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0361-0128/11/3933/1-31 1 Introduction GOLD AT Bendigo is hosted in quartz veins and saddle reef associated with both folds and faults emplaced in a series of Ordovician turbidites. This study investigates whether pyrite textures, chemistry, and trace element zonation can be used to assist in understanding the sequence of ore-forming events. Polished thin sections and polished section blocks en- abled petrographic analysis of sulfide minerals and their host rock, both under the microscope and using the laser ablation- inductively coupled plasma mass spectrometer (LA-ICPMS). Pyrites were acid etched prior to petrographic study, then subjected to LA-ICPMS 4500 analysis at CODES (Hobart, Tasmania) and were analyzed to quantify trace elements using a combination of laser spots and lines. Samples exhibiting zoning were mapped using a technique developed in house (Danyushevsky et al., in press; Large et al., 2009). This tech- nique is a powerful tool in defining separate pyrite growth zones and hence mineralizing fluid events, giving an immedi- ate overview into concurrent trace element availability in an accessible format. This study shows that pyrite growth occurred from sedi- mentation to late metamorphism, and because later pyrite Pyrite and Pyrrhotite Textures and Composition in Sediments, Laminated Quartz Veins, and Reefs at Bendigo Gold Mine, Australia: Insights for Ore Genesis HELEN V. THOMAS, 1,† ROSS R. LARGE, 1 STUART W. BULL, 1 VALERIY MASLENNIKOV , 2 RON F. BERRY, 1 ROD FRASER, 3 SHANE FROUD, 3 AND ROBERT MOYE 1 1 CODES ARC Centre of Excellence in Ore Deposits, Private Bag 126, University of Tasmania, Australia 7001 2 Institute of Mineralogy, Russian Academy of Science, Urals Branch, Miass 3 Bendigo Mining Limited, 66 Ham St, Bendigo, Victoria, Australia 3555 Abstract The various types, textures, and compositional zoning of pyrite in the gold-bearing saddle reefs, quartz veins, and surrounding sedimentary rocks provide new information on the potential source and timing of gold and arsenic and related fluid processes responsible for mineralization at Bendigo. Nodular diagenetic pyrite in the black shale tops to sandstone turbidites is enriched in invisible gold and arsenic with mean values of 0.61 ppm Au, 1,300 ppm As, and Au/Ag <1, based on LA-ICPMS analyses. Other elements enriched in the diagenetic pyrite within the organic-rich shales are Mn, Zn, Mo, Cu, V, Ba, Ag, Cd, Tl, Co, Ni, Bi, Pb, and Te. In contrast, euhedral- and growth-zoned hydrothermal pyrite in the turbidites and bedding-parallel laminated quartz veins contains lower contents of most trace elements but has higher contents of invisible Au and As, especially on the outermost rim of the pyrite. The gold-rich pyrite rims generally become thicker (a few to hundreds of microns) in proximity to the gold-bearing saddle reefs. Pyrite in the reef commonly has the highest levels of invisible Au and As and the lowest levels of other trace elements. It is characterized by Au/Ag >1 and Au/Pb >0.01. In the deepest stratigraphic levels, below known productive gold reefs, diagenetic pyrite in the most car- bonaceous shales has been replaced by pyrrhotite during metamorphism. LA-ICPMS analyses reveal that the disseminated pyrrhotite contains similar levels of Ni and Co to the diagenetic pyrite but is strongly depleted in As and Au. The spatial relationships between organic-rich shales, folded bedding-parallel laminated quartz veins, and gold-arsenic-bearing saddle reefs, combined with the consistent trends in the trace element com- position of pyrite hosted by these three geological elements, is interpreted to indicate that the black shales were an initial source of Au and As, and the laminated quartz veins acted as the initial pathways for hydrothermal fluid flow carrying Au and As from the source shales to the saddle reefs. Maximum gold and arsenic input into the reefs, principally as free gold plus arsenopyrite, occurred late during deformation toward the end of the hydrothermal cycle and is expressed by the Au-As-rich rims to hydrothermal pyrite in the sedimentary host rocks, laminated quartz veins, and reefs. This corresponds with final fold lockup and the development of through-going fault arrays linking adjacent anticlines. The source of Au and As for this final, and most eco- nomically important, fluid-flow event is considered to be from carbonaceous shales deeper in the basin, where original gold-bearing diagenetic arsenian pyrite reacts with organic matter and is converted to pyrrhotite, with release of Au, As, and S to the metamorphic fluid. Corresponding author: email, [email protected] ©2011 by Economic Geology, Vol. 106, pp. 1–31 Submitted: March 9, 2010 Accepted: October 5, 2010 Economic Geology BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS VOL. 106 January–February 2011 NO.1
31

Pyrite and Pyrrhotite Textures and Composition in Sediments, Laminated Quartz Veins,

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  • 0361-0128/11/3933/1-31 1

    IntroductionGOLD AT Bendigo is hosted in quartz veins and saddle reef associated with both folds and faults emplaced in a series ofOrdovician turbidites. This study investigates whether pyritetextures, chemistry, and trace element zonation can be usedto assist in understanding the sequence of ore-formingevents. Polished thin sections and polished section blocks en-abled petrographic analysis of sulfide minerals and their hostrock, both under the microscope and using the laser ablation-inductively coupled plasma mass spectrometer (LA-ICPMS).

    Pyrites were acid etched prior to petrographic study, thensubjected to LA-ICPMS 4500 analysis at CODES (Hobart,Tasmania) and were analyzed to quantify trace elements usinga combination of laser spots and lines. Samples exhibitingzoning were mapped using a technique developed in house(Danyushevsky et al., in press; Large et al., 2009). This tech-nique is a powerful tool in defining separate pyrite growthzones and hence mineralizing fluid events, giving an immedi-ate overview into concurrent trace element availability in anaccessible format.

    This study shows that pyrite growth occurred from sedi-mentation to late metamorphism, and because later pyrite

    Pyrite and Pyrrhotite Textures and Composition in Sediments, Laminated Quartz Veins,and Reefs at Bendigo Gold Mine, Australia: Insights for Ore Genesis

    HELEN V. THOMAS,1, ROSS R. LARGE,1 STUART W. BULL,1 VALERIY MASLENNIKOV,2RON F. BERRY,1 ROD FRASER,3 SHANE FROUD,3 AND ROBERT MOYE1

    1CODES ARC Centre of Excellence in Ore Deposits, Private Bag 126, University of Tasmania, Australia 70012Institute of Mineralogy, Russian Academy of Science, Urals Branch, Miass

    3Bendigo Mining Limited, 66 Ham St, Bendigo, Victoria, Australia 3555

    AbstractThe various types, textures, and compositional zoning of pyrite in the gold-bearing saddle reefs, quartz veins,

    and surrounding sedimentary rocks provide new information on the potential source and timing of gold and arsenic and related fluid processes responsible for mineralization at Bendigo. Nodular diagenetic pyrite in theblack shale tops to sandstone turbidites is enriched in invisible gold and arsenic with mean values of 0.61 ppmAu, 1,300 ppm As, and Au/Ag 1 and Au/Pb>0.01. In the deepest stratigraphic levels, below known productive gold reefs, diagenetic pyrite in the most car-bonaceous shales has been replaced by pyrrhotite during metamorphism. LA-ICPMS analyses reveal that thedisseminated pyrrhotite contains similar levels of Ni and Co to the diagenetic pyrite but is strongly depleted inAs and Au. The spatial relationships between organic-rich shales, folded bedding-parallel laminated quartzveins, and gold-arsenicbearing saddle reefs, combined with the consistent trends in the trace element com-position of pyrite hosted by these three geological elements, is interpreted to indicate that the black shales werean initial source of Au and As, and the laminated quartz veins acted as the initial pathways for hydrothermalfluid flow carrying Au and As from the source shales to the saddle reefs. Maximum gold and arsenic input intothe reefs, principally as free gold plus arsenopyrite, occurred late during deformation toward the end of the hydrothermal cycle and is expressed by the Au-Asrich rims to hydrothermal pyrite in the sedimentary hostrocks, laminated quartz veins, and reefs. This corresponds with final fold lockup and the development ofthrough-going fault arrays linking adjacent anticlines. The source of Au and As for this final, and most eco-nomically important, fluid-flow event is considered to be from carbonaceous shales deeper in the basin, whereoriginal gold-bearing diagenetic arsenian pyrite reacts with organic matter and is converted to pyrrhotite, withrelease of Au, As, and S to the metamorphic fluid.

    Corresponding author: email, [email protected]

    2011 by Economic Geology, Vol. 106, pp. 131 Submitted: March 9, 2010Accepted: October 5, 2010

    Economic GeologyBULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

    VOL. 106 JanuaryFebruary 2011 NO. 1

  • often overgrows earlier generations of pyrite, with carefulpetrography, a near-complete sequence of fluid-flow eventshas been determined. Samples of pyrite from sedimentaryrocks were taken from drill core within and around the reefs,but surficial weathering proved too pervasive to recover vi-able pyrite grains from surface exposures. We show that dif-ferent generations of pyrite have characteristic trace elementpatterns, which can be used as a proxy for fluid-flow events.Early diagenetic pyrites are characterized by a low Au/Agratio with elevations in Bi, Ni, Cu, Mo, V, and Pb, whereaslate hydrothermal pyrites are commonly zoned, with a higherAu/Ag ratio and a distinct Au-As correlation. Hydrothermalpyrite in the sedimentary rocks and laminated quartz veinsclose to gold reefs has distinct Au-As rims that increase inthickness in proximity to the reefs. This new information hasenabled a reinterpretation of the source of gold and the hydrothermal fluid-flow processes that formed the gold-bear-ing saddle reefs at Bendigo.

    Regional GeologyThe host rocks to the Bendigo goldfields in western Victoria

    are an ~3-km-thick Lower and Middle Ordovician successionof mass-flow deposited sandstone and mudstone, with lesser

    suspension deposited mudstone and shale, termed the Castle-maine Group (Fergusson and VandenBerg, 2003). Togetherwith the contemporaneous Adaminaby Group in eastern Vic-toria, these form the Ordovician mud pile (Vandenberg etal., 2000) that records Late Cambrian to Late Ordovicianbasin plain sedimentation outboard of the Gondwanan conti-nental landmass (Cas, 1983). Despite the widespread natureof these units and the diversity of graptolite fauna they host,the Castlemaine Supergroup is largely undifferentiated strati-graphically, with laterally discontinuous channelized struc-tures common, preventing correlation over more than a fewkilometers (Cas et al., 1988). The current across-strike extentof this deep water succession is >500 km, and the pervasivedevelopment of chevron folds during Late Ordovician toEarly Silurian east-west contractional deformation of the Be-nambran orogeny has resulted in overall shortening of up to70 percent (Gray, 1997).

    In western Victoria, the turbidite pile is exposed in twostructural elements separated by the north-southtrendingAvoca fault (Fig. 1). The western Stawell zone comprises aCambro-Ordovician succession termed the St Arnaud Group,and the eastern Bendigo zone the Lower and Middle Ordovi-cian succession termed the Castlemaine Group. The base of

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    FIG. 1. Map showing location of the Bendigo gold deposit, Victoria, Australia, with major structural zones and terraneslabeled. Modified from Vandenberg et al. (2000).

