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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pyrite in the most car-bonaceous shales has been replaced by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
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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).
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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.
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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
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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).
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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
PYRITE AND PYRRHOTITE AT BENDIGO MINE, AUSTRALIA 7
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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
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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.
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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
22 THOMAS ET AL.
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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.
-
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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
26 THOMAS ET AL.
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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
28 THOMAS ET AL.
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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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