Late-kinematic timing of orogenic gold deposits and significance for ...

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because the present structural geometries of: i the deposits, ii the hosting goldfields, and iii ... Importantly, the two giant greenstone-hosted goldfields, Kalgoorlie.
Ore Geology Reviews 17 Ž2000. 1–38 www.elsevier.nlrlocateroregeorev

Late-kinematic timing of orogenic gold deposits and significance for computer-based exploration techniques with emphasis on the Yilgarn Block, Western Australia David I. Groves a,) , Richard J. Goldfarb a,b, Carl M. Knox-Robinson a , Juhani Ojala c , Stephen Gardoll a , Grace Y. Yun a , Peter Holyland c a

Centre for Teaching and Research in Strategic Mineral Deposits, Department of Geology and Geophysics, UniÕersity of Western Australia, Nedlands, Perth, W.A. 6907, Australia b United States Geological SurÕey, DenÕer Federal Center, DenÕer, Colorado, 80225 USA c Terra Sancta, Unit 6, 4 Brodie Hall DriÕe, Bentley, W.A., Australia 6104. Received 4 August 1999; accepted 7 January 2000

Abstract Orogenic gold deposits are a widespread coherent group of epigenetic ore deposits that are sited in accretionary or collisional orogens. They formed over a large crustal-depth range from deep-seated low-salinity H 2 O–CO 2 " CH 4 " N2 ore fluids and with Au transported as thio-complexes. Regional structures provide the main control on deposit distribution. In many terranes, first-order faults or shear zones appear to have controlled regional fluid flow, with greatest ore-fluid fluxes in, and adjacent to, lower-order faults, shear zones andror large folds. Highly competent andror chemically reactive rocks are the most common hosts to the larger deposits. Focusing of supralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes, normally within D 3 or D4 in a D1 –D4 deformation sequence. Reactivation of suitably oriented pre-existing structures during a change in far-field stress orientation is a factor common to many deposits, and repeated reactivation may account for multiple mineralization episodes in some larger deposits. Absolute robust ages of mineralization support their late-kinematic timing, and, in general, suggest that deposits formed diachronously towards the end of the 100 to 200 m.y. long evolutionary history of hosting orogens. For example, in the Yilgarn Block, a region specifically emphasised in this study, orogenic gold deposits formed in the time interval between 40 and 90 m.y., with most about 60 to 70 m.y., after the youngest widespread basic-ultrabasic volcanism and towards the end of felsic magmatism. The late timing of orogenic gold deposits is pivotal to geologically-based exploration methodologies. This is because the present structural geometries of: Ži. the deposits, Žii. the hosting goldfields, and Žiii. the enclosing terranes are all essentially similar to those during gold mineralization, at least in their relative position to each other. Thus, interpretation of

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Corresponding author. Tel.: q6189-380-3838; fax: q6189-380-1178. E-mail address: [email protected] ŽD.I. Groves..

0169-1368r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 6 8 Ž 0 0 . 0 0 0 0 2 - 0

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geological maps and cross-sections and three-dimensional models can be used to accurately simulate the physical conditions that existed at the time of ore deposition. It is particularly significant that the deposits are commonly related to repetitive and predictable geometries, such as structural heterogeneities within or adjacent to first-order structures, around rigid granitoid bodies, or in specific Alocked-upB fold-thrust structures. Importantly, the two giant greenstone-hosted goldfields, Kalgoorlie and Timmins, show a remarkably similar geometry at the regional scale. Computer-based stress mapping and GIS-based prospectivity mapping are two computer-based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provide important geological aids in exploration, both in broad regions and more localized goldfields. Both require an accurate and consistent solid geology map, stress mapping requires knowledge of the far-field stresses during mineralization, and the empirical prospectivity mapping requires data from a significant number of known deposits in the terrane. The Kalgoorlie Terrane, in the Yilgarn Block, meets these criteria, and illustrates the potential of these methodologies in the exploration for orogenic gold deposits. Low minimum stress anomalies, interpreted to represent dilational zones during gold-related deformation, coincide well with the positions of known goldfields rather than individual gold deposits in the terrane, and there are additional as-yet unexplained anomalies. The prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are: Ži. proximity to, and orientation and curvature of, granitoid-greenstone contacts, Žii. proximity to segments of crustal faults which strike in a preferred direction, Žiii. proximity to specific lithological contacts which have similar preferred strike, Živ. proximity to anticlinal structures, and Žv. the presence of preferred reactive host rocks Že.g., dolerite.. The prospectivity map defines a series of anomalous areas, which broadly conform to those of the stress map Ž) 78% correspondence.. The most prospective category on this map covers less than 0.3% of the greenstone belts and yet hosts 16% of the known deposits, which have produced) 80% of known gold. Thus, it discriminates in favour of the larger economically more-attractive deposits in the terrane. The successful application of stress mapping and prospectivity mapping to geology-based exploration for orogenic gold deposits indicates that more quantitative analysis of geological map data is a profitable line of research. The computer-based nature of these methodologies is ideal for the production of an ultimate, integrated, deposit target map, which can be compared to other, more conventional, targeting parameters such as geophysical and geochemical anomalies. Such an integrated strategy appears the way forward in the increasingly difficult task of cost-effective global exploration for orogenic gold deposits in poorly exposed terranes. q 2000 Elsevier Science B.V. All rights reserved. Keywords: orogenic gold deposits; structural control on mineralization; stress mapping; prospectivity mapping; mineral exploration; Yilgarn Block

1. Introduction In a recent review article, Groves et al. Ž1998. suggest that lode-gold deposits worldwide, which have been variously termed mesothermal, greenstone-hosted, slate-belt hosted, turbidite-hosted, Mother Lode-type or gold-only deposits, are a coherent group of deposits with a common origin. They favour the term Aorogenic gold depositB for this deposit class, because all of the deposits ascribed to this class were formed during compressional to transpressional deformation processes at convergent plate margins in accretionary or collisional orogens. The deposits occupy a unique depth range for hydrothermal ore deposits, with gold deposition from 15–20 km depth to within 2–3 km of the surface, where stibnite and cinnabar may become dominant within the same hydrothermal cells ŽFig. 1..

Orogenic gold deposits may occur in the same orogens as gold-rich porphyry-style and epithermal vein-deposits. However, the porphyry and epithermal deposits occur over a very narrow depth range above, and landward of, a continental margin arc, whereas the orogenic gold lodes extend over a continuum of depths in the growing forearc region ŽFig. 1.. Auriferous volcanic-hosted massive sulfide ŽVHMS. deposits may have a gross spatial association with orogenic gold deposits, but typically formed tens of millions of years earlier, and in the oceanic rocks prior to their collision with the growing continental margin. The chemistry of vein-forming fluids is remarkably similar between orogenic gold ores of all ages. Detailed fluid inclusion studies have shown ore fluids consistently to contain from about 5 mol% CO 2 to equal amounts of H 2 O and CO 2 Že.g., Roedder,

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Fig. 1. Schematic diagram showing the setting and nature of orogenic lode–gold deposits. ŽA. Plate tectonic environments of formation of orogenic lode–gold deposits and other largely syn-volcanic or syn-intrusive gold-rich deposit styles. ŽB. Depth profile of orogenic lode–gold deposit. Adapted from Groves et al. Ž1998..

1984; Smith et al., 1984; Goldfarb et al., 1986; Robert and Kelly, 1987; Goldfarb et al., 1989; Diamond, 1990; Ho et al., 1990; de Ronde et al., 1992.. The relatively high CO 2 content of these ore fluids, responsible for the common widespread carbonate-

rich alteration zones, contrasts with those that deposited other gold deposit types. In addition, ore fluids from orogenic gold deposits, especially where hosted in terranes dominated by metasedimentary rock units, may contain nitrogen and methane at the

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mole percent level. The greater the non-aqueous volatile content of the original ore fluid, the more likely the occurrence of at least episodic immiscibility between aqueous and non-aqueous fluids during decreases in temperature into the two-phase fluid field. The transport of gold in these H 2 O–CO 2 " CH 4 –N2 fluids is facilitied by reduced sulfur complexes, as is supported by measurements of minor amounts of H 2 S in some of the studied fluid inclusions ŽGoldfarb et al., 1989; Yardley et al., 1993.. Pressure–temperature conditions of veining, as determined from the above listed and many other fluid inclusion investigations of orogenic gold systems, are concentrated between 1 and 3 kb and 2508C and 3508C. However, conditions for some near-surface gold deposits may be as low as 0.5 kb and 1508C, and as high as 4–5 kb and 7008C for some of the deeper deposits formed within evolving orogens ŽGroves, 1993; Groves et al., 1998.. The relatively non-aqueous volatile-and H 2 S-rich fluids are also characterized by low total-solvent volumes. The above-listed fluid inclusion studies from both Phanerozoic and Precambrian hydrothermal systems consistently report salinities of less than about 6 wt.% equiv. NaCl. The low salinities are responsible for the well-recognized lack of base metal enrichments within orogenic lode–gold deposits ŽKerrich and Fryer, 1981.. A few gold provinces containing veins with many features suggestive of an orogenic gold classification, such as Sabie-Pilgrim’s Rest along the eastern margin of the Transvaal Basin in South Africa ŽBoer et al., 1993; Tyler and Tyler, 1996. and Tennant Creek ŽKhin et al, 1994. and Telfer ŽRowins et al., 1997., contain high-salinity fluids that may have deposited gold ore along with abundant copper. The spatial association between the gold ores and basinal sedimentary sequences appears important in these circumstances. Yardley Ž1997. points out that high-salinity pore waters, such as developed in basinal settings, will persist in the crust until anatexis and, therefore, may be incorporated into many deep-crustal fluid-flow cells. A consistently heavy and narrow range characterizes oxygen isotope measurements of ore-hosting quartz of Precambrian ŽGolding and Wilson, 1987; Kerrich, 1987., Proterozoic ŽOberthur et al., 1996., Paleozoic ŽKontak and Kerrich, 1997., and Mesozoic-Cenozoic age ŽBohlke and Kistler, 1986; Nes-

bitt et al., 1989; Goldfarb et al., 1997.. Most measured values for d 18 O quartz of Precambrian age range between about 11 and 14 per mil, and younger veins are typically 2–3 per mil heavier, with the difference reflecting variations in host rock compositions and Žor. vein formation temperatures. For both age groups, ore fluids are calculated to have d 18 O compositions between 6 and 11 per mil. Values of fluid d D from these studies, when calculated from measurements made on H-bearing hydrothermal silicates, are between about y70 and y30 per mil. Collectively, these data implicate a deep-crustal fluid source for orogenic lode-gold, although whether this is a metamorphic ŽKerrich and Fryer, 1979; Phillips and Groves, 1983; Goldfarb et al., 1988. magmatic ŽWood et al., 1986; Spooner, 1993., or mantle fluid ŽPerring et al., 1987; Cameron, 1988. is still poorly understood after more than a decade of debate on the scenarios. Carbon and sulfur isotope data are highly variable between orogenic gold districts of all ages, indicating local crustal contributions of at least some of the ore fluid carbon and sulfur, as well as variations in the oxidation state of ore fluids at the final depositional site in the case of sulfur ŽKerrich, 1989.. As a result of deposition from low-salinity, H 2 O– CO 2 " CH 4 " N2 fluids, orogenic lode-gold deposits characteristically have high Au:Ag ratios and low base-metal and tin contents Žsee more detailed discussion below.. Lode–gold deposits that have abundant silver ŽAu:Ag ratios less than 1., significant base-metal contents or tin-rich mineral phases should not be classified as orogenic deposits unless there are convincing reasons to do so based on detailed research. Some of the so-called intrusion-related gold deposits Že.g., Mantos de Punitaqui, Chile; Snip, Canada; Pacoy-Pataz, Peru; Palpa-Ocona, Peru; Kidston, Australia; Kari Kolla, Bolivia; of Sillitoe and Thompson, 1998 and Thompson et al., 1999. have metal characteristics that clearly distinguish them from orogenic gold deposits as defined by Groves et al. Ž1998., whereas others have metal ratios Žand other features. appropriate to their classification as orogenic deposits Že.g., Charters Towers, Australia; Ryan Lode, USA; Mokrsko, Czech Republic; Vasil’kovskoye, Kazakhstan.. Most of the lode–gold deposits with relatively high base-metal, silver andror tin contents were emplaced at shallow depths only. They are gold systems that are better linked to

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the continental arc setting of Fig. 1, as many of the referenced ore deposits are spatially associated with continental-margin Andean batholiths. The purpose of the first part of this paper is to briefly review the consistent features of the orogenic gold deposits Žsee Groves et al., 1998 for more exhaustive review. and to document that the timing of gold deposition is typically late within the tectonic evolution of its host terranes. Demonstration of late timing is vital to the utilization of geology in exploration because, if demonstrated, it means that the current geometries of the deposits and their enclosing host rocks are essentially the same in map and cross-sectional view as they were at the time of gold mineralization. As the primary controls on the siting of the gold deposits are the geometry Žincluding orientation. of the controlling structures and the nature and geometry of host sequences, methodologies that utilize such geometries are thus a potentially powerful tool in mineral exploration for orogenic lode-gold deposits. The second part of the paper reviews the role of stress mapping and GIS-based prospectivity analysis, two such methodologies, in exploration for these deposits. To provide a consistent background, examples of the application of the two methodologies are given for the Yilgarn Block of Western Australia, but they equally can be applied to other terranes hosting this deposit type.