  • the Castlemanian is defined by the Lancefieldian Group ofcompositionally variable, laterally extensive lithic sandstonesinterbedded with silicious shales (Cas et al., 1988). Prove-nance data indicate that both successions were derived fromconvergent tectonic activity that affected the adjacent Gond-wanan continental margin; the Delamerian and Ross oroge-nies in South Australia and Antarctica, respectively (Gray andWebb, 1995; Turner et al., 1996; Fergusson and Tye, 1999).

    The most comprehensive study of the Castlemaine Group isthat of Cas et al. (1988). The sandstone beds that characterizethe Ordovician succession have been widely interpreted as tur-bidites due to their broadly lensoidal geometry, and the rangeand arrangement of primary sedimentary structures that areconsistent with the Bouma sequence (Cas et al., 1988). Thelack of subsequent reworking and preservation of interveningfine-grained suspension deposits indicates that they record anentirely deep water environment on the basin plain outboardof the continental rise and slope. In a regional sense, one as-pect of the sedimentology of the Castlemaine Group thatstands out is the thickness and abundance of graptolitic blackshales. In the turbidite sequences to the west and east of theBendigo zone such black shales are relatively rare and fine-grained interbeds are more commonly cherts that have faunalassemblages consistent with a truly pelagic origin (Cas et al.,1988). It has been suggested that the Castlemaine Group wasdeposited in an area of oceanic upwelling that favored organicproductivity, while anoxic bottom conditions favored organicpreservation (Fergusson and VandenBerg, 2003).

    Deformation and mineralization in the Bendigo zone

    The Bendigo zone is characterized by classic upright open totight chevron folds with wavelengths of 150 to 500 m, indicat-ing relatively simple east-west compression during the Benam-bran orogeny (Gray and Foster, 1998; Fergusson and Van-denBerg, 2003). Narrow zones of stronger deformation andcleavage development occur in the hanging walls of the fourmajor north-southtrending high-angle reverse faults that sub-divide the zone (Fig. 1) and are interpreted as a linked systemof imbricates that splay off a midcrustal detachment zone (Coxet al., 1991; Gray and Willman, 1991). A recent seismic surveyshows that the original depth of the sedimentary pile may havebeen >4,000 m, now shortened to its present depth of over 10km (Willman et al., 2010; Cayley et al., in press). The Heath-cote fault zone at the eastern edge of the Bendigo zone repre-sents the exhumation of the detachment zone (Fig. 1).

    The Bendigo goldfield is the largest in Victoria and has pro-duced 529 metric tons (t) of vein-hosted gold (Willman andWilkinson, 1992; Fig. 1). It is hosted by symmetric and per-sistent north-southtrending chevron folds that have con-trolled the geometry of the gold reefs (e.g., Vandenberg et al.,2000). The eastern edge of the field is sharply bounded by themore deformed succession in the hanging wall of the west-dipping Whitelaw fault, but there is a gradational decline ingrade and abundance of mineralized structures to the west(Willman and Wilkinson, 1992).

    Geology of the Bendigo DepositThere have been many studies on the geology and mineral-

    ization at Bendigo (e.g., Cas et al., 1988; Sharpe and MacGee-han, 1990; Cox et al., 1991, 1995; Willman and Wilkinson,

    1992; Gao and Kwak, 1995a, b; Arne et al., 1998; Bierlein etal., 1998, 2001, 2004; Foster et al., 1998; Ramsay et al., 1998;Jia et al., 2000, 2001; Schaubs and Wilson, 2002; Schaubs andZhao, 2002; Willman, 2007; Wood and Large, 2007; Boucheret al., 2008) and only a summary of the major features andcurrent genetic ideas are presented here.

    Gold at Bendigo occurs as nuggets and/or hydrothermalgrains of free gold in quartz saddle reefs and associated spurveins. Alluvial gold was exploited in the early history of thedeposit (18511853), but resources were soon exhausted andreef mining began in 1853 (Willman and Wilkinson, 1992).Gold in quartz reefs is often associated with pyrite, arsenopy-rite, galena, sphalerite with minor chalcopyrite, and pyrrhotite(Sharpe and MacGeehan, 1990). Historically, most of the goldcame from the hinge zones and eastern limbs of three anti-clines, the Garden Gully, New Chum, and Hustlers Lines,with the Deborah Line mined mainly from the 1940s onward(Willman and Wilkinson, 1992). The best mineralized areascorrespond to domal culminations in the fold axes especiallyon the eastern limbs (Fig. 2), however, west-dipping fault andassociated extensional veins also host significant gold (Sharpeand MacGeehan, 1990). The latter comprise bedding-parallellaminated quartz veins (locally termed LQ veins) that oftenjoin quartz saddle reefs forming legs (Fig. 2; Willman andWilkinson, 1992). These usually occur at the boundary be-tween sandy and shaly intervals (Tanner, 1989) due to the in-herent competency contrast. They are thought to form earlyin the deformation and/or metamorphic history as bedding-parallel zones of flexural slip (Cox et al., 1991; Jessell et al.,1994), and it is likely that they acted as fluid pathways early inthe deposit history before being overprinted by crosscuttingstructures. These veins can be correlated over kilometers ofstratigraphy along relict bedding planes and commonly de-velop into saddle reefs (Schaubs and Wilson, 2002).

    The turbidite host rocks at the mine consist of thick sand-stone packages from 10 to 120 m thick, separated by packagesof shale, siltstone, and minor sandstone from less than 5 to 50m thick. At least 15 cycles of sandstone to shale have been de-fined in the mine drilling (Fig. 3). Individual turbidites andturbidite packages generally exhibit an upward fining from0.3- to 4-m-thick sandy turbidites, to 5- to 20-cm shale-richturbidites, and then into a sequence of black and gray shaleup to 30 m thick, which define the major named shale units(Figs. 23; Sharpe and MacGeehan, 1990; Boucher et al.,2008). In the current mining area in the Kangaroo Flat sec-tion, productive gold-bearing quartz reefs are confined to theRailway Shale and Big Blue Shale (Fig. 2) but in other areasof the mine gold reefs are developed in Rowes Shale, InnerShale, and Royal Albert Shale (Fig. 3). The siltstones andshales vary from gray to black, depending on grain size andorganic carbon content (Fig. 4). Mineralogy consists ofquartz, sericite with minor carbonate, chlorite, graphite, andpyrite. The gray siltstones and shales contain 0.04 to 0.20 wtpercent organic carbon and 0.5 to 2 wt percent pyrite, whereasthe black shales contain 0.2 to 2.0 wt percent organic carbonand 1.0 to 6.0 wt percent pyrite. The sandstones are light graymassive units (Fig. 4) with a quartz, albite, sericite mineralogyand contain

  • ppm), Zn (1001000 ppm), Cu (50150 ppm), Ni (50140ppm), As (5140 ppm), U (412 ppm), Mo (140 ppm), andAg (0.10.5 ppm). Previous workers (e.g., Cas et al., 1988)have suggested that the regional turbidite pile at Bendigofines upward, with sandstones dominating the lower stratigra-phy. This is not supported by recent drilling, which has inter-sected a package consisting of more than 500 m of turbiditeswhich contains over 30 percent shales and siltstones belowthe stratigraphic level of the Railway Shale (Fig. 3).

    Alteration halos around the veins and reefs are controlledby the porosity and permeability of the immediate host rockand can comprise minor disseminated porphyroblasts ofpyrite and rare arsenopyrite up to several 10s of metersaway from lodes as well as more widespread sericite andchlorite alteration and carbonate poikioblasts (Cox et al.,1991; Li et al., 1998; Bierlein et al., 2004). Chlorite geother-mometry performed by Li et al. (1998) suggested that thechlorite alteration took place in temperatures of 260 to290C. Recent work by Dugdale et al. (2009) suggested thatcarbonate spots at Bendigo initially formed during diagenesisof the sediments by anaerobic oxidation processes associatedwith a fluid containing methane. During metamorphism, the

    diagenetic spots were overgrown by a second stage of meta-morphic carbonate.

    Previous sulfur isotope studies (Jia et al., 2001; Bierlein etal., 2004) demonstrate that sulfides in the gold reefs are nar-rowly constrained from 7 to +8 per mil with a median of+2.5 per mil, compared to pyrite in the sedimentary hostrocks, which shows a larger spread from 23 to +12 per mil.They interpreted this to reflect variable exchange of ore flu-ids with synsedimentary-diagenetic sulfides present in wallrocks during transport of ore fluid to the site of deposition.

    Jia et al. (2000) analyzed fluid inclusions in quartz from theCentral and North Deborah mine at Bendigo. They found thatfluid inclusions from the Au-bearing quartz veins are generallyreduced in the CH4-CO2-H2O-NaCl system, whereas fluid in-clusions in non Au-bearing quartz veins are more oxidized withcompositions in the CO2-H2O-NaCl or H2O-NaCl systemsonly. Recent fluid inclusion studies by Fu et al. (2009) showthat fluid Br/Cl and I/Cl values reflect interaction with, or de-rivation from, sedimentary rocks enriched in organic matterand provide no evidence of mantle or deeply derived compo-nents. Illite b-spacing index studies indicate peak metamorphictemperatures were around 300C (Wilson et al., 2009), which

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    -700 z

    -800 z

    -900 z

    48700 E 48800 E48600 E

    NBD 171

    NBD 161

    NBD 238

    Stratigraphic column and legend

    Gill Reef

    Garden Gully Anticline

    Scale 1:500

    Grace Sand

    Emily Shale

    Noah Sand

    Railway Shale

    Alexandria Sand

    True Grit

    Christine Unit

    Big Blue Shale

    Western Sand

    Western Shale

    Harris Sand

    Gold Reef

    Laminated Quartz Vein

    Fault

    Drill hole trace

    Anticline/Syncline trace

    FIG. 2. Mine stratigraphic section Garden Gully anticline, Kangaroo Flat, Bendigo, redrawn from drill sections providedby Bendigo Mining Ltd. The majority of samples were taken from drill holes shown in the diagram by thick solid lines. Redlines represent laminated quartz veins; a gold reef is located in the anticlinal axis of the Railway shale (colored red). Synclineand anticline axes are indicated with dotted lines, faults are indicated with dashed lines. Additional samples were takenthroughout the entire stratigraphy drilled at Bendigo and are discussed in Thomas et al. (in prep). Units are Bendigonian inage.

  • is somewhat lower than the hydrothermal fluid inclusion tem-peratures of 350 25C determined by Jia et al. (2000).Ar/Ar dating of sericite from a gold-bearing extensional veinfrom the Central Deborah mine gave an age of 439 2 Ma(Foster et al., 1998). Arsenopyrite and pyrite grains from anauriferous quartz vein in the Deborah anticline were dated as438 6 Ma, using the Re-Os method by Arne et al. (2001).

