Granitoid-associated orogenic, intrusion-related, and porphyry style metal deposits in the Archean Yilgarn Craton, Western Australia

Ore Geology Reviews 32 (2007) 157 – 186 www.elsevier.com/locate/oregeorev

Granitoid-associated orogenic, intrusion-related, and porphyry style metal deposits in the Archean Yilgarn Craton, Western Australia P. Duuring a,⁎, K.F. Cassidy b , S.G. Hagemann c a

c

Earth and Ocean Sciences, The University of British Columbia, 6339 Stores Road, Vancouver, B.C. Canada V6T1Z4 b Geoscience Australia, Canberra, A.C.T., 2601, Australia Centre for Exploration Targeting (CET), School of Earth and Geographical Sciences, The University of Western Australia, Nedlands, W.A. 6009, Australia Received 8 April 2005; accepted 10 November 2006 Available online 10 January 2007

Abstract The Yilgarn Craton hosts three types of Archean granitoid-associated metallogenic systems: orogenic gold, intrusion-related, and porphyry systems. These systems may occur in the same terrane and share many broad characteristics, including spatial coincidence with granitoids, gangue silicate mineralogy, metal associations, and local structural controls. Features of Archean orogenic gold systems spatially associated with granitoids include (i) a broad range of granitoid host compositions; (ii) gold mineralization is mostly late with respect to Yilgarn-wide, granitoid emplacement, peak metamorphism, and deformation; (iii) orebodies are structurally controlled, and (iv) gold deposition occurs over a large range of temperatures, pressures, and crustal depths (2 to 15 km) from a CO2rich, low to moderate salinity, reduced ore fluid that was derived from a metamorphic and/or distal magmatic fluid source. These systems do not display deposit-scale vertical or lateral zonation of metals away from granitoid contacts. Host granitoid age, composition, or oxidation state are not important in controlling gold mineralization processes, whereas structural setting and fluid flux are paramount. Archean intrusion-related Au–Mo–Wand Mo ± Au systems in the Yilgarn Craton are commonly small (<10 t Au), and are spatially and temporally associated with felsic intrusions emplaced at <5 to 14 km crustal depths. Orebodies are associated with pervasive alteration in the granitoid-host and proximal supracrustal countryrock. Where available, fluid inclusion and metal association data indicate the involvement of an aqueous, CO2-bearing, moderate to high salinity fluid during ore deposition. Archean porphyry Cu–Mo–Au, Cu–Mo, and Cu–Au systems are spatially and temporally associated with volumetrically small, pervasively altered, felsic plutons and dikes surrounded by altered and mineralized supracrustal countryrock. Lateral zonation of metals away from porphyry centers commonly includes a decrease in Cu:Au and Ag:Au ratios with distance from the source pluton. Microthermometry studies on fluid inclusions in gold-bearing veins suggest that early ore fluids were high-temperature and highsalinity, and that mineralization occurred at <5 km crustal depths. Exploration for Archean granitoid-associated orogenic gold systems in the Yilgarn Craton lies primarily in identifying mineralized structures in granitoids or adjacent supracrustal rocks and predicting low mean stress areas where ore fluids may concentrate. In contrast, exploration strategies for Archean syn-magmatic systems hinges upon establishing temporal connections between granitoid emplacement and associated mineralization, and testing this relationship in coeval granitoids in the same terrane. Fractionation trends in granitoid composition and ore metal associations, using geochemical

⁎ Corresponding author. E-mail address: [email protected] (P. Duuring). 0169-1368/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.oregeorev.2006.11.001

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indices such as Rb/Sr versus Fe2O3/FeO, are demonstrated in Phanerozoic terranes but are presently ill-defined in the Yilgarn Craton. Crustal depth estimation is important in defining areas likely to have preserve shallow-forming, Archean porphyry systems. © 2006 Elsevier B.V. All rights reserved. Keywords: Orogenic; Intrusion-related; Porphyry; Granitoid; Archean; Yilgarn

1. Introduction Phanerozoic terranes host numerous metallogenic systems (e.g., Cu, Au, W, Mo, Sn) spatially associated with granitoids (e.g., Ishihara, 1981; Sillitoe, 1991; Blevin and Chappell, 1992, 1995). A number of these systems are gold-bearing, including deposits that have been variously described as “intrusion-related stockwork-disseminated”, “plutonic-related”, and “intrusionrelated” gold deposits (Sillitoe, 1991; Thompson et al., 1999; Thompson and Newberry, 2000; Lang and Baker, 2001). In these intrusion-related gold systems, deposits generally form proximal to a felsic-intermediate intrusion and mineralization is intimately associated in space and time with the development of magmatic-hydrothermal fluids. Coeval orogenic gold systems in the same terrane (e.g., Yukon–Alaska, Peru–Bolivia, and North Queensland) differ in that metals are concentrated within a variety of host rocks, including granitoids, by depositional processes occurring distal to the site of fluid generation (e.g., Groves et al., 1998; Ridley and Diamond, 2000; Goldfarb et al., 2001, 2005). Difficulties exist in distinguishing intrusion-related from orogenic gold deposits associated with granitoids because of their similar spatial coincidence with granitoids, gangue mineralogy, metal associations, and local structural controls (Goldfarb et al., 2005). In Phanerozoic terranes, intrusion-related and porphyry-style mineralization is spatially and genetically associated with specific granite suites with distinct chemical characteristics (Blevin et al., 1996; Baker et al., 2005). Archean terranes also contain examples of intrusionrelated, orogenic gold, and porphyry systems spatially associated with granitoids. In the Abitibi Subprovince, Canada, in addition to orogenic gold systems, gold mineralization spatially associated with Archean quartzmonzonite to syenite intrusions is interpreted to be intrusion-related (Robert, 1997, 2001). In the Zimbabwe Craton, gold mineralization at the Ford (Buchholz et al., 1998), Freda-Rebecca (Klemm and Krautner, 2000) and Eureca (Selby et al., 2001) deposits is also interpreted to be intrusion-related. Although most gold deposits spatially associated with Archean granitoids in the Yilgarn Craton of Australia are historically considered to be orogenic (Cassidy et al., 1998; Groves et al., 1998;

Hagemann and Cassidy, 2000), recent deposit studies that integrate structural-hydrothermal relationships with high precision dating of magmatic and hydrothermal events (e.g., Boddington, Nevoria, Chalice) have led to the possibility that some of these deposits belong to the intrusion-related or porphyry class. The present paper compares well-documented granitoid-associated orogenic gold systems with possible intrusion-related and porphyry gold systems in the Yilgarn Craton, with the aim of determining a set of defining characteristics for each mineralization style. By focusing on the differences between these deposits, exploration criteria are given for identifying the different styles of Archean granitoid-associated metal deposits in the Yilgarn Craton. Given the commonly smaller tonnage of intrusion-related gold deposits relative to orogenic and porphyry deposits, discrimination between these styles of granitoid-associated deposits is important during the early stages of exploration. 2. Yilgarn Craton geological setting The Yilgarn Craton largely consists of metavolcanic and metasedimentary rocks and granitoids that formed between ca. 3050 and 2600 Ma, with a minor older (to >3700 Ma) component particularly in northwest and southwest gneiss complexes (Myers, 1993, 1995; Barley et al., 1998). The craton has been divided on the basis of structural trends and lithostratigraphic assemblages (e.g., Gee et al., 1981), fault-bounded tectono-stratigraphic terranes (e.g., Myers, 1995), and geophysical character (Whitaker, 1997). This contribution retains the subdivision of Gee et al. (1981, 1986) into granitegreenstone Provinces and Gneiss complexes (Fig. 1), except where Swager (1997) has delineated “terranes” in the southeast of the Yilgarn. The main components of the Craton are the ca. 3.05–2.62 Ga Southern Cross and Murchison Provinces, > 2.76–2.62 Ga Eastern Goldfield Province, ca. 3.20–2.60 Ga high-grade gneiss complexes and greenstones in the southwest, and >3.70– 2.62 Ga Narryer Gneiss complex in the northwest (Gee et al., 1986; Myers, 1995; Nelson, 1997; Barley et al., 1998). A period of late-Archean orogeny, between 2.78 and 2.62 Ga, resulted in stabilization of these terranes to form the present Yilgarn Craton (Swager, 1997; Blewett

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Fig. 1. Geological map of the Yilgarn Craton in Western Australia that shows province subdivisions and the location of orogenic gold, intrusionrelated Au–Mo–W, Mo ± Au, and porphyry Cu ± Mo ± Au systems that are mentioned in the text. Orogenic gold deposits that also contain minor intrusion-related mineralization (e.g., Chalice, Cleo-Sunrise, Granny Smith, Noongal, Porphyry, Tower Hill) are indicated by the intrusion-related gold systems symbol shown in brackets.

et al., 2004). Key chronological events in the geological evolution of the Yilgarn Craton are summarized below and in Fig. 2. Difficulties arise when comparing geochronological events between provinces, within provinces (e.g., Murchison Province: Pidgeon and Hallberg, 2000), and even when comparing depositand regional-scale events (cf. Cassidy and Hagemann, 2001).

2.1. Supracrustal sequences In the Eastern Goldfields Province, compiled ages show that tholeiitic, komatiitic and calc-alkaline volcanic and sedimentary rocks developed dominantly between ca. 2720 and 2660 Ma, with evidence for minor older (> 2760 Ma) volcanism (Nelson, 1997; Barley et al., 1998; Krapez et al., 2000). Several lines of

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Fig. 2. Summary of geochronological events in the Yilgarn Craton, Western Australia. References include Kent and McDougall (1995), Kent and Hagemann (1996), Barley et al. (2001), Krapez et al. (2000), and Witt et al. (2001b) for the Eastern Goldfield Province; Watkins and Hickman (1990), Savage (1992), Wiedenbeck and Watkins (1993), Pidgeon et al. (1994), Schiotte and Campbell (1996), Wang et al. (1996, 1998), Yeats et al. (1996), Witt (1998), Nelson (1999, 2000, 2001), Oliver (1999), Pidgeon and Hallberg (2000), Chen et al. (2001), Ross et al. (2001), and Wyche et al. (2001) for the Murchison and Southern Cross Provinces; and McCuaig et al. (2001) and Stein et al. (2001) for the Saddleback Greenstone Belt.

evidence (detrital and xenocrystic zircons, radiogenic isotopes, geochemical evidence for crustal contamination) suggest that the greenstone succession developed on substantial older crust (e.g., Compston et al., 1986; Nelson, 1997; Krapez et al., 2000; Cassidy et al., 2002a). Coarse clastic sequences unconformably overlie, or are in fault contact with, the greenstone succession and are interpreted to have been deposited after 2655 Ma (Swager, 1997; Krapez et al., 2000). Available geochronological data indicate several periods of volcanism and sediment deposition in the Murchison and Southern Cross Provinces at ca. ?3050– 2930, 2810 and 2760–2720 Ma (Pidgeon and Wilde, 1990; Wang et al., 1998; Pidgeon and Hallberg, 2000). Evidence for older components in the Murchison and Southern Cross Provinces and Narryer Gneiss complex are implicated by detrital and xenocrystic zircon ages that suggest sources > 4300, > 3800 and > 3400– 3100 Ma (Myers, 1995; Pidgeon and Hallberg, 2000). The composite Southwest Gneiss Terrane has been divided into three tectonostratigraphic terranes, although the exact location of the boundaries between each terrane is poorly constrained (Wilde et al., 1996). Each terrane has contrasting supracrustal sequences ranging in age

from the ca. 3200–2800 Ma Chittering, Jimperding, and Balingup Metamorphic Belts, to the ca. 2720–2670 Ma Saddleback greenstone belt (Wilde et al., 1996; Allibone et al., 1998; Wilde, 2001; McCuaig et al., 2001). Quartzites in the Jimperding Metamorphic Belt contain detrital zircons with a spectrum of ages from ca. 3.73 to 3.18 Ga (Wilde, 2001), indicating a complex and variably old provenance. 2.2. Granitoids Granitoids were emplaced during several episodes across the Yilgarn Craton (Bettenay, 1977; Schiotte and Campbell, 1996; Champion and Sheraton, 1997; Nelson, 1997; Cassidy et al., 2002a). Granitoids in the Eastern Goldfields Province were emplaced from ca. 2810 to 2630 Ma with the majority from 2680 to 2650 Ma (Champion and Sheraton, 1997; Nelson, 1997; Fletcher et al., 2001; Dunphy et al., 2003). In the eastern Yilgarn, granitoids are divided on the basis of geological, geochemical and geophysical characteristics into two major (High-Ca, Low-Ca) and three minor (Mafic, HighHFSE, Syenitic) groups (Champion and Sheraton, 1997). High-Ca granitoids, derived principally from