2. Geological characteristics of orogenic lode-gold deposits 2.1. Geology of host terranes Perhaps the single most consistent characteristic of the deposits is their consistent association with deformed metamorphic terranes of all ages. Observations from preserved Archaean greenstone belts and most-recently active, Phanerozoic sedimentary rockdominant metamorphic belts throughout the world indicate a strong association of gold and greenschist facies rocks. However, some significant deposits occur in higher metamorphic-grade Archaean terranes Že.g., McCuaig et al., 1993. or in sub-greenschist grade domains within the metamorphic belts of a variety of geological ages. In the Archaean rocks of Western Australia, some synmetamorphic deposits

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extend into upper amphibolite to granulite facies domains ŽGroves 1993.. In a few areas, where parts of older cratons are reworked during younger orogens, gold ores may be sited also in high-grade metamorphic rocks Že.g., northern, eastern and southern margins of the North China Craton.. However, in these cases, gold ores post-date the metamorphism of immediate host-rocks by more than 1 b.y. Pre-metamorphic protoliths for the auriferous Archaean greenstone belts are predominantly volcano-plutonic terranes of oceanic back-arc basalt and felsic to mafic arc rocks. Terranes dominated by clastic marine sedimentary rocks, generally metamorphosed under greenschist facies conditions, host mostly younger ores. They also are, however, important in some Archaean terranes Že.g., Slave Province, Canada. and Proterozoic Že.g., Ashanti belt, West Africa. terranes. Banded iron formation or ferruginous chert is an additional favourable ore host in many Precambrian gold provinces Že.g., Morro Velho, Quadrilatero Ferrifero, Brazil; Homestake, Northern Black Hills, USA.. 2.2. Deposit mineralogy and alteration The orogenic gold deposits in greenschist facies terranes are typified by quartz-dominant vein systems with F 3–5% sulfide minerals Žmainly Fe-and As-bearing sulfides. and F 5–15% carbonate minerals. Albite, white mica or fuchsite, chlorite, scheelite and tourmaline are also common gangue phases in veins in greenschist-facies host rocks. Vein systems may be continuous over a vertical extent of 1–2 km with little change in mineralogy or gold grade; subtle mineral zoning does occur, however, in some deposits Že.g., Mt Charlotte, Golden Crown, Hill 50 in Western Australia and Alaska-Juneau, SE Alaska, USA.. Gold:silver ratios range from 10 Žnormal. to 1 Žless common., with ore in places being in the veins and elsewhere in sulfidized wallrocks. Gold grades are relatively high, historically having been in the 5–30 grt range; modern-day bulk-mining methods have led to exploitation of lower grade targets. Sulfide mineralogy commonly reflects the lithogeochemistry of the host. Arsenopyrite is the most common sulfide mineral in metasedimentary country rocks, whereas pyrite or pyrrhotite is more typical in metamorphosed igneous rocks. Gold-bearing veins

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exhibit variable enrichment in As, B, Bi, Hg, Sb, Te and W; Cu, Pb and Zn concentrations are generally only slightly elevated above regional backgrounds, although Pb may be anomalous in some deposits. Deposits exhibit strong lateral zonation of alteration phases from proximal to distal assemblages on the scale of metres. Mineralogical assemblages within the alteration zones, and the width of these zones, generally vary with wallrock type and crustal level. Most commonly, carbonates include ankerite, dolomite or calcite; sulfides include pyrite, pyrrhotite or arsenopyrite; alkali metasomatism involves sericitization or, less commonly, formation of fuchsite, biotite or K-feldspar, and albitization; and mafic minerals are highly chloritized. Amphibole or diopside occurs at progressively deeper crustal levels, and carbonate minerals are less abundant. Sulfidation is extreme in banded iron-formation ŽBIF. and ironrich mafic host rocks. Wallrock alteration in greenschist facies rocks involves the addition of significant amounts of CO 2 , S, K, H 2 O, SiO 2 " Na and LILE. 2.3. Structure The strong structural control of mineralization is very apparent at a variety of scales. Many would argue that structure is the single most-important control on gold mineralization. Deposits are normally sited in second-or third-order structures, typically near large-scale, commonly transcrustal, first-order compressional or transpressional structures. Although the controlling structures are commonly ductile to brittle, they are highly variable in type, including: Ža. brittle faults to ductile shear zones with both low-angle to high-angle reverse motion and strike-slip or oblique-slip motion; Žb. fracture arrays, stockwork networks or breccia zones in competent rocks; Žc. foliated zones; and Žd. fold hinges and overturned limbs in ductile turbidite sequences. Mineralized structures have small syn- and post-mineralization displacements Že.g., Ojala et al., 1993., but the gold deposits commonly have extensive down-plunge continuity Žhundreds of metres to as much as 1–2.5 km.. Extreme pressure fluctuations, leading to cyclic fault-valve behavior ŽSibson et al., 1988., result in flat-lying extensional veins and mutually cross-cut-

ting steep fault veins that characterize many deposits Že.g., Robert and Brown, 1986; Miller et al., 1994.. 2.4. Timing of gold mineralization The late timing of formation of orogenic gold deposits is taken, in a relative sense, to suggest that most of the gold veining post-dates regional metamorphism, plutonism and early phases of orogenic deformation in the immediate host rocks. Simultaneously, however, veining is coeval with metamorphism and magmatism at deeper levels within the orogen Že.g., Groves et al., 1995.. Ore fluids may have formed at these deeper levels, or even below the deepest supracrustal rocks, and migrated to retrograding and uplifting parts of the orogen during continuing episodes of deformation Že.g., Sibson, 1985; Phillips and Powell, 1993; Stuwe et al., 1993; Kerrich and Cassidy, 1994.. The deposits are, therefore, commonly late in the overall deformational cycle, although, in some provinces, they may be overprinted by deformational fabrics Že.g., Morasse et al., 1995.. In an absolute time sense, the late timing can represent a range of scenarios from veining just a few million years after regional metamorphism and deformation of host rocks Že.g., Chugach accretionary prism, Gulf of Alaska. to many tens of millions of years after initial collisional and orogenic events affecting the host sequences Že.g., Juneau gold belt, SE Alaska.. Many of the worlds great Phanerozoic orogenic belts evolved over periods of 100 to 200 m.y., with orogenic gold lodes having been emplaced throughout diachronous post-collisional deformation and uplift episodes in the accreted crust. Timing relationships may be more complex in older collisional belts. Robust geochronological data commonly place formation of Precambrian gold deposits some 20 to 70 m.y. after youngest nearby volcanism Že.g., Kerrich and Cassidy, 1994; Yeats and McNaughton, 1997., although deposits sited in some older greenstone belts may be formed in the deformation event related to accretion of younger greenstone belts in the same terrane some hundreds of millions of years after formation of the host rocks Že.g., Yilgarn Block, Australia; Groves et al., 1995.. In exceptional cases Že.g., eastern, northern and southern margins of North China craton; Variscan of southern Europe., Phanerozoic orogenic gold de-

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posits may be emplaced, within reworked Precambrian basement, hundreds of millions to billions of years subsequent to initial deformation of the host rock. This late timing is discussed in more detail below as it is pivotal to the discussion of exploration methodologies based on geological models presented in the second half of the paper, and is critical to their implementation. 2.5. Yilgarn gold deposits as orogenic gold deposits The gold deposits of the Yilgarn Block of Western Australia are the best-documented examples where stress mapping and GIS-based prospectivity mapping have been applied to the exploration for orogenic gold deposits. Because of this, they are chosen for review in this paper. Based on the parameters outlined above, the majority of gold deposits in the Yilgarn Block, including the somewhat controversial porphyry-hosted Že.g., Kanowna Belle. and syenite-hosted Že.g., Jupiter. deposits, fit into the class of orogenic gold deposits. There is no evidence for different ore fluids, significantly different alteration assemblages Žwhere allowance is made for variable host-rock composition and P–T conditions., or different metal ratios to indicate a different fluid source type or genetic model for any of the gold deposits in the eastern or central part of the craton. The only large deposit that is clearly anomalous is Boddington in the western part of the Yilgarn Block, where the Au–Cu–Mo association, the high-temperature biotite-amphibole alteration assemblages in a relatively low-grade greenschist-facies metamorphic environment, and the high-temperature, high-salinity fluid inclusions are unlike those in any other wellstudied Yilgarn gold deposit ŽRoth, 1992.. The timing of mineralization is controversial Že.g., Alibone et al., 1998., but the style of mineralization appears best to fit a diorite porphyry-style model. 3. Timing of relationships in orogenic gold deposits 3.1. RelatiÕe timing from structural eÕidence As mentioned above, most structural studies of orogenic lode-gold deposits indicate ore formation during D 3 or D4 events in terranes for which D 1 –D 3

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or D 1 –D4 deformation events are recognized Že.g., Colvine, 1989; Vearncombe et al., 1989; Cox et al., 1991.. The Eocene Juneau gold belt in southeastern Alaska, USA provides an excellent example of such a relative timing scenario. Gehrels et al. Ž1992. noted four sets of regional fabrics, developed over about 50 m.y., within metasedimentary units of some of the gold-host terranes. These included in order: Ž1. an initial foliation; Ž2. fabric related to ductile thrusting along the major, terrane-bounding first-order shear zones; Ž3. polygenetic, mainly dip-slip structures; and Ž4. later shear fabric related to sill emplacement during regional uplift. Gold veins surround first-order fault zones, but were emplaced in third-order subsidary shear veins and tension fractures during the final of the four deformational events ŽGoldfarb et al., 1991; Miller et al., 1994.. The relatively late timing of gold veins is also evident from more detailed mine-scale structural studies at the deposits. For example, Miller et al. Ž1992. show that veins at the Alaska–Juneau deposit cut schistosity ŽS1., isoclinal folding ŽS2. and crenulation cleavage ŽS3., although boudinaged veins and offset ore zones indicate minor post-gold deformation. Such a relatively late timing for orogenic gold deposits characterizes many orogens. Also within the Cordilleran orogen and along the accretionary prism of southern Alaska, gold lodes are reported to have been emplaced during a fourth stage of five deformational events that have affected the turbidite host rocks of the Chugach terrane ŽNokleberg et al., 1989.. Farther south within the orogen along western North America, metasedimentary rocks of the Foothills terrane in central California were progressively deformed over a period of about 50 m.y. ŽTobisch et al., 1989.. Dating of gold veins of the Mother Lode belt ŽBohlke and Kistler, 1986. indicates that they were emplaced during the last part of the deformation period. In the Tasman orogen, the Victorian goldfields of eastern Australia occur in classic saddle reef structures that were formed during a D2 event, whereas the gold was introduced into these structures during D4 ŽForde, 1991; Forde and Bell, 1994.. Similarly, gold in D2 reef structures in the Meguma terrane, of the Acadian orogen, Nova Scotia, was mainly deposited during D3 deformation ŽRyan and Smith, 1998.. In older sequences, such as