    Despite the long history of mining of these deposits, thereis no consensus on the source of gold. Workers, includingCrawford and Keays (1987) and Keays (1987), suggested thatgold may have been sourced from deep in the Cambrianboninites in the lower part of the greenstone pile that under-lies the turbididic succession, whereas Bierlein et al. (1998)suggested that the gold may have been sourced from Cambrianmetatholeiites and interflow sediments that form the upperpart of the greenstone pile. Suggested sources for the ore-generating fluids are metamorphic fluids originating from thegreenschist-amphibolite boundary at depths >12 km (Phillipset al., 2003), or released from a subduction slab break-off atdepths >200 km (Vos et al., 2007). These previous studies, fo-cused on the source of the gold, have omitted to consider thesource of arsenic, which is in far greater abundance in theores than gold. We provide evidence in this paper that thesource of both gold and arsenic was most likely the thick pileof Ordovician turbiditic sediments that also host the deposits.

    Methodology and Analytical TechniquesAnalytical instrumentation employed in this study consists of

    a New Wave 213-nm solid-state laser microprobe coupled to anAgilent 4500 quadrupole ICPMS. One pyrite sample (NBD171-544.8 fish) was mapped on a slightly different set-up uti-lizing the New Wave 213-nm solid-state laser microprobe cou-pled to an Agilent 7700 quadrupole ICPMS; both housed atthe CODES LA-ICPMS facility at the University of Tasmania.The method is described in detail in Large et al. (2009) and(Danyushevsky et al., in press) and summarized here below.

    The laser microprobe is equipped with a small volume abla-tion cell (~2.5 cm3) characterized by

  • A set of 20 elements was chosen for analysis, with acquisi-tion time for the majority of elements set to 0.002 s; excep-tions were Se (0.004 s), and Ag, Te, and Au (0.04 s). Totalsweep time was ~0.2 s. A 13-s delay was left after each line toallow for cell washout. Background levels and drift were mea-sured on the primary standard before and after every image(Danyushevsky et al., in press). Maps were generally gener-ated over a period of 1 to 2 h where drift in sensitivity is min-imal. Occasionally, larger maps were generated and a driftcorrection was undertaken, with instrument drift consideredto be linear between the standards. Image processing in-volved drift correction (if necessary), application of a medianfilter to remove artefacts generated during processing, sub-traction of background from filtered counts, replacement offiltered counts less than background with the standard devia-tion value for that element; finally, images were produced foreach element using a logarithmic color scale. Redeposition of

    sample back onto the surface as an ablation plume was keptto a minimum by preablating each line immediately prior toanalysis. Some elements are particularly prone to redistribu-tion; this is especially evident in the noise present around na-tive gold inclusions.

    Pyrite and Pyrrhotite Types and TexturesBased on methods developed in previous studies on pyrite

    in sediment- and volcanic-hosted gold deposits (e.g., Muminet al., 1994; Huston et al., 1995; Large et al., 2007, 2009), themorphology and internal structure and zoning of pyrites havebeen used here as a guide to the timing relationship betweenpyrite growth and gold events.

    Diagenetic pyrite in the sedimentary rocks occurs as fine-grained rounded or nodular aggregates aligned parallel to bed-ding, comprised of intergrowths of acicular and very fine cubicpyrite and marcasite (Figs. 5A-C, 6, 7). Diagenetic pyrite is

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    5 cm

    1 cm

    1 cm

    1 cm

    1 cm

    A

    C D

    B

    FE

    FIG. 4. Rock lithologic units at Bendigo. A. Core tray from drill hole NBD 212W1, showing lithologic units in the Rail-way Shale. Dark bands are carbonaceous shale, light bands are gray siltstone. A bedding-parallel laminated quartz vein oc-curs in the middle of the tray. B. Gray sandstone, typical of the massive sandstones in the basal parts of thick turbidite pack-ages. C. Carbonaceous and pyritic shale, typical of the fine tops to the sandstone turbidites. D. Black shale and gray siltstone,with euhedral pyrite developed in the base of the siltstone. E. Example of a bedding-parallel laminated quartz vein (LQ),typical of those which connect to the saddle reefs. These veins are characterized by their laminated texture, consisting ofwhite quartz and dark-colored selvage (chlorite, sericite, graphite). Pyrite present in laminated quartz veins is hydrothermalin origin, typically euhedral and coarse grained. F. Underground photograph of the Gill reef, a typical gold-rich saddle reefat Bendigo (Mine Northing 124357 MN; provided by Bendigo Mining).

  • preferentially hosted in black shale units, which provide theanoxic-reducing environment necessary for their growth. Noframboidal pyrite was observed in this study at Bendigo, but ithas been previously reported by Li et al. (1998).

    Hydrothermal pyrite in the sedimentary rocks is euhedraland commonly internally zoned (Fig. 5D-I), in places over-printing the metamorphic fabric of the host shale which isvery apparent after acid etching (Fig. 5I). In many cases the

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    FIG. 5. Photomicrograph of pyrite types at Bendigo. A. NBD161-437.15, fine acicular-grained pyrite, diagenetic py1 aftermarcasite? B. NBD171-531.5, fine-grained diagenetic pyrite in shale (Py1). C. NBD238-186.5, fine-grained pyrite nodulepy1 (after marcasite?). D. BD171-544.8, partially recrystallized diagenetic py1 nodule, with euhedral hydrothermal pyritepy3 developed in pressure shadows on edges of nodule. E. NBD180W3-338.8, fine-grained cluster of diagenetic py1 over-grown by coarse euhedral hydrothermal py3. F. NBD177-377E, early diagenetic core of py1, followed by a later diageneticovergrowth nodule (py2), overgrown by euhedral hydrothermal py3. G. NBD005-104.1, coarse-grained euhedral hydrother-mal py3, with an outermost rim. H. NBD161-431.3, coarse-grained euhedral hydrothermal py3 aggregate overgrowing a silt-shale boundary. I. NBD186-517.4, zoned euhedral hydrothermal py3, showing the metamorphic fabric of the shale preservedin the overgrowth pyrite; revealed by acid etch. J. NBD171-521.7, euhedral laminated quartz vein pyrite, showing vein sel-vage overgrowth texture. K. NBD180W3-341.6, subhedral laminated quartz vein pyrite exhibiting vein selvage overgrowthtexture. L. NBD238-164.8, euhedral laminated quartz vein pyrite, showing vein selvage overgrowth texture. Scale bars areall 500 m except for pyrites C and I, for which the scale bar is 200 m.

    Diagenetic pyrites

    Diagenetic pyrite cores with euhedral hydrothermal overgrowths

    Hydrothermal pyrites

    Laminated quartz vein pyrites

  • euhedral pyrites have an anhedral porous core composed oforiginal diagenetic pyrite recrystallized during metamor-phism, surrounded by euhedral hydrothermal pyrite, withgrowth zoning commonly overprinting internal metamorphicfabric (e.g., Fig. 5F, I). In these cases the term metamorphic-hydrothermal pyrite is used. Hydrothermal and metamor-phic-hydrothermal pyrite is not confined to any lithology atBendigo, with the pyrites exhibiting the most evidence ofgrowth zoning occurring in sandstone units, presumably re-flecting their relatively high effective porosity.

    Pyrite from laminated quartz veins is euhedral to anhedraland commonly displays a pervasive structural overprint,which is parallel to the lamination in the veins (Fig. 5J-L).This texture of pyrite growth, overprinting the laminatedquartz bands, indicates that the pyrite has grown late in thehistory of vein development.

    In the quartz-rich gold-bearing saddle reefs, arsenopyrite ismore abundant than pyrite, with lesser sphalerite, minorgalena, and chalcopyrite. Reef pyrite occurs as euhedral crys-tals in association with arsenopyrite. Free gold is present asblebs, from 10 m to several millimeters across, commonlyassociated with sphalerite and galena, or occurring at the con-tact between pyrite and arsenopyrite crystals (Fig. 6). Sharpeand MacGeehan (1990) reported that 95 percent of the reefgold production at Bendigo has been free gold, with 5 percentfrom roasted sulfide concentrates; most probably gold-bear-ing arsenian pyrite. The highest gold grades are commonlypresent toward the margins of the reef where wall-rock sel-vages and graphitic material are more abundant (Cox et al.,

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    FIG. 6. Photomicrograph of sulfide textures in a gold-rich sample from the Gill reef. Subhedral pyrite and arsenopyriteshow a fractured texture. Sphalerite and gold are present within pyrite and interstitial to pyrite grains.

    FIG. 7. Photomicrographs of disseminated pyrrhotite in black shales. A.Disseminated pyrrhotite aligned parallel to cleavage. B. Different texturalstyles of pyrrhotite in shales: LHS = rounded pyrrhotite after framboidal ag-gregate of pyrite, RHS = blocky pyrrhotite after aggregates of coarse cubicpyrite.

  • 1995). In these zones the reef has a laminated texture similarto the laminated quartz veins, and after acid etching the ar-senopyrite shows an overprinting relationship analogous tothat observed in the laminated quartz vein pyrite.

    Recent deep drilling at the Kangaroo Flat mine at Bendigohas revealed that carbonaceous shales intersected in thelowest stratigraphy contain fine-grained cleavage-parallelpyrrhotite, rather than pyrite. Magnetic susceptibility read-ings on the drill core indicate that this pyrrhotite-bearing in-terval is about 300 m thick (Fig. 3). Disseminated pyrrhotiteis concentrated in the most carbonaceous black shales,whereas the light-colored siltstones and sandstones containdisseminated pyrite. Some shale beds contain both dissemi-nated pyrite and pyrrhotite. Under the microscope (Fig. 7A),the pyrrhotite commonly occurs in elongate patches about 0.5to 2 mm long, aligned parallel to cleavage. Minor inclusionsof chalcopyrite and sphalerite are common within the elon-gate pyrrhotite patches. Textural relationships indicate thatthe pyrrhotite has replaced diagenetic pyrite in the shales(Fig. 7B). Because the pyrrhotite is softer than pyrite, it ismore easily recrystallized and deformed during the metamor-phism and deformation and is aligned in the cleavage.

    Trace Element Associations and Zoning within the Sulfide Types

    A total of 371 quantified LA-ICPMS spot and line analyseshave been completed on pyrite from Bendigo for the follow-ing suite of elements: Mg, Al, Ca, Ti, V, Cr, Mn, Fe, Co, Ni,Cu, Zn, As, Se, Sr, Zr, Mo, Ag, Cd, Sn, Sb, Te, Ba, La, W, Pt,Au, Tl, Pb, Bi, Th, and U. Mean values for each major pyrite

    type are given in Table 1 and selected analyses for each majorpyrite type are given in Table 2. The full suite of analyses in-cludes 41 for nodular diagenetic pyrite in sediments, 272 foreuhedral hydrothermal pyrite in sediments, 43 for laminatedquartz vein pyrite and 25 for gold reef pyrite. In addition tothe spot analyses a further 45 pyrite grains have been mappedby LA-ICPMS to determine trace element zonation and rela-tionships to timing of gold. Six of the pyrite trace elementmaps are presented here (Figs. 813).

