American Mineralogist, Volume 82, pages 182-193, 1997

Gold-bearingarsenianpyrite and marcasiteand arsenopyritefrom
CarlinTrendgolddeposits
and laboratorysynthesis
Mrcuanr, E. Fr-nnr' lNn A. H.lvrrn MuvrrN2
rDepartmentof Earth Sciences,University of WesternOntario, London, Ontario, CanadaN6A 5B7'?Departmentof Geology, Brandon University, Brandon, Manitoba, CanadaR7A 6A9

Ansrntcr
Invisible gold in natural and synthetic arsenian pyrite and marcasite correlates withanomalousAs contentand Fe deficiency,and high contentsof invisible gold in most naturaland all synthetic arsenopyritecorrelate with excessAs and Fe deficiency. As-rich, Fe-deficient arsenopyritesynthesizedhydrothermally contains up to 3.0 wt%oAu uniformly distributed in growth zones of light backscatteredelectron contrast. At the Deep Star gold
deposit, Carlin Trend, Nevada, the sulfide compositions apparently span the full range ofmetastability from FeS, to near FeAsS (40 atvo S); arsenianpyrite containsup to 0.3'7wt%oAu, but arsenopyritehas excessS and is relatively Au poor. Observed minimum Fe contents are 29.1 atvoin arsenianpyrite and marcasitefrom the Deep Star depositand 31.3
at%oin synthetic arsenopyrite.We suggestthat invisible gold in arsenianpyrite and marcasite and arsenopyritefrom sediment-hostedgold deposits representsAu removed fromore fluids by chemisorptionat As-rich, Fe-deficient surface sites and incorporatedinto thesolids in metastablesolid solution. However, the oxidation state of invisible gold (Auo,
Au'*) remains uncertain becausethe chemisorptionprocessis intrinsically nonsystematicin terms of crystal-chemicalparametersand does not result in definitive atomic substitution
trends.

INrnolucrroN
In sediment-hostedgold deposits and some mesothermal lode-gold deposits, a substantial fraction of Au ispresentas "invisible gold" within grains of iron sulfide
and sulfarsenideminerals (principally pyrite and arsenopyrite). Previous studies have establishedthat invisible
gold is spatially associatedwith local As enrichment inpyrite grains(Wells and Mullens l9l3;Fleet et al. 1989;
Cook and Chryssoulis1990;Bakken et al. 1991;Fleet etal. 1993; Arehart et al. 1993; Mumin et al. 1994; Michel
et al. 1994).Fleet et al. (1993)concludedthat Au is probably incorporatedinto metastablesolid solution in arsenian pyrite via As-rich growth surfaces.Similarly, Arehart
et al. (1993) suggestedthat Au in arsenianpyrite is present as a chargedspecies(Au.* ) and was probablydeposited with As as coupledsubstitutionsin pyrite. However,the chemical stateof invisible gold in pyrite remainscontroversialbecause(l) the substitutionreaction(s)has notbeen demonstratedthrough element correlations,(2) submicroscopic particles of Au (colloidal gold) may be present (e.g.,Bakkenet al. 1989;Cook and Chryssoulis1990;
Mao 1991; Fleet et al. 1993), and (3) available
spectroscopic techniques yield ambiguous results fortrace amountsof Au (e.g., Mcissbauerspectroscopy:Cathelineauet al. 1989;Wagneretal. 1994;X-ray photoelectron spectroscopy(XPS): Mycroft et al. 1995).
Fleet et al. (1989, 1993)and Maclean (1991)developed a potassium permanganatestaining technique forooo3404x/97l0102-0I 82$05.00