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the Paleoproterozoic Birimian orogenic belt of West Africa, gold ores were similarly emplaced during D4 events within an extensive history of deformational episodes ŽMilesi et al., 1992.. It is critical to realize that various workers use D1, D2, . . . nomenclature to describe a variety of features that are not always equivalent from one area to another. Although many studies note that gold is D3 or D4 in timing, and thus is emplaced late during deformation in an orogen, some studies indicate a D1 or D2 event as being temporally associated with formation of ores in some relatively small orogenicgold provinces. For example, Murphy and Roberts Ž1997. indicate early D1 Variscan gold-forming events in the Iberian Massif of Spain, Stowell et al. Ž1996. favour a D2 timing in the Blue Ridge of the Appalachian orogen, southeastern US, and LeAnderson et al. Ž1995. describe some D2 veining in the Pan-African goldfields of Saudi Arabia. It is important to realize that the features described by these authors are relatively broad-scale features, such as a D1 regional metamorphic event. This may still be a relatively late feature in the deformation history of the rocks Žconfirmed for Blue Ridge by T.H. Bell, pers. comm., 1999., but either is much more well-developed or completely overprints early pre-accretionary to collisional structures that certainly must have been present prior to metamorphism. Such complexities of relative timing between gold provinces highlight the need for absolute dating of ores and deformation events to confidently establish the temporal setting of gold ores in the evolving orogen. Generally in brittle-ductile regimes, orogenic-gold vein formation is commonly related to reactivation of earlier-formed faults or shear zones in a contrasting far-field stress regime Že.g., Goldfarb et al., 1991; de Ronde and de Wit, 1994. or to reactivation of tightly-folded, Alocked-upB folds Že.g., Cox et al., 1995.. In some cases, there is evidence for overprinting foliation on pre-existing gold deposits in brittleductile regimes Že.g., Morasse et al., 1995., but, in such cases, there is little evidence to suggest that there has been drastic changes to the structural geometry of the deposits after formation. In more ductile regimes, gold-bearing veins, particularly where hosted by shear zones, are commonly deformed, but they still formed late in the overall

deformational history of the host terrane Že.g., Knight et al., 1993; Bloem et al., 1994.. Thus, most field evidence suggests that orogenic lode-gold deposits formed under conditions of low effective stress and limited displacement during reactivation of earlierformed structures. The nature and orientation of quartz veins and their selective development as vein arrays in more competent units in the host successions suggest the existence of supalithostatic pressures during mineralization Že.g., Robert and Brown, 1986; Sibson et al., 1988; Miller et al., 1994; Groves et al., 1995.. 3.2. Geochronological eÕidence In many terranes, the relative structural evidence has been confirmed by absolute dating of gold mineralization based on robust geochronology Že.g., U–Pb studies. andror consistent data from other techniques Že.g., Ar–Ar plateaus, Pb–Pb or Sm–Nd techniques.: see, for example, review by Kerrich and Cassidy Ž1994.. It is not the object of this paper to thoroughly review the extensive and, in places, controversial literature on this topic. However, as the examples of repetitive structural geometry, stress mapping and prospectivity mapping presented below are mainly from the Yilgarn Block of Western Australia, the geochronological evidence from there is summarized in some detail. Subsequently, less detailed data, which confirm the same recurring temporal patterns between gold-forming hydrothermal events and other processes in developing orogens are briefly discussed from other terranes. 3.3. Geochronology : Yilgarn block The ages of gold mineralization of Yilgarn orogenic lode-gold deposits ŽFig. 2. have been constrained by three different methods. A maximum age has been determined by dating felsic intrusive rocks that are cut by gold-bearing veins andror are altered, a minimum age is defined by the age of crosscutting felsic dykes, and the age of mineralization has been dated directly by using appropriate, gold-related minerals Že.g., hydrothermal zircon, titanite, rutile, mica and garnet. in relevant alteration zones. A summary of these ages is given in Fig. 3.

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Fig. 2. Simplified geological map of the Yilgarn Block, Western Australia, showing the location of gold deposits discussed in the text. Adapted from Groves Ž1993. and Myers Ž1993..

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Fig. 3. Diagram illustrating geochronological constraints on orogenic lode–gold deposits of the Yilgarn Block, Western Australia. Ages are shown as "2 s . Shaded area is 2640–2625 Ma, interpreted to be the period of a widespread gold mineralizing event in the Yilgarn Block. Greenstone belt ages are not shown, but the youngest mineralized mafic and ultramafic rocks are ca 2700 Ma in age. Adapted from Yeats and McNaughton Ž1997..

Most of the larger gold deposits are hosted by ca 2.7 Ga greenstone belts in the Norseman-Wiluna Belt, although some deposits are also hosted by greenstone belts as old as 3.0 Ga in the Murchison and Southern Cross Provinces ŽFig. 2.. The best constraints on the maximum age of gold mineralization are provided by pre-mineralization granitoid plutons and felsic porphyry dykes and sills, including diorite at Granny Smith, with a U–Pb in zircon SHRIMP age of 2665 " 5 Ma ŽCampbell et al., 1993. and tonalite and leucogranite at Lawlers with a SHRIMP age of 2666 " 3 and 2666 " 7 Ma, respectively ŽFletcher et al., in Yeats and McNaughton, 1997.. Various porphyry bodies which are cut by gold-bearing veins at the Mount Charlotte, Mount Percy, Racetrack and Porphyry deposits have U–Pb in zircon SHRIMP ages between 2670 and 2660 Ma ŽYeats et al., in press.. Thus, gold mineralization in these deposits must be younger than about 2660 Ma.

Minimum ages for gold mineralization are provided by dykes and sills which post-date gold deposits, or are not gold mineralized, but show minor alteration of similar type to that of the adjacent gold deposit, or are slightly offset by late movement on gold-bearing structures. Such intrusions are extremely rare in the Norseman–Wiluna Belt. However, in the more western Yilgarn provinces and the southern most tip of the Norseman–Wiluna Belt, U–Pb in zircon SHRIMP dates late-to post-mineralization pegmatite dykes at 2623 " 7 Ma at Mt Gibson ŽYeats et al., 1996., 2637 " 7 Ma at Westonia ŽKent et al., 1996. and 2637 " 3 Ma at Griffins Find ŽQiu et al., 1997.. Similarly, Sm–Nd isochrons date late-to post-mineralization pegmatite intrusions at Westonia at 2640 " 11 Ma, at Nevoria at 2628 " 10 Ma and Scotia at 2620 " 36 Ma ŽKent et al., 1996.. At Corinthia, Bloem et al. Ž1995. record a Pb–Pb isochron age of 2620 " 6 Ma for a post-mineraliza-

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tion pegmatite dyke. Thus, the minimum age for mineralization of some Yilgarn deposits lies between 2620 and 2640 Ma. Exceptions to this are provided by U–Pb in zircon SHRIMP ages of porphyry dykes which are interpreted to cut gold mineralization at Jundee and Mt McClure in the northeastern Eastern Goldfields Province to the east of the Keith–Kilkenny Fault, in an area interpreted to be a distinct terrane within the province Že.g., Myers, 1993.. These dykes have ages of 2656 " 7 Ma and 2663 " 4 Ma, respectively ŽYeats et al., in press., which are virtually indistinguishable from the ages of porphyry bodies that are cut by gold mineralization in the Norseman–Wiluna Belt to the west of the Fault. This suggests that at least some of the gold deposits east of the Keith– Kilkenny Fault are older than the majority of gold deposits to the west of the Fault and older than other deposits within the same eastern terrane Že.g., Porphyry, Granny Smith; Fig. 2.. Direct dating of gold deposits by robust techniques, such as U–Pb or Pb–Pb of alteration minerals, and by less-robust Ar–Ar techniques for muscovite, mainly produce ages in the range ca. 2625 to 2640 Ma, consistent with the maximum and minimum age constraints provided by most pre-and post-mineralization intrusive rocks. Examples include a U–Pb in zircon SHRIMP age of 2627 " 13 Ma for Mt Gibson ŽYeats et al., 1996., a Pb–Pb age of rutile from Victory at Kambalda of 2627 " 14 Ma ŽClark et al., 1989., Pb–Pb ages of titanite from Griffins Find and Reedys of 2636 " 3 Ma ŽBarnicoat et al., 1991. and 2639 " 4 Ma ŽWang et al., 1993., respectively, and Ar–Ar muscovite ages of 2629 " 9 Ma at the Golden Mile ŽKent and McDougall, 1995., 2622 " 12 Ma at Matilda ŽKent and Hagemann, 1996., and 2639 " 16 Ma at Big Bell ŽMueller et al., 1996.. Significantly younger ages, which are still geologically reasonable, include an Ar–Ar muscovite age of 2602 " 8 Ma for Mt Charlotte ŽKent and McDougall, 1995. and U–Pb in titanite SHRIMP ages of minor, second-stage mineralization at Big Bell at 2614 " 2 Ma ŽMueller et al., 1996. and Lawlers at 2590 " 9 Ma ŽFletcher et al., in Yeats and McNaughton, 1997.. An Ar–Ar muscovite age of 2590 " 9 Ma for East Lode, Wiluna ŽKent and Hagemann, 1996. appears unreasonable in that it postdates the interpreted age of cratonization at ca 2600

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Ma. whereas the hosting faults are D 3 or D4 structures. It thus appears that gold mineralization took place between about 2660 and 2610 Ma, with the most voluminous and widespread episode between 2640 and 2630 Ma. This is some 40 to 90 m.y. after the latest major episode of basic-ultrabasic magmatism, and late during the major episode of craton-wide felsic magmatism. The geochronological data are entirely consistent with the relative structural data, both confirming the late timing of gold mineralization. To the east of the Keith–Kilkenny Fault, some gold formed at the old end of the 50 m.y. long period of hydrothermal activity, suggesting that this terrane ŽKurnalpi Terrane of Myers, 1993. could have been subjected to an earlier gold mineralization event, although structural data from the apparently older deposits suggest that they are still late within local deformational sequences. 3.4. Geochronology: other terranes worldwide The growing body of absolute geochronological data from orogenic gold deposits worldwide is consistent with relationships in the Yilgarn Block, Že.g., see summaries in Kerrich and Cassidy, 1994; Groves et al., 1998; McCuaig and Kerrich, 1998.. Most orogens tend to evolve over about 100- to 200-m.y.long periods, and it is during the later deformational events that gold veining occurs, generally forming a diachronous pattern across the entire evolving orogen within the uplifting and cooling parts of the host terranes. In the Precambrian, outside of Western Australia, timing of events are particularly well-established for the Superior Province in central Canada ŽKerrich and Cassidy, 1994., where ages of Late Archean rocks range from ) 2.9 Ga to ca. 2.65 Ga, among a variety of amalgamated terranes. The last accretionary events included the ca. 2.7 to F 2.67 Ga progressive collision of the Quetico, Abitibi-Wawa, Pontiac, and Minnesota River Valley terranes on to the southern margin of the previously formed part of the Superior Province ŽCard, 1990.. Also, within the gold-rich Abitibi–Wawa terrane itself, there is a younging southward of the greenstone sequences from 2730– 2720 Ma in the north to as young as 2680 Ma in the south ŽJackson and Cruden, 1995.. Deformation, re-

