, Coffs Harbour and Nambucca oroclines (Cawood, 1982b, 1985). Were they originally straight arcs and accretionary complexes that have subsequently been deformed? And if so what caused this large-scale deformation. This continues to attract research interest as outlined by Rosenbaum (this volume). By the Early Permian the relatively high angle convergence plate margin that persisted throughout the Carboniferous evolved into either a transform or highly oblique convergent margin (Aitchison & Flood, 1992). Consequently several elongate, rapidly filled sedimentary basins developed along the PMFS either during or just after serpentinite emplacement. Most of these basins are fault-bounded but rare unconformable contacts over older Devonian rocks are present (Allan & Leitch, 1990). Sedimentary rocks of these basins are generally assigned to the Manning Group (Voisey, 1957; Mayer, 1972). The Manning Group dissects Devonian and Upper Carboniferous sedimentary rocks and is intruded in part by Middle Permian plutons (Leitch, 1974). Rare fossils found in the Manning Group sedimentary rocks indicate an age of latest Carboniferous to Early Permian (Runnegar, 1970; Mayer, 1972; Price, 1972). Large quantities of detritus were shed rapidly from adjacent uplifted basement rocks into rapidly subsiding basins and the New England would have truly been a dynamic place to be at the time particularly with the eminent intrusion of voluminous granite batholiths soon after.
Granites The story of granites in the New England has a rich history and has culminated in the granites database as presented in this volume by Bruce Chappell. This is a lifelong compilation of geochemical and geochronological data of almost every single granitic pluton in the New England. The magmatic history spans the Carboniferous to the Triassic and includes the intrusion of I-type, S-type and A-type granites respectively. The relationship, or lack of, between these granite supersuites and the evolving continental arc and the underlying subducting slab is still difficult to resolve but is becoming clearer as more precise zircon ages are obtained. A significant number of granite-related contributions are made in this volume by the likes of Chappell, Champion, Vernon, Bryant, Jeon and Blevin. There is no doubt that granites play an integral part of the formation of significant mineral deposits in the New England and the Chappell database will undoubtedly be used for many years to come as an exploration tool.
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Mineralisation The lithological ingredients and geological processes required to cook up rich ore deposits in the Earth’s crust are varied and complex but some of the essential ingredients undoubtedly include; i)
magma as the source of heat, metals and fluids
ii)
metamorphism and alteration
iii)
hydrothermal fluids to scavenge and mobilize metals
iv)
deformation, fluid pathways and structural traps
v) diverse host rocks that provide contrasting chemical conditions that may favour the precipitation of dissolved metals from hydrothermal solutions, and lastly, vi)
erosion to expose the deeper crustal level ore bodies.
There is no doubt that the New England Orogen has all of the ingredients required to form world-class ore bodies. Certainly, significant mineral deposits have been discovered and mined as at Mount Morgan and Hillgrove. Despite this, there have been relatively few world-class discoveries and the area has struggled to attract the exploration investment that has been so lavish in other regions such as the Lachlan Orogen, Mt Isa, Broken Hill and the greenstone belts of W.A. It is worth contemplating the reasons, or lack, for this. Is there some fundamental geological ingredient or tectonic process lacking within the New England Orogen? If so, what is it? Is it more of a circular exploration paradigm revolving around the notion that a big deposit is yet to be discovered in the region so why spend money exploring there? It is worth highlighting that exploration in the Lachlan Orogen was very limited until the “discovery” and development of mines at Cadia/Ridgeway, North Parkes, and Lake Cowal. Since then many of the large exploration companies have moved back into the region and there has been a focus on deep exploration drilling within the Ordovician volcanic packages of the Macquarie Arc to discover high-tonnage, low-grade porphyry-related ore bodies. But without the initial lead by Newcrest to risk mining a very low-grade, marginal deposit at Cadia Hill the other exploration successes such as Ridgeway and Cadia East would not have happened and N.S.W. would not be enjoying the royalties it currently earns from these deposits. More recently there has been exploration success at McPhillamys, which appears to be a different style of deposit again, related to an extensive shearzone. Thirty years earlier almost all exploration in the Lachlan was focused on the discovery of VMS-style mineralisation in the Silurian volcanics as at Woodlawn and Captains Flat. This highlights the sentimental way in which companies can be blinkered by the latest vogue mineral deposit model and potentially blind to new styles of deposits in traditionally “unfashionable” regions. Other reasons for the lack of exploration success in the NEO may relate to the rough and often inaccessible terrain or the thick cover of Tertiary alluvium and volcanics in places and the lack of suitable soil or regolith mediums with which to conduct meaningful and widespread surface geochemical programs. Another factor to consider is whether the New England Orogen has experienced enough erosion to expose the high temperature, deeper crustal level porphyry and hydrothermal systems that are exposed in the older Lachlan Orogen. I suspect that the last point is an important one to consider in future exploration programs and that much of the New England may be exposed at a level transitional between the low-temperature, near surface epithermal systems common in active orogens and the deeper, high-temperature hydrothermal systems found associated with mid- to upper-crustal level porphyry-style deposits. A new exploration paradigm closely linked to well-founded tectonic interpretations is required to adequately test the New England. Indeed, the fact that there is so much “smoke” in the form of small, high-grade deposits and alluvial workings, suggests that the New England Orogen may represent a relatively under-explored part of Australia and possibly a new frontier for rejuvenated, successful exploration.
Tertiary Widespread erosion took place before the eruption of the Tertiary basalts as evident by the irregular unconformable contact between the Tertiary conglomerates and the underlying Paleozoic strata. The last major tectonic event to occur in the NEO was the eruption of extensive basalt flows due to southward migrating hotspot activity during the Tertiary (Johnson, 1989). Erosion and dissection continued after eruption of the Tertiary basalts thus exposing the underlying strata of the New England orogen and forming the present structurally controlled geomorphology. It is quite likely that intraplate, neotectonic activity continues to shape the landscape in more subtle ways than during the Paleozoic-Mesozoic when eastern Australia was part of an extensive convergent margin. xi
Conclusions Although the rocks of the NEO have not changed over the past 100 years, geological interpretations have. Peter Flood (this volume) outlines the major phases of thinking relating to the NEO. It will be interesting to see how a new generation of young geologists with access to more readily available and accurate geochronological and geochemical analytical tools will interpret the ever-increasing amount of data flooding in from this fascinating orogen. We begin this volume with a visually abstract portrayal of the evolution of Eastern Australia. This series of five paintings is kindly provided by John Jackson and beautifully encapsulates the connection between time, geological events and cross-cutting relationships, which is still fundamental to any study of the Earth and its geological history.
References Aitchison, J.C. & Flood, P.G. 1992. Early Permian transform margin development of the southern New England orogen, eastern Australia (eastern Gondwana). Tectonics, 11, 1385-1391. Aitchison, J.C., and Ireland, T.R. 1995, Age profile of ophiolitic rocks across the Late Paleozoic New England orogen, New South Wales, Australia: Implications for tectonic models. Australian Journal of Earth Sciences, 42, 11-23. Aitchison, J.C., Ireland, T.R., Blake, M.C. Jr & Flood, P.G. 1992. 530 Ma zircon age for ophiolite from the New England orogen: oldest known rocks from eastern Australia. Geology 20, 125-128. Aitchison, J.C., Stratford, J.M. & Buckman, S. 1997. Geology of the Upper Barnard region: evidence of Early Permian oblique-slip faulting along the Peel-Manning Fault System. In Tectonics and Metallogenesis of the New England Orogen; Ashley, P.M. & Flood, P.G., (eds.) Geological Society of Australia, Special Publication, 19,188-196. Allan, A.D. & Leitch, E.C. 1990. The tectonic significance of unconformable contacts at the base of Early Permian sequences, southern New England Fold Belt. Australian Journal of Earth Sciences, 37, 43-49. Benson, W.N. 1911. A preliminary account of the geology of the Nundle district, near Tamworth, N.S.W. Australia. Assoc. Advancement Sci. Rep., 13, 100-106. Benson, W.N. 1912. Geology of the Nundle district. Report of the Australia and New Zealand Association for the Advancement of Science, 12, 100. Benson, W.N. 1913a. The geology and petrology of the Great Serpentine Belt of New South Wales. Part i. Introduction. Proceedings of the Linnean Society of New South Wales, 38, 569-596. Benson, W.N. 1914a. The geology and petrology of the Great Serpentinite Belt of New South Wales. Part ii. The geology of the Nundle district. Proceedings of the Linnean Society of New South Wales, 38, 490-517. Benson, W.N. 1914b. The geology and petrology of the Great Serpentinite Belt of New South Wales. Part iii. The geology of the Nundle district. Proceedings of the Linnean Society of New South Wales, 38, 662-724. Benson, W.N. 1915a. The dolerites, spilites and keratophyres of the Nundle district. Part iv. Proceedings of the Linnean Society of New South Wales, 40, 121-173. Benson, W.N. 1917. The geology and petrology of the Great Serpentinite Belt of New South Wales. Part vi. A general account of the geology and physiography of the western slopes of New England. Proceedings of the Linnean Society of New South Wales, 42, 223-283. Benson, W.N. 1918a. The geology and petrology of the Great Serpentine Belt of New South Wales. Appendix to part vi. The Attunga district. Proceedings of the Linnean Society of New South Wales, 42, 693700. Benson, W.N. 1918b. The geology and petrology of the Great Serpentine Belt of New South Wales. Part viii. The extension of the Great Serpentine Belt from the Nundle district to the coast. Proceedings of the Linnean Society of New South Wales, 43, 593-599. Blake, M.C. Jr, & Murchey, B. 1988b. A California model for the New England fold belt. In: Kleeman J.D. (ed.), New England Orogen Tectonics and Metallogenesis, University of New England, Australia, pp 20-31.
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Cawood, P.A. 1982a. Tectonic reconstruction of the New England Fold Belt in the Early Permian: an example of development at an oblique slip margin. In: Flood, P.G. & Runnegar, B. (eds.) New England geology, Voisey Symposium Volume: Armidale N.S.W., University of New England, 25-34. Cawood, P.A. 1982b. Structural relations in the subduction complex of the Paleozoic New England Fold Belt, eastern Australia. Journal of Geology., 90, 381-392. Cawood, P.A. 1982c. Correlation of stratigraphic units across the Peel Fault System. In: Flood, P.G. & Runnegar, B. (eds.) New England Geology, University of New England, Armidale, 55-61. Cawood, P.A. 1983. Modal compositions and detrital clinopyroxene geochemistry of lithic sandstones from the New England Fold Belt, east Australia: a Paleozoic forearc terrane. Geological Society of America Bulletin, 94, 1199-1214. Cawood, P.A. 1984. A geochemical study of meta-basalts from a subduction complex in eastern Australia. Chemical Geology, 43, 29-47. Cawood, P.A. & Leitch, E.C. 1985. Accretion and dispersal tectonics of the southern New England Fold Belt, eastern Australia. In: Howell, D.G. (ed.) Tectonostratigraphic terranes of the Circum-Pacific region, Circum-Pacific Council for Energy and Mineral Resources. Earth Science Series, 1, 481-492. Crawford, A.J. 1993. Late Proterozoic - Cambrian tectonic evolution of the Lachlan Foldbelt in SE Australia, South Island New Zealand and northern Victoria Land, Antarctica. In: Flood, P.G. & Aitchison, J.C. (eds.) New England Orogen, eastern Australia. The University of New England, Armidale, pp. 147-155. Crook, K.A.W. 1980. Fore-arc evolution in the Tasman Geosyncline: the origin of the south-eastern Australian crust. Journal of the Geological Society of Australia, 27, 215-232. Day, R.W., Murray, C.G. and Whitaker, W.G., 1978. The eastern part of the Tasman Orogenic Zone. Tectonophysics, 48, 327-364. Degeling, P.R., Gilligan, L.B., Scheibner, E., & Suppel, D.W.1986. Metallogeny and tectonic development of the Tasman Fold Belt System in New South Wales. In: Scheibner, E. (ed.), Metallogeny and Tectonic Development of eastern Australia, Ore Geology Reviews, 1, 259-313 Dewey, J.F. & Bird, J.W. 1970. Plate tectonics and geosynclines. Tectonophysics, 10, 625-638. Evans, P.R. & Roberts, J. 1980. Evolution of central eastern Australia during the late Paleozoic and early Mesozoic. Journal of the Geological Society of Australia, 26, 325-340. Fergusson, C.L. 1984a. Tectono-stratigraphy of a Paleozoic subduction complex of central Coffs Harbour Block of north-eastern New South Wales. Journal of the Geological Society of Australia 31, 217-236. Fergusson, C.L. 1984b. The Gundahl Complex of the New England Fold Belt, eastern Australia: a tectonic melange formed in a Paleozoic subduction complex. Journal of Structural Geology, 6, 257-271. Fergusson, C.L. 1985. Trench floor sedimentary sequences in a Paleozoic subduction complex. Journal of Structural Geology, 42, 181-200. Fergusson, C.L. & Flood, P.G. 1984. A late Paleozoic subduction complex in the Border Rivers area of southeast Queensland. Proceedings of the Royal Society of Queensland, 95, 47-55. Flood, P.G. 1988. New England Orogen: Geosyncline, Mobile Belt and Terranes. In: Kleeman, J.D. (ed.) New England Orogen Tectonics and Metallogenesis, Armidale, Australia, University of New England, 7-10. Flood, P.G. & Aitchison, J.C., 1988. Tectonostratigraphic terranes of the southern part of the New England Orogen, eastern Australia. In, Kleeman J.D. (ed.) New England Orogen: Tectonics and Metallogenesis, University of New England, Armidale, Australia, pp 7-10. Flood, P.G. & Aitchison, J.C. 1993a. Understanding New England geology: the comparative approach. In: Flood, P.G. & Aitchison, J.C. (eds.) New England Orogen, eastern Australia. The University of New England, Armidale, pp 1-10. Flood, P.G. & Aitchison, J.C. 1993b. Recent advances in understanding the geological development of the New England Province of the New England Orogen. In: Flood, P.G. & Aitchison, J.C. (eds.) New England Orogen, eastern Australia. The University of New England, Armidale, pp 61-68.
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Harrington, H.J. 1974. The Tasman Geosyncline in Australia. In: Denmead A.K., Tweedale, G.W. and Wilson, A.F. (eds.). The Tasman Geosyncline - a symposium, Geological Society of Australia, Queensland Division, Brisbane. Pp. 383-407. Harrington, H.J. & Korsch, R.J. 1985a. Tectonic model for the Devonian to Middle Permian of the New England Orogen. Australian Journal of Earth Sciences, 32, 163-179. Harrington, H.J. & Korsch, R.J. 1985b. Deformation associated with the accretion of the Gympie terrane in eastern Australia. In, Leitch, E.C. (ed.) Third Circum-Pacific Terrane Conference Extended Abstracts. Geological Society of Australia Abstracts, 14, 104-108. Howell, D., G., 1995. Principles of terrane analysis; new applications for global tectonics. Topics in the Earth Sciences, 8. Chapman and Hall, London, United Kingdom, 245 pp. Johnson, R.W. Knutson, J. & Taylor, S.R. 1989. Intraplate volcanism in eastern Australia and New Zealand. Cambridge University Publishing, Cambridge, England. 408 pp. Kay, M. 1951. North American geosynclines. Geological Society of America Memoir 48, 143 pp. Korsch, R.J. & Harrington, H.J. 1981. Stratigraphic and structural synthesis of the New England Orogen. Journal of the Geological Society of Australia 28, 205-226. Korsch, R.J. & Harrington, H.J.1987. Oroclinal bending, fragmentation and deformation of terranes in the New England orogen, eastern Australia. In: Leitch, E.C. & Scheibner, E. (eds.) Terrane Accretion and Orogenic Belts, American Geophysical Union, Geodynamic Series 19, 129-139. Korsch, R.J. & Harrington, H.J., Murray, C.G., Fergusson, C.L., Flood, P.G. 1990. Tectonics of the New England Orogen. Bureau of Mineral Resources, Australia, Bulletin, 232, 35-52. Korsch, R.J., Wake-Dyster, K.D., and Johnstone, D.W. 1993. The Gunnedah Basin - New England Orogen deep seismic reflection profile: implications for New England tectonics. In: Flood, P.G. & Aitchison, J.C. (eds.) New England Orogen, eastern Australia. The University of New England, Armidale, pp 85-100. Leitch, E.C. 1969. Igneous activity and diastrophism in the Permian of New South Wales. Geological Society of Australia Special Publication,. 2, 21-37. Leitch, E.C. 1974. The geological development of the southern part of the New England Fold Belt. Journal of the Geological Society of Australia 21, 133-156. Leitch, E.C. 1975. Plate tectonic interpretations of the Paleozoic history of the New England Fold Belt. Geological Society of America Bulletin 86, 141-144. Leitch, E.C. 1980. The Great Serpentinite Belt of New South Wales: diverse mafic-ultramafic complexes set in a Paleozoic arc. In: Panayiotou, A. (ed.) Proceedings of the International Ophiolite Symposium, Cyprus, Cyprus Geological Survey, Nicosia, Cyprus. Pp 637-648. Leitch, E.C. & Cawood, P.A.1980. Olistoliths and debris flow deposits at ancient consuming plate margins; an eastern Australian example. Sedimentary Geology, 25, 5-22. Mayer, W. 1972. Paleozoic sedimentary rocks from the southern New England: a sedimentological evaluation. Ph.D. Thesis, Univ. New England, Armidale, Australia. Murray, C.G., Fergusson, C.L., Flood, P.G., Whitaker, W.G. & Korsch, R.J. 1987. Plate tectonic model for the Carboniferous evolution of the New England Fold Belt. Australian Journal of Earth Sciences 34. 213236. Packham, G.H., & Leitch, E.C. 1974. The role of plate tectonic theory in the interpretation of the Tasman Orogenic Zone. In: Denmead, A.K., Tweedale, G.W. & Wilson, A.F. (eds.) The Tasman Geosyncline - A Symposium, Geological Society of Australia, Queensland Division. pp. 129-156. Price, I. 1972. A new Permian and Upper Carboniferous (?) Succession near Woodsreef, N.S.W., and its bearing on the paleogeography of Western New England. Proceedings of the Linnean Society of New South Wales, 97 202-210. Roberts, J. & Engel, B.A. 1980. Carboniferous paleogeography of the Yarrol and New England Orogens, eastern Australia. Journal of the Geological Society of Australia, 27, 167-187. Roberts, J. & Engel, B.A. 1987. Depositional and tectonic history of the southern New England Orogen. Australian Journal of Earth Sciences 34, 1-20. xiv
Runnegar, B. 1970 Eurydesma and Glendella gen. nov. (Bivalvia) in the Permian of eastern Australia. Bureau of Mineral Resources, Australia. Bulletin, 116, 83-106. Scheibner, E. 1973. A plate tectonic model of the Paleozoic tectonic history of New South Wales. Journal of the Geological Society of Australia 20, 405-426. Scheibner, E. 1985. Suspect terranes in the Tasman fold Belt System, eastern Australia. In: Howell, D.G. (ed.) Tectonostratigraphic terranes of the Circum-Pacific Region. Circum-Pacific Council for Energy and Mineral Resources. Earth Science Series 1, 493-514. Scheibner, E. 1989. The tectonics of New South Wales in the second decade of application of the plate tectonic paradigm. Journal and Proceedings of the Royal Society of New South Wales, 122, 35-74. Schuchert, C. 1916. The problems of continental fracturing and diastrophism in Oceanica. American Journal of Science, 42, 91-105. Voisey, A.H. 1957. Further remarks on the sedimentary formations of New South Wales. Journal and Proceedings of the Royal Society of New South Wales, 91, 165-188. Voisey, A.H. 1959. Tectonic evolution of north-eastern New South Wales, Australia. Journal and Proceedings of the Royal Society of New South Wales 97, 65-72.
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NEO 2010
“Migrating in Time”
John H. Jackson
This is a series of five paintings by Dr. John H. Jackson (known as “The Rock Doctor”) highlighting the geological evolution of “The Green Cauldron” landscapes that are located in South East Queensland and North East New South Wales. The Green Cauldron contains some of Australia’s most significant landscapes and includes The Scenic Rim, The Border Ranges, and Mount Warning. It also covers parts of the Clarence Morton Basin and the Texas/Coffs Harbour Mega-fold. The geostory told by these paintings contain part of Australia’s journey from Antarctica to Asia. The paintings were completed in April 2010 and were first exhibited at Brisbane’s Convention and Exhibition Centre in May of the same year. Each painting represents a significant tectonic chapter in the four hundred million year old, geo-construction of Australia’s Green Cauldron.
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Painting One “MIGRATING IN TIME 400 to 300 MILLION YEARS AGO” At 400 million years ago man’s evolution was still taking place in the oceans and eastern Australia was connected to both New Zealand and Antarctica but had separated from North America. A hot plume of the earth’s mantle, The Pacific Super-plume had risen from the super hot, liquid, outer core to just under the crust and tore apart the super-continent “Rodinia” of which Australia was a part. The Pacific Super-plume then set about creating the floor of the Pacific Ocean. Painting One represents part of the western continental shelf, slope, subduction zone and deep sea floor of this newly forming ocean. Brisbane would have been on the deep sea floor of this newly forming ocean. On the left of the picture is the blue continental shelf containing round, red volcanic centres in a chain of deep purple islands surrounded by pale mauve beaches. To the right of the islands are three pale mauve and deep purple sediment fans. These fans are made mostly from volcanic debris. They spread east over the midblue continental slope, across the thin, north-south striking, pink subduction zone and out onto the dark blue and jasper coloured deep sea floor.
Painting Two “MIGRATING IN TIME 290 MILLION YEARS AGO” By 290 million years ago eastern Australia and New Zealand were being deformed and uplifted from the ocean. Furthermore, eastern Australia was being bent into a huge mega-fold by compressing the continent from the north and the south. This giant mega-fold bent the marine and volcanic rocks into a huge “S-bend” that can be traced from west of Stanthorpe, around through Mount Barney, down to Coffs Harbour, up to the Gold Coast and on to Brisbane. The result was a shortening of Australia’s eastern continental crust by some 500 kilometres.
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Painting Three “MIGRATING IN TIME 250 MILLION YEARS AGO” This is the time of earth’s biggest life extinction. In the region of The Green Cauldron, large masses of hot granite magma were intruded into the rocks of the megafold. Since then the Solar System has done one revolution of the Milky Way and vines have been planted on the Granite Belt of South East Queensland. Mid-red to pale red “blobs” of granite magma are intruding the rocks of the mega-fold.
Painting Four “MIGRATING IN TIME 160 MILLION YEARS AGO” Prior to New Zealand and Antarctica being split from Australia the area of The Green Cauldron was split into one or more rift valleys. These rift valleys were filled with volcanic debris, sands, clays and substantial organic matter from peat swamps and forests. In the top right, sloping down to the mid-right, is a multicoloured area representing a fault plane that forms the eastern boundary of a rift valley. To the left of the fault plane are pale yellow pebbles and pale yellow sand grains. Pale green ribbons occur left of the sand grains and represent silt and clay. The darkest greens are areas of peat swamps, lakes and lagoons.
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Painting Five “MIGRATING IN TIME 17 MILLION YEARS AGO” At the time of this painting New Zealand had been split away from Australia during the opening of the Tasman Sea, Australia had attempted to split from New Guinea with the opening of the Coral Sea and Antarctica had been left behind on the South Pole. After Australia left Antarctica it had initially migrated north at around 71 kilometres per million years. As it migrated its underside was being cooked by “mantle hotspots” initiated during the opening of the Coral Sea. The hot spots sometimes broke through the crust leaving an “icing” of hot volcanic lava. At first Australia’s northwards journey towards Melanesia and South East Asia was reasonably fast but at 26 million years ago it changed to a westerly drift and slowed to about 26 kilometres per million years. Running into the world’s largest igneous province (now located around the Solomon Islands) had interrupted the northward journey of the Australian Continental Plate. The Pacific Superplume made this huge igneous “rock” during a cataclysmic outpouring of magma 122 million years ago. It took Australia 3 million years to navigate around this “gigantic rock” but it slowed travel time enough for the hot spot beneath to put the “icing” on The Green Calderon and produce a monument of landscapes that represent the first docking of Australia with Melanesia and South East Asia. After navigating this “giant rock” Australia speed up to 61 kilometres per million years and continued north (reference: 2008 “Rapid change in drift of the Australian plate records collision with Ontong Java plateau” Knesel K.M., Cohen B.E., Vasconcelos P.M. & Thiede D.S. Nature vol., 454/7). The hot spot that put the “icing” on The Green Calderon is now located beneath Bass Strait. The dark mauve ribbon that runs from top to bottom is the underside trace of the “mantle hot spot” as Australia moved across it. The deep purple and red “flowerets” represent isolated or overlapping volcanic provinces with their associated yellow centres. The red is for “sticky”, viscous magma/lava such as rhyolite and the deep purple is for “runny” magma/lava such as basalt. The top, small, isolated, volcanic province is Flinders Peak at 26 million years old. The central, overlapping, left to right provinces constitute The Green Cauldron and range in age from 26 to 23 million years with some late stage volcanic activity up to 20 million years. The isolated volcanic province below The Green Cauldron is located at Belmore New South Wales and the bottom most province is at Ebor New South Wales. Brisbane is located in the top right, Coffs Harbour in the bottom right and Stanthorpe just left of centre on the big red granite “blob”. John H. Jackson, “The Rock Doctor” (April, 2010)
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NEO 2010
Accretionary complexes in eastern Australia and New Zealand: matching their sediment sources and destinations C.J. Adams GNS Science, Private Bag 1930, Dunedin, New Zealand
Keywords: New England Orogen, Coastal Block, Eastern Province New Zealand, Torlesse Terrane, detrital zircon patterns, provenance studies
Introduction Accretionary complexes form a major part of the late Paleozoic-Mesozoic margin of Eastern Gondwanaland. A Devonian-Carboniferous sector in eastern Australia (Coastal, Texas-Woolomin, Beenleigh and Coffs Harbour Blocks) falls within the New England Orogen (NEO) (Holcombe 1997a, b, Glen 2005), whilst that in New Zealand makes up the Eastern Province, now seen as a collage of mainly Permian to Cretaceous tectonostratigraphic terranes (Torlesse, Waipapa and Caples Terranes) along the length of both North and South Islands (Fig. 1). The Australian and New Zealand sectors are of comparable size, but whilst the sedimentary rocks of the New England Orogen accretionary wedge have probable sediment sources in major contemporaneous magmatic arcs immediately inboard to the west (Auburn and Connors magmatic arcs, Campwyn Volcanics, and early Paleozoic-Proterozoic rocks of the hinterland), those in the Eastern Province of New Zealand appear to have no such local source. In the latter, the accretionary rocks of the Torlesse Terrane (Carboniferous-Cretaceous) are derived from continental sources of plutonic and metamorphic rocks (MacKinnon 1983), whose ages cannot be matched with suitable sources in the Western Province of New Zealand (Cambrian-Devonian) or correlative rocks in formerly adjacent Antarctica. The Torlesse sediment sources must be dominated by Permian-Triassic granitoids, and to a minor extent by early PaleozoicProterozoic igneous rocks. For this reason, the Torlesse Terrane (and companion Waipapa and Caples Terranes), are generally considered as ‘suspect’ terranes originating at the eastern Australian continental border. This then suggests that there should have been some evolutionary transition between the accretionary wedges at the Eastern Gondwanaland margin, from an autochthonous Devonian-Carboniferous sector in the New England Orogen, to the now allochthonous, Permian-Triassic sector of the New Zealand Eastern Province. Between these two main convergent phases, there is an Early Permian, extensional phase recognised only in the New England Orogen (Holcombe et al. 1997b, Korsch et al. 2009a), and well exemplified by the Nambucca Block (Fig. 1)(Leitch et al. 1979). Recent detrital zircon studies in the New England Orogen and New Zealand Eastern Province accretionary rocks have now established the principal components of their sediment source regions, and the age databases in both regions are sufficiently detailed to examine here the nature and significance of the transitional Early Permian period (Cawood et al. 1999, 2002, Pickard et al. 2000, Wandres et al. 2004a, b, Adams et al. 2007, 2009a, b, Korsch et al. 2009b).
Detrital zircon age components in Torlesse Terrane, New Zealand The large detrital zircon age databases for New England Orogen and New Zealand Eastern Province accretionary rocks have generally been presented in the form of probability density diagrams, but also analysed more rigorously, by treating only those zircon age components as significant which comprise >4 individual ages and ≥4% of the total dataset. These data are here compiled (Fig. 2) in stratigraphic age order, or where this is uncertain, the maximum stratigraphic age (as constrained by the youngest zircon age component). Thus, New Zealand age data from the Torlesse Terrane (Fig. 2A) reveal the youngest, though not necessarily major, age components often fall within the depositional age range (where known), whilst the major components are invariably Late Permian to Early Triassic, and endure over 150 million years (Late Permian to mid-Cretaceous) of accretionary deposition. This pattern suggests a very major, voluminous 5
NEO 2010 sediment source such as a slowly-eroding cordilleran-type batholith, accompanied by smaller contributions from contemporary active margin volcanism.
Fig. 1: The accretionary complexes of the New England Orogen (left) and New Zealand (right). Detrital zircon age patterns are shown ( insets) for sandstones NAMX1 of the Nambucca Block, and CONZ1 of the earliest Torlesse Terrane at Conical Mt., Otago.
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NEO 2010
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A
New Zealand Eastern Province Torlesse Terrane n=209 of N=213
244 255 264
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1234 1564 1580 1754
0 0
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detrital zircon component age (Ma)
Fig. 2: Significant detrital zircon age components in sandstones from the accretionary wedges of (A) Torlesse Terrane, New Zealand, and (B) New England Orogen, Eastern Australia. Ages (and errors at 95% confidence) of individual zircon age components are plotted on the horizontal axis, and then the sample sets are ordered vertically by stratigraphic age where known, or otherwise the maximum stratigraphic age as given by age of youngest detrital zircon age component. The height of individual databoxes is set according to the proportion of that component as a percentage of the total set, for which a scale bar is shown at the right side.
Fig. 3: Combined histogram and probability diagrams of significant detrital zircon age components in sandstones in accretionary wedges from (A) Torlesse Terrane, New Zealand, (B) volcaniclastic successions of Wandilla Formation, Coastal Block and Coffs Harbour Block, New England Orogen and (C) quartzose successions in Shoalwater Formation, Coastal Block and Beenleigh Block, New England Orogen. Age components >600 Ma are stacked at right side.
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NEO 2010 Torlesse Terrane sandstones also commonly contain significant proportions (>20% total) of earlier Paleozoic and Precambrian zircons, and in the Triassic and Permian examples especially, these invariably contain significant age components of Carboniferous and Cambrian-Ordovician age (Fig. 2A). It is likely that these are inherited from the major cordilleran granitoid sources postulated above, or older provinces of their hinterland. A histogram (Fig. 3A) summarises the grouping of these age components. Whilst it is clear that Middle Triassic-Late Permian components (230-265 Ma) are particularly prevalent, Early (340-360 Ma) and Late (300-330 Ma) Carboniferous and Cambrian-Ordovician (490-510 Ma) groups are significant. Although there is no fossil evidence, the oldest Torlesse rocks might predate the exhumation of the major Late Permian-Early Triassic sources. In the southernmost Torlesse Terrane, at Conical Mountain, Otago, South Island (Fig. 1), a possible Middle Permian greywacke, occurring above (contact uncertain) a very local (300 m (Gilligan et al., 1992). Mineralisation is representative of a crustally shallow version of the orogenic Au deposit spectrum and is hosted in laterally and vertically persistent, steeply-dipping vein and breccia systems over an area of ~30 km2 straddling the Bakers Creek gorge system. Host rocks are dominantly metamorphosed mudstone-siltstone-sandstone, with subordinate monzogranite and diorite. Mineralised structures are dominated by quartz and carbonate, containing stibnite, arsenopyrite, pyrite, gold and adularia; immediate host rocks have strong hydrothermal alteration haloes of sericite, carbonate, quartz, arsenopyrite and pyrite (Ashley and Craw, 2004). Mineralised rock and mill tailings typically contain Sb and As values in the hundreds of ppm to low percent range. Owing to the presence of significant carbonate, the depth of weathering is shallow and no acid mine drainage occurs; effluent and stream waters have pH values of 6-8.5 (Ashley et al., 2003). Mining and processing has caused physical and chemical dispersion of Sb and As into Bakers Creek with high solubilities of Sb and As in nearneutral effluent fluids (Ashley et al., 2003).
Contamination of the Bakers Creek-Macleay River-floodplain system Stream sediments have been significantly contaminated by Sb and As from Hillgrove to the Pacific Ocean. The stream system is a high-energy, gravel bed type to the tidal limit, allowing efficient and rapid transport of particulates. Antimony concentrations exceed background by >100x for 50 km downstream of Hillgrove and remain close to, or above 10x background to the floodplain (Ashley et al., 2007). A similar pattern is demonstrated by As, but at an order of magnitude lower. In Bakers Creek, some of the Sb and As are retained in detrital heavy minerals, but with progression downstream, the elements are also associated with HFO (Tighe, 2005) and possibly with clay minerals, organics, neoformed sulphides and jarosite. Major sediment transport occurs during flood events. Flights of modern and older terraces are common along the Macleay and many reflect the prior dumping of contaminated sediment during floods. Coring of sediment profiles on terraces has demonstrated that post-mining floods have deposited layers of sediment that are significantly more contaminated (3-10x) than currently active stream sediments. These deposits, dumped by historic major floods (e.g. 1893, 1949, 1950, 1963, 2001) imply that more-highly contaminated sediment has been flushed down the river system in earlier years. The terrace deposits will provide a reservoir of contaminated material accessible to future high floods and associated erosion. The Macleay floodplain has a veneer of flood-deposited silt and mud with low to moderate Sb and As contamination over 90% of the area. Antimony and As concentrations over 6-8% of the area exceed national environmental investigation levels (Tighe et al., 2005). Coring of floodplain sediments has demonstrated that Sb contamination is mostly in the top 20-40 cm and although As is also concentrated in the same interval, its values remain elevated at greater depth, in part due to its sequestering in Fe sulphides in potential acid sulphate soils (ASS). Dating of sediments by 210Pb has indicated modern ( 102-103 years. In the floodplain/estuary environment, a fertile and agriculturally productive region is affected by widespread low-moderate Sb and As soil contamination. There is demonstrated uptake into pastures and potentially into crops (cycling into grazing animals and humans). Local water contamination has occurred, leading to potential aquaculture implications. There is little that remediation could achieve both practically and economically in the catchment. More research needs to be undertaken on 25
NEO 2010 biogeochemical pathways and toxicology, especially for Sb, leading to development of appropriate management strategies.
Environmental and human health risks This investigation has raised questions regarding environmental and human health risks, some of which are currently unquantifiable due to uncertainty about the toxicological effects of Sb. The main risks are inherent in the lower Macleay and floodplain, where there is greater population density and intensive landuse, and stem from agriculture and grazing, and potentially from drinking water supplies. Is the exceedence of soil Sb and As environmental investigation levels in areas of the catchment important for environmental protection (e.g. for grazing and dairying)? There is no current soil health investigation level for Sb. Could the higher soil Sb levels pose a health threat (e.g. via grazing and cropping on the floodplain and river terraces)? What effect do pasture Sb and As levels have on grazing animals? Is food chain transfer and bio-accumulation occurring? Are the elevated stream water concentrations of Sb and As affecting the biota and is there a risk to drinking water supplies? These are some of the questions still to be answered. Effective management (planning and action) of the extensive Sb and As contamination can only be successfully achieved by a thorough scientific understanding of the issue by statutory authorities (health, environment, land and water, agriculture), local councils and research providers.
Acknowledgements This investigation has been supported financially and logistically over many years by the NSW Environmental Trust, Straits Hillgrove Gold, NSW Department of Infrastructure and Industry, and NSW National Parks and Wildlife Service.
References Ashley, P.M. and Craw, D., 2004. Structural controls on hydrothermal alteration and gold-antimony mineralization in the Hillgrove area, NSW, Australia. Mineralium Deposita, 39, 223-239. Ashley, P.M., Craw, D., Graham, B.P. and Chappell, D.A., 2003. Environmental mobility of antimony around mesothermal stibnite deposits, New South Wales, Australia and southern New Zealand. Journal of Geochemical Exploration, 77, 1-14. Ashley, P. and Graham, B., 2001. Heavy metal loadings of streams in the Macleay River catchment. Geological Survey of New South Wales Open File Report GS2001/303, 129 pp. Ashley, P.M., Graham, B.P., Tighe, M.K. and Wolfenden, B.J., 2007. Antimony and arsenic dispersion in the Macleay River catchment, New South Wales, Australia: a study of the environmental geochemical consequences. Australian Journal of Earth Sciences, 54, 83-103. Atkinson, G. 1999. Soil landscapes of the Kempsey – Korogoro Point 1:100 000 sheet. NSW Department of Land and Water Conservation, Sydney. Craw, D., Falconer, D. and Youngson, J.H. 2003. Environmental arsenopyrite stability and dissolution: theory, experiment, and field observations. Chemical Geology, 199, 71-82. Gilligan, L.B., Brownlow, J.W., Cameron, R.G. and Henley, H.F. 1992. Dorrigo-Coffs Harbour 1:250 000 Metallogenic Map. SH/56-10, SH/56/-11. Metallogenic study and mineral deposit data sheets. Geological Survey of New South Wales, Sydney. Lottermoser, B.G., Ashley, P.M., Muller, M. and Whistler, B.D., 1997. Metal contamination due to mining activities at the Halls Peak massive sulphide deposits, New South Wales. Geological Society of Australia Special Publication 19, 290-299. NEPC (National Environment Protection Council), 1999. Assessment of site contamination. Schedule B (1) Guideline on the investigation levels for soil and groundwater. http://www.ephc.gov.au/pdf/cs/cs_01_inv_levels.pdf.
26
NEO 2010 NWQMS (National Water Quality Management Strategy), 2000. Australian and New Zealand guidelines for fresh and marine water quality. Paper No. 4. Australian and New Zealand Environment and Conservation Council, Agriculture and Resource Management. Smith, K.L. and Huyck, H.L.O., 1999. An overview of the abundance, relative mobility, bioavailability, and human toxicity of metals. In Plumlee, G. S. & Logsdon, M. J. (eds). The Environmental Geochemistry of Mineral Deposits. Part A: Processes, Techniques, and Health Issues. Reviews in Economic Geology, Volume 6A, Society of Economic Geologists, Littleton, pp. 29-70. Straits Resources Limited, 2009. June annual report resources and reserves. http://www.straits.com.au/aboutstraits/resources-and-reserves.html Telford, K., Maher, W., Krikowa, F., Foster, S., Ellwood, M.J., Ashley, P.M., Lockwood, P.V. and Wilson, S.C., 2009. Bioaccumulation of antimony and arsenic in a highly contaminated stream adjacent to the Hillgrove Mine, NSW, Australia. Environmental Chemistry, 6, 133-143. Tighe, M. 2005. The fate and behaviour of antimony in a coastal floodplain system, with comparisons to arsenic. PhD thesis, University of New England (unpublished). Tighe, M., Ashley, P.M., Lockwood, P. and Wilson, S. 2005. Soil, water and pasture enrichment of antimony and arsenic within a coastal floodplain system. Science of the Total Environment, 347, 175-186.
27
NEO 2010
Exploration and opportunities in the New England Orogen, NSW Robert G. Barnes Geological Survey of New South Wales, Industry and Investment NSW.
The New England region in NSW hosts a wide range of commodities and deposit types. Historically the region has been a major gold, tin and antimony producer. Other important commodities include metals such as copper, silver, molybdenum, tungsten, bismuth, lead, zinc, arsenic, and mercury, and industrial minerals and gems including sapphire (and ruby), diamond, limestone, diatomite, quartz and topaz (including emerald). The recent boom in commodity prices has led to the re-evaluation of numerous deposits and mineral deposit types across virtually all mineral commodities but large areas of the southern NEO remain under-explored. The Geological Survey of NSW has undertaken numerous and wide-ranging mineral deposit studies across the region, including a series of metallogenic and mineral systems studies (e.g., Henley et al. 2001, Downes et al. 2009; Lewis and Downes 2008). These provide a framework within which the deposits can be described. In addition, NSW government funded exploration initiatives have generated regional geophysical surveys. These surveys and their interpretations have provided new insights into the geology and mineralisation (e.g. Brown 2006). The dominant systems types in the southern New England Orogen (sNEO) are magmatic, magmatic hydrothermal, hydrothermal with variable magmatic input, metamorphic hydrothermal, sedimentary and residual. Numerous clusters of deposit occur near major structures. Mineral deposits and mineral systems of the sNEO are described in metallogenic notes (e.g. Gilligan et al., 1992) and in review articles such as Barnes et al (1988), Gilligan and Barnes (1990), and Stroud et al. (1999). The oldest mineral systems are associated with the Great Serpentinite Belt along the Peel Fault and ophiolite bodies extending to Port Macquarie and the Gordonbrook Serpentinite on the western margin of the Clarence-Moreton Basin. These rocks contain magmatic chromitite and minor amounts of platinum group metals. Around Port Macquarie, lateritic weathering has produced elevated nickel, cobalt and scandium values which have been of exploration interest (Whitehouse et al., 2006). Carboniferous basement sediments and interbedded greenstones host massive sulphide base metal deposits such the Gulf and Dungowan copper mines. Manganese horizons occur throughout the Carboniferous accretionary complex sediments and probably precipitated in the deep ocean. Exploration for these types of basemetal deposits is complicated by the intense deformation of many of the host sequences. Small early Permian basins host volcanic massive sulphide deposits at Halls Peak and just over the Queensland border at Texas. These are small but very high grade base metal deposits. The predominantly Carboniferous accretionary complex rocks in the sNEO host several hundred small to medium-sized metamorphic (orogenic) gold and antimony deposits. The largest gold-antimony-tungsten system at Hillgrove contains over 200 individual veins and has been worked intermittently for over 110 years. Regionally, deposits are concentrated in secondary structures adjacent to major crustal sutures (for example adjacent to the Peel Fault) or terrain boundaries as is the case at Hillgrove. Deposits also favour structural settings including in contact metamorphosed sediments around intrusives (for example at Lionsville). Exploration has concentrated around known deposits at Hillgrove and Biesldown (Dorrigo) and along the Peel Fault. In places late-stage lamprophyric and felsic dykes occur in the same structures as the hydrothermal gold and or antimony rich veins. The Late Permian Wandsworth Volcanic Group is considered to be the eruptive equivalents of Late Permian intrusives and includes areally extensive sub-areal rhyodacitic ignimbritic sheets. These rocks are host to granitic veins in places, for example Glen Eden, but little syngenetic mineralisation has been reported. The 28
NEO 2010 volcanics potentially cover older intrusive related mineral systems. The Drake Volcanics in the northeast include proximal volcanics in a marine setting and host mineralogically complex epithermal gold-silver-base metal deposits. They have been the focus of exploration interest for many decades. Increased commodity prices have led to the re-evaluation of remnant resources in previously mined areas (Rex Minerals, 2010). The magmatic hydrothermal systems include a vast array of deposits related to granites (sensu lato) (e.g. Blevin, 2010a.b). These include tin veins and disseminations in and around the Mole Granite), Mo-W-Bi pipes (e.g. Kingsgate, Deepwater) and intrusive-related gold deposits (Timbarra and possibly Glen Elgin). Most of the mineralising granites are highly fractionated I-type granites and most are Late Permian to Early Triassic in age. Recent dating (Blevin and Norman, 2010) has shown that tin systems range in age from the Latest Carboniferous / Early Permian in age (Giants Den) through into the Triassic near to the coast. Metal-bearing fluids derived from granites have also produced large base metal deposits including the Cangai copper deposit, large tin base metal and silver deposits (Conrad, Webbs Silver, Webbs Consols, Ottery tin arsenic, Rivertree silver field). Recent drilling of the larger deposits are has led to new resource estimates at Conrad (Malachite Resources Ltd, 2008) and Webbs Silver (Hobby, 2010) amongst others. Skarn deposits are rare in the sNEO, the best known example being at Attunga where new high grade zones have been discovered which include tungsten, copper and gold (Vickery et al., 2010). The Clarence River suite bodies in Tabulam region east of Drake are rather oxidised and have potential for intrusive-related copper gold systems. Tertiary (Neogene) post-orogenic deposits in the region include important historic concentrations of gold (e.g. Uralla, Glen Elgin) and world class tin deposits at Emmaville. Other significant deposits include diamonds (Copeton, Bingara), sapphires and rubies (Inverell, Glen Innes, Barrington Tops) and a range of industrial minerals including diatomite (e.g Barraba). Deposits types include deep leads and residual concentrations. Extensive basaltic flows with lateritic bauxite are being evaluated in the Inverell region (Australian Bauxite Ltd, 2010). The sNEO has not been subject to the intensity of mineral exploration typical of the Lachlan Orogen and older Curnamona Craton. Large areas remain relatively unexplored.
References Australian Bauxite Limited, 2010. Resource increases to 36 million tonnes at Inverell. Announcement to Australian Stock Exchange. http://www.australianbauxite.com.au/Announcements/ASX%20JORC%20Resource%20Update%2002Sep10 .pdf, accessed 5 October 2010. Barnes R.G., Brown R.E., Brownlow J.W., Gilligan L.B., Krynen J. and Willis I.L., 1988. A review of the mineral deposits of the New England Orogen, New South Wales. In: Kleeman, J.D. (ed) New England Orogen —Tectonics and Metallogenesis. Department of Geology and Geophysics, University of New England, Armidale, 211-227. Blevin, P., 2010a Eastern Australian granites: Origins and metallogenesis. In: Lewis, P.C. (ed) Mines and Wines 2010. Mineral Exploration in the Tasmanides. Australian Institute of Geoscientists, Bulletin 52, 1-4 Blevin, P., 2010b Igneous metallogenic contrasts between the northern and southern New England Orogen, eastern Australia. This volume Blevin, P. L. and Norman, M., 2010. Cassiterite—the zircon of mineral systems? A scoping study. Geological Society of Australia, 2010 Australian Earth Sciences Convention (AESC) 2010, Earth systems: change, sustainability, vulnerability. Abstracts 98, 399-400. Brown, R.E., 2006. The Inverell Exploration NSW geophysics – new data for exploration and geological investigations in the northern New England area of New South Wales. Geological Survey of New South Wales Quarterly Notes 121. Downes, P.M., Lewis, P., Blevin, P., Forster, D.B., Whitehouse, J. and Barnes, R., 2008. Mineral Systems in New South Wales (poster). Geological Survey of New South Wales, Maitland. Gilligan, L.B., and Barnes, R.G., 1990. New England Fold Belt, New South Wales – Regional Geology and Mineralisation, In: Hughes, F.E. (ed), Geology of the Mineral Deposits of Australia and Papua New Guinea. Australian Institute of Mining and Metallurgy, pp. 1417-1423. 29
NEO 2010 Gilligan, L.B., Brownlow, J.W., Cameron, R.G. and Henley H.F., 1992. Dorrigo-Coffs Harbour 1:250,000 Metallogenic Map SH/56-10, SH/56-11. Metallogenic Study and Mineral Deposit Data Sheets. 509 pp. Geological Survey of New South Wales, Sydney. Henley, H.F., Brown, R.E., Brownlow, J.W., Barnes, R.G. and Stroud W.J., 2001. Grafton-Maclean 1:250 000 metallogenic map SH/56-6 and SH/56-7: Metallogenic Study and Mineral Deposit Data Sheets. Geological Survey of New South Wales, Sydney, xii + 292 pp with CD ROM. Hobby, D., 2010. The Webb’s silver deposit. In Lewis, P.C., Mines and Wines 2010 Mineral Exploration in the Tasmanides. Australian Institute of Geoscientists Bulletin 52, 19-24. Malachite Resources Ltd, 2008. Conrad Silver Project: Resource Upgrade. ASX announcement http://www.malachite.com.au/pdf/asx/2008/MAR%20-%20ConradResourceUpgrade%2016Dec08.pdf Accessed 5 October 2010. Lewis, P. and Downes, P.M., 2008. Mineral systems and processes in New South Wales: a project to enhance understanding and assist exploration. Geological Survey of New South Wales Quarterly Notes 128. Rex Minerals, 2010. http://www.rexminerals.com.au/projects/gold Accessed 29 September 2010. Stroud, W.J., Barnes, R.G., Brown, R.E., Brownlow, J.W. and Henley, H.F., 1999. Some aspects of the metallogenesis of the Southern New England Fold Belt. In: Flood, P.G. (ed.) New England Orogen. Regional Geology Tectonics and Metallogenesis. University of New England, Armidale, 365-371. Whitehouse, J., Brownlow, J.W., Burton, G.R., Ferguson, A.C., Glen, R.A., Lishmund, S.R., MacRae G.P., Malloch K.R., Oakes, G.M., Paterson, I.B.L., Pienmunne, J.T., Ray, H.N. and Watkins, J.J., 2006. Industrial Mineral Opportunities in New South Wales. Geological Survey of New South Wales, NSW Department of Primary Industries, Bulletin 33. Vickery, N., Leigh, J. and Oates, M., 2010. Attunga skarn project: Latest exploration findings and interpretations. In. Lewis, P.C. (ed) Mines and Wines 2010. Mineral Exploration in the Tasmanides. Australian Institute of Geoscientists Bulletin 52, 147-154.
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NEO 2010
The geology and mineralisation of the New England Orogen in Queensland P.R. Blake1 & I.W. Withnall1 1
Geological Survey of Queensland, Department of Employment, Economic Development and Innovation.
Keywords: magmatic arc, forearc, accretionary wedge, mineralisation, Queensland
Introduction The Queensland portion of the New England Orogen is adjacent to the coast and extends from the border, northward, almost to Townsville. Although elements of the NEO range in age from Neoproterozoic to Mesozoic, most of the terranes are Devonian to Early Permian and are interpreted to have evolved in a convergent plate margin tectonic setting with a west-dipping subduction system. The terranes can be divided into volcanic arc, forearc basin and accretionary wedge. The Connors and Auburn Arches in the west are the volcanic arc, the central Yarrol Terrane is the forearc basin, and the accretionary wedge is made up by the Coastal, Yarraman, North D'Aguilar, South D'Aguilar Beenleigh, and Texas terranes (Murray 1986). The present boundary between the forearc basin strata to the west and the subduction complex assemblages to the east is a major fault zone with serpentinite lenses known as the Yarrol Fault (Murray, 1988). The Gympie Province is interpreted as an exotic terrane (Fig. 1). Numerous tectonic models have been proposed for the development of the NEO (e.g., Leitch, 1975; Cawood 1982, 1983; Murray et al., 1987). Most infer a long-lived, east-facing convergent plate margin setting, with progressive accretion of younger rocks at the eastern margin of Gondwanaland. The orogen was most likely island arc-related from the Cambrian to the Middle Devonian, changing to a continental margin magmatic arc from the Late Devonian onwards. The subsequent history of the orogen involved strike-slip faulting and major oroclinal bending. Large amounts of volcanism and plutonism took place in the Late Permian and Early Triassic (Korsch et al., 1990).
Late Neoproterozoic to earliest Ordovician The only definitive record of Neoproterozoic-Cambrian material in the Queensland NEO is represented by the Princhester and related ophiolites, which are ca. 565 Ma in age (Bruce et al., 2000). The ophiolite has depleted MORB-like trace element characteristics that suggest formation as oceanic crust at a Neoproterozoic ocean spreading ridge (Bruce et al., 2000) and it was probably accreted to the NEO in the late Palaeozoic (Murray and Blake, 2005)
Ordovician The only known occurrence of Ordovician rocks within the Queensland NEO is from the Yarrol Province where an exposure of siltstone, limestone and conglomerate with Ordovician conodonts and trilobites occurs near Santa Glen homestead to the south of Awoonga Dam.
Silurian to Middle to early Late Devonian Late Silurian to Middle Devonian rocks are referred to as the Calliope Volcanic Arc and consist of shallow marine volcaniclastic sediments with various amounts of calcalkaline felsic to mafic volcanic rocks. Geochemical studies suggest formation in an island arc setting (Offler and Gamble, 2002; Murray and Blake, 2005). On the basis of coral faunal assemblages, Blake (2010) interpreted the Calliope Volcanic Arc Assemblage to be made up of at least three island arcs separated from each other that the time of their formation. If exotic, the Calliope must have reached its present position by the end of the Middle Devonian; 31
NEO 2010 the accretion event being represented by an unconformity with overlying Late Devonian forearc basin strata (Korsch et al., 1990). The Silverwood Group is considered to be a southern extension of the Calliope Volcanic Arc (Day et al., 1978). Ocean basin cherts were deposited in the Coastal Block during this period, producing the Doonside Formation between ~418Ma and ~360Ma (Champion et al., 2009).
Fig. 1 The extent of the New England Orogen in Queensland with A showing the different terranes and B showing the tectonic settings interpreted for the terranes. Figures are modified from Champion et al. (2009).
Late Devonian to Early Permian The accretion of the Calliope Island Arc to the Gondwana margin heralded the initiation of the NEO as an Andean-style continental margin with a westerly dipping subduction zone, responsible for the development of a magmatic arc in the west, flanked by a forearc basin and accretionary wedge in the east (Cawood, 1982; Murray et al., 1987). This Andean-style arc remained active until almost the end of the Carboniferous (Roberts et al., 1995). In Queensland the magmatic arc is represented by the Connors and Auburn Arches, the forearc basin is represented by the Yarrol Terrane, and the accretionary complex represented by the Coastal, Yarraman, North and South D’Aguilar, Beenleigh and Texas blocks. In the Auburn Arch, a metamorphic basement of unknown age is intruded by deformed early Carboniferous granitic rocks. The lower part of the overlying succession, the Torsdale Volcanics, consist mainly of felsic ignimbrite with SHRIMP dates in range 325-300Ma. These are overlain by the Camboon Volcanics, which pass upwards through a succession of alternating felsic ignimbrite and mafic lava packages into dominantly basaltic rocks. Unconformities marked by conglomerates that locally overlie small granitic bodies have been observed, but these may be local and/or diachronous and the dominantly mafic and felsic successions may overlap in age. Numerous plutons ranging from gabbro to granite were emplaced synchronously with the volcanic rocks (Withnall et al., 2009; Champion et al., 2009). SHRIMP U-Pb zircon dating suggests at least three major magmatic episodes in the Connors Arch (Withnall et al., 2009). An early Carboniferous (older than ~330Ma) mafic to felsic magmatic event is represented by volcanic and minor plutonic rocks in the Connors Arch. A late Carboniferous felsic magmatic event at around 310-320Ma is represented by extensive ignimbritic units and small granitic plutons. These are separated by a regional unconformity from a more heterogeneous assemblage of volcanic and sedimentary 32
NEO 2010 rocks that contain some local unconformities. SHRIMP U-Pb zircon ages range from 300 to 285Ma. Most of the plutonic rocks (gabbro to granite) of the Urannah Batholith were probably also emplaced during this interval. The oldest rocks in the interval (Leura Volcanics) show a complete range of compositions from basalt through andesite to rhyolite, but the younger rocks (Lizzie Creek Volcanic Group) tend more towards bimodality (Withnall et al., 2009; Champion et al., 2009). Late Devonian rocks in the western part of the Yarrol Province are a mixture of andesitic to dacitic (possibly some rhyolitic) ignimbrites and lavas, hyaloclastites, and volcaniclastic sediments, and are thought to be part of the Late Devonian arc. Farther east, marine sediments dominate with few primary volcanic rocks, and the eastern part of the Yarrol Province appears to be entirely deep marine, and represents the forearc sequence. The Late Devonian rocks unconformably overlie the Calliope Volcanic Arc (Korsch et al., 1990) and the unconformity was the result of the accretion of the Calliope Volcanic Arc with the continent (Murray et al. 2003). Further evidence for a major tectonic event at the boundary between the Middle Devonian and Late Devonian rocks is the occurrence of large allochthonous blocks of Early and Middle Devonian limestone in the base of the Late Devonian sequence, particularly the Mount Alma Formation (Blake, 2010). On a regional scale, the Late Devonian rocks of the Yarrol Province are considered to represent a transition from an intraoceanic setting (Calliope Arc) to a continental margin setting in the Carboniferous (Murray and Blake, 2005). The Early Carboniferous Rockhampton Group was deposited in a forearc basin (Murray et al., 2003) and is considered to have been originally continuous with the forearc basin in the southern NEO (Korsch et al., 1990). It covered the former Late Devonian arc, as well as older basin strata to the east. The sedimentary rocks are mainly siltstones, sandstones, oolitic limestones, ignimbrites and conglomerates. The Coastal, Yarraman, North and South D’Aguilar, Beenleigh and Texas terranes in the northern NEO are interpreted as continuous with the accretionary wedge rocks preserved in the southern NEO, and grew oceanwards by accreting trench-fill volcaniclastic turbidites (derived from a magmatic arc) and minor amounts of oceanic crust (basalt, chert, mudstone) (Korsch et al., 1990). The Beenleigh Block consists of greywacke, argillite and Early Carboniferous chert (Aitchison, 1988). Similar strata occur in the South D’Aguilar Block (Holcombe et al., 1997b). In contrast, the North D’Aguilar Terrane is a composite terrane, containing high-level accretionary wedge rocks as well as ophiolitic components of an accretionary wedge that were subducted to more than 18km before 315 Ma, and were subsequently exhumed in the lower plate below a latest Carboniferous, low angle normal fault (Little et al., 1992, 1995; Holcombe et al., 1997b). Similar-aged accretionary wedge successions are recorded in the Yarraman and Coastal Blocks. The Texas Block consist of deformed and dismembered volcanogenic siltstone and sandstone, basalt, chert and minor conglomerate. Along the Gondwanan rim, a transition took place from active accretion in the Carboniferous to widespread extension in the Early Permian (Leitch, 1988; Holcombe et al., 1997a; Withnall et al., 2009). The youngest volcanics in the Connors and Auburn Arches have been considered to represent a superimposed, Early Permian subduction-related episode, although associated forearc basin or accretionary wedge elements have not been recognised (Holcombe et al., 1997a). Withnall et al. (2009) showed that although there is no major break, the geochemical patterns, although still arc-like, show some evidence of crustal thinning, suggesting the onset of extension. The main units are the Camboon Volcanics, Lizzie Creek Volcanic Group and the Nogo and Narayen beds. This onset of extension has been interpreted in terms of eastward retreat of the subducting slab, and migration of the volcanic arc offshore (Holcombe et al., 1997a). Thus, by the Early Permian, much of the NEO was the site of numerous backarc extensional basins filled with marine and terrestrial deposits and mafic-silicic volcanics. In Queensland, these are represented by the Rookwood Volcanics, which have MORB-like geochemistry. The Carmila beds and related units that occur at the top of the Lizzie Creek Volcanic Group and Camboon Volcanics show evidence of deposition in a rift environment (Withnall et al., 2009). This is also the time that the Berserker Group, comprising felsic volcanics and volcaniclastics and marine sediments, was deposited in the Yarrol Province. An inferred Early Permian backarc setting for the NEO requires that an arc environment existed outboard at this time. The Early Permian volcanics of the Gympie Terrane have a chemical signature indicative of an arc setting (Sivell and McCulloch, 1997). The primitive chemical and isotopic character of these units implies a juvenile island-arc terrain, isolated from the influence of continental crust (Sivell and Waterhouse, 1988). This is consistent with their intimate association with primitive oceanic backarc (Cedarton and Cambroon 33
NEO 2010 Volcanics) basalts (Sivell and McCulloch, 1997), which suggests that a well-developed backarc separated the primitive Gympie island arc from the NEO in the Early Permian. Detrital zircon data from Gympie (Korsch et al., 2009) suggest that the Gympie arc was attached back to eastern Australia at the end of the Permian or Early Triassic. The time of oroclinal bending that produced the Texas-Coffs Harbour Oroclines has been the subject of much debate. Some authors consider that it occurred in the Late Carboniferous (310-300 Ma, Murray et al., 1987), Early Permian (290-280 Ma; Fergusson and Leitch 1993), and Early to mid-Permian (280-265 Ma; Korsch and Harrington, 1987). Offler and Foster (2008) suggest that development of the oroclines took place between 273-260 Ma. Champion et al. (2009) consider oroclinal bending most likely took place after Early Permian backarc extension but prior to the main phase of the Hunter-Bowen Orogeny.
Late Early Permian to Middle Triassic The late Early to early Late Permian (~265-262 Ma) saw another change in the subduction system, when a continental margin magmatic arc was re-established and the backarc changed from an extensional to a contractional regime (Korsch and Totterdell, 1995). This led to the formation of a retroforeland fold-thrust belt west of the magmatic arc (Korsch et al., 1997). The late Early to early Late Permian saw the renewed onset of subduction-related magmatism with voluminous Late Permian to Early (and Middle) Triassic intrusive and extrusive magmatism occurring throughout the NEO (Gust et al., 1993; Holcombe et al., 1997b; Van Noord, 1999) in a continental margin magmatic arc setting. Volcanism was predominantly andesitic (Holcombe et al., 1997b). Widespread intrusive magmatism also occurred at this time. These granites extend from the New England Batholith (Shaw and Flood, 1981), in the south, at least up to Rockhampton in the north (e.g., Murray, 2003). About half of the exposed granitoids in the northern NEO have K-Ar ages between 270 Ma and 230 Ma (Gust et al., 1993; Murray, 2003), although most of these are reset ages. The granitoids are widely distributed and are predominantly of intermediate to felsic, I-type, composition (Bryant et al., 1997; Murray, 2003). They are compositionally very similar to the earlier (Late Carboniferous to Early Permian) magmatism (Murray, 2003). The Late Triassic (c. 230 Ma) saw a switch in geodynamics back to an extensional, probably backarc environment. This resulted in a change in plutonism to A-type granites, bimodal volcanism and development of extensional basins with coal-bearing successions. This also marked the timing of effective cratonisation of eastern Australia.