    Trace metals in pyrite may occur in several ways: (1) as in-visible solid solution within the pyrite structure, (2) within in-visible nanoinclusions of other sulfides or elements, (3) withinvisible micron-sized inclusions of other sulfides, or (4) withinvisible micron-sized inclusions of silicate or carbonate miner-als. There has been no attempt in this study to remove the ef-fects of micron-sized inclusions on the pyrite analyses in Ta-bles 1 and 2. However, inspection of the LA-ICPMS outputtraces from each laser spot analysis enables an estimation ofwhether a particular trace element occurs within a homoge-neous invisible or nano-sized inclusion, or alternatively aslarger isolated micron-sized inclusions in the pyrite (Fig. 12;Maslennikov et al., 2009). However it is not possible to de-termine by LA-ICPMS whether a particular element is pre-sent as nanoparticles in pyrite or as solid solution in the pyritestructure. As with previous studies (e.g., Large et al., 2007,2009), this study has found that As, Mn, Co, Ni, Se, and Moare commonly homogeneously distributed in the pyrite, ei-ther within the pyrite structure or as nanoinclusions, whereasAu, Ag, Cu, Zn, Sb, Te, Tl, Pb, and Bi may be either invisible,as in group (1) above, or present within metallic or sulfide

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    TABLE 1. Mean Trace Element Values for Each Major Pyrite Type Determined by LAICPMS

    Pyrite type Reef Hydrothermal Hydrothermal Laminated Laminated DiageneticRim/core (all) (rim) (core) quartz vein (rim) quartz vein (core) (all)

    No. of analyses (ppm) 25 167 96 23 20 43Au 5.65 1.24 0.65 4.37 2.44 0.61Al 5022.27 3538.88 3717.30 453.68 1092.84 23910Ti 902.19 995.06 1335.09 449.68 339.61 2775.73V 7.27 5.72 7.29 1.97 3.85 35.18Cr 17.16 16.68 20.02 6.96 11.27 46.69Mn 8.41 36.34 12.61 4.00 7.41 1065.73Co 65.16 198.06 200.34 36.08 169.76 765.19Ni 385.55 537.08 404.97 442.73 171.32 1236.17Cu 65.16 97.18 108.05 73.24 15.40 1127.44Zn 5.67 28.42 128.15 2.93 3.30 499.11As 6581.14 1852.55 1546.89 4961.71 3974.50 1325.79Se 13.95 33.73 56.62 20.74 33.32 66.88Zr 18.72 36.16 21.07 9.17 10.13 26.56Mo 0.20 0.74 2.56 0.73 0.21 11.47Ag 1.11 1.25 4.52 0.70 0.82 4.53Sn 0.44 0.54 0.57 0.50 0.20 1.89Sb 25.86 92.08 82.14 75.72 47.49 215.21Te 0.36 1.94 2.95 3.07 1.40 5.08Ba 47.63 30.72 34.15 4.70 9.99 216.47La 3.76 12.20 5.54 4.79 0.18 35.85W 2.40 1.72 1.96 1.93 0.96 2.96Pt 0.01 0.05 0.05 0.03 0.04 0.06Tl 0.20 0.17 0.16 0.08 0.11 0.75Pb 290.14 426.06 2079.45 315.42 138.99 1156.76Bi 38.91 33.86 38.37 14.58 11.44 81.07Th 1.21 3.70 2.32 1.27 2.21 13.94U 0.52 1.70 0.75 0.26 0.48 4.66

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    TABLE 2. Selected Trace Element Analyses for Each Major Pyrite Type and Free Gold from Reef Determined by LA-ICPMS

    Sample no. Py type Au Al Ti V Cr Mn Co Ni Cu

    0911-83.80A Reef 4.51 13.64 6.12

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    238-177.525 Laminated quartz vein 101.31 0.20 3.56 0.03 1.64 916.46 253.32 0.23 0.24161-449.8 Laminated quartz vein 0.39 0.07 0.37 0.04 0.05 4.70 2.49 0.21 0.01161-449.8 Laminated quartz vein 2.99 0.35 3.21 0.04 0.04 14.55 3.66 1.00 1.60161-437.15 Diagenetic 104.82 1.60 2.61 0.03 1.79 585.82 188.78 39.55 9.06171-544.8 Diagenetic 134.71 0.44 3.74 0.08 1.19 884.83 23.84 1.06 1.24171-531.5 Diagenetic 321.19 0.31 3.64 0.04 0.71 860.64 227.43 3.02 1.08180W3-338.8 Diagenetic 118.60 0.14 2.15 0.04 0.25 1628.08 53.46 2.63 3.03186-273.3 Diagenetic 11.07 4.73 1.11 0.01 0.07 1535.52 69.92 1.27 0.160911-80.90 Arsenopyrite 4.86 0.34 43.99 0.01 0.04 24.65 28.33 2.96 3.050911-80.90 Arsenopyrite 0.15 0.01 0.28 0.01 0.01 5.46 8.56 0.01 0.010911-83.80 Arsenopyrite 0.53 0.02 0.51 0.00 0.00 15.74 21.92 0.02 0.020911-76.65 Arsenopyrite 58.50 0.12 1.72 0.02 1.25 615.50 280.24 0.49 1.610911-85.85 Arsenopyrite 15.79 0.45 2.84 0.00 0.64 76.41 88.28 3.67 3.74

    FDD0911 = 80.90 Free Au x x 0.38 0.38 x 0.96 5.79 x x

    Trace element concentrations in ppm

    TABLE 2. (Cont.)

    Sample no. Py type Ba La W Pt Tl Pb Bi Th U

    FIG. 8. Trace element LA-ICPMS map of diagenetic pyrite NBD238-164.6. This aggregate has a rounded nodular mor-phology common to diagenetic pyrites. It exhibits some zoning, with the pyrite core displaying elevated Ag, Au, Co, Mo, andCu. The next internal zone is characterized by elevated Se, Te, Bi, and patchy Pb. Au is present throughout the pyrite nod-ule but is more elevated in the core. These elements are all part of a suite commonly enriched in diagenetic pyrites atBendigo. Arsenic is enriched as a small inclusion of arsenopyrite in the core which has smeared out during laser ablation. Ni,Co, and As form an outer most rim.

  • microinclusions; the suite V, Ti, Al, Cr, Zr, Sn, Ba, W, Th, andU is present in pyrite within nonsulfide microinclusions (Fig.14).

    Composition of nodular diagenetic pyrite in sediments

    The fine aggregates of nodular diagenetic pyrite in theblack shale facies are commonly enriched in a full range ofchalcophile elements (Figs. 89). A previous preliminarystudy, reported in Large et al. (2009), found that diageneticpyrite at Bendigo was enriched in Ni, Cu, Mo, Ag, Bi, and Pbcompared to later hydrothermal pyrites. In the more com-

    prehensive study here the list of elements enriched in diage-netic pyrite has been extended to include: Mn, Zn, Mo, Cu,V, Ba, Ag, Cd, Tl, Co, Ni, Bi, and Pb (Fig. 15).

    Gold content in diagenetic pyrite in the shales varies from0.02 to 1.73 ppm with a mean of 0.61 ppm. Arsenic contentvaries from 20 to 6,500 ppm with a mean of 1,300 ppm. In agiven LA-ICPMS spot analysis the As and Au counts are rel-atively uniform, with no spikes, suggesting that the Au and Asare present in solid solution, or as invisible nanoparticles inthe pyrite (Fig. 14). However, the contents of As and Au donot exhibit any obvious correlation (Fig. 16A). The only trace

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    FIG. 9. Trace element LA-ICPMS map of diagenetic pyrite in the center of a quartz nodule in an organic-rich shale(NBD177-377C). The pyrite is enriched in Ag, Ni, Bi, Pb with lesser Cu, As, Au, and Se. The Pb, V, and Ti maps show theoutline of the quartz nodule that surrounds the pyrite.

  • element which shows a clear positive correlation with gold indiagenetic pyrite is Bi (Fig. 16B). Trace element mappingshows that some nodular diagenetic aggregates have a core

    enriched in Au, Ag, Mo, Cu, Se, and sometimes Bi, Te, andPb, compared to the outer bulk of the pyrite aggregate (e.g.,Fig. 8).

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    FIG. 10. Trace element LA-ICPMS map of euhedral hydrothermal pyrite from a siltstone, sample NBD171-544.8. Cycliczoning present in most elements indicates a protracted growth history associated with changes is hydrothermal fluid compo-sition. Round spots defining a v-shape enrichment pattern in Ni, Cu, and Zn are laser burn holes and should be disregarded.

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    FIG. 11. Trace element LA-ICPMS map of pyrite in sample NBD180W3-345.8. The euhedral outline and cyclic growthzoning indicate a hydrothermal origin for this pyrite. The pattern of the growth zoning and trace element distribution arecomplicated by the fact that the crystal overgrows a siltstone-shale boundary. The boundary is aligned E-W and occurs ap-proximately in the center of the pyrite, with the upper uniformly zoned shale portion defined by elevated Co, Ni, V, and Bi,and the lower sandstone portion by elevated Pb. Gold is highest in the core of the pyrite, with later growth cycles also ele-vated in Au, As, Pb, Bi, and Cu.

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    FIG. 12. Trace element LA-ICPMS map of pyrite crystal NBD171-521.7. This pyrite is from a laminated quartz vein dis-tal to a productive gold reef, as shown in Figure 2, drill section. Pyrite in this laminated quartz vein overgrows the internalvein selvages and shows an inherited fabric, revealed by acid etching. The mapping highlights both growth zoning in thepyrite and the internal vein selvage structure. Gold is elevated on some crosscutting selvage bands (also Pb, Bi, V, Ag, Co,and Ni) and also as an outermost narrow rim (with As).

  • Composition of euhedral hydrothermal pyrite in sediments

    The euhedral pyrites, which occur in the sedimentary rockssurrounding both the gold reefs and the bedding parallel lam-inated quartz veins, are enriched in Au and As but depletedin all other elements compared to the nodular diageneticpyrite (Fig. 15). The LA-ICPMS mapping shows that most ofthe hydrothermal pyrites exhibit cyclic zoning of certain trace

    elements (e.g., As, Ag, Co, Pb, Bi, Te, and Au; Figs. 10, 11),indicating fluctuating composition of the hydrothermal fluid.Au content varies from 0.01 to 21.6 ppm with a mean of 1.03ppm, whereas As varies from 1 to 12,900 ppm with a mean of1,740 ppm As. Spot analyses conducted on both the rims andthe cores of euhedral pyrites (Table 1) show that some rimsare enriched in both Au and As compared to the cores (Fig.17). The pyrite rims average 1.24 ppm Au and 1,850 ppm As,

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    FIG. 13. Trace element LA-ICPMS map of pyrite in sample FDD0911-79.65 from the gold-rich Gill reef. The euhedralpyrite is zoned from an Ni-rich core to an Au-Asrich outer zone. Spotty elevated Ag, Pb, Bi, Se, Te on the lower right-handside of the pyrite is due to inclusions of galena carrying Pb, Bi, Se, and Te.