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gional metamorphism, and voluminous calc-alkaline magmatism were widespread in the terrane at 2700– 2670 Ma, with much of the deformation along the major Porcupine–Destor and Larder Lake–Cadillac crustal shear zones at ca. 2680–2670 Ma. Because absolute dates on gold ores cluster near the end of the 30-m.y.-long deformation range ŽKerrich and Cassidy, 1994. and gold shows a spatial association with these faults that formed at about this same time ŽJackson and Cruden, 1995., it is apparent that the major orogenic gold ores of the Superior Province are also late kinematic. Not all gold deposits in the Superior Province may have formed by ca. 2670 Ma. There are a series of more controversial younger dates on some of the gold deposits within the Abitibi–Wawa terrane that give ages of mineralization of 2630 Ma and younger ŽKerrich and Cassidy, 1994.. Final ductile deformation and formation of subvertical faults, prior to cratonization, also occurred within the southern Superior Province to as late as 2660–2640 Ma ŽJackson and Cruden, 1995.. This is certainly an expected deformational event, as final collision and subduction of the Pontiac and Minnesota River Valley terranes seemingly occurred throughout this period. Therefore, other episodes of fluid flow and gold veining after the initial G 2670 Ma event are not inconsistent with the late tectonism. As far back as the Middle Archean, gold veining is tied to late orogenic events. In the Barberton greenstone belt of South Africa, collisional processes are recognized between about 3230 and 3080 Ma, but gold veining did not occur until about 3084 Ma, when a major change to strike-slip tectonics occurred Žde Ronde and de Wit, 1994.. The Proterozoic Birimian belt in western Africa evolved between 2190 and 2080 Ma, with most of the deformation concentrated during the last 40 m.y. ŽOberthur et al., 1998.. The most extensive period of metamorphism and magmatism in this period was at about 2105 Ma, with the Ashanti belt gold veins being emplaced at G 2098–2085 Ma near the end of Eburnian orogenesis. Late timing of vein emplacement also characterizes many of the Phanerozoic orogens. In the Variscan of southern Europe, orogeny, and subsequent extension and uplift extended from 440 to 280 Ma ŽRey et al., 1997.. Orogenic lodes within areas

of basement uplifts, such as the Bohemian Massif, Massif Central, and Iberian Massif, were deposited coevally with the later part of 360–290 Ma magmatism ŽBouchot et al., 1989; Le Guen et al., 1992; Murphy and Roberts, 1997; Stein et al., 1998.. In the southern part of the Cordilleran orogen of North America, initial folding of sedimentary rocks and associated ductile deformation occurred at about 160–150 Ma in the Foothills terrane ŽTobisch et al., 1989., with gold veining some 25–50 m.y. after initial deformation ŽBohlke and Kistler, 1986.. Initial terrane collision and resulting deformation of rocks that host the ores of the Juneau gold belt began at about 100–90 Ma ŽGehrels et al., 1992., with ore deposition some 35–45 m.y. later during changes in plate motions at about 55 Ma ŽGoldfarb et al., 1991.. 3.5. Implications of late timing The late timing of most Archaean, Proterozoic, Paleozoic, Mesozoic and Tertiary orogenic lode–gold deposits worldwide carries with it implications that the present structural geometry within the deposits, and the structural geometry within the host terranes, approximate those at the time of gold-deposit formation. Post-ore strike-slip faults may offset coeval gold deposits on opposite sides of major structures, but the within-terrane structural geometry is little affected. Transtensional and uplift-related normal faulting is normally the only significant feature superimposed on the late-forming orogenic gold deposits. Thus, geological maps, and computer-based methodologies based on their analysis, can be used as a predictive tool in gold exploration where the structural controls on mineralization are understood and repetitive structural geometries are produced.

4. Structural geometry of orogenic lode-gold deposits Structure is widely considered to be the single most dominant control on localisation of lode-gold deposits Že.g., Sibson et al., 1988; Hodgson, 1989; Cox et al., 1991.. Hence, the structural geometry of mineralized environments should be predictable given

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the late timing of mineralization. This is well known for strike-slip faults and shear zones in brittle-ductile transpressional regimes Že.g., Hodgson, 1989; Sibson, 1990., where deposits occur in predictable dilational sites, such as in dilational jogs and at overstepping faults. For example, this is well illustrated for the Wiluna gold deposits in the Yilgarn Block ŽHagemann et al., 1992.. Younger gold deposits, such as those in the Fairbanks area of central Alaska and the Chicagoff district of SE Alaska, may also be sited along cross faults between first-order strike-slip faults ŽLeLacheur, 1991; Goldfarb et al., 1997.. In compressional brittle–ductile regimes, the lode-gold deposits are commonly sited in dilational zones where the shear zones or faults are more gently dipping than elsewhere. For example, this occurs at the Granny Smith ŽOjala et al., 1993. and Sons of Gwalia ŽSmith and Gardoll, 1997. deposits in the Yilgarn Block. Flower structures Že.g., at the Ashanti deposits, Ghana. are also predictable sites of gold mineralization in compressional regimes, as are suitably oriented structures in zones of heterogeneous stress fields around rigid granitoid plutons Že.g., Granny Smith, Yilgarn; Ojala et al., 1993; Kensington, SE Alaska; Miller et al., 1995. or complex batholith margins Že.g., Coolgardie, Yilgarn; Knight et al., 1993: Willow Creek, south–central Alaska; Goldfarb et al., 1997., or in the pressure shadows at the terminations of elongate batholiths: see summary diagrams in Groves et al. Ž1995.. Fluctuations in local stress fields can lead to a complex pattern of coeval shear and extensional vein networks ŽMiller et al., 1994; Nguyen et al., 1998.. Apart from such deposits preferentially hosted in structural heterogeneities near major crustal faults or shear zones, other orogenic lode-gold deposits may be confined to specific lithologies Ži.e., they are essentially stratabound. within complex lithostratigraphic sequences. These can also show repetitive and predictable structural geometries. Markedly linear greenstone belts are bounded by crustal-scale shear zones, forming elongate crustal blocks in dominantly granitoid-gneiss terrains. Greenstone belts with a large percentage of relatively competent greenstone lithologies oriented at a high angle to the maximum principal far-field stress that was active during progressive deformation, are the most common sites of large gold deposits Že.g., Groves and

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Barley, 1994.. This is because these more competent rock units tend to fail selectively in this orientation with respect to the far-field stress ŽRidley, 1993.. Within the linear greenstone belts, structurally isolated units of the most competent volcanic rocks especially tend to form zones of structurally-enhanced permeability at all scales. The geometry of these structurally isolated units varies, but there are repetitive geometries shown schematically on geological maps. In the Yilgarn Block, for example, one repetitive geometry is shown in Fig. 4, where broadly NW-trending competent units are isolated by broadly NE-trending late faults or shear zones. These isolated blocks are selectively fractured as the confined ore fluids reach supalithostatic pressures, leading to the formation of large vein arrays and associated alteration. The mineralization is typically brittle in style, although local stress reorientation can lead to shear failure ŽNguyen et al., 1998.. In brittle–ductile regimes, thick units of such competent rocks may be structurally isolated by complex interactions between early compressional Že.g., thrust. structures and later transpressional structures to form curvilinear blocks which close on one or both ends. Such structural geometries are the sites of highly anomalous fluid flux and can produce giant gold deposits. Example of such geometries are shown in Fig. 5, which illustrates the overall similar structural geometry of the two giant orogenic lode-gold goldfields, Kalgoorlie and Timmins, in Late-Archaean greenstone belts. The geometry of the geological setting of the Pampalo deposit from Finland ŽNurmi et al., 1993. is shown to illustrate how similar structural geometry may also characterise a smaller deposit. In Proterozoic and Phanerozoic sedimentary rock-dominant terranes, gold deposits are, in places, located in fold hinges in belts that are oriented at a high angle to the far-field stress, because the weaker rocks, such as graded greywacke units, selectively fail in such fold zones where the layering is oriented sub-parallel to the far-field stress ŽRidley, 1993.. Examples are especially well-documented for the Pine Creek region ŽNorthern Territory, Australia., the Meguma terrane ŽNova Scotia, Canada. and the Bendigo and Ballarat districts ŽVictoria, Australia.. The deposits are commonly sited in anticlinal hinges and particularly at domal culminations Že.g., Thomas, 1953; Kontak et al., 1990; Partington and Mc-

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Fig. 4. Schematic diagram showing examples of structurally controlled lode–gold deposits from the Yilgarn Block, shown in the same orientation at the same scale. The NE-trending faults have divided the sequence into a number of isolated or semi-isolated blocks of competent units that are selectively mineralized in a brittle–ductile to brittle regime. Adapted from Knight et al. Ž1996., Groves et al. Ž1997. and Newton et al. Ž1997..

Naughton, 1997.. Other deposits may be located in, and above, reactivated thrust ramps at the more gently-dipping segments of steep reverse faults that are active late in, or postdate, the major folding episode Že.g., Cox et al., 1991.. Commonly, the gold deposits in greywacke sequences are sited in Alocked-upB structures, such as steep reverse faults or tight overturned anticlines, which show extreme fault-valve behavior, and are reactivated and fail under high fluid pressure relative to minimum principal stress Že.g., Sibson et al., 1988; Cox et al., 1991.. Anticline-hosted lode-gold deposits thus tend to show repetitive structural geometries in similar rock sequences, with common parameters for deposits in greywacke sequences including low dihedral angles, overturned back limbs, front-limb parallel thrusts, which break through on to the back limb, and doubly plunging closures Že.g., Nesbitt, 1991..

In summary, it is evident that orogenic lode-gold deposits, hosted both in Precambrian greenstone belts and Precambrian to Phanerozoic accretionary sedimentary belts, commonly show predictable and repetitive structural geometries. Most importantly, their late timing relative to the evolution of the host terranes means that this geometry is depicted on geological maps andror cross sections derived from geological mapping, which may be combined with interpretation of remotely sensed images and airborne geophysical data sets. Exploration guides include structural heterogeneities near major fault zones, competency contrasts within the broad lithostratigraphic sequences themselves, and major fold hinge zones. This is the geological basis for the computer-based exploration technologies, stress mapping and prospectivity mapping, which are described and discussed below and are advantageous

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Fig. 5. Comparison of the structural geometries of the two giant greenstone-hosted orogenic lode–gold deposits at Kalgoorlie, Western Australia, and Timmins, Canada, and a smaller gold deposit, Pampalo from Finland. Note the similar geometry of competent units surrounded by less competent units on a variety of scales caused by the subsequent folding and faulting of early-formed thrust faults. Adapted from Nurmi et al. Ž1993., Phillips et al. Ž1996., and Groves et al. Ž1997..

for recognition of such favourable geometric patterns.