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melting of lower-mid crust source regions, were emplaced from 2810 to 2655 Ma, whereas the midcrustally-derived Low-Ca granitoids were emplaced from 2655 to 2630 Ma (Champion and Smithies, 2001; Champion and Cassidy, 2002). Volumetrically minor granitoid groups, High-HFSE (2720–2670 Ma), Mafic (2690–2650 Ma), and Syenitic (2665–2640 Ma: Champion and Sheraton, 1997; Champion and Cassidy, 2002), are spatially restricted to specific domains within the eastern Yilgarn. Granitoid types in the eastern Yilgarn show a trend with time to increasingly potassic (higher LILE contents) compositions (Champion and Smithies, 2001), reflecting a major change from lower crust and/or mantle sources to produce the High-Ca and Mafic granitoids, to crustal melting at low pressure for the LowCa granitoids (Champion and Sheraton, 1997). Granitoids in the Murchison and Southern Cross Provinces have emplacement ages of ca. 3010–2920, 2810 and 2760–2620 Ma (e.g., Wiedenbeck and Watkins, 1993; Wang et al., 1995; Savage et al., 1995; Schiotte and Campbell, 1996; Qiu et al., 1997; Pidgeon and Hallberg, 2000; Cassidy et al., 2002a). Recent dating of granitoids in the composite Southwestern Gneiss Terrane has indicated a complex history, with emplacement from > 2750 to 2600 Ma, with the majority < 2700 Ma. Zircons from these granitoids contain 2690–2650 Ma cores surrounded by rims with ages of 2650–2620 Ma (Nemchin and Pidgeon, 1997). In the Lake Grace Terrane, emplacement of charnockitic granitoids coincided with granulite-facies metamorphism at ca. 2640 Ma (Nemchin et al., 1994). 2.3. Metamorphism In the Eastern Goldfields Province, peak regional metamorphism coincided with the last stages of regional deformation and felsic magmatism (e.g., Ridley, 1993; Ridley et al., 1997; Groves et al., 1998; Barley et al., 2001). The timing of peak metamorphism is constrained on textural grounds to be synchronous with the main episode of granitoid emplacement (2670–2640 Ma). Metamorphic hornblendes from metavolcanic rocks at Coolgardie in the Kalgoorlie Terrane yield average 40 Ar–39Ar plateau ages of ca. 2650 Ma (Napier et al., 1998). At Chalice, peak regional amphibolite-facies metamorphism is interpreted to coincide with Stage I gold mineralization at 2644 ± 8 Ma (Bucci, 2001; Bucci et al., 2004). The timing of peak metamorphism elsewhere across the Yilgarn Craton is poorly constrained, with only a few precisely dated metamorphic events. In the Southern Cross Province, at least two metamorphic events are

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invoked (Dalstra et al., 1999); an early (ca. 2970 Ma) metamorphic event is reported in the Ravensthorpe belt (Ridley et al., 1997), whereas a second high-strain, higher-grade metamorphic event is interpreted to coincide with differential uplift and granitoid emplacement between 2690 and 2640 Ma (Dalstra et al., 1999), although Mueller and McNaughton (2000) extend this event to ca. 2780 Ma. Peak metamorphism coincided with voluminous granitoid emplacement at ca. 2720–2650 Ma in the Murchison Province. In the southwest Yilgarn Craton, granulite-facies metamorphism was synchronous with ca. 2640–2620 Ma granitoid emplacement (e.g., Barnicoat et al., 1991; Nemchin et al., 1994), and intrusive and volcanic rocks at Boddington were metamorphosed to upper-greenschist to lower-amphibolite-facies at ca. 2640 Ma (McCuaig et al., 2001). 2.4. Deformation Different histories in each of the granite-greenstone terranes in the Yilgarn Craton make it invalid to use a common deformation sequence for the craton. There are, however, enough similarities to indicate that for most of their history, the Murchison and Southern Cross Provinces acted as a single entity (Cassidy and Champion, 2004), whereas recent studies in the Eastern Goldfields Province indicate complexity previously unrecognized (e.g., Witt, 2001; Barley et al., 2001; Blewett et al., 2004; Weinberg et al., 2005). The unifying deformation feature of the Yilgarn Craton is that the entire craton records a major east–west compressive orogenic event superimposed on previously deformed cratonic elements. The deformation sequence is best deciphered for the southern part of the Eastern Goldfields Province (Kalgoorlie Terrane), where most researchers recognize two phases of compressive deformation separated by a period of extension that formed deep marine basins (e.g., Swager, 1997; Krapez et al., 2000). The first event for the Eastern Goldfields Province (i.e., D1-EGP) involved south-over-north thrust stacking and recumbent folding (e.g., Swager and Nelson, 1997). The absolute timing of D1-EGP is poorly constrained to later than ca. 2690 Ma, the age of the upper mafic volcanic sequence in the Kalgoorlie Terrane (Bateman et al., 2001). Based on the intrusion of the Kanowna Belle Porphyry within a D1-EGP structure, D1-EGP is interpreted to have a minimum age of 2656 ± 6 Ma, at least in the Kalgoorlie Terrane (Barley et al., 2001; Ross et al., 2004). Regional upright north– northwest-trending folds formed during D2-EGP coincided with the uplift of granitoid-core anticlines and development of a pervasive metamorphic foliation. The D2-EGP is

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considered to post-date late siliciclastic basins (e.g., Kurrawang, Merougil, Penny Dam Sequences) that have maximum depositional ages of ca. 2655 Ma (Swager, 1997; Krapez et al., 2000), although Blewett et al. (2004) provide evidence for a more complex D2-EGP that included an episode of extension during which the late basins were deposited. The D2-EGP deformation progressed to transpressive strike-slip deformation during D3-EGP (Swager, 1997). Regional compression and metamorphism occurred from 2660 to 2630 Ma and was possibly diachronous (Blewett et al., 2004). Regional metamorphism reached peak temperatures during late D2–D3-EGP and coincided with syn-D3-EGP granitoid emplacement (e.g., Swager and Nelson, 1997). 2.5. Metallogenic episodes The Yilgarn Craton contains a variety of metallogenic events ranging from komatiite-associated Ni–Cu sulfide (ca. > 2930 Ma Maggie Hays; ca. 2700 Ma Kambalda, Perseverance, Mt. Keith) and volcanichosted massive sulfide (VHMS) (ca. 2950 Ma Golden Grove and Mt Gibson in southern Murchison Province; 2690 Ma Teutonic Bore in northern Eastern Goldfields Province), syn-magmatic Cu–Au ± Ag mineralization (2970 Ma Ravensthorpe), magmatic PGE and V–Ti (?2800 Ma Windimurra, Barrambie, Weld Range), orogenic gold, and a variety of intrusion-related Cu, Au, W, Mo, and Sn mineralization styles (Barley et al., 1998). The main phase of orogenic gold mineralization across the Yilgarn Craton is considered late-tectonic relative to their immediate host rocks (Fig. 2; cf. Groves et al., 1995) but is syn-orogenic with respect to on-going deep-crustal thermal processes (Groves et al., 1998). The main gold mineralization event in the Eastern Goldfields Province took place between 2650 and 2625 Ma, syn- to post-granitoid emplacement and regional peak metamorphism (Groves et al., 2000, 2003). Robust dating of host rocks, post-ore rocks, and the direct dating of minerals associated with mineralization show that gold mineralization is diachronous, at least for the Eastern Goldfields province (Vielreicher et al., 2003; Blewett et al., 2004), and that large deposits, such as the Golden Mile deposit, may contain more than one episode of gold mineralization associated with several different mineralization types and processes (Robert et al., 2005). Recent age constraints for gold mineralization in the Yilgarn include >2668 ± 9 Ma epithermal and < 2650 Ma orogenic gold events at Kanowna Belle (U–Pb on zircon: Ross et al., 2004), 2654 ± 8 Ma gold mineralization at Cleo (U–Pb on monazite and xenotime: Brown

et al., 2002), 2650 ± 6 Ma gold mineralization at Wallaby (U–Pb on monazite and xenotime: Salier et al., 2004), 2644 ± 8 Ma Stage I and 2624 ± 7 Ma Stage II gold mineralization at Chalice (U–Pb on titanite: Bucci, 2001), and 2631 ± 9 Ma mineralization at Revenge (U– Pb on monazite: Nguyen, 1997 in Vielreicher et al., 2003). SHRIMP U–Pb analysis of gold-related monazite and xenotime constrain gold mineralization to 2650 ± 7 Ma at Mount Morgans, 2649 ± 11 Ma at Jubilee, 2657 ± 21 at Jupiter, and 2653 ± 6 Ma at Granny Smith (Salier et al., 2005). In the Golden Mile deposit, early Fimistontype mineralization is constrained by a 2676 ± 3 Ma preore feldspar porphyry dike and a 2663 ± 10 Ma post-ore feldspar-hornblende porphyry dike (U–Pb on zircon: Gauthier et al., 2004). Disseminated sulfide-rich mineralization of the Oroya Shoot is considered slightly younger, but related, to Fimiston-type lodes (Bateman and Hagemann, 2004). Later, Mt. Charlotte-type mineralized quartz–carbonate veins cut the Fimiston-type lodes (Clout et al., 1990). The attempts at dating gold mineralization at Mt. McClure and Jundee illustrate the importance of confidently defining relative timing relationships between felsic dike emplacement and gold mineralization. Previously interpreted >2663 ± 4 Ma and >2656 ± 7 Ma respective ages for orogenic gold mineralization at Mt McClure and Jundee (Yeats et al., 1999, 2001) have recently been re-evaluated, based on the reinterpretation of cross-cutting relationships between goldbearing veins and lamprophyre dikes, to be <2649 ± 7 Ma and <2657± 6 Ma, respectively (Vielreicher et al., 2001; Baggott et al., 2005). Other time constraints on gold mineralization in the Yilgarn includes 2636± 8 Ma for a syn-to post-mineral intrusion at Westonia (U–Pb on zircon: Kent et al., 1996), 2636± 1.2 Ma for skarn-related mineralization at Nevoria (U–Pb on allanite in garnet: Mueller et al., 2004), 2662 ± 5 Ma for skarn-related mineralization at Big Bell (U–Pb on garnet: Mueller et al., 1996), and 2707 ± 17 Ma and 2623 ± 9 Ma mineralization events at Boddington (Re–Os on molybdenite; Stein et al., 2001; McCuaig et al., 2001). 3. Orogenic gold deposits spatially associated with granitoids in the Yilgarn Craton Orogenic gold deposits hosted in, or spatially associated with, granitoids in the Yilgarn Craton, as well as other Archean terranes (e.g., Superior, Slave, Zimbabwe), represent a subgroup of the orogenic gold deposit class (Groves, 1993; Cassidy et al., 1998; Groves et al., 1998; Goldfarb et al., 2005) and show minor divergence from the broader deposit classification due to the effects of granitoid host rock rheology and

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chemistry on gold depositional processes. The spatial correlation between granitoids and orogenic gold deposits in the majority of orogenic gold provinces is largely due to their both being products of collisionalaccretionary processes at convergent margins (Groves et al., 2003; Goldfarb et al., 2005). This section briefly summarizes the characteristics of Archean granitoid-associated orogenic gold deposits in the Yilgarn Craton in light of new data from recent detailed deposit studies. Table 1 shows key properties of several granitoid-associated gold deposits in the Yilgarn, including the recently described Federal/Golden Cities (Federal: Phillips and Zhou, 1999; Golden Cities: Kehal and Stephens, 2000; Zhou et al., 2003, Mason, 2004), Tarmoola (Duuring et al., 2001, 2004), Jupiter (Duuring et al., 2000), Wallaby (Salier et al., 2001, 2004), Chalice (Bucci et al., 2002, 2004), and Celtic and Wonder North deposits (Winzer, 2001). 3.1. Granitoid host rock characteristics Orogenic gold deposits are spatially associated with all granitoid groups in the Yilgarn Craton (Table 1). Interestingly, Mafic group granitoids host the majority of known deposits (e.g., Granny Smith, Lawlers, Golden Cities) despite them being less volumetrically abundant than High-Ca and Low-Ca groups (these latter groups represent 80 % of all granitoids in the Yilgarn: Champion and Cassidy, 2002; Cassidy et al., 2002a), and despite Low-Ca and Syenitic group crystallization ages (i.e., 2665–2630 Ma) more closely overlapping the main gold mineralization event (i.e., 2650–2630 Ma) in the eastern Yilgarn (Cassidy et al., 1998). Mafic and Syenitic granitoids are broadly associated with large fault systems throughout the Eastern Goldfields and central Murchison provinces (Smithies and Champion, 1999). Mineralized granitoids vary from being internally homogeneous, such as the massive trondhjemite at Tarmoola (Duuring et al., 2001), to exhibiting internal heterogeneities, including grain size variation (e.g., granodiorite at Federal: Zhou et al., 2003), or containing large countryrock xenoliths (e.g., Great Eastern: Cassidy, 1992). In most deposits mineralization is relatively late, cutting regional penetrative fabrics in the granitoids, such as shear zones, folds, and foliation. Granitoids are commonly cut by dikes of variable composition ranging from late-stage aplites and syenogranites (Lawlers) to intermediate-mafic (e.g., Tarmoola, Jupiter, Wonder North). Although gold-bearing veins post-date the dikes in most deposits, dikes of variable composition cross-cut mineralization at several deposits, principally