Mineralisation The oldest and most significant mineral deposit in the Queensland NEO is the Mount Morgan Cu-Au deposit in the Yarrol Province. As a world-class ore body, producing a total of 50Mt at 0.72% Cu and 4.99g/t Au (Ulrich et al., 2003) between 1882 and 1981, it has been studied numerous times and has been assigned to various conflicting deposit models. It has been interpreted as a VHMS deposit (e.g. Frets & Balde, 1975; Taube, 1986) as well as having epigenetic origins related to the ~381Ma (Golding et al., 1993) Mount Morgan Tonalite (Arnold and Sillitoe, 1989) and Permian intrusions (Cornelius, 1969). The Mount Morgan deposit is hosted by a roof pendant of thermally metamorphosed volcaniclastic, siliciclastic and carbonate rocks within the Mount Morgan Tonalite, which is at most several million years younger than the host sequence that is part of the Capella Creek Group within the Calliope Volcanic Arc. Ulrich et al. (2003) advocated a hybrid origin for the deposit, involving deposition of barren massive sulphide at or near the seafloor, and overprinted by quartz-chalcopyrite-pyrite stockwork sourced from the Mount Morgan Tonalite. More recently, Murray and Blake (2005) have also supported the VHMS model, noting the similarity between the geochemistry of the volcanic host rocks and Mount Morgan Tonalite with modern day volcanics of the Izu-Bonin arc, and suggesting that the orebody may have formed in a submarine caldera system. The interpretation of an Andean-type continental arc for the Connors-Auburn Arch, accompanied by the fore-arc in the Yarrol terrane and the accretionary wedge comprised of the Coastal, Yarraman, North D'Aguilar, South D'Aguilar, Beenleigh and Texas blocks, suggests considerable scope for metallogenesis. The Late Devonian and Carboniferous rocks of the Connors-Auburn Arch should be prospective for porphyry copper, epithermal and other intrusion-related systems. However, the preservation of these deposits is governed by the depth of exhumation, and given that no significant deposits of these types have been found suggests that, if formed, they may have been stripped by erosion. 34
NEO 2010 However, the younger part of the Connors-Auburn Arch, the Late Carboniferous to Early Permian volcanics are known to host epithermal-style gold deposits and porphyry copper systems. A prominent example of the former is the Cracow Goldfield which has produced 26t of gold, and is hosted in the gently-dipping Late Carboniferous-Early Permian basaltic to rhyolitic Camboon Volcanics (Murray, 1986; 1990). In general, orebodies occur as open-space vein fillings, which dip vertically to sub-vertically and are structurally controlled (Dong and Zhou, 1996). Farther north, a series of small epithermal gold deposits have been prospected in the Camboon Volcanics near Rannes and at Mount Mackenzie within the Lizzie Volcanic Group west of Marlborough (Garrad & Withnall, 2004a, b). The latter is an advanced argillic highsulphidation system and exploration is ongoing. Near Nebo, Waitara is a porphyry copper type system with a late overprint of stockwork molybdenum mineralisation. Significant results include 99m at 0.34% Cu equivalent (Midas Resources Ltd, 2008). Epithermal vein systems also occur in the area. At the northern end of the NEO in the Collinsville area, the Lizzie Creek Volcanic Group hosts the Silver Hill Au-Ag epithermal deposit which is currently undergoing feasibility studies by Conquest Mining Ltd as part of its Mount Carlton Project. The area hosts several other subeconomic epithermal deposits such as at Crush Creek. The Early Permian Chalmers Formation in the Yarrol Province hosts the Mount Chalmers Cu deposit which is interpreted as a small VHMS deposit. The Chalmers Formation is ~277Ma (Crouch, 1999) and is part of the Berserker Group. The Mount Chalmers deposit produced 1.4Mt grading 1.99% copper, 20.8g/t Ag and 3.38g/t Au with minor zinc and lead (Taube, 1990). The Mount Chalmers is interpreted to belong to the highsulphidation sub-class of VHMS deposits. Taube & van der Helder (1983) described several small VHMS prospects in the Chalmers Formation and Crouch (1999) described small lode gold deposits of unknown age to the south of the Mount Chalmers deposit. The Mount Chalmers deposit is interpreted to have been formed upon thinned continental crust on the eastern margin of a back-arc extensional basin formed on the continental margin (Crouch, 1999). The Middle to Late Triassic Gympie gold province, located 100 to 300km north of Brisbane, is the most significant mineral province in the New England Orogen (Champion et al., 2009). The largest mineralisation system in this province is the Gympie goldfield which is the sixth largest historical Au producer in Australia. A total of 125t of gold, at an average grade of 29g/t were produced between 1867 and 1927. The Gympie goldfield covers an area 4km by 10km and consists of an extensive, mesothermal, orogenic quartz vein system hosted within the Permo-Triassic mafic to intermediate island arc volcanics and sediments of the Gympie Group (Cranfield et al., 1997). As well as the Gympie goldfield, this province contains numerous smaller vein and breccia Au deposits, including Mount Rawdon and North Arm. These deposits appear to be, in part, low sulphidation epithermal deposits. Significant Au-Ag production has also come from open cut mining of a subvolcanic breccia system at Mount Rawdon with results of 26.4Mt at 1.15g/t Au and 4.4g/t Ag (Denaro et al., 2007). The mineralisation lies adjacent to the intersection of the ENE-trending Swindon Fault and a SSE-trending structure parallel to the Yarrol Fault. These fault trends were important in localising structural complexity and mineralisation at Mount Rawdon and other prospects in the area (Angus, 1996). Brooker & Jaireth (1995) described Mount Rawdon as a transitional deposit, displaying both epithermal and porphyry characteristics. In the Kilkivan area in the southeast Queensland, straddling the northern part of the North D’Aguilar Block and the eastern margin of the Esk Trough, a variety of small deposit types are hosted in Palaeozoic metamorphic rocks; Permian to Triassic granites and porphyries, and Triassic andesites and rhyolites (Brooks et al., 1974; Bischoff, 1986; Murray, 1986; Nash, 1986). The majority of these deposits are epigenetic hydrothermal type, and all mineralization appears to be genetically related to intrusive events. An example is the Barambah epithermal deposit in the Mount Marcella Volcanics, currently being explored by ActivEx Ltd. Other mineralisation styles such as skarns are also associated with these intrusives, but they are uneconomic. Another significant intrusion-related deposit is the Whitewash Cu-Mo deposit in the Rawbelle Batholith southwest of Monto. It is being explored by Aussie Q Resources Ltd. The mineralisation is hosted by the Late Permian Wingfield Granite. The granite shows only weak propylitic alteration, but numerous northeast trending quartz-clay-sericite zones up to 3m wide and over 300m long coincide with the geochemical centre of the prospect. The mineralisation is strongly fracture controlled, as is the associated alteration. Inferred reserves of 71.5Mt at 0.1% Cu and 0.03% Mo have been announced.
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References Aitchison, J. C., 1988. Early Carboniferous (Tournaisian) Radiolaria from the Neranleigh-Fernvale beds near Brisbane. Queensland Government Mining Journal 89, 240-241. Arnold, G.O. and Sillitoe, R.H., 1989. Mount Morgan gold-copper deposit, Queensland, Australia: evidence for an intrusion-related replacement origin. Economic Geology 84, 1805-1816. Bischoff, K., 1986. Mineralisation in the Kilkivan area. In: Willmott, W.F., (ed.) Field Conference Geological Society of Australia; The Southern Burnett District; 1986 Field Conference, Brisbane, Queensland, Australia, 1986, 21-31 Blake, P.R., 2010. Devonian corals of the Yarrol Province, eastern-central Queensland. Memoirs of the Association of Australasian Palaeontologists 38, 191 p. Brooker, M., and Jaireth, S., 1995. Mount Rawdon, Southeast Queensland, Australia; a diatreme-hosted gold-silver deposit. In: Walshe, J.L., McQueens, K.G. & Cox, S.F. (eds) A special issue on the metallogeny of the Tasman Fold Belt system of eastern Australia, Economic Geology and the Bulletin of the Society of Economic Geologists 90, 1799-1817. Brooks, J.H., Syvret, J.N. and Sawers, J.D., 1974. Mineral resources of the Kilkivan district. Geological Survey of Queensland Report 60, 1-49. Bruce, M.C., Niu, Y., Harbort, T.A. and Holcombe, R.J., 2000. Petrological, geochemical and geochronological evidence for a Neoproterozoic ocean basin recorded in the Marlborough terrane of the northern New England Fold Belt. Australian Journal of Earth Sciences 47, 1053-1064. Bryant, C.J., Arculus, R.J. and Chappell, B.W., 1997. Clarence River Supersuite; 250 Ma Cordilleran tonalitic I-type intrusions in eastern Australia. Journal of Petrology 38, 975-1001. Cawood, P.A., 1982. Structural relations in the subduction complex of the Palaeozoic New England Fold Belt, eastern Australia. Journal of Geology 90, 381-392. Cawood, P.A., 1983. Modal composition and detrital clinopyroxene geochemistry of lithic sandstones from the New England Fold Belt (east Australia): A Palaeozoic forearc terrane. Geological Society of America Bulletin 94, 1199-1214. Champion, D.C., Kositcin, N., Huston, D.L., Mathews, E. and Brown, C., 2009. Geodynamic Synthesis of the Phanerozoic of Eastern Australia and implications for Metallogeny. Geoscience Australia Record 2009/18, 254 p. Cornelius, K.D., 1969. The Mount Morgan mine, Queensland - a massive gold copper-pyritic replacement deposit. Economic Geology 64, 885-902. Cranfield, L.C., Shorten, G., Scott, M. and Barker, R.M., 1997. Geology and mineralisation of the Gympie Province. Geological Society of Australia Special Publication 19, 128-147. Crouch, S., 1999. Geology, tectonic setting and metallogenesis of the Berserker Subprovince, northern New England Orogen. In: Flood, P.G. (ed.) Regional Geology, tectonics and metallogenesis - New England Orogen, 233-249. Department of Earth Sciences, University of New England, Armidale, New South Wales, Australia. Day, R.W., Murray, C.G. and Whitaker, W.G., 1978. The eastern part of the Tasman Orogenic Zone. Tectonophysics 48, 327-364. Denaro, T.J., Cranfield, L.C., Fitzell, M.J., Burrows, P.E., and Morwood, D.A., 2007. Mines, Mineralisation and mineral exploration in the Maryborough 1:250 000 map sheet area, south-east Queensland. Queensland Geological Record 2007/01, 294 p. Dong, G.Y. and Zhou, T., 1996. Zoning in the Carboniferous-Lower Permian Cracow epithermal vein system, central Queensland, Australia. Mineralium Deposita 31, 210-224. Fergusson, C.L. and Leitch, E.C., 1993. Late Carboniferous to Early Triassic tectonics of the New England Fold Belt, eastern Australia. In: Flood, P.G. & Aitchison, J.C. (eds.), New England Orogen, eastern Australia, NEO ‘93 Conference Proceedings, University of New England, 53-59.
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NEO 2010 Frets, D.C. and Balde, R., 1975. Mount Morgan copper-gold deposit. Australasian Institute of Mining and Metallurgy 2, 1557-1560. Garrad, P.D. and Withnall, I.W., 2004a. Mineral occurrences and District Analysis– Banana, Theodore and Scoria 1:100 000 Sheet areas, central Queensland. Queensland Geological Record 2004/2. Garrad, P.D. and Withnall, I.W., 2004b. Mineral occurrences – St Lawrence and Port Clinton 1:250 000 Sheet areas, central Queensland. Queensland Geological Record 2004/7. Golding, S.D., Huston, D.L., Dean, J.A., Messenger, P.R., Jones, I.W.O., Taube, A. and White, A.H., 1993. Mount Morgan gold-copper deposit; the 1992 perspective. Proceedings Australasian Institute of Mining and Metallurgy Centenary Conference, 30 March-4 April, Adelaide, 95-111. Gust, D.A., Stephens, C.J. and Grenfell, A.T., 1993. Granitoids of the northern NEO: their distribution in time and space and their tectonic implications. In: P.G. Flood & J. Aitchison (Eds), New England Orogen, eastern Australia. University of New England, Armidale, Australia, 565-571. Holcombe, R.J., Stephens, C.J., Fielding, C.R., Gust, D., Little, T.A., Sliwa, R., McPhie, J. and Ewart, A. 1997a. Tectonic evolution of the northern New England Fold Belt: Carboniferous to Early Permian transition from active accretion to extension. In: Ashley, P. M. & Flood, P. G. (eds.) Tectonics and Metallogenesis of the New England Orogen. Geological Society of Australia Special Publication 19, 66-79. Holcombe, R. J., Stephens, C.J., Fielding, C.R., Gust, D., Little, T.A., Sliwa, R., Kassan, J., McPhie, J. and Ewart, A., 1997b. Tectonic evolution of the northern New England Fold Belt: the Permian - Triassic HunterBowen event. In: Ashley, P.M. & Flood, P.G. (eds.) Tectonics and Metallogenesis of the New England Orogen. Geological Society of Australia Special Publication 19, 52-65. Korsch, R.J. and Harrington, H.J., 1987. Oroclinal bending, fragmentation and deformation of terranes in the New England Orogen, eastern Australia. In: Leitch, E.C. & Scheibner E. (eds). Terrane Accretion and Orogenic Belts. American Geophysical Union Geodynamics Series 19, 129-139. Korsch, R.J. and Totterdell, J.M. 1995. Structural events and deformational styles in the Bowen Basin. In: Follington, I.W., Beeston, J. W. & Hamilton, L.H. (eds.) Bowen Basin Symposium 1995...150 Years On...Proceedings, 27-35. Geological Society of Australia, Coal Geology Group, Brisbane. Korsch, R.J., Harrington, H.J., Murray, C.G., Fergusson, C.L. and Flood, P.G., 1990. Tectonics of the New England Orogen. In: Finlayson, D. M. (ed.) The Eromanga- Brisbane Geoscience Transect: a guide to basin development across Phanerozoic Australia in southern Queensland. Bureau of Mineral Resources Bulletin 232, 35-52. Korsch, R.J., Johstone, D.W. and Wake-Dyster, K.D., 1997. Crustal architecture of the New England Orogen based on deep seismic reflection profiling. In: Tectonics and Metallogenesis of the New England Orogen. Geological Society of Australia Special Publication 19, 29-51. Korsch, R.J., Adams, C.J., Black, L.P., Foster, D.A., Fraser, G.L., Murray, C.G., Foudoulis, C. and Griffin, W.L., 2009. Geochronology and provenance of the Late Palaeozoic accretionary wedge and Gympie Terrane, New England Orogen, eastern Australia. Australian Journal of Earth Sciences 56, 5, 655-686. Leitch, E.C. 1988. The Barnard Basin and the Early Permian Development of the southern part of the New England Fold Belt. In: Kleeman, J. D. (ed.) New England Orogen - Tectonics and Metallogenesis. Department of Geology and Geophysics, University of New England, Armidale, 61-67. Little, T.A., Holcombe, R.J., Gibson, G.M., Offler, R., Gans, P.B. and McWilliams, M.O., 1992. Exhumation of late Palaeozoic blueschists in Queensland, Australia, by extensional faulting. Geology 20, 231-234. Little, T.A., McWilliams, M.O. and Holcombe, R.J., 1995. 40Ar/39Ar thermochronology of epidoteblueschists from the North D’Aguilar Block, Queensland, Australia: timing and kinematics of subduction complex unroofing. Geological Society of America Bulletin 107, 520-535. Leitch, E.C., 1975. Plate tectonic interpretation of the Palaeozoic history of the New England Fold Belt. Geological Society of America Bulletin 86, 141-144. Midas Resources Ltd, 2008. Midas Resources, First Quarter Activities Statement. Report to Australian Stock Exchange, 21 October 2008.
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NEO 2010 Murray, C.G., 1986. Metallogeny and tectonic development of the Tasman Fold Belt system in Queensland. Ore Geology Reviews 1, 315-400. Murray, C.G., 1988. Tectonostratigraphic terranes in southeast Queensland. In: Hamilton, L.H. (ed) Field Excursions Handbook for the 9th Australian Geological Convention. Geological Society of Australia, Queensland Division, Brisbane, 19-51. Murray, C.G., 1990.Tasman Fold Belt in Queensland. In: F.E. Hughes (ed.) Geology of the Mineral Deposits of Australia and Papua New Guinea. Monograph 14. Australasian Institute of Mining and Metallurgy, v. 2, 1434-1450. Murray, C.G., 2003. Granites of the northern New England Orogen. In: Blevin, P., Jones, M. & Chappell, B. (eds) Magmas to Mineralisation: The Ishihara Symposium. Geoscience Australia Record 2003/14, 101-108. Murray, C.G. and Blake, P.R., 2005. Geochemical discrimination of tectonic setting for Devonian basalts of the Yarrol Province of the New England Orogen, central coastal Queensland: an empirical approach. Australian Journal of Earth Sciences, 52, 993-1034. Murray, C.G., Fergusson, C.L., Flood, P.G., Whitaker, W.G. and Korsch, R.J., 1987. Plate tectonic model for the Carboniferous evolution of the New England Fold Belt. Australian Journal of Earth Sciences, 34, 213236. Murray, C.G., Blake, P.R., Hutton, L.J., Withnall, I.W., Hayward, M.A., Simpson, G.A. and Fordham, B.G., 2003. Discussion. Yarrol terrane of the northern New England Fold Belt: forearc or backarc? Australian Journal of Earth Sciences, 50, 271-278. Nash, C.R., 1986. Permo-Triassic tectonic evolution and metallogenesis of the Rockhampton-Maryborough area, Queensland; a photogeological investigation. In: Scheibner, E. (ed.) Metallogeny and tectonic development of Eastern Australia, Ore Geology Reviews, 1, 401-412. Offler, R. and Gamble, J., 2002. Evolution of an intra-oceanic island arc during the Late Silurian to Late Devonian, New England Fold Belt. Australian Journal of Earth Sciences, 49, 349-366. Offler, R. and Foster, D.A. 2008. Timing and development of oroclines in the southern New England Orogen, New South Wales. Australian Journal of Earth Sciences, 55, 331-340. Roberts, J., Claoue-Long, J., Jones, P.J. and Foster, C.B., 1995. SHRIMP zircon age control of Gondwana sequences in late Carboniferous and Early Permian Australia. In: Dunnay, R.E. & Hailwood, E.A. (eds.) Nonbiostratigraphical methods of dating and correlation. Geological Society Special Publication 89, 145174. Shaw, S.E. and Flood, R.H., 1981. The New England Batholith, eastern Australia: geochemical variations in space and time. Journal of Geophysical Research, 86, B10530-B10544. Sivell, W.J. and McCulloch, M.T., 2001. Geochemical and Nd-isotopic systematics of the Permo-Triassic Gympie Group, southeast Queensland. Australian Journal of Earth Sciences, 48, 377-393. Sivell, W.J. and Waterhouse, J.B., 1988. Petrogenesis of Gympie Group volcanics: evidence for remnants of an Early Permian Volcanic arc in eastern Australia. Lithos, 21, 81-95. Taube, A., 1986. The Mount Morgan gold-copper mine and environment, Queensland: a volcanogenic massive sulfide deposit associated with the penecontemporaneous faulting. Economic Geology, 81, 13221340. Taube, A, 1990. Mount Chalmers gold-copper deposits. Australasian Institute of Mining and Metallurgy Monograph, 14, 1493-1497. Taube, A. and van der Helder, P. 1983. The Mount Chalmers mine and environment - a Kuroko-styel volcanogenic sulphide environment. In: Permian geology of Queensland. Geological Society of Australia, Queensland Division, Brisbane, 387-399. Ulrich, T., Golding, S.D., Kamber, B.S., Khin Zaw and A. Taube, 2003. Different mineralization styles in a volcanic-hosted ore deposit: the fluid and isotopic signatures of the Mt Morgan Au–Cu deposit, Australia, Ore Geology Reviews, 22, 61-90
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NEO 2010 van Noord, K.A.A., 1999. Basin development, geological evolution and tectonic setting of the Silverwood Group. In: Flood, P.G (editor). New England Orogen. Regional Geology, Tectonics and Metallogenesis. University of New England, Armidale, 163-180. Withnall, I.W., Hutton, L.J., Bultitude, R.J., von Gnielinski, F.E. and Rienks, I.P., 2009. Geology of the Auburn Arch, southern Connors Arch and adjacent parts of the Bowen Basin and Yarrol Province, central Queensland. Queensland Geology, 12. Yarrol Project Team 1997. New insights into the geology of the northern New England Orogen in the Rockhampton -Monto region, central coastal Queensland: progress report on the Yarrol Project. Queensland Government Mining Journal, 98, 11-26. Yarrol Project Team 2003. Central Queensland Regional Geoscience Data-Yarrol GIS. Queensland Department of Natural Resources and Mines, Brisbane.
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Igneous metallogenic contrasts between the northern and southern New England Orogen, eastern Australia Phillip L. Blevin Geological Survey of New South Wales, Industry and Investment NSW, Maitland, NSW, Australia.
Keywords: granites, metallogeny, tin, copper, gold, oxidation state, geothermal.
Introduction Granites can be classified according to metallogenic potential using a scheme based on compositional character, degree of compositional evolution, degree of fractionation, and oxidation state (Blevin et al., 1996; Blevin, 2004). This scheme is based on empirical and theoretical considerations and satisfactorily describes the known distribution of granite-related mineralisation in eastern Australia. Parameters that are most important in determining the overall metallogenic “flavour” of intrusive igneous suites include: granite type, compositional evolution, degree of fractionation, and oxidation state. Granite type (I, S, A; metaluminous, peraluminous or peralkaline) is important as a guide to the abundance of elements and volatiles in the granites and their behaviour during fractionation processes. Mineral zonation around granites can also be understood in terms of such a model (Fig. 1). The degree of compositional evolution can be monitored by the K/Rb ratio (Blevin, 2004; Blevin and Chappell, 2010). A granite can be regarded as compositionally evolved if its chemistry is no longer compatible with mantle materials. Three stages can be recognised. First generation or primary granites are those directly derived from mantle materials. They typically consist of oceanic plagiogranites and tonalitic rocks in primitive island arc settings. Second “generation” granites are derived via remelting or remagmatisation of mantle-derived rocks and their related materials. They are typically located in continental margin arc settings. Third cycle granites are the products of the fusion of these materials and their associated rocks within the crust. S-type granites are typically of this type, however in the New England Orogen S-types have also been derived from juvenile arc sediments (Jeon et al., 2010). Fig. 1. Mineral deposit and district zonation patterns as a function of compositional and oxidation properties of mineralised granites (see also Blevin, 2004).
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Fig. 2 (left). Average K/Rb ratios of granites from the New England Orogen. Note marked contrast between the nNEO and sNEO. S-types in the sNEO are also relatively unevolved in terms of K/Rb compared to those of the Lachlan Orogen (Blevin and Chappell, 2010).
K/Rb (plutons) >400 250 - 400
Fig. 3 (left, below). Average values of ΔOx for NEO plutons. Strongly oxidised (0.8 > ΔOx > 0.3);; moderately oxidised (0 < ΔOx < 0.3);; moderately reduced (0 > ΔOx > -0.5); strongly reduced (ΔOx < -0.5). ΔOx = 0 designates magnetite-ilmenite series boundary. ΔOx = log10(Fe2O3/FeO) + 0.3 + 0.03*FeOt. See Blevin (2004).
200 - 250 150 - 200 100 - 150 33 - 100
0 50 100
200 Kilometers
HGU (µWm-3) 5
ΔOx (plutons) >0.8
0 50 100
200 Kilometers
0.3 - 0.8 0.1 - 0.3 -0.1 - 0.1 -0.7 - -0.1 < -0.7
0 50 100
200 Kilometers
Fig. 4 (above). Radioactive heat generation in a rock can be expressed as A in units of μWm-3 (microwatts per cubic metre) (Rybach, 1976) and is determined from the relationship A = d*10-2*(9.52*U + 2.56*Th +3.48*K) where U and Th are in ppm; K is in weight percent, and d represents the density of the rock (here taken as 2.8 tonnes per cubic metre). The relative oxidation state of the magma is of 41
NEO 2010 paramount importance in controlling the partitioning behaviour of many ore elements. A division of granites into ilmenite- and magnetite-series can be very useful as Cu, Mo and Au deposits are typically related to magnetite-bearing (magnetite-series) granites whilst Sn (±W) deposits occur with magnetite-free (ilmeniteseries) granites. Oxidation state is largely inherited from source although the effects of wall rock interaction can be locally important. Also, there is a general trend to lower relative oxidation state in granites from arc settings through continental margin settings to those of continental interiors. As a consequence, S-types are almost invariably reduced.
Granites of the northern NEO The northern NEO (nNEO) extends from the Clarence–Moreton Basin to just south of Townsville and west to the Bowen Basin. Four age groups of granite are present: Middle–Late Devonian (minor); midCarboniferous–Early Permian; Late Permian–Late Triassic; and Early Cretaceous (Murray, 2003). Devonian igneous rocks of the nNEO, such as the Mount Morgan Tonalite Complex share with the Ordovician of the Lachlan Orogen the most unevolved compositions and highest K/Rb ratios of all intrusive igneous units in eastern Australia (Fig. 2). The Mount Morgan Tonalite Complex has trace element and REE signatures indicative of an island arc derivation while the presence of a reasonably continuous compositional range within the Mount Morgan Tonalite Complex suggests fractionation of magmas sourced from the fusion of basalt as the most likely origin. Post-Devonian magmatism in the northern NEO comprises moderately evolved compositions (200 < K/Rb < 400; Blevin and Chappell, 2010). Granites of the Yarrol Province and the Urannah Batholith are on average less compositionally evolved in terms of K/Rb than granites further to the south (ie. have many units with K/Rb values between 300 and 400). Granites of the Auburn Province (Rawbelle Batholith) are less evolved than those of the Moonbi Supersuite (MSS) of the New England Batholith in the southern NEO (sNEO). As a group, the granites of the nNEO are overwhelmingly I-type, lack significant zircon inheritance and are isotopically unevolved (Murray, 2003). Copper–Au mineralisation dominates all the age groups in the nNEO but is generally low grade (Horton, 1978). Evolved fractionated I-types, rare S-types, and sporadic A-types of Permo-Triassic and Cretaceous age are also present, though minor, in the nNEO. Copper-Au, Cu-Mo and Mo type mineralisation dominates the granite-related metallogeny of the nNEO. Although these associations are present with granites of all ages, they are best developed in the Devonian, Late Permian to Late Triassic and Early Cretaceous. Interestingly, while the mid Carboniferous to Early Permian granites of the nNEO are poorly mineralised, volcanics of this age are host to significant epithermal Au mineralisation.
Granites of the southern NEO The granites of the sNEO fall into two distinct groups in terms of K/Rb (Fig. 2). The Clarence River Supersuite (CRSS) has K/Rb ratios around 250 to 350, while all other supersuites in the NEB have markedly lower K/Rb ratios. In this regard the CRSS granites are more typical of the nNEO in general than of the south. Many of the granites of the sNEO (Stanthorpe and related granites, and Permo-Triassic leucogranites) are similar in terms of compositional evolution to that of the Carboniferous I-types of the Georgetown– Herberton region of far north Queensland, however differ significantly in terms if isotopic evolution. The MSS consists of oxidised, high-K hornblende–biotite (augite) granites. By contrast, the I-type Uralla Supersuite is generally lower in K2O, P2O5, light REE and Ba and higher in Y (and HREE) than the Moonbi suite intrusions. The Uralla granites have features that are transitional between typical S- and I-type granites, and can be classified as ilmenite-series (Fig 3). Highly fractionated leucogranites are I-type in character, and are now known to be both Triassic (Mole) and Permian (Gilgai) in age (Cross and Blevin, 2010). The Permian leucogranites are oxidised (Gilgai, Parlour Mountain) while some of the Triassic examples are either oxidised (Dumboy–Gragin), or reduced (Mole). The sNEO also hosts two major supersuites of S-types – Bundarra and Hillgrove. The leucogranites comprise by far the most important group of mineralised granites in the sNEO having produced Sn, W, Mo, Ag, As, Bi, Cu, Pb, Au, fluorite, beryl and topaz. The most important mineral commodity associated with the sNEO granites is Sn with around ~200 kt having been produced, largely from alluvial sources. Hardrock Sn production has been substantially less and has been won from disseminated greisen deposits, quartz-cassiterite-chlorite veins, and sulphide lodes often bearing stannite. Large tonnagelow grade mineralisation in the form of stockwork and/or sheeted vein systems in wall rocks overlying granite cupolas and ridges are present at the Sundown and Taronga Sn prospects. Minor Mo mineralisation is 42
NEO 2010 widespread within the leucogranites as showings, disseminations, pipes and veins. Stockwork and breccia hosted ores are also present at Glen Eden. Numerous polymetallic (Ag, Pb, Zn, Cu, As, Sb) vein deposits are also closely associated with the Sn mineralised leucogranites. Gold is also commonly reported from the leucogranites in association with Mo-Bi (+ Ag) deposits, and as lodes and disseminations. These granites are also important as they are high heat producing in terms of their elevated K, Th and U contents (Fig. 4)
Comparison with adjacent regions To the north west of the nNEO, the extensive granites of Cambro-Ordovician, Silurian–Devonian and Permo-Carboniferous ages are developed. These intrude both Proterozoic and Palaeozoic terranes. Most of north Queensland’s intrusion-related mineralisation is associated with I- and S-type granites of the Kennedy Igneous Province. These voluminous Permo-Carboniferous granites are typically high-K, high SiO2 (>70 weight percent) and often fractionated. A-types are also present but are unmineralised. The mineralisation is dominated by Sn and/or W deposits and related base metal systems. Significant deposits of the “intrusionrelated gold deposit” type are also present (e.g. Kidston, Red Dome). A significant break in the compositional character of granite magmatism occurs just north of Townsville from more evolved in the north to less evolved and fractionated granites to the south. Cu–Mo–Au centred systems dominate in the oxidised, more unevolved Permian of the Ravenswood Batholith, and may represent the true end of “New England magmatism” (Blevin et al., 1999).
One orogen, two metallogenies A significant feature of the NEO is the presence of a distinctive Sn-W-Mo dominated granite metallogeny in the sNEO and a Cu-Mo-Au dominated metallogeny in the north. Both these show a repeat of these metallogenies with time. In the nNEO Cu-Mo-Au gold is repeated in the Late Devonian, Permo-Carb. Permo-Triassic and Cretaceous; where Sn dominated mineralisation occurs in the sNEO in rocks of Earliest Permian, Mid-Late Permian, and Lower, to mid Triassic (see Blevin, this volume). Compositional parameters of granites that have an effect on ore commodities observed in the NEO include compositional evolution (as expressed as K/Rb), and oxidation state. Granite type is also important as fractionation pathways for certain elements are controlled by mineral stability and element solubilities sensitive to variation in alumina saturation. I-types overwhelmingly dominate in the nNEO (>95% or plutons), but are more equally balanced in the south. Similarly, there is a strong contrast in such parameters as compositional evolution (K/Rb) and oxidation state. The nNEO granites are compositionally less evolved and more oxidised generally than the granites of the sNEO (Figs 2, 3) and this is reflected in the Cu dominant metallogeny compared to the lithophile dominated metallogeny in the more evolved and fractionated granites of the sNEO. These metallogenic differences do not change with time in either the northern or southern sections of the NEO and suggest long lived contrasts in the nature of the source materials producing both the granites and their related mineralisation. Other consequences of these variations include heat generation potential (Fig. 4), where the granites of the sNEO typically have higher K, Th and U contents and higher heat generation potential, even in granites that otherwise do not show extended fractional crystallisation (e.g. Moonbi Suite).
Summary The New England Orogen has a diverse igneous metallogeny with all major commodity associations (Cu-Au, Cu-Mo, Mo, W and Sn) being present. What is intriguing though is that the metallogenic variation is geographically specific with contrasting metallogenic associations present between the northern (nNEO) and southern (sNEO) segments. The general metallogenic character between these two zones do not appear to change significantly through time from the Carboniferous to the Cretaceous. New compilations and interpretations of granite chemistry from north and south strongly reinforce the importance of magmatic controls on the observed distribution of granite related mineralisation. The distribution of strongly fractionated granites (and thus high heat producing) also follow this trend with high heat producing granites strongly concentrated in the sNEO. This has not only implications for the geothermal potential of the two segments of the orogen, but also for the distribution of high heat producing elements within the crust more generally.
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Acknowledgements Granite geochemical data used in the abstract is courtesy of B. W. Chappell. Published with the permission of the acting Director, Geological Survey of New South Wales.
References Blevin, P.L., 2010. Old friends in a whole new light: A new chronology for the igneous metallogeny of the southern New England Orogen, New South Wales. This volume Blevin, P.L., 2004. Redox and compositional parameters for interpreting the granitoid metallogeny of eastern Australia: Implications for gold-rich ore systems. Resource Geology, 54, 241-252. Blevin, P.L. and Chappell, B.W., 2010. The use of the K/Rb ratio as a metallogenic discriminant: the example of the eastern Australian granites. Zeitschrift für Geologische Wissenschaften, 38, 235-242. Blevin, P.L., Chappell, B.W., and Allen, C. M., 1999. Permo-Carboniferous magmatism in North Queensland: where does New England end? In: Flood, P.G. (ed) New England Orogen. Regional geology tectonics and metallogenesis. The University of New England, Armidale, 297-299. Blevin, P.L., Chappell, B.W., and Allen, C.M., 1996. Intrusive metallogenic provinces in eastern Australia based on granite source and composition. Transactions of the Royal Society of Edinburgh: Earth Sciences, 87, 281-290. Cross, A., and Blevin, P. L., 2010. Summary of results for the joint GSNSW – GA geochronology project: New England Orogen and Sydney–Gunnedah Basin April – July 2008. Geological Survey Report, GS2010/778. Horton, D.J., 1978: Porphyry-type copper-molybdenum mineralization belts in eastern Queensland, Australia. Economic Geology, 73, 904-921. Jeon, H., Williams, I.S. and Chappell, B.W., 2010. Zircon U-Pb and O isotopic evidence for the age and source of the S-type Bundarra Supersuite granites, southern NEO. This volume. Murray, C., 2003. Granites of the northern New England Orogen. In: Blevin, P.L., Jones, M. & Chappell, B. W. (eds). Magmas to Mineralisation: The Ishihara Symposium. Geoscience Australia Record 2003/14, 101108. Rybach, L., 1976. Radioactive heat production in rocks and its relation to other petrophysical parameters. Pure and Applied Geophysics 114, 309-317.
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Old friends in a whole new light: A new chronology for the igneous metallogeny of the southern New England Orogen, New South Wales Phillip L. Blevin Industry & Investment New South Wales, Maitland, New South Wales, Australia.
Keywords: New England Orogen, granites, mineralisation, geochronology, volcanics, zircon, tin deposits
Introduction The pattern of igneous metallogeny observed in the southern New England Orogen (NEO) is a function of the distribution and age relationships of the granites, basic relationships of which have changed little since the landmark contribution of Shaw and Flood (1981). More recently Blevin (1996) separated the Stanthorpe and related satellitic stocks from the leucoadamellite group of Shaw and Flood (1981) into the Stanthorpe Granite Group, although assignment of these units into a discrete supersuite is clearly appropriate on spatial, age and compositional grounds. The supersuite classification allowed for a simple attendant metallogenic classification. Blevin and Chappell (1993) used granite compositional parameters such as granite types, degree of fractionation and oxidation state to characterise the metallogenic fertility of the main supersuites for various ore commodities. While some modest mineralisation (Mo, Cu, W, Au) is associated with the Permian Moonbi Supersuite, significant granite-related mineralisation is deemed to be only associated with the Triassic leucogranites. The leucogranites comprise by far the most important group of mineralised granites in the NEO having produced Sn, W, Mo, Ag, As, Bi, Cu, Pb, Au, fluorite, beryl and topaz. Mineralisation related to the leucogranites can be divided into Mo, Sn, polymetallic vein and Au dominant associations. Such was the primacy of the assumptions that most if not all mineralised fractionated granites were restricted to the Triassic that Sn mineralisation in the Bundarra Supersuite is sometimes considered to be associated with younger rocks. The advent of a new campaign of dating of the mineralised granites of the NEO, and the development of new techniques such as the U-Pb dating of cassiterite (Blevin and Norman, 2010) allow for significant progress to be made in understanding the stratigraphic relationships and timing of magmatism and mineralisation events. The present study has combined direct dating of granites using U-Pb zircon dating by SHRIMP, with direct dating of the ore minerals molybdenite (Re-Os), and cassiterite (U-Pb). This enables the mineralisation event to be directly dated (i.e. the transport and deposition of ore metals). This approach is particularly useful in areas of geological complexity and as an independent check on magmatic crystallisation ages, and where mineralisation is located away from potential igneous sources.
The Tingha-Gilgai-Elsmore plutonic association The Tingha Monzogranite, Gilgai Granite and Elsmore Granite (Fig. 1) occur in close spatial relationship with each other. The Tingha Monzogranite is a typical a member of the Uralla Supersuite while the Gilgai and Elsmore Granites have been assigned by previous workers to the Triassic leucoadamellite association of Shaw and Flood (1981). However, zones of mingling and hybridisation between the Tingha and Gilgai units are extensively developed in the Kings Gap area, and demonstrate that the magmas were interacting with each other while hot. Subsequently, a campaign of SHRIMP dating was undertaken in conjunction with Geoscience Australia (Cross and Blevin, 2010) to establish the relative timing relationships. Three U-Pb ages determined for the Gilgai Granite were 252.2 ± 1.8 Ma; 252.6 ± 1.8 Ma; and 251.8 ± 1.7 Ma. The Tingha Monzogranite 45
NEO 2010 returned an age of 251.3 ± 1.7 Ma. These ages are indistinguishable from each other and confirm a Permian age for the Gilgai Granite and suggest that in places, both the Gilgai and the Tingha magmas were being emplaced synchronously. Elsewhere, sharp intrusive contacts are also present. The two intrusions contrast markedly in their chemistry and oxidation state, with the Tingha Monzogranite being reduced (typical of the Uralla Supersuite), while the Gilgai Granite is strongly oxidised, felsic and strongly fractionated. Geophysical imagery (NSW Geological Survey data) suggests that the strongly magnetic Gilgai Granite cannot be extensively developed under the nonmagnetic Tingha Monzogranite, particularly in areas where the Tingha Granite is host to tin mineralisation.
Fig.1. Location of main plutons mentioned in the text.
Tin mineralisation and the Bundarra Supersuite Small to medium-sized hard rock and alluvial tin deposits are associated with the Bundarra Supersuite. The presence of microgranites and porphyritic phases physically associated with the hard rock occurrences at Giants Den and The Glen, and the textural similarity of these rocks with the Gilgai Granite have lead to questions as to the relationship of these mineralising phases with the host Bundara Supersuite. Dating (U-Pb) of zircons by SHRIMP from the porphyritic microgranite at Giants Den by Cross and Blevin (in prep) has yielded an age of 288.7 ± 1.6 Ma, indistinguishable from other ages from the Bundarra Supersuite (Black, 2007; Cawood et al, 2010). The mineralogical associations of these intrusions are also similar and the deposits can be regarded as genetically related to the S-type Bundarra Supersuite.
Age of the Parlour Mountain - Red Range and Oban River intrusions The Parlour Mountain Granite is associated with the significant Booralong molybdenite deposits in addition to smaller Sn and Au occurrences. A zircon U-Pb SHRIMP age of 254.7 ± 1.6 Ma (Cross and Blevin, 2010) supports a temporal spatial association with Permian magmatism for this granite, and is supported by the age of the cross cutting Regional Felsic Dyke Swarm (252.5 ± 1.8 Ma, Cross and Blevin, op. cit.). The tectonic significance (cf. Flood et al., 1991) of the Regional Felsic Dyke Swarm is not clear as the swarm does not appear to represent a significant time hiatus between earlier plutons that are cut by the dykes, and later plutons which cut the swarm (e.g. Gwydir River Adamellite at 254.7 ± 1.6 Ma; Cross and Blevin, op. cit). An age determination of 243 ± 2 Ma (Marc Norman, ANU, pers commun, 2008) using the Re-Os method on a molybdenite from the Perrins mine within the Gwydir River Adamellite is indistinguishable from the U-Pb SHRIMP zircon age, supporting a genetic relationship between mineralisation and the Uralla Supersuite.
46
NEO 2010 The Red Range and Oban River Leucoadamellites, located to the east of Glen Innes, are associated with Sn, Mo and gem mineralisation. Both units have previously been regarded as Triassic. New U-Pb zircon ages by Cross and Blevin (in prep) for the Red Range (253.2 ± 1.5 Ma) and Oban River (252.1 ± 1.4 Ma) Leucoadamellites indicate Permian rather than Triassic affinities. The results are also indistinguishable from the ages obtained for the Gilgai Granite, and are the same as ages of the Uralla and Moonbi Supersuites in general.
The Coastal Granites Tin mineralisation is associated with granites of the Gundle Tin Belt which have been incorporated by Bryant and Chappell (this volume) into a somewhat informal “coastal” supersuite association. Dating of the largest of the mineralised plutons (Glen Esk) yielded an age of 221.0 ± 1.6 Ma (Cross and Blevin, in prep). Elsewhere within this province, the Valla and Yarrahapinni plutons are associated with a zoned Mo, Au, Ag, and As district. The Valla and Yarrahapinni plutons yielded zircon U-Pb SHRIMP ages of 230.6 ± 1.6 Ma and 228.4 ± 1.7 Ma, respectively (Cross and Blevin, op cit), and support a close genetic relationship between these two plutons. The younger (post-Mole) ages for granites along the “eastern fall” and coast of the southern NEO indicates that rather than granite magmatism having terminated in a single early Triassic flourish, plutonism occurred episodically at ~240 (Round Mountain), ~230 and ~220 Ma, and finishing (at least as current dating indicates) at 212.3 ± 1.6 Ma with the emplacement of the subvolcanic intrusions into the Lorne Basin (see Cross and Blevin, 2010).
Drake and Halls Peak: age of mineralised volcanic packages Gold-Ag and Ag-Pb-Zn mineralisation is also associated with Permo-Triassic volcanics in the southern NEO. These include the Drake epithermal Au-Ag deposits and the massive sulphide style mineralisation associated with the Halls Peak Volcanics. Cawood et al. (2010) obtained an age of 292.6 ± 2.0 Ma (U-Pb zircon by SHRIMP) for the system at Halls Peak. At Drake, a recent preliminary Ar-Ar age of 261.4 ± 1.9 Ma on alteration sericite (David Phillips Melbourne University, pers commun, 2010) has established that the epithermal style mineralisation there is related in time to the age of the host volcanics (264.4 ± 2.5 Ma, Cross and Blevin, 2010). These results indicate that the mineralisation at Drake does not represent a superimposed event related to younger magmatism. The setting of the volcanics and their mineral systems also vary, with the Halls Peak and Drake volcanics being shallow marine, while the younger volcanics of the Wandsworth Volcanic Group are dominantly terrestrial.
Discussion The metallogenic story of the southern NEO has evolved in step with new understandings of the timing of magmatism and their metallogenic associations. The most important change from the recent dating work is that the important Gilgai Granite is now established to be Permian in age (Cross and Blevin, 2010) and to have been emplaced synchronously with the Tingha Monzogranite. The importance of the new ages for the Gilgai, Parlour Mountain, Red Range and Oban River plutons is the reassignment of important mineralising events to the Permian rather than the Triassic. Confirmation of tin mineralisation ages in the early Permian (Bundarra Supersuite) through to the Upper Triassic (Glen Esk) has also established that mineralisation events associated with magmatism in the southern NEO are not confined to one or two key periods, but are present essentially over the entire range associated with intermediate to felsic magmatism. What is also intriguing is that the association of Uralla Supersuite plutons with oxidised and fractionated granites is not restricted to the Tingha and Gilgai plutons. The Oban River and Red Range leucoadamellites appear to intrude, or are intruded by, the Wards Mistake pluton. Vickery et al (this volume) also report a fractionated intrusion (Newholme) within the Mount Duval pluton. The Parlour Mountain Granite is also spatially associated with the Uralla Supersuite, and at the south western extent of the supersuite, the Balala Adamellite is spatially associated with the felsic Glenifer Monzogranite. The extent of any genetic relationship that may exist between the Uralla Supersuite and these more fractionated magmas, and the role of any relationship in determining the unusual Sn-Mo metallogeny observed in many of these systems, are new questions that remain to be investigated.
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Acknowledgement Published with the permission of the Acting Director, Geological Survey of New South Wales.
References Black, L.P., 2007. SHRIMP U-Pb zircon ages obtained during 2006/07 for NSW Geological Survey projects. Geological Survey Report, GS2007/298. Blevin, P.L., 1996. Internal evolution and metallogeny of Permo-Triassic high-K granites in the TenterfieldStanthorpe region, southern New England Orogen, Australia. Geological Society Of Australia Abstracts 43, 94-100. Blevin, P.L., and Chappell, B.W., 1993. The influence of fractionation and magma redox on the distribution of mineralisation associated with the New England Batholith. In: Flood, P.G. & Aitchison, J.C. (eds). New England Orogen, Eastern Australia. The University of New England, Armidale, 423-429. Blevin, P. L. and Norman, M., 2010. Cassiterite—the zircon of mineral systems? A scoping study. Geological Society of Australia, 2010 Australian Earth Sciences Convention (AESC) 2010, Earth systems: change, sustainability, vulnerability. Abstracts 98, 399-400. Bryant, C.J. and Chappell., B.W., 2010. New England Batholith: unravelling the compositional diversity. This volume. Cawood, P., Leitch, E., Merle, R., and Nemchin, A., 2010. Earliest Permian non-collisional orogeny and basin formation in the southern New England fold belt sector of the Terra Australis Orogen. Geological Society of Australia, 2010 Australian Earth Sciences Convention (AESC) 2010, Earth systems: change, sustainability, vulnerability. Abstracts 98, 70. Cross, A., and Blevin, P. L., 2010. Summary of results for the joint GSNSW – GA geochronology project: New England Orogen and Sydney–Gunnedah Basin April – July 2008. Geological Survey Report, GS2010/778. Flood, R. H, Shaw, S. E., and Farrell, T. R., 1991. Plutonic, volcanic and metamorphic rocks of the New England Batholith. Excursion Guide. Second Hutton Symposium on Granites and related Rocks, Canberra, 1991. BMR Record 1991/23. Shaw, S. E. and Flood, R. H., 1981. The New England Batholith, eastern Australia: geochemical variations in time and space. Journal of Geophysical Reearch, 86, 10530-10544 Vickery, N., Blevin, P.L., Brown, B., Rutten, R, and Ashley, P., 2010. Definition of a new member of the Uralla Supersuite, New England Batholith: the Newholme Monzogranite and its distinction from the Mount Duval Monzogranite. This volume.
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The New England Orogen – Beginnings of geological knowledge David Branagan School of Geosciences, University of Sydney, Australia
Keywords: Explorers, Leichhardt, Clarke, David, Andrews, Voisey
Introduction Although the New England Orogen extends well into Queensland the early European geological studies were carried out mainly in the New South Wales portion, and this paper concentrates on this region. The study is largely sequential, rather than thematic, noting geological observations by the earliest explorers and knowledgeable ‘travellers’; the beginning and continuation of government surveys; specific studies from the beginning of the twentieth century by staff and students of the University of Sydney. The final section deals briefly with the early years of geology at the then New England College. Through these 125 years the main themes of New England geology were developing: age relationships, granite variation, volcanism, serpentinite occurrences, the distribution and origin of ore deposits, geomorphology, glaciation, palaeontology, structure and tectonism, While significant contributions to the knowledge of New England Geology were made by others, the major figure was T.W. Edgeworth David who first visited the region in 1883 – 84, studying geology related to tin, gold and coal. His last paper specifically on the region appeared in 1930, and his students ensured continuing significant research.
Explorers In 1818 the first European incursion into New England was led by John Oxley. His party travelled via the Warrumbungles, north of the Nandewars to the Peel (Tamworth). At the upper Apsley River he had to divert, before reaching the headwaters of the Hastings River and thence to Port Macquarie. (Fig. 1). Thus Oxley crossed the Tamworth Trough, the Peel Fault, the south end of the Woolomin-Texas Block and the Hastings Block. He recognised the changes from the Gunnedah Basin sandstones, noting in the Tamworth area ‘granite, coarse porphyry, freestone and whinstone’ Near Walcha Road there was a marked change, the rocks being hard blue and shiny’. The rocks at the Apsley and Tia Falls were all slate’. Oxley wrote ‘[here] the country seems cleft in twain … a mark of the vast convulsions this country must at one period have undergone’: a distinctly ‘catastrophist’ idea. Some of Oxley’s rock samples, including a granite, were described by Buckland (1821). In 1827 Allan Cunningham’s expedition from Scone recorded sandstone on the northern side of the Liverpool Range, the rocky nature of Currabubula Creek, sandstone The hills near Warialda were of a ‘bold and rocky character’. He named ‘a very sharp cone’, Braco Peak on Mastermans Range. Here ‘large sandstone masses rolled down’ lay on pudding-stone containing large pebbles of quartz and jasper. Next day the rock formation changed to a porous rock, related to trap, containing quartz nodules.’ Three weeks later, travelling northeast the party reached a ‘broken country’, with a geological structure not previously seen: a hard granite with preponderant quartz unusually large. Cunningham tried to return south via a more easterly route but was frustrated near Tenterfield by the rough country (Fig.1).
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Fig. 1 New England Geological Exploration, 1818 – 1886
Ludwig Leichhardt, European visitor In the 1840s New England was settled by squatters who made the geography known. One, Sir John Jamison, sent fossils to W.B. Clarke, as did Surveyor James Burnett who noted limestone in the Peel valley and ‘trap’ on the Dividing Range. Paszkowski (1997) thought Strzelecki travelled as far as Narrabri, returning south through Tamworth and Nowendoc (Fig. 1), late 1842 to April 1843, but his unpublished Geological Map (Branagan 1986) only shows geology as far as the Liverpool Range, and Strzelecki (1845, p. 57) merely wrote, probably second-hand: ‘At the 300 of latitude [this value is clearly wrong], the granitoite chain divides the sources of the river Peel, running to the westward, from those of the Hastings, flowing NE towards Port Macquarie’. 50
NEO 2010 Ludwig Leichhardt travelled from Glendon north to the Liverpool Ranges, which he noted ‘forming a sweeping arc of basalt around the basin of the Hunter … the feldspathic and pyroxenic porphyries constitute a subordinate series closer to the sea’. His extensive traverse, in 1843 – 44, (Leichhardt, 1855, 1867 –8) was the most comprehensive through New England by a geological observer, particularly on the return journey from Wide Bay south from Tenterfield and Armidale, to Apsley Falls, the Manning, and Gloucester to Newcastle. His numerous cross-sections are particularly enlightening (Fig. 2).
Fig. 2 Summing up the Geology, he saw extensive sandstone containing fossil trees, along the western foot of mountains of quartz rock and granite. The granite formed a high tableland stretching south for 114 miles, until broken through by basalts. At the flanks were hills of quartz and porphyry, which intruded the granite in dykes and irregular masses. The basalt or phonolitic rocks, which intruded the granite, formed the Ben Lomond Range, 32 miles north to south, rising 1 000 feet above the high land of the granite. … The granitic mountain ranges show rounded outlines, and not those bold forms we are accustomed to see in granitic rocks in other districts. This is caused by the excess of feldspar and the great size of the components, so that the rock crumbles easily. The forms and combinations of the blocks are often very striking; we see mighty masses balanced on a weak support … (Fig. 3). The Phonolite and Basalt mountains show the usual character: conical hills or elongated ridges, with nearly rectilinear backs and short sharp slopes. In the granitic district we can distinguish first, the coarse-grained reddish granite … then a whitish one, harder and with smaller components, quartz, feldspar, mica evenly mixed … further on a kind of Pegmatite … and a blackish granite. Lastly several feldspar porphyries … the conglomerates and pudding stone seem … to be only local formations. ‘The talcose schist and slate, forming the fall of the Apsley, belong to the Eastern Coast system extending from the Gloucester to the Bunya Bunya chain, and even to Wide Bay … from the moderate hilly country at Apsley Falls (Fig. 4) we could scarcely suspect being near a cleft nearly 300 feet deep. The fall is twenty miles from the coastal land. The mountains extend thus far without a break and then drop quickly and steeply. It is probable that floods have gradually hollowed out the channel, and that the fall itself is presently travelling slowly [upstream], till it [breaks through the slaty area, and remains stationary at the (if not harder, at least) less cleaved basalt and phonolite’. Leichhardt’s uniformitarian interpretation of the evolution of the eastern edge of the plateau contrasts notably with Oxley’s earlier ideas. Near Mt. George he noted ‘an orbicular serpentine banded by quartz rock’ … then met a conglomerate or pudding stone which extended ‘nearly far down the Gloucester’.
Government geological interest The beginning of gold mining in New South Wales, and a rapid spread from the central west to New England, in the 1850s saw the Government become involved, with the appointment of Samuel Stutchbury. Undertaking a regional survey, he also had to act as a skilled prospector reporting on ‘finds’ and potential sources. He entered New England territory in May, 1853 (Fig. 1) remaining essentially in the Tamworth Belt, continuing northerly into Queensland (Moreton Bay and finally to Gladstone). He did not move onto the New England tableland as that ‘important gold district’ was being examined by Rev W.B. Clarke. ‘I have turned more to the westward, for the purpose of extending the knowledge of as large a portion of the colony as early as possible.’ Near Bingara he recorded serpentine and noted it penetrated by a ‘close grained syenite 51
NEO 2010 standing in irregular tors; … with iron ore (chromite?) in isolated masses.’ He was disappointed in not finding native or ores of copper. Gold was dispersed through the soil flats. Just north serpentine gave way to porphyritic rocks, which prevailed as far north as Molroy. Near Gravesend Stutchbury recorded two occurrences of limestone, containing corals (?Favosites fibrosa) and crinoid remains, and he felt there was no intervening fault. Stutchbury’s real contributions to New England geology belong in Queensland, and must be left for now (Bryan 1954; Branagan 1975).
Clarke set out to examine the headwaters of the Manning, Macleay, Hastings, Clarence and Tweed flowing east, and the Peel, Namoi, Gwydir, Macintyre and Condamine flowing west. By 6 November 1852 Clarke was camped on the Peel River, where he found the country ‘had the classic features of the gold country west of the Blue Mountains and in Victoria, with thick beds of quartz.’ Clarke continued to the Hanging Rock diggings, reporting that the gold occurred capriciously as lumps near the surface or as fine dust in granite (?) not quartz. Clarke confirmed Leichhardt’s observations, noting that the ranges north of Armidale were largely of granite, ‘through which trappean basalts and greenstones had been forced’. Clarke then went east into the Clarence Basin, recognising its synclinal character and equating the ‘carboniferous’ formations with the Wianamatta of the Sydney Basin. He described the unconformity between the basin rocks and the older formations (including slate, schist, serpentine, granite and limestone), which further south he had nominated as being, in at least, ‘Silurian’. Clarke made several ascents of the ‘Great Escarpment’ west of the Clarence and thought it was the direct result of faulting, and probably continuous with that along the escarpment in the Apsley– Macleay River region. Between Grafton and Tabulam he recognised 16 units, particularly ‘siliceous slate and serpentine, in stony ridges … and, a little further north, at Yulgilbar ‘granite with serpentine, and compact feldspar, on the hills grey cornean’. West from Tabulam to Tenterfield he went from quartz sandstone to granite, ophite and green schist pebbles, flinty slate, granite, ophite and siliceo-felspathic schist to pegmatite on the tableland, and finally granite, coming essentially to the point where Cunningham first noted New England granite back in 1828. While the work of Stutchbury and Clarke were by their nature broad surveys, Ferdinand Odernheimer’s mapping of the Cordillera Gold Co’s leases at Nundle in 1855 was one of the first detailed mapping undertaken in New England, the series of individual beds cropping out in the Peel River and tributaries being carefully identified. Odernheimer did other less detailed mapping further south suggesting Manning rocks were possibly the same age.
Tin miners, Wilkinson 1870s, David 1883-84 Government geological work in New England was re-ignited in the 1870s, when tin mining was wellestablished, but gold was still of interest, Government Geologist C.S. Wilkinson travelling to Uralla to examine the diggings (Wilkinson 1878). Next year Lamont Young went to the Bingara goldfield, noting ‘Silurian strata passing from well-defined slate into serpentinite’, visiting the Prince of Wales Copper mine, reporting gold reefs, extensive basalt at the head of the valley and fossiliferous limestone (Young 1879) 52
NEO 2010 Wilkinson began the study of the tin fields in 1881, but after a preliminary study he passed the project over to Edgeworth David, who began his first major work in 1883 around Vegetable Creek, completing his mapping by August 1884. Apart from the tin-mining he examined coal near Ashford', a silver lode near Emmaville, and the Uralla goldfield (1886). He published papers on laterite formation and on ‘points of eruption’ (David, 1886, 1887). The ideas and publications of one of the Geological Survey’s best-known members, E.C. Andrews, concerning geomorphology, based to some degree on his observations in New England (1904), had a profound influence on ideas of uplift and erosion in the period from 1910 to the late 1950s. And an oddity for the Survey was E.J. Kenny’s (1922) reporting on Oil Search near Tamworth.
The University of Sydney contributions David’s move to the University of Sydney in 1891 meant less direct involvement in the region, but he maintained his interest, leading excursions, and publishing on specific aspects (including diamonds) (David 1897, 1907, 1931), incorporating aspects of its geology in publications on structure and tectonics (David, 1905, 1911). Numerous David students ensured continuing development of New England geology. These include Cotton (1909, 1910, 1914) dealing with tin, other ores and diamonds. Cotton and Walkom (1913) Carboniferous-Devonian near Tamworth. Better known is the ground-breaking work by Benson (1913 to 1920) on the Great Serpentine Belt. H.I. Jensen’s (1914) significant book on soils included a considerable amount on New England geology, and included controversial geology. The 1930s mark the final period of David’s direct influence, through the publications of two great friends and rivals: Voisey, mainly North Coast (1934 to 1939 and 1942) and Carey, the Werrie Basin (1934, 1935, 1937, 193), Carey & Browne (1938), Carey & Osborne (1939).
New England College, 1938, begins the ‘local’ activity This adjunct of the University of Sydney was established in 1938 under Warden Edgar Booth. He had to battle ‘scepticism, inertia and actual hostility’ (Drummond 1959). Geology and Geography began the following year with Alan Voisey, fresh from experience in northern Australia, as Lecturer. Elizabeth M Basnett (later Robbins) was appointed Assistant Lecturer in 1942. The advent of WW2 saw little development until 1946. The modern story of New England geology is dealt with in other papers of this Symposium.
References Branagan, D.F., (ed.) 1973. Rocks, Fossils, Profs. Department of Geology & Geeophysics, University of Sydney. Science Press, Sydney. Branagan, D.F., 1984. Dr. Odernheimer’s Map. Seventh Australian Geological Convention, Geological Society of Australia Abstracts 1984, 12, 78. Branagan, D.F., 1986. Strzelecki’s Geological Map of Southeastern Australia: an eclectic synthesis. Historical Records of Australian Science, 6, 375 – 392. Bryan, W.H., 1954. Samuel Stutchbury and some of those who followed him. Queensland Government Mining Journal, 55, 641 – 646. Buckland, W., 1821. Notice on the geological structure of a part of the island of Madagascar, with observations from the interior of New South Wales collected during Mr Oxley’s expedition in 1818 Transactions of the Geological Society, London, 1st series, 5, 476 – 481. Clarke, W.B.,1853. Letter from the Rev. W.B. Clarke … on the Geology of the Clarence District and the adjoining regions, Report IX, 24 June 1853. NSW Legislative Council Papers. Drummond, D.H. 1959. A University is born. The Story of the Founding of the University College of New England, Angus & Robertson, Sydney. Jensen, H.I., 1914. The Soils of New South Wales. Department of Agriculture, New South Wales, Government Printer, Sydney, 199 pp. Leichhardt. L.,1855. Beiträge zur Geologie von Australien von Professor H. Girard. H.W. Schmidt, Halle. 53
NEO 2010 Leichhardt, L., 1867– 68. Notes on the Geology of parts of New South Wales and Queensland. (Translated by G.H. Ulrich). The Australian Almanac. Pt 1, 1867, 29 – 55; Pt 2, 1868, 29 – 52. Oxley, J., 1820. Journals of two expeditions into the interior of New South Wales in 1817 – 18. John Murray, London. Paszkowski, L., 1997. Sir Paul Edmund de Strzelecki. Australian Scholarly Publishing, Kew Victoria. Strzelecki, P.E., 1845. Physical Description of New South Wales and Van Diemen’s Land. Longman, Green and Longmans, London. Stutchbury, S., 1853. Eleventh tri-monthly Report from the Geological Surveyor to the Colonial Secretary. NSW Legislative Council Papers. Wilkinson, C.S., 1876. Government Geologist’s Report. Annual Report NSW Dept of Mines. , Govt Printer, Sydney. Young, H. L., 1879. The Bingara Goldfield. Annual Report of the Department of Mines, NSW for the Year 1878. Government Printer, Sydney, pp. [For Benson, Carey, Cotton, David & Voisey publications see Branagan (1973)]
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Potential exploration uses of high resolution geophysical data in the southern New England Orogen, NSW R.E. Brown Industry and Investment, NSW,Geological Survey of New South Wales, Armidale
Keywords: geophysics, radiometric, aeromagnetic, gold, diamond, sapphire, bauxite, palaeochannel, faults, fractures, dykes, granite, basalt.
Abstract
L PEE
High resolution airborne geophysical surveys conducted during 1996, 1998, 2001 and 2002 have produced detailed radiometric and magnetic data over a large area of the western New England Orogen in New South Wales. Images produced from these data have been used for geological and mineralisation studies, revealing many previously unidentified geological features, and enhancing our knowledge of some previously recognized structures and geological units. Image processing techniques have further enhanced the useability of the geophysical images. This was initially utilised for diamond exploration, whereby point aeromagnetic anomalies were enhanced as potential focal points for hard rock and associated alluvial 3 diamond exploration. This technique proved successful, with traces of diamond located in a previously unexplored region southwest of 5 4 Bingara. Filtering of aeromagnetic data has proven useful for the interpretation of possible palaeochannels beneath 1 deep leads, and for the possible buried granitoids beneath felsic and basaltic 2 volcanic cover. em nd Be
r ee
n Li
m ea
t en
co
r do rri
FA UL T
Localities in text 1. 2. 3. 4. 5.
Tilbuster-Puddledock area Enmore-Melrose area Webbs silver mine Conrad mine Tingha-Gilgai area
Fig. 1. The location and structural setting of the three high resolution airborne geophysical surveys flown within the southern New England Orogen in New South Wales.
Aeromagnetic images are useful for the accurate definition of fractures, faults and dykes spatially associated with mineralisation. These features commonly do not crop out. Relationships between known orogenic gold occurrences and sets and intersections of faults and fractures can be determined from aeromagnetic images in various sites along the Peel fault, in the Enmore-Melrose and Tilbuster-Puddledock areas, and near Uralla. Similar relationships are apparent in granite-related polymetallic deposits hosted by the Gilgai and 55
NEO 2010 Tingha granitoids, and in metasedimentary rocks spatially associated with the Webbs silver mine lode. Aeromagnetic images are also useful for identifying fractures associated with the Bendemeer Lineament, some of which are associated with orogenic gold occurrences. Radiometric images have proven useful for the identification of previously unmapped leucogranite bodies. Fractionated leucogranites are the most significant source of granite-related Sn, W, Au and Ag-rich polymetallic mineralisation in the southern New England Orogen. Image manipulation of radiometric data has successfully demonstrated the relationship and spatial distribution of a suite of basaltic volcanics which have sourced the rich alluvial sapphire deposits of the Inverell-Glen Innes area. There is also some potential for the identification of previously unmapped bauxitic laterite and sapphire-bearing volcaniclastics deposits using image manipulation of radiometric data.
Introduction The southern New England Orogen in New South Wales is partially covered by 3 high resolution aeromagnetic and radiometric surveys released by the Geological Survey of New South Wales (Fig. 1): 1. The Peel Survey, a composite of data acquired by CRAE Pty Ltd (Terrill and Kitson 1996) with data acquired in 1998 by the New South Wales Geological Survey under the Discovery 2000 initiative (Brown 2001). 2. The Peel South Survey was flown in 2001 (Brown 2002, 2003a) as part of the Exploration NSW initiative, and 3. The Inverell Survey was flown in 2002 as part of the Exploration NSW initiative (Brown 2003b, 2006) The surveys were flown using fixed-wing aircraft and global positioning system (GPS) control. East-west flight lines were used with 250 m interline spacing at a nominal 60 m ground clearance. The surveys acquired potassium, thorium and uranium radioelement; magnetic; and digital elevation data. The three survey areas were sited over areas considered to have significant potential for the discovery of mineral deposits. The areas were also considered to have existing geological mapping of a generally low standard, which would benefit from reinterpretation using high resolution geophysics. Each area was also capable of being flown by fixed wing aircraft, a feature which excluded large areas of the New England (Fig. 2). Since the release of the geophysical survey data it has been extensively used by:
the mineral exploration industry, academics and students in the fields of geology and geography, the Namoi Catchment Management Authority, and the New South Wales Geological Survey.
The mineral exploration industry has used the geophysical data as a tool for geological mapping, for the delineation of potentially mineralisationhosted fractures, and for the identification of potentially diamondiferous mafic dykes and
Fig. 2. The location of the present three high resolution geophysical surveys and the proposed Grafton-Tenterfield survey, superimposed on a gradient-coloured digital elevation model. White and yellow shades represent high elevations, whereas dark blue is low elevation.
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NEO 2010 plugs. Academics and students from a number of universities have used the data as teaching material, for research on specific rock units and intrusives, for mapping soil types and variability, and for student geological mapping exercises and theses. One academic group has trialled the use of radiometric data for identifying soil types associated with high carbon content. The Namoi Catchment Authority has used radiometric data to map sediment types associated with major watercourses, relating them to their provenance and to water chemistry. The Geological Survey has used the geophysical data as a major tool in mapping the Manilla 1:100 000 sheet, and in providing advice to clients in academia and the mineral exploration industry. It is intended to fly an additional area of the southern New England Orogen in the 2010-2011 financial year. The initial area to be flown will extend southward from the state border and include the Warwick-Tweed Heads and Grafton Maclean 1:250 000 sheet areas. The new survey will be known as the Grafton-Tenterfield survey (Fig. 2).
Use of airborne geophysical survey data for mineral exploration There are numerous options for utilising the high resolution geophysical survey data for mineral exploration. These include: 1. 2. 3. 4. 5. 6. 7.
identifying unmapped or concealed, potentially diamond-bearing mafic dykes and plugs accurate definition of potentially mineralised fractures and faults identification of outcropping and unmapped, fractionated leucogranitoid bodies identification of shallowly buried granitoids potentially associated with cupola-style mineralisation delineation of shallowly buried, basalt-capped deep leads identification of broad alteration zones within reactive rocks delineation of the extent of primary sapphire host rocks from which commercially viable sapphire placers may be derived 8. identification of unmapped bauxitic laterite sheets, and better definition of the outcrop extent of known bauxites.