polished sectionsof ore minerals and used it to map the
As distribution in pyrite grains from a wide variety ofgold deposits.The resulting complex optical micrographs
revealed both multistage and episodic rock-fluid activity
in in situ gold-deposit systems.An early application of
their surface-stainingprocedureshowedthat the compact
rounded pyrite grains in quartz-pebbleconglomerategold
ore from the Witwatersrand basin of South Africa wereindeed detrital in origin (Maclean and Fleet 1989), thus
providing further evidence for a detrital origin for thisimportantclassof gold deposit.From Fleet et al. (1989,1993),Maclean (1991),Mumin et al. (1994),and unpublished work, authigenic arsenianpyrite appearsto be
associated with sediment-hosted,epithermal, and lowtemperaturemesothermalgold depositsin carbonate-bearing host rocks that have not been metamorphosedbeyond
middle low-gradeconditions.
Fleet et al. (1989) reported As contentsin pyrite fromgold deposits of up to 8 wt%o(5 at%o),far in excess ofthe maximum As content expected from laboratory experimentsin the dry Fe-As-S system (Clark 1960) extrapolated to low and very-low geological temperatures.
They demonstratedan inverse correlation of As with S
consistentwith the solid solutionFe(S,-,As,),for the natural arsenianpyrite of their study. However, the compositions of natural and syntheticarsenopyrite(FeAsS)have
been interpreted as indicating both a ligand and a cationrole for As (e.g., Johan et al. 1989; Wu and Delbove