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deposits in amphibolite facies metamorphosed terranes (e.g., Westonia). Granitoids spatially associated with orogenic gold mineralization commonly contain primary magnetite ± titanite and, where the data are available, have Fe2O3/ FeO ratios of ∼ 0.3 to 3 (Table 1; Fig. 3), suggesting that they formed from oxidized melts. Granitoid Rb/Sr ratios of ∼ 0.05 to 5 imply that the host intrusions only reached moderate degrees of fractionation. Fig. 3 clearly shows that the composition of granitoids spatially associated with gold mineralization is not similar to that of intrusions hosting gold mineralization in Phanerozoic terranes. The majority of High-Ca, Low-Ca, and Mafic granitoids have either higher degrees of fractionation or lower Fe2O3/FeO ratios, with all individual suites showing a large variation in Fe2O3/FeO ratio. High-Ca and Low-Ca granitoids are compositionally more similar to granitoids associated with porphyry-style Cu–Mo and/or W–Mo deposits (Fig. 3A). Some syenitic granitoids overlap the field of intrusions associated with Phanerozoic porphyry Cu–Au deposits (Fig. 3D). However, although data have been culled to exclude intensively altered material, the potential effects of alteration and metamorphism on the Fe2O3/FeO ratio and degree of fractionation have not been completely assessed. The possibility that the redox state of specific granitoids is important in oxidizing the reduced hydrothermal ore fluid and precipitating gold is equivocal. Phillips and Zhou (1999) and Zhou et al. (2003) argue that the more mafic granitoids (i.e., those with higher Fe/Fe + Mg + Ca ratios) are better geochemical traps for gold due to interaction between the granitoid host and hydrothermal fluids, causing destabilization of the gold bisulfide complex, wallrock sulfidation, and gold deposition. By their reasoning, granitoid composition may be a useful exploration parameter in that tonalite, trondhjemite (e.g., Tarmoola), hornblende-bearing granodiorite (e.g., Federal/Golden Cities), and hematite-altered syenite (e.g., Jupiter and Wallaby) are potentially better chemical hosts than granitoids that have lower Fe/Fe + Mg + Ca ratios (e.g., monzogranite and granite). However, recent thermodynamic modeling by Bastrakov et al. (2000) and Mason (2004) on the effect of variable granitoid host FeOtotal composition on gold mineralization via fluid/wallrock interaction demonstrates that gold deposition efficiency is still achieved at relatively low FeOtotal contents (e.g., FeOtotal ∼ 1.0 wt.%). Most granitoid FeOtotal contents are therefore capable of triggering gold deposition and are equally prospective. Hence, the effect of granitoid composition on gold mineralization is probably minor

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Table 1 Structural and ore fluid characteristics of selected Archean orogenic gold deposits that are spatially related to granitoids in the Yilgarn Craton, Western Australia

Resource (t Au) Granitoid host Composition

Celtic/Wonder North

Lady Bountiful

Tarmoola

Jupiter

Wallaby

Granny Smith (Stage II)

Porphyry

Great Eastern

Chalice (Stage 1)

Westonia

43.55 Hornblende and biotite Granodiorite

12.7 Granodiorite ± monzoGranite ± syenogranite Bundurra Batholith

11.36 Biotite granodiorite

∼116 Trondhjemite

6.08 Quartz alkalifeldspar Syenite

∼ 220 Monzonite, syenite, Carbonatite

49.77 Quartz diorite

10.58 Biotite ± hornblende Quartz monzonite

14.74 Biotitehornblende Tonalitegranodiorite

18 Biotite monzogranite

27.67 Biotitehornblende Tonalite gneiss

Ap, tn, zr, mt, il Magnetite

Mt, tn, zr

2666 ± 7 Supracrustal xenoliths

>2664 ± 8 Mafic and ultramafic amphibolite

> 2640 Maficultramafic amphibolite

Intermediate (7–10) Upper-greenschist to loweramphibolite Post-peak

Deep (12–14) Mid-to upperamphibolite Syn-to post-peak Metamorphism, postgranitoid intrusion 2644 ± 8

Deep (10–15) Upperamphibolite

Tn, to

∼0.13 None

Mt, ilm, ap, mon, zr Magnetite Oxidized (∼ 1.83) ∼ 0.57 Lamprophyre, hornblende porphyry, mafic xenoliths

Crustal depth (km) Metamorphic grade of supracrustal sequence Timing of mineralization Relative

Not applicable

Post-granitoid

Greenschist (granitoids and mafic xenoliths) Post-peak

∼0.08 Mafic sill

Mt, ilm, ap, tn, mon, zr Magnetite Oxidized (1–2) 0.18 to 0.28

∼0.11 2700 ± 11 Komatiite, tholeiitic Tholeiitic basalt, basalt, sedimentary quartz-feldspar rocks, quartz diorite porphyry

Shallow to inter. (< 5–7) Lower-midgreenschist

Shallow to inter. (2–10) Greenschist

Shallow to inter. (<5–7) Greenschist

Shallow to inter. (< 5–7) Greenschist

Post-peak

Post-peak

Post-peak

Metamorphism, postgranitoid intrusion <2667 ± 8 diorite dikes Brittle

Post-peak

Post-contact

Post-peak

Metamorphism, postgranitoid intrusion 2657 ± 21

Metamorphism, postgranitoid intrusion 2650 ± 6

Metamorphism, postgranitoid intrusion 2635a; 2653 ± 6b

Brittle

Brittle-ductile

Brittle

Metamorphism, Metamorphism? postgranitoid Postgranitoid intrusion intrusion 2646 ± 25; minor 2592 ± 9 Brittle-ductile Brittle-ductile

Ductile

Along granitoid contact in komatiite and NW-striking veins in granitoid

Syenite and NW-and NE-striking, shallowdipping shear zones

Shallow to moderately NE-dipping fault zone sets in altered syenite and conglomerate Brittle subparallel quartz vein sets

Along granitoid contact and fault zones within sedimentary rocks

Flexure of irregular shear zone

Intersection of shear zone sets

Foliation parallel veins

Intersection of shear zone with flexure at footwall contact

Conjugate fractures in granitoid. Veins oriented subparallel to bedding in sedimentary srocks

Shear zone-hosted veins

Brittle-ductile shear zone hosted quartz veins; discrete veins ± breccia

Foliation parallel veins

Foliation parallel veins and quartz reefs

Qz–cb

Qz–ank ± mu ± ru

Qz–ank–mu ± chl ± ru ± to

Stage I: Qz–bt– cc ± kf ± tr ± mu, Stage II: qz–cc– chl ± mu ± ab ± hm

Qz–ab– cpx–tn– gt–am ± bt ± cc

Stage I: qz– kfbt–cpx ± mu ± hb ± pc ± cc Stage II: qz– cpx ± hb ± bt ± kf ± pc ± tn ± cc

Metamorphism? Postgranitoid intrusion

Structural regime

Brittle-ductile

Brittle-ductile

Ore shoot control

Intersection of shear zone sets

Intersection of NWstriking, anatomizing shear zones and NEstriking fault zones

Brittle to brittle-ductile Intersection of fault zone sets

Mineralized vein type

Brittle-ductile shear zone hosted quartz veins

Shear zone-hosted quartz veinlet network

Brittle en echelon NW-striking veins quartz vein sets in granitoid. Veins oriented subparallel to granitoid margins and foliation in komatiite

Shear zonehosted, discrete, and ladder veins

Qz ± cb

Qz–cc–wm– chl ± ank

Qz–cc–mu ± chl ± ru

Qz–cc–ank– ab ± ser

Qz–ank–mu ± ab ± chl ± cc ± fu ± ep

Shallow to inter. (< 5–8)

Felsic volcanic rocks

Uppergreenschist

Metamorphism, postgranitoid intrusion

Absolute (Ma)

0.09 to 0.13 2665 ± 4 Sedimentary ± volcanic rocks

Magnetite

Intermediate (5–10) Mid-uppergreenschist

Intrusion

Vein mineralogy Silicate, carbonate minerals

2664 ± 3 Conglomerate

Granodiorite

Syn-to post-peak Metamorphism, postgranitoid intrusion < 2640 felsic dikes Ductile

P. Duuring et al. / Ore Geology Reviews 32 (2007) 157–186

Accessory minerals Granitoid series Fe2O3/FeO Rb/Sr Absolute age (Ma) Other hosts

Federal/Golden Cities

Ore minerals

Metal association

Wallrock alteration

Py–cpy

Au–Cu–Pb– Bi–Ag– Sb–Te Mu–bt–qz– ab ± ep ± hm ± chl ± cb ± tn

Au–Cu–As–Mo– Pb–V–Sn

Stage II: cpy– sph–ga–tell (Pb–Bi–Au)–sch Au–Ag–Cu–Pb– W–Bi–As–Mo– Zn–Te–Sb Qz–ank–cc–mu ± ab ± chl

Py–cpy–sph– ga–mo tell (Pb–Au–Bi) –sch Au–Ag–Bi–Te– Cu–Pb–Zn– Mo–W Qz–cc–ank– ab ± ser

Py

Py–po–cpy–ga

Py ± cpy ± ga ± mo

sph–apy–mo–tell

Dol–ab–qz– py–mu–fu ± ser ± xe ± mon

Au–Ag–As– Sb–Bi–Te–W

Au–Ag– As–Mo

Qz–mu–ank– ab–py ± ru ± chl ± kf

Stage I: py ± po ± cpy ± ga ± sch, Stage II: py ± cpy ± ga ± tell Au–Ag–As–W– Pb–Mo

Po–py– cpy–mt

Po–py–ga– cpy–sch–wfr mo ± apy ± tell

Au–Cu–W

Au–Ag–W– Cu–Pb–Mo– Bi–Te Stage I: qz– pc–kf–bt–cpx– po–tn–hb–ilm

Qz–cpx–pc– kf–tn–po– py–mt ± gt ± hb ± sch ± bt

Absent

Absent

Absent

Absent

Absent

Absent

Absent

250 to 440

Celtic: 314 ± 31 Wonder North: 307 ± 7 < 1 to 2

Stage I: 300 ± 50 Stage II: 250 ± 50

Stage I: 225 to 400 Stage II: 300 ± 50

200 to 400

270 ± 50

325 ± 50

350 ± 50

Stage I: 425 ± 50 ∼600 Stage II: 280 to 415

Stage I: ∼ 2 Stage II: 0.5 to 2.0 CO2–H2O– NaCl ± CH4 0.06 to 0.19 7 to 10

Stage I:<1–3 Stage II: 0.5 to 3.0

<1 to 2

1.9 ± 0.6

0.7 to 2.6

∼2

Stage I: 2.0 ± 0.7 Stage II: 0.5 to 2.0

CO2–H2O– NaCl ± CH4 0.01 to 0.44 <5.5

CO2–H2O– NaCl ± CH4 Moderate

H2O–CO2– NaCl ± CH4 0.05 to 0.30 3 to 6

CO2–H2O– NaCl ± CH4 0.21 to 0.59 <2

CO2–H2O– NaCl Moderate Low-moderate

CO2–H2O–NaCl

5.3 to 5.6 −43 to−40 −4.0 to −2.0

5.1 to 5.5 −37 to − 40 −3.5 to 0.5 −35 to − 9 (qz, mu) 5.9 to 7.5 (qz)

5 to 6

5.9 −37 −2.5 to −0.5 −104 to −44 (qz, mu) 3.1 to 7.6 (qz, cc)

5 to 6 −35 to −32 −2.5 to −0.0

5.3 to 5.7 −33 to − 31.5 −2.0 to −0.5

−1.8 to 2.8

−10 to 10 (py)

Sulfidation ± phase separation

Fluid-wallrock interaction

Stage I: sulfidation Stage II: phase separation

Fluid:wallrock interaction

Ojala (1995); Ojala et al. (1993, 1997); Cassidy et al. (1998); b Salier et al. (2005)

Allen (1987); Cassidy (1992); Bucci et al. Cassidy Cassidy et al. (1998); (2002, 2004) et al. (1998) Fletcher et al. (1998). δ34S: pers. com. P. Hodkiewicz (2001)

Cassidy (1992); Kent et al. (1996); Cassidy et al. (1998); Hagemann and Cassidy (1999)

∼2

Solutes

H2O–CO2– NaCl

Reference

Qz–ab–mu– cc ± chl ± py ± ru ± hm

Stage I: py

Qz–ab–mu– Stage I: qz–bt– ank–py–ru ± pc–cc–py– chl ± kf ± to ± hm kf–mu ± chl ± tn ± mt ± ilm Stage II: qz-chlab-cc-mupy ± ep ± hm ± ru Absent Absent

Pressure (kbar)

X(CO2) Salinity (eq. wt.% NaCl) pH log f O2 log aH2S δDfluid (‰; SMOW) δ18Ofluid (‰; SMOW) δ34SH2S (‰; CDT) Gold deposition mechanism

Qz–cc–wm– chl–py ± ank

Stage I: py– po–cpy Stage II: py– tell–ga– cpy–sph Au–Ag–As– Te–Bi–W–Pb

∼6

3.4 to 8.7 (cc)