Diamond exploration. Immediately after its release, the Peel geophysical data were used extensively (by a number of exploration companies) for diamond exploration in the western New England Orogen. Exploration focussed on bullseye magnetic anomalies which are interpreted to be mafic plugs (Fig. 3). Activities were directed to the region about the Bingara diamond field, where alluvial diamonds were mined during the 19th century. No hard rock source was identified during mining, and previous pre-Peel geophysics exploration relied upon stream sediment sampling to identify possible kimberlitic intrusions. Numerous previously unidentified plugs were identified, and subsequent exploration was successful in demonstrating a spatial relationship between diamond and plugs to the south of Bingara (Rimfire n.d.) Alluvial diamond occurrences in the Copeton area are spatially associated with dolerite dykes. More than 10 diamonds were recovered from at least one of these dykes during mining in the 19th century (Brown & Stroud 1997). The dykes are only exposed in underground deep lead workings. An apparent northerly-trending dyke swarm is evident in the aeromagnetic images through the Copeton area. Similarly-orientated linear anomalies, interpreted to be magnetic mafic dykes, also occur in the Watsons Creek
Fig. 3. Enhanced first vertical derivative magnetic map of the Bingara region. Green symbols are recorded diamond occurrences. The orange, transparent polygons are Tertiary igneous rocks.
57
NEO 2010 area, where small numbers of alluvial diamonds have been found. Possible dykes are apparent to the west of Bingara in the vicinity of alluvial diamond occurrences (Fig. 3). Mineralised fractures and faults. Aeromagnetic images display linear anomalies of low magnetic response which are interpreted to be fractures and faults. These structures are particularly well shown in first vertical derivative (1VD) images. Examination of 1VD images proximal to areas of structurally controlled orogenic gold and granite-related mineralisation commonly shows structural features which may relate to the control of the mineralisation. Many orogenic gold-antimony-quartz-carbonate veins spatially associated with the Peel Fault, and in the Tilbuster-Puddledock and Enmore-Melrose areas show a close relationship between the orientation of the veins and apparent fractures visible in the 1VD images. Silver-rich polymetallic veins associated with the Mole Granite and Gilgai-Tingha granitoids also shows a strong correlation between vein orientations and lineaments visible in 1VD images. In particular, many interpreted fractures are apparent about the 8 km long Conrad lode and spatially associated, minor polymetallic veins (Fig. 4). Unmapped leucogranitoids. A number of previously unmapped, outcropping leucocratic granitoids have been identified to the south of the Gilgai Granite (Brown 2001). Outcrop areas of some bodies exceed tens of hectares. These granitoids are spatially associated with the regional felsic dyke swarm (Brown et al. 1992; Brown 2001) and minor occurrences of Sn and polymetallic mineralisation. The subsurface extent of these granitoids is unknown, and their potential for significant, cupola-associated mineralisation has not been investigated. Shallowly buried granitoids. Filtering of the Inverell Survey magnetic data using proprietary methods (Brown 2006) was successful in identifying possible shallowly buried granitoid plutons. At least one of these possible bodies is spatially associated with stockwork Sn mineralisation (Grant and Faulkners mine). Most interpreted granitoids are covered by felsic volcanics of the Late Permian Wandsworth Volcanic Group. No further research has been undertaken to validate the presence of plutons. Geophysical modelling of the interpreted granitoids may reveal their three dimensional geometry, thereby indicating any potential for cupola-style mineralisation. Deep leads. Extensive sheets of Tertiary basaltic lavas cover large areas of the New England Orogen. Some of these sheets are locally associated with deep leads hosting cassiterite, gold, diamond or sapphire deposits. The areal extent of deep leads beneath the basalt sheets is poorly defined, and previous exploration for these palaeochannels has relied upon shaft sinking, drilling and limited use of geophysics. The use of proprietary filtering techniques on the Inverell Survey aeromagnetic data (Brown 2006) was apparently successful in identifying one previously unidentified palaeochennel beneath basaltic lavas (Fig. 5). Although there has been no attempt to validate this possible deep lead, the potential for using various filtering techniques (i.e. overburden filters) to enhance shallow magnetic anomalies has been highlighted by this example. Alteration zones. Broad alteration zones within reactive rocks such as the extensive granitoids and felsic volcanics of the southern New England Orogen could potentially be identified using radiometric images. Subtle to intense alteration would result in addition or depletion of K, Th or U, resulting in local radiometric anomalies. These anomalies, and their associated alteration zones, may be accompanied by hydrothermal, epithermal or mesothermal mineralisation. Although no attempt has been made to investigate the possibility of identifying broad alteration zones, small areas of apparent alteration (or weathering) are evident on aeromagnetic images as linear zones accompanying fractures in many plutons. Broad areas of apparent subtle deuteric alteration have been interpreted using radiometric and aeromagnetic images in parts of the Gilgai-Tingha pluton (J. Leigh, pers comm.. 2010). Sapphire exploration. The Inverell Survey radiometric images have provided a tool for the relatively accurate areal subdivision of the Tertiary lava and volcaniclastics sheets of the Inverell-Glenn Innes region (Brown 2006, Vickery et al. 2007). The radiometric images have provided a reliable framework for distinguishing different eruptive suites, of which the Maybole Volcanic Suite (Vickery et al. 2007) is genetically related to the occurrence of commercial alluvial sapphire deposits. The extent of the Maybole Volcanic Suite on radiometric images provides an exploration tool for focussing sapphire exploration activities beyond the present, or traditional areas of prospecting. Further refinement of the radiometric images using image processing algorithms may assist in identifying the basal volcaniclastic 58
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6688000
and lava facies of this volcanic suite which are recognised as the primary hosts to sapphires (Brown & Pecover 1986a,b; Pecover 1987, Oakes et al. 1996).
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59
NEO 2010 Bauxitic laterite exploration. There has been sporadic exploration for bauxitic laterite in the New England region for many decades. Exploration for this commodity is presently widespread in the Inverell region, where the latasols are spatially associated with Tertiary lavas, volcaniclastics and sediments. Previous mapping of the extent of the laterites is generally inaccurate and incomplete. The presence of maghemite with some laterites precludes the use of aeromagnetic images as a means of distinguishing the lateritic rocks from basalt. Image processing of radiometric images using algorithms designed to highlight the high Th, very low K and low to moderate U characteristics of the laterite may assist with identifying its regional extent. Initial use of discriminatory algorithms on radiometric images has been promising, but requires field validation to adequately test and refine the technique.
Conclusion The present three airborne geophysical surveys flown by the New South Wales Geological Survey offer potential for assisting with mineral exploration in the southern New England Orogen. Initial and potential uses for the geophysical images include the identification of potential magmatic diamond hosts; identifying the controlling and host structures for orogenic gold and granite-related polymetallic veins; identifying unmapped and concealed granitic plutons which may be associated with cupola-style mineralisation; identifying concealed deep leads; accurately defining the extent of primary sapphire host rocks which may produce economic accumulations of alluvial sapphires; and mapping the extent of bauxitic laterites. To achieve maximum benefit from the data, image processing or geophysical modelling of the data is required to enhance the exploration target. Proposed additional airborne geophysical surveys over the following years will significantly increase the number of potential exploration targets in the southern New England Orogen. These surveys will possibly also make available new exploration techniques using images derived from the survey data. The geophysical data will also provide an excellent tool for reinterpreting and remapping the geology of the region, thereby providing a significantly improved platform for mineral exploration.
Acknowledgement. Published with the permission of the Acting Director, Geological Survey of NSW.
References Brown, R.E., 2001. Peel Discovery 2000 geophysics – providing keys to exploration in the western New England region of New South Wales. Geological Survey of New South Wales - Quarterly Notes 111, 1–15. Brown, R.E. (compiler), 2002. Peel south geoscience data package, version 1. Geological Survey of New South Wales, Sydney. Published on CDROM. ISBN 0 7313 9299 X Brown, R.E., 2003a. Peel South Exploration NSW geophysics – interpretation of new data for exploration and geological investigations in the western New England area of New South Wales. Geological Survey of New South Wales - Quarterly Notes 114, 1–28. Brown, R.E. (compiler), 2003b. Inverell geoscience data package, version 1. Geological Survey of New South Wales, Sydney. Published on CDROM. ISBN 0 7313 9227 2 Brown, R.E., 2006. Inverell Exploration NSW geophysics – new data for exploration and geological investigations in the northern New England area of New South Wales. Geological Survey of New South Wales - Quarterly Notes 121, 1–38. Brown, R.E. and Pecover, S.R., 1986a. The geology of the “Braemar” sapphire field. Geological Survey of New South Wales, Report GS1986/270 (unpublished). Brown, R.E. and Pecover S.R., 1986b. The geology of the Kings Plains sapphire deposit. Geological Survey of New South Wales, Report GS1986/271 (unpublished). Brown, R.E. and Stroud, W.J., 1997. Inverell 1:250 000 metallogenic map SH/56-5: metallogenic study and mineral deposit data sheets. Geological Survey of New South Wales, Sydney, 576 pp. Brown, R.E., Brownlow, J.W. and Krynen, J.P., 1992. Manilla-Narrabri 1:250 000 Metallogenic Map SH/56-9, SH/55-12: metallogenic study and mineral deposit data sheets. Geological Survey of New South Wales, Sydney, 319 pp. 60
NEO 2010 Oakes, G.M., Barron, L.M. and Lishmund, S.R., 1996. Alkali basalts and associated volcaniclastic rocks as a source of sapphire in eastern Australia. Australian Journal of Earth Sciences 43, 289-298. Pecover, S.R., 1987. Tertiary maar volcanism and origin of sapphires in northeastern New South Wales. pp 13-21. In Extended abstracts from seminar on Tertiary volcanics and sapphires in the New England district. Geological Survey of New South Wales, Report GS1986/271 (unpublished). Rimfire N.D. Bingara Diamond Project. URL http://www.rimfire.com.au/bingara.htm, accessed 23/08/2010. Terrill, J.E. and Kitson, S.E., 1996. First annual report for the period ending 19 April 1996, EL 4822 Gwydir NSW. Volumes I and II. CRA Exploration Pty. Limited Report No. 21924, Geological Survey of New South Wales, File GS1997/065 (unpublished). Vickery, N.M., Dawson, M.W., Sivell, W.J., Malloch, K.R. and Dunlap W.J. 2007. Cainozoic igneous rocks in the Bingara to Inverell area, northeastern New South Wales. Geological Survey of New South Wales Quarterly Notes 123, 1–31.
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Mid Permian to mid Triassic development of the southern New England Orogen Jeff Brownlow Industry & Investment NSW, Armidale, NSW, Australia
Keywords: recurrent, deformation, deflation, thermal, cooling, subduction
Introduction The New England Orogen (NEO) was transformed in the latest Carboniferous-latest Triassic from a forearcaccretionary complex to a continental mass onlapped by fringing basins (Brownlow, 1999). That transformation was attributed to the effects of five episodes of geological activity. These episodes were inferred from evidence of a recurrent sequence of igneous, deformational and geomorphic events affecting the NEO and fringing Sydney-Bowen, Esk, Ipswich and Lorne Basins (Brownlow, 1999). New isotopic dates are used to refine the previous interpretation for the mid Permian to the mid Triassic (ca. 270 to 220 Ma) for the southern NEO (sNEO). That interval encompasses the later three episodes of geological activity previously identified, plus an additional (mid Permian) episode tentatively identified herein (Fig. 1 a-e). The viability of subduction as an underlying cause is further tested.
Background The recurrent geological activity previously identified in the NEO and fringing basins (Brownlow, 1999) comprises the following sequence of events:
Deformation and related uplift manifest as folding, cleavage development, unconformity or a basinal record of prograding, NEO-derived clastic wedges lacking juvenile tuffs; Deflation manifest as a lull in NEO-derived sedimentation into fringing basins and/ or a change to a dominantly continental sedimentary influence; Thermal activity manifest as volcanism and granitoid intrusion associated with regional uplift and a basinal record of tuffaceous, regressive sedimentation, or as volcanism associated with rift or extensional basin development; Cooling and deflation manifest by a waning of igneous activity, loss of thermal uplift and/ or by formation of sag basins in place of previous extension or rifting.
The three post-mid Permian episodes affecting the NEO and fringing basins (Brownlow, 1999) span the later Permian, Early-Middle Triassic and Late Triassic. The time ranges correspond to those inferred from the stratigraphic record of deformation and volcanism in fringing basins. Granitoids generally do not directly affect fringing basins, so their stratigraphic age ranges are less clear. However, available dating and overprinting relationships suggest intrusion during or after volcanism, with some later granitoids from one episode intruding after (mobilised by?) deformation at the start of the succeeding episode.
Revised interpretation Timescales for the Permian and Triassic have now been extensively revised by the International Commission on Stratigraphy and are being further refined through access to high-precision isotopic dating (Ogg et al., 2008, Mundil et al., 2010). Increasing numbers of SHRIMP U/Pb zircon dates are now also becoming available in NEO (carried out by Dr Andrew Cross at Geoscience Australia mainly through a joint project with the Geological Survey of New South Wales). A few high-precision dates have also been produced by Ar/Ar (Offler and Foster, 2008) and ID-TIMS U/Pb dating (e.g., Schaltegger et al., 2005). 62
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Fig. 1. a-d. Palaeogeographic maps for: (a) Late Triassic; (b) Early-Middle Triassic, (c) later Permian; and (d) mid Permian. e. Time space plot for the southern New England Orogen using the Sydney Basin, and SE Queensland basinal sequences as reference stratigraphies (see text for details).
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NEO 2010 Some new dates confirm the previous interpretation, e.g., the later Permian and Middle Triassic ages for the Dundee Rhyodacite and Round Mountain Leucoadamellite (Brownlow, 1999). However, consistent with Rb/Sr dates (Shaw, 1994), recent SHRIMP U/Pb dating supports a younger, Late Triassic, assignment for several Coastal belt granitoids (Gilligan et al., 1992) and an older assignment of the Oban belt granitoids (Gilligan et al., 1992), rather than the Middle Triassic previously proposed (Brownlow, 1999). In addition, a date from the Drake Volcanics (SHRIMP U/Pb date 264.4 ± 2.5 Ma, Cross, pers. comm.) is within error of the date of cleavage formation in the Nambucca Block (260-264 Ma, Offler and Foster, 2008), so their assignment to the later Permian episode (Brownlow, 1999) is uncertain. The net effect is that the three episodes proposed previously are confirmed but modified. However, NNWtrending folds, volcanic and granitoid belts in NE New England suggest the influence of an additional, mid Permian episode overlapping the later Permian episode. The four revised episodes are described below in reverse order to best illustrate this interpretation.
Late Triassic episode The Late Triassic episode remains most clearly demonstrated stratigraphically in SE Queensland by intense deformation of the Middle Triassic Esk Basin north of Brisbane (Campbell et al., 1999), and by ensuing widespread volcanism (e.g., Brisbane Tuff, Chillingham Volcanics), associated granitoids, and coal measure sedimentation with interbedded volcanics in the Ipswich Basin (Fig. 1a; Day et al., 1983). Numerous small granitoids in the southeastern part of the sNEO are attributed to the Late Triassic episode (Fig. 1a). Those forming the Coastal Belt were previously inferred to be Middle Triassic, in part because they occur along strike from the southern extension of the Middle Triassic Esk Basin at Nymboida (Gilligan et al., 1992). However, SHRIMP U/Pb dating (Cross, pers. comm.) has established a early Late Triassic age for the Valla and Yarrahapinni adamellites (230.6 ± 1.6 Ma and 228.4 ± 1.7 Ma respectively) and slightly younger age for the Smokey Cape Adamellite (218.4 ± 1.6 Ma). The latter is tentatively included here as a very late product. The Glen Esk Adamellite is also Late Triassic (221.0 ± 1.6 Ma SHRIMP U/Pb, Cross, pers. comm.), consistent with Rb/Sr dating on a nearby pluton (Shaw, 1994). These two plutons seem to be part of a NW-trending belt of small, leucocratic granitoids, some of which are Sn-bearing. Tentatively included here are an unnamed, Sn-bearing pluton at Willi Willi and the Sn-mineralised Kellys Creek Leucoadamellite near Nymboida, respectively as northern and offset northern extensions to this belt. The NW-trending belt centred on the Glen Esk Adamellite is normal to the NE-elongation of the Valla Adamellite suggesting NE-SW shortening.
Early-Middle Triassic episode The Early-Middle Triassic episode (Fig. 1b) is mostly clearly demonstrated by a combination of basal Triassic folding affecting the Gunnedah Basin, and by Middle Triassic thermal activity manifest as thick, volcanic-rich sedimentation in the Esk Basin in SE Queensland (Campbell et al., 1999) and its subsurface extension to Nymboida (northern New South Wales). Supporting stratigraphic evidence for the Early-Middle Triassic episode comes from the Sydney-Gunnedah Basin sequence of coarse, NEO-derived, tuff-poor sedimentation in the lower Narrabeen Group, dominantly continental influence in the upper Narrabeen Group and Hawkesbury Sandstone, and sparsely-tuffaceous, NEO-sourced sedimentation in the Wianamatta Group (Herbert, 1980), which are respectively interpreted as indicating deformation-related uplift, deflation and thermally-related uplift (Brownlow, 1999). Granitoids attributed to this episode (Fig. 1b) are now limited to a belt extending north and south from the Round Mountain Leucoadamellite. SHRIMP U/Pb dating (237.9 ± 1.7 Cross, pers. comm.) has confirmed its age as late Middle Triassic, and similar to the Rb/Sr dates on the Chaelundi Complex and Dandahra Creek Leucogranite (Shaw, 1994). Many of these granitoids are of similar size, many are leucocratic, and many are Sn-bearing (Fig. 1b). The composite Carrai pluton (Carrai Granodiorite and smaller Daisy Plains Leucoadamellite) with its associated Sn mineralisation plus the Werrikimbe caldera (leucocratic) are included here because of similar size, proximity and partly leucocratic compositions. The WSW-trending Tenterfield Fault and widespread, subparallel fractures in later Permian volcanics and some Permo-Triassic granitoids in central New England are tentatively attributed to Early Triassic deformation and suggest WSW-ENE shortening, or approximately normal to the arguably contemporary, Mooki Thrust and adjacent folding in the western Tamworth Belt (Fig. 1b). 64
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Later Permian episode The later Permian episode (Fig. 1c) is most clearly demonstrated stratigraphically by the Permian Wandsworth Volcanic Group in central sNEO and its distal influence on the Singleton Supergroup in the Sydney-Gunnedah Basin, as well as by the concurrent inception of a parallel belt of shoshonitic volcanism in the southern Sydney Basin (Brownlow, 1999). Widespread preceding deformation is suggested by: (a) cleavage formation in the Nambucca Block at 260-264 Ma (Offler and Foster, 2008), (b) rise of the Lochinvar Anticline in the northern Sydney Basin (cf. Collins, 1991), and (c) the sub-Porcupine unconformity in the Gunnedah Basin (Tadros, 1993). The Sydney-Gunnedah Basin sequence below the Singleton Supergroup (Herbert, 1980) provides supporting evidence of deformation and uplift (manifest as coarse, prograding clastic wedges of the Muree Sandstone and correlative Nowra Sandstone and upper Porcupine Formation), followed by deflation (manifest as finer grained sedimentation of the Mulbring Siltstone and correlative Berry and Watermark Formations). Granitoids intruding during or after Permian volcanism in the NNE-trending zone of thermal activity (Fig. 1c) include a belt of Permo-Triassic (U/Pb SHRIMP dating, Cross pers., comm.), mainly mesocratic granitoids of the Moonbi and Uralla supersuites (Bryant et al., 2003) near the SE (outboard) side and the parallel Gilgai-Mole-Stanthorpe belt of Permo-Triassic to earliest Middle Triassic (U/Pb SHRIMP and TIMS dating, Cross pers., comm., Schaltegger et al., 2005), Sn-mineralised leucogranitoids on the NW (inboard) side. Additional Uralla Supersuite granitoids occur in between and one occurs in the inboard belt at Gilgai. Activity extended further SE to include Au ± Sb mineralisation and lamprophyres (e.g., Hillgrove, Rockvale, Enmore) and may have extended to the NW to include the Triassic Sn-bearing Dumboy-Gragin Granite. Structures attributed to the later Permian episode (Fig. 1c) include widespread NE-trending folds, NE-E trending folds and cleavage in the Nambucca Block, WNW-trending left-lateral faults such as the Manning River Fault System, and NS-trending folds such as the Gloucester Basin and Muswellbrook Anticline (Fig. 1c). Early NW-SE to WNW-ESE shortening probably produced widespread NNE to NE trending folds. Later shortening from the same or a more easterly direction is likely to have produced significant left-lateral faulting on the Manning River, and on the curved Peel Fault and other faults to the north. Early folds to the north may have been rotated clockwise to a more easterly direction. Northerly trending folds to the south suggest anticlockwise rotation of early folds, wrench-related oblique folding, and/ or orthogonal folding due to more easterly oriented shortening. Early folding probably accompanied rise of the NE-trending Lochinvar Anticline in Muree Sandstone-Mulbring Siltstone times (cf. Collins, 1991). Later NS-trending folding affected correlatives of the Tomago Coal Measures (lower Singleton Supergroup) and probably coincided with a local unconformity at its top (cf. Collins, 1991).
Mid Permian episode The later Permian and subsequent episodes account for most activity in the sNEO subsequent to the mid Early Permian age Owl Gully Arc (Briggs, 1998). However, various anomalies remain (Fig. 1d). Firstly, granitoids of the later Permian Clarence River Suite (Bryant et al., 1977) and the Permo-Triassic Oban belt of granitoids trend respectively normal and oblique to the NNE-trending zone of later Permian activity. Secondly, the Dummy Creek Conglomerate, Glenmore Formation, the lower, non-volcanic part of the Rhyolite Range Beds (all sNEO) and distal tuffs in the Wandrawandian Siltstone (southern Sydney Basin) predate the Muree Sandstone (Briggs, 1998), so predate the inferred beginning of the later Permian episode. Thirdly, the Drake Volcanics (264.4 ± 2.5 Ma, SHRIMP U/Pb, Cross, pers. comm.) may predate cleavage formation in the Nambucca Block (260-264 Ma, Ar/Ar, Offler and Foster, 2008) and rise of the Lochinvar Anticline in the Sydney Basin. However, their relative ages depends on uncertain stratigraphic relationships at Drake, and on whether cleavage formation in the Nambucca Block was a brief event or the culmination of a more protracted deformation. Finally, NE strikes at the southern end of the Drake Volcanics belt and in the Dummy Creek Conglomerate may indicate deposition before folding early in the later Permian episode. Collectively, these anomalies suggest an earlier, mid Permian episode of geological activity in a NNWtrending belt in NE New England (Fig. 1d). Deformation and uplift marking its onset is indicated by an unconformity between the Drake Volcanics and underlying, NNW-trending, steeply folded sedimentary and volcanic rocks of mid Early Permian age (cf Briggs, 1998). The stratigraphic expression of this episode in the Sydney-Gunnedah Basin is unclear but may have occurred during upper Branxton Subgroup times and sourced distal tuffs in the Wandrawandian Siltstone in the southern Sydney Basin. The volcanic expression is also unclear, but may have included the Drake Volcanics, and the upper, volcanic-rich part of the Rhyolite 65
NEO 2010 Range beds. The later Permian, mesocratic, Clarence River granitoids and the Permo-Triassic, predominantly leucocratic (some Sn-bearing), granitoids of the northerly trending Oban belt are interpreted as outboard and inboard belts occurring either side of the NNW-trending belt of Drake Volcanics (Fig. 1d).
Synthesis New isotopic dates support arguments for recurrent activity in the Permo-Triassic development of the sNEO and fringing basins (Brownlow (1999), but suggest refinements including recognition of a fourth, mid Permian episode in addition to later Permian, Early-Middle Triassic and Late Triassic episodes previously identified. These four episodes exhibited a recurrent pattern of activity (Brownlow 1999) comprising: deformation and uplift, deflation, thermal activity including late granitoids, then cooling and deflation in places with basin development. Successive episodes commonly overlap – a key factor in deciphering the geological development in the region. The extent of temporal overlap varies, but is especially marked between the mid Permian and later Permian episodes. These episodes also exhibited spatial recurrence in geological activity, e.g., concentration of thermal activity in elongate, grossly asymmetric zones orthogonal to regional shortening. Moreover, inboard granitoids commonly differ from outboard granitoids by being larger, younger?, typically leucocratic and commonly Sn-bearing (cf Weber and Scheibner, 1977, Brownlow, 1999).
Discussion Subduction has long been favoured as the fundamental process driving the Permo-Triassic development of the sNEO (e.g., Veevers et al., 2000, Jenkins et al., 2002). The overall shape, scale and asymmetry of thermal zones and their distribution broadly parallel to structural trends is grossly arc-like (cf Tatsumi & Eggins, 1995). The geochemistry of granitoids, andesites and basalts (e.g., Chappell, 1994, Jenkins et al., 2002) and metallogenesis (Weber and Scheibner, 1977) of the sNEO are also arc-like. However, no model yet matches the temporal and spatial recurrence of geological activity described herein. Critically, that recurrent activity does not disprove subduction, but does constraint its potential mode of operation, viz:
Low-angle subduction of the north Japan type (Tatsumi and Eggins, 1995) may explain the parallel belts of igneous rocks and mineralisation produced by these four episodes. Associated accretionary complexes would be offshore, so their absence onshore does not rule out subduction. The four geological episodes suggest four separate subduction episodes, with the onset of each (or immediate precursor activity) causing regional deformation and related uplift. The delay before the onset of volcanism (“deflation” event herein) suggests an evolving subduction process requiring time for a slab to penetrate to sufficient depth to initiate volcanism. Deflation is consistent with cessation of prior dynamic uplift upon slab breakage and/ or downdragging of an overlying plate by a subducting plate after subduction initiation. Contrasting shortening directions exhibited by the first three episodes suggests that any subduction associated with each was complete before the next began, because otherwise subducting slabs would interfere with each other. Slab breakoff may also be necessary to avoid interference. Late (especially inboard) igneous activity need not be due solely to subduction, but may involve additional processes such as post-subduction asthenospheric instability or mobilisation by later deformation. Cross-arc variation in metallogenesis (particularly Sn in inboard leucogranites) may reflect preferential, early (shallower) breakdown of slab-borne hydrous silicates and late (deeper) breakdown of F-enriched silicates, with F acting as a carrier or scavenger of Sn (Weber and Scheibner, 1977). Differences in the widths of thermal zones, the typical size of granitoids and the intensity of mineralisation between episodes suggest difference in subduction rates and dips, differences in slab maturities, buoyancies and water contents and/ or differing degrees of crustal thickening over time.
Conclusion Further analysis incorporating new isotopic dates suggests that four overlapping episodes of recurrent activity (deformation, deflation, thermal activity, cooling) controlled the mid Permian to mid Triassic development of the sNEO and fringing basins (cf Brownlow, 1999). Subduction is a plausible driving mechanism for each episode, which collectively constrain its potential mode of operation. Differences 66
NEO 2010 between episodes, such as the widths of successive thermal zones and the abundance of thermal products and metal (e.g., Sn) endowments, suggest differences in factors such as subduction rate and dip. Post-subduction mantle processes and/or later crustal reactivation may also have been important in determining the composition and metal endowment of late magmatism.
Acknowledgements Thanks to Cameron Ricketts for comments on the manuscript, and Andrew Cross (Geoscience Australia) for extensive discussion about geochronology. Published with the permission of the Director, Geological Survey of New South Wales.
References Briggs, D.J.C., 1998. Permian Productidina and Strophalosiidina from the Sydney – Bowen Basin and New England Orogen: systematics and biostratigraphic significance. Association of Australasian Palaeontologists, Memoir, 19, 1-258. Brownlow, J.W., 1999. Recurrent patterns in the Permian-Triassic development of the NEO and its environs. In: Flood P.G. (ed.), New England Orogen: Regional Geology, Tectonics and Metallogenesis, NEO ’99 Conference. Earth Sciences, The University of New England, Armidale, 127-136. Bryant, C.J., Cosca, M.A. and Arculus, R.J., 1997. 40Ar/39Ar ages of Clarence River Supersuite intrusions from the northern portion of the New England Batholith, southern New England Orogen. In: Ashley, P.M. & Flood, P.G. (eds.) Tectonics and metallogenesis of the New England Orogen. Geological Society of Australia, Special Publication, 19, 242-253. Bryant, C.J., Chappell, B.W. and Blevin, P.L., 2003. Granites of the southern New England Orogen. In: Blevin, P., Jones, M. & Chappell, B.W. (eds.) The Ishihara Symposium: Granites and Associated Metallogenesis, GEMOC, Macquarie University, July 22-24 2003. Geoscience Australia Record, 2003/14. Campbell, L.M., Holcombe, R.J. and Fielding, C.R., 1999. The Esk Basin – a Triassic foreland basin within the northern New England Orogen. In: Flood, P.G. (ed.), New England Orogen: Regional Geology, Tectonics and Metallogenesis, NEO ’99 Conference. Earth Sciences, The University of New England, Armidale, 275-284. Chappell, B.W.1994. Lachlan and New England: fold belts of contrasting magmatic and tectonic development. Journal and Proceedings of the Royal Society of New South Wales, 127, 47–59. Collins, W.J. 1991. A reassessment of the ‘Hunter-Bowen Orogeny’: tectonic implications for the southern New England Fold Belt. Australian Journal of Earth Science, 38, 409–423. Day, R.W., Whitaker, W.G., Murray, C.G., Wilson, I.H., Grimes, K.G., 1983. Queensland Geology. A companion volume to the 1:2 500 000 scale geological map (1975). Geological Survey of Queensland. Publication 383. Gilligan, L.B., Brownlow, J.W., Cameron, R.G. and Henley, H.F., 1992. Dorrigo-Coffs Harbour 1:250,000 metallogenic map, SH/56-10, 11: Metallogenic Study and Mineral Deposit Data Sheets. Geological Survey of New South Wales, Sydney, 509 pp. Herbert, C., 1980. Depositional development of the Sydney Basin. In: Herbert C. & Helby R.(eds), A Guide to the Sydney Basin. Geological Survey of New South Wales, Bulletin, 26, 11-52. Jenkins, R.B., Landenberger, B., Collins, W.J., 2002. Late Palaeozoic retreating and advancing subduction boundary in the New England fold belt, New South Wales. Australian Journal of Earth Sciences, 49 (3), 467-489. Mundil, R., Pálfy, J., Renne, P.R. and Brack, P., 2010.The Triassic timescale: new constraints and a review of geochronological data. In: Lucas, S.G. (ed.) The Triassic Timescale. Geological Society, London, Special Publications, 334, 41–60. Offler, R. and Foster, D.A, 2008. Timing and development of oroclines in the southern New England Orogen, New South Wales. Australian Journal of Earth Sciences, 55, 331-340. Ogg, J.G., Ogg, G. and Gradstein, F.M., 2008. The Concise Geologic Time scale. Cambridge University Press, 150 pp. 67
NEO 2010 Schaltegger, U., Pettke, T., Audetat, A., Reusser, E., Heinrich, C.A., 2005. Magmatic to hydrothermal crystallization in the W-Sn mineralized Mole Granite (NSW, Australia); Part I, Crystallization of zircon and REE-phosphates over three million years; a geochemical U/Pb geochronological study. Chemical Geology, 220 (3-4), 215-235. Shaw, S.E., 1994. Eastern Australia. Appendix 3. Permian-Triassic radiometric dates of granitoids and associated volcanics form the southern New England Fold Belt. In: Veevers, J.J., Powell, C.McA. (eds.), Permian– Triassic Pangean Basins and Foldbelts Along the Panthalassan Margin of Gondwanaland. Geological Society of America Memoir, 184, 147–159. Tadros, N.Z., 1993. Tectonic interpretations. In: Tadros N.Z.(ed.), The Gunnedah Basin, New South Wales. Geological Survey of New South Wales, Memoir Geology, 12, 47-54. Tatsumi, Y. and Eggins, S., 1995. Subduction Zone Magmatism. Blackwell, Oxford, 211 pp. Veevers, J.J. (ed.), 2000. Billion-Year Earth History of Australia and Neighbours in Gondwanaland. GEMOC Press, Sydney.400 pp. Weber, C.R. and Scheibner, E., 1977. The origin of some Permo-Triassic metal zones in the New England region, New South Wales. Geological Survey of New South Wales, Quarterly Notes, 26, 1-14.
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TIMS U-Pb and SHRIMP U-Pb zircon dating of the Dundee Rhyodacite, northern New England, NSW Jeff Brownlow1 and Andrew Cross2 1. Industry & Investment NSW, Armidale, NSW, Australia 2. Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
Keywords: Dundee Rhyodacite, ID-TIMS, SHRIMP, Wandsworth Volcanic Group
Introduction The Dundee Rhyodacite (Barnes et al., 1991) is a distinctive and widespread ignimbrite that forms the uppermost preserved subdivision of the Wandsworth Volcanic Group (WVG) (Barnes et al., 1991) in the northern New England Tablelands of New South Wales and southernmost Queensland. The WVG represents the early products of widespread Permo-Triassic igneous activity that affected much of the southern New England Orogen (sNEO) producing dominantly terrestrial felsic volcanism, voluminous granitoids as well as abundant Sn-W-Mo-Au-Sb and other base metal mineralisation (Henley et al., 2001; Bryant et al., 2003). The Dundee Rhyodacite is broadly Permo-Triassic in age. The unit is unfossiliferous and its biostratigraphic context only establishes that it is post-Kungurian (latest Early Permian) (Briggs, 1998). Current isotopic age constraints for the Dundee Rhyodacite differ. Available K-Ar (Evernden and Richards, 1962; Shaw, 1994) and LA-ICP-MS U-Pb (Belousova et al., 2006) dates strongly support a Permian age, but conflict with the more widely quoted Rb-Sr dates (Shaw, 1994) which indicate an Early-Middle Triassic age (Veevers, 2000). Therefore, the aim of this study was to more firmly establish the crystallisation age of the Dundee Rhyodacite and in doing so, establish a minimum age for the WVG, a maximum age for the granitoids that intrude it, and help constrain evolutionary and tectonic models for the sNEO. Furthermore, a robust age determination for the Dundee Rhyodacite may also test suggestions that WVG volcanism was a source of distal air fall tuffs in the Singleton Supergroup and correlatives in the Sydney-Gunnedah Basin (Brownlow, 1979; Shaw et al., 1991; and Briggs, 1998).
Geological context The sNEO occurs north-east of the Hunter-Mooki Fault System in north-eastern NSW and extends into southern Queensland. It was a complex, active zone of deformation, volcanism, plutonism and mineralisation linked to renewed magmatism during the later Permian and earlier Triassic. In contrast, the adjoining Sydney-Gunnedah Basin to the west was a contemporary zone of quasi-continuous, terrestrial and shallow marine sedimentation, relatively undisturbed by tectonism (Veevers, 2000). The WVG is the youngest, preserved, stratified rock sequence in the sNEO, and comprises extensive, mainly flat-lying to gently-dipping felsic volcanic rocks. The duration of WVG volcanism is not well established but some infer that it exceeds 15 My (e.g., Briggs, 1998, Henley et al, 2001). The Middle-Late Permian Singleton Supergroup and correlatives are a coal measure sequence that extends throughout the Sydney-Gunnedah Basin and the Bowen Basin in Queensland (Veevers, 2000). Abundant interbedded felsic tuffs and tuffaceous sediments characterise the Singleton Supergroup and correlatives and help distinguish them from the overlying, terrestrial, red-bed-bearing, sparsely tuffaceous Narrabeen Group and correlatives (e.g., Shaw et al., 1991). A recent CA-ID-TIMS U-Pb zircon age of 252.2 ± 0.4 Ma from a tuff at the top of the Singleton Supergroup equivalent in the Bowen Basin (Mundil et al., 2006), is indistinguishable from the most recent estimate of 252.3 Ma for the Permo-Triassic Boundary (PTB) (Mundil et al., 2010). This date indicates that deposition of tuffs and the host coal measures extended to at least until the end of the Permian, consistent with traditional, biostratigraphic and carbon-isotope-based 69
NEO 2010 assessments of the PTB in Eastern Australia (e.g., Veevers, 2000). The duration of volcanism recorded by the tuffs of the Singleton Supergroup may exceed ~13.5 My based on an ID-TIMS U-Pb zircon age of 266 ± 0.4 Ma for a tuff in the Thornton Claystone (lower Singleton Supergroup) (Gulson et al., 1990). Alternatively, the duration may be little more than ~7 My based on more recent SHRIMP U-Pb zircon ages for tuffs slightly above the stratigraphic level of the Thornton Claystone elsewhere in the Sydney–Bowen basin (e.g., Michaelsen et al., 2001).
Dundee Rhyodacite The Dundee Rhyodacite (Flood et al., 1980; Barnes et al., 1991) is typically a distinctive, remarkably uniform, blue-grey, medium-grained, densely welded, crystal-rich, pyroxene-bearing, hornblende-biotite dacite ignimbrite. Geochemically, typical Dundee Rhyodacite is a normative hornblende-biotite granodiorite with < 1% relative regional variation in major components (Flood et al., 1980). Its petrogenetic affinity is with the (“low-temperature”, I-type) Moonbi Plutonic Suite (Flood et al., 1980) or with mesocratic granitoids of the Moonbi Supersuite (Bryant et al., 2003). The Dundee Rhyodacite crops out extensively between Glen Innes in northern New England and Wallangarra in southern Queensland in a 100 x 30 km, NNE-trending zone that overlaps the northern parts of the New England Batholith (Barnes et al., 1991). The greatest preserved thickness of the Dundee Rhyodacite exceeds 2 km, and its composition and petrography is remarkably uniform. Only the Coombadjha outlier displays significantly greater complexity, manifest as a thin (~50 m), petrologically-related precursor sequence of ignimbrite, porphyritic dacite lava, bedded tuff and breccia plus a later ring dyke (the Moonta Gully Monzogranite) respectively underlying and intruding typical Dundee Rhyodacite (Barnes et al., 1991). McPhie (1988) attributed emplacement of typical Dundee Rhyodacite (her “normal facies”) to a large magnitude but low violence eruption, centred on or near the Coombadjha Volcanic Complex, that produced a low eruption column and relatively poorly fluidised, slow moving pyroclastic flows. These flows must have been voluminous to blanket the current outcrop area of the Dundee Rhyodacite to sufficient thickness to leave densely welded outliers locally 2 km or more thick. Stratigraphically, the Dundee Rhyodacite occurs at the preserved top of the Wandsworth Volcanic Group where it typically overlies the Emmaville Volcanics. Its base is generally conformable, although minor angular discordance is recognised locally on underlying units of the WVG, and it locally overlaps onto Early Permian metasedimentary rocks (Barnes et al., 1991). Several Permo-Triassic igneous plutons and porphyritic ring dykes intrude the Dundee Rhyodacite, including the Bungulla and Sandy Flat Monzogranites plus the Billyrimba, Bolivia Range and Pyes Creek Leucomonzogranites (Barnes et al., 1991).
Previous dating Previous attempts to radiometrically constrain the age of the Dundee Rhyodacite include K-Ar, Rb-Sr, SHRIMP U-Pb zircon and LA-ICP-MS U-Pb zircon dating. Evernden and Richards (1962) reported a K-Ar age of ~242 Ma which indicated a late Permian age by contemporary timescales, but would now be interpreted as Middle Triassic (cf Mundil et al., 2010). That date has since been revised (revised decay constant and corrected spike) to 253 ± 2.5 Ma (F. Della-Pasqua pers. comm. in Shaw, 1994), which is within error of the recommended contemporary PTB of 252.3 Ma (Mundil et al., 2010). Multiple Rb-Sr dates averaging 247.25 ± 1.5 Ma and ranging from ~248.7 to ~245.8 Ma (Shaw, 1994) have been widely quoted and underpin the Early Triassic age interpretation of Veevers (2000). Black (2007) interpreted an Early Triassic age from a SHRIMP U-Pb zircon study, however, that result was adversely affected by poor analytical conditions with five separate 206Pb/238U calibrations across two different instrumental sessions yielding mutually inconsistent magmatic crystallisation ages ranging from 250.4 ± 1.9 to 254.6 ± 2.4 Ma. More recently, a LA-ICP-MS U-Pb zircon date of 257.6 ± 2.5 Ma for the Dundee Rhyodacite by Belousova et al. (2006) indicates a Late Permian age.
Dating results The new TIMS date of 254.34 ± 0.34 Ma for the Dundee Rhyodacite is approximately ~2 My older than the recommended age of the PTB at Meishan in SE China (252.3 Ma; Mundil et al., 2010). Thus, this new date is unequivocally Permian. It is also approximately 2 My younger than the interpolated WuchiapingianChanghsingian boundary age of 256 Ma (Mundil et al., 2004), indicating a Late Permian, probably mid Changhsingian age for the Dundee Rhyodacite. The new SHRIMP U-Pb date of 254.1 ± 2.2 Ma, the LA70
NEO 2010 ICP-MS U-Pb zircon date (257.6 ± 2.5 Ma; Belousova et al., 2006) and the recalculated K-Ar date from the Wyberba outlier (253 ± 2.5 Ma; Shaw, 1994) are all broadly consistent with the new TIMS date. In contrast, the new TIMS date is distinctly older than the earlier, and widely quoted, Early-Middle Triassic Rb-Sr dates of Shaw (1994) (248.7 - 245.8 Ma, averaging 247.25 ± 1.5 Ma). New U-Pb zircon data for the DRD have been acquired via both SHRIMP and ID-TIMS. The zircons targeted for SHRIMP analysis were taken from mineral separates prepared as part of the study by Black (2007), and were analysed using Geoscience Australia’s SHRIMP IIe and procedures described by Cross et al. (2009). The zircons selected for ID-TIMS analysis were plucked directly from the grain mount (GA6014) originally analysed by Black (2007). ID-TIMS analyses were conducted by Richard Friedman (University of British Columbia) using procedures outlined by Viruete et al. (2008). These zircons were not subjected to chemical abrasion (as defined by Mattinson, 2005) prior to analysis. The SHRIMP U-Pb results comprise 19 analyses, all with the same radiogenic 206Pb/238U within their analytical errors (MSWD = 1.11) and combine to give an age of 254.1 ± 2.2 Ma. Six ID-TIMS analyses were obtained three of which have identical radiogenic 206Pb/238U and combine to give 254.34 ± 0.34 Ma. This is our best estimate for the igneous crystallisation age of this rock. Two other fractions have concordant but slightly younger 206Pb/238U (~253 and ~249 Ma) while another fraction is slightly discordant with an age of ~261 Ma (Fig. 1). It is likely that the younger fractions have undergone minor post-crystallisation loss of radiogenic Pb and that the older fraction represents inheritance from a marginally older Permian source.
Fig. 1. Concordia diagram showing ID-TIMS dating of six zircon fractions plucked directly from grain mount GA6014 (Black 2007).
Discussion The new TIMS date of 254.34 + 0.34 Ma for the Dundee Rhyodacite is approximately ~2 My older than the recommended age of the PTB at Meishan in SE China (252.3 Ma; Mundil et al., 2010). Thus, this new date is unequivocally Permian. It is also approximately 2 My younger than the interpolated WuchiapingianChanghsingian boundary age of 256 Ma (Mundil et al., 2004), indicating a Late Permian, probably mid Changhsingian age for the Dundee Rhyodacite. The new SHRIMP U-Pb date of 254.1 + 2.2 Ma, the LAICP-MS U-Pb zircon date (257.6 ± 2.5 Ma; Belousova et al., 2006) and the recalculated K-Ar date from the Wyberba outlier (253 ± 2.5 Ma; Shaw, 1994) are all broadly consistent with the new TIMS date. In contrast, the new TIMS date is distinctly older than the earlier, and widely quoted, Early-Middle Triassic Rb-Sr dates of Shaw (1994) (248.7 - 245.8 Ma, averaging 247.25 + 1.5 Ma).
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NEO 2010 As the Dundee Rhyodacite is the uppermost unit of the WVG, the preserved WVG must be entirely Permian, and cannot be partly or wholly Triassic. Moreover, as crystallisation of the Dundee Rhyodacite predates the PTB by ~2 My, granitoids that intrude it could be as old as latest Permian. The mesocratic, Moonbi-type Bungulla Monzogranite is the most likely candidate for emplacement in this 2 My interval, given its petrogenetic affinity with the Dundee Rhyodacite and its emplacement before some or all of the other intruding, mainly leucocratic granitoids. A latest Permian age for those leucocratic granitoids is also possible. The new Dundee TIMS date also falls within the range of dates now available for tuffs in the Singleton Supergroup and correlatives in the Sydney-Gunnedah and Bowen Basins (~252.2 to ~259 Ma, possibly extending to ~266 Ma). Thus the new age supports the inferred correlation of the WVG with tuffs of the Singleton Supergroup (Brownlow, 1979; Shaw et al., 1991; Briggs, 1988), and is consistent with the more specific correlation of the Dundee Rhyodacite with tuffs of the upper (Newcastle Coal Measures) subdivision of the Singleton Supergroup (Briggs, 1998). However, proof of cogenesis requires the additional demonstration of unique compositional linkages. Such links are important to establish because the contrasting geology of these two neighbouring provinces (sNEO and the Sydney-Gunnedah Basin) arguably preserves complementary records of a shared geological history, and understanding one provides insight into the other.
Fig. 2. Radiometric dates from the Dundee Rhyodacite and recent, published U-Pb dates from tuffs of the Singleton Supergroup interval in the Sydney-Bowen Basin. Dundee Rhyodacite dates: 1a (Evernden and Richards 1962); 1b (Evernden and Richards 1962, modified by F Della-Pasqua pers. comm. in Shaw (1994), 2a-f (Shaw 1994); 3. (Belousova et al., 2006); 4a-b (herein). Basinal tuff dates: 5 - Platypus Tuff (Michaelsen et al., 2001); 6 - Thornton Claystone (Gulson et al., 1990); 7 – tuff at top of Singleton Supergroup equivalent Bowen Basin (Mundil et al., 2006). Colour code: K-Ar (purple), RbSr (blue); LA-ICP-MS (green); SHRIMP (orange); and ID-TIMS (red), showing error ranges. PTB = Permo-Triassic boundary.
Conclusions The new Dundee TIMS date and supporting SHRIMP date have: 1. Confirmed the earlier, corrected K-Ar dating (Evernden and Richards, 1962; F. Della-Pasqua pers. comm. in Shaw, 1994) and has demonstrated unequivocally that the Dundee Rhyodacite is Late Permian in age and not Early Triassic. Therefore, all underlying units of the WVG must be Permian.
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NEO 2010 2. Established sufficient time gap between the Dundee Rhyodacite and the Permo-Triassic boundary to accommodate intrusion by granitoids such as the Bungulla Monzogranite (mesocratic, Moonbi-type granitoid). 3. Confirmed emplacement of the Dundee Rhyodacite during the later part of the interval of tuffaceous volcanism recorded in the Singleton Supergroup of the Sydney Basin (Brownlow, 1979; Barnes et al., 1991; Shaw et al., 1991; Briggs, 1998). Co-genesis is indicated but proof requires demonstration of a unique provenance linkage.
Acknowledgements Thanks to Dave Champion and Simon Bodorkos for comments on the paper and to Cameron Ricketts for comments on earlier versions of the manuscript. Published with the permission of the Director, Geological Survey of New South Wales and Executive Director, Geoscience Australia.
References Barnes, R.G., Brown, R.E., Brownlow, J.W. and Stroud, W.J., 1991. Late Permian volcanics in New England - the Wandsworth Volcanic Group. Geological Survey of New South Wales, Quarterly Notes, 84, 1-36. Belousova, E.A., Griffin, W.L. and O'Reilly, S.Y., 2006. Zircon crystal morphology, trace-element signatures and Hf-isotope composition as a tool for petrogenetic modelling: examples from eastern Australian granitoids. Journal of Petrology, 47, 329-353. Black, L.P., 2007. SHRIMP U-Pb zircon ages obtained during 2006/07 for NSW Geological Survey projects. Geological Survey of New South Wales Report GS2007/298. Briggs, D.J.C., 1998. Permian Productidina and Strophalosiidina from the Sydney – Bowen Basin and New England Orogen: systematics and biostratigraphic significance. Association of Australasian Palaeontologists, Memoir, 19, 1-258. Brownlow, J.W., 1979. Discussion: The Reids Mistake Formation at Swansea Heads, New South Wales. Geological Society of Australia, Journal, 26, 319-322. Bryant, C.J., Chappell, B.W. and Blevin, P.L., 2003. Granites of the southern New England Orogen. In: Phil Blevin, Mel Jones & Bruce Chappell (eds.) The Ishihara Symposium: Granites and Associated Metallogenesis, GEMOC, Macquarie University, July 22-24 2003. Geoscience Australia Record, 2003/14. Cross, A.J., Purdy, D.J., Bultitude, R.J., Dhnaram, C.R. and von Gnielinski, F.E. 2009. Joint GSQ-GA NGA geochronology project, New England Orogen and Drummond Basin, 2008. Queensland Geological Record 2009/03. Evernden, J.F. and Richards, J.R., 1962. Potassium-argon ages in eastern Australia. Geological Society of Australia, Journal, 9, 1-49. Flood, R.H., Shaw, S.E. and Chappell, B.W., 1980. Mineralogical and chemical matching of plutonic and associated volcanic units, New England Batholith, Australia. Chemical Geology, 29, 163-170. Gulson, B.J., Diessel, C.F.K., Mason, D.R. and Krogh, T.E., 1990. High precision radiometric ages from the northern Sydney Basin and their implication for the Permian time interval and sedimentation rates. Australian Journal of Earth Sciences, 37, 459-469. Henley, H.F., Brown, R.E., Brownlow, J.W., Barnes, R.G. and Stroud, W.J., 2001. Grafton Maclean 1:250 000 Metallogenic Map SH/56-6 & SH/56-7: Metallogenic Study and Mineral Deposit Data Sheets. Geological Survey of New South Wales, Sydney, xii + 292pp with CD-ROM. Mattinson, J.M., 2005. Zircon U-Pb chemical abrasion (‘CA-TIMS’) method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220, 47-66. McPhie, J., 1988. Source(s) of the Dundee Rhyodacite ignimbrite: implications for the existence of Late Permian cauldrons in the southern New England Orogen. In: Kleeman J.D. (ed.), New England Orogen —
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NEO 2010 Tectonics and Metallogenesis. Symposium, Department of Geology and Geophysics, The University of New England, Armidale, 145-149. Michaelsen, P., Henderson, R.A., Crosdale, P.J., Fanning, C.M., 2001. Age and significance of the Platypus Tuff Bed, a regional reference horizon in the Upper Permian Moranbah Coal Measures, North Bowen Basin. Australian Journal of Earth Sciences, 48, 183-192. Mundil, R., Ludwig, K.R., Metcalfe, I. and Renne, P.R., 2004. Age and timing of the Permian mass extinctions: U/Pb geochronology on closed-system zircons. Science, 305, 1760-1763. Mundil, R., Metcalfe, I., Chang, S. and Renne, P.R., 2006. The Permian – Triassic boundary in Australia: new radio-isotopic ages. In: 16th Goldschmidt Conference Melbourne. Geochimica et Cosmochimica Acta, 70 (18), Supplement 1 A119. Mundil, R., Pálfy, J., Renne, P.R. and Brack, P., 2010. The Triassic timescale: new constraints and a review of geochronological data. In: Lucas, S. G. (ed.) The Triassic Timescale. Geological Society, London, Special Publications, 334, 41–60. Shaw, S.E., 1994. Eastern Australia. Appendix 3. Permian-Triassic radiometric dates of granitoids and associated volcanics form the southern New England Fold Belt. In: Veevers, J.J., Powell, C.McA. (eds.), Permian– Triassic Pangean Basins and Foldbelts Along the Panthalassan Margin of Gondwanaland. Geological Society of America Memoir, 184, 147–159. Shaw, S.E., Conaghan, P.J. and Flood, R.H., 1991. Late Permian and Triassic igneous activity in the New England Batholith and contemporaneous tephra in the Sydney and Gunnedah Basins. Advances in the Study of the Sydney Basin, 25th Symposium, Department of Geology, University of Newcastle, 44-51. Veevers, J.J. (ed.), 2000. Billion-Year Earth History of Australia and Neighbours in Gondwanaland. GEMOC Press, Sydney.400 pp. Viruete, J.E., Joubert, M., Urien, P., Friedman, R., Weis, D., Ullrich A. and Pérez-Estaún, A., 2008. Caribbean island-arc rifting and back-arc basin development in the Late Cretaceous: Geochemical, isotopic and geochronological evidence from Central Hispaniola. Lithos, 104, 378-404.
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NEO 2010
New England Batholith: unravelling the compositional diversity Colleen J Bryant1 and Bruce W Chappell1 1
School of Earth and Environmental Sciences, University of Wollongong, Australia
Keywords: granite, New England Batholith, supersuite, granite
Introduction The New England Batholith (NEB), covers some 16,000 km2, and extends from Barrington Tops into Queensland, and dominates the central parts of the southern New England Orogen. The understanding of this complex has evolved over time through the efforts of numerous workers. While some studies have considered the batholith as a whole (e.g. Shaw and Flood 1981; Hensel et al., 1985), others have been restricted to particular aspects (e.g. Bryant et al., 1997; Chappell 1978; Landenberger 1995). In the process, there have been numerous attempts to describe, understand and classify granites of the NEB. The most seminal classification, as marked by its longevity, is that of Shaw and Flood (1981). This classification system remains the basis of that used today, and we refer the reader to Shaw et al., (this volume) for an overall account of plutonism within the NEB. The extensive database assembled by Chappell (this volume) has afforded a unique opportunity to reexamine the compositional variations within the NEB. While the dominant structure of the granite classification system for the NEB remains unchanged, our work demonstrates the existence of finer-scale chemical variations that with the aid of more intensive zircon dating initiatives have the potential to contribute significantly to our understanding not only of the granite petrogenesis but the structure and formation of the whole southern NEO. Granites of the NEB that share generally similar compositional characteristics, e.g. I- or S-type, similarity of incompatible element abundances, etc, are placed in specific supersuites, generally following the subdivisions of Shaw and Flood (1981). These supersuites are not as well defined as those of the Lachlan Fold Belt (LFB), and unlike the LFB, recognition of separate suites within those larger units is uncommon for the NEB. Volcanic rocks are an important part of the whole NEB complex, but they are commonly extremely altered and not suitable for chemical analysis, although the Dundee Rhydacite near the northern end of the NEB is a conspicuous exception to that. Also, present space requirements preclude discussion of the volcanic rocks, as it does any significant consideration of the petrogenesis of the granites of the NEB.
Bundarra Supersuite The Bundara Supersuite incorporates the early Permian S-type granites that occur along the western margin of the NEB. The granites are mostly, but not invariably, coarse-grained and are frequently porphyritic. They have low Fe3+/Fe2+, consistent with the white K-feldspar, and ilmenite as the opaque mineral phase. The Alsaturated character is evident in the widespread presence of muscovite and cordierite, and less commonly garnet, and large apatite crystals (P soluble in the peraluminous melt). The granites are highly felsic (typically > 71.5 wt % SiO2), not significantly fractionated, chemically coherent, and characterised by high abundances of Al and P, and low concentrations of Nb and Sr. There are comparatively low abundances of Th, U, and LREE/HREE compared with many NEB I-type granites, and the comparatively primitive Sr and Nd isotopic compositions are consistent with these rocks having been derived from young sedimentary sources that were derived from a relatively depleted arc (Jeon et al., this volume). Eleven distinct units are now recognised in the Bundarra Supersuite, although detailed subdivisions have only been made in the south, where there are better exposures. Six units are recognised within the approximately 50 km wide near circular mass at the southern end of the batholith. Chappell (1978) mapped 75
NEO 2010 the Banalasta, Glenclair, Pringles and Tilmunda intrusions, which he defined as belonging to the Banalasta Suite. This suite has subsequently been restricted to only include Banalasta and Pringles. The Banalasta Monzogranite, well exposed along the road from Bendemeer to Glenclair homestead, is the most mafic unit in the supersuite, and also the most southerly. The Banalasta Suite is characterised by comparatively high Al, Sr, Pb, Ba and Ga, and low Th and Zr relative to other members of the supersuite. The Glenclair, Watsons Creek and Tilmunda Monzogranites are all characterised by a trend of decreasing P with increasing SiO2, and have a greater abundance of elements that tend to be more concentrated in ferromagnesian phases including higher Fe, Mg, Ti, Sc, Cr, V, Ni, and Cu. Despite subtle inter-pluton variations these three units are collectively referred to as the Glenclair Suite. Roumalla, a newly defined unit located to the north of the area mapped by Chappell (1978), is characterised by lower Sr and P than Banalasta. Watsons Creek, also a newly defined unit, occurs further to the west, with the northern boundary likely occurring southwest of Kingstown. Five units are recognised in the elongate northern portion of the supersuite. Namoi Tops, which remains unsampled, occurs immediately to the north of the Watsons Creek unit and extends to a north-northeast running boundary near Bakers Creek, across which there are marked changes in the airborne radiometric signature data (Kovarch, 1993). The Copeton Monzogranite occurs to the north of this lineament and extends north to the basalt cover west of Inverell. This unit is undoubtedly composite but has not been subdivided. The Sepoy Monzogranite, which is relatively fine-grained, was used in the construction of Copeton Dam. The Graman Monzogranite extends from the basalt cover to the west of Inverell to a Carboniferous screen along Mandoe Creek. The Linton Monzogranite extends north from that screen to the northern extent of the supersuite close to the Queensland border, near Texas. Variations are evident in major and trace element compositions from south to north, with units from the northern portion of the supersuite being characterised by lower Sr and somewhat lower Al, Pb, Cu, Co and V and, higher Rb, and Sn than the southern units. These variations imply subtle corresponding variations in the composition of the granite sources. Variations in the oxygen isotopic compositions of inherited zircon cores suggest that to some extent these geochemical variations pre-date the sedimentary cycle, potentially reflecting spatial variations in the earlier Carboniferous arc itself (Jeon et al., this volume).
Hillgrove Supersuite The Hillgrove Supersuite comprising approximately 20 units, is a moderately to strongly deformed early Permian S-type supersuite in the central portion of the NEB (contrast with the Bundarra Supersuite, which is never deformed). These granites are compositionally more diverse than those of the Bundarra Supersuite, comprising granodiorites through to monzogranites, ranging from 62 to 76% SiO2. Petrographically, they are characterised by russet-red biotite that often contains exsolved needles of rutile, and variably, minor almandine-rich garnet, actinolitic amphibole and cummingtonite. Although characterised by low Ca, Na and Sr, consistent with an S-type affinity, these granites are less peraluminous, and contain lower Al and P than those of the Bundara Supersuite. Higher Th, U, P, Ce and Zr relative to the Bundarra Supersuite suggest derivation from components more enriched in these elements. This may reflect more enriched arc source rock components prior to weathering and/or the inclusion of more evolved sedimentary components. The Hillgrove Supersuite is characterised by greater interpluton chemical variability than the Bundarra Supersuite, with only one suite being recognised. The Rockvale Suite includes the Rockvale and Tobermory units. It is possible that Kimberley Park and Argyll define another suite although more analyses would be required to confirm this. Despite the lack of suites, there are definable geochemical associations within the supersuite, based on Sr, Nb, Ti, Zr, and oxidation states. In some cases, plutons with similar chemical compositions have a coherent geographical distribution. For example, plutons with relatively high Sr are primarily located along the eastern margin of the supersuite. Intrusions with high Sr tend to have high Mn and low to moderate K and Rb, Zr and Th, and low As. Plutons with low Na tend to be located in the geographical centre of the Hillgrove Supersuite, whereas those with high Na are located at the southernmost and northernmost extensions. In contrast, the variations in oxidation state do not have a consistent geographic distribution.
Moonbi Supersuite The Moonbi Supersuite (MSS; 255-244 Ma; Shaw et al., 1991), as currently defined is the most extensive and complex supersuite in the NEB, with the northern and southern extensions of the supersuite, being geographically separated by the Uralla and Hillgrove Supersuites in the centre of the NEB. While these granites from these two regions share sufficient characteristics to warrant them being grouped, including 76
NEO 2010 high abundances of K, Rb, Sr, Ba, Pb, Th, and U, high oxidation states (euhedral titanite and magnetite), and an abundance of large pink K-feldspar phenocrysts, distinct compositional and petrographic and differences are evident between these two parts of the NEB. Granites from the southern portion of the MSS are typically more mafic than their northern counterparts, and tend to be characterised by higher concentrations of Sr, Ba, Pb, Th, and U. Granites from the northern portion of the NEB are compositionally and texturally more variable. Based on work by Blevin and Chappell (1996), those northern granites have been subdivided into three groupings, Bungulla-type granites (BTG), Stanthorpe-type granites (STG) and Ruby Creek-type granites (RCTG). The BTG represent the most mafic group, compositionally being most similar to granites from the southern portion of the NEB. Areally the most significant group in the supersuite, the STG, which outcrop in the Tenterfield-Stanthorpe region, comprise coarse- to medium-grained equigranular to mildly porphyritic leucogranites. The Stanthorpe Granite is the largest intrusion within the STG The RCTG comprises the numerous small, intrusions and dykes that occur within and associated with STG granites. These rocks are highly felsic, and highly texturally variable, including medium to fine-grained equigranular pale pink to white leucocratic granites as well a variety of texturally diverse porphyritic, aplitic, microgranitic and pegmatitic phases, and commonly contain miarolitic variants. The RCTG are strongly enriched in Ga, Rb, Y, Nb, and Sn, as a consequence of extensive fractional crystallisation, and are associated with Sn, Mo and base metal mineralisation in the Sundown-Kilminster-Stanthope-Tenterfield region. Compositional differences between the BTG, STG and RCTG are consistent with them being part of a fractionating magma series, with compositional variations in the more mafic members being controlled dominantly by feldspar fractionation. Collectively, the MSS is characterised by considerable compositional diversity ranging from 55 to 77% SiO2. Only two suites are recognised, the Moonbi Suite (Moonbi and Bendemeer; Chappell, 1978) and the Bungulla Suite (Bungulla and Undercliffe Falls).
Uralla Supersuite The Uralla Supersuite (USS; 255-244 Ma; Shaw et al., 1991) is an rather heterogenous collection of granites that occur within the central portion of the NEB. They are on average more mafic than the Moonbi Supersuite and contain both a higher proportion of mafic rocks and fewer highly fractionated granites. Granites of the USS are typically equigranular and speckled black and white in colour, although porphyritic variants do occur. Ferromagnesian phases variably include actinolite, hornblende, red-brown biotite, clinopyroxene, and abundant orthopyroxene (mafic rocks only). Broadly, the USS granites are intermediate between those of the Moonbi and Clarence River Supersuites in terms of their K, Ca, Pb, Th, U, Ba, P, and LREE abundances, although overlaps do exist. They possess characteristics common amongst reduced I-type granites including not only lower redox states, but also lower normative diopside, and higher initial 87Sr/86Sr ratios (~0.7046-0.707; Shaw and Flood, 1981) relative to other New England I-type granites. Until recently the Uralla Supersuite has been the most poorly understood of all the granite supersuites in New England. Interrogation of the geochemical data finds that order exists within this seeming geochemical chaos, with the identification of a number of geochemical associations. These groupings, which do not conform to a classical definition of suites, include: 1. Mafic granites (Shalimar Tonalite, Terrible Vale Porphyritic Microtonalite, Kentucky Diorite, Harnham Grove Porphyritic Microtonalite, Wilhelmshohe Tonalite, Manuka Farm Porphyritic Microtonalite, Khatoun Tonalite, Back Creek Tonalite and mafic rocks of the Wongalee Complex). These granites primarily occur in the southern portion of the NEB, adjacent to the southern exposures of the Moonbi Supersuite, and in many ways granites in this group have characteristics that are transitional between dominant Uralla and Moonbi Supersuite characteristics. These granites have characteristics typical of mafic rocks including very high Mg (> 10% MgO), Fe, Ni, Cr, and Cu. Differentiation trends support the operation of fractional crystallisation processes and some rocks may be cumulates. Unlike most mafic rocks the mafic USS are reduced. 2. Mt Duval grouping (Mt Duval, Booralong, Glenore, Gwydir River, Tingha, The Basin, Glenreach and possibly Exmouth). These are intermediate to felsic in composition. Of all the Uralla granites, these are the most geochemically similar to granites of the Moonbi Supersuite, being more oxidised and having higher K, Rb, P, Th, U, and LREE than other compositionally equivalent USS plutons. They also have high Mg, Cr and Ni relative to other intermediate to felsic Uralla granites. This unit includes one suite (Mt Duval Suite), this being composed of the Mt Duval and Booralong units.