r82

FLEET AND MUMIN: AU IN PYRITE, MARCASITE, AND ARSENOPYRITE

183

Mountains, Nevada, and includes severalvery large gold
deposits(Post-Betze,Genesis,Gold Quany, Carlin) andnumerous smaller ones, including the Deep Star deposit.
The gold deposits in the Deep Star-Genesis-Post-Betze
areaare spatially associatedwith the Goldstrike intrusion,
a composite dioritic to granodioritic stock of Cretaceousage (about 150 m.y.), which has locally metamorphosed
the overturned middle Paleozoic strata to hornfels andmarble with extensiveexo- and endoskarns.The ore mineralogy consists of pyrite, arsenianpyrite, arsenianmarcasite, arsenopyrite, realgar, and orpiment. The age ofmineralizationhas been constrainedto the period between
120 to 35 m.y., althoughthereremainsconsiderablecontroversy on the precise timing in individual deposits(Christensen1993; Rota 1993). Higher level and lower
temperature ores are disseminatedand clearly replacement type in nature, and are located where permeable
stratigraphicunits provided channelwaysfor mineralizing
fluids. The ore mineralogy is very-fine-grainedpyrite andmarcasite. With increasing depth in the ore system or
proximity to structural feeder zones, changesin both ore
textures and mineralogy suggestprogressivelyincreasing
temperaturesof deposition.Deeper level and higher temperature ores are largely structurally controlled and associatedwith fault breccias.The ore mineralization is oflarger grain size and consists of pyrite, marcasite, andincreasingly abundant arsenopyrite.At the Gold Quarry
InoN sur-rrlEs AND SuLFARSENTDES
FRoM
and Genesis deposits the fault breccias associatedwith
Clnr,rN TnrNn DEPosrrs
the deeperlevel ores evidently representstructuralfeeder
The prominent ranking of the United Statesin world zones for the higher level replacementores. Supergene
gold production is due mostly to output from the sedi- oxidation of sulfides along the Carlin Trend extends toment-hosted(Carlin-type) depositsof Nevada. The geol- variable depths of up to 300 m.
ogy and origin of these important gold depositswere reThe Deep Star deposit,from which most of the present
viewed recently by Berger and Bagby (1991) and samples were obtained, consists of deeper level, faultore only. The depositis hostedwithin exoChristensen(1993). The gold depositsare both passive breccia-related
replacementsand hydrothermal and tectonic brecciasand and endoskarnsof the contact metamorphic aureole asoccur predominatelyin Paleozoicand early Mesozoic ma- sociatedwith the Goldstrike intrusion. Host rocks for therine sedimentaryand volcanic rocks, and particularly in mineralization include calc-silicate hornfels and marble
carbonaceous,finely laminated silty carbonates,which of the Popovich Formation, and strongly altered portions
are commonly dolomitic, and carbonate-bearing silt- of the intrusion. Alteration and ore minerals include
quartz, sericite, dolomite, kaolinite, montmorillonite. pystones. There is often a close spatial association withgranodioritic or alkaline igneous intrusions of Jurassic- rite, marcasite,arsenopyrite,realgar, orpiment, and trace
Cretaceousage. The unusual and characteristicore min- amountsof other sulfides.The Au:Ag ratio is high (up toeralogyof invisible gold in As-rich rims of pyrite grains 50:l), and the Au gradeincreaseswith the presenceandhas been extensively studied (e.g., Wells and Mullens abundanceof very-fine-grainedquartz-kaolinitealteration
1973; Radtke 1985; Bakken et al. l99l; Arehart et al. and high pyrite-marcasite content. Abundant ultra-fine1993). Bakken et al. (1991) used secondaryion mass grained pyrite and marcasite give ore rocks a sooty apspectrometry(SIMS) to show that Au was associatedwith pearance,which can be mistaken for graphite.
As- and Sb-bearing rims of auriferous pyrite from the
Samplesof gold sulfide ore from the Deep Star, Gold
Post deposit. Colloidal gold particles were not identified Quarry, Genesis,and Carlin depositswere obtained durat 20-50 A resolution in their parallel transmissionelec- ing a visit to the Newmont Gold Mining operations attron microscopy (TEM) study. Arehart et al. (1993) stud- Carlin. Nevada.in December1994.Polishedthin sections
ied samplesfrom the Chimney Creek, Gold Acres, Gold typical of unoxidized moderate- and high-grade faulr
Quarry, Post-Betze,and Rain deposits,using scanning breccia ore from drill holes DS-3, DS-9, DS-63 (Deepelectronmicroscopy (SEM) and SIMS. They reportedthat Star deposit), BSN-389 (North Star deposit), CAT-699
largely with arsenianpyrite present (West Genesisdeposit),QRC-644 and QRC-1322(Goldthe Au was associatedas rims on, and narrow veinlets in, Au-free pyrite.
Quarry deposit),and hand samplesfrom the Genesisopen
The Carlin Trend follows the line of the Tirscarora pit were investigated.Polished thin sectionstypical of1989). For arsenopyritefrom the Central Massif, France,
Johanet al. (1989) reportedenrichmentin Au and Sb of
up to 1.5 and 1.6 wtvo, respectively,and a marked deficiency of Fe, which was interpretedto representthe complex solid solution (Fe,Au,As,Sb)As,*,S,*,and a substitution mechanismof 2As = Au (or Sb) + Fe, In apparent
contrast,Wu and Delbove (1989) synthesizedcompositionally zoned arsenopyritewith up to 1.7 wt% Au in thesystemFe-As-S-O-NaCl(KCl)-H,Oat 500'C and 2 kbar,
and reported a substitutionmechanismof Fe : Au. Moregenerally, the role of As and the mechanismsof valencecompensation in pyrite-marcasite-arsenopyritegroupcompoundsremain mysterious(e.g., Tosselet al. 1981;
Wijeyesekeraand Hoffmann 1983). Kuranti (1941) iscredited with documenting solid solution of Au in synthetic pyrite from chemical analysisand unit-cell spacing,but no details are available and his experimentshave notbeen repeatedwith modern analytical procedures.
We presently report on Au-bearing arsenianpyrite andmarcasite and arsenopyrite from Carlin Trend deposits,
Nevada, and from laboratory synthesis.The study further
documentsthe extraordinary control of surfacechemistryon the uptake of Au and As by these minerals and the
crystallizationof metastablesolid solutions.However,theprecise oxidation state of the Au incorporated into these(Au0,{ut+) remainsuncertain.
sulfidesand sulfarsenides