−0.6 to 3.7 Fluid-wallrock interaction

Wallrock sulfidation

Phillips and Zhou Winzer (2001) (1999); Kehal and Stephens (2000); Zhou et al. (2003); Mason (2004)

Stage II: phase separation

Cassidy and Bennett (1993); Cassidy et al. (1998)

Fluid:wallrock inter-action; temperature and pressure decrease Nelson (1996); Duuring et al.; (2001,2004); Fletcher et al. (2001)

13.2 to 14.7 (dol) −9.8 to 0.5 (ga, py) Fluid-wallrock interaction

13.0 to 15.5 (dol) −0.7 to 6.1 (py) Fluid-wallrock interaction

Duuring et al. (2000); Salier et al. (2005)

Salier et al. (2001, 2004, 2005); Drieberg et al. (2004)

a

Absent

∼4 to 6

Stage I: 600 ± 50 Stage II: 600 ± 25 3 to 4

CO2–H2O– NaCl ± CH4 Variable 5 to 10

Moderate ∼5

I: 4.2 to 5.8, II: 1.9 to 4.2

Stage II: qz-pckf-bt-hb-po ± mu ± cd ± sil ± ilm Absent

−55 to −69

5.0 to 6.0 − 19 to −17.5 0.0 to 1.0 − 63 to −29

6.4 to 8.6

7.3 to 8.9

2.2 to 3.5 (py, po, mo)

P. Duuring et al. / Ore Geology Reviews 32 (2007) 157–186

Deposit-scale zonation Ore fluid Temperature (°C)

Py–cpy ± ga ± bi ± bis ± ara ± tell

Abbreviations: ab, albite; am, amphibole; ahd, anhdrite; ank, ankerite; ara, aramayoite; ap, apatite; apy, arsenopyrite; bi, native bismuth; bis, bismuthinite; bt, biotite; cb, carbonate; cc, calcite; cd, cordierite; chl, chlorite; cpy, chalcopyrite; cpx, diopside; Cr, chromiumrich; dol, dolomite; ep, epidote; fu, fuchsite; ga, galena; gt, garnet; hb, hornblende; hm, hematite; ilm, ilmenite; inter, intermediate; kf, alkali-feldspar; mo, molybdenite; mon, monazite; mt, magnetite; mu, muscovite; pc, plagioclase; po, pyrrhotite; py, pyrite; qz, quartz; ru, rutile; sch, scheelite; ser, sericite; sil, sillimanite; sph, sphalerite; tell, tellurides; tn, titanite; to, tourmaline; tr, tremolite; wfr, wolframite; wm, white mica; xe, xenotime; zr, zircon. Deposits are listed from left to right in order of increasing regional peak metamorphic facies.

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Fig. 3. Covariate plots A to F show the redox state (Fe2O3/FeO) and degree of fractionation (Rb/Sr) for important granitoid hosts to orogenic-, intrusion-related-, and porphyry-styles of mineralization in the Yilgarn Craton. Granitoid host data (from Cassidy et al., 2002b) are divided into four groups based on their compositional affinities (Fig. 3A–D; High- and Low-Ca granitoids, Mafic granitoids, Mafic porphyries, and Syenites; cf. Champion and Sheraton, 1997). Granitoid composition fields are also shown for the Ravensthorpe (Fig. 3E; data from Witt, 1998) and Boddington (Fig. 3F; unpublished data) deposits. Each covariate plot displays: (i) a dashed rectangle that represents the compositional range for granitoid hosts to Phanerozoic intrusion-related Au systems in Eastern Australia (after Blevin, 2004), (ii) an irregular-shaped dotted line that defines the compositional range for granitoid hosts to Phanerozoic porphyry Cu–Au systems in Eastern Australia (after Blevin et al., 1996), and (iii) an ellipse that shows the compositional range for granitoid hosts to intrusion-related Au–Bi systems (after Baker et al., 2005).

and certainly less important than structural setting and fluid flux. In the Archean St. Ives gold camp near Kambalda in Western Australia, Neumayr et al. (2003, 2004, sub-

mitted for publication) report that hydrothermal sulfide– oxide–gold mineral assemblages indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time. Reduced and

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oxidized alteration assemblages (pyrrhotite–loellingite and magnetite–pyrite, hematite–pyrite, respectively) in individual deposits occur in distinct parts of the camp but, significantly overlap locally and specifically in the largest deposits (e.g., Victory-Defiance deposit). Deposits that contain oxidized mineral assemblages are focused on gravity lows, which are interpreted to reflect abundant felsic porphyritic intrusions of about 1000 m below the present surface. The spatial distribution and temporal sequence of sulfide–oxide–gold assemblages indicate the presence of at least two broadly synchronous or sequential, spatially restricted, hydrothermal fluids with variable redox states. Gold grades are apparently highest where the redox state of the hydrothermal fluid switches from relatively reduced pyrrhotite–pyrite to relatively oxidized magnetite–pyrite and hematite– pyrite, both in space and time. 3.2. Relative timing of granitoid emplacement and gold mineralization Integrated studies on the geochronology of plutonism, peak metamorphism, and gold mineralization in individual granitoid-associated gold deposits are rare. Fig. 4 and Table 1 summarize available granitoid emplacement and gold mineralization ages for granitoidassociated gold deposits in the Yilgarn Craton. In all of these orogenic gold deposits, granitoid emplacement ages are older than main-stage gold events, with minimum age differences between these two events ranging from 14 m.y. at Tarmoola to 2 m.y. at Granny Smith (Fig. 4). At the Wallaby deposit, TIMS U–Pb analysis on magmatic titanite in syenite constrains the timing of magmatism to 2664 ± 3 Ma, whereas SHRIMP U–Pb analysis on gold-related phosphate minerals constrains gold mineralization to 2650 ± 6 Ma, thereby demonstrating their incongruence (Salier et al., 2004). Despite these chronological relationships, Drieberg et al. (2004) cite the presence of pegmatites, miarolitic cavities, and magmatic-hydrothermal breccia pipes as evidence for a genetic link between syenite magmatism and gold mineralization at Wallaby. 3.3. Structures, veins, wallrock alteration, and ore fluid characteristics Orogenic gold deposits that are spatially associated with granitoids are mainly sited along granitoid contacts, are hosted by the intrusions and adjacent supracrustal countryrock (e.g., Tarmoola, Granny Smith, Porphyry), but also occur exclusively within granitoid intrusions up to 6 km in horizontal distance from granitoid margins

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(e.g., Federal/Golden Cities, Wonder North). Most deposits are hosted in sub-greenschist-facies metamorphic terranes, corresponding to temperatures of below 450 °C, pressures of 2 to 3 kbar, and crustal depths of less than 10 km (Table 1), However, gold deposition took place over a range of crustal temperatures (250° to ∼600 °C), pressures (∼1 to 6 kbar), depths (2 to 15 km), and metamorphic facies (lower-greenschist to upper-amphibolite; Fig. 5). The depth at which gold deposition occurs influences (i) the nature of goldbearing structures (i.e., ductile vs. brittle), (ii) hydrothermal vein and wallrock alteration mineral assemblages, and (iii) the relative timing of gold mineralization with respect to granitoid emplacement and regional metamorphism (cf. Groves, 1993). Structures that post-date the emplacement of the hostgranitoid are the primary means for transporting and focusing ore fluids from source areas to depositional sites. Granitoid bodies commonly cause rheological heterogeneity in surrounding ductile supracrustal rocks, which influences the regional stress field and provides loci for hydrothermal fluid flow (Cassidy et al., 1998; Witt and Vanderhor, 1998). The shape of the granitoid with respect to the orientation of the regional stress field determines local areas of low mean stress, fluid focusing, and gold deposition (Groves et al., 1995; Duuring et al., 2001). High-grade ore shoots commonly occur in structures that are oriented parallel to granitoid margins (e.g., Tarmoola, Granny Smith, Porphyry), within flexures in fault or shear zones (e.g., Wonder North), at the intersection between two structures (e.g., Federal/Golden Cities), or where structures intersect favorable lithologies (e.g., syenite dikes at Jupiter). Isotropic granitoids commonly deform as massive, competent bodies with gold-bearing structures that strike subparallel to the direction of σ1 (e.g., Jupiter, Lady Bountiful, Tarmoola). Gold-bearing vein types include extensional and extensional-shear vein sets, shear zone-hosted, ladder veins, stockwork veinlets, and vein breccias; several of these structures may be present in individual deposits (Table 1). Brittle to brittle-ductile structures are common in low- to mid-greenschist grade deposits, whereas brittle-ductile to ductile structures prevail in higher metamorphic grade deposits. Using the descriptive classification for gold mineralization styles of Robert et al. (2005), granitoid-associated orogenic gold deposits commonly host quartz-carbonate veins (e.g., Tarmoola, Granny Smith, Westonia) and to a lesser extent, disseminated-stockwork styles of mineralization (e.g., Wallaby). Sulfidic replacement styles are rare in that they involve the sulfidic replacement of banded iron formations (e.g., Nevoria) or else they form in epizonal environments (Robert et al., 2005). Sulfide-rich veins

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Fig. 4. Summary of age constraints for granitoid emplacement and mineralization events in granitoid-associated metal deposits in the Yilgarn Craton. Geochronological data are presented in decreasing mean age of the host pluton; geochronological techniques are indicated by symbols. A. Orogenic deposits. B. Syn-magmatic deposits. Data are from the following sources: Tarmoola (Nelson, 1996; Fletcher et al., 2001), Cleo (Brown et al., 2002), Great Eastern (Fletcher et al., 1998), Granny Smith (Campbell et al., 1993; Salier et al., 2005), Wallaby (Salier et al., 2004, 2005), Chalice (Bucci, 2001; Bucci et al., 2004), Mt Mulgine (Oliver, 1999), Tower Hill (Fletcher et al., 2001; Witt et al., 2002), Boddington (McCuaig et al., 2001; Stein et al., 2001), and Nevoria (Mueller et al., 2004). Note that the Great Eastern mineralization age is shown as a dashed line to indicate the lower precision of that date. The age constraint for orogenic Au at Tarmoola is based on the age of a pre-mineralization porphyry dike (Fletcher et al., 2001).

and semimassive to massive sulfide lenses are also rarely expressed in granitoid-associated deposits. Hydrothermal alteration assemblages developed during gold mineralization are temperature-and pressuredependent (Table 1; McCuaig and Kerrich, 1998). Greenschist-facies deposits have vein and hydrothermal proximal wallrock minerals that mainly include white mica, albite (An < 5), ankerite, calcite, and chlorite, with minor tourmaline and biotite. Greenschist to lower-

amphibolite-facies deposits contain mostly hydrothermal biotite, chlorite, actinolite, alkali-feldspar, plagioclase (An < 20) and calcite, whereas mid-to upper-amphibolitefacies deposits comprise mostly hydrothermal amphibole, clinopyroxene, biotite, alkali-feldspar, plagioclase (An20–30), and calcite. Gold-bearing veins have the ore assemblage of mainly pyrite, chalcopyrite, and galena, with lesser arsenopyrite, tellurides, pyrrhotite, molybdenite, sphalerite, and scheelite, and rare magnetite and

P. Duuring et al. / Ore Geology Reviews 32 (2007) 157–186

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Fig. 5. Schematic cross-section that shows the distribution and nature of orogenic gold deposits that are spatially associated with granitoids in the Yilgarn Craton. Abbreviations: ab, albite; ank, ankerite; bt, biotite; calcic am, calcic amphibole; cc, calcite; chl, chlorite; cpx, clinopyroxene; kf, alkali feldspar; pc, plagioclase; po, pyrrhotite; py, pyrite; tn, titanite; wm, white mica.

wolframite (Table 1). Pyrite is more common at low metamorphic grades, whereas pyrrhotite-dominated assemblages occur at higher grades. Deposit-scale zonation of hydrothermal minerals, metals, or fluid compositions with vertical or lateral distance from granitoid hosts is largely absent in granitoid-associated orogenic gold deposits in the Yilgarn Craton (Table 1). Hydrothermal mineral assemblages in wallrock only occur on the scale of cm to tens of meters. Despite hydrothermal mineral assemblages that vary with depth and metamorphic grade, chemical properties of gold-bearing hydrothermal fluids are relatively uniform (Table 1). Fluid properties include (i) H2O– CO2 ± CH4 ± N2 ± H2S components, (ii) a high CO2 (X [CO2] mainly 0.1 to 0.3), (iii) low to moderate salinity (< 10 equiv. wt.% NaCl), and (iv) a reduced, neutral to slightly alkaline (5.0 to 6.0) character. Relative to background element concentrations in unmineralized granitoid rocks (cf. Perring et al., 1990), Au in orogenic granitoid-associated deposits is commonly enriched by up to a factor of 105; Ag is enriched by 103; As, W, Bi, Mo, Te, and Sb by 102 (Table 2). Lead, Cu and Zn are locally enriched by up to 102 times background levels. Westonia is atypical in that W was present in sufficient

concentrations for it to be recovered as a by-product during gold mining (Mueller, 1988). Hydrothermal fluid histories commonly comprise multiple stages of hydrothermal influx, which may invoke strain hardening, crack-seal vein deformation, and vein and hydrothermal wallrock mineral deposition (e.g., Tarmoola, Lady Bountiful). The spatial and temporal association of gold with sulfide minerals in wallrock, in combination with findings from fluid inclusion and thermodynamic studies, suggest that gold was transported as a reduced sulfur species and was deposited late in the hydrothermal fluid history of each deposit (e.g., Tarmoola, Lady Bountiful) due to a variety, and often a combination of, processes, including fluid-wallrock interaction, fluid phase separation, and fluid mixing. Geochemical and isotopic tracers (i.e., O, H, S, C, Pb) in orogenic gold deposits (Table 1) do not give definitive evidence for the hydrothermal fluid source (cf. Ridley and Diamond, 2000) but they do suggest that the fluids are deeply-sourced, wallrockequilibrated, metamorphic or magmatic fluids that interacted with granitic rocks along the flow path (Cassidy et al., 1998). Shallow-level deposits contain fluids that potentially have a minor contribution from