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NEO 2010 3. Wards Mistake grouping (Wards Mistake, Cottesbrook, Taylor’s, Hell Hole, Ottery). These granites are located at the northern extent of the Uralla Supersuite. This group is relatively homogenous, and have low P, Ti, K, U, Th, Rb, Mg, Cr and Ni and higher Na, and Al than granites in the Mt Duval grouping. On average they also tend to have higher Ca and lower La, Pb, Sn, As and Cu when compared to those granites. The Wards Mistake Suite consists of the Wards Mistake and Cottesbrook units. 4. Llangothlin grouping (Llangothlin, Aberfoyle River, Wellingrove and Blackfellows Gully). These are intermediate granites that are characterised by very low Mg, Ni, Cr, low P, relatively low V, Rb, Th and U, and high Na, LREE, Zr, and Ba when compared to other Uralla Supersuite granites. The Uralla, Balala and Yarrowyck units are compositionally distinct both from each other and from the groupings described above.
Clarence River Supersuite The Clarence River Supersuite (CRSS) consists of a number of small, low-K, mainly Permian units (255-244 Ma; Shaw, Conaghan, and Flood, 1991; Bryant et al., 1996) located at the north-eastern and southern extensions of the NEB. Our current definition of the CRSS is based on that of Shaw and Flood (1981), but Barrington Tops has been subdivided into the Omadale Brook and Gummi Plain intrusions (cf. Eggins and Hensen, 1987), and several low-K intrusions identified by Hensel (1982) have been included. However, membership to the CRSS remains debateable depending on the selection criteria adopted. Koreelan Creek is nominally included in the CRSS but is somewhat transitional between the CRSS and intrusions from the northern portion of the MSS. With age of 290 Ma (Bryant et al., 1996), and reduced composition, the Kaloe Granodiorite is anomalous, and also could be excluded. Granites of the CRSS are compositionally diverse, ranging from gabbro to monzogranite but with dominant tonalite, diorite and granodiorite. The rocks are typically speckled black and white in colour, consisting of early formed mafic minerals (cpx ± opx followed by amphibole then biotite) and highly An-rich plagioclase (~An90), with interstitial quartz and K-feldspar. Distinctive features of the CRSS include high Ca, and low abundances of incompatible elements (LILE, HFSE, LREE, Th, U, Pb. P). CRSS granites are typically highly oxidised (excluding Kaloe). Isotopically, they are amongst the most primitive in eastern Australia. Textures and compositions are consistent with crystallisation from high temperature melts. These granites are similar to Permian-Triassic granites from the Yarrol Province in Queensland and to the Cordilleran granites of the western Peninsular Ranges Batholith of California. At least some of the intrusions may be the end-product of mantle-derived arc magmatism. Nevertheless, there is considerable geochemical heterogeneity, and multiple sources and styles of petrogenesis appear to be involved. No suites are recognised within the CRSS, but there are two broad geochemical associations; primitive CRSS plutons and a tonalite-trondhjemite grouping (TTG). The primitive group (Towgon Grange, Jenny Lind, Omadale Brook) are the most mafic, the most incompatible element depleted and the most isotopically primitive granites in the CRSS. The TTG (Duncans Creek, Mt Ephraim, high-Si Kaloe) are dominated by tonalitic and trondhjemitic compositions that are characterised by high Al, Ga, Na/K, Na/Ca, and Sr, and low Ca, K, Fe, Sc and V, and very low concentrations of Y and the HREE. There are a series of other granites that share some similarities with, but are disparate from, these groups. Dumbudgery Creek Granodiorite, Bruxner and Cullens Creek (new assigned member) appear to incorporate a component similar to the primitive CRSS granites, tending to have low concentrations of P, HFSE, Sr, Ba and REE, but being characterised by higher concentrations of K, Rb, Th and U, which implies more significant incorporation of crustal components. The integration of mantle and crustal components is particularly complex in the Dumbudgery Creek. Gummi Plain and low-Si Kaloe are distinct from other CRSS granites, having high concentrations of HFSE, P, REE and Ga, but low concentrations of K, Rb, Pb, Th, U and Ba. These features require a different petrogenesis and/or source materials than other CRSS granites, that are unrelated to crustal contamination.
Low-K granites proximal to the northern MSS There are a number of low-K intrusions, including Mt You You, Four Bull, Greymare, Maryland, Herries, and Newton Boyd that occur around the margins of the BTG-STG-RCTG mass. These intrusions are characterised by lower Ti, K, Rb, Th, U and Pb, and in some cases higher Ba than the BTG samples at equivalent SiO2 contents. Koreelan Creek and Mascotte arguably could be included in this grouping. These 78
NEO 2010 intrusions currently remain unclassified and available data indicate that they do not form a coherent geochemical association.
Leucogranites Shaw and Flood (1981) first used the leucogranite grouping to describe virtually all highly felsic granites in the northern portion of the NEB, including granites that are now classified as belonging to the MSS. The leucogranite grouping as used here only relates to leucogranites that are not already included within the MSS, being primarily located in the central portion of the batholith. They are characterised by widespread textural variability, and include pegmatitic, aplitic and phases with many miarolitic cavities. While these melts may have been generated by partial melting close to minimum-temperature melt compositions, many have undergone extensive fractional crystallisation in which there was concentration of incompatible elements (marked by high Rb/Sr) and fluid exsolution (marked by the presence of pegmatite and miarolitic textures). Their geochemical characteristics, including low P, and increasing Y, Pb and Th with fractionation, imply a derivation from I-type source rocks (cf. Chappell, 1999). However, the presence of near minimum melt compositions and fractional crystallisation to some degree limit the capacity to accurately identify the source rocks, and hence their affinity with a particular I-type supersuite. There are a series of leucogranites located to the west of the USS. Of these, The Basin, Parlour Mountain and Honeysuckle Creek appear to be part of the USS. Myanbah and Webbs Consols may also form part of that supersuite. The Mole, Gilgai and Delungra granites are all strongly fractionated leucogranites, and distinct from other leucogranites in the area. The high LREE in these intrusions are somewhat unusual as most leucogranites in the NEB evolve toward low concentrations of these elements. The strong fractionation of these intrusions make their assignment particularly difficult. Although there is some suggestion that the petrogenesis of Gilgai may be related to that of Tingha, the affiliation of all three remains uncertain. Another group of leucogranites occur in the eastern portion of the batholith that are geographically associated with both USS and MSS granites. These include Oban River, Kingsgate, Glen Garry, and Yellow Gap. The Kingsgate intrusion is virtually indistinguishable from the Oban River pluton. These two intrusions are chemically similar to the Sandy Flat, Nonnington and Mt Jonblee intrusions from the NMSS. As such Oban River and Kingsgate are now classified as belonging to the Moonbi Supersuite. Glen Garry may also belong to the Moonbi Supersuite. Yellow Gap remains unclassified. The classification of both Glen Garry and Yellow Gap can only be assessed with further data.
Coastal Granites Granites within the coastal belt to the east of the main body of the NEB are younger than other I-type granites in the NEB (235-222 Ma; Shaw et al., 1991), being classified as post-orogenic granites by Hensel et al. (1985). These intrusions were formerly included in the Gundle Granite Belt, the Coastal Granite Belt and granites of the Laurieton-Taree area. Few of the Coastal granites have been mapped or studied in detail. Available chemical analyses are largely restricted to the major, easily accessible plutons. Granites within the coastal belt include both I-types and A-types. The Coastal I-type granites are mineralogically, petrographically and to a perhaps a lesser extent chemically diverse, and currently not identified as a supersuite. However, most are characterised by comparatively low concentrations of K, Rb, Th, U, and Pb, being similar to the CRSS intrusions in these respects. However, the coastal granites are distinct having have higher Nb, P, Zn, Cr and to a lesser extent Zr and Ti than the CRSS. The Coastal A-type granites include the A-type granites from the Chaelundi Complex, the Smokey Cape Monzogranite, and a series of shallow-level intrusive rocks that occur in the Laurieton-Taree area. These granites are Triassic in age, similar to the Coastal I-type granites. The Chaelundi Complex is the most well documented of these intrusions (Landenberger and Collins, 1996). The Chaelundi A-type granites can be subdivided into two groups (< 74.5 wt % SiO2 and > 74 wt % SiO2). The Smokey Cape and Laurieton granites are compositionally similar both to each other and to the intermediate Chaelundi A-type granites. These intermediate A-type granites are characterised by a number of distinguishing features including low concentrations of Ca, Mg and V, and very high concentrations of Zr compared to other NEB I-type granites at equivalent SiO2. The A-types have U, Th and Pb in the Coastal A-type granites, similar to that observed in the coastal I-type granites, suggesting perhaps that this is an intrinsic feature of the crust in this area.
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Comparisons with the Lachlan Fold Belt Significant differences exist between NEB granites and the Silurian-Devonian granites of the LFB, although the MSS conspicuously resembles the Carboniferous granites of the northeastern LFB. Isotopically, NEB granites are significantly more primitive than their LFB counterparts, indicating rapid recycling of the crust since its production from the mantle, and perhaps more of a direct mantle component. Also, apart from the BSS, granites of the NEB are compositionally diverse, often over short distances. This significant heterogeneity contributes to a lack of recognised suites, in contrast to the LFB where precisely defined I-type suites can sometimes be recognised over distances of up to 200 km, and the two largest S-type supersuites both cover areas of around 9,000 km2. This in itself may reflect the comparatively juvenile nature of the crust beneath the NEB, a situation that contributes to limited opportunity for crustal homogenisation. Despite the lack of suites, there are what appear to be strongly geographically controlled geochemical associations. This is reflected in the distribution of the supersuites themselves, in the internal variations within supersuites, in the gradations in the geochemical compositions across supersuite boundaries, and in geochemical characteristics of granites (particularly is it relates to Pb, Th, U, and the alkali elements).
References Blevin P. L. and Chappell, B. W., 1996. Internal evolution and metallogeny of Permo-Triassic high-K granites in the Tenterfield-Stanthorpe region, southern New England Orogen, Australia, Mesozoic Geology of the eastern Australia Plate Conference – Geological Society of Australia, Extended Abstracts, 43, 94-100. Bryant C. J., Arculus, R. J. and Chappell, B. W., 1997. Clarence River Supersuite: 250 Ma Cordilleran tonalitic I-type intrusions in eastern Australia, Journal of Petrology, 38, 975-1001. Bryant C. J., Cosca M. A. and Arculus R. J., 1997. 40Ar/39Ar ages of Clarence River Supersuite intrusions from the northern Portion of the New England Batholith, southern New England Orogen, In: Ashley P. M. and Flood P. G. (eds), Tectonics and Metallogenesis of the New England Orogen, Geological Society of Australia Special Publication, 19, 242–253. Chappell B.W., 1978. Granitoids from the Moonbi district, New England Batholith, eastern Australia,. Journal of the Geological Society of Australia, 25, 267-283. Chappell, B.W. 1999. Aluminium saturation in I- and S-type granites and the characterization of fractionated haplogranites. Lithos, 46, 535-551. Eggins S. and Hensen B.J., 1987. Evolution of mantle-derived, augite-hypersthene granodiorites by crystal fractionation: Barrington Tops Batholith, eastern Australia, Lithos, 20, 295-310. Hensel H.D., 1982. The mineralogy, petrology and geochronology of granitoids and associated intrusives from the southern portion of the New England Batholith. PhD thesis, University of New England. Hensel H.D., McCulloch M.T. and Chappell B.W., 1985. The New England Batholith: constraints on its derivation from Nd and Sr isotopic studies of granitoids and country rocks, Geochimica et Cosmochimica Acta, 49, 369-384. Kovarch A., 1993. Geographical Information Systems for regional scale geological analysis: The Manilla 1:250,000 map area, a case study, BSc (Hons) thesis, University of New England. Landenberger B. and Collins W.J., 1996. Derivation of A-type granites from a dehydrated charnockitic lower crust: evidence from the Chaelundi Complex, eastern Australia, Journal of Petrology, 37, 145-170. Landenberger B., Farrell T.R., Offler R., Collins W.J. and Whitford, D.J., 1995. Tectonic implications of Rb-Sr biotite ages for the Hillgrove Plutonic Suite, New England Fold Belt, NSW., Australia, Precambrian Research, 71, 251-263. Shaw S. E., Conaghan P. J. and Flood R. H., 1991. Late Permian and Triassic activity in the New England Batholith and contemporaneous tephra in the Sydney and Gunnedah Basins, In: Advances in the study of the Sydney Basin, 25th Newcastle Symposium, University of Newcastle, NSW., 44-51. Shaw S.E. and Flood R. H., 1981. The New England Batholith, Eastern Australia: Geochemical variations in space and time, Journal of Geophysical Research, 86, 10530-10544.
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Hawkwook Iron-Titanium-Vanadium Project Southeast Queensland Peter Buckley1, Paul Ashley2 1
Eastern Iron, Level 1, 80 Chandos Street, St Leonards, NSW 2065
2
Paul Ashley Petrographic and Geological Services, 37 Bishop Crescent, Armidale, NSW 2350
Keywords: Hawkwood, layered intrusion, iron-ore, titanium, vanadium, magnetics.
Introduction The Hawkwood iron-titanium-vanadium project is a developing magnetite, iron ore exploration project in the southeastern Queensland portion of the New England Orogen. The project is a joint venture between Eastern Iron Limited as operator and Rugby Mining. Vanadium-bearing titaniferous magnetite rich gabbro and magnetite rich layers are hosted by the layered intrusive complex of the Hawkwood Gabbro. Eastern Iron has completed a low-level, high resolution airborne magnetic survey over the project area identifying a 12 kilometre long magnetic anomaly. Ground geophysical traverses across the interpreted magnetite-bearing layers followed by interpretation and modelling of the results were also completed prior to percussion drilling. An exploration target tonnage is estimated from the shallow drilling completed to date of the order of 300-400 million tonnes of gabbro with 20-25% contained magnetite. Additional tonnage potential exists at depth and in a further 5 kilometres of untested magnetic anomaly.
Location and rationale for exploration The Hawkwood project area is located 45 kilometres southwest of Mundubbera in southeast Queensland. The project area is freehold open grazing land, well serviced with power and roads. Mundubbera is connected by sealed highway to other nearby regional centres and by rail to the main north-south trunk line some 200 kilometres to the east and 250 kilometres northeast to the export port of Gladstone (Fig. 1).
Figure 1. Location of Eastern Iron Hawkwood Project, rail and bulk export infrastructure.
At Hawkwood, Eastern Iron is targeting a resource large enough to support an iron ore mining and processing operation capable of producing at least 2 million tonnes of iron ore concentrate per annum within a large, layered titanomagnetite body. The availability of existing road, rail and export port infrastructure is a key part of Eastern Iron’s exploration rationale as it provides any potential development of an iron ore operation at 81
NEO 2010 Hawkwood with a significant advantage compared to other proposed magnetite iron developments in Western and South Australia The rail line to Maryborough and Gladstone is currently not in use but has been maintained and is the subject of recent studies for re-activation to meet haulage requirements of other potential bulk users in the region including proposed mineral and coal developments. Port facilities at Gladstone offer any potential development at Hawkwood ready access to overseas export markets. The port currently has a total throughput of more than 75 million tonnes annually with major cargoes including coal, alumina, aluminium and cement and is the third largest coal export port in Australia. Planned expansions will accommodate a doubling of the current coal export capacity and exports from the proposed new LNG terminals. The Gladstone Ports Corporation have advised Eastern Iron that subject to environmental, community and commercial conditions being met there is available capacity for iron ore shipments of 1-2 Mtpa with an additional 5-6 Mtpa available from existing facilities as other planned expansions come on line.
Geological setting The Hawkwood Gabbro forms an ovoid body of about 100 km2, adjacent to the Delubra Quartz Gabbro, with the two masses possibly being genetically related, e.g. the Hawkwood body is simply a more mafic and better layered portion of the Delubra mass. The intrusions form a small component of the large Rawbelle Batholith, of late Permian to early Triassic age. The Hawkwood Gabbro has intruded early Permian mafic to intermediate volcanic and sedimentary rocks, and is overlain by Jurassic strata of the Surat Basin and by Tertiary basalt and laterite (Whitaker et al., 1974; 1980). Rock type identified in the Hawkwood Gabbro include hornblende-, orthopyroxene- and olivine-bearing gabbro, pyroxenite, anorthosite, diorite and monzonite. The magnetite-rich accumulations appear to be commonly associated with hornblende-bearing gabbro. Magnetite is a primary igneous mineral and has concentrated by magmatic accumulation in layers in amounts from a few per cent to almost massive magnetite (Fig. 2). Layers vary from less than a metre to tens of metres in thickness.
Figure 2: Cut slab of magnetite-rich rock from surface float. Typical magnetite grainsize is several millimetres.
Examples of this style of magnetite occurrence occur widely and include the Bushveld igneous complex, South Africa, Duluth Complex, northeastern Minnesota, United States, Skaergaard intrusion of east Greenland, Stillwater igneous complex, southwestern Montana, United States. Similar magnetite deposits at Balla Balla and Windimurra in Western Australia form continuous magnetite rich layers over several kilometres long and aggregate to resources of several hundred million tonnes. Typically these deposits contain some titanium and vanadium that can be valuable by-products or, in the case of the Windimurra vanadium deposit, be the main target commodity. Examples of other magnetite-bearing layered intrusions in central and southeast Queensland include the Goondicum, Wateranga and Eulogie Park bodies (Day et al., 1983).
Exploration Magnetite bearing layers outcrop locally and ephemeral streams and drainage channels often carry significant amounts of detrital magnetite. This outcropping magnetite content of the gabbro was initially recognised by Tennent Minerals in 1964 and six magnetic anomalies were outlined within a mineralised zone extending north-easterly in an area 1000m long and up to 600m wide. Three core holes to test the magnetiterich gabbro returned assays results of up to 0.3% Cu (1% chalcopyrite in a 0.9m interval), 350ppm Ni (1% pyrrhotite), 25% magnetite and 2% Ti (Bryan, 1965). In a similar area during 1968, the Queensland Government completed 473m of stratigraphic drilling, intersecting several bands of magnetite up to 25m 82
NEO 2010 thick with 25% Fe, 2% Ti and 0.1-0.2% V. Laboratory tests suggested a 55% Fe and 4-5% Ti product was possible with wet separation (Brooks, 1968). In 1971, Theiss carried out further drilling that indicated a potential resource of 590,000t of magnetite in ore grading 23% from a pit with an ore:waste ratio of 1:0.63 to a depth of 40m (Johnson et al., 1971).
Fig. 3: Location of the Hawkwood project area overlayed with airborne geophysics.
Since acquiring the project in January 2010, Eastern Iron has completed a low level high resolution airborne magnetic survey over the project area, significantly improving the existing regional survey and successfully identifying a 12 kilometre long magnetic anomaly interpreted to be coincident with magnetite-bearing rocks within the underlying layered intrusive complex (Fig. 3). Ground geophysical traverses across the magnetitebearing layers followed by interpretation and modelling of the results were also completed. Outcrop across the project area is largely obscured by a tertiary weathering surface and Eastern Iron has recently completed a bedrock-drilling program to determine the source of the magnetic anomaly and obtain some preliminary indications of the grade and size of a potential magnetite iron-vanadium exploration target tonnage at Hawkwood. A total of 19 holes for 1,848m of reverse circulation drilling were completed across the area. Drill holes were located on lines previously surveyed by ground magnetics (Fig. 4). The drilling, conducted on lines around 1 kilometre apart, successfully discovered thick sections of magnetite-bearing gabbro or ferrigabbro, that are widespread throughout the anomaly area and appear to be continuous over the 7 kilometres of strike length so far tested. The widespread occurrence of magnetite-bearing gabboric rocks and the lack of distinctive geological marker horizons has made correlation between the drilling and the magnetic modelling uncertain. Interpretation of the drilling results has inferred a relatively shallow northerly dip for the magnetic layers, which will be confirmed by follow up, oriented diamond core drilling (Fig. 5). Drill hole depths varied from 60 – 120m. During drilling 1m samples were collected and measured for magnetic susceptibility. Two metre composites were assayed by XRF fusion for Al2O3, As, Ba, CaO, Co, Cr2O3, Cu, Fe, K2O, MgO, Mn, Na2O, Ni, P, Pb, S, SiO2, TiO2, V, Zn, LOI (water content or Loss On Ignition). Drilling results received to date indicate wide downhole intersections of ferrigabbro ranging from 12% to 35% Fe and averaging 18% Fe across all holes. The presence of vanadium and titanium may substantially increase the value of any product that may be mined from this deposit. Ferrigabbro intersected by drilling ranges from a few metres up to 112m intersected HWRC03 and includes thinner bands of massive and semi massive magnetite.
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NEO 2010 Assuming at least two ferrigabbro layers of 40m thickness continuous over seven kilometres and mineable to 200m gives an exploration target tonnage range of 300-400Mt (assumed tonnage factor of 3.3 tonnes/cubic metre). Based on the results to date this has an estimated magnetite content of 20-25%. Further resource potential exists within the five kilometres of untested strike of the magnetic anomaly which extends north from the western end of the prospect area. (De Ross, 2010)
Fig. 4: Location of the drill collars along the Hawkwood project target horizon. Background is a total magnetic intensity image of the airborne magnetic survey
The resource potential will need to be tested by further drilling. In particular, core drilling to confirm the dip of the layers and an indication as to what extent the intersected intervals represent true widths. Preliminary testwork of magnetite concentrates assay up to 61% Fe, 1.1% V2O5 and 7.88% TiO2 (Buckley, 2010) and Davis tube analysis also will be carried out on selected drilling intervals to provide samples for definitive concentrate analysis and estimates of mass recoveries to concentrate. Eastern Iron was also recently granted an exploration license over the Auburn area some 12km west of Hawkwood (Fig. 3). This area is underlain by layered intrusive similar to the Hawkwood/Delubra Gabbro and will be assessed for its potential to host magnetite deposits over the coming months.
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Petrographic data and implications Petrographic observations were made on samples from the strongly magnetically anomalous zone (drill core and surface outcrop/float). The primary rocks are medium to coarse grained and composed of cumulus grains of plagioclase, clinopyroxene, orthopyroxene, olivine and hornblende (in varying amounts), with intercumulus FeTi oxides (magnetite, ilmenite) and sulphides (trace to minor chalcopyrite, pyrrhotite) and traces of a spinel phase (e.g. hercynite). In the magnetite-rich zones, the proportion of magnetite + ilmenite ranges from ~20% to approaching 100%, with the ratio of magnetite to ilmenite typically being 5:1 to 10:1. Where richer in FeTi oxides, the hosting rock is commonly richer in hornblende. It is evident that imposed processes acted on the magmatic rocks. These include hydrothermal and/or metamorphic processes, with local strong replacement of the igneous minerals by amphibole (e.g. magnesio-hornblende), biotitephlogopite, sodic plagioclase, clinozoisite and pyrite. Supergene oxidation has strongly modified nearsurface rocks, with replacement of silicates by clay and goethite, and magnetite by hematite (martitisation). Petrographic observations have indicated that magnetite is relatively “pure” and only has a little finely exsolved ilmenite and/or spinel (volumetrically less than a few %). Elsewhere, magnetite and ilmenite form discrete medium to coarse grains implying reasonably good liberation on crushing/milling (Fig. 6). Replacement of magnetite by hematite is likely to be a supergene oxidation effect and probably does not occur below the zone of oxidation. Magnetite is the most likely repository for V values in the magnetitebearing rocks. V is strongly lithophile (hence partitions into oxide minerals on igneous crystallisation), especially if oxidation conditions are suitable to stabilise magnetite (as is the case at Hawkwood). In igneous oxide minerals, V is typically in the V3+ oxidation state and substitutes readily into magnetite, e.g. as Fe2+(Fe3+,V3+)2O4.
Figure 5: Interpreted drill section looking north at 7147400mN (GDA 94), see Figure 4 for location.
Conclusions Exploration by Eastern Iron has identified a large tonnage vanadium-iron bearing magnetite body within the Hawkwood gabbro in southeast Queensland approximately 240 kilometres by rail from the Port of Gladstone, Australia’s third largest coal export port. Eastern Iron completed a low level, high-resolution airborne magnetic survey over the project area identifying a 12 kilometre long magnetic anomaly. Ground geophysical traverses across the interpreted magnetite-bearing layers were completed, followed by interpretation and modelling of the results. 85
NEO 2010 A bedrock drilling program confirmed that titanium and vanadium bearing magnetite was the source of the magnetic anomaly. Preliminary indications of the grade and size of a potential iron ore exploration target tonnage at Hawkwood are in the order of 300-400Mt (assumed tonnage factor of 3.3 tonnes/cubic metre). Based on the results to date this has an estimated magnetite content of 20-25%. Preliminary petrographic observations have indicated that magnetite is relatively “pure” with little finely exsolved ilmenite and/or spinel (volumetrically less than a few %). Elsewhere, magnetite and ilmenite form discrete medium to coarse grains implying reasonably good liberation on crushing/milling. Hawkwood will be the focus of further drilling, metallurgical evaluation work and concentrate marketing in the months ahead.
Fig. 6: Photomicrograph of intercumulus magnetite (pale grey-brown) and subordinate ilmenite (more obvious brown), enclosing cumulus olivine (dark grey) and goethite-altered sulphides. The pale grey rims on some magnetite grains are composed of supergene hematite. Plane polarised reflected light, field of view 1 mm across.
References Brooks, J.H., 1968. Departmental Drilling Programme, Magnetite Deposits, Hawkwood Area, Queensland Government Mineral Journal 69, 537-545 pp. Bryan, W.B., 1965. Geology of a magnetite anorthosite complex near Hawkwood Station, Queensland. Unpublished report to Tennent Minerals Development Pty Ltd. Buckley, P.M., 2010. High Grade Magnetite - Vanadium Assays, Hawkwood Joint Venture, Queensland. Australian Securities Exchange, Eastern Iron Limited Market Release (ASX Code EFE): 18 February 2010. Day, R.W., Whitaker, W.G., Murray, C.G., Wilson, I.H. and Grimes, K.G., 1983. Queensland geology: a companion volume to the 1:2 500 000 scale geological map (1975). Geological Survey of Queensland Publication 383, 194 pp. De Ross, G., 2010. Hawkwood Iron-Vanadium Project - Drilling Update. Australian Securities Exchange, Eastern Iron Limited Market Release (ASX Code EFE): 30 September 2010. Johnson, G.J. and Chiu Chong, E.S., 1971. Technical Report on Hawkwood Magnetite Prospect. Unpublished report to Thiess Peabody Mitsui Coal Pty. Ltd. 86
NEO 2010 Whitaker, W.G., Murphy, P.R. and Rollason, R.G., 1974. Mundubbera, Queensland, 1:250 000 geological series map. Geological Survey of Queensland Report 84, 113 pp. Whitaker, W.G., Murphy, P.R. and Rollason, R.G., 1980. Geology of the Mundubbera 1:250 000 sheet area, SG 56-5. Second edition. Geological Survey of Queensland.
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Cu mineralisation associated with intrusive phases of the Gamilaroi and Weraerai terranes at Barry Station, southern New England Orogen Solomon Buckman1, Tom Line1, Jonathan Aitchison2, Allen Nutman1 1
Affiliation eg. School of Earth and Environmental Sciences, University of Wollongong, Australia 2
Department of Earth Sciences, University of Hong Kong, Hong Kong SAR, China
Keywords: island arc, porphyry Cu/Au mineralisation, intrusive, Gamilaroi, Weraerai
Introduction The Tamworth Block or Belt (Korsch, 1977) includes Devonian-Carboniferous lithologies west of the PMFS that were thought to represent a forearc basin deposited adjacent to a continental margin arc (Murray et al. 1987). The Tamworth Belt was pre-empted by the use of the lithostratigraphic unit - Tamworth Group (Crook, 1961a, b). The name 'Gamilaroi terrane' was introduced by Flood and Aitchison (1988) to describe intra-oceanic island arc related rocks found within the Tamworth Belt but forming a separate lithostratigraphic identity. It is now recognised that two arc-related sequences are present west of the Peel Manning fault system: 1) a Devonian intra-oceanic island arc sequence - the 'Gamilaroi terrane' (Morris, 1988) which is devoid of any continental (Gondwana) sediment influx, and contains low K, Ti and Zr subvolcanic rocks (Aitchison and Flood, 1993); and 2) a Carboniferous continental margin arc (high K values), which developed over the Gamilaroi terrane, forming an overlap sequence (Fig. 1). The Gamilaroi terrane at Barry can be recognised through the presence of meta-felsic subvolcanic rocks and was the focus of the day 2 pre-conference fieldtrip. It is juxtaposed against rocks of the Djungati terrane and Manning Group to the west, and the Weraerai terrane and Manning Group (Back River) to the east. Gamilaroi subvolcanic rocks are distinguished from those of the Weraerai terrane by the presence of more acidic rocks such as diorite, trondhjemite and granodiorite as opposed to the exclusively gabbro, leucogabbro and ultramafics of the Weraerai terrane; also there is a lack of extensive quartz veining commonly found in Weraerai rocks. The felsic intrusives of the Gamilaroi commonly contain xenoliths of fine-grained basalt. The Gamilaroi terrane also contains a range of volcanic rocks including dacite, rhyolite and basalt that are intercalated with tuffs, tuffaceous cherts, volcanigenic sediments and limestone. Rocks of the Gamilaroi terrane have been metamorphosed to prehnite-pumpellyite facies and at Barry are associated with several minor sites of Cu mineralisation at the junction of the Barnard and Back Rivers. This study will focus on the stratigraphy of an undocumented part of the Gamilaroi and Weraerai terranes at Barry Station as detailed by the field mapping of Buckman (1993). The style and timing of mineralisation associated with intrusive phases and represents part of an ongoing honours project in which we will undertake zircon dating to further constrain the age of key intrusive phases of the Gamilaroi and Weraerai terranes at Barry.
Stratigraphy Felsic subvolcanic rocks form the base of the Gamilaroi terrane (Fig. 2) and have been dated as Late Silurian in age elsewhere (Kimbrough et al. 1993; Aitchison & Flood, 1993). Silurian fossils are unknown in the Gamilaroi terrane so the felsic igneous rocks may represent the base of a Silurian-Devonian succession (Flood & Aitchison, 1992). The subvolcanic rocks at Barry are overlain by up to 400 m of volcanic rocks including dacite, rhyolite and basalt. Overlying the volcanic and subvolcanic (in part) rocks is a thin (~10-40 m) unit of high-density mass flow conglomerates and breccias (G.R. 413062). This is, in turn, overlain by up to 700 m of tuffaceous chert, volcanigenic sediment, basalt and limestone. Conglomerates
88
NEO 2010 Conglomerates and breccias directly overlie the felsic subvolcanics and basalts as a unit about 30 m thick (G.R. 412061). Conglomerate clasts are composed almost entirely of siliceous radiolarian tuff and basalt clasts in a sandstone-mud matrix (Fig. 3, D). The presence of felsic igneous clasts as well as tuff and basalt clasts within these conglomerates indicates this unit formed as a result of reworking of underlying strata as well as erosion of the subvolcanic intrusive rocks. These deposits display little or no bedding indicating rapid chaotic deposition from high-density mass-flows. The conglomerates continue to be interstratified with the overlying sedimentary rocks at higher stratigraphic levels. Fig. 1. Geology of Barry Station and the location of Cu mineralisation as mapped by Buckman (1993).
Siliceous tuffs and volcanigenic sandstones Grey, brown and green siliceous tuffs comprise the dominant lithology of the Gamilaroi terrane at Barry. Radiolarians were observed in hand specimen but could not be extracted due to intense recrystallisation and deformation. Radiolarians collected from other areas of the Gamilaroi terrane indicate the entire Gamilaroi terrane sequence may have accumulated during the Devonian (Aitchison et al. 1992; Aitchison & Flood 1993). Black volcanigenic sandstones are observed near the top of the preserved section and contain detrital clinopyroxene (Fig. 3, E) indicating close proximity to a volcanigenic source at the time of deposition. These volcaniclastic sandstones are commonly deposited as thinly bedded graded turbidite sequences (G.R. 401087).
Limestones Limestones are commonly interstratified with the tuffs and volcanigenic sediments of the Gamilaroi terrane. Outcrops range in size from 1 km long and 100 m wide (G.R. 401084), to pebble size. There is no transition through increasingly carbonate-rich, marly sediments between the clean limestone blocks and the surrounding strata. The surrounding tuffs and sediments probably formed in a deep marine environment as evident by the presence of radiolarian ghosts in siliceous tuffs, so the shallow marine limestone blocks are interpreted as being deposited as olistostromal blocks that slumped into deep marine sediments. Limestone breccias are often observed on one side of the limestone blocks and may represent the rubble generated as the block slumped. The limestones have been heavily recrystallised, forming intricate stylolite patterns (Fig. 89
NEO 2010 3, A & B). Fossils including sponges, corals, bryozoans, crinoids and brachiopods are abundant (Fig. 4) and have usually been silicified. However, preservation is poor due to recrystallisation and deformation. Recognisable brachiopods extracted from the limestone allochthons include: Howellella sp., (Fig. 4, 1 & 2) Silurian (Llandovery) to Devonian (Emsian); Atrypa sp. Silurian (Llandovery) to Gamilaroi terrane stratigraphy Devonian (Frasnian); Gypidula sp. Silurian Djungati terrane f (Wenlockian) to Devonian f Peel-Manning fault (Frasnian); and possibly Volcaniclastic sediments Cyrtina sp. (Fig. 4, 3 & 4) Devonian (Gedinnian) to Permian. From ten Lower Limestone blocks Limestone limestone samples Devonian source unknown processed with acetic acid, Basalt, brecciated only one poorly preserved basalt conodont was recovered Dolerite from the limestones (Fig. Maximum thickness Tuffaceous chert 4). ~ 1 km
Lower Devonian Breccia and conglomerate Basalt, dolerite Dacite, rhyolite Lower Silurian Felsic subvolcanic and intrusive rocks
f Cambrian
f
Peel-Manning fault Weraerai terrane
Fig. 2. Generalised stratigraphy of the Gamilaroi terrane at Barry. Age constraints for the limestone blocks provided from brachiopod fauna. The ages of the felsic intrusives of the Gamilaroi terrane and the Weraerai terrane are taken from data published by Aitchison and Flood (1992) and Aitchison et al. (1992). Note that the Weraerai terrane is not part of the Gamilaroi sequence.
Corals are abundant in the limestones and best preserved in the limestone breccias. This is probably due to the surrounding matrix partially sealing the limestone and protecting it from the fluids responsible for recrystallisation. Species are tentatively identified as: Xystriphyllum sp. (Upper Silurian - Middle Devonian) (Fig. 3 A); Phillipsastrea sp. (Lower - Upper Devonian); and Heliolites sp. (Middle Ordovician – Middle Devonian) (Fig. 3, C). A variety unidentifiable solitary rugose corals are also present in the limestones.
Igneous petrology Detailed descriptions of the mineralogy and geochemistry of igneous rocks within the Gamilaroi terrane are provided by Vallance (1969), Offler (1982), Cross, (1983), Morris (1988), Cawood and Flood (1989) and Aitchison and Flood (1993). Regionally extensive subvolcanic and volcanic rocks (Fig. 5) are intercalated with lower Tamworth Group sedimentary rocks. Geochemical analyses indicate that they are altered subalkaline volcanic arc andesites with subordinate dacites (Cawood & Flood, 1989). These igneous rocks show similarities to modern intra-oceanic volcanic island arc settings (Aitchison & Flood, 1993). Aitchison and Ireland (1995), recently determined a Late Silurian age for a tonalitic rock from the Gamilaroi terrane. Basaltic intrusives and extrusives (Fig. 5, A & B) generally occur stratigraphically above the meta-felsic subvolcanic rocks. The felsic subvolcanic rocks (Fig. 5, C) are observed intruding the overlying volcanic and volcanigenicsediments (Fig. 5, D) at G.R. 412061. The Gamilaroi igneous rocks have undergone low grade, prehnite-pumpellyite facies, metamorphism.
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Cu mineralisation Several Cu prospects occur around the periphery of the intrusive (trondhjemite) core of the Gamilaroi terrane (Fig. 6) at the junction between the Barnard and Back Rivers. A narrow, shear zone on the eastern margin of the trondhjemite is intensely brecciated by hydraulic fracturing and quartz veins are associated with malachite staining due to the weathering of chalcopyrite. In some cases the Cu mineralisation is disseminated throughout the trondhjemite suggesting that it is closely related to the intrusion and is indicative of porphyrystyle mineralisation. At Mount Morgan’s mine in southern Queensland a very similar style of Cu/Au mineralisation is associated with Gamilaroi terrane equivalent rocks. Outcrops of deep level intrusive phases of the Gamilaroi terrane are rare in the southern New England Orogen while outcrops of volcanic phases are more common. This suggests that deeper crustal levels of the Gamilaroi terrane are yet to be exposed by erosion in the southern portions of the New England Orogen and as such this region may have the potential to host Mount Morgan style mineralisation at depth beneath the Gamilaroi terrane volcanics and the Cu prospects associated with rare intrusive phases outcropping along deeply incised valleys at Barry warrant further exploration. Fig. 3. Sedimentary rocks of the Gamilaroi terrane: A) Partly recrystallised shallow marine limestone. Stylolites form around corals. Tangential and cross section of ?Xystrophyllum, sp. (Lower Devonian) (G.R. 408069); B) Limestone containing a variety of coral fragments and showing evidence of extensive stylolitisation. Corals are best preserved immediately adjacent the stylolites (G.R. 408069); C) Limestone breccia containing a silicified coral; Order Heliolitida (M. Ordovician - M. Devonian) (G.R. 409063); D) Photomicrograph of a high-density mass-flow deposit overlying the felsic intrusives and extrusives. Dominant clasts are fragments of siliceous radiolarian tuffs. Rare basalt fragments (top right) are also present. Felsic subvolcanic clasts are observed in hand specimen (F.O.V. = 2 mm) (G.R. 413062);; E) Photomicrograph of volcanigenic sandstone deposited by turbidity currents, and dominated by volcanic quartz, plagioclase and clinopyroxene (F.O.V. = 2 mm) (G.R. 402087).
Fig. 4. Brachiopods and a conodont from limestones in the Gamilaroi terrane. 1) Order Spiriferida - Howellela sp. pedicle valve (scale top 6 mm) 2) Howellela sp. brachial valve (scale top 5 mm): Biconvex with incurved pedicle umbo; large interarea, with open delthyrium; subquadrilangular outline; coarse plications; interior with strong divergent dental plates and short brachiophore plates; no medium septum (Murray, 1985). 3) Order Spiriferida: possibly Cyrtina sp., pedicle valve, interior with prominent teeth and sockets (scale top = 1.5 cm) 4) Pedicle valve (scale top = 1.5 cm) exterior. 5) Order Pentamerida: Gypidula sp. (scale top = 1 cm) 6) Gypidula sp. brachial valve (scale top = 1 cm). No ribs, smooth surface, prominent medium septum. 7) Unidentified conodont (scale bottom = 1 mm).
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D
Fig. 5 Thin section analyses of the igneous rocks of the Gamilaroi terrane. A) Dolerite intrusive within volcanigenic sediments high in the stratigraphic section. Plagioclase has been albitised. Pyroxenes remain largely unaltered. F.O.V. 2 mm. G.R. 402072. B) Brecciated basaltic tuff showing a fining upwards of the large plagioclase and clinopyroxene phenocrysts. F.O.V. 2 mm. G.R. 398077. C) Altered felsic subvolcanic rock from the base of the Gamilaroi stratigraphy. The veins running down the plate contain quartz and prehnite (high birefringence). Small amounts of clinopyroxene have been preserved in a dominantly quartz and plagioclase rock.. F.O.V. 2 mm. G.R. 411067. D) Dacitic tuff containing large phenocrysts of quartz and plagioclase in a fine-grained matrix of quartz and feldspar. Overlies the felsic subvolcanic rocks. F.O.V. 2 mm. G.R. 412066.
Interpretation
E
The felsic intrusives represent the oldest in situ rocks of the Gamilaroi terrane (Kimbrough, et al. 1993; Aitchison & Flood 1993) and are overlain by a Devonian sedimentary sequence (Aitchison et al. 1992a). Geochemical data from the meta-basalts and felsic intrusives (keratophyres) at Barry and from other areas of the Gamilaroi terrane (Offler, 1982; Cross, 1983; Morris, 1988; Cawood & Flood, 1989; Aitchison & Flood, 1993) indicates they are subalkaline volcanic arc andesite with subordinate dacite. Comparisons with modern intra-oceanic island-arc settings, for example, the Lau Basin, Scotia Arc and Marianas Trough, indicates the Gamilaroi terrane formed in an intraoceanic island-arc setting (Cawood & Flood, 1989; Aitchison & Flood, 1993). The presence of both felsic and basic lavas within the succession suggests the Gamilaroi underwent rifting either in a fore-arc, intraoceanic arc or back arc setting during the Middle to Late Devonian (Aitchison, 1993). Recent ODP results indicate sedimentation rates in modern island arc environments are typically high (Fujioka et al. 1990; Berger, 1976). It is likely that the sedimentary rocks observed within the Gamilaroi terrane also accumulated rapidly as indicated by the presence of high density mass-flow deposits and thick sequences of volcanigenicsediments and felsic tuffs
intercalated with olistostromal deposits of limestone. Previous and existing models interpret the rocks of the Tamworth Belt (Gamilaroi terrane) as forming in a long-lived Cambrian-Permian forearc basin (Leitch, 1975; Cawood, 1983; Murray et al. 1987) as a result of westward dipping subduction throughout the Paleozoic. In the light of further geochemical data and revised dating, Aitchison et al. (1992) and Aitchison and Flood (1993) suggest that the oceanic crust between the Gamilaroi terrane and Gondwana was subducted eastward under the western margin of the Gamilaroi terrane arc. This system would facilitate the accretion and preservation of the Gamilaroi terrane into an upper plate position. Such interpretations warrant further investigations as to the age and nature of Silurian-Devonian
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NEO 2010 intrusives and related Cu/Au mineralisation associated with island-arc terranes (Gamilaroi terrane) along the entire New England Orogen.
Fig. 6 A) Brecciated quartz vein associated with Cu mineralisation and B) malachite staining on the surface of intrusive trondhjemite at the junction of the Barnard and Back Rivers.
References Aitchison, J.C. 1993. Evolution of the eastern margin of Australian plate: possible correlatives in Australia, New Caledonia and New Zealand. In Flood, P.G. & Aitchison, J.C. (eds) New England Orogen, eastern Australia. The University of New England, Armidale, pp 665-669. Aitchison, J.C., and Flood, P.G., 1995, Gamilaroi Terrane a rifted Devonian intra-oceanic island arc assemblage, NSW, Australia, in Smellie, J., ed., Volcanism associated with extension at consuming plate margins, Geological Society of London Special Publication 81, 155-168. Aitchison, J.C., Flood, P.G., & Spiller, F.C.P. 1992. Tectonic setting and paleoenvironment of terranes in the southern New England orogen, eastern Australia as constrained by radiolarian biostratigraphy. Palaeogeography, Palaeoclimatology, Palaeoecology, 94, 31-54. Aitchison, J.C., Ireland, T.R., Blake M.C. Jr & Flood, P.G. 1992. 530Ma zircon age for ophiolite from the New England orogen: oldest known rocks from eastern Australia. Geology, 20, 125-128. Aitchison, J. C., & Ireland, T.R., 1995. Age profile of ophiolitic rocks across the Late Paleozoic New England orogen, New South Wales, Australia: Implications for tectonic models. Australian Journal of Earth Sciences, 42: 11-23 Berger W. H. 1976. Biogenic deep sea sediments: Production, preservation and interpretation. In: Riley. J.P. and Chester, R. (eds.) Chemical Oceanography, Academic Press, London, NY, San Fransisco, pp. 266-388. Buckman, S. 1993. The geology north of the Barnard River at Barry Station: Evidence of Early Permian strike-slip faulting and basin development. Unpubl. Honours thesis, University of Sydney. Cawood, P.A. 1983. Modal compositions and detrital clinopyroxene geochemistry of lithic sandstones from the New England Fold Belt, east Australia: a Paleozoic forearc terrane. Geological Society of America Bulletin, 94, 1199-1214. Cawood, P.A. & Flood, R.H. 1989. Geochemical character and tectonic significance of Early Devonian Keratophyres in the New England Fold Belt, eastern Australia. Australian Journal of Earth Sciences, 36, 297-311. Crook, K.A.W. 1961a. Stratigraphy of the Tamworth Group (Lower and Middle Devonian), TamworthNundle District, N.S.W. Journal and Proceedings of the Royal Society of N.S.W. 94, 173-188. Crook, K.A.W. 1961b. Post-Carboniferous stratigraphy of the Tamworth-Nundle district, New South Wales. Journal and Proceedings of the Royal Society of N.S.W., 94, 209-213.
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NEO 2010 Cross, K.C. 1983. The Pigna Barney ophiolitec complex and associated basaltic rocks, northeastern New South Wales, Australia. unpubl. Ph.D. Thesis, University of New England, Armidale, Australia. Flood, P.G. & Aitchison, J.C., 1988. Tectonostratigraphic terranes of the southern part of the New England Orogen, eastern Australia. In, Kleeman J.D. ed. New England Orogen: Tectonics and Metallogenesis, University of New England, Armidale, Australia, pp 7-10. Flood, P.G. & Aitchison, J.C. 1992. Late Devonian accretion of the Gamilaroi terrane to Gondwana: provenance linkage provided by quartzite clasts in the overlap sequence. Australian Journal of Earth Sciences, 39, 539-534. Fujioka, K., Taylor, B., Janecek, T.R., Aitchison, J. C., et al., 1992. Proceedings of the Ocean Drilling Program, Science Reports, 126: College Station, TX (Ocean Drilling Program) 709 pp. Korsch, R.J. 1977. A framework for the Paleozoic geology of the southern part of the New England Geosyncline. Journal of the Geological Society of Australia, 23, 339-355. Kimbrough D.L., Cross, K.C. & Korsch, R.J. 1993. U-Pb isotopic ages for zircons from the Pola Fogal and Nundle granite suites, southern New England Orogen. In Flood P.G. and Aitchison J.C. (eds.) New England Orogen, eastern Australia, pp. 403-412. University of New England, Armidale. Leitch, E.C. 1975. Plate tectonic interpretations of the Paleozoic history of the New England Fold Belt. Geological Society of America Bulletin, 86, 141-144. Morris, P.A. 1988. Petrogenesis of fore-arc metabasites from the Paleozoic of New England, eastern Australia. Mineralogy and Petrology, 38, 1-16. Murray, C.G., Fergusson, C.L., Flood, P.G., Whitaker, W.G. & Korsch, R.J. 1987. Plate tectonic model for the Carboniferous evolution of the New England Fold Belt. Australian Journal of Earth Sciences, 34. 213236. Offler, R. 1982. Geochemistry and tectonic setting of igneous rocks in the Glenrock Station area, N.S.W. Journal of the Geological Society of Australia, 29, 443-445. Vallance, T.G. 1969. Recognition of specific magmatic character in some Palaeozoic mafic lavas in New South Wales.Geological Society of Australia Special Publication, 2, 163-167.
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Silica-carbonate (listwanites) related gold mineralisation associated with epithermal alteration of serpentinite bodies Solomon Buckman1 and Paul Ashley2 1
School of Earth and Environmental Sciences, University of Wollongong, Australia 2
School of Earth Sciences, University of New England, Australia
Keywords: serpentinite, listwanite, silica-carbonate, carbonatization, epithermal, gold
Introduction The New England Orogen is host to the Great Serpentinite Belt, which occurs along the Peel-Manning Fault System and generally divides rocks of the Tamworth Belt (Gamilaroi terrane) to the west from the accretionary complex rocks of the Djungati terrane (Woolomin Beds) to the east. The serpentinite melange contains exotic blocks of ophiolitic material of Early Cambrian age and HPLT blueschist/eclogite facies metamorphic rocks of Ordovician age (Aitchison and Ireland, 1995). However, there is no evidence of the emplacement of serpentinite melange until the Early Permian as evident by the presence of serpentinite detritus in the Manning Group diamictites. These sediments were deposited contemporaneously with serpentinite emplacement within localised extensional settings associated with strike-slip faulting along the Peel-Manning Fault System (Aitchison et al., 1997). The subsequent intrusion of granite batholiths from the Early Permian to Triassic were the heat engines that drove hydrothermal cells responsible for widespread epithermal mineralisation in the New England Orogen which resulted in the carbonatization of serpentinite protoliths to form silica-carbonate alteration zones or “listwanites”. Many of these listwanite bodies contain gold mineralisation in the New England and the telescoped natures of listwanite-related mineralization in other regions suggest that this style of alteration presents a significant exploration target. This paper is a review of the silica-carbonate (listwanite) alteration process and how it relates to serpentinite bodies in the New England Orogen. The term "listwanite" (alternate spellings - listwaenite, listvaenite or listvanite) was first introduced by Rose (1837) to describe the silica-carbonate alteration of serpentinite in the Urals. Since then the term "listwanite" has remained in use in Eurasia, whilst in America, Canada and Australia, the self-descriptive term "silicacarbonate" has become more prevalent. The terms "listwanite" and "silica-carbonate" are synonymous and encompass all forms of carbonatization from the carbonate-rich to silica-rich phases. Listwanites are host to world-class gold deposits, such as McLaughlin’s Mine in California, but have received little exploration attention in Australia. Listwanites are relatively abundant around the margins of the Great Serpentinite Belt (ophiolitic Weraerai terrane) and may represent a potentially new and rewarding exploration target. The alteration develops along faults that intersect bodies of serpentinized ultramafic rocks and is locally host to sub-economic grades of gold at Nundle, Barraba and Bingara. This paper reviews silica-carbonate alteration processes and the behaviour of gold in listwanites of the New England. The alteration of serpentinized ultramafic rocks to silica-carbonates is caused by the migration of carbon dioxide-rich fluids along faults, and is sometimes referred to as “carbonatization”. Silica-carbonate alteration is commonly observed around the margins of dismembered ophiolite sequences and has been reported by a number of authors worldwide, notably because it is often host to gold, mercury, magnesite and occasionally, base metal mineralisation (Ash and Arksey, 1990a; 1990b; Ashley, 1997; Ashley and Hartshorn, 1988; Auclair et al., 1993; Barnes et al., 1973; Bohlke, 1989; Buisson and Leblanc, 1985; 1986; Efremov, 1952; Knopf, 1906; Madu et al., 1990; Ploshko and Bogdanova, 1963; Pohl, 1990; Sherlock and Logan, 1995; White, 1967).
95
NEO 2010 The formation of silica-carbonate rocks requires a precursor body of serpentinite before this particular style of alteration can evolve. However, the silica-carbonate alteration can migrate into adjacent nonserpentinized/ultramafic rocks. Therefore, it is important to understand a little about the initial serpentinization process. Serpentinization occurs as a result of the hydration of ultramafic rocks, in particular the magnesium silicates such as olivine, pyroxene and amphibole (Klein and Hurlbut, 1985). The reaction can follow different paths depending on the quantity and composition of the fluids and host rock, as shown in equations 1-4. A thorough review of the processes involved in serpentinization has been published by (O'Hanley, 1996).
Serpentinization The serpentinization process commonly occurs in supra-subduction zones, where wet oceanic crust is being subducted beneath island-arc material. As the oceanic slab descends the pressure and temperature increases resulting in dehydration of the down-going lithologies. The fluids released by dehydration interact with the surrounding ultramafic, mafic and pelagic sediments and the trapped mantle wedge of the overriding plate contains ultramafic rocks that can be readily altered to serpentinite. The serpentinization process is exothermic and can produce temperatures of up to 300C (Moody, 1976; Wenner, 1972; Wenner and Taylor, 1971; 1973; 1974). The reaction also results in a substantial increase in volume, which can force the ductile serpentinite into faults and fractures where it develops its characteristic schistose fabric. Serpentinite is found extruding out of mud-type volcanoes within present day fore-arc regions, such as the Marianas region (Fryer, 1985, 1992; Fryer and Fryer, 1987; Fryer and Mottl, 1992; Fryer et al., 1999). Serpentinite seamounts or “mud volcanoes” occur 50-120 km from the trench axis in the 200 km wide Mariana forearc. The serpentinite seamounts are interpreted as diapirs and consist of fragments of serpentinite in a serpentine mud composed of serpentine, chlorite, clay and carbonate (Fryer and Mottl, 1992; Lagabrielle et al., 1992).
Silica-carbonates (Listwanites) Listwanites form as a result of the chemical reaction between serpentinite and CO2-rich fluids. These fluids usually migrate along faults or fractures along the contact of serpentinite and the adjacent country rocks. Freshly broken listwanites have a green-orange colour due to the presence of fuchsite and ferro-magnesium carbonates respectively. The weathered surface of listwanites usually has a gossanous boxwork texture and a brown-red colour due to the preferential breakdown of the ferro-magnesium carbonates (Fig. 1). Silica-carbonates, as their name suggests are composed of varying proportions of silica - in the form of quartz or chalcedony, and carbonates - in the form of magnesite (with varying proportions of Fe and Ca). Other, less abundant minerals, commonly found in silica-carbonates include chlorite, fuchsite (Cr-rich mica), talc, fluorite, residual serpentine and chromite, and sulfides such as pyrite, chalcopyrite and arsenopyrite. Studies by Barnes et al (1973) indicate that silica-carbonate rocks from the Coast Ranges, California, are commonly supersaturated with magnesite, dolomite and sometimes siderite and ankerite. All of the samples are undersaturated in brucite, diopside, pyroxene and serpentine (chrysotile) which is the major precursor mineral. The persistence of serpentine in some silica-carbonates is probably an indication that the volume of water required to remove all of the serpentine was not sufficient. The alteration of serpentinite in the presence of silica-rich hydrothermal fluids might be expected to form talc as in reaction 4. However, most silica-carbonates are undersaturated in both talc and tremolite (phases which are richer in silica than serpentine) despite the common occurrence of opaline silica in the silica-carbonates. This is because talc is not a stable mineral at the low temperatures of the springs providing the fluids. Instead the H+ produced by the partially dissolved CO2 gas (reaction 6) is removed by the reaction with serpentine (reaction 8), releasing Mg2+ and SiO42- into solution. The chemical reaction involved in the early stages of silica-carbonate alteration can be summarized by reaction 4 (Sherlock and Logan, 1995). Tuysuz and Erler (1993) undertook detailed geochemical studies of silica-carbonates in the Kagizman region, NE-Turkey, and divided the silica-carbonates into two categories, Phase 1 and 2, based on the degree of alteration: Phase 1 silica-carbonates are much more widespread and partly envelop phase 2 silica-carbonates. Phase 1 silica-carbonates are carbonate-dominated and contain relict serpentinite fragments, while phase 2 silicacarbonates are rich in silica and often host to economic grades of gold. These two contrasting assemblages (Phase 1 and 2) have been noted by many authors describing silica-carbonates but with varying terminology. 96
NEO 2010 Phase 1 silica-carbonates are also referred to as “talc-carbonate schists", while phase 2 silica-carbonates have been termed “birbirite” (Auclair et al., 1993).
Fig. 1. A) The distinct orange listwanite alteration in outcrop formed on the margins of the Sartuohai ophiolitic melange in NW China and host to significant gold mineralisation locally (Buckman, 2000). B) Listwanite slab from Sartuohai showing early stage carbonate veining and a green fuchsite + silica matrix and late stage quartz/chalcedony veins in white (width of slab is 20 cm).
Phase 1 silica-carbonates Phase 1 silica-carbonates are composed of varying proportions of carbonates, principally dolomite, ankerite dolomite, and rarely siderite, calcite magnesite and serpentinite. According to Tuysuz and Erler (1993), generation of Phase 1 silica-carbonates begins where the value of MgO decreases to about 32% and they persist to MgO content of 15% while the silica concentrations can drop to as low as 5%. They calculated that the transition from serpentinite to Phase 1 listwanites involves an overall volume increase of 63% based on the assumption is immobile during the alteration process. This equates to a loss in MgO (~28%) and SiO2 (~36%). In contrast, there is an overall increase in the concentration of CaO (96%) and CO2 (95%) in the form of carbonate minerals. The mean trace element contents indicate phase 1 silica-carbonates are enriched relative to serpentinite in Ni (13%), Co (27%), Cu (78%), and As (87%), while Au decreases by about 29% from serpentinites to phase 1 silica-carbonates. The decrease in gold concentration highlights an important process because it identifies the Phase 1 listwanites as the alteration stage at which gold is freed from serpentinite and dissolved into solution. For economic gold deposits to form it is vital that this process of gold scavenging occur through a large volume of source rocks and that the gold-rich fluids are channelled into a relatively small zone in which pressure, temperature and chemical conditions favour the precipitation of gold from solution. Such conditions usually occur closer to the surface as a result of the interaction with meteoric water and decreases in temperature and pressure. These conditions also give rise to the formation of Phase 2 silica-carbonates.
Phase 2 silica-carbonates Phase 2 silica-carbonates consist predominantly of silica in the form of quartz, chalcedony and rarely opal. Carbonate minerals are either minor constituents or totally absent. Phase 2 silica-carbonates are characterised by the introduction of silica. The same two assumptions as above apply (Tuysuz and Erler, 1993), and based on a volume increase, MgO loss is 36% from phase 1 to phase 2 silica-carbonates and CaO loss is about 21%. There is a major increase in Au (52%), in the transition from phase 1 to phase 2 silica-carbonates with very little increase in Cu (7%) and As (3%).
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NEO 2010 Assuming a constant volume, phase 2 silica-carbonates show a loss in MgO (78%) and CaO (35%), but gains in SiO2 (30%), CO2 (94%), and K2O (98%). In the trace elements, there are gains in Au (21%), Cu (76%) and As (84%). Co and Ni contents of phase 2 silica-carbonates relative to serpentinite decrease by 48% and 66% respectively (Tuysuz and Erler, 1993). Chemical reactions involved in the serpentinization process (1-3) (O'Hanley, 1996) and the carbonatisation (listwanite) process (5-10). olivine
water
1)
2Mg2SiO4 +
3H2O
2)
3Mg2SiO4 +
4H2O + SiO2
olivine
orthopyroxene
serpentinite
3Mg2SiO4 +
MgSiO3 + H2O
Mg3Si2O5(OH)4
Serpentine +
silica talc
3)
4)
serpentinite
brucite
Mg3Si2O5(OH)4 +
Mg(OH)2
2Mg3Si2O5(OH)4
+
Mg3Si2O5(OH)4 + 2SiO2 Mg3Si4O10(OH)2 +
water H2O
Serpentinite + carbonic acid + silicic acid magnesite + water + silica 5)
Mg3Si2O5(OH)4 + 3H2CO3 (aq) + (n)H4SiO4 (aq) 3MgCO3 + (5+2n)H2O +(2+n)SiO2 Carbon dioxide + water carbonic acid +
H2O H+ + HCO3-
6)
CO2
7)
HCO3- H+ + CO32-
8)
6H+ + Mg3Si2O5(OH)4 3Mg2+ + 2H4SiO4 + H2O MgFe-carbonate + silicic acid silica + carbon dioxide + water + Mg, Fe ions
9)
MgFe(CO3)2 + 4H+ + SiO44- SiO2 + 2CO2 + 4H2O + Mg2+ + Fe2+
10)
nMg2+ + mFe2+ + 4HCO3- {n/(n+m)}Mg{m/(n+m)}Fe(CO3)2 + 2H2O + 2CO2
Fig. 2. A) A carbonate-rich listwanite from the Sartuohai 1 Gold Mine. The carbonates have pseudomorphed the original S-C fabric (dextral) of the schistose serpentinite. Small (grey-blue) patches of amorphous silica have started to develop and overprint the carbonates. B) A silica-rich listwanite (phase 2) in which chalcedony is replacing the earlier formed carbonates. This marks the transition from alkaline to acidic conditions and often corresponds with gold mineralisation.
98
NEO 2010 According to Barnes et al., (1973), if CO2 is present in the system as a gas, it controls the pH maintaining acidic conditions. Serpentinite will react to yield Mg2+, Fe2+ and SiO4- in solution (reaction 8). The silica released into solution as silicic-acid precipitates as amorphous silica at lower temperatures, which in time forms -cristobalite. Based on chemical potentials, magnesite, dolomite, and siderite or ankerite may be expected to precipitate from solution because the H+ is being consumed in the reaction with serpentinite (reaction 8) leaving the excess HCO3- to react with the Mg2+ and Fe2+ ions (reaction 10) to precipitate ferroan magnesite. The production of excess water and carbon dioxide in reaction 10 provides positive feedback to the carbonic acid reaction in reaction 6, thus accelerating and sustaining the process. Calcite only precipitates rarely because saturation with calcite is less common than saturation with magnesite or dolomite. The above reactions form part of an open system, in which, two processes are competing, the consumption of H+ by the reaction with serpentinite (reaction 8), and the supply of H+ by solution of CO2 (reaction 6) (Barnes et al., 1973). However, if the serpentinite reservoirs adjacent to the channels of fluid flow become exhausted before CO2 is diminished, the pH of the fluid will remain low due to the high PCO2, and because the H+ is no longer being consumed in the breakdown of serpentinite. There will no longer be a steady supply of Mg2+ because the breakdown of serpentinite is no longer occurring, which means the precipitation of carbonate minerals will also cease. This is point at which the Phase 1 listwanites begin the transformation to silica-rich, Phase 2 listwanites. The fluids become increasingly acidic, due to the high PCO2 and they are rich in dissolved silica from the initial breakdown of serpentinite (reaction 8). These acidic fluids begin to dissolve the existing carbonate assemblage and replace them with almost pure silica. In the process more CO2 is released (reaction 9) thereby further driving the carbonic acid reaction (reaction 6) and increasing the acidity of the fluids. The Mg2+ and Fe2+ released into solution by the breakdown of the carbonates are probably channelled along the fault and precipitated as carbonates in more suitable conditions (higher pH and temperature). This may explain why small listwanite zones can be found along faults that extend into surrounding country rock with no initial serpentinite protolith. In contrast to reaction 9, the CO2 flux in the system may be insufficient to maintain free CO2 gas throughout the flow path due to; 1) a low CO2 flow from the source, 2) a long reaction path (serpentine only), or 3) the presence of brucite, pyroxene or olivine (Barnes et al., 1973). The presence of serpentine alone yields pH values of more than 8 (Barnes and O'Neil, 1969). Brucite yields even higher pHs and pyroxenes and olivine yield the highest pHs of all. As the pH increases, so does the CO32- activity (reaction 7). The result is the abundant deposition of carbonate minerals (chiefly magnesite because Mg2+ is much more abundant in serpentine than Ca2+ or Fe2+). If the high PCO2 fluids react with pyroxene or olivine, the pH values may exceed 11 and the silica yielded by hydrolysis will be largely ionised as SiO42-. Mg2+ will be immobile with very high CO32- activities and magnesite would be expected to form with very little or no silica. This process probably explains the origin of many silica-carbonate hosted magnesite deposits (Barnes et al., 1973). The silica-carbonate alteration process is largely dependent on the outcome of intermediate reactions and the rate at which they occur. Of most importance is the total concentration of carbon dioxide within the system as this affects the pH of the circulating fluids. High pH favours the precipitation of carbonates while low pH favours the precipitation of silica (Barnes et al., 1973). Carbonate minerals form under high temperatures (>100C) and high pH (>8) but they can be easily redissolved by low-temperature and weakly acidic solutions. The opposite is true for silica, as it is more readily ionised in hot basic solutions but precipitates as amorphous silica or quartz veins at low temperatures and pH. Most hydrothermal or metamorphic events naturally progress from hot to cold as the heat source diminishes. Thus the precipitation of silica usually marks the final stages of silica-carbonate alteration and cross-cuts all previous mineral assemblages (Fig. 1). The stability of silica, once precipitated, is high enough that it can be regarded as an irreversible reaction at the relatively low temperatures associated with silica-carbonate alteration. The carbonates on the other hand can are readily dissolved and re-precipitated so there may be several generations of carbonates within a single listwanite. This is the reason why most listwanites evolve along a linear path of silica enrichment and carbonate diminishment, after the initial formation of the Phase 1 listwanites. This process often results in the formation of a silica core surrounded by the intermediate ferromagnesium carbonates. The hydrothermal system involved in silica-carbonate alteration is open to two components, water and carbon dioxide. Based on isotopic data (O18 and C13), Barnes et al., (1973) favour the interpretation of locally derived meteoric waters mixing with metamorphic derived CO2. The extent of the alteration depends on the influx of gas and water from their respective sources. Stable isotope data also indicate that temperature plays a significant role in determining the composition of the listwanite. The isotopic compositions of silica-rich cores of listwanites (Auclair et al., 1993; Sherlock et al., 1993) show that they 99
NEO 2010 formed at low temperatures while the surrounding carbonate assemblage formed at much higher temperatures.