5. Stress mapping and its application to orogenic gold deposits 5.1. Introduction Stress mapping is a numerical modelling technique developed to target structurally-controlled hydrothermal mineralization located in dilatant Žor low stress. sites. It is based on rock mechanics principles and stress–strain relationships ŽHolyland 1990a,b; Holyland and Ojala 1997.. The technique is used to transform strain data, in the form of a solid geology map, to stress data. In order to use geological maps directly as an input, it must be shown that the present

rock geometries are those that existed at the time of mineralization. As discussed in detail above, this is certainly the case for the orogenic gold deposits globally, and for those of the Yilgarn Block, specifically. The strong structural control of the orogenic gold deposits, particularly the association with fault and shear zones, indicates that they are characterized by channelized fluid flow and high fluid:rock ratios. Under medium-to high-grade metamorphic conditions, fluid pressures are buffered close to, or commonly exceed, lithostatic pressures and fluid flow is generally upward ŽEtheridge et al., 1984.. Focusing of the upward fluid flow into a discrete channelway, as required to form an ore deposit, is due to lateral fluid-pressure gradients. These lateral gradients may be induced structurally by either variations in fracture permeability of active fault zones, or by variations in mean rock stress. Mineralized extensional

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veins indicative of supralithostatic fluid pressures are common in orogenic gold systems ŽSibson et al., 1988., and these are compatible with fluid focussing into zones of low mean rock stress. Relative fluid pressures are higher in these zones than local lithostatic rock pressure, but absolute fluid pressures are lower than in the surrounding rocks ŽRidley, 1993.. Sites of low mean stress can, therefore, be simultaneously sites of fluid focusing and of low effective mean-stress. At most crustal levels characterized by orogenic-gold vein formation, rocks are too plastic to allow larger-scale free convection, and flow must be driven by fluid pressure gradients. At higher crustal levels, ambient fluid pressures are closer to hydrostatic pressures, but fluid pressures along major conduits are still likely to be in excess of hydrostatic pressure and probably episodically exceed lithostatic pressures. Consequently, fluid flow is strongly controlled by permeability which, in turn, is influenced by rock heterogeneities. Although high permeability pathways can be lithologically determined, structurally controlled ore deposits are more common in crystalline rocks and, in general, they have formed in reactivated fault systems Že.g., Phillips, 1972; Sibson et al., 1975, 1988.: this is certainly the case for many orogenic gold deposits. A failure of a pre-existing weakness, or intact rock, can be initiated by an increase of differential stress, by an increase in the pore fluid pressure, or by a combination of both. In most cases, stress changes lead to decrease of mean stress or, more commonly, decrease of minimum principal stress. A model in which fluid focusing in the crust is due to variations in mean stress, or in which failure of pre-existing weaknesses and, therefore, enhanced fracture permeability, is due to lowered mean or minimum stress, best explains the wide variety of structural settings of many vein-type mineral deposits, including orogenic gold deposits. Thus, a technique that measures variations in rock stress, specifically where the geometry of the ore-hosting structure is complex, has the potential to generate viable exploration targets. Complex geometry Že.g., variations in orientations of structures. and large rheological contrasts are important because different structures and rock types accommodate strain via different modes and different rates, resulting in heterogeneous stress distribution and hence, potentially

compressional and dilational zones in the same domain. 5.2. Stress mapping principles Stress mapping is a form of computer-based structural analysis which examines the variation in local strain and stress within an inhomogeneous terrane resulting from the imposition of a regional stress field Že.g., Holyland and Ojala, 1997.. When stressed, an inhomogeneous material develops zones of strain partitioning whose components vary with the geometry and rheological properties of discontinuities and blocks, with variable deformation behavior between discontinuities. The modelling considers only elastic and elasto-plastic stresses and strains, and, therefore, deformation dilatency due to viscous strains is not considered. Stress mapping as a basis for prediction of hydrothermal fluid flow is based on the following reasonable assumptions. Low minimum principal Ž s 3. stress indicates proximity to failure and therefore, the likelihood of deformation-enhanced increased permeability. This is more important in modelling of high crustal-level deformation because absolute fluid pressure is higher in the fluid conduits Žcommonly faults. than in the wall rocks, and the fracture permeabilityrrock permeability ratio is higher than at deeper levels. At depths of more than a few kilometers, fluid pressure is consistently close to lithostatic pressure and the control on fluid pressure is mean stress. Variations in mean stress will cause variations in fluid pressure. Fluid flow is both upwards and towards zones of low mean stress. Combined with the knowledge that structurally controlled orogenic gold mineralization is commonly late in the tectonic history of a terrain and that typical Archaean orogenic gold deposits show evidence of fluid overpressuring Že.g., Sibson et al., 1988; Groves et al., 1995., this enables stress analysis of two-dimensional map patterns of rock units and faults to predict those zones of low mean Ž sm s Ž s 1 q s 2 q s 3 .r3. or minimum principal Ž s 3 . stress, provided that the orientation of the far-field stress can be reasonably assumed.

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The most severe limiting assumption of two-dimensional stress modelling is that the plane of a map does not accurately reflect the stress pattern in an area with complex three-dimensional geometry ŽHolyland et al., 1993.. In many terranes, such as those of the Yilgarn Block, this may not be a critical restriction, since the vast majority of the recorded gold-hosting structures are near vertical ŽHronsky et al., 1990; Libby et al., 1990.. The accuracy of the three-dimensional geological interpretation becomes especially important in deposit-scale modelling where the extent of interest of the vertical dimension is similar to horizontal dimensions ŽHolyland and Ojala, 1997. andror where deposits are controlled by changes in dip of the hosting structures. However, the example presented below is restricted to two dimensions. 5.3. Computer program Details of the computer program are described elsewhere ŽHolyland, 1990d. and only a brief summary is presented here. Two-dimensional modelling is by a distinct element code ŽUDEC program. and the method has three distinguishing features which make it well suited for discontinuum modelling ŽHolyland, 1990a,b.. These are: 1. An assemblage of blocks, which interact through corner and edge contacts, is simulated. 2. Discontinuities are regarded as boundaries between blocks; discontinuity Žfault. behavior is prescribed for interactions between these. 3. An explicit timestepping Ždynamic. algorithm is utilized that does not limit displacements or rotations, and assumes general non-constitutive behavior for both the matrix and discontinuities. Modelling can be done at any scale and, in addition to minimum, maximum and mean stresses, total displacement and fault displacements can be computed. The total displacement is the amount of movement experienced by the rock blocks, and their movement directions. 5.4. Input for stress mapping Stress mapping requires an accurate geological map or, if a three-dimensional model is desired, then

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information on the subsurface is also critical. It also requires estimates of the magnitudes and orientations of the far-field horizontal stresses and rock and fault deformation properties. Geological map data are converted to a solid-geology computerized base, which provides continuous lithological and structural information Že.g., Fig. 6.. The modelled area is treated as a mosaic of polygonal blocks Žrock units. and joins Ždiscontinuities: contacts, faults and shear zones.. When external stress is applied to this system, the blocks are juggled and deform internally until equilibrium is attained. This identifies areas of lower stress Ži.e., dilation zones., which represent favoured areas for ore-fluid flux and hence mineralization, and therefore targets in exploration. 5.5. Example of two-dimensional stress mapping: kalgoorlie terrane 5.5.1. Strategy The geological map used to illustrate the value of stress mapping in exploration is the 1:250,000 solid geology map of the Kalgoorlie Terrane ŽSwager and Griffin, 1990.. Fig. 6 is a simplified scale-reduction version of this map. This map was chosen for a variety of generic reasons. First, the terrane is a mature one in terms of geological mapping and mineral exploration, such that there is a large geological database. Second, the area is uniformly covered by airborne geophysics, which has aided interpretation of the solid geology map in widespread areas of deep regolith. Third, the map was produced by a small team of experienced geologists, and hence is a relatively consistent product. From the viewpoint of demonstration of the potential, or otherwise, of the technique, it is also important that there are numerous gold deposits and goldfields that have produced more than one million ounces of gold in the terrane ŽFig. 6.. Finally, detailed research by a number of workers in the terrane has confirmed the late timing of the gold deposits Žsee summary in Witt, 1993.. At the 1:250,000 scale of the solid geology map, it is not expected to define the location of individual gold deposits, normally - 1 km2 in area, although the giant Golden Mile deposit Žapprox. 8 km2 . is an exception. Rather, the output from the technique at this scale will define the location of major goldfields,

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potentially containing several individual gold deposits. 5.5.2. Application For the stress mapping, discontinuities on the 1:250,000 scale map of the Kalgoorlie Terrane were classified according to their nature, length and continuity as first- or second-order D 3 –D4 faults, D1 thrusts or lithological contacts. In this order, the discontinuties define a relative stiffness scale from lowest Žfirst-order fault. to highest Žlithological continuity. stiffness. First-order structures include regional-scale faults that generally have strike lengths of more than 10 km Že.g., Ida Fault, Boulder-Lefroy Fault: Fig. 6.; whereas second-order structures are of shorter strike length. Rock types on the geological map of the Kalgoorlie Terrane are grouped by their interpreted geomechanical properties into undifferentiated mafic rocks, undifferentiated ultramafic rocks, undifferentiated sedimentary rocks Žincluding sedimentary rocks and felsic volcanic rocks. and felsic intrusive rocks. The felsic intrusive rocks are treated as the most competent rock type, followed by the mafic rocks, sedimentary rocks and finally ultramafic rocks. The rock parameters used reflect more than ten years experience of stress mapping in greenstone belts. The maximum principal stress Ž s 1 . direction used in the simulation is E–W. This is based on the gross greenstone belt geometry and published structural studies Že.g., Swager 1989., and more detailed structural studies of the orogenic gold deposits themselves Že.g., Knight et al., 1993.. The technique is relatively insensitive to small differences Ž- 308. in orientation of s 1. Studies of orogenic gold deposits in the Yilgarn Block and elsewhere indicate low

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displacements on controlling structures during mineralization, as discussed above. Therefore, the stress modelling was carried out with relatively low imposed compressive stresses, 100 Mpa for s 1 , and 70 Mpa for s 3 , such that overall strain was less than 0.25% after the models were cycled until a steady state of deformation was reached, corresponding to a constant stress distribution where no new stress anomalies developed. The northern and southern haloes of the map, with some overlap to avoid edge effects, were stressed separately due to size and shape constraints. This has led to small differences in the magnitude of the stress anomalies in each half of the map, although, in areas of overlap, the stress anomalies are in identical locations. 5.5.3. Results Sites of low minimum principal stress Ž s 3 . simulated in the model for the Kalgoorlie Terrane are shown in Fig. 7. Low stress anomalies constitute less than ten percent of the map area, and are typically related to changes in strike of first-order faults, largely from the dominant NNW trend to a more restricted NW trend, and to intersections of two or more first-order faults or of first-order and secondorder faults. Despite the relatively large scale of the modelling, anomalously-low minimum stress zones define all of the major goldfields in the area, including the Kalgoorlie ŽGolden Mile, Mt Charlotte, Mt Percy., Coolgardie ŽBayleys, Tindals, Three Mile Hill., Kanowna Belle, Mt Pleasant ŽLady Bountiful, Mt Pleasant., New Celebration ŽNew Celebration, Jubilee, Golden Hope., Paddington and Kambalda–St Ives ŽVictory–Defiance, St Ives, Junction. and Higginsville goldfields. Most of these are adjacent to first-order faults in the modelled map ŽFig. 7..

Fig. 6. A digitized solid-geology map of the Kalgoorlie Terrane. The geology is simplified to facilitate modelling. The position of major goldfields, or the major deposits within them, is shown for reference. For the geomechanical units, felsic intrusive rocks are treated as the most competent rock type, followed by undifferentiated mafic rocks, undifferentiated sedimentary rocks Žincludes felsic volcanic rocks. and, finally, undifferentiated ultramafic rocks. First-order structures are modelled as having the lowest stiffness and lithological contacts the highest stiffness. The maximum principal stress, s 1 is simulated at E–W, based on the history of the Yilgarn Craton and the place of gold mineralization within it. Abbreviations for mines in the terrane : BA, Broad Arrow; Bd, Bardoc; Bl, Bayleys; Ch, Chalice; CR, Cave Rocks; Du, Durkin; Gh, Golden Hope; GM, Golden Mile; GS, Gimlet South; Hg, Higginsville; Hu, Hunt; Jn, Junction; Ju, Jubilee; Kb, Kanowna Belle; Kd, Kundana; Ki, Kintore; Ku, Kunanalling; LB, Lady Bountiful; Mtc, Mt Charlotte; MtM, Mt Morgan; MtP, Mt Pleasant; MtPe, Mt Percy; NC, New Celebration; Pad, Paddington; RW, Prince of Wales; Ot, Otter; Re, Revenge; Si, Siberia; Stl, St Ives; TMH, Three Mile Hill, Ti, Tindals; VD, Victory–Defiance.

