7.8 Bulk rock chemistry of anthophyllite samples and amphibolite samples . ...... previously unrecognised minerals, anthophyllite and stilpnomelane, as well as ...
Mineral Paragenesis and Geometallurgy of the Karlawinda Deposit, Western Australia Matthew Ross Murray Ferguson BSc
School of Earth Sciences
CODES Centre of Excellence in Ore Deposits 2012 Supervisors: Prof. A. Crawford and Prof. R. Large
A research thesis submitted in partial fulfilment of the degree of Bachelor of Science with Honours (Economic Geology)
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Date: 23rd April 2013
Matthew Ross Murray Ferguson
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i Mineral Paragenesis and Geometallurgical Characteristics of the Karlawinda Deposit
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Date: 23rd April 2013
Matthew Ross Murray Ferguson
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ii Mineral Paragenesis and Geometallurgical Characteristics of the Karlawinda Deposit
Abstract
Abstract The Bibra gold-only deposit at Karlawinda is hosted in an upper amphibolite faciesmetamorphosed quartz-rich turbidite sequence, which now comprises psammites, psammopelites, and schists, intruded by dolerites that are now amphibolites. The host sequence is younger than ca 2.46 Ga and dolerite sills crystallised ca 2.25 Ga prior to mineralisation and metamorphism. This study aims to provide further understanding of the lithology, petrography, geochemistry, gold mineralisation and geometallurgy of the Bibra deposit. The textural and structural siting of gold and anhydrous mineral assemblages can be accounted for by the formation of the deposit at greenschist facies, followed by prograde metamorphism to high-metamorphic grade. Gold mineralisation is associated with sulfides and deformed quartz-plagioclase veins that occur predominantly in poorly sorted sandstone units. Wall-rock silicification has occurred in mineralised horizons, in addition to the formation of plagioclase, calcite, ankerite, pyrite, arsenopyrite, petzite and other telluride minerals, electrum and native gold. Silicates and sulfides encapsulate ~75% of gold-containing grains observed in this study, which include native gold (0.5m thick sandstone beds are characterised as thick and are interpreted as the deposits from high-concentration, locally erosive turbidity currents (Lowe, 1982). Thinly bedded sandstones and siltstones are deposits of low-concentration and diluted turbidity currents with siltstones becoming more prevalent away from the main flow axis, until mudstones become dominant in interlobe positions (Prélat et al., 2009). The main controls on sediment supply are channel migration or abandonment and depositional topography. 27 Mineral Paragenesis and Geometallurgical Characteristics of the Karlawinda Deposit
Chapter 3: Geology of the Bibra Deposit Grecula et al. (2003) and Prélat et al. (2009) found that flows are ground-hugging and sensitive to subtle depositional gradients, with lobe switching controlled by infilling and generation of depositional relief until subsequent turbidity currents are forced to find a new depression to fill. The thickness of mudstone units at Bibra indicates that quiet conditions may have persisted for relatively long periods of time, much longer than the time taken to deposit sandy units. A poorly permeable seal over a fluid reservoir with fluid supplied from below is necessary for reaching critical fluid pressure, overcoming lithostatic loading and the strength of rocks for hydraulic fracturing to occur (Ostapenko and Neroda, 2007). Intercalated rocks of different permeability and competency facilitate hydraulic fracturing and are favourable as fluid- and ore-controlling structures. Intercalated sandstones, amphibolites and mudstones at Bibra meet these criteria and it is likely that tensile hydraulic fracturing occurred in sandstone units in response to tectonic activity and the high fluid pressures built up under the amphibolite seal. At the Tokur deposit, Russia, economic veins are hosted in the footwall of the main ore controlling fault, hosted in bedded sandstone capped by siltstones and shales. Hydraulic fracturing occurred in the most permeable, fluid saturated areas in the sandstone (Ostapenko and Neroda, 2007). These relationships between a major fault, poorly permeable capping, permeable units and veining are evident at Bibra in KBD026. At the Bendigo deposit, ore zones occur above sandstones capped by shales. Hydraulic fracturing and delamination occurred due to contrasts in bedding, permeability and competency of rocks in structural trap sites (Ostapenko and Neroda, 2007). As a result, ore bodies are localised in delaminated cavities in the anticline hinges beneath shale caps and in ladder veins hosted in thin sandstone interlayers. This suggests that saddle reefs may occur in anticline hinges at Bibra if delamination between sandstones and amphibolites has occurred (Figure 3.7). KBD026 may have intercepted a spur reef-like occurrence of mineralisation at Bibra that may be inferred from the cryptic, thin and sheeted to ladder-like of veining in
Figure 3.7: Types of quartz reef at Bendigo. After Wilkinson (1988).