Gold in listwanites Based on geochemical studies of silica-carbonates from ophiolite complexes in Liguria, Morocco and Saudi Arabia, (Buisson and Leblanc, 1987) concluded that silica-carbonates are similar in style to Archean carbonatized ultramafic rocks with economic grades of gold being reached in sulfide-rich zones and late quartz veins. Ultramafic rocks (upper mantle peridotites) are believed to be the main source of gold (Pipino, 1980), which is contained within the accessory opaque minerals (sulfides, chromite, and magnetite). Gold contents in serpentinized mantle peridotite range between 3 and 5 ppb. Large-scale hydrothermal systems operating during the late stages of tectonic emplacement leach gold from the opaque minerals and transport it in a CO2-S-As-Cl-Na-K-B-rich solution as thio-arsenic complexes. The hydrothermal fluids are focussed along tectonic contacts. As the system evolves the acid gold-bearing solutions precipitate silica-pyritearsenides and gold when entering into the reducing and alkaline carbonatized rocks. Silica-carbonates are on average 5 – 20 times more enriched in gold (average 0.02 – 0.1 ppm Au) than the surrounding ultramafic rocks (between 0.001-0.010 ppm) (Buisson and Leblanc, 1987). Economic grades of gold are usually associated with sulfides, sulfarsenides or arsenides (Buisson and Leblanc, 1985; 1986; 1987; Leblanc, 1986; Leblanc and Billaud, 1982). The gold is usually bound to the sulfides, with pyrite grading between 10 and 50 ppm, and Co-arsenides between 10 and 100 ppm. Small gold grains are observed around liminitized pyrite grains as a result of weathering and oxidation. Trace element analyses show strong positive correlations between Au and As, and quite commonly K. Elements such as Ba, Sb, B, Bi, Ag, and Cu may also be associated with high gold values. Buisson and Leblanc (1987) concluded that during the serpentinization process half the gold is concentrated into magnetite and secondary sulfides. These opaque minerals are subsequently destroyed in talc-carbonate alteration zones (Phase 1 listwanites), which results in gold being released from the serpentinite wall rocks and transported in solution. The gold- and silica-rich fluids are transported to higher crustal levels where the change in conditions (lower temperature, pH and O2) results in the precipitation of gold, quartz and sulfides.
Mercury in listwanites Silica-carbonate mercury deposits have been widely reported from the California region and account for 85% of mercury production in the United States (Davis, 1966; Sherlock and Logan, 1995). These mercury deposits generally have low gold concentrations (< 5 ppb) however, there have been rare occurrences of elevated gold concentrations (Vredenburgh, 1981), most notably in zones associated with petroliferous chalcedonic veins (Peabody and Einaudi, 1992; Sherlock and Logan, 1995). Three styles of mercury deposit occur in northern California: (1) Those associated with the Geysers Geothermal System, such as the Culver Baer deposit (Peabody and Einaudi, 1992), composed of magnesite-chalcedony-cinnabar veins hosted within silica-carbonate altered serpentinite. (2) Fractured silica-carbonate containing veins of cinnabar and no other gaunge minerals, and: (3) a less common style of deposit containing disseminated cinnabar in limonitic altered greywacke.
Platinum in listwanites Fischer et al., (1987) touched on the prospect of PGE mineralisation associated with serpentinites and carbonatized ophiolitic rocks at Bou-Azzer, Central Anti-Atlas. Platinum, palladium and gold are believed to be concentrated as Ni-Fe sulfides or arsenides along with chromite in small residual basaltic magma chambers. The process of serpentinization destroys the ultramafic mineral assemblage resulting in the formation of lizardite, chrysotile and magnetite. The process induces a decrease in oxygen fugacity and allowing PGE's and gold to be concentrated in magnetite. The serpentinization process does not seem to affect the overall PGE concentration (Oshin and Crocket, 1982) so the magnetite PGE patterns are representative of PGE distribution in the primary surrounding host-rocks. Gold is often found in much lower concentrations in magnetite compared to the PGE’s. During subsequent silica-carbonate alteration of serpentinite the oxides and sulfides are destroyed by acidic hydrothermal solutions (Buisson and Leblanc, 1987) and PGE's and gold are transported in solution as Cl, S or As complexes (Bischoff and Seyfried, 1978). Gold will precipitate from this solution at low temperatures in a reducing environment but the behaviour of PGE's in this process is not well understood. As PGE concentrations in magnetite of the 100
NEO 2010 original serpentinite are greater than gold, it would be expected that PGE concentrations would also be high in the sulfides and arsenides found in most listwanite gold deposits. However, analytical data show that PGE contents are generally low and do not correlate with gold. It appears that the PGE were less mobile than Au during this type of hydrothermal alteration. This is accounted for, as the solubility of Pt and Pd as Clcomplexes is low between temperatures of 25C and 300C. In addition, platinum readily forms arsenides and gold does not, indicating that the amount of platinum that can be transported in As-rich fluids is very low compared to gold. PGE’s probably occur as discrete pockets of mineralization, such as sperrylite (PtAs2) as described by (Hudson and Donaldson, 1984) at Kambalda. There, the PGE mineralization is interpreted as a product of low temperature hydrothermal activity (300C). As yet there is no satisfactory explanation as to what happens to the PGE's when magnetite is destroyed in the carbonatization process. Maybe platinum is precipitated at much deeper levels within a fault-system where temperatures are much higher (300C+).
Fig. 3. Plots of the major elements vs. silica content for listwanite rocks of Tangbale, Sartuohai and the New England Orogen. In the case of MgO extra data has been added from analyses taken from Northern California - McLaughlin, Knoxville, Soda Springs (Sherlock and Logan, 1995), and from Turkey (Tuysuz and Erler, 1993). 101
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Fig. 4. A scatter-graph of Au verse SiO2/(CaO + MgO) for silica-carbonates from different locations. An exponential curve of best fit has been applied to each location and an R2 statistical value for each curve is stated. Early phase listwanites are carbonate-rich and silica-poor (93000 islands and seamounts in the Pacific Ocean (Hillier and Watts, 2007). Most oceanic intraplate volcanoes are a product of hotspot volcanism or mantle plumes, which bring hot material from the Transition Zone, lower mantle or core-mantle boundary to melting conditions at the base of the lithosphere, and produces long chains of volcanoes that become progressively older as a function of distance from the active volcanism locus (Koppers et al., 2010a; Staudigel and Clague, 2010). Transport of the islands away from the hotspot, cessation of volcanism, thermal subsidence and flexure of the lithosphere lead to submersion of volcanic edifices (commonly 160 seamounts have subducted or accreted on average in the past 475 Ma along each 1 km-long segment of the Pacific/Panthalassa margin. As a consequence, seamount subduction/accretion may represent an unrecognized major process during the long-term evolution of convergent margins. A major limitation of our understanding of the formation and evolution of oceanic islands is related to restriction of our observations (and sampling) to superficial layers of volcanic edifices. Deep drill cores are rare and, despite the island of Hawaii is considered as the archetypical example of an oceanic island, penetration of “only” 3.1 km or ~30 % of the total height of the volcanic edifice has been achieved by drilling (e.g. Garcia et al., 2007). Hawaiian islands are composed of >95% tholeiitic igneous rocks, with minor emplacements of alkalic igneous rocks in the earliest and latest stages of volcanic evolution (Clague and Dalrymple, 1987). However, many oceanic islands are in apparent disagreement with the textbook model of Hawaiian volcanoes. In particular, intraplate volcanoes formed in the South Pacific experienced repeated events of subsidence and uplift, as well as rejuvenated events of alkalic magmatism several tens of Ma after initial cessation of volcanism (e.g. Koppers et al., 2003). South Pacific Islands have apparently a more alkalic composition than Hawaiian Islands, but it is unclear if this is related to a true compositional distinction or a sampling bias during superficial dredging and on-land field studies. Clearly, there is a strong need to access deeper levels of oceanic islands (Koppers et al., 2010b). The New England Orogen exposes unusually extended accreted Paleozoic sequences (several hundred of km wide) (e.g. Coney et al., 1990; Aitchison et al., 1999) and, thus, can be regarded as a unique example of a large accretionary complex preserved along an ancient convergent margin. Ordovician to middle Carboniferous accreted oceanic islands preserved under low metamorphic conditions occur at least at 10 localities (Fig. 1). The accreted oceanic islands have been recognized based on the occurrence of mafic igneous rocks with ocean island basalt (OIB)-like geochemical affinities, and their association in the field with pure shallow-water limestones that formed in a near-equatorial latitude (Flood and Aitchison, 1994; Flood et al., 1994; Flood and Aitchison, 1995; Wilkie, 1996; Flood, 1999b). We show here that: (1) accretionary complexes in eastern Australia offer a remarkable access to some ancient oceanic islands; and (2), occurrences of accreted oceanic islands can be efficiently recognized and studied through a combination of tectonostratigraphic observations and geochemical analyses. We propose access to oceanic islands in accretionary complexes can provide a new insight into the mode of formation of oceanic islands and their possible role during the long-term evolution of convergent margins. Recognizing accreted oceanic islands in the New England Orogen Recognition of ancient oceanic islands in accretionary complexes of the New England Orogen is achievable through a combination of field observations and geochemical analyses. Volcanic oceanic islands have a specific lithologic assemblage that includes submarine/subaerial mafic lava flows and related intrusives, volcaniclastic slope deposits (e.g. basaltic breccias), shallow-water limestones, and hemipelagic-pelagic cover sediments (Staudigel and Clague, 2010) (Fig. 2). This lithologic assemblage is not encountered in the “normal” oceanic crust, continental basement, volcanic arcs or ophiolites, which typically contain extended pelagic sequences, siliciclastic desposits, pyroclastic deposits and/or mantle-related ultramafic rocks. Geochemistry of igneous rocks from oceanic islands or “OIB” is generally dissimilar from that of igneous rocks in other volcanic systems. Notably, most OIB have an enriched pattern on a Primitive Mantlenormalized multielementary diagram (Fig. 3). This pattern is clearly distinct from that of the bulk of the MidOcean Ridge Basalts (MORB) (Fig. 3A). Enriched MORB are similar to OIB in terms of trace element contents but have distinct differentiation trends. Trace elements patterns of supra-subduction igneous rocks are characterized by Nb-Ta and Ti negative anomalies and U, Pb and Sr positive anomalies that are generally not observed in OIB (Fig. 3B). In eastern Australia, igneous rocks produced by intraplate volcanism in the Cenozoic have trace element and radiogenic isotope affinities that support an OIB-type source as the dominant component for most of these rocks (O’Reilly and Zhang, 1995). These rocks have a Zr-Hf negative anomaly on a Primitive Mantle-normalized multi-elementary diagram (Fig. 3C), which has been interpreted to reflect the occurrence of amphibole in the sub-continental mantle (O’Reilly and Zhang, 1995). This Zr-Hf 108
NEO 2010 negative anomaly does not appear in “true” OIB and can be used to distinguish between accreted OIB and continental intraplate igneous rocks with OIB-like signatures. New field observations and geochemical analyses of accreted oceanic islands have been made following preceding approach at Ashford Caves (~29°12.044’S / 150°58.861’E), Riverton Limestone Quarry (~S29°1.336’S / 151°28.332’E) and The Pinnacle Quarry (29°0.509’S / 151°21.367’). Each site provides a distinct insight into the formation and evolution of oceanic islands preserved in the New England Orogen.
Fig. 2: Field examples of accreted oceanic islands in the New England Orogen. (A) shallow-water deposit composed of very well-rounded fragments of vesiculated basalts embedded in a carbonate matrix (Ashford Caves, lens cap is ~4cm). This sediment occurrence likely records emergence of an oceanic island. (B) ~1.5 m-thick basaltic dyke crosscutting an atoll-related shallow-water limestone (Riverton Limestone Quarry). Insert shows recrystallization of the limestone at the contact with the dyke. (C-D) Accreted olistostromes interpreted to have form through dismemberment of incoming guyots in the trench (The Pinnacle, Tenterfield). (C) “The Pinnacle” (arrow) is a large block of shallow-water limestone (olistolith) outlined in the landscape by weathering process. (D) Nearby limestone quarry that exposes another limestone olistolith embedded in accreted silicilastic sediments (height of the wall is ~7 m).
Results As previously shown by Flood and Aitchison (1995) and Wilkie (1996) typical oceanic island(s) sequences are exposed at Ashford Caves, with, notably, vesiculated lava flows, subvolcanic-intrusives, and Carboniferous shallow-water limestones. An ironstone layer occurs in association to shallow-water limestones and has been interpreted based on its geochemical composition as a hydrothermal product originally formed in a seamount (Wilkie, 1996). Although contact metamorphism by Permian arc-related intrusives locally occurs, accreted sequences generally remained under low metamorphic conditions and are remarkably-well preserved in comparison to OIB occurrences in other ancient accretionary complexes (e.g. Doubleday et al., 1994; Xenophontos and Osazawa, 2004; Timpa et al., 2005; John et al., 2009). In addition, we observed sandy to pebbly shallow-water deposits composed of very well-rounded fragments of vesiculated basalts in a limestone matrix, which we interpret as the record of the emergence of a volcanic island (Fig. 2A). Our preliminary geochemical results show that 2 tholeiitic basalts, 1 alkali basalt and 1 109
NEO 2010 trachyandesite have typical OIB-like signatures, with limited or no alteration effects (Fig. 3). Concentrations of the most incompatible elements (Fig. 3), and Rb-Sr and Sm-Nd radiogenic isotope composition of another sample (Flood and Aitchison, 1995) suggest a HIMU source for the OIB.
Fig. 3: Primitive Mantle-normalized multielementary diagrams illustrating the composition of 6 igneous rocks from accreted oceanic islands in the New England Orogen (whole rock samples, LA-ICP-MS analyses carried out on glass disks at the ANU). Typical compositional ranges of igneous rocks from distinct tectonic settings are illustrated for comparison: (A) global MORB dataset (Jenner and O’Neill, pers. com. 2010); (B) Mariana Arc (Central Island Province) as a typical example of a tholeiitic intra-oceanic arc with enriched signatures (compiled from the GEOROC online database); (C) Cenozoic intraplate lavas in eastern Australia (O'Reilly and Zhang, 1995); and (D-E), OIB endmembers (EM1, EM2 and HIMU, expressed as ±1σ of the average of the analyses compiled by Willbold and Stracke, 2006). Retentivity index shows the relative efficiency of elements to remain immobile in a fluid-rock system (based on mobility indexes in Kogiso et al. (1997), plotted following the procedure given in Willbold and Stracke (2006)). At Riverton Limestone Quarry, atoll-related limestones locally contain dykes of alkali basalts (Fig. 2B). 2 dykes have been analyzed, which display both OIB and supra-subduction-like geochemical signatures. Association of these dykes with pure, shallow-water limestones supports an oceanic, intraplate origin of the dykes, and suggests existence of a late stage, post-atoll volcanic activity in the oceanic island. As previously suggested for some Samoan basalts (Jackson et al., 2007), supra-subduction-like geochemical signatures in one of the dykes is likely due to occurrence of a recycled sediment component in the OIB source. At The Pinnacle Limestone Quarry, large blocks of pure shallow-water limestones are embedded within siliciclastic sequences of the accretionary complex (Fig. 2C-D). We interpret the blocks as olistoliths formed during partial collapse of guyots in the vicinity of the trench as a response to flexural bulging of the subducting lithosphere (e.g. Sano and Kanmera, 1991). 110
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Conclusions Our results show a combination of field observations and geochemical analyses allow the recognition of accreted oceanic islands in the New England Orogen. The studied accreted oceanic islands include submarine and near-shore deposits, as well as volcanic intrusives. Our preliminary results suggest existence of a post-atoll volcanic activity in some islands, which to our knowledge has not been observed in the ocean so far. Despite some occurrences of accreted oceanic islands may represent mass-wasting products in the vicinity of the trench (e.g. The Pinnacle Quarry), occurrences of more extended sequences (e.g. Ashford Caves) point toward dismemberment of seamounts during subduction. Such occurrences have the potential to provide an access to the roots of intraplate oceanic volcanoes, which is needed to better understand the formation of intraplate oceanic volcanoes and associated mineralization processes. Studying the distribution and mode of preservation of the islands in the New England Orogen as well as the development patterns of their hosting accretionary complexes is expected to provide valuable constraints on long-term effects of seamount subduction/accretion along the Gondwana Margin.
References Aitchison, J.C., Flood, P.G., Stratford, J.M.C. and Davis, A.M., 1999. Conceptual advances in understanding the New England Orogen. In: Flood, P.G. (ed.), Regional geology, tectonics and metallogenesis: New England orogen: papers presented at a conference held at The University of New England, Armidale, 1-11. Buchs, D.M., Baumgartner, P.O., Baumgartner-Mora, C., Bandini, A.N., Jackett, S.-J., Diserens, M.-O. and Stucki, J., 2009. Late Cretaceous to Miocene Seamount Accretion and Mélange Formation in the Osa and Burica Peninsulas (Southern Costa Rica): Episodic Growth of a Convergent Margin. In: James, K. Lorente, M.A. and Pindell, J. (eds), The Origin and Evolution of the Caribbean Plate. Geological Society of London, Special Publication, 328, 411-456. Clague, D.A. and Dalrymple, G.B., 1987. The Hawaiian-Emperor volcanic chain, Part I, Geologic evolution. In: Decker, R.W., Wright, T.L. and Stauffer, P.H. (eds), Volcanism in Hawaii. U.S. Geological Survey Professional Paper. U.S. Government Printing Office, 5-54. Coney, P.J., Edwards, A., Hine, R., Morrison, F. and Windrim, D., 1990. The regional tectonics of the Tasman orogenic system, eastern Australia. Journal of Structural Geology, 12, 519-543. Dominguez, S., Lallemand, S.E., Malavieille, J. and von Huene, R., 1998. Upper plate deformation associated with seamount subduction. Tectonophysics, 293, 207-224. Doubleday, P.A., Leat, P.T., Alabaster, T., Nell, P.A.R. and Tranter, T.H., 1994. Allochthonous Oceanic Basalts within the Mesozoic Accretionary Complex of Alexander Island, Antarctica - Remnants of ProtoPacific Oceanic-Crust. Journal of the Geological Society, 151, 65-78. Flood, P.G., 1999a. Development of northwest Pacific guyots: General results from Ocean Drilling Program legs 143 and 144. Island Arc, 8, 92-98. Flood, P.G., 1999b. Exotic seamounts within Gandwanan accretionary complexes, Eastern Australia. In: Flood, P.G. (ed.), Regional geology, tectonics and metallogenesis: New England orogen: papers presented at a conference held at The University of New England, Armidale, 23-29. Flood, P.G., 2001. The 'Darwin Point' of Pacific Ocean atolls and guyots: a reappraisal. Palaeogeography Palaeoclimatology Palaeoecology, 175, 147-152. Flood, P.G. and Aitchison, J.C., 1994. Accreted Seamounts within Late Palaeozoic Accretionary Prism of New England Orogen, Eastern Australia. IGCP 321 Gondwana Dispersion and Asian Accretion, Fourth International Symposium, 28-29. Flood, P.G. and Aitchison, J.C., 1995. Accreted Seamounts within Late Palaeozoic Accretionary Prism of New England Orogen, Eastern Australia: Initial Results. In: Seccombe, P.K. and Ashley, P.M. (eds), Centre for Isotope Studies, Research Report 1993-1994. CSIRO, North Ryde, Sydney, 105-107. Flood, P.G., Aitchison, J.C. and Lenox, P.G., 1994. Limestone-capped seamounts within the Latc Palaeozoic accretionary prism of the New England Orogen, Eastern Australia. Geological Society of Australia Abstracts, 36, 110-111.
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NEO 2010 Garcia, M.O., Haskins, E.H., Stolper, E.M. and Baker, M., 2007. Stratigraphy of the Hawai'i Scientific Drilling Project core (HSDP2): Anatomy of a Hawaiian shield volcano. Geochemistry Geophysics Geosystems, 8, Q02G20. Hein, J.R., Conrad, T.A. and Staudigel, H., 2010. Seamount Mineral Deposits. Oceanography, 23, 184-189. Hillier, J.K. and Watts, A.B., 2007. Global distribution of seamounts from ship-track bathymetry data. Geophysical Research Letters, 34, L13304. Jackson, M.G., Hart, S.R., Koppers, A.A.P., Staudigel, H., Konter, J., Blusztajn, J., Kurz, M., and Russell, J.A., 2007. The return of subducted continental crust in Samoan lavas. Nature, 448, 684-687. John, T., Scherer, E., Schenk, V., Herms, P., Halama, R., and Garbe-Schönberg, D., 2009, Subducted seamounts in an eclogite-facies ophiolite sequence: the Andean Raspas Complex, SW Ecuador. Contributions to Mineralogy and Petrology, 159, 265-284. Kogiso, T., Tatsumi, Y. and Nakano, S., 1997. Trace element transport during dehydration processes in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts. Earth and Planetary Science Letters, 148, 193-205. Koppers, A.A.P., Staudigel, H., Pringle, M.S. and Wijbrans, J.R., 2003. Short-lived and discontinuous intraplate volcanism in the South Pacific: Hot spots or extensional volcanism? Geochemistry Geophysics Geosystems, 4, 1089. Koppers, A.A.P. and Watts, A.B., 2010a. Intraplate Seamounts as a Window into Deep Earth Processes. Oceanography, 23, 42-57. Koppers, A.A.P., Yamazaki, T. and Geldmacher, J., 2010b. Louisville Seamount Trail-Implications for geodynamic mantle flow models and the geochemical evolution of primary hotspots. Integrated Ocean Drilling Program, Expedition 330, Scientific Prospectus. O’Reilly, S. and Zhang, M., 1995. Geochemical characteristics of lava-field basalts from eastern Australia and inferred sources: Connections with the subcontinental lithospheric mantle? Contributions to Mineralogy and Petrology, 12, 148-170. Ranero, C.R. and von Huene, R., 2000. Subduction erosion along the Middle America convergent margin. Nature, 404, 748-755. Sano, H. and Kanmera, K., 1991. Collapse of ancient oceanic reef complex; what happened during collision of Akiyoshi reef complex? Sequence of collisional collapse and generation of collapse products. Journal of the Geological Society of Japan, 97, 631-644. Staudigel, H. and Clague, D.A., 2010. The Geological History of Deep-Sea Volcanoes. Oceanography, 23, 58-71. Staudigel, H., Koppers, A.A.P., Plank, T.A. and Hanan, B.B., 2010. Seamounts in the Subduction Factory. Oceanography, 23, 177-181. Timpa, S., Gillis, K.M. and Canil, D., 2005. Accretion-related metamorphism of the Metchosin Igneous Complex, southern Vancouver Island, British Columbia. Canadian Journal of Earth Sciences, 42, 14671479. Watts, A.B., Koppers, A.A.P. and Robinson, R.T., 2010. Seamount Subduction and Earthquakes. Oceanography, 23, 166-173. Wilkie, A.M.D.J., 1996. The Kwiambal Terrane - An Accreted Seamount in the Southern New England Orogen. BSc Hons. Thesis, University of New England, Armidale, 151 pp. Willbold, M. and Stracke, A., 2006. Trace element composition of mantle end-members: Implications for recycling of oceanic and upper and lower continental crust. Geochemistry Geophysics Geosystems, 7(4): Q04004. Xenophontos, C. and Osozawa, S., 2004. Travel time of accreted igneous assemblages in western Pacific orogenic belts and their associated sedimentary rocks. Tectonophysics, 393, 241-261.
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Geochemistry and isotope systematics of Carboniferous to Triassic felsic magmatism in northeastern Australia – putting the New England Orogen in its place D.C. Champion1, R.J. Bultitude2, P. L. Blevin3 1 2
Onshore Energy and Minerals Division, Geoscience Australia, Canberra, Australia
Geological Survey of Queensland, Department of Environment, Economic Development and Innovation, Brisbane, Australia 3
Geological Survey of NSW, Industry and Investment NSW, Maitland, Australia
Keywords: New England Orogen, Tasman Orogenic Zone, granites, geochemistry, isotopes, crustal growth
Introduction The New England Orogen (NEO) forms the easternmost part of Australian continent, and is one of several orogenic belts identified within the Tasman Orogenic Zone of eastern Australia. The NEO borders parts of the Lachlan, Thomson and North Queensland Orogens (Fig. 1), although contacts are largely obscured by the Sydney-Gunnedah-Bowen Basin system and other cover rocks. The NEO consists of a collage of terranes and has a complex history that extends from the Neoproterozoic to the Mesozoic (e.g., Murray, 2003; Glen, 2005), although most of the exposed rocks are Devonian or younger. Several tectonic models have been proposed for the orogen. Most invoke an accretionary origin related to a long-lived, convergent margin setting. By the Late Devonian, the NEO was part of mainland Australia and there is general agreement that the orogen acted as an Andean-type continental margin, from this time onwards, though varying between arc and backarc (e.g., Glen, 2005). A significant feature of the NEO in this convergent margin setting was voluminous Carboniferous to Triassic magmatism, the products of which form a major component of the orogen. Importantly, magmatism during this time was not confined to the NEO. Carboniferous to late Triassic felsic magmatism (ca. 350–220 Ma) (post-Kanimblan Orogeny to Hunter-Bowen Orogeny) affected a large part of the Tasman Orogenic Zone, extending in a broad belt from central New South Wales (Bathurst region) to Torres Strait, straddling the Lachlan, Thomson, New England and North Queensland Orogens (Fig. 1), as well as extending into the Proterozoic basement west of the Tasman Orogenic Zone in north Queensland (Fig. 1). The geochemical and isotopic characteristics of these magmatic rocks and their regional variations, therefore, have the potential to provide significant information regarding the nature and age of the crust in these orogens, as well as constrain the relationship between the development of the NEO and the neighbouring orogens. This is particularly true for the northern boundary of the NEO, where Proterozoic basement, and the Thomson and North Queensland Orogens are exposed in close proximity to the NEO.
NEO magmatism Although the tectonic environment of the NEO is interpreted to have switched from arc to backarc at ca. 305 Ma and back to arc some time about ca. 265 Ma (e.g., Glen, 2004), we largely follow Murray (2003) in subdividing the Carboniferous–Triassic NEO granites into two groups: 1) those of Early Carboniferous to Early Permian age (pre-Hunter-Bowen Orogeny), and 2) those of Late Permian to Triassic age (syn-HunterBowen Orogeny). As pointed out by Hensel et al. (1985), Bryant et al. (1997, 2003), Allen et al. (1998), Murray (2003), the NEO granites are dominated by medium- to high-K, I-type, largely granodioritic rocks. They have a broad compositional range (diorite to leucogranite), however, and are commonly intimately associated with mafic intrusives. Notably, Murray (2003) showed that the Carboniferous to Triassic granites of the northern NEO have remarkably similar major- and trace-element geochemistry, although, on average, 113
NEO 2010 they appear to become more silica-rich with decreasing age. The southern NEO differs in some respects from the northern NEO, for example, 1) pre-~305 Ma granites have not been identified in the southern NEO, 2) (Late Carboniferous?–) Early Permian S-type granites form a significant component of the southern NEO, and 3) the southern NEO contains relatively voluminous Late Permian–Early Triassic, high-K, variably fractionated, I-type granites (e.g., Bryant et al., 2003).
Fig. 1. Distribution of felsic magmatism in eastern Australia for: A) the Carboniferous to Permian and; B) Late Permian to Triassic periods. C) Orogens of the Tasman Orogenic Zone as used in this paper, following Glen (2005) and Champion et al. (2009). Regions shaded yellow in A) and B) show approximate outline of magmatism, pink corresponds to areas of sedimentation. Pink lines in A) and B) show orogen boundaries as defined in C). I, S, and A refer to types of granite magmatism (see Table 1). Fig.s adapted from Champion et al. (2009).
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Age
Etheridge Province
Late Permian to Triassic
North Qld
Thomson
New England
Lachlan
Orogen
Orogen
Orogen
Orogen
(S)?
Ig If It M A
Permian
Ig If A (M)
S Ig If A
If Ig (M)
Ig IF It M S
Mid Carboniferous toEarly Permian
If Ig (It M)
If Ig (M S)
Ig If (M)
Ig It If M S
Early to mid Carboniferous
If
(If, M A??)
If (M)
Ig It If M If Ig (northern NEO)
1670 ± 200
1380 ± 201
1050 ± 101 1210-930 (18) 1
680 ± 191
T2DM for I & A-type units Median ± SD (Ma) Range (Ma) (# of samples)
(30)
(50)
Data sources
1,2,3,4
1, 2, 4
(92) 1,5,6,7
1090 ± 128 1280-920 (6) 1
Table 1. Range of granite types and respective Nd signatures for the Carboniferous to Triassic granites of eastern Australia. It, Ig and If = tonalitic, granodioritic, or granitic (monzogranite and syenogranite) I-types. S = S-types, A = A-types, M = mafic rocks (largely gabbro to diorite). Items in bold are the dominant granite type for that age period and region; items in parentheses indicate only minor or uncommon component. Depleted mantle (two-stage) model ages (T2DM) reported for I- and A-type felsic magmatism within each geological region. Granite data from Hensel et al. (1985), Chappell et al. (1988), Champion and Chappell (1992), Allen et al. (1997), Bain and Draper (1997), Bryant et al. (1997, 2003), Champion and Bultitude (2003), Murray (2003), Donchak et al. (2007), and Withnall et al. (2009). Nd isotopic data sources as follows: 1: GA, GSQ and ANU unpublished data; 2: Black and McCulloch, 1990; 3: Black et al. 1992; 4: Champion and Chappell (1992); 5: Hensel et al., 1985; 6: Bryant et al., 1997; 7: Allen et al., 1997.
Other magmatism Carboniferous to Early Permian magmatism in the North Queensland, Thomson and Lachlan Orogens and the Etheridge Province was dominated by medium- to high-K, locally extensively fractionated, high-silica, Itype granites (e.g., Chappell et al., 1988; Champion and Chappell, 1992; Bain and Draper, 1997; Champion and Bultitude, 2003; Table 1, Fig. 1). The magmatic activity also produced minor but widespread A-type granites and volcanics, especially in the Etheridge Province and western part of the North Queensland Orogen (e.g., Bain and Draper, 1997), and abundant S-type granites in the North Queensland Orogen (Champion and Bultitude, 2003). The granites, irrespective of type, generally show a restricted silica range (mostly >65%) and associated mafic rocks form only a minor component, in marked contrast to the NEO. Magmatism commenced at ca. 345 Ma in the Lachlan and Thomson Orogens and Etheridge Province, and slightly later in the North Queensland Orogen (ca. 335 Ma; see Table 1). Magmatism in the north Queensland region shows an overall decrease in age to the east, and a switch from I- and A-type to S-, I- and A-type just east of the boundary between the Etheridge Province and the North Queensland Orogen (Champion and Bultitude, 2003). Late Permian magmatism was mainly confined to the North Queensland Orogen (dominated by S-type granites), and parts of the Thomson Orogen (I- and A-types), especially the eastern Thomson Orogen close to the NEO (Table 1). Youngest magmatism in the Etheridge Province is ca. 280 Ma (Bain and Draper, 1997). No significant Triassic magmatism has been recorded outside of the NEO.
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Fig. 2. Two-stage neodymium depleted mantle model ages for Carboniferous to Triassic felsic I- and A-type granites of eastern Australia. Refer to Table 1 for data sources. Pink and black dotted lines are orogen boundaries and extent of Sydney-Gunnedah-Bowen Basin system, as shown in Fig. 1C. 116
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Regional comparisons Although there is significant overlap in geochemical characteristics between granites of the various orogens and provinces, the range of compositions and the degree of variability is most pronounced within the NEO compared to similar-aged granites in the older cratonic hinterland. The extended compositional range (e.g., in SiO2, LILE, LREE, HREE) and the more even distribution of compositions across that range displayed by granites of the NEO contrast markedly with those elsewhere. For example, within the North Queensland Orogen and Etheridge Province , more than 70% of the intrusives are characterised by high-silica, high-LILE compositions. Consequently, most contrast between granites of the NEO and other regions is reflected in the LILE and other incompatible elements, e.g., Sr, Th, and ratios involving these elements, e.g., Rb/Sr, Na/K, Sr/Y, K/Rb. There are some exceptions to this generalisation. Firstly, as pointed out by Chappell et al. (1988), the Carboniferous granites of the Lachlan Orogen have many similarities to those in the NEO, including units with elevated Sr, Sr/Y, K/Rb, Na/K. Secondly, the granites within the southern NEO have a significant component of high-silica, moderately to strongly fractionated compositions with many similarities to the Carboniferous granites in north Queensland.
Regional isotopic signatures A significant amount of published and unpublished Sm-Nd isotopic data now exists for the Carboniferous to Triassic granites of eastern Australia (see Table 1; Fig. 2). The Nd data for the I- and A-type granites show pronounced regional trends from relatively evolved in the Etheridge Province through to more juvenile within the NEO. This is particularly evident in north Queensland. Depleted mantle model ages, for example (Fig. 2), range from an average of 1670 Ma in the Etheridge Province, to 1380 Ma in the North Queensland Orogen, 1050 Ma in the Thomson Orogen, and 680 Ma in the NEO (Table 1). A similar trend is evident between the northern Lachlan Orogen and the NEO. These data complement and mirror the general results from other isotopic systems. For example, Webb and McDougall (1968), in a comprehensive survey of eastern Queensland magmatism, showed that the NEO rocks have significantly more juvenile initial Sr isotopic signatures than rocks in the Thomson Orogen and Etheridge Province to the north and northwest. Subsequent studies in the southern NEO (e.g., Hensel et al., 1985; Bryant et al., 1997) confirm this more juvenile Sr signature for the NEO, in line with the Nd isotopic data.
Discussion The relationship between geochemistry and isotopic signatures is complex – a reflection of the processes and potentially multiple sources involved in granite formation. Although there appears to be a broad correlation between granite chemistry and isotopic signature for the rocks in question, in detail this is not the case. This is clearly demonstrated by the strongly-fractionated, high-silica, granites in the southern NEO (e.g., Stanthorpe Supersuite; Donchak et al., 2007) and similar rocks in north Queensland (e.g., O’Briens Creek Supersuite; Champion and Chappell, 1992) which, despite having very similar chemistry, have distinctly different isotopic signatures, e.g., εNd of ~+3.5 and -7.0 to -11.0, respectively. This and other examples from the region indicate granite generation involved a significant component of increasingly older crustal material away from the NEO, consistent with the exposed geology (Fig. 2). Conversely, the large compositional range evident within the NEO granites (especially in LILE and LREE contents), coupled with their variable but generally juvenile isotopic signatures, imply there was a variety of source components (both crustal and mantle, e.g., Bryant et al., 1997), involved in their genesis. There was, as pointed out by Webb and McDougall (1968), little significant involvement of much older crustal material. This suggests that the NEO resulted from significant new crustal growth in the Palaeozoic and that the granites and associated volcanics in the NEO are largely the result of reworking of young crust (plus or minus variable mantle input; also see Bryant et al., 1997). The isotopic data, therefore, are consistent with both the known geology of eastern Australia, and tectonic models for the Tasman Orogenic Zone that have eastern Australia growing in the Palaeozoic and Triassic by accretion on to the eastern margin of Gondwana (e.g., Glen, 2005). The pronounced isotopic gradients evident in north Queensland, from the northern NEO to the Etheridge Province, are largely a function of this convergent margin being apparently effectively pinned in this region throughout the Palaeozoic. As a result successive magmatic events and orogenies were confined to the same general region, albeit with some eastwest migration. This is in distinct contrast to the much wider and more complex Lachlan Orogen to the south 117
NEO 2010 (e.g., Glen, 2005). The NEO represents a dominantly juvenile crustal block that formed to the east of the older orogens and effectively unified eastern Australian.
Acknowledgements Thanks to Russell Korsch and Natalie Kositcin for comments on earlier versions of the manuscript. Published with the permission of the Executive Director, Geoscience Australia and the Director, Geological Survey of New South Wales.
References Allen C. M., Williams I. S., Stephens C. J. and Fielding C. R., 1998. Granite genesis and basin formation in an extensional setting: the magmatic history of the northernmost New England Orogen. Australian Journal of Earth Sciences, 45, 875-888. Allen C. M., Wooden J.L. and Chappell B. W., 1997. Late Paleozoic crustal history of central coastal Queensland interpreted from geochemistry of Mesozoic plutons: the effects of continental rifting. Lithos, 42, 67-88. Bain J. H. C and Draper J. J. (eds), 1997. North Queensland Geology. AGSO Bulletin 240, and Queensland Department of Mines and Energy Queensland Geology 9. Black L. P. and McCulloch M. T., 1990. Isotopic evidence for the dependence of recurrent felsic magmatism on new crust formation: an example from the Georgetown region of northeastern Australia. Geochimica et Cosmochimica Acta, 54, 183-196. Black L. P., Bultitude R. J., Sun S. S., Knutson J. and Blewett R. S., 1992. Emplacement ages of granitic rocks in the Coen Inlier (Cape York): implications for local geological evolution and regional correlation. Bureau of Mineral Resources Journal of Geology and Geophysics, 13, 191-200. Bryant C. J., Arculus R. J. and Chappell B. W., 1997. Clarence River Supersuite: 250 Ma Cordilleran tonalitic I-type intrusions in eastern Australia. Journal of Petrology, 38, 975-1001. Bryant C. J., Chappell B. W. and Blevin P.L, 2003. Granites of the southern New England Orogen. In: Blevin P., Jones M. and Chappell, B. (eds) Magmas to Mineralisation: The Ishihara Symposium, Geoscience Australia Record, 2003/14, 9-12. Champion D. C. and Chappell B. W., 1992. Petrogenesis of felsic I-type granites: an example from northern Queensland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 83, 115-126. Champion D. C. and Bultitude R. J., 2003. Granites of north Queensland. In: Blevin P., Jones M. & Chappell, B. (eds) Magmas to Mineralisation: The Ishihara Symposium, Geoscience Australia Record, 2003/14, 19-24. Champion D. C., Kositcin, N., Huston D. L., Mathews E. and Brown C., 2009. Geodynamic synthesis of the Phanerozoic of eastern Australia and implications for metallogeny. Geoscience Australia Record, 2009/18. Chappell B. W., White A. J. R. and Hine R., 1988. Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences, 35, 505-521. Donchak P. J. T., Bultitude R. J., Purdy D. J. and Denaro T.J., 2007. Geology and mineralisation of the Texas Region, south-eastern Queensland. Queensland Geology, 11. Glen R. A., 2005. The Tasmanides of eastern Australia. In: Vaughan A. P. M., Leat P. T. and Pankhurst R. J. (eds) Terrane Processes at the Margins of Gondwana. Geological Society of London Special Publication 246, 23-96. Hensel H. D., McCulloch M. T. and Chappell B. W., 1985. The New England Batholith: constraints on its derivation from Nd and Sr isotopic studies of granitoids and country rocks. Geochimica et Cosmochimica Acta, 49, 369-384 Murray C., 2003. Granites of the northern New England Orogen. In: Blevin P., Jones M. & Chappell, B. (eds) Magmas to Mineralisation: The Ishihara Symposium, Geoscience Australia Record, 2003/14, 101-108. Webb A. W. and McDougall I., 1968. The geochronology of the igneous rocks of Eastern Queensland. Australian Journal of Earth Sciences, 15, 313-346. 118
NEO 2010 Withnall I. W., Hutton L. J., Bultitude R. J., Von Gnielinski F. E. and Rienks I. P., 2009. Geology of the Auburn Arch, southern Connors Arch and adjacent parts of the Bowen Basin and Yarrol Province, central Queensland. Queensland Geology, 12.
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A chemical database for the New England Batholith Bruce W. Chappell1 1
School of Earth and Environmental Sciences, University of Wollongong, Australia
Keywords: granite database, New England Batholith
Introduction The New England Batholith (NEB) is a major component of the southern New England Orogen. It covers an area of 16,000 km2, with additional parts covered by Tertiary basalt. Presumably because of the association with Sn mineralisation, the earliest studies of the New England Batholith were carried out on the northern part by the NSW Geological Survey (Andrews et al., 1907). Modern studies of the rocks of the NEB were pioneered in an honours thesis on the Uralla Granodiorite by Ron Vernon in 1956 (Vernon, 1961). My own studies of the NEB commenced during an honours thesis in 1958. Stirling Shaw commenced his studies in the Tenterfield region in 1960. During the 1960’s numerous further studies of the NEB were initiated and in 1969 a long-delayed account of much of the earliest work was published in the “Geology of New South Wales” (Journal of the Geological Society of Australia, vol. 16). Completion of my own BSc honours project coincided with the publication of the most significant publication ever on granites, that of Tuttle and Bowen (1958) (T&B). For the previous few decades, there had been a great dispute about whether granites are in fact igneous rocks, or whether they result from transformation of older rocks in the solid state – the process of granitisation. T&B unambiguously resolved this “Granite Controversy” in favour of a magmatic origin and that work marked the beginning of the modern era in the study of granites, in which a magmatic origin has been broadly accepted. Other controversies were unresolved in 1958. In the Armidale department, the view was that the continents were fixed and stationary, which meant that in today’s terms, much of the New England geology was poorly understood, even though the geological elements that comprised the region were becoming better known.
Granites as magmatic products Tuttle and Bowen (1958) showed that the more felsic granites have compositions that match those of silicate melts that in the laboratory are in equilibrium with quartz and feldspars at the lowest possible temperatures. This is illustrated by the granites of the Bundarra Supersuite of the NEB in Fig. 1.
Fig. 1. Compositions of 47 granites of the Bundarra Supersuite plotted as the normative Quartz, albite, and orthoclase components on a Tuttle and Bowen diagram, with the minimum-temperature compositions at 50 and 100 MPa also shown. That granites have a magmatic origin, 120
NEO 2010 involving such an equilibrium, rapidly became widely accepted. Such an equilibrium can result from two processes. First, favoured by Bowen, is the process of fractional crystallisation, in which various minerals, including quartz and feldspars are progressively removed from a less felsic melt, so that the melt evolves in equilibrium with those minerals. Second, recognised as a possibility but nor favoured by Bowen, is a process in which the melt forms from the melting of quartz and feldspars and is in equilibrium with those minerals. For the more felsic granites, the second process of partial melting became widely accepted.
Formation of the less felsic granites It was probably reasonable to extrapolate from the T&B experiments, but not completely rigorous to do so. If the T&B model was universally applicable, then the most common granitic rocks should be the monzograntes (adamellites) containing approximately equal amounts of quartz, Na-plagioclase and Kfeldspar. While such rocks are important, and in the NEB more so than in most granite terranes, the dominant rocks worldwide are less felsic granodiorites and in some areas tonalites are important. Both of those types of granite have higher plagioclase/K-feldspar ratios than the monzogranites. Also, in some cases of which there are important examples in the NEB, less quartz is present and the rocks trend towards monzogranites and monzodiorites. All of these other granites are magmatic, but they do not conform to the T&B experiments. There are four possible explanations for this: 1. Partial melting within the crust may extend to higher temperatures and more mafic compositions, if a T&B component becomes depleted in the source rocks during partial melting. Additional amount of elements such as Ca and Fe are carried in solution in the magma at those higher temperatures. 2. Felsic partial melts may entrain unmelted source material (restite) so that the more mafic elements are carried in minerals in suspension in the magma.. 3. The more mafic granites could be the result of intermediate levels of fractional crystallisation of mafic magmas, perhaps from the mantle. 4. The more mafic granites could be the products of mixing or mingling between a felsic melt and of mafic material from the mantle, which perhaps contributed to that melting. Which of these processes operated in specific cases, and whether (2) ever happens at all, is controversial. The view of this author is that all can take place, but (4) probably not on a large scale (Chappell, 1996) and it cannot apply to granites sourced from sedimentary rocks (S-type) which become more “sedimentary” in composition (peraluminous) as they become more mafic. The best single line of evidence for (2) is either the occurrence of older zircon, or continually decreasing Zr contents as the granites become more felsic, both of which show that the melts involved in production of the granite were saturated in zircon. In the less felsic granites this means that the temperatures of the melts were too low for the magmas to have been completely molten, and the presence of entrained mafic minerals is inferred.
High- and low-temperature granites Granites that evolved by (1) and (3) did so at relatively high temperatures and Chappell et al. (1998, 2004) have referred to them as high-temperature, while (2) comprise the low-temperature granites. In general, high-temperature granites are much more likely to be related to mineralisation (although Sn deposits related to low-temperature granites, such as those of the northern NEB, are a notable exception), for the following reasons (from Chappell et al., 2004): 1. During the partial melting process at higher temperatures, the solubility of trace metals in the melt will be greater than at lower temperatures. 2. Granite suites that evolve by fractional crystallisation concentrate incompatible trace metals at that magmatic stage, enhancing the probability of later significant mineralisation (Blevin and Chappell 1992). High-temperature suites, particularly those that are monzonitic rather than tonalitic, are more likely to undergo fractional crystallisation, which may increase the abundances of some rare elements in the evolving melt, but will also cause the abundances of others to decrease. 3. H2O contents of the melts will also increase with such fractionation, so that mineralising fluids are more likely to be released (cf. the low-temperature suites as discussed by White (2001)). 4. High-temperature magmas are also largely molten magmas, and because of both the higher temperatures and that larger fraction of melt, will have a higher heat capacity and would introduce more heat into the country rocks during cooling.
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I- and S-type granites The proposal by Chappell and White (1974) that there are two fundamental groups of granites, the I- and Stypes, formed from magmas derived respectively by the partial melting of igneous and sedimentary source rocks, arose from detailed studies in the Berridale-Koszciusko region of the Lachlan Fold Belt (LFB). But there was an early hint of this in my own PhD study (Chappell, 1966) on granites of the southern NEB, where there was a marked difference between the hornblende-bearing rocks south of Bendemeer and the muscovite-bearing granites which they had intruded further north. In that thesis, the production of the former was attributed to partial melting of mafic rocks near the base of the crust, while the latter “resulted from the partial melting of sialic crustal rocks”. That thesis was mainly concerned with the petrogenesis of the hornblende-bearing granites, and the muscovite-bearing rocks were not studied in any detail. Flood and Shaw (19757 recognised that those muscovite-bearing granites, which they assigned to the Bundarra Plutonic Suite, are one of two groups of S-type granites in the NEB, the other being the Hillgrove Plutonic Suite on the eastern side of the batholith. It is likely that the S-type granites of the LFB were derived by partial melting of marine sediments, whereas Flood and Shaw (1977) proposed that the Bundarra granites, having relatively primitive Sr isotopic compositions, were derived from a young sedimentary pile of predominantly volcanic character. This supported the view strongly held by Allan White that I-type granites are not derived from buried volcanic rock sequences (White and Chappell, 1988), but rather from “underplated” or infracrustal igneous rocks (Chappell and Stephens, 1988). Chappell (1984) proposed the use of the alternative terms infracrustal and supracrustal for the I- and S-type granites. Both types are important components of the NEB, as they are of the earlier Delamarian and Lachlan belts of eastern Australia. A-type granites are present in the NEB but very restricted in their occurrence (Landenberger et al. 1996).
Fractionated granites In a real sense, all granites are fractionated rocks, that is they were derived by various processes from source material that had a different composition, i.e. they were fractionated during their formation. But it is useful, particularly in an economic sense, to discriminate those granites that have undergone enrichments or depletions in certain trace elements, generally by a process of fractional crystallisation, as fractionated granites. The elements that are enriched in this way may be of economic significance, and such magmatic enrichment at the same time as H2O and other volatile elements increase in abundance, may be an important early stage in developing mineral deposits. Such enrichments may be useful in a regional sense, to identify provinces that potentially contain mineralisation (Blevin and Chappell, 2010) and also to assess the likelihood of mineralisation being associated with individual granite plutons or suites. The granites of large parts of the NEB are unfractionated in this sense, but there are examples of extreme fractionation in parts of the northern NEB in the Mole granites and the felsic granites of the Stanthorpe area.
The chemical database A chemical database has been assembled for the granites of the NEB, comprising some 1500 major and trace element analyses. This is part of a project to provide a comprehensive set of analyses of the granites of eastern Australia, with the NEB data being available in 2010, the granites of the LFB in 2011, the northern New England Orogen in 2012, and Far North Queensland at a later date. These reports will include chemical and locality data, and the rocks will be assigned to plutons or lithological units and grouped into suites and supersuites where appropriate. The suites will be assigned a status as I-, S-, or A-type and low- or hightemperature in origin. Much of the stimulus and support for the production of this information came initially from projects sponsored by AMIRA and supporting mineral exploration companies. It is recognised that a chemical analysis does not fully define a granite, and that complete characterisation also requires knowledge of both the major and accessory minerals that are present with their abundances and compositions, and information on the texture of the rock. When this project started five decades ago, the production of major element analyses was both time consuming and not particularly precise. The measurement of trace element abundances was in its infancy and quite imprecise. Modal analysis of thinsections was the preferred method of determining granite compositions, again a lengthy and somewhat inaccurate way of characterising coarse-grained rocks. The quality of the data was improved by measuring mineral abundances on slabs of rock, but basically this provided the quartz and feldspar abundances needed for a traditional classification, and little else. There was a transformation of the methods of instrumental chemical analysis during the 1960’s, with the most important contributions being made in the CSIRO Division of Soils (Norrish and Hutton, 1969), and X-ray fluorescence spectrometry (XRF) rapidly became 122
NEO 2010 the method of choice for the determination of the major elements and most minor elements (> 1 ppm; Z> 20) in rocks. A strong impetus to the analytical techniques was provided by the Apollo Lunar program, in which the laboratory was involved. The XRF method has been used for this project to provide data of consistent quality, and this work would not have been possible without the early support in the development of analytical methods by Keith Norrish. The quality of these analytical methods has enabled the fine detail of many of the granites to be established, with grouping of plutons into suites. The trace element data is valuable both in a petrogenetic sense and for assessing the likely metallogenic potential of the granites. The samples analysed for this report are dominantly granites of various kinds, with some more mafic rocks. Aplites, dyke rocks, and porphyries are represented. Enclaves are mainly from the Moonbi area and the Clarence River granites. The initial samples were obtained for the PhD thesis of Chappell (1966). Other samples are from the theses of Hensel (1982) and Bryant (1992). Many other samples have been collected, by those three authors, by Phil Blevin and David Champion, and extensively by Robert Bultitude. Additional sample powders have been made available by David Keith of the University of New England from UNE honours theses, mainly to cover gaps in the sampling. Preliminary accounts of the petrological (Bryant and Chappell) and metallogenic (Blevin) implications of these data are provided elsewhere in this volume.
References Andrews, E.C., Mingaye, J.C.H. and Card, G.W. 1907. The geology of the New England Plateau, with special reference to the granites of northern New England. Part IV. Petrology. Records of the Geological Survey of New South Wales 8, 196-238. Blevin, P.L. and Chappell, B.W. 1992. The role of magma sources, oxidation states and fractionation in determining the granite metallogeny of eastern Australia. Transactions of the Royal Society of Edinburgh: Earth Sciences 83, 305-316. Blevin, P.L. and Chappell, B.W., 2010. The use of the K/Rb ratio as a metallogenic discriminant: the example of the eastern Australian granites. Zeitschrift für Geologische 38, 235-241. Bryant, C.J. 1992. Clarence River Suite granitoids: petrography and geochemistry. BSc (Hons) thesis, The University of New England. Chappell. B.W. 1966. Petrogenesis of the granites at Moonbi, New South Wales. PhD thesis, The Australian National University. Chappell, B.W. 1984. Source rocks of I- and S-type granites in the Lachlan Fold Belt, southeastern Australia. Philosophical Transactions of the Royal Society of London A 310, 693-707. Chappell, B.W. 1996. Magma mixing and the production of compositional variation within granite suites: evidence from the granites of southeastern Australia. Journal of Petrology 37, 449-470. Chappell, B.W., Bryant, C.J., Wyborn, D., White, A.J.R. and Williams, I.S. 1998. High- and lowtemperature I-type granites. Resource Geology 48, 225-236. Chappell, B.W. and Stephens, W.E. 1988. Origin of infracrustal (I-type) granite magmas. Transactions of the Royal Society of Edinburgh: Earth Sciences 79, 71-86. Chappell, B.W. and White, A.J.R. 1974. Two contrasting granite types. Pacific Geology 8, 173-174. Chappell, B.W., White, A.J.R., Williams, I.S. and Wyborn, D. 2004. Low- and high-temperature granites. Transactions of the Royal Society of Edinburgh: Earth Sciences 95, 125-140. Flood, R.H. and Shaw, S.E., 1977. Two “S-type” granite suites with low initial 87Sr/86Sr ratios from the New England Batholith, Australia. Contributions to Mineralogy and Petrology, 61, 163-173. Hensel, H.D. 1982. The mineralogy, petrology and geochronology of granitoids and associated intrusives from the southern portion of the New England Batholith. PhD thesis, The University of New England. Landenberger, B. and Collins, W.J. 1996. Derivation of A-type granites from a dehydrated charnockitic lower crust: evidence from the Chaelundi Complex, eastern Australia. Journal of Petrology, 37, 145-170. Norrish, K. and Hutton, J.T. 1969. An accurate X-ray spectrographic method for the analysis of a wide range of geological samples. Geochimica et cosmochimica Acta, 33, 431-453.
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NEO 2010 Tuttle, O.F. and Bowen, N.L. 1958. Origin of granite in the light of experimental studies in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O. The Geological Society of America Memoir 74. Vernon, R.H., 1961. The geology and petrology of the Uralla area, N.S.W. Journal and Proceedings of the Royal Society of New South Wales 95, 23-33. White, A.J.R. 2001. Water, restite and granite mineralisation. Australian Journal of Earth Sciences 48, 551555. White, A.J.R. and Chappell, B.W. 1988. Some supracrustal (S-type) granites of the Lachlan Fold Belt. Transactions of the Royal Society of Edinburgh: Earth Sciences 79, 169-181.
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The Wongwibinda Complex: A HTLP metamorphic terrain S.J. Craven GEMOC Key Centre, Dept. of Earth & Planetary Sciences, Macquarie University, Sydney, 2109, Australia.
Keywords: HTLP metamorphism, migmatite, metapsammite, metapelite, TerraneChron, U/Pb isotopes, Hf isotopes.
Introduction. The Late Carboniferous Wongwibinda Complex (WC) is a region of high-temperature, low-pressure (HTLP) metamorphic rocks that occur in association with granites of the Hillgrove Plutonic Suite within the Tablelands Complex of the southern New England Fold Belt (Fig. 1; Binns, 1966; Vernon, 1982; Farrell, 1995). The WC exhibits a metamorphic progression from relatively unmetamorphosed sedimentary rocks to highgrade schists with migmatites abutting the Abroi Granodiorite/Gneiss. This project aims to understand the tectonic processes that generate HTLP terrains by studying the evolution of the WC. An interdisciplinary approach is being employed studying the interaction between deformation, metamorphism, partial melting and magmatism. Fig. 1. Location of the WC.
Methods. The Wongwibinda Complex is composed mainly of the Girrikool Beds and their metamorphic equivalents, the Ramsbeck Schists and associated migmatites. The Girrikool Beds are a thick (15-20 km) turbidite sequence of interbedded, siliceous, intermediate- to fine-grained psammites and pelites (Fig. 2; Korsch, 1978).
Fig. 2. Geology of the WC.
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NEO 2010 In order to better confine the age and character of this sequence, U-Pb and Hf-isotopes were measured from zircon separated from rock samples and a TerraneChron drainage survey from the complex. EMP monazite chemical dating of some of the higher-grade metamorphic equivalents of the Girrikool Beds was also carried out. The monazite age of metamorphism of an unfoliated cordierite hornfels, a cordierite augen schist and a migmatite was determined. Zircon U/Pb ages were also determined for associated S-type granites, the Abroi, Rockvale and Tobermory plutons. Analyses were carried out employing a combination of EMP and LA-ICPMS.
Results The TerraneChron survey identified well-defined populations at ~40 Ma, ~250 Ma, and ~290 Ma (Fig 3a) and a broad population at ~330 ± 20 Ma (Fig 3b). A small number of grains yielding Proterozoic and Archean ages (Fig 3a) are likely to have been inherited by the magmatic/volcanic rocks from which the Girrikool sediments were derived. Detrital zircon grains from a weakly metamorphosed metapsammite (333.7±3.6 Ma) (Fig 3c), an unfoliated metapelitic cordierite hornfels (330.6±7.5 Ma) (Fig 3c) and a migmatite (325.9±6.4 Ma) (Fig 3e) adjacent to the Abroi (Figs 3g,h) exhibit U-Pb age distributions similar to the TerraneChron alluvial samples and suggest deposition of the sedimentary package by 320 Ma. A second migmatite sample yields a younger age distribution (306.9±8.6 Ma) (Fig 3f). EMP monazite chemical dating was completed on unfoliated cordierite hornfels (311±8.4 Ma), cordierite augen schists (292±2.8 Ma) and migmatites (297±4.3 Ma) (Fig 4). Two zircon concentrates from strongly foliated/gneissic and relatively unfoliated samples of the Abroi Granodiorite/Gneiss returned U-Pb ages within error: 291.2 ± 2.3 Ma and 293.6 ± 3.5 Ma respectively. Each sample has few inherited grains. Zircon grains show no evidence of metamorphic effects such as overgrowths of new zircon or dissolution of igneous zircon. U-Pb zircon ages were also determined for the Rockvale (296.4 ± 3.2 Ma) and Tobermory (299.5 ± 2.7 Ma) adamellites.
Fig. 3. Probability-density distribution and histogram plots for combined, TerraneChron, alluvial zircons (green) and selected rock types (red) from the WC.
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Fig. 4. Cumulative frequency histograms and isochrons from EMP monazite chemical dating of different metamorphic events. Field observations and mapping around three of the Hillgrove Suite plutons in the WC show that high-grade metamorphic rocks are limited to being located adjacent to the central portion of the Abroi Granodiorite/Gneiss and everywhere else, the contact aureoles are biotite grade at most (see also Leitch, 1978).
Discussion The TerraneChron survey identified well-defined populations at ~40 Ma, ~250 Ma, and ~290 Ma and a broad population at ~330 ± 20 Ma. Respectively, the first three populations reflect known ages of Tertiary Basalts, I-type plutons, and S-type plutons. The broad peak at ~310–350 Ma is interpreted as provenance for the sedimentary rocks being an Early to Late Carboniferous volcanic arc. The younger age population identified in one migmatite sample is interpreted as reflecting Pb disturbance of the detrital grains during metamorphism, on the basis of a spread of ages from ~360 Ma down to ~290 Ma and much of the data for that sample being > 10% discordant. The metamorphic ages of the regional aureole range from ~320-290 Ma. The youngest metamorphic ages overlap the crystallisation age of the granites at 300-290 Ma. However, field relationships show that the highest-grade rocks are located next to the youngest pluton (Abroi) and the older plutons (Rockvale and Tobermory) have very minor thermal perturbations adjacent to them. These data and observations suggest that granite magmatism was not the primary heat source for metamorphism.
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NEO 2010 The Hf data for the granite samples indicate a mix of juvenile and crustal components with the juvenile component increasing with time and an average Hf model age of 1.8 Ga. The Hf data from the alluvial detrital zircons also indicates a mix with the juvenile component decreasing with time (Fig 5). The disparity in age data between emplacement of the Abroi at ~293 Ma and early metamorphism of the metasediments at ~311 Ma suggests that metamorphism largely pre-dates the emplacement of the Abroi Granodiorite/Gneiss by at least a few million years and possibly up to 15 million years.
Fig. 5. Initial hafnium values for granites of the Hillgrove Suite plotted at the calculated sample age (red, pink, mauve, blue squares), inherited grains in granites (green circles), and alluvial detrital and detrital zircons from the Girrikool Beds (brown triangles).
Conclusions New detrital zircon data presented here has identified a broad 310-350 Ma age population in the Girrikool Beds, with very few older components. The geographical location of these rocks east of the Early to Late Carboniferous volcanic arc (western edge of the Tamworth Belt; Jenkins et al., 2002) and the age distribution of the detrital zircon are consistent with the sediment provenance being almost exclusively derived from the arc. Field mapping shows there is no relationship between the location of the highest-grade metamorphic rocks and granite plutons, with the exception of part of the Abroi Granodiorite/Gneiss being in contact with migmatites. New monazite geochronology indicates that metamorphism in the highest-grade rocks likely shortly predates the intrusion of the adjacent Abroi Granodiorite/Gneiss. These relationships require a heat source other than advection of heat by the granites to produce the metamorphism and suggest that both the metamorphism and S-type granites are products of a thermal perturbation associated with a rift tectonic setting. The distribution of Hf isotopes is supportive of a Late Palaeozoic advancing retreating subduction model for the New England Fold belt. The sediments of the Girrikool Beds reflect a weakly increasing crustal component in the Carboniferous volcanic arc with time, while the S-type granites of the Hillgrove Suite manifest a weakly increasing juvenile (mantle or young mafic underplate) component with time, reflecting respectively, an advancing, compressional environment and a retreating, crustal thinning, environment. These data will be integrated with structural and metamorphic data to compile a complete temporal and spatial history of the Complex. The further research planned for the Wongwibinda Complex will provide important information about the New England Fold Belt, the geological evolution of the Australian Plate and contribute to wider geodynamic interpretations.
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References Binns, R.A., 1966. Granitic intrusions and regional metamorphic rocks of Permian age from the Wongwibinda district, northeastern New South Wales. Journal of the Proceedings of the Royal Society of N.S.W 99: 5-36. Farrell, T.R., 1992. Deformation, Metamorphism and Migmatite Genesis in the Wongwibinda Metamorphic Complex. Ph.D. Thesis, Department of Geology, University of Newcastle, Newcastle, N.S.W. (unpubl.). Jenkins, R.B., Landenberger, B. and Collins, W.J., 2002. Late Palaeozoic retreating and advancing subduction boundary in the New England Fold Belt. Australian Journal of Earth Sciences 49, 476 - 489. Landenberger, B., Farrell, T.R., Offler, R., Collins, W.J. and Whitford, D.J., 1995. Tectonic implications of Rb-Sr biotite ages for the Hillgrove Plutonic Suite, New England Fold Belt, N.S.W. Australia. Precambrian Research 71, 251 – 263. Vernon, R.H., 1982. Isobaric cooling of two regional metamorphic complexes relate to igneous complexes in southern Australia. Geology 10, 76 – 81.
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Using volcanic facies analysis to unlock the mineral potential of the Drake gold field Grace Cumming1, Geoffrey Lowe2 1 2
Consulting Geologist for White Rock Minerals
White Rock Minerals Ltd, 24 Skipton St Ballarat VIC 3350
Keywords: epithermal, volcanic facies, Permian, gold, silver, copper
Introduction The Drake Goldfield is hosted by the Drake Volcanics and is located approximately an hour’s drive west of Lismore in northern NSW. Gold was first discovered in the region at Mt Carrington in 1886 and the project area saw numerous gold, silver and copper mining activities through to 1908. Open pit gold and silver mining on a small-scale also occurred from 1988 to 1990, after which followed a hiatus in exploration from 1994 to 2006. The Mt Carrington gold-silver-copper project is the primary asset held by White Rock Minerals Ltd (‘White Rock’), a new exploration company which demerged from Rex Minerals Ltd (‘Rex’) in June 2010. The Mt Carrington project was acquired by Rex in 2009 after having completed due diligence on the project under an Option to Purchase Agreement in 2008. Rex remodelled and validated the existing Mineral Resource base for the project to define shallow JORC compliant Inferred Mineral Resources totalling 190,000ozs of gold and 10.5Mozs of silver. Rex also completed substantial exploration programs including geochemical stream sediment and rock chip sampling, induced polarisation and magnetics geophysical surveys, and diamond drilling programs. Rex has also undertaken significant environmental management and rehabilitation programs on the Mt Carrington Mining Leases as part of an approved Mine Operations Plan. Since June 2010 White Rock have continued the exploration work initiated by Rex, which has included work by specialists on the structure and volcanic architecture as it relates to known mineralisation and district wide targeting. Prospecting work also continues with ongoing stream, soil and rock sampling ahead of an aggressive drill program targeting the expansion of known resources and the testing of multiple new targets over the coming 18 months.
Background geology The Mt Carrington project contains substantial precious and base metal mineralisation predominantly hosted by the Permian Drake Volcanics, within the NEFB in north-eastern New South Wales. The Drake Volcanics comprises a 60km long by 20km wide north to northwest-trending sequence of middle Permian acid to intermediate volcanics dominated by volcaniclastic andesitic units. The Drake Volcanics are intruded by contemporaneous sub-volcanic andesite and rhyolite porphyries. The unit has an estimated thickness of 600 metres in the Red Rock area while within the Mt Carrington group of MLs, it is at least 500 metres thick. The Drake Volcanics overlie the older Carboniferous Emu Creek Formation to the east, which comprises predominantly sedimentary rocks that host the gold mineralisation of the Tooloom and Lunatic Goldfields. To the west of Mt Carrington the Drake Volcanics are in faulted contact with late Permian to early Triassic leucocratic granitoids that form part of the New England Batholith. These granitoids host numerous mineralisation occurrences including disseminated gold mineralisation at Timbarra and tin-tungstenmolybdenum mineralisation in the Wilsons Downfall and Stanthorpe areas.