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Fig. 7. Equivalent map to Fig. 6, showing contoured values of minimum principal stress Ž s 3 .. Based on stressing the digital map of the Kalgoorlie Terrane with s , in an E–W orientation. The first-order faults are shown to assist correlation between Figs. 6 and 7. Anomalously low values of s 3 have a strong correlation with the locations of known goldfields in the Terrane, but there are also anomalies which potentially define as-yet undiscovered goldfields or gold deposits.

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Smaller deposits not sited within low stress anomalies Že.g., Siberia, Prince of Wales. do not have significant faults or fault intersections nearby ŽFig. 7., and it is probable that less significant controlling structures were simply not shown on the 1:250,000 scale map which was modelled. 5.6. Discussion The two-dimensional, regional-scale stress modelling of the Kalgoorlie Terrane shows the ability of stress mapping to define zones of low minimum principal stress Ždilation zones., which correspond very well with locations of known goldfields. It also defines several new target areas of high prospectivity. The modelling of palaeo-stress patterns in the Kalgoorlie Terrane is in agreement with the earlierdescribed orogenic gold model in which the fluid flow and location of mineralization are mainly controlled by major structures and rheological heterogeneities. The fact that this simulation successfully located the major goldfields is attributed to the fact that the gold lodes formed late in the tectonic evolution of the Kalgoorlie Terrane. As a result, the goldfields remain in essentially the same spatial setting as that when they were first formed, in close proximity to complexities in the first-order structures of the terrane. Numerical modelling of rock deformation is not an exact science and there will always be controlling variables which are not fully understood. That is, rock mechanics models are Adata-limitedB, and there are seldom enough data to simulate the rockmass behaviour unambiguously ŽStarfield and Cundall, 1988.. However, two-dimensional modelling of the Kalgoorlie Terrane indicates that results which are useful in exploration for orogenic gold deposits can be obtained with a good geological database. Critical to the successful modelling are reasonably accurate input parameters for rock properties developed from extensive experience in the terranes, and the far-field stress field. In the example from the Kalgoorlie Terrane, stress mapping demonstrates which faults and which segments of the faults are most likely to be dilational at a regional scale, and hence the predicted location of major goldfields. Such knowledge also assists with future data collection and interpretation, because, for example, it defines the

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direction and magnitude of fault strikes, which are likely to be critical in controlling the siting of goldfields and gold deposits. Thus, stress mapping can focus attention on those geological parameters that are critical to goldfield location and thereby aid in discriminant data collection of value for conceptual exploration. It is evident that stress mapping is a viable tool for exploration of broad crustal tracts to define zones of greatest favourability for orogenic gold deposits, provided that there are adequate geological maps and other geological data available. The technique also has been applied successfully on a regional scale to Archaen rocks in the Mt Pleasant area north of Kalgoorlie ŽHolyland et al., 1993., in the goldfields of the Murchison greenstone belt of South Africa ŽVearncombe and Holyland 1995., in a segment of the Midlands greenstone belt of Zimbabwe Žas reported in Campbell and Pitfield, 1994., and to the Rouyn–Noranda district, Abitibi, Canada ŽJebrak et ´ al., 1995.. In all of these cases, the majority of known gold deposits are coincident with broad low minimum-stress anomalies and, most significantly, additional anomalies were defined, identifying the most permissive areas for further exploration. As discussed above, the regional stress mapping is capable of defining anomalies at the scale of goldfields. Once these potentially mineralized areas have been defined, the next stage is to carry out a more detailed analysis at a smaller scale Že.g., 1:25,000 to 1:50,000 scale. in order to define more specific gold targets within the defined potential goldfields. At this scale, it is important to further subdivide the rock units in terms of their strength parameters. For example, dolerite, the host to many of the large gold deposits in the Kalgoorlie Terrane, is simply one rock type grouped within the category undifferentiated mafic rocks in the stress model at 1:250,000 scale shown in Figs. 6 and 7. This can be done in two dimensions, as for the example of the Kalgoorlie Terrane above, or in three dimensions if there is sufficient drilling data in the area ŽHolyland and Ojala, 1997.. An example of such a stress mapping study is that of the area surrounding the Granny Smith gold deposit in the Eastern Goldfields Province of the Yilgarn Block ŽFig. 2.. In this case, low minimum stress anomalies and anomalous local s 1 orientations correlate with changes in orientation

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Fig. 8. Selected spatial relationships identified between orogenic gold deposits and particular geological features in the Kalgoorlie Terrane. Ža. Graph displaying the size–proximity relationship that exists between gold deposits and regional-scale faults. Large deposits are those with ) 1.5 t production and small deposits those with - 1.5 t production. Žb. Results of strike-proximity analysis conducted between gold deposits and lithological contacts. Sections of such contacts within a strike range of between 1208 and 1508 are more prospective than others. Žc. Normalised gold production according to the lithological contact nearest to the gold deposit, with numbers indicating rheological contrast Ž0 s no contrast, 4 s maximum contrast.. Žd. Proportion of discovered gold located within any given distance of an anticlinal hinge. Že. Importance of rock-types based on gold production Žleft graph. and gold production normalised by area of each rock-type Žright graph..

Žboth strike and dip. of a reverse-sheared contact between diorite and metasedimentary rocks, which, in turn, correlate with ore zones in one or both host rocks dependant on the contact configuration ŽHoly-

land and Ojala, 1997: Figs. 5–8.. In this case, progressive three dimensional stress mapping greatly assisted siting of drill-holes during ongoing exploration.

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6. Computer-based prospectivity mapping 6.1. Introduction Since the late 1970s, a computer-based technology has been steadily evolving that has the potential to revolutionize the mineral exploration industry. Geographic Information Systems ŽGIS. are computer-based databases for spatial and associated non-spatial data that provide the necessary tools for the efficient storage and analysis of map and image data. For over a decade, the use of GIS as an exploration tool has been examined by a number of research groups throughout the world ŽBonhamCarter et al., 1989, Knox-Robinson, 1994, Wyborn et al., 1994. and for a number of different deposit types, which importantly include orogenic gold deposits ŽRobinson, 1990; Knox-Robinson, 1994.. Although GIS is a very powerful tool in exploration, there has been a reluctance, until recently, to accept the technology. Reasons for this include the dearth of digital map data, the expense of the GIS software and associated hardware, and the user-unfriendliness of GIS that necessitated the services of a specialized operator. Fortunately, the past few years have witnessed the emergence of so-called ‘desktop’ GIS that are inexpensive, PC-based, and very user-friendly, although less capable than conventional systems. In addition, a number of tools are now available that allow sophisticated exploration-relevant analyses to be applied to stored map data and imagery. Specifically, GIS has been used, in conjunction with typical exploration data sets, to test concepts related to the formation of orogenic gold deposits and to identify and quantify spatial associations between known deposits and other geological features ŽKnox-Robinson, 1994; Knox-Robinson and Groves, 1997; Knox-Robinson and Wyborn, 1997; KnoxRobinson et al., 1996; Yun et al., 1998.. The ultimate aim of the research is to develop the methodology to construct ‘prospectivity’ maps that have a predictive capability in assessing the likelihood that a given area could host an orogenic gold deposit. Attention has focused on regional-scale prospectivity mapping, in which the goal is to identify prospective areas at a goldfield scale using primarily conventional geological map data. An application is presented below for the orogenic gold deposits of the

well-documented Kalgoorlie Terrane in order to link this methodology to the stress mapping technology discussed above. 6.2. Principles of prospectiÕity mapping There are two general approaches to prospectivity mapping: conceptual and empirical ŽKnox-Robinson and Wyborn, 1997.. Using a conceptual approach Že.g., Wyborn et al., 1997., genetic concepts are represented as mappable criteria and these are ultimately integrated into a single prospectivity map. An empirical approach, on the other hand, relies on the existence of a significant number of known deposits that are characterized by a series of defining factors. Such factors are spatially quantified and integrated into a single prospectivity map using the same techniques as for the conceptual approach. The Yilgarn orogenic gold deposits form a coherent group and share a common genesis, but vary in their physical parameters due to differences in structural style, host rock, and degree of alteration, and therefore, it is not appropriate to employ a purely conceptual approach. Fortunately, exploration in the Yilgarn Block has resulted in the discovery of more than 2000 gold deposits. Furthermore, the current structures and geometries on geological maps are similar to those that existed at the time of mineralization, as discussed above. Consequently, an appropriate predominantly empirical methodology has been developed that exploits knowledge of known gold deposits and high-quality map data. The methodology is flexible so that relevant concepts can be tested and, if important, included in the analysis and the subsequent construction of a prospectivity map ŽKnox-Robinson, 1994.. The methodology comprises three main analytical steps of equal importance: identification of spatial relationships on maps, quantification of those identified relationships, and integration of multiple relationships into a single prospectivity map. 6.2.1. Step one: identification of spatial relationships The first step involves the identification of factors that may constrain the location of known gold deposits. Some features, such as crustal-scale faults and shear zones, can control the linearity of orogenic belts, and typically serve as ore-fluid pathways. Since

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fluid pressure variations are important, zones of maximum uplift or extension Ždecreased lithostatic pressure. may be more prospective than others. Along these crustal-scale structures and in uplifted belts, a number of smaller structures can represent zones of enhanced permeability and fluid flow. These include dilational jogs along shear zones, dilation at fold hinges, and portions of deformed lithological contacts that separate rock types of strongly contrasting rheologies. Consequently, particular strike-ranges of faults, shear zones and lithological contacts may be more important than others, and hinge zones may also be highly prospective. In addition, gold deposits may simply be associated with an increased abundance of permeable faults, shears and lithological contacts. Specific rock types that are most competent andror have a very reactive geochemistry Že.g., high FerFeq Mg q Ca ratios. should also be favoured sites for orogenic gold deposits. A number of techniques have been developed to test, amongst others, the above relationships ŽKnoxRobinson, 1994.. At the scale of investigation Žnormally between 1:100,000 and 1:500,000., gold deposits are adequately represented in a GIS as point entities, and other geological structures can be represented as point, line, or polygon entities. With such a representation, there are three broad spatial relationships that can exist between orogenic gold deposits and other geological features. A proximity relationship is one in which deposits are hosted preferentially closer to a feature than expected. An association relationship is one in which deposits are preferentially hosted in a particular polygon type Že.g., a particular rock type.. An abundance relationship is one in which deposits are more likely in areas where there is a high spatial density of a particular feature Že.g., faults or shear zones.. By using the data-query functionality afforded by most GIS, a subset of geological features can be selected to determine, for example, if the strike of faults or shear zones is related to prospectivity. Apart from controlling the location of orogenic gold deposits, particular geological features may, in addition, have a bearing on the size of deposit. Consequently, techniques have also been developed to examine size relative to proximity, association, and abundance relationships. All of the GIS-based software routines employ nonparametric statistical tests, such as the Kolmogorov–

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Smirnov and Chi-square tests, to determine the significance of the spatial relationships ŽKnox-Robinson, 1994; Knox-Robinson et al., 1996.. 6.2.2. Step two: quantification of identified spatial relationships Once critical factors are identified, a geological map or image needs to be constructed that depicts how the spatial relationships between these factors vary. Such an approach uses a metric that denotes the favourability for discovery of new deposits based on the characteristics of the known gold deposits. Suitable metrics include the number of known gold deposits of a given minimum production per square kilometer or, preferably, the tonnage of gold production plus reserves per square kilometer. Even if a statistically significant spatial relationship has not been identified, the contribution of a particular geological feature can be quantified and incorporated into a prospectivity map. Stress maps, discussed above, could also be quantified for inclusion into the final prospectivity map. Vector-based GIS display map features as point, line or polygon entities, and are best suited for the management of discrete data. With such a GIS, a map that depicts a quantified spatial relationship comprises a number of polygon features separated by distinct boundaries. The other kinds of GIS Žrasterand quad-tree-based. store map data as pixilated images, or some encoded equivalent thereof, similar to that produced by a computer monitor. Each cell in the image has a value that denotes the state of a particular feature at that location. With this kind of GIS, a spatial relationship can be quantified as a continuous surface. It is noteworthy that, as geological maps comprise sharp boundaries, a vector GIS is the most suitable for the identification of spatial relationships. However, a raster format is preferred for the quantitative representation of an identified relationship. Fortunately, most GIS software, including desktop packages, are hybrid, and can accept and process, with different degrees of flexibility, spatial data in both map and image form. 6.2.3. Step three: integration of identified spatial relationships The third step is the integration of multiple quantified relationships into a single prospectivity map.