184

FLEET AND MUMIN: Au IN PYRITE, MARCASITE, AND ARSENOPYRITE
50 at% Fe

mercial synthetic pyrite (FeSr), natural arsenopyrite,As,S, and an As-S compositionwith about60 atVoAs (AS1),
g NaCl (or KCl, CaCO,), and 0.01 g H,O (Fig. 1;
0.02
+-! +
Table 1). Synthetic starting materials were preparedfrom+++,
TaAspl+high-purity elements,salts were ACS cenified, and HrOT
was deionized.Starting materialswere generallygranular;
they were not ground and mixed, but some mixing did
occur during loading of the charges.Gold capsuleswere
gold and were the sourceof Au in the experimenAs
Rg orp
S of finetal system.The experimentalconditions varied from 205
Frcunn1. Bulk Fe-As-Scompositions
investigatedin hydro- to 605'C, 1.3 to 1.6 kbar, and 3 to 57 day run times. In
thermalexperiments
at 205-605'C, 1.3-1.6kbar(plussigns). general, ltillingite and westerveldite were investigatedat
Opensquares
areidealcompositions(We),pyrof westerveldite
300-600'C, arsenianpyrite
rhotite/troilite(Po), killingite (Lo), arsenopyrite(Asp), pyrite 500-600'C, arsenopyriteat'C, and FeSr-AsS-S-composiand
marcasiteat300-500(Py),marcasite(Ma),realgar(Rg),andorpimenr(Orp).
tion material at 200-300 'C. All experiments werequenchedin water.Chloride salts were removedby washunoxidized moderate- and high-grade disseminatedore ing in warm water, and the residue was dried in air at'C for l-2 h. The sulfide products were investigatedfrom underground workings immediately beneath the 105
open pit at the Carlin deposit and from the Gold Quany by ore microscopy, potassium permanganate staining,open pit were also investigated. The ore samples were EMPA. and X-ray diffraction.
Fe, S, As, and Au were determined in situ for bothstudied by ore microscopy, potassium permanganatestaining, and electron microprobe analysis (EMPA; natural and syntheticore minerals of this study by EMPA,
using a JEOL JXA 8600 Superprobeoperatedat 20 kV
below).
and20 nA, a beam diameterof 2 p,m,the AuZcr line, andExpnnrlrnNrAt. METHoDs
50 s counts on peak and background for Au. Selected
Ore minerals in the system Fe-As-S-Au were synthe- spot analyseswere repeatedat25 kY and 35 nA with nosized hydrothermally using standard Tuttle cold-seal significant change in the results. Natural pyrite was usedbombsand gold capsules.Chargesconsistedof 0.03-0.08 to calibrate Fe and S in pyrite, arsenianpyrite, marcasite,
g of Fe-As-S, preparedvariously from FeAs, FeS, a com- and pyrrhotite, and arsenopyritewas used for Fe and S
TeeLe 1. Representative
EMPA compositionsfor phases in the Fe-As-S-Ausystem
Phase
Expt.

We
SA20

Po
SA22

Po
SA27

SA21

Fe
As

63.0
0.1
37 1
0 06
100.2

60 4
0.0
39.4
0.03
99.9

27.4
70.4
2.0
0.19
100.0

30.4

Au
Total

43.8
57.2
0.0
0.0
101.0

Fe
As
S
Au

50.7
49.3
00
0.0

49.4
0.0
50.6
0.01

46.8
00
532
001

328
62 9
4.2
0.06

Fe
As
S
FeAs
FeS
Py
As
S
As1

34.8
65.2
0.0
X

50.0
260
240
X
X

33 1

29.7
60.6
9.7
X

r fc)

505
1.6
NaCl

5

LO

Apv
SA18

Apy
SA43

Ma
SA7

Ma
SA7

Py
SA3

Py
SA3

Py
SA3

Py
SA9

'14
7
087
995

wP/o
35 2
242
39 9
0.14
99.4

42.0
1 05
48.9
0.0
1 0 1. 3

38 I
126
49.4
0.35
1 0 1. 2

43 5
70
49.6
0.08
1 0 01

44.8
29
53.0
0 06
1 0 07

440
6.2
49.6
0.06
99.8

43.0
7.7
486
037
997

31.6
41.5
26.6
0 26

32.7
27.7
39 2
0.04

31 1
5.8
63.1
0.0

29.O
70
64.0
0.07

32.2
39
64.0
002

322
1.5
66.3
0.01

325
34
63.8
0.01

32.2
4.3
63.4
0.08

alo/o

P (kbar)
salt
r (d)

tz 3

544
X

Starting composition (atolo;materials)
32 7
30.8
29.8
345
249
79
32.7
44 3
62 3

X

ob5

6.C

70.6

XXX
X

605
16
NaCl

505
1.6
NaCl
6

310
8.8
603
X

X
X
X

505
1.6
NaCl

210

X

XX
X
X
Experimentalconditions
414
400
310
15
t.5
14
NaCl
14

NaCl
57

NaCl
20

X
X

X
X

310
1.4
NaCl
20

405
tc

KCI
7

N o f e . A s l i s A s * S * ;W e : w e s t e r v e l d i t e , P o : p y r r h o t i t e , L o : l o l l i n g i t e , A p y : a r s e n o p y r i t e , M a : m a r c a s i t e , P y : p y r i t e , X : s t a r t i n g m a t e r i a l