130
NEO 2010 The main regional geological feature of interest at Mt Carrington is a circular 15-20km diameter zone of low magnetic intensity with concentric fault and fracture patterns, which occurs, in the central portion of the Mt Carrington Project area (Fig. 1). This feature has been a focus for regional mapping, as it is believed to represent a large volcanic caldera, which has undergone extensive hydrothermal alteration, resulting in destruction of magnetic minerals and a characteristic low magnetic signature. The zone is termed the ‘Drake Quiet Zone’ (“DQZ”) and it is a significant regional exploration target as caldera environments such as these are common settings for epithermal gold-silver mineralisation related to concealed deeper level intrusions elsewhere in the world.
Methodology Exploration in the Drake Goldfield has been focussed on known workings on long-established Mining Leases (at Mount Carrington), which has drawn attention away from an expansive area of the Drake Volcanics with considerable regional mineral potential. White Rock conducted a high resolution airborne magnetic survey over a 400km2 region in the central section of the EL’s, focussed on the DQZ. The magnetic survey has provided a quality dataset on which to base detailed volcanic and alteration facies mapping, as well as detailed structural interpretations and future exploration (Davies, 2010). The DQZ spans approximately 250 square kilometres of forested hills, deep gullies and gorges, which has ultimately discouraged previous detailed mapping attempts. Over an intensive 7-week period from July to September 2010, volcanic facies mapping was completed over approximately 70 square kilometres of area, focussing on known mineralized zones at Mount Carrington, White Rock and Red Rock and peripheral areas to these zones. Field traverses were focussed in and along creek beds, ridgelines and road outcrops during the driest months of the year. Along with detailed mapping, an intensive sampling program was undertaken. The resulting sample library is being utilised as a reference set for future workers as well as providing samples for mineral analysis (petrography and XRD) and geochemical characterisation using a hand held XRF on-site as mapping progresses.
New constraints for the volcanic facies architecture of the Drake Volcanics Detailed volcanic facies mapping in the region has revealed 6 main intermediate to rhyolitic dominated facies associations. These are described, in order from highest to lowest in the stratigraphy. Andesitic lavas, autobreccias and debris flow deposits A sequence of voluminous and laterally extensive flow banded and massive, sparsely feldspar phyric andesite flows flank the succession in the south and south eastern part of the mapped area. These flows appear gradational into monomictic, jigsaw fit and clast rotated, clast supported breccias of the same composition. Massive and bedded polymictic, clast and matrix supported andesitic breccias and volcaniclastic, pebbly and crystal rich sandstones are interbedded and gradational to the lava and autobrecciated domains. These poorly sorted, voluminous beds of polymictic clasts are thought to represent the products of debris and mass flow processes. Rhyolitic volcaniclastic sedimentary, pyroclastic ash-fall and surge deposits An expansive sequence of massive to bedded crystal, pebble and granule rich sandstones and matrix supported cobble breccias occur in the central part of the mapped area. In places, the sandstones contain rare broken feldspar crystals and rounded quartz granules. In the upper portion of massive beds planar bedding is characteristic, and minor truncation and scouring of beds also occur. Bedforms tend to be normally graded with beds of cobble breccias and domains with bedding parallel stratification. The facies appears to be fault bound to the overlying facies. Rhyolitic and dacitic subaqueous pyroclastic flow and dome seated eruption products Bedded rhyolitic and dacitic fiamme bearing facies occur peripheral to dacite and rhyolite bodies and as distinct beds in the stratigraphy. Abundant fiamme may represent flattened glassy or pumiceous fragments formed from explosive volcanism or resedimentation of the glassy pumiceous zones formed from dome growth. The dacitic fiamme breccia facies display characteristics similar to water supported pyroclastic eruptions or Neptunian eruption products (discussed by Allen and Mcphie, 2009) but the products include 131
NEO 2010 clasts and material of differing provenance. The crystal rich, rhyolitic fiamme breccia shows evidence for emplacement by an eruption fed dome seated eruption. Dacite dominated debris flow and alluvial deposits Massive beds of rhyolitic to dacitic lithic breccias and conglomerates are notable for containing boulders (1.5 m diameter), which are clast supported and poorly sorted. The coarse grain-size, planar bedding and absence of cross-bedding in the conglomerates are consistent with deposition from hyperconcentrated flood flows (e.g. Smith, 1986) or mass flows. Deposition is concluded to have occurred in a poorly confined alluvial environment in which upper flow regime conditions dominated. Lower andesite and peperite breccia facies Thick domains of sparsely feldspar and feldspar-amphibole phyric, monomictic andesitic breccia domains with fluidal, peperitic clasts which display chilled margins in a fine volcaniclastic siltstone matrix. Peperite formation involves the in situ disintegration of magma as it intrudes and mingles with unconsolidated or poorly consolidated, typically wet sediments (Skilling, 2001; White, 2000). Intrusions Intrusive complexes are a common feature of the Drake volcanic sequence. Dykes also occur and range in thickness from 0.5 to 50 m but the intrusive complexes are of particular interest. The larger intrusions are dacite, rhyolite and feldspar phyric andesite and these are the most abundant. Rare microdiorite and gabbros were also mapped. The flow-banded rhyolite and dacite intrusions are thought to occur as steep-sided apophysies to a voluminous body at depth (?). The rhyolite and dacite intrusions are spatially and most likely genetically related to the gold, silver, and copper mineralisation in the Drake area, and may be a late evolving and high-level expression of the New England Batholith (?). These rhyolites occur as composites in some domains, consisting of at least two intrusive phases (displayed at the White Rock Silver Mine).
Structural and genetic context for mineralisation An apparent spatial and genetic relationship between numerous exposed and concealed rhyolitic intrusions and mineralisation at Mt Carrington is well constrained. The known mineral camps and alteration intensity has a clear spatial relationship with respect to the interpreted faults and fractures that define the DQZ. The structural data for veins and faults in the Mount Carrington pits also supports the interpretation that deformation associated with the development of a possible caldera, or collapse zone above a shallow laccolithic intrusion, could provide the pathways for the rhyolitic intrusions and mineralisation. The Mt Carrington project contains gold-silver-copper mineralisation which is typical of the intrusion-related low sulphidation epithermal style, and is likely to be derived from resurgent volcanic and magmatic dome activity within the DQZ. Mineralisation is controlled by arrays of steep to shallowly dipping normal faults, breccias and tensional fractures. The main styles of mineralisation are tensional sheeted and stockwork veins, breccia-fill, and lesser disseminated sulphide, hosted by andesitic lavas, volcaniclastic rocks, and flow banded rhyolite intrusions. The individual mineral deposits contain varying proportions of gold, silver and base metal (zinc, copper and lead) sulphide minerals.
Volcanic facies analysis and mineral potential in the Drake gold field Through careful volcanic and alteration facies mapping, favourable rhyolitic-dacitic intrusive domains away from the historical discoveries are being defined elsewhere in the DQZ. Further to this, areas of high alteration intensity and structurally important zones are being discovered with evidence to support a collapse caldera being gathered and scrutinized.
References Allen S. R., McPhie, J., 2009. Products of neptunian eruptions. Geology, 37, 639-642. Davies, B., 2010. Mt Carrington district: structural framework and mineralization. Confidential company report prepared for Rex Mineral Ltd. Skilling I. P., White J. D. L., McPhie J., 2001. Peperite: a review of magma sediment mingling. Journal of Volcanology and Geothermal Research 114, 1-17. 132
NEO 2010 Smith, G.A., 1986. Coarse-grained nonmarine volcaniclastic sediment: Terminology and depositional process. Geological Society of America Bulletin 97, 1–10. White, J.D.L., McPhie, J., Skilling, I.P., 2000. Peperite: a useful genetic term. Bulletin of Volcanology, 62, 65-66.
Fig. 1: Mt Carrington Project - High resolution magnetics and interpreted structure
Fig. 2: Mt Carrington Project – Interpreted volcanic architecture
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Ophiolite origin and obduction: arc-continent collision: an integrated model John Dewey 1
University College Oxford, University of Houston, NUI Galway, University of California Davis
Keywords: ophiolite, obduction, arc-continent collision A kinematic solution to the ”ophiolite enigma, paradox or conundrum” is offered, which is that obducted large-slab full-sequence ophiolite complexes must have originated by organized sea-floor spreading (sheeted dyke complexes) that indicates an origin at oceanic ridges yet have geochemical affinities that link them to fore-arcs. The model is that, following the nucleation of a subduction zone on an oceanic transform/fracture zone, a stable ridge/trench/trench triple junction with a ridge separating hanging-wall plates allows the arcparallel growth and lengthening of a boninitic ophiolite fore-arc, synchronously with the development of a rear-arc oceanic basin at the same ridge. Fore-arc boninites result from high-temperature/low-pressure partial melting of sub-ridge asthenosphere hydrated by water released from the basaltic crust of the subducting plate. The thermal thickening of the fore-arc ophiolite is buffered by its position above the subducting lithosphere. If obduction onto a continental margin is to occur, it must happen within ten to fifteen million years. If the fore-arc ages beyond about twenty million years, the fore-arc lithosphere becomes too thick to obduct and bulldozes the continental margin with which it collides or the thin hot arc lithosphere overrides the forearc, which is subducted, onto the continental margin. The immensely-variable style of arc-continent collision is dependent on the age of the arc/fore-arc and the age and morphology of the rifted margin with which collision occurs. The mafic-protolith basal aureole is formed from oceanic crust in the subduction channel and attached to the base of the ophiolite about five million years after the birth of the ophiolite. It is obducted with the ophiolite and is not formed during obduction. The slip-direction in the subduction channel must be oblique, towards the RTT triple junction, because the oblique motion cannot be partitioned into trench-orthogonal and trench-parallel components. The mid-Ordovician Humberian/Grampian Orogen of Newfoundland and the Caledonides in the British Isles, like the Miocene Bismarck Orogen of New Guinea, and the early Cretaceous Nevadan Orogen of the Sierra Nevada, probably developed by the collision of a supra-subduction zone (SSZ) ophiolite/oceanic arc (s) with a continental margin followed by a flip in subduction polarity, leading to the addition of oceanic arc complexes to the edges of continents and, hence, continental growth. In the Grampian Orogen, imminent collision is heralded by a switch from mafic to silicic magmatism. Fore-arc/successor basins preserve clastic records of collisional events and unroofing of the obducted SSZ ophiolite and underlying metamorphic complexes, ancient zircons from subducted crust appear in the post-collisional arc, and the crust was returned to normal thickness, mainly by extension. The preservation of low-grade rocks in these collisional zones may have been the result of four factors. First, subduction systems commonly show a general subsidence of the over-riding lithosphere resulting from the colder negative buoyancy of the subducting slab(s). Second, the subducting, thinned and stretched, continental margins probably contain substantial amounts of rift-related mafic igneous rocks, which, if converted to eclogite during continental thickening, would contribute to depression of the orogen and reduce erosion. Third, the 12 km-thick obducted arc/supra-subduction-zone ophiolite/arc nappes had an average density of about 3200 kg.m-3, beneath which the evolving orogens were depressed below sea level. Fourth, the Grampian orogen in western Ireland, and probably the Bismarck and Nevadan Orogens, enjoyed a very short (2 Myr) period of late-orogenic extensional denudation, when only very recently-generated staurolite-garnet amphibolites were drawn up beneath an extensional detachment(s) to contribute a pulse of detritus, as the ophiolite/arc hanging wall was drawn down. Subduction flip led to extensional collapse and, probably, delamination/detachment of the eclogitised Laurentian root, which would have generated uplift of the Grampian core from which the high-level obducted sheet was withdrawn. Collision, polyphase deformation, Barrovian metamorphism, erosion of the obducted arc/SSZ ophiolite 134
NEO 2010 nappe, subduction flip, extensional collapse, and the establishment of a continental margin arc with retrocharriage, occurred in less than 20 Myr (478-460 Ma) synchronously with the accumulation of 6 km of clastic sediments a hanging wall basin, the South Mayo Trough, whose detrital heavy minerals and detrital white mica ages record these events precisely. The rapid Barrovian metamorphism (475-468 Ma) of the Dalradian footwall cannot have resulted from crustal thickening and thermal relaxation. Rather, it resulted from the rapid advection of heat into the footwall from the hot obducting arc/ophiolite, probably by the synnappe lateral injection of large volumes of mafic magma (Connemara mafic/ultramafic suite). Newfoundland and the Caledonides differ in that, in the former, the hanging wall arc/ophiolite with a complex precollisional, transtensional, structural/magmatic history is superbly preserved whereas, in the latter, the footwall Barrovian complex is better developed and exposed. The origin of the granite suite, as in most orogens, remains a vexing problem. All, whether I or S, are crustal melts and probably originate above lateorogenic mafic underplates, possibly resulting from lithospheric delamination. None, unlike the TTG suite, originate by the partial melting of mafic/ultramafic protoliths above subducting slabs. A huge amount of field work, by people with structural and field-petrographical skills, is still required to understand the granite problem. It is tragic that Allan White with his enthusiasm, hand-lens and mapping skills, and love of the outcrop, is no longer extant to help us. Bruce Chappell, his granitic geo-half will have to continue the work.
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Conrad Silver Project Michael Donnelly1 1
Malachite Resources Limited
Keywords: Conrad, silver, lead, copper, vein, Gilgai Granite
Introduction The Conrad silver-rich polymetallic quartz-sulphide vein system is located in the New England Fold Belt approximately 20km south of Inverell (Fig. 1). The Conrad mine was the largest silver producer in the New England region of New South Wales, with about 3,500,000 ounces (108,500 kg) of recorded silver production, together with by-product lead, zinc, copper and tin. The mine produced more than 175,000 tonnes of ore at average grades of approximately 600 g/t (20 oz/t) silver, 8% lead, 4% zinc, 1.5% copper and 1.5% tin (Brown and Stroud, 1997). The Conrad mine and surrounding areas are now held under mining leases and exploration titles by Malachite Resources Limited (Malachite), which is conducting drilling and pre-development studies with the aim of bringing the mine back into production. Malachite is also exploring the broader Inverell district for alluvial and hard rock tin resources (Fig. 1). This paper describes the Conrad deposit and the work conducted during 2007-2010 towards redevelopment of a mining operation. The Conrad mine lies within the Early Triassic Gilgai Granite which hosts a large number of polymetallic base metals and silver deposits particularly near the granite’s western margin (Brown and Stroud, 1997). The Conrad quartz-sulphide vein or lode is on a northwest-southeast striking fracture-fault zone that continues southeast into the Tingha Adamellite. This host structure, which can be traced for at least 7.5km with old mine workings intermittently located along its length, is coincident with an aeromagnetic lineament (Brown, 2006). The Conrad mine is located at the northwest end of this structure. The sub-vertically dipping Conrad and King Conrad lodes were mined underground over a 1.4km strike length and to a maximum depth of 260m. The vein is generally 0.6m thick. The King Conrad lode is interpreted as a splay structure off the main Conrad structure. A body of near-surface disseminated and veinlet sulphide mineralisation, from 20 to 40m wide, was discovered by Malachite’s drilling between the Conrad and King Conrad lodes and is referred to as the ‘Greisen Zone’. The Conrad lode differs from the majority of the polymetallic deposits in the district, in its comparatively large size, style of alteration, ore mineralogy and in the persistence of the mineralisation both along strike and at depth (Brown and Stroud, 1993). Within the Gilgai Granite, the lode is hosted by coarse-grained, feldspar megacrystic, biotite-bearing leucomonzogranite intruded by finer-grained, leucogranitic dykes of ‘microgranite’. Sulphide minerals within the quartz-sulphide veins consist of galena, sphalerite, chalcopyrite, arsenopyrite, pyrite, pyrrhotite and minor stannite, and are generally coarse to very coarse grained, with mono-mineralic aggregates up to several centimetres across; this is a very important attribute from a metallurgical perspective. Trace tetrahedrite and argentite-acanthite have been observed in petrographic work by consultant Paul Ashley. Cassiterite is a vein component. These sulphides are generally developed in irregular bands and massive aggregates in vughy, medium to coarse-grained, massive to crustiform-comb quartz with minor sericite/muscovite, chlorite and carbonate. The King Conrad lode is usually 136
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banded with an arsenopyrite rich margin, a mixed sulphide zone and a quartz core. The mixed sulphide zones contain variable proportions of galena, sphalerite, pyrite, pyrrhotite and chalcopyrite. Pervasive quartz, sericite/muscovite and chlorite alteration surrounds the quartzsulphide lodes as a narrow, irregular envelope. This alteration envelope also contains disseminated sulphides and quartz-sulphide veinlets.
Fig. 1. Conrad Silver Project location plan showing simplified geology and malachite tenements 137
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The Conrad lode was discovered in weathered massive sulphide outcrops in 1888. Underground mining commenced in 1891 and continued until 1912, when the mine closed due to industrial relations problems. During this first phase of mining the main metals recovered were silver, lead and tin, with sulphide concentrates produced by simple gravity methods, some of which were smelted on site. The second phase of mining activity commenced in 1947 when Broken Hill South Limited optioned the property, deepened the Conrad shaft, developed two additional production levels, conducted metallurgical testwork, and built a flotation mill. Broken Hill South operated Conrad as a lead mine from 1955 to 1957 but closed the operation when the lead price collapsed in 1957, and the workings have been flooded since then.
Exploration and evaluation activities Malachite has drilled 113 drill holes, totalling 26,800m, aiming to delineate resources within the Conrad lode, King Conrad lode and Greisen Zone that justify the re-development of a mining and processing operation at Conrad. The resource drilling has been conducted over a 2.2km strike length with most holes piercing the lodes between surface and 300m depth, although the deepest hole intersected the Conrad lode almost 500m below surface (Fig. 2). A broad mineralogical zonation is apparent along strike with Ag-Pb-Zn rich mineralisation at the northwest end in the King Conrad and Conrad lodes and a Ag-Cu-Sn-Pb association towards the southeast in an area referred to as the Princess Shoot (Fig. 2). Using drill intersections to illustrate (and from which core was used for metallurgical test work), drill hole CMRD63 intersected the Conrad Lode some 200m northwest of the Conrad Shaft and assayed 2.60m (0.7m true width) at 430g/t Ag, 0.20% Cu, 8.95% Pb, 4.35% Zn, 0.10% Sn and 31g/t In while CMRD93, located 500m southeast of Davis Shaft, assayed 1.02m (0.6m true width) of 382g/t Ag, 1.76% Cu, 2.01% Pb, 0.15% Zn, 1.36% Sn and 23g/t In. Lode mineralisation at Conrad can be seen from these intersections to contain anomalous levels of indium. Although it is not known what indium minerals are present, it is thought indium is associated with sphalerite because of a Zn-In correlation, and also possibly with chalcopyrite because of an apparent Cu-In association in The Princess Shoot where zinc contents are low.
Fig. 2. Conrad Silver Mine longitudinal section
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Tonnes
Ag g/t
Cu %
Pb %
Zn %
Sn %
In g/t
Global resource estimates Conrad Lode – indicated & inferred (1)
2,254,726 106.2
0.22
1.24
0.46
0.23
6.7
King Conrad Lode – indicated & inferred (1)
397,032
100.2
0.13
1.84
0.93
0.19
7.5
Greisen Zone – indicated & inferred (2)
478,643
40.6
0.02
0.88
0.75
0.13
-
Conrad Lode – indicated & inferred (3)
394,022
230.7
0.37
2.49
0.58
0.43
11.8
King Conrad Lode – indicated (3)
64,278
245.4
0.46
3.81
1.54
0.47
15.2
Total high grade resource – indicated & inferred
458,300
232.8
0.39
2.68
0.72
0.44
12.3
High grade resource estimates
Table 1. Conrad Mineral resource estimates (completed in December 2008) (1) Based on a 1.2m width, with no cut off and allowing for historical depletion (2) Based on a 74g/t AgEQ cut off within a wireframe at a notional 10g/t Ag cut off (3) Based on a 1.2m width, with a 300g/t AgEQ cut off and allowing for historical depletion Ag equivalent equation based on December 2008 metal prices: AgEQ = Ag g/t + 22.5*Pb% + 20.0*Zn% +73.3*Cu% + 203.1*Sn% Drill holes into the greisen zone typically intersect zones of disseminated sulphide mineralisation with grades of approximately 40g/t Ag, 0.9% Pb, 0.8% Zn and 0.1% Sn over true widths of 20-35m. The 150m long greisen zone is pyrrhotite rich, strongly sericite/muscovite altered and has low copper content. In December 2008, resource consultants Hellman & Schofield Pty Ltd completed a resource estimation based on 107 drill holes. Estimates were made for the Conrad Lode, King Conrad Lode and the Greisen Zone (Table 1). The figures for the lodes were based on a fixed underground mining width of 1.2m rather than the actual vein width because 1.2m was regarded as the minimum mining width for mechanised mining. The 1.2m width encompasses all of the quartz-sulphide lode plus some of the adjacent alteration envelope. As high grade shoots are expected to be the initial basis for a development of Conrad, mineral resource estimates were made applying defined shapes and a cut off of 300g/t silver equivalent (as defined in the notes to Table 1). The total high grade resource at Conrad of 458,000 tonnes contains 3.4 million ounces of silver Metallurgical test work was conducted by Metcon Laboratories on drill core of the Conrad Lode and its alteration envelope intersected in holes CMRD63 and CMRD93. As described above, these two lode intersections are representative of the Ag-Pb-Zn and Ag-Cu-Sn-Pb ore types respectively. The CMRD63 sample tested comprised hanging wall, lode and footwall. It assayed 146 g/t Ag, 4.78% Pb, 2.03% Zn, 0.14% Cu and 0.11% Sn. The testing aimed to establish the viability of 139
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gravity pre-concentration to allow wider mining widths than the lode and to determine the flotation response of pre-concentrated material and original composite sample. Gravity pre-concentration of crushed -2mm material achieved a 50% weight reduction with losses of only 7% of the lead and 6% of the silver, but with 21% of the copper and 24% of the zinc lost. The gravity concentrate and original composite sample were subjected to a differential float. Recoveries of Pb and Ag to Pb rougher concentrate for the gravity upgraded feed were 93.1% and 80.1% respectively (excluding losses in gravity stage). These are higher than the 83.5% Pb and 62.9% Ag recoveries to Pb rougher concentrate for the original composite feed. Zinc recovery to Zn concentrate for the gravity upgraded feed was 91.5%. Grades of flotation concentrates produced from the gravity upgraded feed were 21% Cu, 79% Pb (with almost 3kg/t Ag) and 47% zinc respectively. It was concluded there was potential for recovering saleable concentrates for copper, lead-silver and zinc. Flotation tests were conducted on a lode sample of CMRD93 drill core, a composite sample of hanging wall, lode and footwall, and a gravity pre-concentrated sample produced from the composite sample. High recoveries of copper, lead and tin into high grade concentrates were achieved by flotation of the lode sample which contained 463 g/t Ag, 2.92% Pb, 0.29% Zn, 2.36% Cu and 1.90% Sn. Silver recovery was also high, at 81.5%, and was split between the copper and lead concentrates. Copper recovery to concentrate was 94.4% with the concentrate assaying 30.3% Cu, and 2,255g/t Ag (32.4% Ag recovery). Lead recovery was 77.8% into a concentrate assaying 80.0% Pb and 9,348g/t Ag (49.1% Ag recovery). Tin was recovered (82.1% recovery) into a tin concentrate, which assayed 51.9% Sn and only 0.08% Cu, indicating that the tin is present as cassiterite rather than stannite. The structure hosting mineralisation at Conrad can be recognised as an aeromagnetic lineament and as a discontinuous line of old workings over a strike distance of 7.5km which to the southeast of Conrad includes the Spicers, Broadhursts, Prima Donna, Borah Extended and Hickies shafts (Brown and Stroud, 1997). Previous exploration along this Conrad Southeast Extension includes the historic mining and prospecting, induced polarisation (IP) surveying in 1969 by Heinrichs Geoexploration, and four diamond drill holes by Base Minerals Limited in the same year testing IP anomalies. Malachite has explored the Conrad Southeast Extension over a 4.5km strike length by geological mapping, rock and soil geochemical sampling, very low frequency electromagnetic (VLF-EM) surveying and ground magnetic surveying. In addition to the Conrad lode, this work outlined a sub-parallel, 1.3km long structure located approximately 100m southwest of the Conrad Lode and which is interpreted to be a splay off the main Conrad structure. Referred to as Coopers Lode, this sub-parallel structure is expressed as sporadic workings, anomalous Ag-As-Cu-Sn rock geochemistry and a continuous conductor indicated from VLF-EM surveying. The Conrad and Coopers structures are continuous across the Gilgai Granite – Tingha Adamellite contact. A 21-hole reverse circulation percussion drilling program totalling 2,070m was completed over a 1.7km strike length in mid-2010 testing the Conrad and Coopers structures for lode and greisen style mineralisation. The Conrad Lode was intersected as a quartz-sulphide vein with surrounding sericite-sulphide alteration halo, while the Coopers Lode was a sulphide lode (mostly pyrite and arsenopyrite) without vein quartz. In the Tingha Adamellite, 10-20m thick zones of sulphidebearing, sericite-chlorite altered greisenous granite were intersected. The drilling did not intersect economic silver and base metals grades as the dominant sulphide minerals were pyrite and arsenopyrite. Best greisen intersection, approximately 150m southeast of the Gilgai Granite – Tingha Adamellite contact was 9m at 28g/t Ag and 0.23% Cu from 83m, while the best Conrad Lode intersection was returned at Spicers Shaft where the quartz-arsenopyrite lode and alteration halo assayed 4m at 42g/t Ag and 8.38% As from 42m.
Discussion The Conrad quartz-sulphide lode is laterally and vertically extensive. Defining high-grade shoots within the Conrad and King Conrad lodes is the key to a re-opening of the Conrad silver mine. The variability in lode width, sulphide content and sulphide mineralogy in relatively wide spaced drill 140
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holes presents challenges in defining shoots that contain sufficient resources to justify mine development. Surface drilling is expensive as long holes are drilled through unmineralised granite to reach the narrow lode, and the silver and base metals content of the lode is variable over short distances, so any individual hole is not necessarily representative of the lode in that vicinity. Malachite’s drilling has identified two high grade zones or shoots which appear to offer the most likely locations for mine re-development based on metal grades and proximity to surface. The King Conrad lode is characterised by a silver-lead-zinc association and more consistently metal-rich intersections. The Princess Shoot has not been previously mined and is characterised by a silvercopper-tin-lead association. Metallurgical test work on drill core of the lode intersected in CMRD93 into the Princess Shoot achieved high recoveries of silver, copper, lead and tin into high grade concentrates. Other high grade shoots are evident from drilling but are at greater depth and are therefore more expensive to define and evaluate by surface drilling.
Conclusions The next major phase of evaluation at Conrad is likely to involve development of underground access to better understand lode geometry, grade variability and ground conditions and to obtain bulk samples for metallurgical test work. Mining studies and limited, relatively shallow diamond drilling will be required in the planning of underground access, and approvals will need to be obtained. Underground development will allow resource drilling to be completed with much lower metres drilled than by surface drilling, and if mine development proceeds the existing exploration access will reduce capital costs and time to commence mining from that decision to mine. Although the Conrad resource is open at depth along its entire strike length, the focus at Conrad will be on defining sufficient high grade resources at relatively shallow depth to justify a decision to develop a mine. The King Conrad Lode and the Princess Shoot contain high grade resources at depths of less than 250m and their upper zones are the expected locations to be targeted in any underground access development.
Acknowledgements This paper is based on the valuable contributions made by current and former Malachite geologists to the current understanding of the mineralisation at Conrad. These include Brad Wake, Brett McKay, Garry Lowder, Bianca Pietrass-Wong, Mark Derriman, Oliver Bayley, William Reynolds, Grahame Bailey and Scott Szulc, and the figures were drawn by Chris Bannerman.
References Brown, R.E., 2006. Inverell Exploration NSW Geophysics – new data for exploration and geological investigations in the northern New England area of New South Wales. Geological Survey of New South Wales, Quarterly Notes, 111, 1-37 Brown, R.E., and Stroud, W.J., 1993. Mineralisation related to the Gilgai Granite, Tingha-Inverell area. In: Flood, P.G., and Aitchison, J.C. (eds), New England Orogen, eastern Australia, NEO ’93 Conference Proceedings, University of New England, Armidale, 431-447 Brown, R.E., and Stroud, W.J., 1997. Inverell 1:250,000 Metallogenic Map: Metallogenic Study and Mineral Deposit Data Sheets. Geological Survey of New South Wales, Sydney.
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Soil geochemistry and pathfinder element distribution associated with the Hillgrove Antimony-Gold-Tungsten deposit, New England Orogen, NSW. Robert Ellsmore1, Solomon Buckman1, Chris Simpson2 1 School of Earth and Environmental Sciences, University of Wollongong, Australia 2 Geology Manager, Straits Hillgrove Mine, Hillgrove, Australia
Keywords: (Hillgrove, geochemistry, gold, antimony, tungsten, pathfinder)
Introduction Antimony was initially discovered at the township of Eleanora in 1876 and mineral exploration in the Hillgrove region has been pursued by a number of companies throughout the latter half of the 20th century. This work is based on a soil geochemistry honours project supported by Straights Resources at Hillgrove. The project will provide valuable information on pathfinder element distribution and soil geochemistry in the Hillgrove district. The Hillgrove exploration tenement contains over 200 individual mineral deposits, containing predominately antimony, gold and tungsten mineralisation. The project aims to identify the geochemical differences of the soil overlying each of the different rock types. Statistical analysis of the soil tests will identify areas with anomalous values for any of 26 trace elements and identify the locations of any anomalies in the study area. To date there is no published data on soil geochemistry in the Hillgrove district, this project will bridge a knowledge gap in the New England Orogen in terms of identifying the geochemical characteristics of soils developed above different rock types on the plateau regions of the New England. This work will have significant practical applications for future geochemical exploration in the region.
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Fig. 1. The Hillgrove mineral field, the Hillgrove mine, owned by Straits Resources and the study area of this project. The project focuses on the Hillgrove mineral field, (Fig.1) situated in the southern region of the New England Orogen. The Hillgrove mineral field is located approximately 30 kilometers East of Armidale on the Hillgrove plutonic suite, the exploration tenements and mining leases of the Hillgrove mineral field are owned by Straits Resources, cover an area 6 kilometers by 9 kilometers in surface dimensions (Switzer, et al, 2004). The aim of this project is to understand the characteristics of the different regolith types in the Hillgrove region and use geochemistry and geostatistics to identify underlying rocktypes and potentially the location of burried mineral deposits. In this work we describe the;
regolith characteristics of each rock type statistically characterise regolith geochemistry for each rock type look for multi-element correlations for each rock type recommend the best pathfinder elements for exploration for the various deposit types at Hillgrove. comment on the mobility of elements and how dispersion, either chemical or physical should be considered when positioning drill holes to test geochemical targets.
Methods The study area contains 873 soil samples taken by Straits Resources that are being used for this project, the samples were taken in two sections of the study area at 25 meter intervals along 25 North-South transects spaced either 50 meters or 100 meters apart, (Fig. 2). Soil samples were also taken along 6 EastWest transects at 25 metre intervals that intersected the North-South transects. The soil samples represent five of the six geological formations found within the study area. Ground truthing was undertaken to determine the contact points for the different soil types present in the study area to produce a soil map. Comparing the soil samples to their locations on the soil map, the soil samples were categorized into five of the six soil types present. The soil types present in the study area are derived from the geological units present in the study area, these are the Girrakool beds, a quaternary alluvium, a tertiary basalt, a quartzite and a lamprophyre dyke no soil samples were overlying the Hillgrove Adamellite. Domaining the soil samples into the different soil types provides the foundation for the majority of this project.
Figure 2. An outline of the study area, boundaries for the geological units and the locations of the soil samples.
Twelve test pits were dug in the study area, two Fig. 2. Outline of the study area, red line, contact test pits for each geological unit. Where possible points of the various geological units, coloured the two test pits for each unit were dug along lines and the locations of the soil samples, yellow strike and away from the strike of known dots. mineralization zones. Soil profiles were recorded and soil samples taken for each horizon. The physical and chemical attributes of each profile have been taken to characterize the different soil types. Geochemical analysis of the soils samples, based on the soil types was undertaken to produce a number of results. The data was analyzed to identify the best pathfinders for gold, antimony and tungsten on each of the five soil types that had soil samples taken. Box and whisker plots were used to find anomalous values for a variety of elements on each of the 5 soil types. The anomalous values are to be used for geospatial analysis to create element zonation maps for a gold, antimony, tungsten and a number of pathfinder elements. 143
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Results
Arsenic ppm.
The anticipated results and outcomes for this project will show the diversity both physically and chemically between the regolith overlying each of the different rock types in the Hillgrove mineral field. Geochemical analysis has identified which pathfinder elements are most useful for identifying each of the major mineral deposits in the field and potentially within each rock types present. For the lamprophyre dyke, the best pathfinder for gold is silver, with an R-squared correlation value of 0.863, tungsten and antimony are the best pathfinders for each other with a correlation value of 0.917. In the quaternary alluvium, gold and antimony are the best pathfinders for each other, the R-squared correlation value is 0.687, and this value isn’t high enough for the elements to be considered pathfinder elements for each other. For tungsten, the best pathfinder is arsenic, the R140 R² = 0.9572 squared correlation has a value of 0.936. In the basalt soil, there are 120 a number of elements that are good pathfinders, for gold, silver, arsenic, bismuth, lead, mercury, 100 tungsten, zinc, cadmium and tin all have R-squared values over 80 0.8, (Fig 3.). For antimony in the basalt soil, no element has an Rsquared correlation value greater 60 than 0.8, although there are a number that have values between 40 0.6 and 0.8, these elements are gold, mercury, lead, zinc, 20 tungsten and tin, the highest of these values is 0.723 for arsenic. A number of pathfinder elements 0 for tungsten in a basalt soil have 0 0.02 0.04 0.06 0.08 0.1 values greater than 0.8, these are Gold ppm. gold, silver, arsenic, bismuth, mercury, lead, zinc, cadmium and tin, the highest R-squared correlation value is 0.99 for lead. Figure 3. Bivariate scatterplot of Gold vs Arsenic in basalt soil. For the quartzite soil there is no element with an R-squared correlation value greater than 0.8 the highest value is 0.621 for antimony. Tungsten has 2 pathfinder elements in the quartzite, antimony has an R-squared value of 0.917 and arsenic has a value of 0.944, for the antimony, tungsten is a viable pathfinder element as previously identified, with a value of 0.917, arsenic is also a pathfinder with a value of 0.941. On the Girrakool Beds, the R-squared correlations are much smaller, the highest correlation for gold is 0.131 from antimony, tungsten’s highest value comes from mercury, a value of 0.242 and the highest value for antimony is 0.374. Table 1. Results from box and whisker plots for gold on the 5 soil types, (figure. 4) Indicates the start points both high and low for anomalous values and the number of anomalies in each soil type.
From the data on R-squared correlations for gold, antimony and tungsten a number of pathfinder elements have been identified for the different soil types. Antimony is a useful pathfinder for gold on several of the soil types as are silver and arsenic. Useful pathfinder elements for antimony across the range of soil types are arsenic and tungsten. For tungsten, the best pathfinder element overall is antimony, although there is no single element that has a very high correlation in more than 2 of the soil types. 144
NEO 2010 Using box and whisker plots, anomalous values for each of the elements tested in the soil samples for each soil type were found. The box and whisker plot indicates the point at which the soil sample values become anomalous. The box and whisker plots for gold, (Fig. 4) show the range of values of gold for each soil type, the maximum and minimum anomalies, where applicable and if they fit within the graphs range.
Discussion The accuracy of the R-squared correlations for four of the five soils is questionable considering the small sample size of the soil samples taken. A larger number of soil samples taken on the soils for the lamprophyre dyke, quartzite unit, tertiary basalt and quaternary alluvium may result in reductions to the R-squared correlation values for the best pathfinder elements. This would coincide with the much smaller R-squared values found for the Girrakool Beds soil. The geospatial analysis has yet to be completed, but when it is it will identify the locations of the study area that have the highest readings of each of the important elements in regards to the project. Using the values obtained from table 1, the element zonation maps
Conclusions Multi-element soil geochemistry was undertaken across zones of mineralisation at Hillgrove to show the lateral variations in element concentration associated with various rock types and across zones of mineralisation. The potential of identifying underlying rock types using trace element signatures in surface soils has the benefit of identifying areas that are host to favourable host lithologies to mineralisation as well as identifying zones of cover such as the Tertiary basalts in which surface geochemistry is ineffective. Vertical profiles showed significant variations in metal concentrations according to the different horizons sampled. 0.1
Gold ppm.
References
0.09
0.06
Switzer, C. K, Ashley, P. M, Hooper B and Roach B, 2004. The Hillgrove gold-antimonytungsten district, NSW, Australia. Australasian Institute of Mining and Metalurgy Publication, 5, p 381-383.
0.05
.
0.08 0.07
0.04 0.03 0.02 0.01
Min
Quartzite
Basalt
Quaternary Alluvium
Lamprophyre Dyke
Girrakool Beds
0
Max
Fig. 4. Box and whisker plot of gold in the five soils
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The Doonba dunite deposit, Barraba, NSW Peter English1, Paul Ashley2 1
P.W. English and Associates, 22 Moyes St, Armidale, NSW 2350
2
Paul Ashley Petrographic and Geological Services, 37 Bishop Crescent, Armidale, NSW 2350
Keywords: dunite, olivine, serpentine, cement, refractories, agriculture
Introduction The Doonba dunite deposit occurs in the Great Serpentinite Belt of northern NSW and represents a potential large resource of industrial grade dunite (olivine-rich rock). Dunite can be utilised in a range of industrial and agricultural applications and the Doonba deposit has been investigated for commercial exploitation.
Geological setting The deposit occurs about 18 km NE of Barraba, northern NSW within the Great Serpentinite Belt (GSB). In the area, the GSB comprised a dismembered suite of ophiolitic rocks, including variably serpentinised harzburgite, dunite and wehrlite, together with extensive gabbro (Bryeley, 1990; Brown et al., 1992). The ophiolitic sequence is approximately 1.5 km wide and strikes just west of north. Immediately to the west of the deposit, the GSB abuts with fault contact against the Carboniferous age Caroda Formation (Brown et al., 1992), with this contact being portion of the Peel Fault, a regionally extensive terrane boundary. The rocks at the dunite deposit were first recognised as being olivine-rich and little-serpentinised by Breyley (1990) and potential for refractory-type olivine was mentioned in Brown et al. (1992). Detailed mapping of the deposit region by Breyley (1990) and mapping and magnetometry by Henshaw (2001) have outlined an area dominated by weakly serpentinised dunite of approximately 4 hectares (Fig. 1). Much of the area to the south, east and north is occupied by variably serpentinised wehrlite (containing a little thinly layered chromitite) (Fig. 1). To the west of the dunite mass and possibly tectonically emplaced, is a mass of massive and schistose serpentinised harzburgite. This body occupies the western segment of the ophiolitic mass and has an inferred (covered) fault contact against the Caroda Formation (Fig. 1). The dunite and wehrlite masses have cumulate textures and are interpreted to have formed by crystal settling processes, e.g. from a gabbroic magma chamber. On the other hand, the harzburgite to the west of the dunite is likely to represent a sliver of mantle tectonite.
Rock type, mineralogy and geochemistry The dunite is a massive grey-green, dense rock (specific gravity ~3.1 g/cm3) composed of abundant coarse olivine . Mostly, it appears little serpentinised, although it is cut by numerous narrow shear zones in which there is more or less complete serpentinisation. Near surface, pale brown weathering rinds have developed on the dunite (Fig. 2), but these effects only penetrate downwards a few metres and most of the deposit consists of fresh rock. Petrographically, the dunite is composed of abundant coarse olivine (original grainsize up to 12 cm), with a little interstitial clinopyroxene and chromite. Olivine has been strongly fractured and serpetninisation has proceeded along the fractures forming the typical mesh texture of massive serpentinites, although much olivine remains (typically between 40-80 volume %) (Fig. 3). The main serpentine mineral that has formed is likely to be lizardite, although minor chrysolite could be present. Because serpetninisation is not advanced, the rock remains a greenish colour and very little magnetite has formed, so that magnetic susceptibility readings remain low, in contrast to the serpentinised harzburgite to the west.
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NEO 2010 Breyley(1990) found that the olivine in the dunite was forsteritic in composition (Fo90-91), with up to 0.5% NiO. Whole rock analyses of the dunite by Breyley (1990) confirmed very high MgO contents (42-43%), with very low ( 0.4). In particular, there is a hiatus between the inheritance ages and those from the melt-precipitated zircon at about the age of the Carboniferous-Permian boundary (299 Ma), as a result of which the two groups of zircon are clearly separated. Of 56 inheritance analyses from the Banalasta pluton, 36 gave Carboniferous ages in the range ~ 345–300 Ma and four Ordovician-Devonian ages (~ 465–360 Ma). Twenty four of 75 inheritance analyses from the Linton pluton gave Carboniferous ages of ~ 350–300 Ma. That granite also contained some much older inheritance than in the Banalasta pluton, Proterozoic to Cambrian age (n=8; ~ 1575–495 Ma). The 18Ozrc values of the Carboniferous inherited zircon were also measured. They showed a large range from 5 to 10 ‰, consistently more primitive than the melt-precipitated zircon. Banalasta inheritance had both a more heterogeneous and higher mean of 18Ozrc value. The range of 18Ozrc in the Carboniferous inheritance from the Bundarra Supersuite, and particularly from the Banalasta pluton, appears to increase from the early to late Carboniferous (Fig. 2). The minimum values are anchored around the mantle value (18Ozrc=5.3 ± 0.3 ‰, Valley, 2003), but the maxima increase up to the level found in the S-type granites of the LFB (Ickert et al., 2008; Kemp et al., 2009). The inheritance ages are mostly Carboniferous, ~ 350–300 Ma, a range that closely coincides with the age range of Carboniferous volcanic rocks (~ 350–300 Ma; Roberts et al., 2006 and references therein). Other older inheritance was probably derived from minor clastic sediments associated with the volcanic detritus. These inheritance ages eliminate Devonian arc materials as possible sources for the Bundarra Supersuite. The source must have been Carboniferous volcanic deposits. If so, the isotopic compositions are representative of a Carboniferous arc-chain formed in a subduction system. The high range of 18Ozrc indicates some crustal contamination of primitive magma probably derived from the mantle. The amount of crustal material incorporated into that magma appears have increased as the arc magmatism continued. Roberts & Engel (1987) found compositional changes in the volcanic detritus (dacitic-rhyolitic) from early to late Carboniferous that could have resulted from crustal contamination. Contamination possibly gave rise to the higher heterogeneity and increasing maximum 18Ozrc values from the early to the late Carboniferous, particularly in the southern region near the current location of the Banalasta pluton. The greater heterogeneity of the inheritance in Banalasta correlates with the higher dispersion of 18Ozrc values in the melt-precipitated rims from that sample, probably because the granite melts retained some memory of the original source rock heterogeneities.
References Bryant, C.J., Chappell, B.W., Bennett, V.C. and McCulloch, M.T., 2004. Lithium isotopic compositions of the New England Batholith: correlations with inferred source rock compositions. Transactions of the Royal Society of Edinburgh: Earth Sciences, 95, 199-214. Flood, R.H. and Shaw, S.E., 1977. Two “S-type” granite suites with low initial 87Sr/86Sr ratios from the New England Batholith, Australia. Contributions to Mineralogy and Petrology, 61, 163-173. Glen, R.A., 2005. The Tasmanides of eastern Australia. Geological Society, London, Special Publications, 246, 23-96. Gray, D.R. and Foster, D.A., 2004. Tectonic evolution of the Lachlan Orogen, southeast Australia: historical review, data synthesis and modern perspectives. Australian Journal of Earth Sciences, 51, 773-817. Hensel, H.D., McCulloch, M.T. and Chappell, B.W., 1985. The New England Batholith: constraints on its derivation from Nd and Sr isotopic studies of granitoids and country rocks. Geochimica et Cosmochimica Acta, 49, 369-384.
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NEO 2010 Ickert, R.B., Hiess, J., Williams, I.S., Holden, P., Ireland, T.R., Lanc, P., Schram, N., Foster, J.J. and Clement, S. W., 2008. Determining high precision, in situ, oxygen isotope ratios with a SHRIMP II: Analyses of MPI-DING silicate-glass reference materials and zircon from contrasting granites. Chemical Geology, 257, 114-128. Kemp, A.I.S., Hawkesworth, C.J., Collins, W.J., Gray, C.M. and Blevin, P.L., 2009. Isotopic evidence for rapid continental growth in an extensional accretionary orogen: The Tasmanides, eastern Australia. Earth and Planetary Science Letters, 284, 455-466. McCulloch, M.T. and Chappell, B.W., 1982. Nd isotopic characteristics of S- and I-type granites. Earth and Planetary Science Letters, 58, 51-64. O'Neil, J.R., Shaw, S.E. and Flood, R.H., 1977. Oxygen and hydrogen isotope compositions as indicators of granite genesis in the New England Batholith, Australia. Contributions to Mineralogy and Petrology, 62, 313-328. Roberts, J. and Engel, B.A., 1987. Depositional and tectonic history of the southern New England Orogen. Australian Journal of Earth Sciences, 34, 1 - 20. Roberts, J., Offler, R. and Fanning, M., 2006. Carboniferous to Lower Permian stratigraphy of the southern Tamworth Belt, southern New England Orogen, Australia: Boundary sequences of the Werrie and Rouchel blocks. Australian Journal of Earth Sciences, 53, 249-249. Shaw, S.E. and Flood, R.H., 1981. The New England Batholith, Eastern Australia: Geochemical Variations in Time and Space. Journal of Geophysical Research, 86, 10530-10544. Tuttle, O.F. and Bowen, N.L., 1958. Origin of granite in the light of experimental studies in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O. New York. Valley, J.W., 2003. Oxygen Isotopes in Zircon. In: Hanchar, J.M. & Hoskin, P.W.O. (eds.) Zircon. Mineralogical Society of America and The Geochemical Society, 53:1, 343-380. Wilkinson, J.F.G., 1969. The New England Batholith. In: Packham, G. H. (ed). Geology of New South Wales. Journal of Geological Society of Australia, 16(1), 271-278. Williams, I.S., 1998. U-Th-Pb Geochronology by Ion Microprobe. In: McKibben, M.A., Shanks III, W.C. & Ridley, W.I. (eds.) Reviews in economic geology: Applications of microanalytical techniques to understanding mineralizing process. Society of Economic Geologists, 1-35.
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Geochemical and geochronological evolution of the Tamworth Belt, southern New England Orogen R.J. Korsch1, P.A. Cawood2,3, A.A. Nemchin4 1
Onshore Energy & Minerals Division, Geoscience Australia, Canberra, Australia
2
School of Earth and Environment, University of Western Australia, Perth, Australia
3
Present address: Department of Earth Sciences, University of St Andrews, Scotland 4
Department of Applied Geology, Curtin University, Perth, Australia
Keywords: New England Orogen, Tamworth Belt, geochemistry, Nd isotopes, SHRIMP geochronology
Introduction The New England Orogen contains a geological record dominated by subduction-related rocks, indicating that the orogen has been part of, or adjacent to, convergent plate margins of eastern Gondwanaland from at least the Cambrian until the end of the Early Cretaceous (~95 Ma). In the Late Devonian, the Orogen records the change from an island arc setting to an Andean-style convergent continental plate margin (e.g., Flood & Aitchison 1992; Skilbeck and Cawood, 1994). The rock record prior to the Middle Devonian is fragmentary, but the Late Devonian to Carboniferous components of the continental margin magmatic arc, forearc basin and accretionary wedge system are well preserved in the New England Orogen, with the Lachlan Orogen, Thomson Orogen and Drummond Basin to the west being in a backarc setting at this time. This system ended in the Late Carboniferous, with the subduction zone stepping to the east (Cawood, 1984). Nevertheless, until at least the Early Cretaceous, the Australian component of the continental margin of East Gondwanaland faced the Proto-Pacific (Panthalassan) Ocean, and has been interpreted to form part of a subduction-related convergent plate margin (e.g., Powell 1984; Cawood 2005; Glen 2005). Here, we examine aspects of the southern New England Orogen from the Cambrian to the Early Permian to further document the nature of the convergent plate margin over this period of time. We are interested especially in the Tamworth Belt, where the changeover is recorded from the Cambrian-Late Devonian island arc setting, to the development of the Devonian-Carboniferous continental margin in a convergent plate setting, with its well-developed forearc basin and accretionary wedge. The island arc component is referred to as the Gamilaroi Terrane by Aitchison and Flood (1995) and Offler and Gamble (2002). We have sampled sandstones from Cambrian to Late Carboniferous stratigraphic units in the Tamworth Belt (Table 1), and, to complement the work of Korsch et al. (2009), from the accretionary wedge from about Woolomin around the Texas Orocline to Jackadgery. We have undertaken thin section petrography and whole rock geochemistry on 32 samples, and Nd isotopes and SHRIMP geochronology on detrital zircons from selected samples.
Results The sandstone samples are predominantly quartz-poor, volcanic-derived, lithic sandstones, which are poorly sorted and contain angular grains. The presence of ferromagnesian minerals (pyroxene, hornblende) in some samples indicate that these are first-cycle sediments that have not been reworked (see also Korsch, 1984; Cawood, 1991). The samples were analysed for major, trace and rare-earth element whole rock geochemistry in the Geochemistry Laboratory at Geoscience Australia. Geochemically, the sandstones from the Cambrian to the Early Carboniferous (Tournaisian) are rather primitive, with SiO2 contents in the range 55-63%;; most fall in the island arc or active continental margin fields on the tectonic discrimination diagram of Roser and Korsch (1986). The discriminant function plot of Roser and Korsch (1988) indicates that the samples have been derived from a mafic to felsic igneous protolith. Younger samples are more silica-rich (Caroda 196
NEO 2010 Formation 67%;; Emu Creek Formation 65%;; Silver Spur beds 79%), all falling in the active continental margin field on the tectonic discrimination diagram of Roser and Korsch (1986). εNd isotope analyses were undertaken on the majority of samples by Roland Mass at Melbourne University. Three Cambrian and Ordovician samples have very positive values (εNd +7.5 to +5.6), indicating derivation from a juvenile source terrane. Three Devonian samples and the sample from the Early Carboniferous (Tournaisian) Tangaratta Formation are also juvenile (εNd +6.6 to +5.6), but four younger samples are isotopically more evolved (εNd +2.7 to -1.3). Eleven samples of sandstones from the Tamworth Belt, ranging in age from Middle Cambrian to Early Carboniferous, plus one from the Late Carboniferous Emu Creek Formation and one Early Permian sandstone from the Silver Spur beds were processed to separate detrital zircons prior to SHRIMP geochronology. Four samples yielded no zircon: Middle Cambrian Murrawong Creek Formation, MiddleLate Cambrian Pipeclay Creek Formation, Ordovician Haedon Formation, and the Early Devonian (Late Emsian-Eifelian) Silver Gully Formation. Three other samples yielded only scarce zircon: Early Devonian (Emsian) Drik Drik Formation (2 grains), Early Devonian (Emsian) Northcotte Formation (10 grains), and the Late Devonian Baldwin Formation (7 grains). The six younger samples all yielded abundant zircon (Table 1). Table 1. Lithological units sampled in this study, arranged in stratigraphic age from oldest at the bottom to youngest at the top. FORMATION
STRATIGRAPHIC AGE
ZIRCON YIELD
Silver Spur beds
Early Permian
abundant
Emu Creek Formation
Late Carboniferous
abundant
Caroda Formation
Early Carboniferous
abundant
Tulcumba Sandstone
Early Carboniferous
abundant
Tangaratta Formation
Early Carboniferous
abundant
Keepit Conglomerate
Late Devonian
abundant
Baldwin Formation
Late Devonian
rare
Silver Gully Formation
Early Devonian
nil
Northcotte Formation
Early Devonian
rare
Drik-Drik Formation
Early Devonian
rare
Haedon Formation
Ordovician
nil
Pipeclay Creek Formation
Middle-Late Cambrian
nil
Murrawong Creek Formation
Middle Cambrian
nil
The sample from the Keepit Conglomerate has a unimodal peak with an age of ~366 Ma, consistent with its Famennian stratigraphic age. No older grains were recorded. A similar situation was found for the sample from the Tangaratta Formation, where the unimodal peak of ~357 Ma is consistent with its Early Carboniferous (Tournaisian) stratigraphic age; again, no older grains were recorded. The sample from the Tulcumba Sandstone also contained no older grains and had a unimodal peak of ~347 Ma, which is late Tournaisian, consistent with its Early Carboniferous (Tournaisian) stratigraphic age. 197
NEO 2010 The sample from the Caroda Formation has a unimodal peak at ~352 Ma, which, following Gradstein et al. (2004), is in the Tournaisian, and is significantly older than its Visean stratigraphic age. The sample contained only one older zircon grain, of ~415 Ma. Roberts et al. (2004) report a SHRIMP U-Pb zircon age of 342 ± 4 Ma from the Barney Springs Andesite member of the Caroda Formation, which is consistent with its Visean stratigraphic age. To the south, the Merlewood Formation is an approximate time equivalent of the Caroda Formation (Mory, 1981). Roberts et al. (2006) report SHRIMP U-Pb zircon ages for three ignimbrites within the Merlewood Formation: ~352 Ma Isismede Ignimbrite Member (3 samples), ~343 Ma Burnewang Ignimbrite Member (3 samples), and ~327 Ma Wheelihans Gap Ignimbrite Member (1 sample). The ~352 Ma age determined here from the sandstone sample in the Caroda Formation is identical to the ages from the Isismede Ignimbrite Member of the Merlewood Formation, supporting the lateral correlation of the two formations. Although the SHRIMP age for the Tulcumba Sandstone is younger than that from the Caroda Formation, they are within error of each other. A sample from the Emu Creek Formation, in the Drake area, considered to be part of the forearc basin which was folded around the Texas Orocline, has a dominant peak with an age of ~323 Ma, which is consistent with its stratigraphic age of Late Carboniferous (Namurian-Westphalian). The sample also had eight older grains, with the oldest being Mesoproterozoic. A sample from the Silver Spur beds, in the Texas area, has a more complicated distribution of detrital zircon ages, with a main peak at ~320 Ma, and minor peaks at ~290 Ma and ~350 Ma, along with several older grains, the oldest being of Mesoproterozoic age. As the Silver Spur beds are Early Permian, most of the detritus in this sample has been recycled from older sources.
Discussion Geochemical and isotopic indicators in the sandstones from the Tamworth Belt, such as the low silica contents and juvenile εNd isotopic values, indicate the succession from the Cambrian Murrawong Creek Formation to the Late Devonian Baldwin Formation, which forms the Gamilaroi Terrane, was derived from a juvenile, volcanic arc source. It is interesting to note that detrital zircon is either absent or extremely scarce in these sandstones. These are the rocks that are considered to have been related to the island arc setting outboard of the Gondwana margin, an interpretation supported by Leitch and Cawood (1987), who showed that volcanic clasts from a conglomerate in the Murrawong Creek Formation were derived from a low-K island arc. The abundance of detrital zircon in the sandstones, first seen in the Keepit Conglomerate, reflects a significant change in the chemistry of the volcanism in the source region. The Keepit Conglomerate is considered to be the basal unit of the forearc basin related to the convergent continental plate margin, and marks the changeover from the island arc to continental margin setting. All samples analysed from the Keepit Conglomerate to the Early Permian Silver Spur beds contain abundant zircon, in distinct contrast to all samples older than the Keepit Conglomerate. The ~323 Ma (Late Carboniferous) age of the sandstone from the Emu Creek Formation is essentially identical to SHRIMP ages from the Clifden Formation in the Tamworth Belt determined by Roberts et al. (2003), and indicates that the forearc was still dominated by first-cycle volcanic detritus well into the Late Carboniferous. It is not until the Early Permian that we see an abundance of zircon grains with SHRIMP ages older than the depositional age determined by paleontology, suggesting a change in tectonic environment from one dominated by an active volcanic arc to one where there was considerable recycling of older material from the source area into the depocentre. In summary, the changeover from a Cambrian to Late Devonian tectonic setting dominated by the island arc environment of the Gamilaroi Terrane, to one of an Andean-style convergent continental plate margin from the Late Devonian to the Late Carboniferous is recorded by the change in the provenance and geochemistry of the sandstones, in particular, the influx of detrital zircons at the start of the convergent continental margin system.
Acknowledgements We thank Chris Foudoulis and his team at Geoscience Australia for the careful zircon separations on the forearc basin samples, and Bill Pappas and Liz Webber for the excellent geochemical analyses. U-Pb zircon 198
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References Aitchison, J.C. and Flood, P.G., 1995. Gamilaroi Terrane: a Devonian rifted intra-oceanic island-arc assemblage, NSW, Australia. In: Smellie, J.L., ed., Volcanism associated with extension at consuming plate margins. Geological Society, London, Special Publication, 81, 155-168. Cawood, P.A., 1984. The development of the SW Pacific margin of Gondwana: correlations between the Rangitata and New England orogens. Tectonics, 3, 539–553. Cawood, P.A., 1991. Characterization of intra-oceanic magmatic arc source terranes by provenance studies of derived sediments. New Zealand Journal of Geology and Geophysics, 34, 347-358. Cawood, P.A., 2005. Terra Australis Orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic. Earth-Science Reviews, 69, 249–279. Flood, P.G. and Aitchison, J.C., 1992. Late Devonian accretion of the Gamilaroi Terrane to eastern Gondwana: provenance linkage suggested by the first appearance of Lachlan Fold Belt derived quartzarenite. Australian Journal of Earth Sciences, 39, 539–544. Glen, R.G., 2005. The Tasmanides of eastern Australia. In: Vaughan, A.P.M., Leat, P.T. and Pankhurst, R.J., eds., Terrane processes at the margins of Gondwana. Geological Society, London, Special Publication, 246, 23–96. Gradstein, F.M, Ogg, J.G. and Smith, A.G., 2004. A geological time scale 2004. Cambridge University Press, Cambridge. Korsch, R.J., 1984. Sandstones compositions from the New England Orogen, eastern Australia: implications for tectonic setting. Journal of Sedimentary Petrolog,y 54, 192-211. Korsch, R.J., Adams, C.J., Black, L.P., Foster, D.A., Fraser, G.L., Murray, C.G., Foudoulis, C. and Griffin, W.L., 2009. Geochronology and provenance of the Late Paleozoic accretionary wedge and Gympie Terrane, New England Orogen, eastern Australia. Australian Journal of Earth Sciences, 56, 655-685. Leitch, E.C. and Cawood, P.A., (987. Provenance determination of volcaniclastic rocks; the nature and tectonic significance of a Cambrian conglomerate from the New England fold belt, eastern Australia. Journal of Sedimentary Petrology, 57, 630-638. Mory, A.J., 1981. A review of Early Carboniferous stratigraphy and correlations in the northern Tamworth Belt, New South Wales. Proceedings of the Linnean Society of New South Wales, 105, 213-236. Offler, R. and Gamble, J., 2002. Evolution of an intra-oceanic island arc during the Late Silurian to Late Devonian, New England Fold Belt. Australian Journal of Earth Sciences, 49, 349-366. Powell, C.McA., 1984. Late Devonian and Early Carboniferous: continental magmatic arc along the eastern edge of the Lachlan Fold Belt. In: Veevers J. J., ed., Phanerozoic earth history of Australia, Clarendon Press, Oxford, 329–340. Roberts, J., Wang, X. and Fanning, M., 2003. Stratigraphy and correlation of Carboniferous ignimbrites, Rocky Creek region, Tamworth Belt, Southern New England Orogen, New South Wales. Australian Journal of Earth Sciences, 50, 931–954. Roberts, J., Offler, R. and Fanning, M., 2004. Upper Carboniferous to Lower Permian volcanic successions of the Carroll-Nandewar region, northern Tamworth Belt, southern New England Orogen, Australia. Australian Journal of Earth Sciences, 51, 205-232. Roberts, J., Offler, R. and Fanning, M., 2006. Carboniferous to Lower Permian stratigraphy of the southern Tamworth Belt, southern New England Orogen, Australia: boundary sequences of the Werrie and Rouchel blocks. Australian Journal of Earth Sciences, 53, 249-284.
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Genesis and age of magmas of the Hillgrove Batholith, southern New England Orogen Bill Landenberger 1, Seann McKibbin 2 and Bill Collins 3 1
School of Environmental & Life Sciences, The University of Newcastle, Australia 2
3
Research School of Earth Sciences, Australian National University, Australia
School of Earth and Environmental Sciences, James Cook University, Australia
Keywords: Late Carboniferous, magmagenesis, tectonics.
Introduction Intrusive igneous suites in the southern New England Orogen span in age from the latest Carboniferous to late Triassic (Fig. 1). The Hillgrove Batholith (informal name) constitutes the oldest intrusive batholith within the accretion-subduction complex of the southern New England Orogen. The batholith is defined here as comprising The Hillgrove Supersuite and the Bakers Creek Supersuite (informal name). The Hillgrove Suite was originally defined by Binns et al. (1967) and several member plutons have been added since by Pogson and Hitchins (1973), Shaw and Flood (1981) and Gilligan et al. (1992). The latter definition included 23 plutons including the Blue Knobby, Campfire, Eastlake, Enmore, Gara, Garibaldi, Glenifer, Gostwyck, Henry River, Hillgrove, Ingleba, Kimberley Park, Kookabookra, Murder Dog, Rockvale and Tobermory monzogranites, together with the Abroi, Argyll, Dundurrabin, Rockisle, Tia and Winterbourne granodiorites. The Hillgrove Supersuite as defined here, includes all of these with the tentative addition of the Harnham Grove Porphyritic Microtonalite which has an age (Black, 2007) within error of other members of the supersuite. The Hillgrove Supersuite is dominated by S-type granites (Flood and Shaw, 1977). In addition to feldspars and quartz, the only other major phase occurring in most Hillgrove Supersuite granitoids is biotite, which forms up to 20% in the more mafic samples. Ilmenite is the sole opaque phase. Some members of the supersuite bear minor almandine garnet, while others have minor secondary actinolite and in rare instances, minor primary hornblende. Most members of the supersuite bear some degree of solid state deformation, varying from relatively massive varieties to intensely deformed mylonites along some eastern margins of the batholith. The Bakers Creek Supersuite (informal name) includes a number of mafic to intermediate intrusives and complexes. The group has previously been described as >mafic tholeiitic and calcalkaline intrusives spatially associated with the Hillgrove Suite (Hensel et al. 1985). The supersuite includes the Apsley River, Camperdown, Cheyenne, Dorrigo Mountain, Moona Plains and Sheep Station Creek complexes, the Bakers Creek, Mornington and Woodburn diorites, Barney House Gabbro and Days Creek Gabbro. Compositions across the Bakers Creek Supersuite vary widely from olivine, two-pyroxene gabbros, through hornblende-biotite diorites to mafic hornblende-bearing granodiorites, often all occurring within the one complex. Some complexes (e.g. Sheep Station Creek Complex) even include minor monzogranite phases identical to S-types of the Hillgrove Supersuite. Like the Hillgrove Supersuite, deformation is variably developed, but overall less apparent due to the doleritic textures exhibited by many members of the supersuite.
Age of the batholith Early attempts at dating members of the Hillgrove Batholith yielded widely varying ages. Biotite K/Ar and biotite-whole rock Rb/Sr pairs gave ages varying from 242 Ma up to 296 Ma, which record cooling and/or 201
NEO 2010 uplift ages (Landenberger et al. 1995). Whole-rock Rb/Sr isochron methods yielded ages of 295±25 Ma (Flood and Shaw 1977) to 310±12 Ma (Hensel et al. 1985), but these have poor precision due to variations in initial 87Sr/86Sr ratios within the supersuite (Landenberger et al. 1995). Zircon U/Pb dating (IDTIMS and SHRIMP) has since confirmed a latest Carboniferous intrusive age for the supersuite. These data include 300±4 Ma and 302±4 Ma for the Abroi and Tia granodiorites respectively (Collins et al. 1993), and an age of 303±4 Ma for the Rockvale Monzogranite (Kent 1994). If the Harnham Grove Porphyritic Microtonalite is to be included in the supersuite, this extends the intrusive phase into the earliest Permian (295±2 Ma, Black 2007). The spatial distribution of the batholith and dated samples is shown if Fig. 2. Reliable intrusive ages for the Bakers Creek Supersuite have been similarly elusive, with a single K/Ar age of 291 Ma for the Bakers Creek Diorite (Pogson and Hilyard, 1980) and Rb/Sr whole rock/biotite ages varying from 261 Ma to 284 Ma (Hensel, 1982). Hensel (1982) also inferred these mafic complexes to be younger than the Hillgrove granitoids, suggesting a middle Permian age. As with the Hillgrove Supersuite, zircon U/Pb dating (SHRIMP) has confirmed a latest Carboniferous age. These data include 302±3 Ma (Bakers Creek Diorite), 304±3 (Barney House Gabbro), 305±3 (Days Creek Gabbro) and 295±3 Ma (Charon Creek Diorite); with the latter again inferring the intrusive activity extended into the earliest Permian (Fig. 2). In the few places where reliable contact exposure can be found, field relations demonstrate that the Hillgrove Supersuite granitoids intruded after the mafic complexes of the Bakers Creek Supersuite, with the former showing quenched margins against the latter.