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The majority of papers in the GIS research field Že.g., Reddy, 1987; Bonham-Carter et al., 1989,. tend to concentrate on this end-product step. Consequently, there are several different integration techniques available, ranging from very simple Boolean techniques to more complex index overlay, Bayesian, algebraic, and fuzzy logic methods. Knox-Robinson and Wyborn Ž1997. discuss these in some detail and this is not repeated here. However, the main features of these methods are summarized in Table 1. 6.3. ProspectiÕity analysis of the Kalgoorlie Terrane To provide continuity with the section on stress mapping above, a prospectivity analysis was con-

ducted for the Kalgoorlie Terrane of the Yilgarn Block, based predominantly on a 1:250,000-scale solid geology map produced by Swager and Griffin Ž1990.. Importantly, the geological data and degree of detail of presentation are consistent throughout the entire region: i.e., mineralized areas will not simply be defined on the basis that they contain superior data — a problem inherent in prospectivity mapping.

6.3.1. Identification of spatial relationships A database of more than 230 known orogenic gold deposits was constructed for the area. Of these, a subset of 150 deposits was extracted that included the largest five gold deposits in the region. This

Table 1 Summary of the main techniques used to combine multiple spatial datasets into a single prospectivity map. These techniques are discussed in more detail in Knox-Robinson and Groves Ž1997. Boolean integration Description Input maps can only have two states Žprospective and unprospective.. Boolean logic rules are used to combine datasets. The resultant map has only two states Pros Easy to implement in most GIS Cons Boolean AANDB is too conservative in the definition of prospective areas, whereas Boolean AORB is too liberal Index oÕerlay Description Input maps comprise two or more levels of prospectivity: the higher the prospectivity, the larger the number. Inputs are combined by spatial overlay and summation of index values Pros Easy to implement in most GIS. Better than Boolean method in that much internal structure is retained in the resultant prospectivity map Cons Prospectivity is represented by an ordinal value: A region with an index value of 10 is more prospective than a region with an index value of 5, but not twice as much. Input maps that are split into a large number of prospectivity levels will contribute more to the resultant map Algebraic method Description Essentially an extension of the index overlay method, except that prospectivity values are represented by ratio-scale numbers. That is, a region with an output value of 10 is twice as prospective as a region with an output value of 5 Pros Deposit size can be incorporated into the analysis Cons Difficult to integrate spatial relationships quantified as continuous surfaces Weights of eÕidence Description Uses Bayes’ Rule of conditional probability to update a prospectivity map based upon new information Pros Provides a probabilistic result Cons Difficult to incorporate deposit size into the analysis. Difficult to implement in a vector GIS Fuzzy logic Description Uses the rules of fuzzy logic, a superset of Boolean logic, to combine datasets. Unlike Boolean logic, in which a variable can have only one of two possible states Ž‘0’ or ‘1’., fuzzy logic variables can have an infinite number of values between ‘0’ and ‘1’, inclusive Pros Can be used to integrate spatial relationships that have been quantified as continuous surfaces Cons Dependent on the fuzzy logic rule used to combine datasets

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subset, which is not used in any of the analyses, is used to test the applicability of the resultant prospectivity map. With the aid of custom-built GIS routines ŽKnox-Robinson, 1994; Knox-Robinson et al., 1996., a number of important spatial relationships are identified in the region. Selected relationships are illustrated in Fig. 8. First, analysis shows that deposits within 3 km of a granitoid-greenstone contact are controlled by a different set of factors than more distal deposits. The locations of granitoid-proximal gold deposits are spatially related to the strike of the granitoid-greenstone contact, with the strike range of 116—1468 being the most prospective. This is similar to the preferred strike of lithological contacts Žsee Fig. 8b. and fault segments. In addition, the curvature of the granitoid-greenstone contact is important, with sections of higher curvature being more prospective than others. This may be due to zones of dilatancy being produced in pressure shadows around rigid granitoid bodies during post-emplacement deformation. Distal to the granitoid-greenstone contacts, different factors control the location of gold deposits. Crustal-scale faults and shears, below referred to simply as faults, provide the strongest control on the location of granitoid-distal gold deposits. Not only are gold deposits more likely to occur close to faults, but fault-proximal deposits tend to be larger ŽFig. 8a.. The strike of faults is also important, with significantly more deposits than expected located close to sections of faults striking within the range of 119–1508. Lithological contacts are identified as the second main control, although a size-proximity relationship cannot be identified. Sections of lithological contacts that strike within the range of 125–1488 are more prospective than others ŽFig. 8b.. There is also a positive abundance relationship between the location of gold deposits and lithological contacts, with gold deposits more likely to be sited where there is a high spatial density of lithological contacts. Furthermore, lithological contacts that separate rocks of strongly contrasting rheologies are more important than others ŽFig. 8c. About 70% of deposits are in anticlinal zones or broad horst-like structures, and 75% of gold occurs within 1.5 km of an anticlinal hinge ŽFig. 8d.. Finally, for both granitoid-distal and granitoid-proximal deposits, mafic rocks, particularly

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differentiated mafic intrusions, tend to be the most prospective host rock ŽFig. 8e.. 6.3.2. Quantification and integration of identified spatial relationships For each of the identified spatial relationships, one or more images show how the distribution of known gold deposits relates to the features of interest. In the case of faults, one raster image quantifies the identified proximity relationship between known gold deposits and sections of faults that strike within the range 119–1508. Prospectivity is depicted in terms of known gold per square kilometer. Similarly, the second image quantifies the relationship between known gold deposits and the remaining faults. The contribution of faults to overall prospectivity is then determined by the combination of these two images using a simple maximum operator. That is, each output pixel is assigned the larger of the two corresponding input pixel values. The other identified spatial relationships are quantified in a similar way. Finally, the quantified factors are scaled to fit the range between 0 and 1, and are integrated using a new technique, termed vectorial fuzzy logic ŽKnoxRobinson, in press.. Fuzzy logic ŽZadeh, 1973. represents a superset of conventional Boolean logic, and is perfectly suited to the task of combining multiple spatial datasets, especially those that are represented as continuous surfaces. A number of fuzzy logic combinatorial operators have been used to combine exploration datasets into a single prospectivity map ŽBonhamCarter, 1995; D’Ercole, 1998., but, owing to limitations in these simple functions, a new operator, the gamma function, is normally used in prospectivity mapping ŽBonham-Carter, 1995.. Although the gamma function is superior to conventional Boolean and simple fuzzy logic techniques, it has a number of limitations ŽKnox-Robinson, in press.. It is unable to address null data values and, more importantly, it is not possible to ascertain from conventional techniques whether areas of intermediate prospectivity are the result of combining several similar input values, or whether all of the input values are low except for one or two extremely high values. These and other limitations have acted as the catalyst for the development of the new vertical fuzzy-logic technique in which a measure of confi-

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Fig. 10. A section of the prospectivity map for the Kalgoorlie area, presented as a perspective view. The area represented by this image is illustrated in Fig. 9.

dence can be incorporated into the analysis ŽKnoxRobinson, in press.. The resultant prospectivity map of the Kalgoorlie Terrane is presented in Fig. 9, with an artificial

illumination to enhance the visualization of prospectivity. Both height and color are related to the calculated prospectivity: purples and blues and topographic lows are areas of low prospectivity, whereas

Fig. 9. Prospectivity map of Kalgoorlie Terrane generated using a new vectorial fuzzy logic technique ŽKnox-Robinson, in press.. Purples and blues represent low prospectivity, greens and yellows represent moderate prospectivity, and reds and whites represent high prospectivity. An artificial sun-angle has been applied to this image, such that topography is proportional to prospectivity also: i.e., high topography corresponds to high prospectivity.

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reds and whites and topographic highs represent highly prospective regions. Gold deposits with more than five tonnes of total contained gold are depicted on this map as red circles. A section of the prospectivity map, represented by the orange box on Fig. 9, is presented in Fig. 10 as a perspective view.

7. Comparison of stress and prospectivity maps Prospectivity maps generated using stress mapping have a conceptual premise: the foundation of this method is that areas of low minimum stress are those most likely to host goldfields. Consequently, to construct a prospectivity map using stress mapping, only a solid-geological map is required and a database on the siting and size of gold deposits in the terrane is not needed. In contrast, a prospectivity map generated using the fuzzy logic method has a predominantly empirical basis. Although a number of concepts related to gold mineralisation are tested, this method involves the analysis of a subset of known gold deposits, and the relationship of these deposits to particular geological features. Without a significant number of known gold deposits, a prospectivity map of this type cannot be constructed. Comparison of prospectivity maps for the Kalgoorlie Terrane demonstrates that the stress mapping and fuzzy logic methods are complementary. The majority of low minimum stress centres Ž) 70%. are coincident with areas considered highly prospective on the fuzzy logic prospectivity map. In the northern part of the Terrane, where the greenstone belt is very narrow, there are a few areas where centres of low minimum stress correspond to the position of small goldfields at Bardoc and Siberia but are represented by zones of only moderate prospectivity on the fuzzy logic map. However, in other areas, the fuzzy logic map provides a better definition of the location of known gold fields. The best example is that of the Coolgardie goldfields. In this region, areas of low minimum stress are centred on undifferentiated mafic and ultramafic rocks, bounded by first-order faults and surrounded by undifferentiated sedimentary rock. The fuzzy logic method correctly highlights the area further west as the most prospective, based on the proximity to the granitoid and the curvature of the greenstone–granitoid contact.

It can be demonstrated that prospectivity maps, constructed using the above methodology, have definite predictive capability with the aid of the subset of known deposits that were not used in any of the analyses, and that the results closely parallel those of stress mapping. For the prospectivity map constructed for the Kalgoorlie Terrane, which implemented an algebraic method of integration ŽKnoxRobinson, 1994., the most prospective category occupies less than 0.3% of the greenstone belt, yet hosts more than 16% of known gold deposits. This represents a 55-fold increase for the likelihood of the discovery of an orogenic gold deposit. Most significantly, this area defined by the GIS approach accounts for more than 80% of known gold production for the region ŽKnox-Robinson and Groves, 1997., demonstrating that the prospectivity map also discriminates in favour of the most-desired larger gold deposits in the region. Furthermore, only deposits that lie within the northern half of the terrane Žnorth of 6,570,000 mN. were used in the analysis, and subsequent construction of the prospectivity map. Known prospective areas in the south, such as the Kambalda–St Ives goldfield, are highlighted as being prospective, thus indicating that the high prospectivity areas identified were correctly defined to best identify favourable tracts for gold deposits in the Kalgoorlie Terrane. However, although similar factors control the location of gold deposits in all of the constituent greenstone belts of the Yilgarn Block, specifics of these factors, such as important strikeranges and important lithological contacts, differ due to factors such as differences in far-field stress directions and different lithostratigraphic sequences. Consequently, it is not possible to export identical factors from one terrane and apply them, directly, to another, although the same principles will apply. As shown above, GIS are useful tools for the quantitative analysis and integration of diverse exploration data sets for late-hydrothermal deposits, such as the orogenic gold deposits. However, although GIS have a good topological foundation and are designed to effectively store and manipulate the spatial relationships between map features, they are not well suited for the analysis of the shapes of objects. As noted above, structural geometry Žessentially shape. appears to be a significant factor in the focussing of ore-fluids and the formation of deposits.