  • compared to an average 0.65 ppm Au and 1,550 ppm As forthe cores. Conversely the average cores are significantly en-riched in Ag, Zn, Mo, and Pb relative to the average rims. Theeuhedral pyrite cores have a similar mean As/Au ratio, Au/Agratio, and Au, Ag, Se, and Pb content to the nodular diage-netic pyrites (Fig. 18A-B), indicating that within the shales,the cores to the euhedral pyrites most probably formed by re-crystallization of diagenetic pyrite. In contrast, the euhedralovergrowth and rim pyrite has a markedly different chemistry,suggesting that it was precipitated from a hydrothermal fluid.

    The LA-ICPMS trace element map in Figure 11 is of a hy-drothermal pyrite that has grown across a shale-sandstonebedding plane. The zoned pyrite on the shale side of the con-tact is relatively enriched in Co, Ni, Bi, and V compared tothe other half of the pyrite on the sandstone side of the con-tact. Lead is the only trace element enriched on the sand-stone side of the contact. These differences suggest that cer-tain elements in hydrothermal pyrite, such as Co, Ni, V, Bi,and Pb, are partly sourced from the immediate surroundingsedimentary matrix.

    Composition of pyrite and arsenopyrite in the gold quartz saddle reefs

    Pyrite in the gold reefs is commonly the most enriched ininvisible gold and arsenic of all the pyrite types, with a similarAs/Au ratio to that shown by euhedral pyrite in both lami-nated quartz veins and the sedimentary rocks proximal to thegold reefs (Fig. 17A). Although enriched in Au and As, thereef pyrite is commonly low in most other trace elements, inparticular, Co, Cu, Zn, Mo, Sb, and Te (e.g., Fig. 13). It isgenerally characterized by a high Au/Ag ratio (90% of the reefpyrite has Au/Ag >1, Fig. 17B) and a high Au/Pb ratio (67%of the reef pyrite has Au/Pb >0.01; Fig. 18A), with low Se andTe contents (Fig. 18C).

    Compared to the pyrite, arsenopyrite in the reefs is onlymarginally more enriched in invisible gold (mean of 2.15 ppmAu in arsenopyrite vs. mean of 1.66 ppm Au in pyrite, Table1). Other elements elevated in the arsenopyrite are Co (mean315 ppm), Ni (770 ppm), Mo (14 ppm), Sb (168 ppm), Te (5ppm), W (6 ppm), and Pb (190 ppm). LA-ICPMS mapping

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    cou

    nts

    secondsFIG. 14. LA-ICPMS counts for a laser line burn across a pyrite aggregate (AP02A58), showing the various types of re-

    sponse possible for major and minor elements in the pyrite and matrix. Fe and S exhibit a relatively flat response typical ofmajor elements in homogeneous pyrite. Au and As show elevated counts on the margins of the pyrite and decrease towardthe center. They clearly define an Au-Asrich rim on the pyrite. The relatively smooth pattern of the counts for Au and As(with no spikes), and their parallel trends, suggest that these elements are zoned, either in solid solution in the pyrite or asnanoparticles. Zircon shows a very spiky pattern indicating presence of many microinclusions of a Zr-rich mineral such asmonazite. Lead is elevated and spiky, suggesting abundant microinclusions of galena in the pyrite, with a larger inclusion ev-ident toward the end of traverse. Silver is three orders of magnitude less than Pb, but shows a very similar pattern, suggest-ing the Ag is present in the structure of the galena inclusions. Antimony is elevated in the center of the pyrite and lower onthe margins, indicating zonation in the reverse order to As. The relatively smooth profile suggests that Sb is in the structure,or as nanoparticles in the pyrite. For most of the traverse Bi parallels Sb, but toward the end it shows a major spike similarto Pb and Ag. This suggests some Bi is in the structure of pyrite (same as Sb, As, Au), but some is present in the galena in-clusions (same as Pb and Ag).

  • shows no clear growth zoning in this arsenopyrite. However,some trace elements (e.g., Bi, Ag, V, Cu) are concentrated ininclusion-rich zones, which follow the lamination of the reef(Fig. 19), whereas others are enriched in fine cracks that cutacross the lamination (e.g., Pb, Co, Ni).

    Composition of pyrite in the laminated bedding-parallel quartz veins

    Pyrite in the laminated quartz veins, which form legs off thesaddle reefs and extend down the fold limbs parallel to bed-ding, has a characteristic texture and trace element distribu-

    tion (Fig. 12) quite unlike that of the other pyrite types. Thepyrite is euhedral but commonly fractured parallel to the lam-ination in the veins. Many of the trace elements (Pb, Bi, Ag,Co, and Au) are aligned along the fractures and appear tooverprint the growth zoning in the pyrite (Fig. 12). The LA-ICPMS spot data shows that their composition overlaps thefields for pyrite in the quartz saddle reefs and pyrite in thesediments (Figs. 17, 18). Commonly the pyrite in laminatedquartz veins that are within 50 to 70 m of the saddle reefs hasdistinct Au-Asrich rims (Fig. 20) with Au/Ag >1, Au/Pb>0.01, and low Se and Te, similar to pyrite in the saddle reefs.

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    FIG. 15. Spidergram illustrating the mean enrichment of trace elements in diagenetic pyrite compared with metamorphichydrothermal pyrite in the sediments at Bendigo. Note the most enriched elements are Mn, Cu, Mo, Zn, Ba, Tl, V, and U.As and Au are the only two elements that are more enriched in the metamorphic hydrothermal pyrite.

    0

    0.5

    1

    1.5

    2

    0 50 100 150 200 250 300

    y = 0.3021 + 0.0037552x R2 = 0.5241

    Au

    pp

    m

    Bi ppm

    0

    0.5

    1

    1.5

    2

    0 1000 2000 3000 4000 5000 6000 7000

    y = 0.58149 + 2.2153e-5x R2 = 0.0087031

    Au

    pp

    m

    As ppm

    A B

    FIG. 16. LA-ICPMS spot analyses for diagenetic pyrite. A. Au-As shows that no clear relationship is present in the dia-genetic pyrite data. B. Au-Bi shows a rough linear relationship, with R2 = 0.5241.

  • Laser traverses at right angles to the laminations in theveins (Fig. 21) show that the narrow 0.1- to 2-mm black bandsthat define the laminations are composed of very fine grainedchlorite, sericite, and graphite (Fig. 4D). They are enriched inAu, Zn, Pb, Mn, Mo, As, Ni, Ag, and Bi, the same suite of el-ements enriched in diagenetic pyrite in the sediments. In Fig-ure 21 it is evident that Au is enriched by a factor of 10 to 100times in the black chloritic bands compared to the whitequartz bands. Although these laminated quartz veins rarelycontain ore-grade gold, there is a strong gold signature in thechloritic bands that likely represents the passage of gold-bear-ing fluids along the veins.

    Composition of pyrite and pyrrhotite in lower stratigraphy below currently mined gold reefs

    In contrast to the upper stratigraphy at Bendigo which hoststhe currently mined gold reefs, the lower stratigraphy belowthe Ethan Shale (Fig. 3) contains both disseminated pyrite andpyrrhotite. This is despite the similarity in lithologic units tothe upper stratigraphic sandstone-shale turbidite package.Diagenetic pyrite (n = 90) from the lower carbonaceous tur-bidites contains a similar range and mean of gold to diageneticpyrite in the upper stratigraphy but a lower level of arsenic(Table 3). In contrast, metamorphic pyrrhotite developed inthe carbonaceous shales by the replacement of diageneticpyrite is depleted in several trace elements, in particular Au(mean 0.04 ppm) and As (mean 4 ppm; Fig. 22). Other ele-ments that are depleted relative to the diagenetic pyrite, in-clude Cu, Zn, Sb, and Te (Fig. 23; Table 3). The trace ele-ments that are not significantly depleted in metamorphic

    pyrrhotite compared to diagenetic pyrite are Ti, V, Co, Ni, Se,Mo, Ag, Sn, and W. (Fig. 23).

    Discussion of Pyrite Composition and Ore GenesisThis study has revealed a number of features concerning

    the pyrite developed in the quartz reefs, veins, and sedimen-tary rocks at Bendigo that need to be taken into account informulating a genetic model for the deposit:

    1. The black shales that are interleaved with turbiditesandstone beds at Bendigo contain syngenetic invisible gold,which is present within bedding-parallel diagenetic arsenian-pyrite aggregates or nodules (Fig. 24A). The Au and As con-tent of this pyrite averages 0.61 and 1,325 ppm, respectively.These diagenetic pyrites are also enriched in a suite of traceelements, including Mn, Zn, Mo, Cu, V, Ag, Tl, Co, Se, Ni, Bi,and Pb, which are commonly enriched in diagenetic gold-bearing pyrite from some other gold provinces, for example,Carlin, Nevada, and Lena province, Siberia (Large et al.,2009).

    2. Similar shale units stratigraphically below the knowngold-bearing quartz reefs contain both disseminated pyriteand pyrrhotite. Textural evidence indicates that the pyrrhotiteformed during metamorphism by the replacement of diage-netic pyrite in specific organic-rich beds (Fig. 22). In contrastto pyrite, the pyrrhotite is devoid of invisible gold and arsenic.

    3. Both the sandstones and the shales contain euhedralhydro thermal pyrite that is concentrated around the saddlereefs and related bedding-parallel laminated quartz veins. Thecores of many of these euhedral pyrites have trace element

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    0.001

    0.01

    0.1

    1

    10

    100

    1000

    1 10 100 1000 104 105

    Diagenetic sedimentary pyriteMetamorphic hydrothermal core pyriteMetamorphic hydrothermal rim pyriteLaminated quartz vein core pyriteLaminated quartz vein rim pyriteGold reef pyrite

    Au

    pp

    m

    As ppm

    0.001

    0.01

    0.1

    1

    10

    100

    0.001 0.01 0.1 1 10 100

    Au

    pp

    mAg ppm

    Au/Ag

    = 1A B

    FIG. 17. LA-ICPMS spot analyses on pyrite. A. Au-As: Diagenetic pyrite field (black) overlaps with the broader meta-morphic pyrite core field (blue). The pyrite in saddle reefs (red) forms a distinct field with good As-Au correlation. The meta-morphic hydrothermal rim pyrite proximal to gold reefs also plots in this field. B. Au-Ag: Diagenetic pyrite (black) forms arelatively tight field, with 86 percent of data having Au/Ag 1. The metamorphic pyrite (blue) forms a diffuse field that overlaps the diagenetic and reef pyrite.

  • affinities with the diagenetic pyrite, whereas the outer zonesand particularly the pyrite rims have a closer textural andchemical affinity to the pyrite in the laminated quartz veinsand gold-bearing saddle reefs (Fig. 24B).

    4. In the sediments and laminated quartz veins close togold-bearing saddle reefs (within 5075 m), the pyrite hasAu-Asrich rims of similar composition to the pyrite withinthe saddle reefs (Fig. 24C). These rims are similar, althoughthicker and of lower gold tenor, to those seen in Carlin de-posits, Nevada (Hofstra and Cline, 2000).

    5. The black graphite-bearing bands in the laminated quartzveins are enriched in Au, As, and the same suite of trace ele-ments that characterize the diagenetic pyrite in the shales.