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Table 2 Highest element abundances for gold-bearing veins in selected granitoid-associated orogenic gold deposits, compared with average background element concentrations for unmineralized granitoids in the Yilgarn Craton

V Cr Co Ni Cu Zn As Rb Sr Mo Ag Sn Sb Te Ta W Au Pb Bi Reference

Lady Bountiful

Tarmoola

Jupiter

Celtic/ Wonder North

Granny Smith (Stage II)

Unmineralized granitoids in the Yilgarn Craton

127 43

19 93 124 4228 5400 148 90 36 404 16 417

93 32

235 194 65 168 561 150 24 77 173 100

40 36 20 39 31 54 26 86 860 2 0.5 3 0.5 0 0.6 45 1.93 22 0.5 Ojala (1995)

75 30 10 15 25 52 1.9 110 440 1 0.05 2 0.2 0 2.3 2.7 0 15 0.2 Perring et al. (1990)

33 204 59 71 866 150

0.2

16 99 197 3 87 496 12.2 0.5 2 3

27 41 <7

110 47 46 78 38 Cassidy and Bennett (1993)

0.2 500 265 2860 29 Duuring (2002)

2 319 243 293 3 Duuring (2002)

50 <5 Winzer (2001)

Element concentrations are given in parts per million.

surface meteoric waters (e.g., Granny Smith: Ojala, 1995). 3.4. Differences with orogenic gold deposits hosted by supracrustal rocks Although granitoid-associated orogenic gold deposits share many characteristics with orogenic gold deposits hosted by supracrustal rocks, there are subtle differences in structural style and ore fluid characteristics (cf. Cassidy et al., 1998; Groves et al., 1998). For instance, goldbearing structures in granitoids (e.g., fault zones and stockwork veinlets) are commonly more brittle than those structures that occur in supracrustal rocks (e.g., ductile to ductile-brittle shear zones) at the same metamorphic grade. This is largely caused by contrasting competency between granitoids and supracrustal rocks under similar metamorphic conditions. This relationship is well illustrated at Jupiter by laterally continuous gold-bearing shear zones that display brittle fabrics in syenite but brittle-ductile fabrics in basalt countryrock (Duuring et al., 2000). A similar difference in structural style was noted for the Lady Bountiful deposit (Cassidy and Bennett, 1993). Furthermore, corresponding wallrock hydrothermal alteration and gold mineralization zones are wider in the host granitoids (syenite at Jupiter; granodiorite at Lady Bountiful) than in the surrounding metabasaltic and

doleritic host lithologies due to the extensively developed brittle fracture network in the granitoids. Supracrustal rocks are more likely to have existing anisotropy, such as a regional penetrative foliation, ductile to ductile-brittle shear zones, or compositional layering (in the example of sedimentary rocks). Heterogeneity in granitoids may include syn-magmatic phase boundaries, countryrock clasts, cross-cutting intrusions or brittle to brittle-ductile structures; where present, regional penetrative foliation in granitoids is generally weakly developed. Existing anisotropy in supracrustal rocks commonly influences the location and orientation of orebodies, through reactivation and channelization of ore fluids to produce narrow orebodies associated with shear zones at a high angle to σ1. In contrast, the more isotropic granitoids commonly have gold-bearing structures that are more likely to be oriented subparallel to σ1. At Tarmoola, a regional penetrative foliation is strongly developed in talc-carbonate-altered serpentinite but is not evident in the trondhjemite. Consequently, orebodies in the serpentinite are associated with brittle-ductile shear zones that are oriented parallel to the schistosity (i.e., N-striking), whereas gold-bearing veins in the trondhjemite are oriented parallel to the direction of σ1 (i.e., E-striking). Hydrothermal wallrock alteration assemblages vary with differing host rock chemistries and metamorphic

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grade but they are mostly characterized by the major addition of CO2, H2O, K, S, and Si, with the minor addition/removal of Ca and Na (McCuaig and Kerrich, 1998). At Tarmoola, carbonate minerals are present in greater abundance in komatiite wallrock than in trondhjemite due to the reaction between hydrothermal fluid CO2 and the higher proportion of ferromagnesian wallrock minerals in the komatiite (Duuring et al., 2004). In addition, carbonate minerals hosted by komatiite contain higher proportions of Fe and Mg relative to carbonate minerals in trondhjemite wallrock, whereas gold-bearing veins and proximal hydrothermal alteration zones in trondhjemite have greater absolute concentrations of Mo and W, relative to komatiitehosted gold-bearing veins and proximal hydrothermal alteration zones. 4. Other mineralization styles associated with granitoids in the Yilgarn Craton 4.1. Archean intrusion-related Au–Mo–W and Mo ± Au systems Intrusion-related gold systems in Phanerozoic terranes are well documented compared to intrusion-related gold systems in Archean terranes, including the Yilgarn Craton. The characteristics of Phanerozoic intrusionrelated gold systems are presented in several recent deposit-scale studies (e.g., Shotgun: Rombach and Newberry, 2001; Timbarra: Mustard, 2001, 2004; Petráckova hora: Zacharias et al., 2001) and are effectively summarized in review papers by Thompson et al. (1999), Lang et al. (2000), Thompson and Newberry (2000), Lang and Baker (2001), Baker et al. (2005), and Goldfarb et al. (2005). Phanerozoic intrusion-related gold systems are mostly small, low-grade (< 1 g/t) deposits that are genetically related to reduced, calc-alkaline, fractionated, I-type granitoids cut by goldbearing pegmatite and aplite dikes (e.g., Emerald Lake: Duncan, 1999). Importantly, the granitoid hosts to Phanerozoic intrusion-related gold systems are chemically distinct from granitoid hosts to porphyry-style systems; Phanerozoic intrusion-related Au–Bi systems are commonly associated with relatively reduced (low Fe2O3/FeO ratios of 0.1 to 0.6) and highly fractionated (high Rb/Sr ratios of 0.1 to 1) granitoids, compared with the more oxidized and less fractionated granitoids associated with porphyry systems (Blevin et al., 1996; Baker et al., 2005). The Timbarra deposit in Eastern Australia is atypical of intrusion-related gold systems in that, although the granitoid host is highly fractionated, it is weakly oxidized (Mustard, 2001, 2004). Structures are

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commonly important in transporting carbonic hydrothermal fluids, containing Au–Bi–W–Mo–Te ± Sb, from the pluton into surrounding countryrock (e.g., Clear Creek: Stephens et al., 2000, 2004; Scheelite Dome: Mair et al., 2000; Donlin Creek: Ebert et al., 2000 and Goldfarb et al., 2004; Fort Knox: Bakke, 1995). Sulfides are not common and hydrothermal alteration zones are weakly developed and spatially restricted. Metal zonation is locally apparent, with more Ag ± base metals located in more distal areas to intrusion-related systems or in epizonal examples (Goldfarb et al., 2005). Like orogenic gold systems, intrusion-related gold systems display a wide variety of mineralization styles that depend on the (i) depth of intrusion emplacement and gold deposition, (ii) lateral distance from the parent intrusion, (iii) countryrock host type, and (iv) structural controls (Thompson and Newberry, 2000). Deposits occur at a wide range of temperatures (< 200° to > 600 °C: Lang and Baker, 2001) and pressures (0.5 to 3.0 kbar: McCoy et al., 1997; Baker and Lang, 1999; Lang and Baker, 2001), which relate to depths of gold deposition (< 2 to 10 km) and the proximity to the parent intrusion. The deposits are located in a tectonic setting that is inboard of recognized convergent plate margins, in provinces that contain W and Sn mineralization (Mitchell and Garson, 1981; Sawkins, 1984). Granitoids associated with intrusion-related gold mineralization in the Tombstone belt of the Yukon display a mixed mantle and crustal signature, suggesting that they formed due to the interaction between mantle-derived mafic alkaline magmas and crustal components that they intrude (Mair et al., 2003). The crustal component is considered important in causing the reduced oxidation state of the melts and for increasing the potential for gold to be concentrated (Goldfarb et al., 2005). The coincidence of these unusual granitic parent magmas with an inboard tectonic setting suggests that the settings for intrusion-related gold systems may be rare in the geological record (Groves et al., 2005) and certainly less common than orogenic gold systems. Hence, the occurrence of analogous Archean intrusion-related systems in the Yilgarn Craton would require the coincidence of several factors, including magmas in a similar tectonic setting, their preservation, and lastly their recognition as being different from orogenic gold systems associated with granitoids. Not surprisingly, there are few documented examples of intrusion-related systems in the Yilgarn Craton. This section describes likely examples and compares their characteristics with Phanerozoic examples. Boddington: The Boddington deposit is located in the Saddleback greenstone belt of the Southwest Gneiss

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Terrane (Fig. 1) and is one of the largest gold resources in the Yilgarn Craton (current gold resource and past production >26 Moz Au: McCuaig et al., 2001). Dioritic to andesitic intrusive and volcanic rocks (ca. 2715 to 2690 Ma) are intruded by ca. 2675 Ma granodioritetonalite intrusions and a 2612 ± 3 Ma monzogranite pluton, with associated aplite and pegmatite dikes (Allibone et al., 1998; McCuaig et al., 2001). Two metallogenic events are recognized at Boddington. An early, 2707 ± 17 Ma (Re–Os on molybdenite: Stein et al., 2001), Cu–Au–Mo–Bi event is associated with quartz– albite–molybdenite ± clinozoisite ± chalcopyrite veins and broad, silica–biotite alteration and in early dioritic to andesitic rocks, which are cut by ductile shear zones. A later, 2623 ± 9 Ma (Re–Os on molybdenite: Stein et al., 2001), Au–Mo event is defined by quartz– albite–molybdenite ± muscovite ± biotite ± fluorite ± clinozoisite ± chalcopyrite, clinozoisite–sulfide–quartz– biotite, actinolite ± sulfide ± quartz, carbonate–chlorite– sulfide veins which cut all other lithologies and are centered around the late monzogranite intrusion (McCuaig et al., 2001). Metals are zoned around the late monzogranite pluton, with an inner zone of Au, Cu, Mo, Bi, and W mineralization surrounded by an outer zone of Pb, Zn, and Ag mineralization (McCuaig et al., 2001). Whole-rock geochemistry on the intrusive rocks at Boddington indicates that the early dioritic phases have Fe2O3/FeO and Rb/Sr ratios that overlap the field of ‘intrusion-related’ Au–Bi mineralization (Fig. 3F; Baker et al., 2005). The dioritic suite at Boddington is the only known suite to fall within the Baker et al. (2005) Au–Bi field and contrasts with the late monzogranite that has a higher degree of fractionation and relative oxidation (Fig. 3F). With the exception of the late monzogranite, all lithologies at Boddington have been subject to moderate to intense hydrothermal alteration and are overprinted by regional metamorphism; the effect of these processes on igneous Fe2O3/FeO and Rb/Sr ratios is unknown. Roth and Anderson (1993) propose that fluid inclusions associated with the early mineralization event consist of H2O–CaCl2–NaCl and contain about 15 to 21 wt.% CaCl2 and 10 to 15 wt.% NaCl (total salt content of 37 wt.%). They homogenized at 280 to 340 °C and, if a pressure correction of 90 °C is applied, suggest a trapping temperature of about 370 to 430 °C. McCuaig et al. (2001) and Hagemann et al. (2002) summarize new fluid inclusion data on the second mineralization event, including sulfide-rich veins that control most of the high-grade gold mineralization. This fluid inclusion data are consistent with exsolution of a volatile phase directly from a magma (at > 600 °C, > 60

equiv. wt.% NaCl, 1.5 kbars), followed by the cooling, depressurization, and boiling of the fluid (at ca. 300 to 550 °C, 0.6 to 0.15 kbar), and the incursion of meteoric water (ca. 150–300 °C, < 10 equiv. wt.% NaCl) (McCuaig et al., 2001). Rare, isolated aqueous-carbonic, carbon–dioxide–methane and pure methane inclusions of unknown timing may be evidence for early metamorphic fluids and, the fact that they display cogenetic liquid and vapor-rich end-members, may suggest that they were trapped during phase immiscibility at about 350 to 400 °C. Importantly, there is no evidence that these inclusions are accompanied by significant gold or other metal precipitation. Several genetic models exist for Boddington, including (i) a deformed and metamorphosed syn-magmatic porphyry deposit (Roth, 1992), (ii) a post-peak metamorphism, shear zone-hosted deposit (Allibone et al., 1998), and more recently, (iii) a two stage model that includes an early porphyry-style Cu–Au–Mo event overprinted by an intrusion-related Au–Mo event (McCuaig et al., 2001). Several features are consistent with a magmatic-hydrothermal model for both the early ‘porphyry-style’ and late ‘intrusion-related’ mineralization, including (i) contemporaneous emplacement of intrusive suites (early diorites, late monzogranite) and formation of Au ± Cu ± Bi ± Mo-bearing veins, (ii) fluid inclusion data indicating involvement of high-temperature, high-salinity fluids of probable magmatic origin for both early and late gold-bearing veins, (iii) metal association (Cu, Au, Bi, Mo, W) and zonation, and (iv) hydrothermal alteration assemblages consistent with the involvement of saline aqueous fluids. Features typical of orogenic gold systems, such as a spatial relationship to a crustal-scale (first-order) fault zone network, limited lateral metal zonation with low concentrations of molybdenum and base-metals (especially Cu), and the involvement of a low-salinity, CO2bearing fluid, are not present at Boddington. Although high-grade Au veins are associated with the second mineralizing event, the overall proportion of gold related to each event is unclear with additional studies required to determine whether Boddington represents early ‘porphyry-style’ or late ‘intrusion-related’ Cu– Au–Bi–Mo mineralization. Understanding the relative timing of regional metamorphism to both mineralizing events may determine which event was dominant and provide a critical element in conceptual exploration models for this type of deposit. The affinity of the early dioritic intrusions with intrusions associated with Phanerozoic Au–Bi mineralization may indicate that the early mineralization is indeed ‘intrusion-related’ rather than ‘porphyry-style’.