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Current research is addressing this limitation inherent in current GIS, and has progressed towards the development of new techniques and software tools for the quantitative analysis of shape and geometry with respect to mineralization potential, particularly for orogenic gold deposits.

8. Potential of shape analysis and artificial intelligence in prospectivity mapping From the above, all evidence suggests that structural geometry, and the shape of mappable geological units that result from it, are the most important parameters which can be used to define the location of orogenic gold deposits. Although the shapes and geometries of map features are stored within the GIS, there are very few tools available within present-day systems that allow for their quantitative analysis in relation to the location of gold deposits. This deficiency in GIS is being addressed by the development of a number of software tools that allow aspects of shape and geometry to be quantified ŽElliott, 1997; Gardoll et al., in press.. The output from these tools can be used to highlight areas that share similar shape and geometry properties, and can be incorporated into a prospectivity analysis and subsequent prospectivity map. Another technique gaining favourablilty is the application of Artificial Neural Networks ŽANNs.. An ANN represents a type of adaptive computing system that can learn from the data and is particularly suited to the tasks of pattern recognition and classification. As such, ANNs are used routinely for optical character recognition and speech recognition, and, by their very nature, they are also suited to the task of prospectivity mapping. Unlike the conventional three-step approach to prospectivity mapping, in which the importance of each spatial feature is examined individually, a neural network analyzes all data simultaneously. Essentially, using a neural network requires the construction of a ‘training set’ that is representative of both mineralized and unmineralized areas Žoutput values.. Geological information is collected for each of these locations, such as distance to nearest fault and rock-type Žinput values.. The ANN iteratively attempts to build a network that maps the input values to the associated output value

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for each record in the training set. This is known as the ‘training’ of the network. Once trained, the validity of the network is tested. If adequate, information for every location is presented to the trained neural network, and a prospectivity map is constructed from the output values. There is a wealth of different ANN structures, philosophies and algorithms available, and, for a particular kind of network, there are a large number of parameters that can be changed. One of the biggest challenges in the use of ANNs is to determine the best configuration for prospectivity mapping. This is at an early stage of investigation, yet is showing signs of promise ŽBrown et al., in press..

9. Conclusions The following conclusions can be drawn from this integrated study of the nature, controls and timing of orogenic gold deposits, and the application of these parameters to computer-based exploration methodologies for the deposit class. Ž1. Orogenic gold deposits represent a coherent group of widely distributed deposits in accretionary or collisional orogens of all ages. They formed over an extended depth range Ž- 5 to ) 15 km., although most commonly in greenschist-facies host rocks, from a relatively uniform, deeply sourced, low-salinity H 2 O–CO 2 " CH 4 " N2 fluid with low a H 2 S and, as a result, have high Au:Ag ratios and low basemetal contents and a distinctive geochemical signature of enhanced Si, K " Na, LILE, CO 2 , H 2 O, Au, As, B, Sb, Te and W. They differ from other gold-rich deposit groups in the same terranes, which formed at uniformly shallow depths Žnormally 5 km to the surface. from highly saline low-CO 2 fluids Že.g., VHMS and porphyry-style deposits. or acidic lowCO 2 fluids Žepithermal deposits. and which, characteristically, are either base-metal Ž"Sn " Mo. andror silver enriched. Ž2. The orogenic gold deposits are epigenetic. Structure is the first-order control on deposit distribution, as high ore-fluid flux into permeable structures or fractured rock bodies is an essential ore control. In many terranes, first-order crustal-scale faults and shear zones control the regional distribution of the deposits, which, however, are normally

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sited in, or adjacent to, second-and third-order structures where fluid focussing was most intense. Other important controls on the siting of the ores are fold hinges, competency contrasts between adjacent rock units, with vein arrays preferentially developed in the most competent units, and chemically reactive host rocks which commonly coincide with the most competent units Že.g., dolerite or BIF with high FerFeq Ma q Ca ratios.. Focusing of supralithostatic ore fluids into dilatent zones in suitable structures and host rocks appears to be the dominant control. Ž3. Structural studies in the terraces which host orogenic gold deposits normally indicate their relative timing as towards the end of the deformational cycle ŽD 3 or D4 in a D 1 –D4 cycle., after accretion or collision, and normally during uplift and exhumation of the orogen. Reactivation of pre-existing, suitably orientated fault and shear zones during a change in far-field stress is a common scenario. Ž4. Robust geochronological studies indicate that most orogens evolved over about 100 to 200 m.y. periods, and that orogenic gold deposits formed diachronously over the terranes in the latter half of this evolutionary period, thus confirming the relative structural timing. In Archaean greenstone terranes, such as those of the Yilgarn Block, the orogenic gold deposits formed some 20–80 m.y. after the youngest major volcanic event, but in younger, largely sedimentary, terranes, the time lag may be longer. In rare instances, the orogenic gold deposits may be related to deformation events that post-date cratonization of the host terrane by more than 1 b.y. Ž5. The late timing of the orogenic gold deposits has important implications for exploration methodologies which are geology-based. It means that the structural geometries of the gold deposits, hosting goldfields and enclosing terranes, as shown in geological maps and sections, and three-dimensional models, where available, are essentially those which were developed at the time of gold mineralization. This is pivotal to the use of geology-based qualitative methodologies and more-quantitative computer methodologies for exploration for this deposit class. Ž6. As expected from their dominant structural control and late timing, there are repetitive predictable patterns to the siting of orogenic gold deposits. Deposits or groups of deposits are commonly sited near structural heterogeneities within, or adja-

cent to, first- or lower-order faults and shear zones, adjacent to contacts across which there are large competency contrasts, in the pressure shadows of rigid granitoid plutons or batholiths, or in fold-hinges, commonly those Alocked-upB by pre-mineralization deformation. The two largest greenstone-hosted goldfields, Kalgoorlie and Timmins, show a remarkably similar structural geometry, attesting to the significance of this parameter in geology-based exploration. Ž7. Computer-based stress mapping is one methodology which can utilize the dominant structural control and late timing of orogenic gold deposits to quantify the expected siting of dilational zones during late deformation of a gold-mineralized terrane. The geological requirements are an accurate solid-geology map of uniform quality and knowledge of the far-field stress at the time of gold mineralization. All rock units, faults and contacts are assigned strength or stiffness parameters, and the stress modelling is carried out in two-dimensions using a distinct element code to determine displacements and movement vectors, as well as the distribution of minimum, maximum and mean stresses. The stress map of the Kalgoorlie Terrace in the Yilgarn Block of Western Australia, used as an example of the potential of stress mapping, clearly demonstrates that low minimum-stress anomalies coincide with known goldfields at the 1:250,000 scale of the modelling. This supports a model in which the location of gold deposits is strongly controlled by the distribution and orientation of first-order structures. There are also additional anomalies that may represent as-yet undiscovered deposits. The next step would be to perform two-dimensional stress mapping at a smaller scale on more-detailed solid-geology maps in the areas Žgoldfields?. defined by the regional low minimum-stress anomalies. Three-dimensional stress mapping could be performed if sufficient drill data are available. Ž8. GIS-based prospectivity mapping is another computer-based methodology that relies on current structural geometry, as shown on geological maps, representing that at the time of mineralization. Thus, it is ideally suited to the search for orogenic gold deposits. Using an empirical approach, it is possible to semi-quantitatively define the spatial relationship between parameters in the map data and the gold deposits, quantify them, and then integrate them into

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a prospectivity map. Again using the Kalgoorlie Terrace 1:250,000 scale map as an ideal example, the prospectivity analysis confirms expected relationships, with the siting of orogenic gold deposits controlled by such factors as proximity to, and orientation and curvature of, granitoid-greenstone contacts, proximity to segments of the crustal-scale faults which strike in a preferred north-westerly direction, proximity to certain lithological contacts which strike in a similar direction, proximity to anticlinal hinges or horst-like structures, and the presence of chemically-reactive host rocks such as differentiated mafic Ždolerite. sills. The prospectivity map, produced from the interpretation of these parameters, closely conforms to the stress map in defining the major goldfields, although there are differences in detail. In this map, the most prospective category occupies less than 0.3% of the greenstone belts, yet hosts over 16% of known gold deposits with over 80% of known gold production from the region. This demonstrates the potential of the methodology on a regional scale, particularly in the targeting of the larger, most-desired, orogenic gold deposits in the analyzed terrace. Ž9. The success of both the qualitative structural geometry approach and the semi-quantitative stressmapping and GIS-based prospectivity mapping in regional targeting for orogenic gold deposits, at least for the terranes so far researched, indicates that quantitative analysis of geological map data is a potentially profitable line of research. Stress mapping has an advantage in previously unexplored terranes as it relies wholly on prediction of dilatent sites and requires no database of existing gold deposits such as is required by the empirically based prospectivity mapping. Artificial neural networks and quantitative shape analysis are two additional methodologies currently being investigated. Ž10. The ultimate deposit target map should be an integration of as many geologically-based prospectivity guides as possible, such that coincident stress anomalies, high prospectivity categories and, in the future, critical shape parameters can be compared to other, more conventional, targeting parameters such as geophysical and geochemical anomalies. In combination, these methodologies should be capable of pinpointing potentially mineralized zones containing orogenic gold deposits in poorly exposed terraces

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with complex regolith or thick superficial cover, the ever increasing locations of modern global exploration. Acknowledgements We gratefully acknowledge colleagues in the Centre for Strategic Mineral Deposits at the University of Western Australia, who have contributed to our collective knowledge of orogenic gold deposits and the Yilgarn Block and to development of prospectivity mapping methodology. Thanks are due to Terra Sancta for use of stress mapping software packages and permission to publish. We are grateful for funding for gold research from many supportive Australian mining and exploration companies. The paper has been improved by the reviews of Tim Bell and Peter Laznicka. References Alibone, A.H., Windh, J., Etheridge, M.A., Burton, D., Anderson, G., Edwards, P.W., Miller, A., Graves, C., Fanning, C.M., Wysoczanski, R., 1998. Timing relationships and structural controls on the location of Au–Cu mineralization at the Boddington Gold Mine, Western Australia. Econ. Geol. 93, 245– 270. Barnicoat, A.C., Fare, R.J., Groves, D.I., McNaughton, N., 1991. Synmetamorphic lode–gold deposits in high-grade Archean settings. Geology 19, 921–924. Bloem, E.J.M., Dalstra, H., Groves, D.I., Ridley, J.R., 1994. Metamorphic and structural setting of amphibolite-hosted gold deposits between Southern Cross and Bullfinch, Southern Cross Province, Yilgarn Block, Western Australia. Ore Geol. Rev. 9, 183–208. Bloem, E.J.M., McNaughton, N.J., Groves, D.I., Ridley, J.R., 1995. An indirect lead isotope age determination of gold mineralization at the Corinthia mine, Yilgarn Block, Western Australia. Aust. J. Earth Sci. 42, 447–451. Boer, R.H., Meyer, F.M., Robb, L.J., Graney, J.R., Vennemann, T.W., 1993. The nature of gold mineralization in the SabiePilgrim’s Rest goldfield, eastern Transvaal, South Africa. Univ. of Witwatersrand. Econ. Geol. Res. Unit, Inform. Circ. 262, 33 pp. Bohlke, J.K., Kistler, R.W., 1986. Rb–Sr, K–Ar, and stable isotope evidence for the ages and sources of fluid components of gold-bearing quartz veins in the northern Sierra Nevada foothills metamorphic belt, California. Econ. Geol. 81, 296– 322. Bonham-Carter, G.F., 1995. Geographic Information Systems for Geoscientists: Modelling with GIS. Computer Methods in the Geosciences 13 Pergamon, New York.

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