N W
E
100
S
ren Cl a
0
km
eton Mor ce -
Surat Basin
Figure 1. Batholiths of the New England Orogen
Basin
Tablelands Complex
LEGEND Mesozoic basins
Peel
Area of Fig. 2
Late Permian - Triassic granitoids
—
o Tamw
Barrington Tops Batholith (mid Permian) Bundarra Batholith (early Permian) ning Man
rth
Early Permian basins
Belt
Hillgrove Batholith (latest Carboniferous) Nambucca Basin
Serpentinite Carboniferous volcanic centres
lt Fau
Accretion/subduction complex Forearc basin sequences
ki – Moo Hunter
Sy st em
ni ng
Gloucester Basin
Th
Sydney
M an
ru st
B
in as
Pacific Ocean Myall Basin
Basin Newcastle
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Figure 2. Map of the Hillgrove Batholith showing zircon ages
LEGEND
305 ±3 Ma
300 ±4 Ma
Guyra
295 ±3 Ma
303 ±3 Ma
Hillgrove Supersuite Zircon U-Pb ages (SHRIMP & IDTIMS)
Dorrigo
Ebor
Bakers Creek Supersuite Zircon U-Pb ages (SHRIMP)
Bellingen
Armidale Hillgrove
302 ±3 Ma
Uralla
0
20 km
304 ±3 Ma
40 N
Post-Triassic faults
Roads
295 ±2 Ma
Late Permian faults Walcha
Latest Permian-Triassic
Granites & volcanics
Early-middle Permian
Sediments and volcanics Hillgrove Supersuite
Latest Carboniferous Hillgrove Batholith
302 ±4 Ma
Bakers Creek Supersuite Accretion complex metasediments and metavolcanics
Pre- late Carboniferous
Nowendoc
Figure 3. Radiogenic isotope results from New England metasediments, and Hillgrove Supersuite granites 10
Gabbro Initial ratios calculated @ 300 Ma
Amphibole-bearing Hillgrove Supersuite
6
"Fertile" greywackes All metasediments (Tablelands Complex) Bakers Creek Supersuite (depleted end-member)
4 2
Nd
Hillgrove Batholith
Hillgrove Supersuite
8
Bundarra Suite (Hensel et al. 1985)
0 -2 Greywackes
-4 -6
Pelites
-8 -10 0.702
0.704
0.706
0.708
0.71
0.712
0.714
0.716
0.718
8786
Sr/Sr
initial
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Magma sources, isotopes and contamination Hillgrove Supersuite Despite relatively low initial 87Sr/86Sr initial ratios, low positive εNd values, low ASI values and high Na2O contents, the majority of Hillgrove Supersuite granitoids are considered as S-type granites (Flood and Shaw 1977, Landenberger 2001). The geochemistry and isotopic signatures simply reflect the relatively juvenile composition and depleted isotopic character of the available metasedimentary sources. More specifically, source rocks for these granites can be narrowed down to volcanogenic greywackes with SiO2 contents in the range of 63-67%. Only greywackes in this compositional range overlap with the Sr and Nd isotopic ratios of the granitoids (Fig. 3). Calculated melt fertilities of various potential source rocks, based on Q-Ab-Or compositions relative to the ternary eutectic at 5 kb, also indicate that these greywackes are the most likely sources to produce large volumes of partial melt. Major and trace element modelling indicate that the more mafic magmas of the suite (68-70% SiO2) were produced by ~48% partial melting of the intermediate greywacke source, under water-undersaturated conditions involving biotite breakdown at granulite facies conditions and mid crustal (~5 kb) depths (Landenberger, in prep). Trace element patterns for the granitoids bear remarkable similarity not only to the accretion complex greywackes, but also to the earlier Carboniferous volcanics which are their provenance (Fig. 4). While there is limited isotopic variation within the supersuite, the more unusual amphibole-bearing members have slightly more depleted Sr and Nd isotopic ratios. This is evident in Fig. 3, and the variation is consistent with minor contamination by magmas of the Bakers Creek Supersuite. Bakers Creek Supersuite Compositional variation is much greater within the Bakers Creek Supersuite, ranging from olivine gabbros (down to ~46%SiO2) to hornblende-biotite granodiorites (~67% SiO2) and even granophyres. Trace element patterns and isotopic compositions are equally variable. The most mafic members of the supersuite are olivine, two-pyroxene tholeiitic gabbros with high modal plagioclase (up to An90) and ophitic textures. These gabbros also contain minor (up to 5% modal) but ubiquitous interstitial phlogopite and brown magnesian hornblende. Ilmenite is commonly the sole opaque phase. These gabbros also have highly depleted initial isotopic ratios. One gabbro from the Sheep Station Creek Complex has the most depleted isotopic ratios for any intrusive within the New England Orogen (87Sr/86Sr 0.7026, εNd 9.55, Fig. 5). MORB-normalised trace element patterns are relatively flat, but display small prominent negative Nb-Ta anomalies (Fig. 6), suggesting mantle derivation in a juvenile back-arc basin setting (Jenkins et al. 2002). Like the gabbros, the Bakers Creek Suite diorites are also dominated by plagioclase, and characteristically have ophitic textures. The plagioclase in these diorites is not as calcic as that in the gabbros, and often display oscillatory zoning and corroded cores. The ferromagnesian minerals present are dominantly hornblende and biotite, but augite is also present in most specimens, showing variable breakdown to secondary actinolite. Quartz is a common interstitial phase and ilmenite is usually the only opaque phase. Geochemically, the diorites are more calc-alkaline in character. Multi-element patterns are remarkably similar to the greywackes and Hillgrove granitoids (Fig. 6). Trace element compositions plot between the gabbros and greywacke/granitoid compositions, but are consistently closer to the latter. While the diorites remain relatively low in silica contents, trace element contents are indicative of a strong crustal influence, either through magma mixing processes, or by direct contamination by metasedimentary components of the accretionary prism. Contamination is also supported by the isotopic ratios. In contrast to the isotopically depleted gabbros, initial ratios of most of the diorites are indistinguishable from the Hillgrove granitoids and the fertile greywackes (Fig. 5). Although major element considerations preclude large proportions of bulk assimilation or mixing, the higher (up to 10 times) elemental concentrations of Nd and Sr in the crustal components swamp the isotopic ratios of the gabbro end-member. Other members of the supersuite form a hyperbolic array between the most depleted gabbros and crustal contaminants. Modelling demonstrates that the bulk composition and isotopic ratios of the diorites can be produced through crustal contamination/mixing processes combined with fractional crystallisation (McKibbin et al., in prep).
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Figure 4. Trace element trends for Carboniferous volcanics, Hillgrove Suite, source greywackes and melt modelling
10000 Rock/N-type MORB (Sun & McDonough, 89)
Modelled partial melt 48% Hillgrove Supersuitite granitoids (latest Carboniferous) Melt-fertile greywackes accretionary prism Carboniferous volcanics (Kuttung Arc)
1000 100 10 1 .1 .01
Sr Cs Ba Th Ta La P Zr Sm Gd Dy Er Lu Cr K Rb Pb U Nb Ce Nd Hf Eu Ti Y Yb Sc Ni
Figure 5. Sr and Nd isotopic results for late Carboniferous magmas & sources 10
Hillgrove Supersuite Initial ratios calculated @ 300 Ma
Barney Houses Gabbro
Bakers Creek Supersuite
6 Apsley River Complex
4 2
Nd
0
Hillgrove Batholith
8
Big Bull Gabbro
"Fertile" greywackes Charon Creek Diorite
All metasediments (Tablelands Complex)
Bakers Creek Diorite Camperdown Complex
Days Creek Gabbro Mornington Diorite
Woodburn Diorite
-2 Greywackes
-4 -6
Pelites
-8 -10 0.702
0.704
0.706
0.708
0.71
0.712
0.714
0.716
0.718
8786
Sr/Sr
initial
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Figure 6. Trace element patterns for gabbros and diorites of the Bakers Creek Supersuite compared with granitoids of the Hillgrove Supersuite. 1000 Bakers Creek Supersuite gabbros Bakers Creek Supersuite diorites Melt-fertile greywackes
Rock/N-type MORB (Sun & McDonough, 89)
100
Hillgrove Supersuite (S-type)
10
1
.1
.01
Rb Ba Th U K Nb Ta La Ce Sr P Zr Hf Ti Y Sc Cr Ni
Conclusions Intrusion of the Hillgrove Batholith began in the latest Carboniferous and extended into the earliest Permian. During this period, mafic magmas of the Bakers Creek Supersuite intruded mostly prior to the granitoids of the Hillgrove Supersuite, as supported by field relations. Once large-scale partial melting in the mid crust was underway to produce the more voluminous S type magmas, buoyancy considerations would preclude mafic magmas proceeding to higher levels. The chemistry and isotopic ratios of the most uncontaminated gabbros demonstrate that magma generation occurred in a juvenile backarc setting. This is in agreement with the slab-rollback tectonic setting outlined in Jenkins et al. (2002). Although the production of S-type granitoids of the Hillgrove Supersuite can be modelled on derivation from accretionary prism greywackes, and only in rare instances show minor mantle end-member contamination, the mafic magmas of the Bakers Creek Supersuite show highly variable degrees of crustal contamination. The mantle end-members of this supersuite are highly depleted and demonstrate derivation from a depleted asthenospheric mantle wedge. In contrast, the more evolved diorites have trace element patterns and isotopic ratios similar to the S-type granites, consistent with a combination of fractional crystallisation and contamination by fertile crustal components or S-type magmas.
References Binns, R.A., Chappell, B.W., Flood R.H., Gunthorpe R.J., Hobson E., Neilson M.J., Ransley J.E. and Slade M.J., 1967. Geological map of New England 1:250,000 - New England Tableland, southern part, with explanatory text. University of New England. Armidale. NSW. Black, L.P., 2007. [2005844055] OZCHRON Geochronology Database. Geoscience Australia, Canberra, Australia. http://www.ga.gov.au/ Collins, W.J., Offler, R., Farrell, T.R. and Landenberger, B., 1993. A revised Late Palaeozoic - Early Mesozoic tectonic history for the southern New England Fold Belt. In: Flood, P.G. & Aitchison, J.C. (eds) New England Orogen, eastern Australia University of New England, Department of Geology and Geophysics, conference volume 1993 pp. 69-84. 206
NEO 2010 Flood, R.H. and Shaw, S.E., 1977. Two S - type granite suites with low initial 87Sr/86Sr ratios from the New England Batholith, Australia. Contributions to Mineralogy and Petrology, 61, 163-173. Gilligan, L.B., Brownlow, J.W., Cameron, R.G. and Henley, H.F., 1992. Dorrigo - Coffs Harbour 1:250000 Metallogenic Map and explanatory notes. Geological Survey of New South Wales, Sydney. Hensel, H.D., 1982. The mineralogy, petrology and geochemistry of granitoids and associated intrusives from the southern part of the New England Batholith. Unpubl. PhD thesis, University of New England, Armidale, N.S.W. Hensel, H.D., McCulloch, M.T. and Chappell, B.W., 1985. The New England Batholith : constraints on its derivation from Nd & Sr isotopic studies of granitoids and country rocks. Geochimica et Cosmochimica Acta, 49, 369-384. Jenkins, R.B., Landenberger, B. and Collins, W.J., 2002. Late Palaeozoic retreating and advancing subduction boundary in the New England Fold Belt, New South Wales. Australian Journal of Earth Sciences, 49, 467-489 Kent, A.J.R., 1994. Geochronology and geochemistry of Palaeozoic intrusive rocks in the Rockvale region, southern New England Orogen, New South Wales. Australian Journal of Earth Sciences, 41, 365-379. Landenberger, B., 2001. Sediment maturity and the fertility of S-type granite sources: A lesson from the Hillgrove Suite, New England batholith. In: Chappell B.W. and Fleming P. (eds) S-type Granites and Related Rocks - AGSO Record 2001/2, 71-72. Landenberger, B. In prep. Sediment maturity and the fertility of S-type granitoid sources: An example from the southern New England Orogen, Australia. (Perspectives on Granite Petrogenesis: the Origin and Evolution of the Concept of I- and S-type Granites and Related Rocks Allan J. R. White Special Issue). Landenberger, B. and Collins, W.J., 2000. Gabbroids, basalts and lamprophyres of the New England Batholith: A legacy late Carboniferous - Middle Triassic arc migration. Geological Society of Australia Abstracts 59, 289. Landenberger, B., Farrell, T.R., Offler, R., Collins, W.J. and Whitford, D.J., 1995. Tectonic and metamorphic implications of Rb/Sr biotite dates for the Hillgrove Plutonic Suite, New England Orogen, NSW, Australia. Precambrian Research 71, 251-263. McKibbin, S.J., Landenberger, B., Collins, W.J., Foden, J.C. and Fanning, M. In prep. Mixing and contamination of Late-Carboniferous magmas in the New England Orogen. Pogson, D.J. and Hilyard, D., 1980. Results of isotopic age dating related to Geological Survey of New South Wales investigations, 1974-1978. Records of the Geological Survey of New South Wales 20, 251-273. Pogson, D.J. and Hitchins, B.L., 1973. New England 1:500,000 Geological sheet, Geological Survey of NSW, Sydney. Shaw, S.E. and Flood, R.H., 1981. The New England Batholith eastern Australia: geochemical variations in time and space. Journal of Geophysical Research 86, 10530-10544.
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A chapter in the orogenic history of an accretionary orogen: the Early Permian transition in the southern New England Fold Belt Evan Leitch1, Peter Cawood2,3, Renaud Merle4, Alexander Nemchin4 1
Department of Environmental Sciences, University of Technology, Sydney, NSW 2007, Australia 2
School of Earth and Environment, University of Western Australia, WA 6009, Australia
3
Present address: Department of Earth Sciences, University of St. Andrews, North Street, St. Andrews KY16 9AL, UK 4
Department of Applied Geology, Curtin University, WA 6845, Australia
Keywords: New England Fold Belt, Early Permian, Gondwanide Orogeny, zircon ages
Introduction The geological history of the southern New England Fold Belt spans the Palaeozoic during which time it developed as an accretionary orogen fronting the Eo-Pacific. Throughout most of its history it was the site of convergent plate margin tectonics with a well-defined division in the Devonian and Carboniferous into a western magmatic arc – forearc basin terrain (Tamworth Belt) and an eastern accretionary subduction wedge (part of Tablelands Complex). At the end of the Carboniferous the tectonic framework changed with the cessation of convergent margin activity replaced in the Early Permian (sensu lato) (ca. 299 Ma – 270 Ma) by widespread compressive deformation, metamorphism, the emplacement of S and I type granites, and rift basin evolution. This comprised the earlier part of the Gondwanide Orogeny in East Gondwana that we refer to as the Tablelands Phase. Subsequent compressive deformation, metamorphism and silicic magmatism extending from ca.270 Ma to 230 Ma comprises the Hunter-Bowen Phase (Korsch et al. 2009a) and is not considered in detail here.
New SHRIMP zircon ages The principal new data presented here are SHRIMP(II) U-Pb zircon ages (Table 1) determined in the John de Laeter Centre of Mass Spectrometry at Curtin University. All were determined with reference to the CZ3 zircon standard. Full analytical details and a more complete discussion of the results are currently being prepared for publication (Cawood et al. in prep). In discussing these results the numerical timescale adopted for the Permian is that of Wardlaw et al. (2004). Ages, rounded to the nearest 1 Ma, are: CarboniferousPermian boundary 299 Ma; Asselian 299 – 295 Ma; Sakmarian 295 – 284 Ma; Artinskian 284 Ma – 276 Ma; Kungurian 276 Ma – 271 Ma. Biostratigraphically determined ages have been fitted to this time scale by way of the correlations between Australian brachiopod and palynological zones and internationally recognized stages as indicated by Briggs (1998).
The end of subduction associated with the Keepit arc and the Tablelands subduction wedge The ending of subduction linked to the eruption of magmatic arc rocks along the western (Keepit) arc and the construction of the Tablelands subduction accretion wedge occurred at about the Carboniferous-Permian boundary. Although the western part of the New England Fold belt was dominated by volcanic activity throughout the Carboniferous and the Early Permian there was, coincident with a stratigraphic break separating the rocks of the two periods, a major change in the composition of the rocks (Jenkins et al. 2002; Roberts et al. 2006). Those of Carboniferous age are predominantly calc-alkaline dacites and rhyolites, with 208
NEO 2010 less widespread andesites whereas those of Early Permian age are a bimodal suite with the basalts having a geochemical affinity with back arc magmas. SHRIMP zircon ages indicate that arc magmatism terminated between about 306 Ma and 291 (Roberts et al. 2006). Subduction complex volcanic sandstones sourced from the Keepit arc have zircon provenance ages as young as 320 Ma (Korsch et al. 2009b). A maximum age for the subduction complex protolith of migmatites of the Wongwibinda Complex is provided by relic detrital zircon grains which have ages as young as ~320 Ma (S. Craven and N. Dazco in Leitch et al. 2010) and this maximum could be as young as ~305 Ma on the basis of the youngest inherited zircons in the S-type granites (Cawood et al. in prep).
Deformation close to the Carboniferous-Permian boundary The importance of compressive deformation in New England at about the Carboniferous-Permian boundary has been a matter of some debate. However there is now a body of evidence from across the fold belt indicating a widespread phase of compressive deformation of this age. The evidence is clearest in the Tablelands Complex where foliations and folds of likely or established late Carboniferous age are unconformably overlain by Early Permian rocks, have been radiometrically dated, and/or are transgressed by Early Permian granites. At Wongwibinda in the eastern Tablelands Complex multiply deformed HTLP regional metamorphic rocks include syn-metamorphic folds and foliations dated at 297 ± 2 Ma (Asselian) by electron microprobe (EMP) chemical dating of monazite in migmatites (Daczko and Craven in Leitch et al. 2010). We have dated the Abroi Granodiorite (Table 1), that was emplaced in the Wongwibinda Complex during the waning stages of deformation (D1-D2 of Farrell 1988) at ~290 Ma (Sakmarian) . It is noteworthy that foliations formed in the Wongwibinda Complex during D1 and D2 dip very steeply (Farrell 1988) and Danis et al. (2010), in a detailed study of metamorphism at Wongwibinda, could find no pressure gradient across the ca. 10 km wide complex, a gradient that might have been expected if the deformation had been extensional and foliations initially formed with a subhorizontal orientation and only steepened during later (Hunter-Bowen) deformation. Dirks et al. (1992) described a sequence of deformation episodes in the Tia Complex that included HTLP metamorphism of up to amphibolite grade in a compressive environment (D3 – D5) during the latter stages of which the ~296 Ma Tia Granodiorite (Table 1) was emplaced. Imbricate thrust and melange structures in the western Tablelands Complex are overprinted by a sub-vertical cleavage that is axial planar to steeply plunging, sinistral macroscopic folds (Cawood 1980; Corbett, 1976). In the north the folds are transgressed by plutons of the 292 ± 2 Ma Bundarra Suite (Table 1) the age of which thus provides a younger age limit on folding and foliation development. Further evidence for regional deformation and metamorphism in the Tablelands Complex in the Early Permian is provided by the truncation of the regional foliation in the Tablelands Complex at its unconformable contact with Early Permian Barnard Basin strata in the Manning region (Leitch et al. 2010), in the southwest Nambucca Slate Belt (Degeling and Runnegar 1979) and in the Texas region (Lennox and Flood 1997). Cao (1994) documented two cleavages and associated folds in the northeast Tamworth Belt, the younger of which he considered on geometric grounds to be a correlative of the pre-Bundarra Suite structures of the western Tablelands Complex. This suggests Late Carboniferous or earliest Permian deformation. Early Permian rocks rest unconformably on Carboniferous rocks. Allan and Leitch (1990) estimated that more than 4500 m of sedimentary rocks were removed from the eastern Tamworth Belt prior to the onset of Early Permian deposition. The degree to which these unconformities result from compressive deformation versus tectonic stripping during Early Permian rifting (see below) remains to be established.
S-type granites Two S-type granite suites have been recognized in southern New England Fold Belt. The Bundarra Suite forms a meridonal belt in the west of the Tablelands Complex associated with chert-greenstone abundant accretionary rock assemblages. The Hillgrove Suite is more widely distributed in a broad northeast trending zone in the central part of the Tablelands Complex, emplaced into accretionary rocks that are predominantly trench-fill turbidites. In the past the Hillgrove Suite has been accorded Pb/U ages of about 300 Ma. Our SHRIMP zircon measurements on 6 samples from 5 separate plutons (Table 1) indicate a spread of ages from 296 Ma to 289 Ma, (late Asselian to middle Sakmarian). The Bundarra Suite has previously yielded slightly younger ages than the Hillgrove Suite, largely on the basis of Rb/Sr biotite dates that averaged 275 Ma (Shaw and Flood 1981). Measurements on zircon concentrates from two samples located in the southern half of the suite have 209
NEO 2010 yielded ages of 288 Ma and 292 Ma (middle Sakmarian) (Table 1). Thus emplacement of the two S-type suites was at least in part contemporaneous. Table 1. New U/Pb SHRIMP(II) zircon ages Sample Number
Rock Unit
Location*
Age**
NE1/05
Bundarra Suite
053997 Mundowey
292.3 ± 1.5 Ma
(9036-I-S) NE3/05
Bundarra Suite
885533 Drummond
287.8 ± 3.6 Ma
(9037 I-S) NE8/05
Abroi Granodiorite
093232 Jeogla
290.2 ± 2.5 Ma
(9336-IV-N) NE74/07
Abroi Granodiorite
191366 Maiden Creek
289.3 ± 1.9 Ma
(9337-3-S) NE13/05
Dundurrabin Granodiorite
560592 Dundurrabin
290.3 ± 5.5 Ma
(9437-IV-S) NE75/07
Rockvale Granodiorite
032380 Thalgarrah
292.6 ± 2.4 Ma
(9237-2-S) NE77/07
Tia Granodiorite
775538 Brackendale
295.7 ± 2.8 Ma
(9235-IV-S) NE79/07
Kilburnie Monzogranite
477545 Weabonga
288.6 ± 1.5 Ma
(9135-I-S) NE66/07
Kaloe Granodiorite
546367 Camelback
291.9+/-2 Ma
(9439-3-S) NE23/05
NE25/05
NE27/05
NE76/07
Barrington Tops Granodiorite
608717 Cobark
Duncans Creek Trondhjemite
311215 Nundle
Duncans Creek Trondhjemite
314213 Nundle
Halls Peak Volcanics
074982 Big Hill
267.2 ± 1.4 Ma
(9234-3-S) 252.1 ± 3.2 Ma
(9135-III-S) 249.4 ± 2.9 Ma
(9135-III-S) 292.6 ± 2.0 Ma
(9336-IV-S) *Grid reference, 1:25 000 Sheet Name, Sheet Number in brackets. Maps published by Central Mapping Authority of New South Wales. **Analytical uncertainties shown at 2-sigma level.
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NEO 2010 The present distribution of the two S-type suites is in divergent belts, this is probably largely a result of postEarly Permian oroclinal folding. Unfolding of the Texas-Coffs Harbour orocline and the removal of the affects of similar age deformation from the south-central Tablelands Complex may yield a distribution of Hillgrove Suite plutons parallel to but several tens of kilometres east of the Bundarra belt (Collins et al. 1993).
Early Permian I-type granites The presence of Early Permian I-type granites in the New England Fold Belt was first reported by Bryant et al (1997) who reported a 40Ar/39Ar plateau age of 293.1+/- 1.8 Ma for the Kaloe Granodiorite. We sampled the same locality from which we obtained a zircon SHRIMP age of 291.9+/-2.0 Ma confirming the Bryant et al determination (Table 1). I-type plutons of similar age have recently been dated in the Texas region with SHRIMP U-Pb zircon ages of ~298.1 ± 2 Ma for the Jibbinbar Granite, 291.5 ± 2.2 Ma for the Bullaganang Granite, and 279.6 ± 2.6 Ma for the Greymare Granodiorite (Cross et al. 2009; Donchak et al. 2007). These data indicate that Early Permian (Asselian-Artinskian) I-type granites are more widespread than earlier recognized and were focused along the western margin of the Tablelands Complex prior to oroclinal folding.
Early Permian basinal deposition Within the southern New England Fold Belt Early Permian sequences up to several thousand metres thick occupy a series of fault blocks and slivers that Leitch (1988) proposed had once formed part of a north northeast-trending rift basin he termed the Barnard Basin. Basin fill consists of clastic, commonly deep water rocks, notably diamictite (pebbly mudstone and sandstone), interstratified with conglomerate, sandstone and siltstone. Mafic and silicic lava, tuff and breccia are a minor component but occur widely. The most extensive sequences occur in the Manning and Nambucca structural blocks. Thinner sequences occur in the eastern Tamworth Belt, the western part of the Tablelands Complex and on the Texas Block, where elongate slices of Early Permian rocks are commonly bounded on one side by an unconformity with older rocks and on the other by faults the latter possibly a normal structure during basin subsidence but reactivated with reverse movement during later compressional deformation. Both biostratigraphic (Briggs 1998) and radiometric dating indicate that deposition commenced in the Asselian or earliest Sakmarian. A younger limit on the commencement of the Barnard Basin is provided by the U-Pb zircon age reported herein of 292.6 ± 2.0 Ma for the Halls Peak Volcanics (Table 1). The youngest rocks so far identified as part of the basin sequence are Late Artinskian. Briggs (1998) has indicated a widespread stratigraphic break in New England in the latest Artinskian-Early Kungurian he interpreted as a product of mid-Permian tectonism and this may have brought to an end sedimentation in the Barnard Basin.
Discussion and conclusions Recent dating indicates a need to refine the tectonic timetable for the latest Carboniferous – Early Permian interval in the southern New England Fold Belt. Rounding ages and ignoring the small analytical uncertainties the following picture emerges for the Tablelands orogenic phase: the end of plate convergence associated with the western magmatic arc and subduction accretion in the Tablelands Complex occurred immediately before (post-305 Ma) the onset of widespread compressive deformation and HPLT metamorphism in the eastern Tablelands Complex (around 296 - 297 Ma). The latter episode overlapped with the emplacement of the oldest dated Hillgrove Suite granites (ca. 296 Ma). Plutons of the latter suite continued to be emplaced (293Ma – 288 Ma) during extensional opening of the Barnard Basin (commenced by at least 293 Ma). Bundarra Suite plutons (292 Ma – 288 Ma) were emplaced contemporaneously with at least the younger dated Hillgrove Suite bodies and there is no difference in age between the oldest S- and Itype bodies. Deposition in the Barnard Basin probably ceased at about 275 Ma at which time there was an influx of detritus from the north into the Sydney Basin. The timetable outlined entails a refinement of recent plate tectonic schemes for New England in the Early Permian. For example the three sequential stages portrayed by Jenkins et al. (2002 Fig. 7(b) to (d)) include critical phenomena that would now seem to be substantially isochronous, as also do the second and third tectonic cycles of Collins et al. (1993).
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NEO 2010 There is little evidence for any collisional event, for example the accretion of an exotic terrane or more specifically defined buoyant element, as the cause of the shut-off of subduction related magmatism and accretion in New England, and associated deformation. As in modern non-collisional orogens compressive deformation was most likely the product of advance of the over-riding plate relative to the down-going plate (Schellart 2008). This deformation was of short duration and immediately succeeded by Early Permian rifting of the Barnard Basin and the ongoing emplacement of S- and I- type granites that had commenced during the waning stages of deformation. Eastward movement of the magmatic arc has been proposed but the location of any Early Permian arc is disputed. It might have been manifest briefly by the belts of Early Permian granites in the Tablelands Complex but other magmatic rocks of this age are described as of backarc affinity. The opening of the Barnard Basin was part of a much larger extensional episode that affected the eastern third of Australia (Leitch 1988; Korsch et al. 2009b) and may not have been a by-product of contemporary convergence. The changes outlined lie within the period of major collisional orogenesis that led to the final assembly of Pangea and ultimately it is likely that the Gonwanide Orogeny including both the Tablelands and Hunter-Bowen Phases were linked to this major ‘far-field’ plate reorganisation (Cawood and Buchan 2007).
References Allan, A.D., and Leitch, E.C., 1990, The tectonic significance of unconformable contacts at the base of Early Permian sequences, southern New England Fold Belt. Australian Journal of Earth Sciences, 37, 43 - 52. Briggs, D.J.C., 1998, Permian Productidina and Strophalosiidina from the Sydney-Bowen Basin and New England Orogen: systematics and biostratigraphic significance. Australian Association of Palaeontologists, Memoir , 19, 1 -258. Bryant, C.J., Cosca, M.A., and Arculus, R.J., 1997, 40Ar/39Ar ages of the Clarence River Supersuite intrusions from the northern portions of the New England Orogen. Geological Society of Australia Special Publication, 19, 242-253. Cao, X., 1994, Structure and tectonics of the eastern Tamworth Belt, Manilla, NSW. Unpublished M.Sc (Hons) thesis, Macquarie University, NSW, 1-213. Cawood, P.A., 1980, The geological development of the New England Fold Belt in the Woolomin-Nemingha and Wisemans Arm regions: The evolution of a Palaeozoic fore-arc terrain. Unpublished PhD thesis, University of Sydney, NSW, 1-429. Cawood, P.A., and Buchan, C., 2007, Linking accretionary orogenesis with supercontinent assembly, EarthScience Reviews, 82, 217-256. Cawood, P.A., Leitch, E.C., Merle, R., and Nemchin, A.A., (in prep), Orogenesis without collision: stablizing the Terra Australis accretionary orogen, eastern Australia. Collins, W.J., Offler, R., Farrell, T.R., and Landenberger, B., 1993, A revised Late Palaeozoic-Early Mesozoic tectonic history for the southern New England Fold Belt. In Flood, P.G. and Aitchison, J.C. (eds.), New England Orogen, eastern Australia:, Department of Geology and Geophysics, University of New England, Armidale, 69-84. Corbett, G.J., 1976, A new fold structure in the Woolomin beds suggesting a sinistral movement on the Peel Fault. Australian Journal of Earth Sciences, 23, 401-406. Cross, A.J., Purdy, D.J., Bultitude, R.J., Dhnaram, C.R., and von Gnielinski, F.E., 2009, Joint GSQ-GA NGA geochronology project, New England Orogen and Drummond Basin, 2008. Queensland Geological Record 2009/03. Danis, C.R., Daczko, N.R., Lackie, M.A., and Craven, S.J., 2010, Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history. Australian Journal of Earth Sciences, 57, 357-375. Degeling, P.R., and Runnegar, B., 1979, New Early Permian fossil localities at Halls Peak and their regional significance. Geological Survey of New South Wales, Quarterly Notes, 36, 10-13. Dirks, P.H.G.M., Hand, M., Collins, W.J., and Offler, R., 1992, Structural-metamorphic evolution of the Tia Complex, New England Fold Belt; thermal overprint of an accretion-subduction complex in a compressional back-arc setting. Journal of Structural Geology, 14, 669-688. 212
NEO 2010 Donchak, P.J.T., Bultitude, R.J., Purdy, D.J., and Denaro, T.J., 2007, Geology and mineralisation of the Texas Region, south-eastern Queensland, Department of Mines and Energy, Queensland Geology, 11. Farrell, T.R., 1988, Structural geology and tectonic development of the Wongwibinda Metamorphic Complex. In Kleeman, J.D. (ed.), New England Orogen; Tectonics and Metallogenesis, University of New England, Armidale, 117-124. Jenkins, R.B., Landenberger, B., and Collins, W.J., 2002, Late Palaeozoic retreating and advancing subduction boundary in the New England Fold Belt, New South Wales. Australian Journal of Earth Sciences, 49, 467-489. Korsch, R.J., Adams, C.J., Black, L.P., Foster, D.A., Fraser, G.L., Murray, C.G., Foudoulis, C., and Griffin, W.L., 2009a, Geochronology and provenance of the Late Paleozoic accretionary wedge and Gympie Terrane, New England Orogen, eastern Australia. Australian Journal of Earth Sciences, 56, 655-685. Korsch, R.J., Totterdell, J.M., Cathro, D.L., and Nicoll, M.G., 2009b, Early Permian East Australian Rift System: Australian Journal of Earth Sciences, 56, 381-400. Leitch, E.C., 1988, The Barnard Basin and the Early Permian Development of the southern part of the New England Fold Belt. In Kleeman, J.D.(ed.), New England Orogen, Tectonics and Metallogenesis, University of New England, Armidale, 61-67. Leitch, E.C., Daczko, N.R., and Fergusson, C.L., 2010, Overview of the southeast New England Fold Belt, Specialist Group in Tectonics and Structural Geology Field Guide No. 15, Geological Society of Australia, 1-39. Lennox, P.G., and Flood, P.G., 1997, Age and structural characterization of the Texas megafold, southern New England Orogen, eastern Australia, Geological Society of Australia Special Publication, 19, 161-177. Roberts, J., Offler, R., and Fanning, M., 2006, Carboniferous to Lower Permian stratigraphy of the southern Tamworth Belt, southern New England Orogen, Australia: boundary sequences of the Werrie and Rouchel blocks. Australian Journal of Earth Sciences, 53, 249-284. Schellart, W.P., 2008, Overriding plate shortening and extension above subduction zones: A parametric study to explain formation of the Andes Mountains. Geological Society of America Bulletin, 120, 1441-1454. Shaw, S.E., and Flood, R.H., 1981, The New England Batholith, eastern Australia: Geochemical variations in time and space. Journal of Geophysical Research, 86B, 10530-10544. Wardlaw, B.R., Davydov, V., and Gradstein, F.M., 2004, The Permian Period, In Gradstein, F.M., Ogg, J.G., and Smith, A.G. (eds.), A Geologic Time Scale 2004, Cambridge University Press, 249-270.
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Emplacement and deformation recorded in the Hastings Block - constraints from serpentinite bodies and structures within and adjacent to the block. Paul G. Lennox1, Robin Offler2 1 2
School of Biological, Earth and Environmental Sciences, University of New South Wales, Australia
Discipline of Earth Sciences, School of Environmental & Life Sciences, University of Newcastle, Australia
Keywords: Hastings Block, enigmatic, translation, rotation, orocline.
Introduction The Hastings Block (HB) consists of mainly Devonian to Carboniferous arc-derived sedimentary and volcanic rocks and an overlying Early Permian sedimentary sequence located on the outboard edge of the subduction complex and the northern margin of the fore-arc Tamworth Belt (TB) of the southern New England Orogen. It is bounded by faults on four sides against the Nambucca, Yarrowitch and Port Macquarie blocks and the TB. Serpentinites are present in many of these faults and contain structures, which constrain the later movement histories of them. The HB is enigmatic and no satisfactory model exists to explain its emplacement. Several models have been advanced which involve the northward displacement of the Hastings Block from the TB along several faults. Some of these models also involve translation, rotation or a combination of both. These will be examined in the light of the structures in the serpentinites, the orientation of sequences in the TB and the palaeomagnetic evidence from the northern HB. Further, the folding and cleavage development within the northern HB will be compared with the adjacent Nambucca Block for evidence that constrain the timing of deformation in this block. For the purposes of this paper the southern HB is considered to have been contiguous with the northern HB during emplacement.
Geology of the Hastings Block According to Roberts et al. (1995), the sequences in the TB and HB were deposited in arc-related settings. The sequences in the HB are similar in age to those in the TB and of similar metamorphic grade (Offler et al. 1997; Roberts et al. 1995). However, the conditions of deposition and the composition of the volcanics are different. In the southern HB, an andesitic provenance persisted in the Early Carboniferous but in the TB a more acid dacitic provenance existed at this time. Further, in the Early Carboniferous, sediments in the HB were deposited in deep water and did not change to shallow water-continental conditions until the Namurian. By contrast in the TB, conditions changed from deep water to shallow water at the Devonian-Carboniferous boundary eventually leading to the deposition of continental deposits. Post Namurian, both the TB and HB experienced a similar history (Roberts et al. 1995).
Timing and evidence of emplacement It is considered that the HB was emplaced sometime between the end of the Namurian and the beginning of the Asselian because there is a depositional hiatus during this period (Roberts & Geeve 1999). The original position of the HB prior to displacement isn’t known. However, we do know that the Hastings Block cannot have been immediately adjacent to the eastern end of the TB because the geology is different (Lennox & Roberts 1988). This difference in characteristics is in part because of translation of the TB by sinistral movement on the Peel-Manning Fault System (Offler & Williams 1987) but cannot be the only factor. Given that the shape of modern island arcs is arcuate rather than straight (Roberts & Geeve 1999), it is likely that the curvature shown by TB is original and that the displaced portion came from the SE.
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Models of emplacement Cawood (1982) proposed that the HB was displaced sinistrally from the TB by a major N-S trending fault based on oolitic limestone present in both blocks. The problem with this model is that the Carboniferous rocks of the TB face east, whereas those in the northern HB face west. Thus the Cawood model fits neither the geology of the eastern end of the TB nor the facing direction of the Carboniferous sequences. Korsch and Harrington (1987) and Pisarevsky et al. (2009) proposed that the Hastings Block formed part of the Manning Orocline. This involved anticlockwise ductile-bending of the southern end of the TB and accretionary complex around this orocline. Mapping of the TB sequences indicates this is unlikely as they are oriented north-south in the hinge zone of this orocline whereas they should be oriented east-west (Roberts & Engel 1987). A variation on the model proposed by Cawood (1982) has been put forward by Lennox and Roberts (1988) who suggested that northward-translation (porpoising) was accompanied by rotation of the HB between N-S-trending, strike-slip faults. The rotation would explain the E facing of the Hastings Block but does not resolve the differences between rock sequences in the TB and HB. Recently, Rosenbaum (2010) on the basis of the distribution of Early Permian granites argued for the presence of the Manning Orocline. However, the few bedding and cleavage trends recorded in the hinge zone of the proposed orocline do not support the existence of this structure. Palaeomagnetic studies by Schmidt et al. (1994) indicated either a 130o clockwise or 230o anticlockwise rotation of the northern HB. The 130o rotation we believe supports the model of Lennox and Roberts (1988) as outlined above. Roberts & Geeve (1999) proposed from palaeomagnetic evidence that four of the six blocks in the southern part of the Tamworth Belt were allochthonous and underwent sinistral rotation on northeast-striking faults with the Hastings Block being temporarily north of the Myall Block before being moved northwestward between the Peel Fault System and another similar striking fault to its present position. This model shows the difficulties of repositioning the Hastings Block if it originally formed part of the TB. Studies of the structures in serpentinites at Yarras within a suture zone on the western margin of the HB by Lennox and Offler (2009) to confirm or negate the sinistral movement required to translate the HB to its current position, indicate dominantly sinistral strike-slip and oblique-slip movement. Such a movement would be consistent with models involving northward-translation of the Hastings Block relative to the accretionary complex rocks to the west. Similar structures in serpentinites and adjacent sediments at Mt George on the southern margin of the Hastings Block indicate the Manning Fault System underwent dextral, strike-slip and then sinistral strike-slip movement (Jenkins & Offler 1996; Lennox & Offler 2009). In view of these observation and other contradictory kinematic histories obtained elsewhere around the margins and within the HB, we believe the serpentinites are recording events later than the emplacement of the HB. We base this on the fact that serpentinites are easily deformed and strained. Thus it is likely that the structures observed in the various serpentinite bodies reflect accommodation of the serpentinites to the Hunter-Bowen Orogeny or later deformation events.
Deformation in the Hastings Block Folding Lennox et al. (1999) showed that the northern HB records evidence for two and possible three deformation events that produced macroscropic and mesoscopic folds but little or no cleavage because of the more siliceous nature of the rock sequences. The overlying Permian units at the northern, northeastern and eastern margins of the northern HB show well-developed pencil to slaty cleavage development. In the northern HB, the first deformation is associated with initially east-west trending folds and cleavage. These structures are overprinted by northwest-trending mesoscopic folds formed at the same time as the Parrabel Dome. The last deformation produced southwest-plunging folds. These early east-west structures correspond to the dominant structures observed in the Nambucca Block (Leitch 1978, Johnston et al. 2002), whilst the later northwesttrending folding corresponds to Leitch’s (1978) D5 folds or Johnston et al. (2002) D3 event. The dominantly E-W trending earlier cleavages (S1 and S2) observed in the Nambucca Block has formed between 273 and 260 Ma according to Offler and Forster (2008). They suggested that S1 and S2 were produced during the Sdirected movement of the Coffs Harbour megafold. The presence of more competent Permo-Carboniferous rocks in the northern HB resulted in contraction of the more incompetent rocks in the Nambucca Block and production of the E-W trending cleavages. By contrast in the northern HB open folds and poorly developed cleavage were produced. 215
NEO 2010 The presence of early east-west structures in the northern Hastings Block and Permian overlap sequences overprinted by northwest-trending folds reflects a change in shortening direction during deformation, namely north-south to northeast-southwest. This may reflect the gradual change from the N-S shortening of the Nambucca Block associated with the southward movement of the Coffs Harbour megafold to the more far field stress field consistent with formation of the megafold involving northeast-southwest shortening. The southern HB consists of two parts; a northern region with predominantly meridional folding, a well defined cleavage in high strain zones adjacent to the Yarrowitch Fault and minor drag folds, and a southern region with two phases of folding in Permian rocks east of Mt George. In this area, meridional, open to tight, upright, gently-plunging macroscopic folds (half-wavelengths 2-5 km) are overprinted by northwestplunging, open to tight macroscopic folds (half-wavelength 100 m – 1 km). This second generation of folding may have formed during sinistral, strike-slip movement on the WNW-striking Khangat Fault. Faulting The northern and southern Hastings Blocks have been extensively faulted with at least four main fault sets developed, namely N-S, NW-SE to WNW-ESE, NE-SW and rare E-W striking faults. Of these faults, the WNW-trending Parrabel Fault in the north, Bagnoo Fault in the centre and Kanghat Fault in the south, dominate. Major displacements occur on meridional and NW-striking faults in the northern and southern Hastings Block (Roberts et al. 1995). In most instances, the meridional faults cut the northwest-striking faults, although there is some overlap in timing, suggesting that meridional faulting continued after movement on most of the NW-striking faults has ceased. The same two sets of faults are present in the central part of the Hastings Block along with well developed NE-SW and rare E-W striking faults. Commonly the NE-striking faults are truncated by either the meridional or NW-striking faults. There is extensive re-arrangement of the fault-bounded blocks near the Werrikimbi Volcanic Complex and between Yarras and Wauchope. The Carboniferous sequence south of the Werrikimbi Volcanic Complex between the N-S striking Cowarral and Pappinabarra faults, youngs to the north, unlike the sequence nearby at Yarras which youngs to the east. This suggests at least 90o anticlockwise rotation of this fault-bounded block between these faults. The stratigraphic facing of the sequence in this fault block is unusual as most of the margins of the northern Hastings Block show facing away from the Parrabel Dome. The sense of movement on the different generations of faults is not well known. However, evidence for sinistral movement on the NW-striking faults indicated by reorientation of bedding, cleavage and axial planes of folds, and shear bands has been documented (Jenkins & Offler 1996). Further, strike-slip and oblique slip have been recorded by Jayko et al. (1993) and Lennox & Offler (2009) on N- and NW-striking faults around Yarras. Timing of movement on these faults in general is difficult to determine, however, in some areas field relationships allow this to be ascertained. For example, N- and NW- striking faults and folding near Yarras are considered by Jayko et al. (1993) to be the result of transpression during the Hunter-Bowen Orogeny (HBO). Reactivation of the NW to WNW-striking faults appear to have taken place later because quartzkaolinite alteration of breccia in the NW-striking Bagnoo Fault, has been produced during an influx of hydrothermal fluids from Triassic granitoids (Leitch & Feenan 1989). This implies that movement on this fault occurred pre 226 Ma the age of the granitoids. Similarly, movement on the WNW- striking Kanghat Fault that forms the southern boundary of the HB, and is part of the Manning Fault System, must have moved after the (HBO) based on the observation that meridionally trending cleavage and folds formed during the HBO are dragged sinistrally into conformity with the Kanghat Fault.
Conclusions The Hastings Block is similar lithologically and in age, and has a similar metamorphic grade to the TB but is enigmatic because it lies outboard of the fore-arc. There appears no simple model, which can explain the current position, deformation and fragmentation. Further studies are needed to determine the time of development of the cleavage in the Hastings Block and to identify evidence for the translation of the block. Further studies of the brittle structures within the mafic igneous rocks in the Yarras suture could provide evidence required to determine the movement history of the block. The structures within the Yarrowitch Block adjacent to the Yarras Suture may also provide evidence as to the movement history of this margin.
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References Cawood, P.A., 1982. Tectonic reconstruction of the New England Fold Belt in the Early Permian: An example of development at an oblique-slip margin. In: Flood, P.G. & Runnegar B. (eds) New England Geology. The University of New England, Armidale 25-34. Jayko, A.S., Blake, M.C. & Aitchison, J. 1993. Sructural uplift of ophiolitic slivers along major faults of the New England orogen. In: Flood, P.G. & Aitchison, J.C. (eds.) New England Orogen, eastern Australia. The University of New England, Armidale, 163-180. Jenkins, R.B. & Offler, R. 1996. Metamorphism and deformation of an Early Permian extensional basin sequence: the Manning Group, southern New England Orogen. Australian Journal of Earth Sciences, 43, 423-436. Johnston, A.J., Offler, R. and Liu, S. 2002. Structural fabric evidence for indentation tectonics in the Nambucca Block, southern New England Fold Belt, New South Wales. Australian Journal of Earth Sciences, 49, 407-421. Korsch, R.J. and Harrington, H.J. 1987. Oroclinal bending, fragmentation and deformation of terranes in the New England Orogen, eastern Australia. In: Leitch, E.C. & Scheibner, E. (eds.) Terrane Accretion and Orogenic Belts. American Geophysical Union Geodynamics Series 19, 129-140. Leitch, E.C., 1978. Structural succession in a late Paleozoic slate belt and its tectonic significance. Tectonophysics, 47, 311-323. Leitch, E.C. & Feenan, J.P. 1989. Quartz-kaolinite rocks from northeastern New South Wales. Australian Journal of Earth Sciences 36, 275-281. Lennox, P. & Offler, R. 2009 Kinematic history of serpentinites in the faulted margins of the Hastings Block, New England Orogen, eastern Australia. Australian Journal of Earth Sciences 56, 621-638. Lennox, P.G. and Roberts, J. 1988. The Hastings Block – a key to the tectonic development of the New England Orogen. In: Kleeman, J.D. (ed.) New England Orogen – Tectonics and Metallogenesis. Department of Geology and Geophysics, University of New England, Armidale, 68-77. Lennox, P.G., Roberts, J. and Offler, R. 1999. Structural analysis of the Hastings Terrane. In: Flood P.G. (ed.) New England Orogen, eastern Australia, University of New England, Armidale 115-124. Offler, R. & Williams, A. 1987. Evidence for sinistral movement on the Peel Fault System in serpentinites, Glenrock Station, NSW. In: Leitch, E.C. & Scheibner, E. (eds.) Terrane Accretion and Orogenic Belts, American Geophysical Union Geodynamics Series 19, 141–151. Offler, R., Roberts, J., Lennox, P. & Gibson ,J. 1997. Metamorphism in Palaeozoic forearc basin sequences, southern New England Fold Belt, NSW, Australia. Proceedings of the 30th International Geological Congress. Beijing, China. 241-250. Offler, R. and Foster, D. 2008. Timing and development of oroclines in the southern New England Orogen, New South Wales. Australian Journal of Earth Sciences, 55, 331-340. Pisarevsky, S.A., Cawood, P.A., Leitch, E.C. and Nemchin, A. 2009. New England Orocline in Late Palaeozoic: Geological and Paleomagnetic Constraints. In: Glen, R.A. & Martin, C. (compilers) International Conference on Island-Arc Continent Collisions: The Macquarie Arc Conference. Geological Society of Australia Abstracts No. 92, 109-110. Roberts, J. & Engel, B.A. 1987. Depositional and tectonic history of the southern New England Orogen. Australian Journal of Earth Sciences, 34,1-20. Roberts, J. & Geeve, R. 1999. Allochthonous forearc blocks and their influence on an orogenic timetable for the Souther New England Orogen. In: Flood P.G. (ed.) New England Orogen, eastern Australia, University of New England, Armidale 105-114. Roberts, J., Leitch, E.C., Lennox, P.G. & Offler, R. 1995. Devonian-Carboniferous stratigraphy of the southern Hastings Block, New England Orogen, eastern Australia. Australian Journal of Earth Sciences, 42, 609-634. Rosenbaum, G. 2010. Alternative models for the formation of the New England oroclines. Australian Earth Sciences Convention, Canberra. 217
NEO 2010 Schmidt, P.W., Aubourg, C., Lennox, P.G. and Roberts, J. 1994. Palaeomagnetism and tectonic rotation of the Hastings Terrane, eastern Australia. Australian Journal of Earth Sciences, 41, 547-560.
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Structural observations from the hinge of Texas Orocline Pengfei Li, Gideon Rosenbaum School of Earth Sciences, The University of Queensland, Brisbane, Australia
Keywords: Texas Orocline, New England Orogen, oroclinal bending, structural analysis
Introduction The New England orogen is the easternmost and youngest component of the Tasmanides of eastern Australia (Glen 2005). The orogen formed from the Devonian to Triassic by subduction and accretion processes, giving rise to a thick accretionary wedge, forearc basin, and voluminous magmatism (Murray et al. 1987). The southern part of the orogenic belt (Fig. 1a) shows remarkable curvatures (oroclines), which are clearly recognized in magnetic images (Fig. 1b). The Texas orocline, defined by the curvature of the dominant fabric, is the largest and most obvious orocline in the southern New England orogen. The geology of the Texas orocline has been studied by Lucas (1960), Flood and Fergusson (1982), Fergusson and Flood (1984), Korsch and Harrington (1987), Lennox and Flood (1997) and Donchak et al. (2007). The eastern limb of the orocline is connected to the western limb of the Coffs Harbour orocline (Korsch 1975, 1993), with the two structures together forming a large Z-shaped structure commonly referred as the Texas-Coffs Harbour Megafold or the Texas-Coffs Harbour oroclines (Korsch et al. 1987; Murray et al. 1987; Offler et al. 2008). In order to better understand the geometry of the Texas orocline and the evolution of oroclinal bending, we have conducted detailed mapping in the area of Mosquito Creek. The location of the study area at the hinge of the orocline enables us to track overprinting fabrics associated with oroclinal bending.
Fig. 1 (a) Geological map of Southern New England Orogen. NB, Nambucca Block; HB, Hastings Block. Rectangle indicates the location of the study area. (b) Total magnetic intensity showing the Texas, Coffs-Harbour and Manning oroclines. 219
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Geology of Mosquito Creek area All rocks in the study area belong to the Texas Beds, which are a relatively monotonous sequence of Carboniferous accretionary wedge metasedimentary rocks. Rocks found in the area include argillaceous slates, carbonaceous slates, volcaniclastic rocks, chlorite schists, cherts and jasper (Fig. 2).
Fig. 2. Geological map of Mosquito Creek area. Bedding (S0) is predominantly sub-parallel to the dominant cleavage. It is steeply dipping or vertical and striking E-W (Fig. 3a). In outcrop, S0 is defined by compositional layering intersecting the dominant cleavage at a low angle (Fig. 4a) Two groups of quartz veins are recognised. Earlier veins are generally oriented N-S, overprinted by the dominant cleavage and form meso- and micro-scale isoclinal folds (Fig. 4b, 5a). Axial planes of these folds are typically oriented ~E-W, parallel to the dominant fabric. A second generation of veins is normally trending E-W, parallel to dominant fabric. Two types of folds are recognised in the outcrop-scale and thin sections. The first type (F1) are vertical isoclinal folds (Fig. 4c), with steeply dipping axial planes trending ~E-W (Fig. 3c). This group of folds is mainly recognised in chert layers, or defined by isoclinal folds in early quartz veins. A later generation of folds is characterised by kink folds (F2) that affected the dominant cleavage. These folds are normally steeply plunging with axial planes oriented NW-SE (Fig. 3d, 5b). A strong slaty cleavage, S1, is developed throughout the study area. It is normally steeply dipping and trending E-W (Fig 3b). This fabric is parallel to the axial planes of isoclinal folds (F1), confirming its origin as an axial plane cleavage. Intersection lineation of cleavage and bedding (L10) is vertical or steeply plunging (Fig. 3e). A spaced crenulation cleavage (S2) is locally found parallel to the axial plane of F2 folds (Fig. 5c). In summary, two generations of structures are recognised in the hinge of Texas orocline. An earlier deformation phase (D1) was responsible for ~E-W trending F1 folds and associated axial plane cleavage S1, which is the dominant fabric throughout the area (Fig. 2). A second deformation phase (D2) is characterised by kink folds (F2) and local crenulations (S2). The orientation of S2 is typically oriented NW-SE. Compared with the dominant regional deformation (D1), D2 represents a relatively low-strain deformation event. 220
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(a)
(b)
(d)
(e)
(c)
Fig. 3 Stereographic projection of structural data from Mosquito Creek area (lower-hemisphere, equal-angle) (a) Poles to bedding S0 (n=36);; (b) Poles to the dominant axial plane cleavage S1 (n=338);; (c) Poles to axial planes of F1 folds (n=7);; (d) Poles to axial plane of F2 folds (n=14) (e) Intersection lineation (L10) of cleavage S1 and bedding S0 (n=30).
Implications to oroclinal bending The Texas orocline is defined by the curvature of the dominant fabric (S1), indicating that this fabric must have developed prior to the development of the orocline. The formation of this fabric involved relatively high strain and probably occurred during the Carboniferous in association with the development of the accretionary wedge. The process of oroclinal bending did not involve the development of penetrative cleavage parallel to the axial plane of the orocline (~NW-SE). However, there is evidence for local F2 folding and associated crenulation cleavage oriented NW-SE (Fig. 2 and Fig. 3d). These structural observations could be attributed to the process of oroclinal bending. The precise timing of the oroclinal deformation (D2) is unknown. We postulate that this deformation occurred during or after the intrusion of Early Permian granitoids (295-285 Ma), which are aligned parallel to the Texas orocline. Late Permian and Triassic magmatism seems to crosscut the oroclinal structure, indicating that oroclinal bending occurred prior to ~260 Ma. The formation of the orocline and associated D2 deformation is therefore attributed to the Early-Mid Permian. This period was characterised by widespread extension (Korsch et al. 2009a), followed by inversion and contractional deformation (Hunter-Bowen orogeny) that commenced at ~265 Ma (Collins, 1991; Holcombe et al., 1997; Korsch et al., 2009b). Whether the Texas orocline formed during extensional tectonics, contractional deformation, or a combination of both, remains an open question.
Acknowledgements This research was funded by the Australian Research Council (DP0986762). We wish to thank Ben Wruck and the residents of Mosquito Creek for their support.
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Fig. 4 Field photographs from the Mosquito Creek area. (a) Interbedding of chert and slate showing oblique orientations of S0 and the dominant cleavage (S1);
(b) N-S quartz veins overprinted by an E-W dominant fabric;
(c) Isoclinal fold (Fl) in chert; (d) Kink fold (F2) overprinting the dominant cleavage in argillaceous slate with a NW-SE axial plane orientating.
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Fig. 5 Photomicrographs from the Mosquito Creek area. (a) Early N-S quartz veins overprinted by the dominant fabric;
(b) F2 kink folds defined by the curvature of dominant cleavage (S1);
(c) Crenulation cleavage (S2) illustrated by overprinting the dominant Fabric.
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References Collins, W.J., 1991. A reassessment of the "Hunter-Bowen orogeny': tectonic implications for the southern New England fold belt. Australian Journal of Earth Sciences, 38, 409-423. Donchak, P.J.T., Bultitude, R.J., Purdy, D.J., and Denaro, T.J., 2007. Geology and mineralisation of the Texas region, south-eastern Queensland, Queensland Geology, 11. Fergusson, C.L. and Flood, P.G., 1984. A late paleozoic subduction complex in the Border Rivers area of southeast queensland. Proceedings of the Royal Society of New South Wales, 95, 47-55. Flood, P.G. and Fergusson, C.L., 1982. Tectono-stratigraphic units and structure of the Texas-Coffs Harbour region. In: Flood, P.G. & Runnegar. B. (eds.) New England Geology, University of New England and AHV Club, Armidale: 71-78. Glen, R., 2005. The Tasmanides of eastern Australia. In Vaughan, A.P.M., Leat, P.T. & Pankhurst , R.G. (eds.) Terrane processes at the margins of Gondwana, Geologcial Society, London, Special Publication, 246, 23–96. Holcombe, R.J., Stephens, C.J., Fielding, C.R., Gust, D., Little, T.A., Sliwa, R., McPhie, J., and Ewart, A., 1997. Tectonic evolution of the northern New England Fold Belt: The Permian-Triassic Hunter-Bowen event. In: Ashley. P.M. & Flood, P.G. (eds.) Tectonics and metallogenesis of the New England orogen, University of New England, Armidale: 52-65. Korsch, R.J., 1975. Structural analysis and geological evolution of the Rock-Coffs Harbour region, Northern New South Wales. Unpublished Ph.D. thesis, University of New England, Armidale. Korsch, R.J. and Harrington, H., 1987. Oroclinal bending, fragmentation and deformation of terranes in the New England Orogen, eastern Australia. American Geophysical Union Geodynamics Series, 19, 129-140. Korsch, R.J., 1993. Reconnaissance geology of the Solitary Islands: Constraints on the geometry of the Coffs Harbour orocline. In: Flood. P.G. & Aitchison, J.C. (eds.) New England Orogen, eastern Australia. University of New England, Armidale, 265-274. Korsch, R.J., Totterdell, J.M., Cathro, D.L., and Nicoll, M.G., 2009a. Early Permian East Australian Rift System. Australian Journal of Earth Sciences, 56, 381-400. Korsch, R.J., Totterdell, J.M., Fomin, T., and Nicoll, M.G., 2009b. Contractional structures and deformational events in the Bowen, Gunnedah and Surat Basins, eastern Australia. Australian Journal of Earth Sciences, 56, 477-499. Lennox, P.G. and Flood, P.G., 1997. Age and structural characterisation of the Texas megafold, southern New England Orogen, eastern Australia. In: Ashley. P.M. & Flood, P.G. (eds.) Tectonics and metallogenesis of the New England orogen, University of New England, Armidale: 161–177. Lucas, K. G., 1960. The Texas area. Journal of the Geological Society of Australia, 7, 229-235. Murray, C., Fergusson, C.L. Flood, P.G., Whitaker, W.G., and Korsch, R.J., 1987. Plate tectonic model for the Carboniferous evolution of the New England Fold Belt. Australian Journal of Earth Sciences, 34(2), 213-236. Offler, R. and Foster, D.A., 2008. Timing and development of oroclines in the southern New England Orogen, New South Wales. Australian Journal of Earth Sciences, 55(3), 331-340.
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Mining into the next century: Environmental advances, opportunities and challenges Bernd G Lottermoser School of Earth and Environmental Sciences, James Cook University, Australia
Keywords: Acid mine drainage, sulphide oxidation, mining, mine wastes, rehabilitation, remediation
Introduction Mankind is feeling the limitations of its science and technology despite all the advances in knowledge on mining environments and the improved practices in mine site rehabilitation and mine waste management. Tailings dams continue to fail and leak; waste rock dumps erode; capping designs of mine waste repositories do not succeed; mine-derived contaminants are dispersed into the biosphere, hydrosphere, pedosphere and atmosphere; predictions on the long-term kinetic behaviour of mine wastes turn out to be incorrect; and the long-term costs of mine site rehabilitation can be staggering (Lottermoser, 2010). As the perturbations have become recognised, our lack of knowledge of fundamental environmental processes at many mine sites has become obvious. The time has come to drastically improve our scientific efforts to understand these processes on all scales. The objective of this paper is to review some of the characteristics, environmental impacts and failed rehabilitation methods pertaining to mining environments. The paper will draw predominantly on examples of mining environments from Australia and elsewhere to illustrate our advances in knowledge, the gaps in our understanding, and the challenges ahead.
Advances in knowledge Acid mine drainage (AMD) resulting from the oxidation of sulphides in mine wastes is a major environmental issue facing the mining industry today. This pollution process has a long history dating back thousands of years when the Rio Tinto mining district of Spain experienced periods of intense mining and the associated production of pyrite-rich wastes and AMD waters (Fig. 1). Mining is not entirely responsible for the generation of AMD and its impact on the Rio Tinto. Historical records refer to the river’s longstanding acidity. The Romans called the Rio Tinto “urbero”, Phoenician for “river of fire”, and the Arab name for it was “river of sulphuric acid”. There is also geological evidence that the sulphide orebodies experienced long-term weathering and erosion at some stage in their geological history. The presence of thick jarosite-rich gossans capping the pyritic ores indicates that acid weathering of outcropping sulphide ores could have produced natural AMD prior to mining. The unique red colour of the river may have attracted the very first miners to the region. Consequently, the water’s conditions today are likely a combination of natural and mining induced AMD. The knowledge that sulphide minerals oxidise at the Earth’s surface and that this process causes the formation of secondary minerals and sulphuric acid is not new to modern science. Early civilisations of Sumeria, Assyria and Egypt were familiar with the salts formed from the oxidation of pyrite and its evaporation (Karpenko and Norris 2002). The Greek philosopher Theophrastus (370-285 BC) recognised pyrite oxidation and the development of acid and sulphide oxidation products (i.e. metal salts). The Greek physician Pedanius Dioscorides (40-90 AD) and the Roman naturalist Pliny the Elder (Gaius Plinius Secundus, 23–79 AD) mentioned the occurrence of sulphate salts in mine workings. Pliny the Elder wrote of “green vitriol” (i.e. melanterite) and “blue vitriol” (i.e. chalcanthite) as well-known substances (Karpenko and Norris 2002). Georgius Agricola (1494-1555 AD) explained the making of metal sulphate salts from pyrite in his renowned textbook on mining and metallurgy (Agricola 1556). He also described metals salts from mine workings, and he knew that the intake of melanterite and chalcanthite can be lethal to organisms.
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NEO 2010 Clearly, the early miners and scholars had some understanding of sulphide oxidation. By comparison, the impacts of sulphide oxidation on receiving waters were rarely studied or reported. Such ground-breaking observations were first documented by Diego Delgado, a 16th century priest, who inspected the Rio Tinto mines and river in southern Spain (Salkield 1987). In 1556, Diego Delgado reported on the state of the Rio Tinto mining district. In doing so, Diego Delgado became one of the first scholars, if not the first, to document fundamental aspects of AMD and its impacts on a stream. Diego Delgado recognised: (a) that pyrite oxidation leads to the formation of AMD products including sulphuric acid and dissolved iron; (b) that iron hydrolyses and forms Fe-rich cements in stream sediments; and (c) that AMD waters are toxic to humans, animals, fish and other aquatic organisms (Salkield 1987). Since Diego Delgado made his ground-breaking observations, there have been uncountable studies and publications on sulphide oxidation and AMD waters. Today, the scientific community has achieved a detailed understanding of the weathering reactions that cause sulphide oxidation and AMD development. More importantly, numerous remediation tools have successfully proven to curtail sulphide oxidation and to remediate AMD waters. Environmental scientists have made some phenomenal advances in their ability to observe and describe mining environments and to develop best practice environmental protection protocols and remediation technologies, particularly for AMD environments.
Fig. 1. Sulphidic waste dumps, slag heaps and abandoned railway infrastructure at Rio Tinto, Spain. The exploitation of sulphidic ores has caused massive AMD flowing into the Rio Tinto.