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Mount Mulgine: Tungsten, Mo, Bi, Ag, Cu, and Au mineralization (∼ 5 t Au: Oliver, 1999) is spatially associated with the fractionated, I-type, 2767 ± 10 Ma (SHRIMP U–Pb on zircon: Oliver, 1999) Mount Mulgine syenogranite in the Murchison Province (Table 3). The syenogranite contains primary magnetite and titanite, has geochemical affinities with High-Ca group granitoids, and intruded the folded supracrustal sequence during regional peak amphibolite-facies metamorphism, resulting in a locally-developed metamorphic foliation in the granitoid (Oliver, 1999). Molybdenite and scheelite occur primarily in stockwork quartz–pyrite–fluorite veins and disseminated within greisen alteration zones in the syenogranite, but are also present in surrounding banded-iron formations (BIF) and metavolcanic rocks. Molybdenum is more spatially restricted to the syenogranite than W, which commonly occurs in supracrustal rocks up to 3 km from the syenogranite margin (Todd, 1995). Studies on fluid inclusions trapped in W–Mo-bearing veins identify both highly saline, aqueous fluid inclusions (Migisha, 1983) and low-salinity, CO2-rich, fluid inclusions (Todd, 1995). The temporal relationship between the two fluid inclusion types has not been determined and, hence, do not constrain the ore fluid source. However, the spatial association between W–Mo and the Mount Mulgine syenogranite, in addition to metal zonation away from the granitoid, suggests these metals were derived from the intrusion (e.g., Watkins and Hickman, 1990; Todd, 1995; Mueller and McNaughton, 2000). Likewise, the Mount Mulgine syenogranite has chemical affinities with ‘intrusion-related’ Cu–Mo and W– Mo systems in Phanerozoic terranes (Fig. 3A). Gold is mainly hosted by shear zones in supracrustal rock located to the west of the Mount Mulgine intrusion (Grigson, 1999; Oliver, 1999). It is possible that synmagmatic Mo–W mineralization is overprinted by later orogenic Au ± W mineralization. Elsewhere in the Murchison Province (e.g., Mount Gibson: Yeats et al., 1996; Reedy's Area: Wang et al., 1995), gold mineralization coincides with the Yilgarnwide, 2650 to 2630 Ma gold mineralization event. For the most part, the gold deposits have features consistent with orogenic gold-style mineralization although several studies have suggested a magmatic origin for at least some of the hydrothermal fluids. For instance, a fluid inclusion study on the Mount Gibson gold deposit (Straub et al., 1995, 1996) provided evidence for primary high-salinity (up to 47 wt.% NaCl equiv.) aqueous inclusions with homogenization and dissolution temperatures of up to 600 °C in quartz veins associated with the orogenic-gold style mineralization.

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These authors suggest a magmatic-hydrothermal fluid source for at least the orogenic part of the Mount Gibson gold system. Nevoria and Corinthian: Debate surrounds the genesis of gold-rich deposits hosted by amphibolitefacies metamorphosed, calc-silicate-altered supracrustal rocks that are spatially associated with magnetite-series granites and granite–pegmatite complexes in the Southern Cross Province. Hagemann et al. (2000) argue that these deposits represent hypozonal orogenic gold deposits, rather than syn-magmatic gold skarns, based on (i) the absence of an universally-present contemporaneous pluton and associated endoskarn, (ii) the lack of calcareous rocks, and (iii) low-salinity (< 10 equiv. wt.% NaCl), CO2-bearing ore fluids. Furthermore, detailed structural-hydrothermal studies at the Copperhead, Yilgarn Star, and Westonia gold deposits suggest that the calc-silicate alteration assemblages are pre-, syn-, and post-orogenic and are not necessarily contemporaneous with spatially associated felsic intrusions or indicative of a magmatic source for gold (Cassidy and Hagemann, 1999; Hagemann and Cassidy, 1999; Hagemann et al., 2000; Witt et al., 2001a; Grainger and Hagemann, 2002). In contrast, the Nevoria and Corinthian deposits are classified as syn-magmatic Au– W ± Mo ± Ag metasomatic skarns based on their metal content, gangue mineralogy (Mueller, 1997), and their spatial and temporal association with late (2640– 2630 Ma) “post-orogenic”, I-type granitic and pegmatite intrusions (Mueller et al., 2004). The mineralized granitoids post-date a major phase of compressional deformation by up to 20 m.y. and amphibolite-facies metamorphism (2775–2724 Ma: Mueller and McNaughton, 2000) by at least 90 m.y., although amphibolite-facies metamorphism is recorded elsewhere in the region at approximately the same time as the emplacement of ‘post-kinematic’ granitoids (Kent et al., 1996). At Nevoria (∼ 12 t Au production: Mueller et al., 2004), 2634 ± 4 Ma pegmatites (SHRIMP U–Pb on zircon: Qiu et al., 1999) and 2635.7 ± 1.2 Ma skarn mineralization zones (TIMS U–Pb on allanite in almandine: Mueller et al., 2004) are more abundant in the upper 80 m of the granite and are more calc-silicate altered and enriched in Au, As, Bi, Cu, W, and Zn relative to less-altered granitoid phases. The skarns are interpreted to have formed from aqueous, CO2-bearing (low-CH4), moderate salinity (∼ 10 wt.% CaCl2 + NaCl) fluids at temperatures of 550 to 600 °C and pressures of 300 to 400 MPa, consistent with 11 to 14 km crustal depths (Mueller et al., 2004). Radiogenic strontium isotope ratios of scheelite (0.7032 to 0.7033) at Nevoria are consistent with a magmatic fluid source, and in

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Table 3 Characteristics of selected Archean intrusion-related Au–Mo–W and Mo ± Au systems plus porphyry Cu ± Mo ± Au systems in the Yilgarn Craton, Western Australia Intrusion-related Au–Mo–W and Mo ± Au systems

Resource (t Au) Granitoid host Type

Mount Mulgine

Tower Hill

Chalice (Stage II)

Granny Smith (Stage I)

Nevoria

Ravensthorpe Tonalite

>808

∼5

∼20

∼1

∼1

∼12

∼4

Monzogranite, aplite, pegmatite

Syenogranite

Biotite monzogranite, felsic porphyry

Biotite monzogranite

Granodiorite

Granite

Tonalite

Dike 2 = Oxidized (∼1.11) Dike 2 = ∼1.43 2626 ± 9 “Post-orogenic” Mafic and ultramafic amphibolite

Oxidized

Oxidized

Reduced (0.36–0.55)

2665 ± 4 “Pre-orogenic”

2634 ± 4 “Post-orogenic” BIF, pegmatites

Redox state (Fe2O3/FeO) Rb/Sr Absolute age (Ma) Relative age Other hosts

Structural regime Ore shoot control

Mineralized vein type

Oxidized

2612 ± 3 (monzogranite) Volcanic, volcaniclastic rocks

Shallow (< 5) Upper-greenschist to lower-amphibolite Stage I: 2707 ± 17 Stage II: ca. 2623 ± 9 Brittle-ductile Intersecting faults, faults and competent rock types

2767 ± 10 “Pre-orogenic” Metavolcanic rocks, BIF, biotite schists, quartz porphyry

2753 ± 6 “Pre-orogenic”? Ultramafic schist

Amphibolite

Amphibolite

W–Mo ± Au: ca. 2767? Orogenic Au–W: 2631 ± 7 Brittle-ductile Mt Mulgine granitoid, shear zones in supracrustal rocks

2755 ± 9 to 2752 ± 9 Brittle-ductile Shear zone along the Raeside batholith contact; plunge of fold hinges and boudinaged veins Shear zone-hosted

0.1 to 0.2 ca. 2990 to 2970 Volcaniclastic rocks, basalt

Deep (12–14) Mid-to upperamphibolite 2621 ± 10

Shallow to inter. (< 5–8) Upper-greenschist

Deep (11–14) Amphibolite

Shallow (< 5) Upper-greenschist to lower-amphibolite ca. 2990 to 2970

ca. 2665?

2635.7 ± 1.2

Ductile Late monzogranite

Brittle Granodiorite

Ductile to ductile-brittle Upper 80 m-thick zone of granite, proximal to pegmatites and skarn alteration

Brittle-ductile Shear zones, proximity to tonalite pluton

Foliation-discordant veins, disseminated gold

Quartz-rich veinlets and microfractures

Skarn in granitoid

Shear zone-hosted in volcaniclastic rocks, minor disseminated metals in tonalite

Shear zone-hosted, stockwork

Stockworks in granitoid and quartz porphyry, shear zone-hosted in supracrustal rocks

Stage I: qz–ab ± cl Stage II: qz–ab ± mu ± bt ± flu ± cl ± am ± cb ± chl Stage I: mo ± cpy ± py Stage II: mo ± cpy ± py ± po ± ga ± sch Inner: Au–Cu–Mo–Bi–W Outer: Pb–Zn–Ag Stage I: qz–bt

Qz–mu–flu

Qz–cb

Qz–ab–am–cpx–tn

Qz

Qz

Qz

Mo–py–sch

Py–cpy–mo–bis–tetra– apy–po–sph–ga–sch

Mo–tell–po–py–mt– cpy–sch

Mo–py–hm

Po–sch–cpy–mal–bi–apy– loel

Cpy–py–po–mt ± tetra ± tell ± sph ± co

W–Mo–Be–Bi ± Au ± Ag ± Cu Qz–mu–py

Au–Bi–Mo–Te

Au–W–Mo–Bi–Sr–Sn– Zn–Cu–V–Rb–Ta–Nb–Th Hm

Al–Au–As–Ag–Bi–Cu–Nb– W–Zn–Te–Mo Hed–ac–al–hb–bt–qz–pc

Au–Cu–Ag ± Zn ± Sb ± Te ± As ± Bi Propylitic and potassic

Deposit-scale zonation

Lateral metal zonation away from granitoid

Lateral metal zoning away from granitoid

Au–Cu–Bi–Sb–Mo–Ni– As–W Qz–cb–chl–py ± bt ± mu ± pc None described

Absent; pervasive hm alteration

Lateral and vertical metal zoning

Reference

Roth (1992); Allibone et al. (1998); McCuaig et al. (2001); Stein et al. (2001)

Oliver (1999); Mueller and McNaughton (2000)

Schiller and Hanna (1990); Fletcher et al. (2001); Witt (2001); Witt et al. (2002)

Bucci (2001); Bucci et al. (2002, 2004)

Skarn concentrated in upper roof zone of granite Qiu et al. (1999); Mueller et al. (2004)

Vein mineralogy Silicate, carbonate minerals

Ore minerals

Metal association Wallrock alteration

Qz–cpx–pc–kf–tn–po– py–mt ± gt ± hb ± sch ± Absent

Ojala et al. (1993); Ojala (1995)

Savage (1992); Witt (1998)

Abbreviations: ab, albite; ac, actinolite; al, almandine; am, amphibole; apy, arsenopyrite; bi, bismuth tellurides; bt, biotite; cb, carbonate; chl, chlorite; cpy, chalcopyrite; cl, clinozoisite; co, cobaltite; cpx, diopside; flu, fluorite; ga, galena; gt, garnet; hb, hornblende; hed, hedenbergite; hm, hematite; inter, intermediate; kf, alkali feldspar; loel, loellingite; mal, maldonite; mo, molybdenite; mt, magnetite; mu, muscovite; pc, plagioclase; po, pyrrhotite; py, pyrite; qz, quartz; sch, scheelite; sph, sphalerite; tell, tellurides; tetra, tetrahedrite; tn, titanite; BIF, Banded Iron Formation.