    6. Although some of the hydrothermal pyrite in the sedi-ments and veins shows weak internal zonation of gold and ar-senic, by far the highest concentrations of Au and As occur in

    the rims of the hydrothermal pyrite, laminated quartz veinpyrite and saddle reef pyrite (Fig. 20).

    It is inferred that the Au-Asrich rims on pyrite within orproximal to the gold reefs are a result of the same gold-pre-cipitating event that formed the free nugget gold in the reefsThis could be possible with the pressure changes between thereef and the sediments causing free gold and arsenopyrite de-position in the reefs, whereas the availability of iron, whichwould be higher in the sedimentary rocks, leading to arsenianpyrite with invisible gold formation in the wall rocks. Someeuhedral hydrothermal pyrites in sedimentary units distal toproductive gold reefs also show this phenomenon, however,in this case Au-As rims are much thinner and less well devel-oped (Fig. 20). Pyrite euhedra that formed earlier in the par-agenetic sequence do not show rims; instead minor gold may

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    0.01

    0.1

    1

    10

    100

    1000

    104

    105

    0.01 0.1 1 10 100 1000 104

    Ni

    pp

    m

    Co ppm

    1

    10

    100

    1000

    0.01 0.1 1 10 100

    Se

    pp

    m

    Te ppm

    0.01

    0.1

    1

    10

    100

    1000

    104

    105

    106

    0.001 0.01 0.1 1 10 100 1000

    Diagenetic sedimentary pyriteMetamorphic-hydrothermal core pyrite Metamorphic-hydrothermal rim pyriteLaminated quartz vein core pyriteLaminated quartz vein rim pyriteGold reef pyrite

    Pb

    pp

    m

    Bi ppm

    0.001

    0.01

    0.1

    1

    10

    100

    0.01 0.1 1 10 100 1000 104 105 106

    Au

    pp

    m

    Pb ppm

    Co/Ni

    = 1

    A

    DC

    B

    FIG. 18. LA-ICPMS spot analyses on pyrite. A. Au-Pb: Reef pyrite has a significantly higher Au/Pb ratio than diageneticpyrite. B. Pb-Bi: Diagenetic pyrite forms a tight cluster, compared to reef pyrite and LQ vein pyrite which is spread over awide Pb and Bi range but with a consistent Pb/Bi ratio. C. Se-Te: Diagenetic pyrite commonly displays the highest Se andTe values, compared with reef pyrite which has the lowest values. Metamorphic and LQ vein pyrites are between the two ex-tremes. D. Ni-Co: Reef and LQ vein pyrites consistently have Co/Ni ratio

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    FIG. 19. Trace element LA-ICPMS map of an aggregate of arsenopyrite in quartz from the Gill reef, Bendigo (samplenumber FDD0911-84.25). The inclusion-rich arsenopyrite, which outlines the structure of the laminated quartz reef, con-tains elevated Ag, Ni, Pb, Bi, V, Cu, and Ti. The complex patterns indicate that some elements are concentrated in inclusion-rich zones (Ag, Bi, V, Mo, Cu), whereas other elements are also elevated in cracks (Co, Ni, Pb).

  • be concentrated in the core of the pyrite, in internal zonesor evenly throughout the crystal (e.g., Figs. 10, 11). This suggests that multiple gold-introducing events occurredthroughout the deposit history at Bendigo, starting with goldintroduced during sedimentation and diagenesis of the sedi-mentary rocks.

    Trends in Au, As, and Ag content of pyrite and native gold

    Plots of the mean values of Au versus As and Au versus Agfor the main pyrite types (Fig. 25) show systematic trends that

    may provide clues regarding the genesis of the gold ores atBendigo. Although there is a significant variation in meanAu and As content for the six pyrite groups, they all plotclose to a mean trend line of As/Au = 1,700. The mean Auand As values for nodular diagenetic pyrite are very similarto the mean for the euhedral hydrothermal core pyrite (Fig.25A), supporting the interpretation that the euhedral coresare recrystallized diagenetic pyrite. Also the mean Au andAs values for the saddle reef pyrite is similar to that for theAu-Asrich rim pyrite in the laminated bedding-parallel

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    FIG. 20. Variation of Au-As rims on various types of pyrite at Bendigo. Rims are thicker proximal to gold-bearing quartzreef structures, particularly in the 0- to 50-m range. Pyrites distal to reef in unmineralized sedimentary units display muchthinner Au-As rims. Sample NBD161-446.05 is from a laminated quartz vein adjacent to a reef, whereas sample NBD171-521.7 is from a laminated quartz vein approximately 100 m from the same reef. Sample NBD177-377E is from a synclinalaxis in the poorly mineralized Western Shale (not shown in drill section). NBD171-544.8 is interpreted as a hydrothermalpyrite, possibly with a diagenetic core from a sedimentary unit more than 100 m from a gold reef. NBD186-775.8 is inter-preted as a hydrothermal pyrite found in the Jude Shale deep in the mine stratigraphy. It shows a diffuse zone of internal Auenrichment on the edge of a zone of As enrichment.

  • PYRITE AND PYRRHOTITE AT BENDIGO MINE, AUSTRALIA 23

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    TABLE 3. Comparison of the Trace Element Composition of Diagenetic Pyrite and Metamorphic Pyrrhotite from the Upper Stratigraphy (Ethan Shale to Inner Shale) and Lower Stratigraphy (below Ethan Shale)

    Diagenetic pyrite Diagenetic pyrite PyrrhotiteLA-ICPMS upper stratigraphy lower stratigraphy lower stratigraphy % loss ((gain))

    when pyrite is replacedTrace element No. of analyses 43 49 63 by pyrrhotite

    Au Range

  • quartz veins, suggesting that they precipitated from the samehydrothermal fluid. Overall the similar mean As/Au ratio ofaround 1,700 for all pyrite types is compatible with the theorythat the hydrothermal fluid that precipitated Au-Asbearingpyrite in the saddle reefs and laminated quartz veins derivedthe Au and As from the diagenetic and recrystallized meta-morphic pyrite in the sediments. This is predicated on the as-sumption that the Au and As are leached from the diageneticpyrite at similar rates and transported together in the samefluid, probably as bisulfide complexes. The mean trend lineon the Au-As plot (Fig. 25A) allows for minor fractionation ofgold into the reef pyrite, as the As/Au ratio decreases system-atically from 2,120 for diagenetic and hydrothermal corepyrite to 1,150 for saddle reef and laminated quartz veinpyrite.

    The trends of mean values on the Au versus Ag plot (Fig.25B) give a very different picture, with a systematic increase inAu/Ag ratio across the diagram from 0.13 in diagenetic pyriteto 43 in saddle reef pyrite. The mean value for diagenetic pyriteis virtually identical to the mean for hydrothermal core pyrite,supporting our previous contention that the hydrothermalcores are recrystallized diagenetic pyrite.The systematic in-crease in Au/Ag ratio from hydrothermal rim pyrite, to lami-nated quartz vein pyrite to saddle reef pyrite, supports a modelof preferential fractionation of gold, compared with silver, intothe hydrothermal fluid, during metal leaching and transport.There are two factors involved which lead to a decoupling ofgold from silver. First, the LA-ICPMS data in Table 3 showthat conversion of pyrite to pyrrhotite in the deep shales re-leases on average 93 percent of the Au from the pyrite but only31 percent of the Ag. The remaining silver is retained in thepyrrhotite structure in the sedimentary rocks. Second, the re-leased gold was most likely transported as a soluble bisulfidecomplex due to the generation of an H2S-rich, but chloride-poor, neutral metamorphic and/or hydrothermal fluid, whereasthe silver would be more likely transported as the relativelyless soluble chloride complex (Phillips et al., 1984). Boththese process lead to an increase in the Au/Ag ratio in the hy-drothermal fluid compared to the source sedimentary rocks.

    Previous studies by Morrison et al. (1991) reported that thefree gold in the reefs at Bendigo has a gold fineness of about950, which equates to an Au/Ag ratio of 19, comparable to theAu/Ag ratio of the reef pyrite of around 41. Recent LA-ICPMSanalyses on free gold from the Gill Reef (reported in Table 2)show a fineness variation of 942 to 967 (Au/Ag = 1630), sim-ilar to Morrisons early data. No free gold has been identifiedin the sedimentary rocks adjacent to the saddle reefs, and thequestion arises, why is the gold in the reefs principally freegold, whereas the gold in the sedimentary wall rocks is invis-ible gold in arsenian pyrite? This probably relates to the avail-ability of nonsulfide iron in the sedimentary rocks compared

    24 THOMAS ET AL.

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    PYRITEppm

    Au = 0.42As = 590Ag = 17

    Te = 14.8

    PYRRHOTITE ppm

    Au = 0.05As =

  • to the quartz saddle reefs. Minor gold in the wall rocks is mostlikely deposited within arsenian pyrite by the process of sulfi-dation (e.g., Kesler et al., 2003). This process is controlled bythe availability of excess Fe, most likely present within ironcarbonate, chlorite, or phengitic muscovite in the sedimen-tary rocks that reacts with H2S in the fluid to form the gold-bearing arsenian pyrite rims. By contrast, in the dilatantquartz-rich reefs there is no excess Fe, except possibly wherewall-rock fragments and slivers are present along the marginsof the reef. The shortage of available Fe in the quartz reefsmeans that sulfidation to form arsenian pyrite is not the mainpathway for gold precipitation, but rather free gold and ar-senopyrite are the ultimate precipitation products.

    Number of gold fluid events

    The presence of invisible gold in both the diagenetic pyritein the black shales, and in the rims of the hydrothermalpyrite, which have markedly different trace element associa-tions, suggests at least two gold events at Bendigo. Weak internal cyclic zonation of Au, As, Ag, Pb, and Bi in some hy-drothermal pyrite (e.g., Fig. 11) suggests that the early syn-metamorphic hydrothermal fluids carried minor gold-arsenicin pulses, but at a much lower level than the late hydrother-mal fluids that formed the Au-Asrich pyrite rims.

    The first, synsedimentary to diagenetic gold event, is ac-companied by a characteristic suite of trace elements (e.g.,Ni, Zn, Ag, Mn, Mo, Cu, V, U) that are known to be elevatedin organic-rich anoxic to euxinic sedimentary environments(Huyck, 1989; Quinby-Hunt and Wilde, 1994; Algeo andMaynard, 2004; Rimmer, 2004; Tribovillard et al., 2006). Theobservation from the laser ICPMS mapping, that some dia-genetic pyrite nodules (e.g., Fig. 8) have the highest values ofAu, Mo, Ag, and Cu in the core, supports the concept thatthe nodules nucleate on organic-rich patches in the shalematrix.

    The later, syndeformation gold event shows a strong Au-As association, with no other consistently elevated traceelement in the pyrite, although Zn, Pb, and Cu occur inother sulfide minerals in the reefs (sphalerite, galena, andchalcopyrite). It is not possible, based on the data pre-sented here, to decide whether the metamorphic hy-drothermal gold-arsenic event introduced new gold and ar-senic to the Bendigo vein system from an external source orsimply that remobilized and reconcentrated gold and ar-senic leached from the preenriched sediments of the firstevent. However, the similar As/Au ratio for pyrite associ-ated with the two events supports (but does not prove) thelatter hypothesis.