Gaps in understanding Conditions within AMD waters are generally toxic to most aquatic biota. Thus, AMD waters are thought to be biologically sterile; however, they are hardly lifeless. While AMD and AMD impacted waters are characterised by a limited diversity of plants, they display a great diversity of microorganisms, including algae, fungi, bacteria, yeast and protists. The conditions in AMD waters are ideal for the proliferation of miroorganisms (so-called “extremophiles”) that can thrive in these environments. The hostile environment also provides a niche for species that produce novel metabolites of potential significance and use to mankind. For example, the acid waters of the Berkeley pit contain microorganisms that generate metabolic compounds with selective anti-cancer activities (e.g. Stierle et al. 2006). In future, bioprospecting of AMD waters may reveal microorganisms and chemicals that could be of considerable benefit to mankind. 226
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AMD waters invariably require treatment, and wetlands are well established treatment options, particularly in North America and Europe (Lottermoser 2010). However, a number of wetlands used to treat AMD waters have failed over time. Also, the accumulation of metals in wetlands creates a metal-rich aquatic environment which may experience changes in its hydrology or climate in the long or short term. A wetland needs a sufficient year-round supply of water that would ensure that the wetland remains in a permanently saturated condition. Drying out of a wetland will lead to the oxidation of biological matter and sulphide minerals, and the development of evaporative metal salts. If metal sulphate salts have formed in the wetland during drying, the minerals will dissolve during the next rainfall event. At the beginning of the next rain period, sulphuric acid, metals and salts are released. These are then flushed through the wetland and into receiving waters (i.e. “first flush” event). Thus, wetlands without sufficient water supply become chemical time bombs and net sources of metals, metalloids, and sulphate. Therefore, wetlands are unsuitable for the treatment of AMD waters over much of inland Australia. Bioreactors (i.e. specifically designed tanks) may be far more appropriate for Australian conditions.
Challenges ahead The establishment of self-sustaining vegetation is an important step in the mine site rehabilitation. The plants fulfil various functions in site reclamation, however, at many mine sites, exposed sulphidic wastes and sulphidic waste-impacted soils have chemical and physical characteristics that are hostile to plant growth (e.g. acidity, salinity, high metal concentrations). The establishment of vegetation on such acid, metal-rich substrates poses particular challenges to plant species. Distinctive plant communities, including metaltolerant plant species (ie. metallophytes), are known to occur over metal-rich soils (e.g. Baker 1981; Baker and Brooks 1989). The metallophytes that inhabit metalliferous substrates display unique growth characteristics, enabling them to thrive in soils containing elevated concentrations of metals and metalloids (Fig. 2).
Fig. 2. Metal-excluding gorse (Ulex europeaus) and heather (Calluna vulgaris) colonising acid, arsenicrich (0.3 wt% As) tailings, Carnon Valley, Cornwall, UK.
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NEO 2010 Individual plant species thereby exhibit their very own distinctive behaviour to deal with the acidity and elevated metal and metalloid values in the substrates. Plants’ reaction to metals in substrates can be assigned to three distinctly different plant behaviours. “Indicator” species display a gradual raise in metal concentrations with increasing metal concentrations in the substrate. Such plants are of use in mineral exploration because the biogeochemical composition of indicator plants reflects the geochemical composition of the soil (e.g. Mitchell grass; Lottermoser et al. 2009). “Accumulator” species accumulate exceptionally high metal concentrations in their biomass. This plant behaviour may be valuable for the phytoextraction or even phytomining of metals. “Excluder” species do not obtain high metal values in their above-ground biomass despite elevated metal concentrations in the substrate. Plants that exclude metals and metalloids are of particular interest in mine site rehabilitation because the exclusion of metals from the above-ground plant biomass is preferable for vegetation covers of mine waste repositories and mine sites (Lottermoser et al. 2009). Geobotanical and biogeochemical studies of mineralised ground, gossans, historic mine sites and mining landscapes have revealed the presence of numerous accumulator plants, which may be used in phytomining (e.g. Sheoran et al. 2009). Phytomining – the use of plants to extract metals from subeconomic ores and wastes – has attracted considerable research activities in recent years. By contrast, minimal attention has been given to the discovery of excluder plant species. This is despite the fact that metal-excluding plants are of particular use in the rehabilitation of metal mine sites and contaminated areas. The exclusion of metals from the above-ground plant biomass reduces the exposure of wildlife and grazing animals to metals and limits the transfer of metals up the food chain. Future research efforts should explore the use and application of metal-excluding plants that are of direct use in phytostabilisation of contaminated land and wastes.
Conclusions More than ever, environmental geochemists have important contributions to make as they provide the data necessary for rational decision-making in critical areas such as mineral resource development, mine waste management and remediation, environmental protection as well as mine, land and waterway rehabilitation. The most urgent problems facing environmental geochemists working on mining environments are:
Quantification of the interactions that control the distribution of metal contaminants in mine wastes and associated soils, sediments, waters and biota.
Better descriptions of the chemistry and mineralogy of mine wastes and an understanding of their long-term behaviour.
Improved predictions on mine drainage chemistry.
Search for innovative, cost-effective remediation and rehabilitation technologies.
There is reason for optimism that the required progress is possible. Such optimism is based on the phenomenal advances in our ability to observe and describe mining environments. However, detailed studies of natural, mined, contaminated and rehabilitated environments are necessary if we are to quantify the variables controlling the containment and dispersal of contaminants and if we are to develop innovative remediation and rehabilitation technologies.
References Agricola, G., 1556. De re metallica. Translated by Hoover H.C., Hoover L.H. (1950). Dover Publications, New York. Baker, A.J.M., 1981. Accumulators and excluders – strategies in the response of plants to heavy metals. Journal of Plant Nutrition, 3, 643–654. Baker, A.J.M., and Brooks, R.R., 1989. Terrestrial higher plants which hyperaccumulate chemical elements – a review of their distribution, ecology and phytochemistry. Biorecovery, 1, 81–126. Karpenko, V., and Norris, J.A., 2002. Vitriol in the history of chemistry. Chemicke Listy, 96, 997-1005. Lottermoser, B.G., 2010. Mine wastes: characterization, treatment, and environmental impacts. 3rd edition Springer-Verlag, Berlin Heidelberg.
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NEO 2010 Lottermoser, B.G., Munksgaard, N.C., and Daniell, M., 2009. Trace element uptake by Mitchell grasses grown on mine wastes, Cannington Ag-Pb-Zn mine, Australia: Implications for mined land reclamation. Water Air and Soil Pollution, 203, 243-259. Salkield, L.U., 1987. A technical history of the Rio Tinto mines: some notes on exploitation from prePhoenician times to the 1950s. The Institution of Mining and Metallurgy, London. Sheoran, V., Sheoran, A.S., and Poonia, P., 2009. Phytomining: a review. Minerals Engineering, 22, 10071019 Stierle, A.A., Stierle, D.B., and Kelly, K., 2006. Berkelic acid, a novel spiroketal with selective anticancer activity from an acid mine waste fungal extremophile. Journal of Organic Chemistry, 71, 5357-5360.
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Environmental geochemical legacies of abandoned metalliferous mine sites, New England Orogen Bernd G Lottermoser1, Paul M Ashley2 1
School of Earth and Environmental Sciences, James Cook University, Australia 2
Earth Sciences, University of New England, Australia
Keywords: Environmental geochemistry, acid mine drainage, metals, metalliferous mines
Introduction Historic mining, mineral processing and smelting of metal ores commonly cause the dispersion of trace metals into surrounding soils, sediments and waters (Lottermoser 2010). At these sites, inappropriate waste disposal practices and unconstrained dispersion of contaminants may lead to metal contamination of soils and sediments well beyond the mine site. Consequently, historic metal mining centres are known for their pronounced geochemical footprint inflicted on the local and regional landscape. Such disturbed and contaminated lands invariably require rehabilitation. In the New England area, there are notable examples of environmental degradation caused by mining and associated smelting operations carried out during the late 19th and early 20th century when there were few or no legislative constraints placed on operations. This study reports on the environmental geochemical legacies of historic metalliferous mine sites in the New England Orogen of northern New South Wales (e.g. Halls Peak, Gulf Creek, Webbs Consols, Collisons, Cangai, Mole River, Hillgrove) and southern Queensland (e.g. Twin Hills, Silver Spur, Mt Perry) (Fig. 1). Abandoned mine sites with differing geologic, physiographic, climatic and floral regimes were sampled for waste rock, ore stockpile, mill tailings and slag materials, soils, stream sediments, waters and vegetation. In this study, chemical analyses of >2000 solid media (waste/ore, tailings, slag, mineral efflorecences, soil, stream sediment, vegetation) and >300 water samples were performed to establish the environmental behaviour of metals and metalloids originating from diverse mineral deposits. The objective of this study was to gain field and laboratory data relating to the mobility of metals (Cu, Pb, Zn, Cd, Fe, Mn, Ag, Hg) and metalloids (As, Sb) at the various mine sites and the impacts of trace element mobility on the environmental quality of soils, sediments, waters and vegetation. This study demonstrates that mineralogical and geochemical properties of mineral deposits as well as climate and topography together with mining and mineral processing practices control the release of contaminants from mine sites and mine wastes. A solid understanding of the environmental geology of mineral deposits is vital to any mining operation, environmental impact assessment or rehabilitation plan. Such knowledge allows the development of effective prediction, prevention and remediation tools necessary for the successful environmental management of these sites.
Metalliferous mineral deposits The New England Orogen displays a diversity of metalliferous mineralisation types, some of which have the potential to produce significant environmental geochemical footprints in the local and regional landscapes. The major ore deposit types that have been investigated include: (1) VHMS deposits; (2) epithermal precious metal (-base metal) deposits; (3) leucogranite-related, As-rich polymetallic vein deposits; (4) intrusionrelated, Cu-rich vein deposits; and (5) orogenic Sb-As-Au deposits (Table 1). Abandonment and neglect of 230
NEO 2010 mined lands led to sites that are characterized by a severely modified (or lack of) vegetation, waste rock heaps, ore stockpiles, tailings dumps, slag and flue residue deposits, disused mining and processing equipment, and ruins. Although the New England Orogen is well known for its historic production of Sn, Mo and Au ores, most of these deposits have low sulphide contents and have not produced major environmental geochemical legacies.
Fig. 1. Location of historic metalliferous mine sites in the New England Orogen. 231
NEO 2010 Table 1. Mineralisation types of New England Orogen and their mineralogical, geochemical and environmental properties. Mineralisation type
Rock types
Major ore minerals
Geochemistry
Acid producing potential
Existing environmental impacts
VHMS deposits (Halls Peak, Gulf Creek, Silver Spur)
Shale, siltstone, diamictite, volcaniclastics, chert, argillite, metabasalt
Sphalerite, galena, chalcopyrite, pyrite (tetrahedrite)
CuPbZnAg
High (due to abundance of pyrite)
AMD waters Dispersion of ore elements into soils, sediments and vegetation Slag dumps
Epithermal precious metal deposits (Twin Hills, Drake)
Metasediments, volcaniclastics
Pyrite, sphalerite, galena (chalcopyrite, tetrahedrite, Agsulphosalts)
AgAu(AsSbHgCdCuPbZn)
Intrusion-related, Cu-rich vein deposits (Cangai, Mt Perry)
Meta-argillite, granite
Pyrite, chalcopyrite
Cu(ZnAsAu)
Leucograniterelated, As-rich polymetallic vein deposits (Collisons, Webbs Consols, Mole River, Conrad, Ottery)
Metasiltstone, meta-argillite, granite
Sphalerite, galena, arsenopyrite, chalcopyrite (stannite, tetrahedrite, cassiterite)
AsZnPbAg(SbSnCu)
Orogenic Sb-As-Au deposits (Hillgrove)
Metasediments, granite
Stibnite, arsenopyrite (pyrite)
SbAsAu
Disseminated Sn, Mo, Au deposits (Timbarra, Emmaville, Glen Eden)
Metasediments, granite
Cassiterite, (molybdenite, pyrite)
Sn, Mo, Au (As)
High (due to abundance of pyrite)
AMD waters
High (due to abundance of pyrite)
AMD waters
High (due to the presence of arsenopyrite, pyrite and pyrrhotite)
As-rich mine waters at acid and near-neutral pH values
Low (due to low total sulphides; carbonate gangue)
SbAs-rich mine waters at nearneutral pH values
Low (due to low total sulphides)
Regolith removal
Dispersion of ore elements into soils, sediments and vegetation
Dispersion of ore elements into soils, sediments and vegetation
Dispersion of ore elements into soils, sediments and vegetation
Dispersion of ore elements into soils, sediments and vegetation
Erosion Stream aggradation
Environmental geochemical legacies Fundamental geological aspects of mineral deposits (e.g. amount and type of metals enriched in the deposit, the kind of sulphide minerals, type of rocks associated with the deposit) exert important and predictable impacts on the environment (e.g. Plumlee 1999). The natural occurrence of elements varies between different ore deposit types of the New England Orogen (Table 1). Certain ores and rocks provide exceptionally high metal and metalloid concentrations to soils, sediments and waters. Different rocks and ores thereby supply different elements. For example, VHMS deposits and their mine sites and wastes provide high Cu, Pb and Zn concentrations and high acidity to mine soils (Gulf Creek, Halls Peak, Silver Spur). Such sites are characterised by acidified, CuPbZn-rich soils, and the vegetation has to adapt to such substrates (Lottermoser 232
NEO 2010 et al. 1997, 1999). Mine waters emanating from waste rock dumps and ore stockpiles possess strongly elevated metal (Cu, Pb, Zn, Fe) and sulphate values (AMD waters). Consequently, stream sediments downstream of VHMS deposits display strongly elevated Cu, Pb and Zn values for some distance downstream. By contrast, leucogranite-related polymetallic vein deposits are enriched in As and base metals (especially Pb and Zn). Hence, unconstrained weathering and erosion of waste rock dumps and ore stockpiles at these sites lead to As enrichment of soils and stream sediments as well as acid or near-neutral pH As-rich mine waters discharging into local streams (Millar 1996; Ashley and Lottermoser 1999; Mercuri 2001; Ashley et al. 2004; Gore et al. 2007; Bell 2008). Furthermore, orogenic SbAsAu deposits have associated metalloid enrichments of soils and significant dispersion trails of As and Sb from the New England highlands to coastal settings (Ashley et al. 2007). Thus, the diversity of New England mineral deposits with differing mineralogy and element enrichments causes distinct environmental signatures in receiving streams, soils and sediments, and these enrichments bring about site-specific adverse effects on local and regional ecosystems. Some of the determined environmental geochemical signatures and impacts of mineral deposits likely occurred naturally for those deposits that experienced weathering prior to mining (e.g. Halls Peak). However, at all mine sites the environmental impacts were exacerbated or largely caused by mining and mine waste disposal practices at that time. Polluted soils in the vicinity of the mine and smelter sites are subject to continuing soil erosion and either support no vegetation, or a depauperate flora with certain species showing minor bioaccumulation of metals and strong resistance to high metal contents in the substrates. Chemical analyses of species colonising the various mine sites demonstrate subdued biogeochemical signatures which reflect the suppressed uptake of ore elements, prevalent at a particular site, into the above-ground biomass of local plant species. Since abandonment, grasses (e.g. Cynodon dactylon, Poa sieberiana, Aristida sp.) and shrubs (e.g. Cassinia sp., Pityrogramma austroamericana) have colonised many of the mine soils (e.g. Ashley et al. 2003). Biogeochemical analyses demonstrate that grasses in particular are opportunistic, pioneering metallophytes. However, metal hyperaccumulator species were not found and, plant species do not acquire high metal concentrations in their biomass despite elevated metal concentrations in the soils. The exclusion of metals from the above-ground plant biomass reduces the exposure of wildlife and grazing animals to metals and limits the transfer of metals up the food chain. Slag dumps occur at several historical smelting sites in the New England Orogen (e.g. Silver Spur, Gulf Creek). The microcrystalline slags contain primary slag phases, relict flux, ore and furnace materials and secondary weathering related minerals. Common primary slag phases are glass, Zn-bearing silicates (olivine, pyroxene, melilite, willemite), with minor amounts of oxide (Zn-bearing spinel), sulphide and metal phases. The slag materials contain wt% concentrations of Zn, Cu and Pb. The slags are undergoing contemporaneous reaction with air and rainwater. The weathering results in the release of metals and metalloids from primary slag phases, and the partial immobilisation of these metals in secondary soluble and insoluble minerals in the slag heaps. Zinc and Cu exhibits pronounced chemical mobility and report together with sulphate into surface seepages. The slag dumps represent long-term sources of metal pollutants, particularly of Zn and Cu, to local ground and surface waters.
Rehabilitation Although mining ceased many decades ago at the investigated sites, contamination of local streams and soils remains and will continue due to the high metal and metalloid content of the exposed ore, waste and slag dumps. The metal and metalloid abundance of many mine waters as well as mine soils and local stream sediments exceed NWQMS (2000) sediment quality criteria and NEPC (1999) contaminated site guidelines (at certain sites by up to several orders of magnitude). Mine seepage waters and impacted streams contain metal and metalloid values well in excess of NWQMS (2000) recommended maximum acceptable concentrations in drinking, stock and irrigation water and water guidelines for freshwater systems. Statutory authority attempts at rehabilitation over the past 10-20 years (Halls Peak, Gulf Creek, Ottery, Conrad, Drake, Mole River) have largely been unsuccessful, resulting in no significant improvements in water quality downstream, and minimal amounts of successful rehabilitation. This failure is due to a management approach to site rehabilitation rather than relying on science. Future rehabilitation efforts of contaminated sites have to consider the mineralogical diversity of exposed wastes and contaminated media and the geochemical processes acting at the various sites. Every mine has produced its own unique waste and contaminated soil and sediment. These materials require mineralogical 233
NEO 2010 and geochemical characterisation prior to site-specific rehabilitation. For example, sulphidic waste rock dumps and ore stockpiles of the VHMS deposits require dry covers to prevent further sulphide oxidation. Acidified topsoils have to be limed and AMD waters emanating from these dumps need treatment (e.g. using bioreactors, anoxic limestone drains). By contrast, rehabilitation of the leucogranite-related, As-rich polymetallic vein deposits has to consider the pronounced mobility of As and Sb at strongly acid, and neutral to alkaline pH conditions. Liming of such sites to near-neutral pH values would exacerbate metalloid leaching and associated contamination problems. Also, any remediation efforts of As-rich ground, involving the addition of phosphate fertilizers and organic amendments (e.g. mulch, sewage sludge), may increase As leaching over time: there is evidence that the addition of phosphates or organic matter to soils leads to desorption of non-bioaccessible As bound to Fe oxides and convert it into bioaccessible forms (e.g. Peryea, 1991; Jackson and Miller, 2000; Lambkin and Alloway, 2003). Therefore, rehabilitation efforts of severely acid, As-rich ground and wastes should focus on the partial neutralization using surface lime application (to a pH value of ~4-5 and associated scorodite precipitation). The addition of iron sulphate amendments to As-Sb contaminated soils may also be pursued as such amendments would result in the coprecipitation of metalloids as relatively insoluble iron oxides and hydroxides (cf. Warren et al. 2003; Spuller et al. 2007). Since abandonment, certain native plant species have colonised some of the exposed wastes and contaminated soils. The establishment of self-sustaining vegetation covers is an important step in site rehabilitation. The presence of such distinct metal- and metalloid-resistant, metal-excluding plant communities in the New England area, developed on mineralised or contaminated ground, is similar to that of other plant communities reported from historic mining and smelting regions of western Europe (e.g. Cornwall, UK; Harz mountains, Germany). The establishment of native vegetation covers, consisting of metal-excluding plants, at historic mine sites demonstrates that abandonment and neglect of mined lands can lead to the natural development of sustainable vegetation covers over contaminated land in the long term.
References Ashley, P.M., and Lottermoser, B.G., 1999. Arsenic pollution at the Mole River mine, northeastern New South Wales, Australia. Australian Journal of Earth Sciences, 46, 861-874 Ashley, P.M., Lottermoser, B.G., and Chubb, A.J., 2003. Environmental geochemistry of the Mt Perry copper mines area, southeast Queensland, Australia. Geochemistry: Exploration, Environment, Analysis, 3, 345-357 Ashley, P.M., Graham, B.P., Tighe, M.K., and Wolfenden, B.J., 2007. Antimony and arsenic dispersion in the Macleay River catchment, New South Wales: a study of the environmental geochemical consequences. Australian Journal of Earth Sciences, 54, 83-103. Ashley, P.M., Lottermoser, B.G., Collins, A., and Grant, C.D., 2004. Environmental geochemistry of the derelict Webbs Consols mine, New South Wales, Australia. Environmental Geology, 46, 596-609. Bell, A.B. 2008. Environmental geochemistry of the derelict Ottery As-Sn mine: site characterisation and rehabilitation recommendations. BScHons thesis, UNE, Armidale, unpubl. Gore, D.B., Preston, N.J., Fryirs, K.A., 2007. Post-rehabilitation environmental hazard of Cu, Zn, As and Pb at the derelict Conrad mine, eastern Australia. Environmental Pollution, 148, 491-500. Jackson, B.P., Miller, W.P., 2000. Effectiveness of phosphate and hydroxide for desorption of arsenic and selenium species from iron oxides. Soil Science Society of America Journal, 64, 1616-1622. Lambkin, D.C., Alloway, B.J., 2003. Arsenate-induced phosphate release from soils and its effect on plant phosphorus. Water Air and Soil Pollution, 144, 41-56. Lottermoser B.G, 2010: Mine wastes: Characterization, treatment, and environmental impacts. TRI Publishing Centre, Skopje, Macedonia (in Albanian) Lottermoser, B.G., Ashley, P.M., Muller, M., and Whistler, B.D., 1997. Metal contamination at the abandoned Halls Peak massive sulphide deposits, New South Wales. In: Ashley P. M. & Flood P. G. (eds), Tectonics and metallogenesis of the New England Orogen. Geological Society of Australia Special Publication 19, 290-299 Lottermoser, B.G., Ashley, P.M., and Lawie, D.C., 1999. Environmental geochemistry of the Gulf Creek copper mine area, northeastern New South Wales, Australia. Environmental Geology, 39, 61-74. 234
NEO 2010 Mercuri, M.D., 2001. Environmental geochemistry of the derelict Conrad base metal mine, northern N.S.W. BScHons thesis, UNE, Armidale, unpubl. Millar, P.M.C., 1996. Reclamation options for the Mole River arsenic mine, Tenterfield, New South Wales. ME thesis, UNE, Armidale, unpubl. NEPC (National Environment Protection Council), 1999. Assessment of site contamination. Schedule B (1) Guideline on the investigation levels for soil and groundwater. NWQMS (National Water Quality Management Strategy), 2000. Australian and New Zealand guidelines for fresh and marine water quality. Paper No. 4. Australian and New Zealand Environment and Conservation Council, Agriculture and Resource Management Peryea, F.J., 1991. Phosphate-induced release of arsenic from soils contaminated with lead arsenate. Soil Science Society of America Journal, 55, 1301–1306. Plumlee, G.S., 1999. The environmental geology of mineral deposits. In: Plumlee G. S. & Logsdon M. S. (eds) The environmental geochemistry of mineral deposits. Part A: Processes, techniques and health issues. Society of Economic Geologists, Littleton. Reviews in Economic Geology, 6A, 71–116. Spuller, C., Weigand, H., and Marb, C., 2007. Trace metal stabilisation in a shooting range soil: Mobility and phytotoxicity. Journal of Hazardous Materials, 141, 378-387. Warren, G.P., Alloway, B.J., Lepp, N.W., Singh, B., Bochereau, F.J.M., and Penny, C., 2003. Field trials to assess the uptake of arsenic by vegetables from contaminated soils and soil remediation with iron oxides. Science of Total Environment, 311, 19-33.
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Geochemical, isotopic and petrographic constraints on the origin and development of the Barrington Tops Batholith I. Meek and B. Landenberger School of Environmental and Life Sciences, University of Newcastle, Newcastle, Australia
Keywords: granodiorite, Barrington Tops, geochemistry, Sr isotope
Introduction The Barrington Tops Batholith (BTB) is an Early Permian, granodioritic suite intruding the Tamworth Belt in the Southern New England Fold Belt (SNEFB), as shown in Fig. 1. It is composed of three plutons of two comagmatic compositions: the two larger plutons, Omadale Brook and Barrington River, are augitehypersthene granodiorite, while the Gummi Plain pluton, to the north-east is a hornblende-biotite granodiorite (Mason & Kaverlieris 1984; Eggins & Hensen 1987). In addition to the granodiorites, Eggins (1984) notes the presence of several phases of dyke and stock intrusion. Two phases of mafic dykes are associated with the plutons, one intruding before the plutons, and one after. In addition, aplitic and microgranodioritic dykes of similar mineralogy to the main suite are known. These felsic intrusives are noted on the margins of the intrusions and are seen in both the granodiorites and their surrounding aureole. Finally, Eggins (1984) notes the presence of quartz diorite intrusives in several locations around the BTB. However, poor outcrop means that the relationship between these and the other intrusives has not been discerned.
Late Palaeozoic- Mesozoic igneous and sedimentary northeasternNSW units of northeastern NSW N W
E
New England Fold Belt Lachlan Fold Belt
S
0
100 ClarenceMoreton Basin
km Tablelands Complex
— —
h rth o rt wo amw T Tam
el Pe Peel
g ning annin M Man
elt B Belt
Nambucca Basin
Surat
LEGEND Mesozoic basins -
tt ul Fa Faul
Basin
Latest Permian Triassic granitoids & volcanics Early-Mid Permian granitoids
ki M Mooki – Moo ter – Hun Hunter
Sy ste m
Early Permian granitoids
M an n
ni n g
in as
a B
Serpentinite Early Permian basins (including Barnard Basin)
Gloucester Basin Th
ru st
Fig. 9: Regional sedimentary and igneous geology of the New England Fold Belt, NSW. Modified after Jenkins et al. (2002).
Myall Basin
Sydney Basin Newcastle
Early Permian volcanics Latest Carboniferous Hillgrove & Bakers Creek suites Carboniferous volcanic centres Carboniferous granites LFB (including Ben Bullen gabbros) SNEFB accretion/subduction complex SNEFB forearc basin sequences LFB Palaeozoic basement Barrington Tops Batholith
The origin of the batholith is debated and is not yet completely understood. Based on geochemical and age constraints, the BTB has previously been included in the Nundle Suite (Hensel et al. 1985), the greater New England Batholith (Eggins & Hensen 1987) and the Clarence River Supersuite (Bryant et al. 1997; Phillips et al. In review). Hensel et al. (1985) use Sr and Nd isotopes and ratios to define the suite as I-type, with a “mantle like” character and Hensen et al. (1992) note the low Pb isotopic 236
NEO 2010 ratio (~207Pb/204Pb = 15.74) of the suite as consistent with an entirely mantle, or mantle with a minor juvenile crustal component source for the BTB. However, the variation in εNd values (+3.3 to +6.1) cited by Hensel et al. (1985) is not consistent with a uniform upper mantle source. Instead Hensel et al. (1985) suggest a composite source combining crustal melting and restite unmixing. A study of magmatic ferromagnesian inclusions in plagioclase cores disputes this (Mason 1986). Petrographic examination of these inclusions found largely isolated crystals, rather than a polycrystalline mass that would be expected to form in the high grade metamorphic environment required for restite unmixing, favouring a magmatic origin. Further, chemical analyses of the inclusions indicate that these phases also occur as crystals in the greater rock mass, implying magmatic crystallisation of both from the same melt (Mason 1986). This argument was further developed by Eggins and Hensen (1987) who suggested that the two-pyroxene granodiorite (Omadale Brook and Barrington River plutons) formed by fractional crystallisation from a liquid parent at least as mafic as a quartz diorite. Such a source is plausible if subduction along the eastern Australian coast during the Permian is taken into account (Jenkins et al. 2002). The unusually hot, dry nature of the BTB (3,500,000 ounces) and base metal production has historically occurred at the Conrad deposit however recent exploration undertaken by Malachite Resources indicates that a considerable economic resource still remains (Donnelly et al 2009). The deposit is primarily hosted within the Early Permian Gilgai Granite (~245Ma) with the SE section of the system extending into the adjacent Tingha Monzogranite. The Gilgai Granite is an I-type leucomonzogranite, geochemically and compositionally similar to the major leuco-granitic suites of the Southern New England Orogen. Textural relationships indicate that the Tingha Monzogranite is slightly older (possibly synchronous) with both plutons intruding the Early Permian Bundarra Plutonic Suite to the west, the Late Permian Wandsworth Volcanics in the east and the Early Carboniferous Sandon Beds in the north and south (Brown and Stroud 1993) . Mineralisation within the Conrad system is primarily represented by a large NW-SE striking sub-vertical vein structure termed the Conrad Lode. Drilling has intercepted the structure at depths of up to 500m and a strike length of 2.2km with mapping and VLF-EM indicating that the strike of the lode continues for a further 2.3km. A significant splay structure termed the King Conrad Lode has been identified at the NW end of the system, diverging west of the Conrad Lode and containing grades of up to 1715 g/t Ag across a 0.5m true width. A further ‘greisen’ type zone exists between the two major lodes, containing a large volume of veinlet hosted and disseminated mineralisation (Malachite Resources Limited 2009). Alteration associated with the lodes is primarily phyllic (quartz-sericite) grading into greisen style (quartzmuscovite) towards the NW end of the deposit. Interstitial carbonates and chlorite occur occasionally with secondary pyrite relatively common within major vein structures and greisen material. Haloes are typically pervasive and display high intensity proximal to major vein structures, often destroying all relict igneous minerals and textures (Ashley 2008). The Conrad system is notably poly-metallic, containing various sulphides and other ore minerals such as galena, sphalerite, chalcopyrite, arsenopyrite, pyrite, pyrrhotite, stannite, cassiterite and argentite-acanthite with likely other silver phases including tennantite and proustite-pyrargyrite. Variations in the relative abundances of these minerals have been recognised, ranging from zoning across individual vein sections to mineralogical changes throughout the extent of the entire system. Determination of the textural relationships, mineralogical and geochemical associations and metal distributions at Conrad is being undertaken through a variety of techniques such as microprobe analysis, petrographic examination and drill core observations as well as various statistical analyses of the drill assay database including grade calculations, metal ratios, metal correlations, element contouring and the plotting of mineralised zones. A generalised zonation pattern has been recognised across the strike of the Conrad deposit, displaying a higher concentration of Cu (chalcopyrite) at the SE end and decreasing towards the NW. The opposite pattern has been identified for Pb (galena) and inferred for Ag due to its frequent mineralogical association. Preliminary interpretation of geochemical data toward the SE end of the section indicates that higher grade Pb and Zn mineralisation may spatially restricted to certain zones, conceptually similar to the ‘ore shoot’ model proposed by BHP south in the 1950’s. 258
NEO 2010 Petrographic observations indicate an early mineralised arsenopyrite dominant phase, later replaced by major ore minerals such as sphalerite, chalcopyrite and galena. Examination of drill core displays a zoning across most lode structures from arsenopyrite rich margins into sphalerite-chalcopyrite-galena (pyritepyrrhotite) dominated zones and further into a relatively barren quartz core. These mineralogical relationships appear to be the result of various fluid pulses passing through the same structure at various times in a ‘crack-seal’ style. Early observations and analyses display distinct metal zonation on various scales within the Conrad system. Further interpretations primarily aim to qualitatively define mineralogical relationships within the system and mineral zoning across vein sections while empirically modelling geochemical changes within the deposit with the scope to delineate higher grade zones.
References Ashley, P.M., 2008. ‘Petrographic Report on Twenty-Six Drill Core Samples from the Conrad Mine, Inverell Area, NSW’ for Malachite Resources, Report #490, Paul Ashley Petrographic and Geological Services Donnelly, M., Meares, R., Bayley, O., Pietrass-Wong, B. & Bannerman, C.J., 2009 ‘Seventh Annual Exploration Report for the Year Ended 26 August 2009’, Conrad Project, NSW, Malachite Resources Malachite Resources Limited, 2009. ‘Malachite Resources Annual Report 2009’, Retrieved 07/06/10, URL: http://www.malachite.com.au/html/annual.htm Brown, R.E. and Stroud, W.J., 1993. ‘Mineralisation Related to the Gilgai Granite, Tingha-Inverell Area’, New England Orogen, Eastern Australia, Conference Paper, University of New England, Armidale
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Analogues to mineral sequestration of CO2: Sources of carbon in magnesite of Attunga Magnesite Quarry, NSW, Australia, a stable isotope study Hans C. Oskierski, Judy G. Bailey, Eric M. Kennedy and Bogdan Z. Dlugogorski Priority Research Centre for Energy, The University of Newcastle, Callaghan, NSW 2308, Australia; Email:
[email protected],
[email protected]
Keywords: mineral sequestration, carbon isotopes, oxygen isotopes, vein magnesite, serpentinite, carbon sources
Introduction Carbon dioxide sequestration or disposal is an essential component of the international effort to stabilise CO2 emissions to the atmosphere. Of the proposed sequestration schemes, mineral sequestration represents the most geologically stable and environmentally benign method for carbon disposal (Lackner et al. 1995). Mineral carbonation mimics natural silicate weathering processes that bind CO2 in stable carbonate minerals. Ultramafic rocks from ophiolite belts, containing high abundances of magnesia as serpentine and olivine, represent the best potential feedstock for mineral carbonation (Metz et al. 2005). At present, research efforts focus on the development of economically viable and energy efficient processes for large-scale industrial implementation of mineral carbonation. These efforts could be assisted by gaining enhanced understanding and characterisation of the natural carbonation of ultramafic rocks. Earlier studies have shown the outstanding potential of serpentinites of the Great Serpentinite Belt for CO2 sequestration. Based on an RCO2 of 2.46 (the number of tonnes of rock required to sequester one tonne of CO2) and geophysical modelling of part of Great Serpentinite Belt in the northern New South Wales, Davis (2008) concluded that 24 × 109 t CO2 could be sequestered, equivalent to 308 y of the total stationary emissions for NSW at 2005 levels. Natural carbonation of the ultramafic rocks of the Great Serpentinite Belt is common and, among others, manifests itself in the development of magnesite deposits and silica carbonate alteration zones (Ashley 1995, 1997). The first step in the understanding of these analogues to mineral sequestration is to identify and trace the reactants. Commonly, cross plots of stable isotopes of carbon (δ13C) and oxygen (δ18O) are used to differentiate between magnesite occurrences and to deduce their sources (Kralik et al. 1989). The sources of carbon cannot always be unequivocally identified, since different processes and mixing can lead to similar, ambiguous carbon and oxygen fingerprints of the magnesite minerals. However, pathways and mechanisms of formation can be constrained if the isotopic fractionations associated with the reaction steps involved are known. In the case of vein deposits of magnesite associated with ultramafic rocks debate has focussed on plant respiration and decay in contrast to metamorphic exhalation as the source of carbon in these deposits (Abu-Jaber and Kimberly 1992). This study seeks to constrain the sources of carbon involved in the carbonation of the ultramafic rocks of the Great Serpentinite Belt that led to the formation of the Attunga magnesite deposit.
Geologic setting The Attunga magnesite deposit is located about 9.5 km northeast of the town of Attunga in the southern part of the New England Orogen in NSW (Fig. 1). It has developed as a decomposition product of serpentinite rocks originally referred to as the Great Serpentine Belt (Benson 1913) and more recently as the Woodsreef Melange (e.g., Brown 2009) or the Great Serpentinite Belt (e.g., Ashley 1997). The Great Serpentinite Belt is interpreted to represent a disrupted ophiolite association (e.g., Yang and Seccombe 1997) which has been variably metasomatised to serpentinite and tectonically emplaced along the Peel Fault (Aitchison and Ireland 260
NEO 2010 1995; Ashley 1997). The Tamworth Belt to the west and the Central Block to the east are juxtaposed along the Peel-Manning Fault system (Brown 2009). The Attunga magnesite deposit occurs at the southern end of a high serpentinite ridge (Fig. 1), apparently distant from areas of silica carbonate alteration. The ridge crops out over an area of approximately 2-3 km long and 600 m wide and is likely to have formed in the late stages of uplift of the New England Orogen (Ashley, personal communication). The superficial deposit consists of compact, high-purity magnesite which developed in serpentinite, filling fractures and cavities in the partly weathered host rock. Magnesite occurs in the form of veins and nodules. The Attunga magnesites resemble Kraubath-type deposits, but are confined to the weathered zone, less regular in form and simpler in mineralogy and depositional history (Facer 2007).
Fig. 1: Sample location (modified from Brown 2009).
Methods Samples were collected during sampling trips in October and November 2009 from the Attunga Magnesite Quarry (Fig. 1). Mineral phases in powdered samples were identified using powder X-ray diffraction (XRD). Data were collected with a scanning step of 0.02° 2θ and counting time of 1 s/step on a Phillips X’Pert MPD diffractometer over a range of 6 to 90° 2θ using Cu Kα radiation. Generated patterns were matched against the International Centre for Diffraction Data® (ICDD) using X’pert Highscore®. Stable carbon and oxygen isotope compositions were determined for 24 specimens from 16 samples from Attunga Magnesite Quarry. To assure high purity, 14 of the specimens were obtained by extracting carbonate from the original samples using a dental drill and subsequent separation of magnetic particles with a rareearth hand magnet. Specimens were analysed using a GV2003 continuous flow mass spectrometer at the University of Newcastle. Aliquots weighing between 0.6 and 0.8 mg of pure magnesite (more for bulk samples) were loaded into septum vials, flushed with He in an acid preparation device and dissolved in 0.5 – 1 mL of 103% phosphoric acid at 72 °C for at least 16 h. The δ13C compositions are reported relative to VPDB and δ18O to VSMOW. The external precision (1σ) for isotopic analyses is 300 °C) for the release of volatiles and that the volatiles are transported in a water-poor liquid that mixes with upper crustal groundwater to precipitate magnesite under near surface conditions. With respect to the theory of soil origin of CO2, during weathering, silicate dissolution in soils is driven by biologically produced carbonic and organic acids (e.g., Stumm et al. 1983) with dissolved CO2 being one of the most abundant acids in soil where plant respiration and decay of organic material raise carbon dioxide levels to 10 to 100 times that of the atmosphere (e.g., Holland 1987). Consequently meteoric waters percolating through soil are enriched in CO2, indirectly accelerating weathering of silicate by fertilising organic activity and the production of organic acids (Brady and Carroll 1994). Pedogenic carbonates form in equilibrium with soil organic matter (Amundson et al. 1994) often exhibiting highly depleted stable isotope signatures of carbon and oxygen (e.g., Knauth et al. 2002). Cerling et al. (1989) found that the δ13C of coexisting soil organic matter and pedogenic carbonate systematically differ by 14 ‰ at 25 °C and that this difference is compatible with isotopic equilibrium between gaseous CO2, aqueous and solid carbonate species when soil CO2 is controlled by diffusive transport. Although this study refers to calcite a similar relationship could be plausible for a “pedogenic” magnesite that acquires carbon from overlying soil. Fig. 2 shows the carbon and oxygen isotopic composition of the Attunga magnesite deposit in the context of other carbonates of the southern New England Orogen. Their signature is identical with samples from Attunga and Borah Creek which have been described as NSW weathering derived magnesites (Ashley 1995). Silica carbonate rock is common along tectonic contacts in the Great Serpentinite Belt. It formed through hydrothermal replacement of serpentinite along the Peel Fault and is interpreted to be derived from a deep seated metamorphic (homogenised crust) and/or magmatic source and thus exhibits higher δ13C and lower δ18O values than the Attunga magnesite (Ashley and Brownlow 1993). The Piedmont magnesite deposit, which has been attributed to hydrothermal alteration of silica carbonate rock (Ashley 1995), is chararterised by heavy δ13C values and a wide range of δ18O values. Some of the cryptocrystalline magnesites at Piedmont show high δ18O values similar to Attunga magnesites but fluid inclusion temperatures indicate formation of Piedmont magnesite at temperatures of 170 – 230 °C and thus from fluids with distinct δ18O values. A sample of cryptocrystalline weathering crust developed from silica-carbonate rock in Piedmont has less heavy δ13C values, intermediate between Piedmont and Attunga signatures and might reflect re-equilibration with the atmosphere during weathering (Brownlow and Ashley 1991; Ashley 1995). Different carbonates from lamprophyres, hydrothermally altered wallrocks and veins at Hillgrove are closely related to the 263
NEO 2010 intrusion of I-type granites of the New England Batholith in the late Permian (Ashley et al. 1994). While δ18O values for these carbonates are significantly lower than for the Attunga magnesites their range of δ13C overlaps. The authors consider the oxidation of graphite, ultimately having an organic origin, as the source of carbon. Based on a temperature of formation of 225 °C, Ashley et al. (1994) calculate the isotopic composition of the mineralising fluid as δ13C of -11.1 ‰ and δ18O of 7.7 ‰, both of which are not compatible with the fluid compositions inferred for the Attunga magnesite deposit.
Conclusions The carbon and oxygen stable isotopic fingerprint of the magnesites from Attunga Magnesite quarry is compatible with the signatures of other cryptocrystalline magnesites developed in ultramafic complexes worldwide as compiled by Kralik et al. 1989. While a hypogene, near-surface formation at low temperatures from meteoric waters is indicated by the oxygen isotope signature, the pathway of carbon into the magnesite is still controversial. Due to the depleted δ13C signature atmospheric CO2, marine limestone, magmatic CO2 and CO2 from mantle degassing can be excluded as dominant sources of carbon in the Attunga magnesites. The most likely source of carbon is organic material that has been enriched in 12C during C3 photosynthesis. A case can be built for either, a carbon supply by ascending CO2 mixing with groundwater of meteoric origin prior to reaction with the ultramafic protolith as well as for descending meteoric waters that acquire CO2 from the respiration and decomposition of soil organic material. Other magnesite occurrences and carbonates in the Great Serpentinite Belt yield significantly different isotopic compositions. Lamprophyre dykes in the Hillgove area of the New England Orogen yield comparable light δ13C but are interpreted to have formed from a fluid with distinctively different isotopic composition (Ashley et al. 1994) and thus no evidence of ascending metamorphic fluids with compatible isotopic composition could be found.
Acknowledgements The first author is indebted to the University of Newcastle for a postgraduate research scholarship. Dr Russell Drysdale assisted in the acquisition of stable isotope data and kindly provided access to IRMS facility at the University of Newcastle. Jenny Zobec assisted in the acquisition of powder diffraction data and the usage of the ICDD database. We acknowledge with gratitude Drs Sasha Wilson (Indiana University Bloomington, USA) and Stephen Crowley (The University of Liverpool, UK) for the provision of samples and many insightful discussions on stable isotope measurement methods. We also thank Professors Paul Ashley (The University of New England, Australia) and Greg Dipple (The University of British Columbia, Canada) for many valuable comments on magnesite deposits in Australia and Canada, respectively.
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NEO 2010 Ashley, P.M., 1995. The Piedmont hydrothermal magnesite deposit, Great Serpentinite Belt, northern NSW: Geochemical and stable isotopic constraints. Research Report, Centre for Isotope Studies North Ryde, NSW, Australia. Ashley, P.M., 1997. Silica-carbonate alteration zones and gold mineralisation in the Great Serpentinite Belt, New England Orogen, New South Wales. In: Ashley, P. M. & Flood, P. G. (eds) Tectonics and Metallogenesis of the New England Orogen. Geological Society of Australia Special Publication, 19, 212225. Benson, W,N., 1913. The geology and petrology of the Great Serpentine Belt of New South Wales. Part 1. Proceedings of the Linnean Society of New South Wales, 38, 490-517. Brady, P.V., Carroll, S.A., 1994. Direct effects of CO2 and temperature on silicate weathering: Possible implications for climate control. Geochimica et Cosmochimica Acta, 58, No. 8, 1853-1856. Brown, R.E., 2009. The newly defined Glen Bell Formation, and a reappraisal of the Wisemans Arm Formation, Halls Creek district, northern NSW. Geological Survey of New South Wales, Quarterly Notes 131. Brownlow, J.W., Ashley, P.M., 1991. Piedmont magnesite deposit – a hydrothermal vein system in the Great Serpentinite Belt. New South Wales Geological Survey, Quarterly Notes, 82, 1-20. Cerling, T.E., 1984. The stable isotopic composition of modern soil carbonate and its relationship to climate. Earth and Planetary Science Letters, 71, 229-240. Cerling, T.E., Quade, J., Wang, Y., Bowman, J.R., 1989. Carbon isotopes in soils and paleosols as ecology and paleoecology indicators. Nature, 341, 138-139. Chacko, T., Deines, P., 2008. Theoretical calculation of oxygen isotope fractionation factors in carbonate systems. Geochimica et Cosmochimica Acta, 72, 3642-3660. Davis, M., 2008. The CO2 sequestration potential of the ultramafic rocks of the Great Serpentinite Belt, New South Wales, Honours Thesis, Department of Earth Sciences, University of Newcastle. Das Sharma, S., Patil, D.J., Gopalan, K., 2002. Temperature dependence of oxygen isotope fractionation of CO2 from magnesite-phosphoric acid reaction. Geochimica et Cosmochimica Acta, 66, No. 4, 589-593. Deines, P., 2002. The carbon isotope geochemistry of mantle xenoliths, Earth-Science Reviews, 58, 247-278. Deines, P., 2004. Carbon isotope effects in carbonate systems. Geochimica et Cosmochimica Acta, 68, No. 12, 2659-2679. Facer, R.A., (ed.) 2007. Industrial mineral opportunities in New South Wales, compiled by Whitehouse, J., Geological Survey of New South Wales Bulletin 33. Golyshev, S.I., Padalko, N.L., Pechenkin, S.A., 1982. Fractionation of stable oxygen and carbon isotopes in carbonate systems. Geokhimiya, 10, 1427-1441. Government of Australia, Bureau of Meteorology (www.bom.gov.au, accessed on 20 Aug 2010). Hoefs, J., 2009. Stable Isotope Geochemistry, Springer-Verlag Berlin Heidelberg. Holland, H.D., 1987. The Chemistry of the Atmosphere and Oceans. Wiley. International Atomic Energy Agency, (IAEA) 1981. Statistical treatment of environmental isotope data in precipitation. Technical reports series No. 206. Knauth, L. P., Brilli, M., Klonowski, S., 2003. Isotope geochemistry of caliche developed on basalt. Geochimica et Cosmochimica Acta, 67, No.2, 185-195. Kralik, M., P. Aharon, et al. 1989. Carbon and oxygen isotope systematics of magnesites; a review. In: Moeller (eds) Monograph Series on Mineral Deposits 28: 197-223. (SEEMS WRONG - CHECK AUTHORS) Lackner, K.S., Wendt, C.H., Butt, D.P., Joyce, E.L., Sharp, D.H., 1995. Carbon dioxide disposal in carbonate minerals. Energy 20, 1153-1170.
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NEO 2010 Metz, B., Davidson, O., de Coninck, H., Loos, M., Meyer, L., 2005. IPCC, 2005: IPCC special report on carbon dioxide capture and storage. Prepared by working group III of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom: 442 pp. Mirnejad, H., Ebrahimi-Nasrabadi, K., Lalonde, A.E., Taylor, B.E., 2008. Mineralogy, stable isotope geochemistry, and paragenesis of magnesite deposits from the ophiolite belt of Eastern Iran. Economic Geology, 103, 1703-1713. Ritger, S., Carson, B., Suess, E., 1987. Methane-derived authigenic carbonates formed by subductioninduced pore-water expulsion along the Oregon/Washington margin. Geological Society of America Bulletin, 98, 147-156. Romanek, C.S., Grossman, E.L., Morse, J.W., 1992. Carbon isotope fractionation in synthetic aragonite and calcite: Effects of temperature and precipitation rate. Geochimica et Cosmochimica Acta, 56, 419-430. Stumm, W., Furrer, G., Kunz, B., 1983. The role of surface coordination in precipitation and dissolution of mineral phases. Croat. Chem. Acta., 46, 593-611. (WRITE SOUCE JOURNAL IN FULL Yang, K., Seccombe, P., 1997 Geochemistry of the mafic and ultramafic complexes of the northern Great Serpentinite Belt, New South Wales: implications for first stage melting. In: Ashley, P. M. & Flood P. G. (eds) Tectonics and metallogenesis of the New England Orogen, Geological Society of Australia Special Publication 19, PAGE NUMBERS Zedef, V., Russell, M.J., Fallick, A. ., 2000. Genesis of vein stockwork and sedimentary magnesite and hydromagnesite deposits in the ultramafic Terranes of southwestern Turkey: A stable isotope study. Economic Geology, 95, 429-446.
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Discontinuous or slow exhumation after subduction evidence from high-pressure rocks in the Peel Manning Fault system Glen Phillips1 1
Discipline of Earth Sciences, The University of Newcastle, NSW, Australia
Keywords: eclogite, blueschist, exhumation, subduction, Tasmanides, Peel Manning Fault system
Introduction Small (< 1 metre) phacoids of high–pressure, low-temperature (HP-LT) metamorphic rocks located in the Peel-Manning Fault system of the southern New England Orogen provide a record of Cambrian subduction and Ordovician exhumation that is difficult to reconcile with tectonic models of the New England Orogen. In this talk, I present new thermodynamically constrained pressure-temperature data from these eclogite and blueschist facies metamorphic rocks. Combined with new thermochronological data and existing geochronological constraints, it can be shown that the high-pressure rocks share a common metamorphic history, and can therefore, be used to reconstruct a pressure-temperature-time path for the Cambrian– Ordovician convergent margin. These data indicate that there was a significant time lag between eclogite (~ 536 Ma) and blueschist facies metamorphism (470 Ma), which took place at crustal depths of 85 km and 20 km, respectively. This ~ 65 million year window indicates that approximately 65 km of exhumation occurred either very slowly (~ 1 mm/yr) or, was discontinuous. This is in stark contrast with rapid, single-stage exhumation models currently proposed for some high-pressure rock localities. Interestingly, the highpressure rocks are also considerably older than the orogen that contains them; which provide an intriguing link between early Palaeozoic subduction and the Permian evolution of the New England Orogen. Finally, the terminal exhumation and exposure of the high-pressure rocks during the Permian was facilitated by localised extrusion along the serpentinite-rich Peel-Manning Fault system.
Field setting, petrography & chronology of high-pressure metamorphic rocks Five known high-pressure metamorphic rock localities have been identified in the southern New England Orogen (Fig. 1). High-pressure metamorphic rocks located at Attunga, Glenrock Station, Gleneden and Pigna Barney are located along the Peel Manning Fault system, whereas those exposed at Port Macquarie probably represent a segment of the fault system that was translated during left-lateral transpression (Roberts et al., 1993). On the basis of mineralogy, the high-pressure rocks can be divided into two main groups, namely: (i) eclogite or (ii) blueschist. Phacoids of eclogite are located at Attunga, Port Macquarie and Gleneden (Fig. 1), where they exist as isolated blocks contained in retrogressed rinds of actinolite and chlorite schist, or glaucophane, phengite (±garnet). The prograde to peak mineral assemblages preserved in the eclogites at all three localities show clear similarities, with the dominant prograde–peak assemblage made up of garnet, rutile, glaucophane and sodic omphacite (Jd45; Table 1). Garnet is invariably zoned, with rimwise increasing pyrope (Mg/(Ca+Mn+Mg+Fe2+ = 8–20) content at the expense of almandine (Fe2+/(Ca+Mn+Mg+Fe2+ = 55– 45) (Allan & Leitch, 1992; Och et al., 2003; this study). Variations in grossular content are limited (Ca/(Ca+Mn+Mg+Fe2+ = 28–32), yet slight core to rim decreases are evident. In addition to the peak phases common to each locality, lawsonite is also present in eclogite phacoids located at Port Macquarie (Och et al., 2003). Retrograde mineral phases identified in the eclogite phacoid from Attunga are a low jadeite omphacite (Jd26), along with actinolite, chlorite, albite and titanite. A transitional retrograde assemblage is also preserved at Port Macquarie, where a phengite, garnet, glaucophane, titanite, albite assemblage define cm–metre-scale retrograde rinds around some of the eclogite phacoids (Och et al., 2003). 267
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Blocks of blueschist are also entrained within serpentenite mélange zones at Glenrock Station and Pigna Barney (Fig. 1). The dominant mineral assemblage preserved in these rocks consists of glaucophane, muscovite, actinolite, albite, pumpellyite and titanite with or without clinozoisite/epidote and chlorite. Relicts of prograde garnet encased by actinolite and chlorite are also evident in the blueschists located at Pigna Barney (Cross, 1983). The presence of these relict garnet supports an earlier, higher-grade history than that indicated by the dominant blueschist assemblage. Interestingly, these relict garnets have identical compositions (py8alm53sp7gr31) to the cores of garnet in the Attunga eclogite. As a result, it is reasonable to assume that the blueschists located at Pigna Barney experienced a similar prograde–peak metamorphic history to that of the Attunga eclogite. Geochronological and thermochronological studies of eclogite and blueschists reveal a two-stage history of formation. An estimate of eclogite facies metamorphism is based on a U-Pb age of 536 ± 18 Ma obtained from SHRIMP analysis of zircons from the Attunga eclogite (Fanning et al., 2002). The relevance of this age is unresolved, as it could represent the timing of protolith emplacement, or of eclogite facies metamorphism. The suggestion that this age may represent the timing of protolith emplacement is based on similar U-Pb zircon dates yielded from plagiogranite and Fig. 1. Simplified map of the an Sm-Nd mineral isochron age calculated from metadiorite located southern New England Orogen in the Peel-Manning Fault system (Aitchison et al., 1992; Sano et showing the high-pressure rocks al., 2004). Timing of blueschist metamorphism is based on a locations. previous K-Ar study of phengite from metabasalts located at Glenrock Station, Pigna Barney and Port Macquarie, which revealed Ordovician ages (491–460 Ma; Fukui et al., 1995). Recent Ar-Ar step heating analyses of phengite from a blueschist at Glenrock Station support this age range and revealed an age of 473±6 Ma (Phillips unpubl.). Single grain, laser ablation Ar-Ar analysis of phengite from the Pigna Barney blueschist also indicate Ordovician ages (~ 470 Ma) (Phillips unpubl.). In addition to supporting an early to middle Ordovician age for blueschist metamorphism, these similarities in the isotopic record strongly support a common metamorphic history for the blueschist facies rocks located throughout the fault system.
A regional P-T-t path of eclogite-blueschist rocks Using a mineral equilibria approach (THERMOCALC; Powell et al., 1998), estimates of pressure and temperature were calculated for prograde and retrograde segments of a regional P-T path for high-pressure rocks. The prograde segment of the P-T path was constrained using phase relationships and mineral composition data from the Attunga eclogite. P-T piercing points for the prograde history were constrained by the stability and AlVI composition of texturally early glaucophane, followed by the core the rim compositions of peak metamorphic garnet. With respect to the segment of the P-T path defined by the garnet compositions, each piercing point represented the intersection between calculated isopleths for almandine and grossular content along a core to rim transect of individual grains. Importantly, this part of the P-T path also crosses the calculated compositional isopleth for Jd44 in omphacite (Fig. 2). Using this approach, the growth of garnet occurred between 20 kbar and 540˚C to 24 kbar and 620˚C. This estimate of peak preserved metamorphism also indicates that eclogite facies metamorphism took place at depths of ~ 85 km in a geothermal gradient of ~ 7˚C/km. It can also be shown thermodynamically that lawsonite would have grown in the Attunga eclogite at these P-T conditions (shown as a solid line in Fig. 2). As a result, it is likely that eclogites exposed at Attunga and Port Macquarie probably record a similar metamorphic history, however owing to different fluid activities during exhumation, lawsonite was only preserved in the eclogite at Port Macquarie. Based on the U-Pb zircon data from the Attunga eclogite, the timing of this eclogite metamorphism is tentatively constrained to the latest Neoproterozoic to Early Cambrian. 268
NEO 2010 Evidence of near isothermal decrease in pressure followed peak preserved metamorphism, which is based on the composition (Jd26) and modal abundance of the low-jadeite omphacite in the Attunga eclogite. Thermodynamic calculations indicate that pressure and temperature conditions of 14-15 kbar and 590˚C were operating during the growth of this mineral phase. These data signify that after peak metamorphism, approximately 35 km of exhumation coincided with an increase in geothermal gradient to ~ 12 ˚C/km. Table 1. A summary of mineral phases and geo/thermochronological data from high-pressure rocks.
Further exhumation of the high-pressure rocks is indicated by the growth of equilibrium glaucophane and pumpellyite in the blueschists at Glenrock Station and Pigna Barney. In combination, mineral equilibria calculations and the AlVI composition of glaucophane indicate that P-T conditions in the range of 5.2-5.4 kbar and 285–300˚C were responsible for the formation of this equilibrium assemblage. Furthermore, white mica associated with the glaucophane-pumpellyite equilibria has 3.6-3.4 Si p.f.u. (Offler, 1999), which overlaps the P-T path delineating this retrograde segment of the regional P-T path. In a temporal sense, ArAr ages of white mica from the blueschists at Glenrock Station and Pigna Barney yield early to middle Ordovician ages. As the estimate of T for the growth of the blueschist assemblage in the rocks from Glenrock Station is below the closure temperature for argon diffusion in white mica (Tc ~ 350˚C), this age can be confidently associated with the crystallisation of the white mica, and therefore, blueschist metamorphism. Fig. 2. A regional P-T path for the evolution of highpressure rocks exposed in the Peel Manning Fault system. Grey ellipses represent P-T piercing points calculated thermodynamically. White squares represent timing constraints based on isotopic data. Si contents are based on calculated isopleths for phengite (Massone & Schreyer, 1987).
Discussion and conclusions Similarities in age, mineral assemblages and phase compositions strongly support that the high-pressure metamorphic rocks located in the Peel-Manning Fault system shared a similar metamorphic history. This history can be summarised as follows: (i) eclogite facies metamorphism at crustal depths of approximately 85 km (geothermal gradient: 7˚ C /km) during the late Neoproterozoic–early Cambrian; (ii) approximately 35 kms of exhumation and heating (geothermal gradient: 12˚C/km) (iii) further exhumation resulted in the growth of glaucophane-pumpellyite bearing assemblages at crustal depths of 20 km (geothermal gradient: 15C˚/km) during the early to middle Ordovician. If this exhumation history was continuous, the current study 269
NEO 2010 indicates that the rate of exhumation was very slow (mm/yr), and in stark contrast to the tectonic time scales estimated by other studies of high-pressure rocks (i.e., 1–10 cm/yr). The difficulty in reconciling the tectonic history of the high-pressure rocks with that of the New England Orogen is the large hiatus in timing between the formation and exhumation of the rocks (i.e., CambrianOrdovician) and their terminal translation and exposure in the Peel-Manning Fault system. Serpentinite detritus located in the basal units of basins located proximally to the Peel Manning Fault system indicate that the exposure of the serpentinite occurred during the early Permian (Allen & Leitch 1990). By association, it is therefore reasonable to assume that the high-pressure rocks were also brought to the surface at this time. The implications of these timing constraints are that after the initial exhumation of the high-pressure rocks during the early to middle Ordovician, these rocks must have resided in the middle crust (~ 20 km depth) for approximately 170 million years. This style of discontinuous exhumation is in strong contrast to many other high-pressure terranes, where exhumation and exposure after peak-preserved metamorphism is rapid and single stage. One potential explanation for these intriguing timing relationships is that the high-pressure rocks record evidence of subduction and exhumation of a long-lived, west-dipping subduction zone that experienced a significant eastward retreat from the Early Palaeozoic Delamerian margin, to the New England convergent margin in the Permian. This being the case, the high-pressure rocks now located in the Peel-Manning Fault system provide a record of: (i) subduction-exhumation during the Delemarian and Lachlan orogenies, as well as (ii) the magnitude of Cambrian to Permian trench retreat of a long-lived west-dipping subduction system that was responsible for the development of the Tasmanides.
Acknowledegments Associate Professor Robin Offler is thanked for his continual support in unravelling the intriguing metamorphic record of the high-pressure rocks in the New England Orogen. The manuscript also benefited from several suggestions made by Robin.
References Aitchison, J.C., Ireland, T.R., Blake, Jr., M.C. and Flood, P.G., 1992. 530 Ma zircon age for ophiolite from the New England orogen: Oldest rocks known from eastern Australia. Geology, 20, 125-128. Allen, A.D., and Leitch, E.C., 1990. The tectonic significance of uncomformbale contacts at the base of Early Permian sequences, southern New England Fold Belt. Australian Journal of Earth Sciences, 37, 43-49. Allan, A.D. and Leitch, E.C., 1992. The nature and origin of eclogite blocks in serpentinite from the Tamworth Belt, New England Fold Belt. Australian Journal of Earth Sciences, 39, 29-35. Cross, K.C., 1983. The Pigna Barney ophiolitic complex and associated basaltic rocks, northeastern New south Wales, Australa. PhD Thesis (Unpubl). University of New England, Armidale. Fanning, C.M., Leitch, E.C., Watanabe, T., 2002. An updated assessment of the SHRIMP U-Pb zircon dating of the Attunga eclogite in New South Wales, Australia; relevance to the Pacific Margin of Gondwana. International Symposium on the Amalgamation of Precambrian Blocks and the role of the Paleozoic Orogens in Asia Sapporo. Fukui, S., Watanabe, T., Itaya, T., Leitch, E.C., 1995. Middle Ordovician high PT metamorphic rocks in eastern Australia: evidence from K-Ar ages. Tectonics 14, 1014-1020. Och, D.J., Leitch, E.C., Caprarelli, G., Watanabe, T., 2003. Blueschist and eclogite in tectonic mélange, Port Macquarie, New South Wales, Australia. Mineralogical Magazine, 67, 609-624. Offler, R., 1999. Origin and significance of blueschist “knockers”, Glenrock Station, NSW. In: P.G. Flood, Editor, New England Orogen, Regional Geology, Tectonics and Metallogenesis, University of New England, Armidale (1999), pp. 35–44. Powell, R., Holland, T.J.B. and Worley, B., 1998. Calculating phase diagrams involving solid solution via non-linear equations, with examples using THERMOCALC. Journal of Metamorphic Geology, 16, 577-588. Roberts, J., Lennox, P.G. and Offler, R., 1993. The geological development of the Hastings Terrane – displaced forearc fragments of the Tamworth Belt. In Flood, P.G. & Aitchison, J.C. (eds) New England Orogen, eastern Australia. The University of New England, Armidale, pp 231-242. 270
NEO 2010 Sano, S., Offler, R., Hyodo, H., Watanabe, T., 2004. Geochemistry and Chronology of Tectonic Blocks in Serpentinite Mélange of the Southern New England Fold Belt, NSW, Australia. Gondwana Research, 7, 817831.
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NEO 2010
Unravelling the New England Orocline: Geological and paleomagnetic constraints Sergei Pisarevsky1,2, Peter Cawood2,3, Evan Leitch4 1 2
School of Earth and Environment, University of Western Australia, Australia 3
4
School of GeoSciences, The University of Edinburgh, UK
School of Geography and Geosciences, University of St Andrews, UK
Department of Environmental Sciences, University of Technology, Sydney, Australia
Keywords: New England orogen, Paleozoic, paleomagnetism, orocline.
Introduction Oroclines are map-view bends of originally quasi-linear lithospheric elements. The doubly vergent New England orocline lies within the Eastern Australian segment of the Terra Australis accretionary orogen and developed during the late Paleozoic to early Mesozoic Gondwanide orogeny that extended along the Pacific margin of Gondwana (Cawood, 2005). The New England segment extends from some 1600 km from north of Sydney to Townsville. It is bounded to the west, and in part overthrusts, the late Paleozoic to Mesozoic Sydney-Gunnedah-Bowen Basin, and is divisible into northern and southern portions by Mesozoic and younger strata of the Clarence-Moreton Basin (Fig. 1). Subduction along the proto-Pacific margin of Gondwana began at the end of the Neoproterozoic and continued throughout the Paleozoic (Cawood, 2005; Cawood et al., 2009). In New England convergent plate margin activity is expressed in the development of a tripartite association comprising the western KeepitConors magmatic arc, the central Tamworth-Yarrol forearc basin, and the eastern Tablelands-Wandilla subduction complex (Cawood and Leitch, 1985; Korsch, 1977; Leitch, 1974).
New England Orocline The orocline is doubly vergent consisting of the northern Texas-Coffs Harbour segment, which shows geologically indicated rotation of up to 180° clockwise, and a southern segment, the Manning-Hastings segment with 150°of anticlockwise rotation (Fig. 1). The doubly vergent character of the orocline has lead to competing models of formation involving dextral motion to form the northern segment (Korsch and Harrington, 1987; Murray et al., 1987; Offler and Foster, 2008) and sinistral motion to form the southern segment (Cawood, 1982). In this paper we show that these opposite senses of apparent motion can be produced during overall sinistral strike-slip movement along this segment of the Gondwana margin. Furthermore, paleomagnetic data indicate that only a sinistral sense of motion is consistent with available paleomagnetic data.
Age of orocline formation The termination of arc related magmatism along the Keepit-Connors magmatic arc occurred at about 306 Ma (Geeve et al., 2002; Roberts et al., 2006). An upper limit on orocline formation is provided by the New England Batholith, which is emplaced across the Texas-Coffs Harbour segment of the orocline. The bulk of the batholith is of Triassic age (