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Crustal depth (km) Metamorphic grade of supracrustal sequence Age of metallogenesis (Ma)

Porphyry systems

Boddington

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combination with the temporal and spatial coincidence between post-orogenic pegmatite intrusions and skarn mineralization, have been used to support an intrusionrelated model for the Nevoria and Corinthian deposits (Mueller et al., 2004). The gold deposits have been compared to the reduced, deep-seated (150 to 250 MPa), Au–W skarns in the North American Cordillera and in the Paleozoic fold belt of the French Pyrenees in terms of their similar silicate gangue (i.e., almandine-rich garnet) and sulfide mineralogy (i.e., bismuth minerals) (Mueller et al., 2004). The post-kinematic granitoids at Nevoria are variably fractionated and relatively reduced (Fig. 3A), similar to some Phanerozoic intrusion-related W–Mo systems in eastern Australia. However, further studies are required to determine if these deposits represent discrete intrusion-related Au–W ± Mo ± Ag systems or if they are akin to other hypozonal orogenic gold systems. Tower Hill: The Tower Hill and Harbour Lights gold deposits in the Leonora district of the Eastern Goldfields Province display relative structural timing relationships and radiometric ages for Mo ± Au-bearing veins that indicate the potential for an early timing for at least some gold deposition (Witt, 2001; Witt et al., 2002), as well as a possible genetic link with felsic magmatism (Witt, 2001). The Leonora gold deposits are located proximal to the eastern margin of the ca. 2760 to 2660 Ma (Witt et al., 2002) Raeside batholith and are hosted by the batholith and adjacent ultramafic schist (Skwarnecki, 1987). At Tower Hill (∼ 20 t Au resource: Witt, 2001), Mo ± Au-bearing quartz veins are located subparallel to an intensely developed regional foliation in ultramafic schist oriented parallel to the granitoid contact. The early veins are folded, boudinaged, and offset by numerous post-mineralization deformation events, including younger Au-bearing veins (Witt et al., 2002). A massive monzogranite pluton at Tower Hill has a SHRIMP U–Pb zircon age of 2753 ± 6 Ma (Fletcher et al., 2001), whereas quartz veins hosting Mo ± Au mineralization have a Re–Os age of between 2755 ± 9 and 2752 ± 9 Ma (Witt et al., 2002). No detailed geochemical, fluid inclusion or stable isotope studies have been performed on the early Mo ± Au hydrothermal fluids; nor is there any documentation of systematic metal zonation away from the Raeside batholith near Tower Hill. The close spatial and temporal association between Mo ± Au mineralization and monzogranitic phases of the Raeside batholith is the main evidence for a magmatic contribution to early Mo ± Au-bearing hydrothermal fluids (Witt et al., 2001b). Chalice Stage II: The main stage of Au–Cu–W mineralization (Stage I, ∼ 18 t Au resource: Bucci,

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2001) at Chalice occurred at 2644 ± 8 Ma (Bucci, 2001; Bucci et al., 2004) and is sited in a shear zone that formed during the regional upright folding of supracrustal units. This style of mineralization coincides with regional amphibolite-facies metamorphism and is analogous to shear zone-hosted orogenic gold mineralization present elsewhere in the Yilgarn Craton. A second minor phase of Au–Bi–Mo–Te mineralization (Stage II, ∼ 1 t Au resource: Bucci, 2001) took place at 2621 ± 10 Ma (Re–Os on molybdenite: Bucci, 2001; Bucci et al., 2004) and is spatially and temporally associated with a slightly oxidized (Fe2O3/FeO = 1.11: Bucci et al., 2002), 2626 ± 9 Ma monzogranite dike (SHRIMP U–Pb on zircon: Bucci, 2001; Bucci et al., 2004). Evidence supporting a magmatic source for Stage II mineralization includes (i) molybdenite-gold veins and disseminated Au in the monzogranite dike, (ii) gold in textural equilibrium with igneous quartz and feldspar, (iii) Stage II monzogranite and molybdenite-gold veins that cut Stage I gold-bearing shear zones, (iv) overlapping absolute ages for Stage II gold and the monzogranite host, and (v) oxygen and hydrogen stable isotope data, which suggest that the Stage II ore fluid was derived from magmatic fluids that were modified during the cooling and interaction with host rock sequences (Bucci, 2001; Bucci et al., 2004). The absence of deposit-scale hydrothermal mineral and ore metal zonation around the monzogranite intrusion may reflect the small size of the Stage II mineralizing event. The more granitophile metal suite of the Stage II fluid, relative to the Stage I fluid, reflects a spatial and temporal association between the ore fluid and felsic intrusions, but does not necessarily imply a genetic relationship (Bucci et al., 2002). Early intrusion-related, barren to subeconomic mineralization and alteration: In several locations across the Yilgarn, there is evidence for minor – generally barren but locally (sub)economic – hydrothermal alteration that is most likely magmatic in origin. For example, at Granny Smith, syn-magmatic Mo ± Au mineralization (∼ 1 t Au resource: Ojala, 1995) is associated with quartz-rich veinlets and fractures and broad hematite alteration in the granodiorite (e.g., Ojala, 1995). Relative to background metal concentrations in unmineralized granitoid rocks (cf. Perring et al., 1990), syn-magmatic Granny Smith mineralization is characterized by Au (enriched by a factor of 102), W (101), and variable, but mostly minor (< 101), Mo, Bi, Sr, Sn, Zn, Cu enrichment (Ojala, 1995). The syn-magmatic gold mineralization is overprinted by quartz–carbonate-white mica vein-and shear zone-related orogenic gold mineralization. Orogenic gold (∼ 50 t Au resource: Ojala,

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1995) is enriched against background levels by up to a factor of 103, W, Ag, and As are enriched by 101, whereas Sb, Ni, Bi, Mo, Sr, Sn, Zn, Cu, and Cr show minor (< 101) enrichment (Ojala, 1995). This style of sub-economic Mo ± Au mineralization and associated syn-magmatic hematite alteration is also present at the Porphyry deposit (Cassidy et al., 1998). At SunriseCleo, quartz–chalcopyrite–molybdenite veins are spatially related to and cut rhyodacite porphyry dikes. The veins are interpreted to have formed at 2663 ± 11 Ma (Re–Os on molybdenite) during the waning stages of felsic magmatism, shortly after porphyry dike emplacement at 2674 ± 3 Ma (SHRIMP U–Pb on zircon: Brown et al., 2002). Porphyry dikes and quartz–chalcopyrite– molybdenite veins are cut by the main 2654 ± 8 Ma orogenic gold event (Brown et al., 2002). Other deposits that display early, possibly syn-magmatic, alteration includes Wallaby (Salier et al., 2004), Jupiter (Duuring et al., 2000), and Darlot-Centenary (Kenworthy and Hagemann, 2005). In each of these deposits, the early and uneconomic magmatic-hydrothermal events are overprinted by orogenic gold systems. In summary, Archean intrusion-related gold systems in the Yilgarn Craton are, with the exception of Boddington, small (<10 t Au) relative to many granitoid-associated orogenic gold deposits. In deposits that exhibit multiple mineralization events, intrusion-related mineralization is subordinate in size to orogenic gold mineralization (e.g., Chalice, Granny Smith, Sunrise-Cleo). Intrusion-related systems are spatially and temporally associated with a range of felsic intrusions (Fig. 6) that were emplaced at intermediate (<5–10 km at Granny Smith and Boddington) to deep (11 to 14 km at Chalice and Nevoria) crustal depths. Similarly to granitoid-associated orogenic gold deposits, orebodies are also hosted by adjacent supracrustal countryrocks (e.g., amphibolite and BIF at Nevoria), although, intrusion-related orebodies are less likely to be primarily controlled by structures associated with crustal-scale shear zones. The timing of intrusionrelated systems varies with respect to orogenic events; some systems pre-date greenstone belt-wide orogenic events (e.g., Mount Mulgine, Tower Hill) and are thus highly deformed (e.g., Tower Hill), whereas others postdate orogenic events (e.g., Nevoria, Chalice Stage II). Archean intrusion-related systems may host a range of ore minerals that include, in decreasing order of common occurrence, pyrite, chalcopyrite, gold, molybdenite, scheelite, pyrrhotite, with minor occurrences of bismuthinite, arsenopyrite, telluride minerals, galena, and sphalerite (Table 3). These minerals reflect a metal association that includes mainly Au, Cu, Mo, Bi, and W. Metal distribution is generally low-grade and associated with

quartz veinlets, microfractures, and pervasive alteration in the granitoid-host (e.g., Boddington, Mount Mulgine, Granny Smith) and within shear zones or veins in proximal supracrustal countryrock (e.g., Tower Hill, Nevoria). Alteration mineral and metal zonation surrounding granitoid plutons are features common to intrusion-related systems but rarely expressed in granitoid-associated orogenic gold systems. Fluid inclusion data from Boddington and Mount Mulgine suggest the involvement of high temperature and moderate- to highsalinity fluids. These fluid compositions for intrusionrelated systems contrast with ore fluids responsible for granitoid-associated orogenic gold systems, which consistently comprise H2O–CO2 ± CH4 ± N2 ± H2S, a high CO2 content [X(CO2) mostly 0.1 to 0.2], low to moderate salinity (< 10 equiv. wt.% NaCl), and are reduced (Table 1). Archean and Phanerozoic intrusion-related systems display similarities in terms of their spatial and temporal affiliation with fractionated felsic intrusions, occurrence over a wide range of crustal depths, metal content, lateral (±vertical) zonation with hydrothermal alteration and metallic mineral assemblages relative to host intrusions, and ore fluid compositions. 4.2. Porphyry Cu–Mo–Au, Cu–Mo, and Cu–Au systems Phanerozoic porphyry Cu ± Au systems are dissimilar to Phanerozoic intrusion-related gold systems in that they are generally associated with dominantly oxidized, magnetite-series and less-fractionated calc-alkalic to alkalic magmatic suites that have affinities with magmatic and/or island arcs (e.g., Thompson et al., 1999; Sillitoe, 2000), whereas the latter are associated with more reduced I-type, calc-alkalic intrusions that contain W, Bi, As ± Sb (Lang and Baker, 2001). Hydrothermal fluids in porphyry systems commonly have a higher salinity but lower levels of methane and CO2 than intrusion-related gold systems (cf. Sillitoe, 1991; Baker, 2002). Although apparently rare, there are several possible examples of Archean porphyry Cu ± Mo ± Au systems in the Yilgarn Craton. Ravensthorpe: The Ravensthorpe greenstone belt occurs in the southern part of the Southern Cross Province (Fig. 1) and includes a ca. 2.97–2.99 Ga calcalkaline complex comprising co-magmatic andesitic volcanic rocks, dacite porphyry dykes and tonalite plutons (e.g., Manyutup tonalite: Savage et al., 1995; Witt, 1998). The co-magmatic relationship is supported by geochemical data, which also indicates that the suite has a range of Rb/Sr and Fe2O3/FeO ratios that roughly overlap the Phanerozoic porphyry Cu–Au field of Blevin et al. (1996; Fig. 3E). Copper–Au–Ag mineralization

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Fig. 6. Schematic plan that shows the lateral distribution of orebodies associated with Archean, syn-magmatic, metallogenic deposits in the Yilgarn Craton.