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    Au, As, Mo, Ag,Cu, Se core

    Ni, Zn, Pb, Te,Ag, Cu, Co, Mn

    B. Cyclic zoned hydrothermal py(weak Ni, Co, As, Bi, Pb, Cu)

    Recrystalliseddiagenetic py(Ag, Zn, Se, Mo,Pb core)

    Au-As-rich rimlow traceelementinner zones

    C.A. Rounded, porous diagenetic pyrite

    FIG. 24. Schematic of textural and chemical evolution of pyrite in shales adjacent to laminated quartz veins and gold-bear-ing saddle reefs at Bendigo.

    0.1

    1

    10

    100

    100 1000 104 105

    Au

    pp

    m

    As ppm

    mean diagenetic pyrite

    mean metamorphic hydrothermal pyrite

    mean core

    mean rim

    mean laminated quartz vein pyrite

    mean gold reef pyrite

    mean core

    mean rim

    As/Au

    = 1700

    A

    0.1

    1

    10

    0.1 1 10

    Au

    pp

    m

    Ag ppm

    mean diagenetic pyrite

    mean metamorphiccore pyritecore

    rimmean gold reef pyrite

    Au/Ag

    = 1

    mean laminated quartz vein pyrite

    mean metamorphicrim pyrite

    B

    FIG. 25. A. Au-As plot of mean LA-ICPMS spot analyses for the six pyrite groupsshows that the mean Au and As val-ues in the diagenetic pyrite and cores to the metamorphic pyrites are very similar. Also shows the contents for reef pyrite andpyrite rims to LQ vein pyrite are very similar. B. Au-Ag plot of mean LA-ICPMS spot analyses for the six pyrite groups. Thedata show a consistent trend from very low Au/Ag ratio in diagenetic pyrite to very high Au/Ag ratio in reef pyrite.

  • Significance of organic-rich black shales at Bendigo

    Gold-bearing saddle reefs and related bedding-parallellaminated quartz veins are generally confined to the shaleunits in the sandstone-shale turbidite package at Bendigo(Thomas, 1953; Willman, 2007; Boucher et al., 2008). TheRailway Shale and Big Blue Shale host most of the productivereefs in the current mining operations (Figs. 23). Theseshale units are dominantly comprised of gray siltstone andblack shale but also contain thin beds of sandstone. The reefsand laminated quartz veins are commonly developed eitherwithin a black shale bed or at a shale-sandstone or shale-silt-stone contact within or at the margins of the shale package.Organic-rich, pyritic black shales are most common along atleast one margin of the reefs and laminated quartz veins. Thisrelationship is considered to reflect flexural slip during earlyfolding having initiated along the weak bedding-parallel blackshale-sandstone contacts or within weak and thin organic-richshale beds. Bedding-parallel fluid-flow and laminated quartzveining was initially confined to these zones of flexural slip inthe shales (e.g., Cox et al., 1991; Ramsay et al., 1998). Manyprevious workers (e.g., Cox et al., 1991; Phillips et al., 2003)considered that the black shales have also acted as chemicaltraps for the gold. This model involves reaction between thehydrothermal fluid and organic matter in the shales to pro-duce a secondary CH4-bearing fluid, which then mixes withthe gold-bearing hydrothermal fluid to precipitate native goldby reduction of the soluble gold bisulfide complex (Cox et al.,1991).

    Rather than the black shales acting as a chemical trap, wesuggest that the black shales may have acted as a source forgold, arsenic, and other trace metals (Zn, Pb, Cu, Sb, Bi, Te)in the reefs. There are several lines of evidence to supportthis proposal: (1) the organic-rich black shales are the princi-pal host for the diagenetic pyrite which is enriched in gold, ar-senic, and a range of other trace elements; (2) no other po-tential source rocks in the district (e.g., mafic volcanics,granites) contain anomalous levels of both gold and arsenic;(3) the bedding-parallel laminated quartz veins are preferen-tially developed either within or at the contacts with thin bedsof organic-rich black shale and are thus proximal to the po-tential Au-As source rocks; (4) metamorphic hydrothermalpyrite in the bedding-parallel laminated quartz veins and theadjacent shales and sandstones have Au-As rims with anAs/Au ratio similar to that of the diagenetic pyrite in theshales; and (5) the laminated quartz veins are folded and linkto the saddle reefs which contain native gold and pyrite withsimilar Au-As rims to those in the veins and shales.

    Significance of disseminated pyrrhotite in the lower stratigraphy

    At upper greenschist and lower amphibolite facies thebreakdown of pyrite to pyrrhotite in sedimentary rocks andgreenstones is facilitated by release of metamorphic water as-sociated with dehydration of chlorite at 450 to 600C.

    FeS2 + H2O FeS + H2S + 0.5O2. (1)

    Previous workers (e.g., Buryak, 1984; Pitcairn et al., 2006;Procenko, 2008; Large et al., 2009; Tomkins, 2010) considerthis to be a key metamorphic reaction for the release of gold,

    arsenic and sulfur and generation of an orogenic gold orefluid. At Bendigo, however, pyrrhotite is developed in the car-bonaceous shales below the gold reefs, at much lower tem-peratures, associated with lower greenschist facies metamor-phism around 250 to 300C (Wilson et al., 2009).

    In a detailed study of the petrology of sulfide-rich graphiticshales and schists from south-central Maine, Ferry (1981)concluded that the metamorphic conversion of sedimentarypyrite to pyrrhotite does not form a clearcut isograd, as sug-gested by Carpenter (1974), but is rather smeared outacross several metamorphic grades from the chlorite to thesillimanite zone. Ferry (1981) found that on average about 40percent of pyrite was converted to pyrrhotite in the chloritezone, about 75 percent in the biotite zone, and over 90 per-cent in the sillimanite zone. He thus concluded that increas-ing temperature is not the sole mechanism that drives thepyrrhotite-pyrite transition, but that reduction of pyrite topyrrhotite by reaction with organic carbon or graphite was acontributing process (eq. 2). These conclusions match our ob-servations at Bendigoa combination of increasing meta-morphic temperature in the lower parts of the stratigraphicsection only enables the pyrite to pyrrhotite conversion totake place in the most organic-rich black shale units.

    2FeS2 + 2H2O + C(graphite) 2FeS + 2H2S + CO2. (2)

    Although graphite is used in this equation, at lower green-schist facies and below, organic matter or CH4 may be the ac-tive reductant. In typical pelitic rocks, with water generatedby dehydration of chlorite, this reaction is limited by the sol-ubility of H2S, which is relatively low (Connolly and Cesare,1993). An alternative way to drive this reaction is by intro-duction of meteoric or formation waters during deformation(e.g., Nesbitt et al., 1989), although there is no direct evi-dence that meteoric or formation waters were involved in oregenesis at Bendigo. Most of the transition to pyrrhotite is con-trolled by Fe availability, from Fe-rich silicates or carbonates.Studies by Hoschek (1984) demonstrated that pyrite is onlyreacted to pyrrhotite in carbonaceous rocks that were rela-tively Fe rich:

    FeS2 + Fe2+ + 0.5C + H2O 2FeS + 2H+ + 0.5CO2. (3)

    In the case of diagenetic arsenian pyrite with 0.0999 molpercent As and 0.0001 mol percent Au, as observed in thegraphitic shales at Bendigo, equation (2) can be rewritten asfollows:

    2Fe(S0.9As0.099Au0.001)2 + 0.492C + 2.192H2O 2FeS + 1.592H2S +0.004Au(HS)2 +

    0.396H3AsO3 + 0.004H+ + 0.492CO2. (4)

    Equation 4 records the reduction of gold-bearing arsenianpyrite by reaction with carbon, during prograde metamor-phism, to form pyrrhotite and H2S, accompanied by the re-lease of gold as the soluble bsulfide complex, and arsenic asthe soluble arsenate species (Heinrich and Eadington, 1986),plus minor hydrogen ions and CO2, to the metamorphic fluid.

    As shown by Cooke and Simmons (2000), a near-neutral(very slightly acidic) fluid, buffered by equation (4), wouldhave the capacity to carry significant gold (10100 ppb). Such

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  • fluids could move laterally, or upsection, away from the zonesof pyrite to pyrrhotite conversion, to deposit the gold and ar-senic in favorable structural sites such as the saddle reefs atBendigo.

    In this discussion, we have concentrated on gold and ar-senic, however our LA-ICPMS results indicate that the sedi-mentary pyrite to pyrrhotite conversion is accompanied bydepletion in certain trace elements (Cu, Zn, Sb, Mn, and Te),and retention or increase in other trace elements (Ag, Ti, Ni,W, V, Co, Sn, and Mo) in the resultant pyrrhotite (Fig. 15). Inthe case of Cu and Zn, petrological studies have shown thatthese elements, on release from the diagenetic pyrite, formdiscrete sulfide phases, chalcopyrite and sphalerite, which areretained as inclusions and intergrowths in the pyrrhotite.Other elements (Au, As, S, Sb, Mn, and Te) are likely to bereleased into the metamorphic fluid and subsequently pre-cipitated as free Au, arsenopyrite, Au tellurides, stibnite, andMn carbonate in and around the ore deposits.

    Source rocks for gold and arsenic

    The ultimate source of gold and arsenic in the Victoriangoldfield is not well constrained. Previous workers have sug-gested that the Cambrian mafic volcanics that underlie theturbidite package may have been originally enriched in goldand thus were a viable source (Crawford and Keays, 1978;Keays, 1987). Phillips et al. (2003) and Phillips and Powell(2010) suggested that the gold-bearing metamorphic fluidswere generated at the greenschist-amphibolite boundary, at adepth of 12 to 15 km, whereas Vos et al. (2007) consideredthat fluids released from a subduction slab break-off at over200 km is the most likely gold source. Li et al. (1998) sug-gested that pyrite framboids may have acted as a source ofgold when recrystallized during metamorphism but assertthat arsenic must have had a different source. No framboidswere observed as part of this study. None of these studieshave considered the source of the arsenic, which is far moreabundant than gold in the ores.