FLEET AND MUMIN: Au IN PYRITE, MARCASITE, AND ARSENOPYRITE

in analysesof arsenopyrite,ldllingite, and westerveldite.
The arsenopyritestandardwas used for As, and pure gold
for Au. We found that our sample of Asp57 of Kretschmar and Scott (1976) exhibited too much spatialvariationin composition for use as a standard,but our sample of
Asp200 was surprisingly homogeneousand was therefore
adopted as the arsenopyrite standard. Back analysis of
this standardover l6 separateEMPA sessionsresultedin
an averageFe contentof 34.53 + 0.18 wt%o(Y. Thibeau,personal communication); Kretschmar and Scott (1976)
reported an averagevalue of 34.51 wt%oand a range from33.8 to 35.3 wt%o.
The minimum detection limit for Au in pyrite fromrepeatedanalysesof the pyrite standardand many analyses of Au-poor arsenianpyrite was about 300 ppm (cf.
Cabri et al. 1989; Cabi 1994, personal communication).
We appreciatedthat the minimum detection limit for Au
is markedly reduced at higher voltages and filament currents: Robinson and Graham (1992) reported values as
low as 10 ppm for Au in sulfides at 40 kY, 470 nA, andcount times of about 30 min. However, becauseof theirfine grain size and susceptibility to beam damage,manyof the materials presently analyzedwere quite unsuited
to these advancedtechniquesof analysis.
About 800 spot analysesof natural and synthetic ore
minerals were made, although several hundred of thesewere reconnaissancein nature. Representativeresults forthe synthetic phasesare given in Table l.
Grain fragments of selected experimental products
were investigated by X-ray precession photography
Trele 1. Extended.
Fe52-AsS-S-compositionmaterial

Phase
Expt

SA31

SA31

SA31

43.8
3.2
52.6
0.0
99.6

33.2
1 34
51 I
013
98.5

zvc

Fe
As
s
Au
Total

46.6
0.0
53.1
0.0
99.8

43 1
43
51.1
0.0
98 5

wl"/"
31.5
20.4
47 0
00
98.9

Fe
As
S
Au

33.5
0.0

31 .8

24 5

31.7

24 7

2.4
65.8
00

1 1I
63 7
0.0

1.7
66.3
00

74
67 1
0.03

200
cu. I

010
994

aP/o

Fe
As
D

FeAs
FeS
Py
As
SX
As1

rfc)