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(∼4 t Au resource: Witt, 1998) and pervasive propylitic and/or potassic alteration occur along the margins of the Manyutup tonalite, as well as up to 2 km from the tonalite margin, where it is associated with narrow (< 2 m wide) shear zones and quartz veins (Witt, 1998). Copper–Au– Ag mineralization is associated with chalcopyrite, gold, pyrite, magnetite, with lesser pyrrhotite, ilmenite, cobaltite, sphalerite, tetrahedrite, and Bi ± Ag tellurides. Locally within the tonalite, sulfide and oxide inclusions are unrelated to fractures and other secondary minerals and may be magmatic inclusions (Witt, 1998). Copper: Au ratios increase with depth in the tonalite (Sofoulis, 1958), whereas Cu:Au and Ag:Au ratios decrease laterally away from the tonalite (Savage, 1992; Witt, 1998). A high-salinity (> 60 equiv. wt.% NaCl), hightemperature (> 550 °C) ore fluid has been identified in Cu–Au–Ag–bearing veins (Savage, 1992). Competing genetic models for Cu–Au–Ag mineralization at Ravensthorpe include (i) a magmatic-hydrothermal origin (Sofoulis, 1958; Savage, 1992), where mineralization is genetically related to the co-magmatic calcalkaline igneous suite, (ii) stratiform sulfide mineralization that is remobilized in adjacent volcanic and volcaniclastic rocks during regional metamorphism and deformation (Marston, 1979), (iii) syn-metamorphic mineralization (cf. Witt, 1993), or (iv) submarine, synvolcanic mineralization (Witt, 1998). The spatial association between Cu–Au–Ag mineralization and the calcalkaline, oxidized Manyutup tonalite, in addition to the pervasive propylitic and local potassic alteration, vertical and lateral zonation of ore metals away from the tonalite, and the presence of high-temperature, high-salinity ore fluids, are all suggestive of syn-magmatic, and possibly porphyry-style, Cu–Au mineralization at Ravensthorpe. Although many features are consistent with a porphyrystyle Cu–Au model, structurally-controlled mineralization away from the pluton margins may reflect remobilization of some of the metals during subsequent regional metamorphism and deformation events. Elsewhere in the Yilgarn Craton, there are several minor Cu ± Au deposits with some characteristics of porphyry Cu ± Au systems. For instance, Majestic (20 kg Au: Witt, 1997) displays vein and alteration characteristics that are similar to porphyry Cu ± Au systems, including stockwork quartz–carbonate–chlorite–epidote–pyrite– gold ± biotite veins in porphyritic felsic dikes and lateral changes in alteration, from an inner chlorite–rich–pyrite ± chalcopyrite–galena (with relict biotite) assemblage to an outer albite–and muscovite–pyrite ± chalcopyrite–galena assemblage (Witt, 1997; Roberts et al., 2004). In summary, Archean porphyry Cu ± Mo ± Au systems are rare in the Yilgarn Craton, probably due to their

shallow depth of formation and their corresponding low potential for preservation (Groves et al., 2005). Where they occur, they are spatially and temporally associated with volumetrically small, pervasively altered, plutons and dikes that are surrounded by altered and mineralized supracrustal countryrock (e.g., Ravensthorpe, Majestic, and possibly early mineralization event at Boddington). In Phanerozoic terranes, porphyry-style systems appear to be restricted to shallow (< 5 km) emplacement depths. Depth estimates for porphyry-style mineralization in the Yilgarn are poorly constrained, however, relative timing criteria indicate that mineralization was prior to regional metamorphic events and associated with intrusive suites that were likely subvolcanic. In contrast, a wide range of crustal depths of formation is estimated for intrusionrelated (< 5 to 14 km) and granitoid-associated orogenic gold systems (2 to 15 km). Countryrock up to 2 km away from porphyry centers may be mineralized, with shear zones and faults in supracrustal rocks acting as conduits to magmatic-hydrothermal ore fluids. Like Archean intrusion-related gold systems, metal ratios and alteration minerals in porphyry systems vary with distance from magmatic centers; however, they also appear to be more enriched in Cu and commonly display relict potassic alteration (e.g., Ravensthorpe and Majestic). Ore minerals occur mainly in quartz veins and shear zones, although disseminated sulfides are also present in granitoids. Metals (Cu, Au, Ag, Pb, Zn) and alteration mineral assemblages (potassic and phyllic) are commonly zoned around the pluton and in adjacent countryrock. Where microthermometry studies have been performed, they indicate a high-temperature and saline ore fluid, consistent with derivation from spatially and temporally associated magmatism. 5. Discussion Exploration for Archean granitoid-associated orogenic gold, intrusion-related, and porphyry systems in the Yilgarn Craton requires being able to confidently distinguish between these mineral systems and then deciding which criteria are effective for their discovery. Important differences include the (i) relative timing between granitoid emplacement and mineralization, (ii) granitoid composition (i.e., granitoid chemistry, oxidation state, and degree of fractionation) and the relationship to particular metal suites, (iii) zonation of alteration minerals and metals around the source intrusion, and (iv) depth of ore deposition. Timing of granitoid emplacement and mineralization: Determining the absolute timing of granitoid emplacement and related mineralization is required to distinguish

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granitoid-associated orogenic gold from intrusionrelated and porphyry systems. While a multi-tool approach to dating benefits independent age determinations, caution must be used when comparing error ranges from different geochronological methods (e.g., comparing SHRIMP U–Pb on zircon ages with Re–Os on molybdenite ages); the total error may be greater than the analytical errors for individual techniques. This creates difficulties in discriminating between magmatic and metallogenic events that have narrow time gaps. The successful identification of several discrete mineralization events in a deposit may resolve initial confusion in classifying “anomalous” deposits (e.g., early orogenic Au overprinted by late intrusion-related mineralization at Chalice). Once the timing (and geochemistry) of intrusion and mineralization events are constrained at the deposit scale, exploration can then be shifted towards examining co-genetic granitoids in the same terrane. Granitoid composition and corresponding metal suites: For syn-magmatic systems, granitoid composition and the oxidation state of the parent magma are commonly associated with particular metal suites in a given terrane (cf. Carmichael, 1991; Blevin et al., 1996; Baker et al., 2005). Phanerozoic magmas emplaced proximal to collisional arc settings in Eastern Australia are commonly more mafic, oxidized, and enriched in Cu ± Au ± Mo ± W (i.e., representing I-type porphyry systems) relative to magmas in more distal areas to the collisional arc (Blevin et al., 1996). Distal areas contain more evolved, S-type felsic magmas that have assimilated, or have interacted with fluids derived from, reduced, graphite-bearing, supracrustal countryrock (cf. Ishihara, 1981) and are subsequently reduced and host Mo (± W) and Sn (± W) mineralization (i.e., representing intrusion-related systems). Intermediate areas between proximal and distal settings are Wbearing and weakly oxidized to reduced (Blevin et al., 1996). Intrusion-related Au–Bi deposits commonly occur in the same setting as W-rich granitoids, however, granitoids associated with Au–Bi mineralized have a greater range of SiO2 and are commonly less fractionated than the W-rich granitoids (Baker et al., 2005). Granitoid and ore metal fractionation trends can be measured by comparing Rb/Sr ratios (a measure of magma fractionation) against Fe2O3/FeO ratios (an indicator of oxidation state); for instance, low Rb/Sr but high Fe2O3/FeO indices suggest a low degree of fractionation and an oxidized magma, favoring Cu– Au to W-dominated metal associations. In the Yilgarn Craton, none of the High-Ca and Low-Ca granitoids display Fe2O3/FeO and Rb/Sr ratios that are compar-

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able to porphyry-style Cu–Au systems in Phanerozoic terranes (Fig. 3). The only exceptions are some Syenitic suites and possibly some of the suites in the Ravensthorpe district. In contrast, the early dioritic suites at Boddington overlap the intrusion-related Au– Bi fields of Blevin (2004) and Baker et al. (2005), whereas the Mount Mulgine syenogranite data overlap the general Mo–W fields of Blevin (2004). Metamorphism and hydrothermal alteration may influence the fractionation and oxidation trends of the granitoids, however, the specific effects are unclear. Blevin et al. (1996) suggest that geochemical fractionation trends may prove to be more readily observed than actual tectonic boundaries because structures may not be exposed at the present surface. Precise trends in mineralized magmatic suites are not presently recognized in the Archean Yilgarn Craton, although, recent integrated geochemical and geochronology studies in the Leonora–Laverton district (Cassidy et al., 2002a) demonstrate broad relationships between granitoid composition and age of emplacement, with granitoids becoming more potassic with time (Champion and Smithies, 2001). Similarly, the temporal overlap between gold mineralization and the emplacement of some mafic and syenitic granitoids in the Eastern Goldfields Province suggests that magmatic and hydrothermal fluids were at least present at the same time and in the same terrane. Interestingly, a spatial and temporal relationship exists between Archean syenites and Au–Mo–Cu in the Superior Province of Canada (Robert, 2001); this association is yet to be fully tested in the Yilgarn Craton (e.g., Wallaby: Salier et al., 2004; Drieberg et al., 2004). This difficulty in identifying precise magmatic-metallogenic trends in Archean terranes is probably due to the incomplete preservation of syn-magmatic systems but it may also be due to fundamental differences between Archean and Phanerozoic tectonic processes. While the overlapping spatial occurrence of orogenic gold and porphyry systems in Archean terranes suggest that collisional arc settings were forming during the Archean and are not restricted to the Phanerozoic, the higher heat flow and greater plume activity during the Archean may be responsible for the anomalously high abundance of granitoids in cratons away from collisional arcs (Groves et al., 2005) and may be the cause of the lack of clear metallogenic trends away from clearly defined collisional arcs. Zonation of alteration minerals and metals: Geochemical zonation of alteration minerals and metals surrounding porphyry and intrusion-related granitoid centers may be used to vector towards the core of

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mineralized intrusive systems. Lateral zonation away from Archean porphyry systems in the Yilgarn Craton may include a decrease in Cu:Au and Ag:Au ratios (e.g., Ravensthorpe), whereas Au–Mo ± Bi ± W ± Cu are enriched in the centers of intrusion-related systems and Pb–Zn–Ag are dominant in distal zones (e.g., Boddington, Mt Mulgine). Structures that focus magmas and ore fluids may complicate otherwise concentrically zoned, geochemical patterns. Depth of ore deposition: Archean porphyry systems, like their Phanerozoic analogues, are restricted to shallow crustal depths. Hence, by estimating formation pressures and depths for presently exposed intrusions and surrounding supracrustal countryrock (using fluid inclusion and mineral chemistry geobarometric techniques) areas of a terrane may be identified that are more likely to preserved porphyry systems. Unfortunately, this is difficult to apply universally because porphyry systems that form during the early constructional tectonic phase of an orogen are commonly eroded or overprinted by subsequent deformation and fluid events, which modify the porphyry systems and preferentially record the properties of the later orogenic fluids. In contrast, orogenic systems that form during deformation and metamorphism of an accreting terrane, or intrusionrelated systems that form more distal to active convergent plate margins, are more likely to preserve fluid inclusion and stable isotope data that reflect their conditions of formation. 6. Conclusions Archean terranes, such as the Yilgarn Craton in Western Australia, commonly host multiple styles of granitoid-associated deposits, including orogenic, intrusion-related, and porphyry systems. Orogenic gold systems are prevalent; they form over a broad crustal depth range (2 to 15 km) and are mostly sited along the margins of granitoid plutons. Competent granitoid bodies within ductile supracrustal rock influence the regional stress field and focus hydrothermal fluids toward low mean stress zones. Granitoid host rock composition has little effect compared to structural controls on the siting of orebodies. Intrusion-related systems are less common and, with the exception of Boddington, are smaller (< 10 t Au) than orogenic gold systems. Intrusion-related systems are spatially and temporally associated with fractionated felsic plutons and dikes. Metal suites hosted by the granitoids and proximal supracrustal countryrock include mainly Au, Mo, Bi, W, and Cu, with minor As, Ag, Zn, Te, Nb, Pb, Sn, and Sb. Metals and hydrothermal alteration mineral

assemblages are laterally zoned away from magmatic centers. Porphyry systems are spatially and temporally associated with volumetrically small, pervasively altered, calc-alkalic to alkalic plutons and dikes. Vertical and lateral zonation of metals away from porphyry intrusions commonly includes an increase in Cu:Au ratios with depth and a decrease in Cu:Au and Ag:Au ratios with lateral distance away from the source pluton. Microthermometry studies on fluid inclusions in goldbearing veins indicate high-temperature (> 550 °C) and high-salinity (> 60 equiv. wt.% NaCl) ore fluids that deposited the metals at shallow crustal depths (< 5 km). Porphyry-style and intrusion-related granitoid hosts in Phanerozoic terranes show characteristic fractionation (Rb/Sr) and oxidation (Fe2O3/FeO) ratios. In the Yilgarn Craton, only some Syenitic suites and possibly some of the suites in the Ravensthorpe district have fractionation and oxidation trends comparable to Phanerozoic porphyry-style Cu–Au systems. Early dioritic suites at Boddington exhibit compositional similarities to intrusion-related Au–Bi systems, whereas the Mount Mulgine syenogranite is comparable to intrusion-related Mo–W granitoids. Exploration for orogenic gold systems lies primarily in identifying mineralized structures in granitoids or adjacent supracrustal rocks and predicting areas of low mean stress and fluid focusing in threedimensional space. Exploration for intrusion-related and porphyry systems hinges mainly in establishing the temporal overlap between granitoid emplacement and mineralization, granitoid compositions, and metallogenic trends across a terrane. Porphyry systems form at shallow crustal depths; hence, the identification of granitoid emplacement depths by geobarometry is potentially important in exploring for porphyry systems. Acknowledgments This paper is based on a doctoral study funded by an Australian Postgraduate Award scholarship and PacMin Corporation Ltd. The Society of Economic Geologists is gratefully acknowledged for the awarding of McKinstry research grants (1999 and 2000) to Paul Duuring. Kevin Cassidy publishes with the permission of the Chief Executive Officer of Geoscience Australia. Elizabeth Colgan helped with drafting the figures. The manuscript benefited from early reviews by Kevin Ansdell, Tim Baker, and Steve Rowins, plus discussions with Richard Goldfarb and with members of the Department of Geology and Geophysics at the University of Western Australia, including Louis Bucci, Elizabeth Colgan, Craig Hart, Paul Hodkiewicz, and John Mair. Reviewers, Tim Baker, Roger Bateman, and

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