28 Mineral Paragenesis and Geometallurgical Characteristics of the Karlawinda Deposit
Chapter 3: Geology of the Bibra Deposit KBD026, which is different from the more massive style expected for saddle reefs (Wilkinson, 1988; Bierlein et al., 2004). Mineralisation at Fosterville is also associated with faults and fractures in sandstone units that result from high fluid pressure and high fluid flux (Cox et al., 1991). Tensile hydraulic fracturing is considered to be an important mechanism for gold emplacement across the Bendigo Zone (Cox et al., 1991; Cox, 1995). In all deposits discussed here, mineralisation is also controlled by domal culminations of regional folds, where fold axes plunge in opposite directions (Bierlein et al., 2004; Ostapenko and Neroda, 2007). It is outside the scope of this investigation to determine the position of the domal culmination of regional folding, although it can be said with confidence that it is not at Bibra as evidence of flattening of stratigraphy is currently lacking. Previous work on progressive rotation of lineations by Clow (2010) may allow determination of the approximate location of the domal culmination. The fault observed in KBD026 is likely a third or fourth order structure that sourced gold bearing fluids from a first or second order structure. The observed fault is likely related to the unnamed fault, found through aeromagnetics, which inflects around Bibra (IGO, 2011). Bibra is located at the point where the fault changes strike from NNE to ENE. These fault geometries match well with first to second order structures in the area such as Fortescue River Fault and the Western Creek Fault (Tyler, 1991). First order faults rarely host economic mineralisation; rather it is lower order faults that are the sites of mineral deposition (Goldfarb et al., 2005). Fluid will be focussed from higher order faults by changes in strike, areas of regional uplift or anticlines, and zones of competency contrast – again, all features present at Bibra. 3.4.3 Discussion - Implications of findings The association of mineralisation with silicified sandstone intercalated between amphibolites provides criteria for detecting favourable mineralisation sites during initial core logging. The association of sulfides with gold mineralised intervals is also useful for recognising mineralisation of this style during core logging. It is likely that mineralisation present in KBD026 is associated with a spur reef due to the relatively low contained gold compared to some other drilling intersections and the apparent ladder-like nature of quartz veining. Higher-grade zones may be located in saddle reef positions in anticline hinges. 29 Mineral Paragenesis and Geometallurgical Characteristics of the Karlawinda Deposit
Chapter 3: Geology of the Bibra Deposit There is also the real and unfortunate likelihood that the domal culmination associated with mineralisation at Bibra has been eroded away. However, stratigraphy below the mineralised horizons at Bibra will still be present and may contain gold mineralisation; the regolith above this culmination may also contain supergene gold mineralisation.
30 Mineral Paragenesis and Geometallurgical Characteristics of the Karlawinda Deposit
Chapter 3: Geology of the Bibra Deposit
A3 FULL PAGE INSERT: Figure 3.8 KBD026 graphic log.
31 Mineral Paragenesis and Geometallurgical Characteristics of the Karlawinda Deposit
Chapter 4: Petrography
Chapter 4 Petrography of the Bibra Deposit 4.1
Introduction
This chapter presents the petrography of the seven key rock types identified in diamond drill core from the Bibra Deposit. Diagnostic mineral assemblages and abundances were used to classify samples. Outcomes are summarised and discussed, highlighting observations and relating groupings back to drill core observations. The documentation of peak and post-peak metamorphic mineralogy in each unit, when linked with their lithostratigraphic position, extent, and mineral and whole-rock geochemistry, underpins further discussion of mineral paragenesis, geometallurgy and the origins of mineralisation and the host package. In addition, a thorough petrographic description of the Bibra Deposit allows for more informed comparisons with other Archean gold deposits. 4.2
Methods
Some 95 samples from Bibra diamond drill core were submitted to the UTAS School of Earth Sciences lapidary department for preparation of standard thin sections, polished thin sections and a polished slab (Appendix 3). Fresh samples were mounted on 76mm x 26mm slides and ground down to 30µ thickness for optical examination, with 20 samples were subsequently polished on a 10µ diamond grinder. Microscopy was carried out using an Olympus BX60 microscope and an Olympus CH2 microscope. Modal abundances of minerals, grain shapes, sizes, conditions, general textural relationships/distributions and associations are accounted for in this study (Appendix 4). Modal abundances have been estimated with aid of vol% charts. Grain shapes are described based on regularity of grain margins, relative proportions of long and short axes, and development of crystal faces. Grain size measurements were made on long axes and are presented as maximum and 1σ value (~66% grains 60 modal% quartz (herein referred to as %), amphibolites contain >50% amphiboles, schists constitute foliated quartz-amphibole-feldspar-biotite assemblages plus other minor minerals and are classified into either quartz- or amphibole-dominated. Individual sample 32 Mineral Paragenesis and Geometallurgical Characteristics of the Karlawinda Deposit
Chapter 4: Petrography names are based on major minerals (>10%) arranged in order of decreasing abundance (Figure 4.2 to 4.6). The gold grade of samples herein classified as ‘mineralised’ exceeds 0.25gt-1 for the interval in which the sample originated; these samples are described separately from similar unmineralised rocks in the host package. All units have been recrystallised to metamorphic assemblages and primary textures are rarely preserved. Some degree of uncertainty is inherent in attempting identification of possible protoliths of high-grade metamorphic rocks from petrographic analysis. The classification scheme presented focuses on the mineralogical constitution of the units as a precursor to further restriction of potential protoliths via geochemical methods. Summaries and full petrographic and mineragraphic descriptions are provided in Appendix 4. Table 4.1: Major rock type mineral compositions. Crystal sizes are presented as 1σ and maximum. Crystal Rock type
size
(Mineralised)