    We maintain that the source of both gold and arsenic is theblack shales in the thick Ordovician turbidite package thathosts and underlies the ore deposits in the Bendigo district.There is over 10 km of structurally thickened turbiditic strata(Willman, 2007) that could have supplied gold and arsenic tothe reefs during metamorphism and deformation. The factthat the final Au-rich rims on the late-stage hydrothermalpyrite are also strongly enriched in As, suggests that this finalgold pulse came from Au-Asbearing source rocks. The Au-As association in the reefs and sedimentary rocks is too inti-mate and coincident to accommodate a model involving dif-ferent source rocks for each metal. Tompkins (2010)demonstrated that mafic volcanics produce one to two ordersof magnitude less H2S than carbonaceous sediments duringmetamorphism. These lines of evidence appear to precludethe mafic volcanic rocks deep in the basin, because they arenot a suitable source rock for As and S, even if they were apossible Au source. Supporting evidence for this sedimentarysource model is (1) oxygen isotope data for over 400 quartzvein samples across the Victorian goldfield (Gray et al., 1991),which indicate a homogeneous hydrothermal fluid in equilib-rium with the local turbidite package; (2) sulfur isotope dataon 150 samples from pyrite in the sediments and quartz reefs

    which indicate that the reef pyrite has an isotopic signatureidentical to the mean value for the sedimentary host rocks(Thomas et al., in prep); (3) Br/Cl and I/Cl values of fluid in-clusions in the quartz reefs reflect fluid interaction with, orderivation from, sedimentary rocks enriched in organic mat-ter, and suggest negligible input of fluids from juvenile man-tle or deeply derived components (Fu et al., 2009); and (4)initial Re/Os ratios of 187Os/188Os = 1.04 0.16 (2), mea-sured on pyrite and arsenopyrite from an auriferous quartzvein, suggest derivation from a crustal source with very littlemantle component (Arne et al., 2001).

    A new genetic model for the gold-rich saddle reefs

    Our model for development of the gold-bearing saddlereefs (Figs. 26, 27) builds upon previous research (particu-larly that of Cox et al., 1991; Willman, 2007) and incorporatesdeductions based on this pyrite study. The process starts dur-ing diagenesis with preconcentration of Au, As, and othertrace elements (Mn, Zn, Mo, Cu, V, Ag, Cd, Tl, Te, Co, Ni, Bi,and Pb) in the organic-rich shale tops of turbidite cycles,where they become locked in diagenetic arsenian pyrite (Fig.26A). At the onset of compressional deformation and folding,flexural slip either within the thin organic-rich and struc-turally weak black shales, or at black shale-sandstone con-tacts, creates high permeability zones which focus silica-satu-rated metamorphic hydrothermal fluids to producebedding-parallel laminated quartz veins within the shalepackages (Fig. 26B). The fluids dissolve diagenetic arsenianpyrite from the shales to release Au and As into the hy-drothermal fluid for passage along the permeable laminatedquartz veins. Continued flexural slip during folding promotesdilation at the fold hinge zones. The hydrothermal fluids arefocused along the laminated quartz veins into the hinge zonesto form the saddle reefs (Fig. 22C; Cox et al., 1991). Gold andarsenic are transferred from the black shales to the reefs viathe laminated quartz veins by this process. However, the thickAu-As rims on pyrite within and proximal to the reefs indicatethat the main episode of hydrothermal gold and arsenic trans-fer occurred during the final stages of deformation. Based onprevious work (e.g., Cox et al., 1991; Schaubs and Wilson,2002; Willman, 2007), this probably occurred during maxi-mum shortening, with the development of throughgoing faultarrays linking adjacent anticlines and finally accompanyingfault lockup.

    At this stage a larger hydrothermal fluid reservoir wouldhave become available to source gold and arsenic from shalesdeeper in the Ordovician basin (Fig. 26D). The release ofmetals from these deeper sedimentary rocks was driven bythe pyrite to pyrrhotite metamorphic conversion, contribut-ing Au, As, and S to the fluid reservoir that was accessed bythe throughgoing fault array (Fig. 27).

    Mass-balance calculation for gold and arsenic

    Our LA-ICPMS analyses on diagenetic pyrites in shalesaway from the ore zones at Bendigo (Table 2) average 0.61ppm Au and 1,300 ppm As, giving an As/Au ratio of 2,131.The background As content of the shale whole rocks, awayfrom ore zones and visible alteration, is 23.4 ppm (Large,unpub. data). Using this value, and the mean measured As/Auratio in the diagenetic pyrite, it is possible to estimate the

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  • original gold content of the shales prior to metamorphism andhydrothermal leaching (23.4 1,000)/2,131 = 10.98 ppb Au.These estimated whole-rock values of about 11 ppb Au and23 ppm As for the premetamorphic Bendigo shales comparefavorably with recent data from Ketris and Yudovitch (2009),who report mean values of 7 ppb Au and 30 ppm As for car-bonaceous black shales generally. Assuming that the completeLower to Middle Ordovician turbiditic sedimentary packageat Bendigo is composed of 20 percent carbonaceous shalescontaining 7 ppb Au, and 50 percent of the diagenetic pyritein the shales becomes recrystallized and converted topyrrhotite during metamorphism, releasing all its containedgold and arsenic, then 1.9 t of gold (plus 6,270 t of As and35,000 t of S) is released to the metamorphic fluid per cubickilometer of sedimentary rock during the pyrite to pyrrhotitetransition. This means about 370 km3 of turbidites are neededto source the 700 t of gold produced from the Bendigo gold-field. Considering the area of the goldfield (7 20 km2), thisequates to sourcing the gold from a total of about 2.7-kmthickness of turbiditic sediments below the deposits. Thissource volume appears feasible, based on an inferred strati-graphic thickness of about 3 km and a structural thickness ofover 10 km for the Lower to Middle Ordovician turbiditepackage (Willman, 2007).

    A local sedimentary Au-As source for Bendigo may also ex-plain why the major gold deposits in the Victorian goldfieldsare all contained in the Lower to Lower Middle Ordovician(Lancefieldian to Castlemanian stages; Phillips et al., 2003).This may relate to the original syngenetic gold and arseniccontent of this interval being greater than that in the younger

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    A. Syngenetic-diagenetic low-level Au-As in shales

    shale

    sandstone

    B. Initial deformation-flexure slip nucleates LQveins in shales, which act as main fluid pathways

    LQ veins

    C. Continued shortening, Au and As leached fromshales and transported via LQ veins to saddle reefs

    saddlereef

    D. Final stages: through-going fault array links todeeper metamorphic basin fluids for final Au-As pulse

    deeper basin fluids

    40 m

    FIG. 26. Schematic evolution of mineralized reefs at Bendigo (modifiedfrom Cox et al., 1991; Willman, 2007). A. Syngenetic accumulation of Au andAs in organic-rich shale. Gold becomes locked in diagenetic arsenian pyrite.B. Initial deformation: Flexural slip on black shale contacts nucleate lami-nated quartz veins, which act as main fluid pathways. C. Continued folding:Au and As are leached from shales and transported via laminated quartz veinsto saddle reefs. D. Final stages of deformation: Throughgoing faults link ad-jacent anticlines and tap deeper metamorphic basin fluids leading to final,and major, Au-As pulse(s).

    METEORICRE-CHARGE ?

    py poreleases Au and

    As to fluid

    AuAs

    AuAs

    200 m

    Zone where pyrite is replaced bypyrrhotite in carbonaceous shales

    Pyrite stable in shales and sandstones

    Gold saddle reefs

    Laminated quartz veins

    Faults and relatedlaminated quartz veins

    Fluid-flow pathways

    FIG. 27. Geologic two-dimensional model, showing the relative positionsof the interpreted Au-As source rocks, Au-bearing saddle reefs, and signifi-cant structures that have focused fluid flow from the source rocks to the reefs(compiled using sections from Cox et al., 1991; and Willman et al., 2007).

  • and older strata. Productive gold deposits may be absent tothe east of the Bendigo zone in the Melbourne zone, if theshales and cherts to the east are lacking in syngenetic gold, oralternatively, as suggested by Wilson et al. (2009), the under-lying Selwyn block shielded the overlying strata from perva-sive metamorphism, deformation, and faulting.

    ConclusionsThis study further establishes the value of conducting an LA-

    ICPMS study into the trace element composition of sulfides(especially pyrite) at the ore-deposit scale. Pyrite is formedthroughout the ore-deposit evolution and enables a near-totaldocumentation of fluid-flow events to be captured, from earlydiagenesis through to peak- and postmetamorphic events.

    Our work has demonstrated the effects of host-rock com-position and texture on the enclosed pyrite. Shales are oftenmore favorable hosts for diagenetic pyrite due to their highorganic content and generally anoxic, reducing conditions; itis inferred that diagenetic and/or early hydrothermal pyritesscavenged trace elements from their immediate organic-richvicinity, due to shales low effective porosity and the early par-agenetic timing of the pyrite. Sandstones and siltstones arehost to later hydrothermal pyritewith a more pervasivefluid flow accommodated by their higher effective porosity.The pyrite present in the sandstones generally exhibits perva-sive growth zoning and as such can provide a better record oflater fluid-flow events through the sedimentary rocks. In or-ganic-rich black shale beds deeper in the section pyrite is par-tially to completely replaced by metamorphic pyrrhotitewhich becomes aligned in the metamorphic fabric.

    Diagenetic pyrites present in shale horizons exhibit a con-sistent association of elevated Bi, Ni, Cu, Mo, Ag, V, and Pb,with a lower Au/Ag ratio than demonstrated in hydrothermalpyrites. Hydrothermal pyrites are commonly euhedral,growth zoned, and exhibit a simpler trace element associa-tion, exemplified by a strong gold-arsenic relationship wherethe concentration of gold retained in the structure of pyrite isdirectly proportional to the arsenic concentration. They alsoexhibit higher Au/Ag ratios, similar to the Au/Ag ratio (orfineness) of the free gold in the saddle reefs. Pyrites adjacentto productive saddle reefs have gold-arsenic rims analogousto the fine (220 m) gold rims seen at Carlin, Nevada (Hof-stra and Cline, 2000; Large et al., 2009). Disseminatedpyrrhotite, formed by the replacement of pyrite in the shalesbelow the gold reefs, is totally devoid of gold and arsenicwhich have been lost to the metamorphic fluid.

    The consistent complex trace element association presentin early pyrite leads to some interesting questions with re-gards to the origin of some of the elements responsible forgenerating the ore deposit in question. Many workers havepostulated that the source of the gold for the Bendigo golddeposits is the Cambrian mafic volcanics which underlie theturbidite pile at a depth of greater than 3 km but neglect toconsider the source of the arsenic and sulfur. We proposethat the initial source of gold, arsenic, and sulfur is from theshale host rocks, generated by the recrystallization and de-struction of early diagenetic pyrite to pyrrhotite. The deepersedimentary strata reaccessed only after fold lockup and theformation of throughgoing structures during the later stagesof the hydrothermal events, which correspond to the peak in

    gold-arsenic deposition. It is not possible, based on the datapresented here, to resolve whether the later hydrothermalgold events introduced new gold to the Bendigo vein systemfrom a different genetic source, or simply remobilized andreconcentrated gold leached from the preenriched sedimentsof the first event. However, we believe that the coupled Au-As relationship observed throughout the pyrite paragenesis inboth the host rocks and the gold reefs at Bendigo is strong ev-idence for a sedimentary source.

    AcknowledgmentsLeonid Danyushevsky is thanked for his help and support

    in leading the analytical developments at the CODES LA-ICPMS laboratory. The authors would like to thank BendigoMining Ltd. for access to samples and information about thedeposit, particularly mine cross sections. This research wasfunded by Bendigo Mining Ltd. and the Australian ResearchCouncil through the ARC Centre of Excellence program.Peter Schaubs and Robert Hough are thanked for their thor-ough and constructive reviews of the manuscript.

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