oo-J

0.0

Starting composition (atolo;materials)
205
19.0
20.5
19.0
CVU
62.0
X
X

X

215

P (kbar)
14
salt
NaCl
(d)
f
57

Experimental conditions
215
1.4
KCI
57

113
oou

0.02

185

(MoKa, \ : 0.71069A). X-ray diffraction (XRD) powder patterns of FeSr-AsS-S-composition material andother polycrystalline experimental products were obtained using a Gandolfi powder camera (CrKct, \ :2.2909A).
ExppnrlrnNrAr- RESULTS
Phasesencounteredin the presentinvestigation of the
Fe-As-S system at Au saturationincluded westerveldite,pyrrhotite, killingite, arsenopyrite, arsenian pyrite andmarcasite, pyrite, arsenic, realgar, orpiment, sulfur, andvarious other compositionsor mixtures on the As-S join,material.In experimentsat
and FeSr-AsS-S-composition
500'C and below, crystal grainsof native gold were notpresent(native gold was presentonly as physical contamination by capsulemetal), and colloidal gold was not suspected during EMPA of sulfides and sulfarsenides.Smallblebs of gold were commonly presentin the products ofthe four experimentsat 605 'C, conflrming the observations of Clark (1960) on the corrosive effect of arseniansulfide fluids on gold capsules.
Progressof reaction was very much dependenton thechernical combination of Fe, As, and S in the starting
composition and on experimental temperature. Also,chemical communication within the chargeswas limitedby the low proportion of fluid to solid and the lack ofintimate mixing of starting materials. At and below 400'C, starting FeS was invariably incompletely reactedwith
excess As and S, tending to be partly replaced and armored by arsenopyrite,marcasite,and pyrite. At 200-400'C, starting pyrite reactedwith excessAs, and S was internally desiccated.At 300-400'C, starting arsenopyrite
alone showed no evidence of reaction with the fluid, either at grain boundariesor along fractures, and retained
primary compositional
zoning.
A starting mixture of FeAs + NaCl + HrO yielded
westerveldite + lcillingite + iron oxide (or fayalite withaddedSiO,) at 500-600'C.
Arsenopyrite crystallized fairly readily by heterogeneous nucleation from mixtures of FeAs and excess As
and S, but the productswere very-fine grainedand locallyoccurred as crystal masses.Crystallization of arsenopyrite from FeS and excessAs was complex and temperature dependent,involving both replacementand heterogeneous nucleation. In general, progress of reaction
varied markedly with communicationin the gold capsule,with the least reaction (and uptake of Au) at the bottomof the charge and furthest away from the bulk of the fluid
phase.Four texturally distinct types of arsenopyritewere
generally present:topotaxial replacementproduct of pyrrhotite (starting FeS transformed into nonstoichiometricpynhotite); euhedralacicular grains forming a rim, overgrowth, or both to arsenopyrite l; isolated or matted
euhedralaciculargrains,usually <10 pm in crosssection(Fig.2a; cf. Wu and Delbove 1989);and euhedralaciculargrains (as arsenopyrite2) forming a rim, overgrowth, orboth to unreplacedpyrrhotite grains. Arsenopyrite I was
the most common product, with replacementoriginating

r86

FLEET AND MUMIN: Au IN PYRITE, MARCASITE, AND ARSENOPYRITE

at grain boundaries of massively aggregatedpyrrhotite.
The euhedral varieties of arsenopyritepreferentially developed from loosely aggregatedstarting materials or at
locationswith locally high fluid/solid ratios; e.9., matted
crystals of arsenopyrite3 developed in fluid pockets orveins within the pyrrhotite nutrients. Experiments at
about 400'C and of 1, 4, 74, and 57 d duration showedthat all four textural varieties developed simultaneously.
With increasein reaction and annealingtime, arsenopyrite
I becamemore pervasive and the heterogeneouslynucleated varieties more euhedral.Also, compositional zoning
present in the proto crystals did not homogenize withannealing.
Arsenian pyrite developed locally as a marginal alteration of starting pyrite and along cleavagetraces of desiccated pyrrhotite in experiments at and above 300 "C.
Marcasite was invariably arsenian and also locally developed with arsenopyrite,marginal to and within desiccated pyrite and as a narrow rim on pyrrhotite. It was
identified petrographically and by the characteristic 110
line in powder XRD patterns of composite pyrite-marcasiteintergrowths.
Material with a bulk composition field of FeS,-AsS-S(Fig. 1), with many compositionsfalling in the more restricted field defined by the FeSr-AsS and FeSr-AsS,tielines,was first observedin minor amountsin experiments
at and above 300 "C. The proportion present increased
with decreasing temperature, and in experiments conducted at about 200'C, with the appropriatebulk composition and starting materials of either FeAs + S or pyrite + As 1 S, FeSr-AsS-S-compositionmaterial was the
major product phase. It occurred as compositionallyzoned botryoidal massesand oscillatory-zonedoolitic
bodies up to 100 pm in diameter (Figs. 2b and 2c). Individual zones were isotropic and homogeneousat theresolution of reflected-light microscopy and EMPA, andgradedin width down to submicroscopicsize. Optical reflectivity varied from high to low, for Fe-rich to Fe-poorcompositions, respectively. However, Gandolfi XRD
powder patterns of both homogeneousand zoned samples, some of which had been analyzed by EMPA andremoved from polished sections,revealed only pure pyrite and orpiment, with trace marcasite and other As-S
phases.Thus, the FeSr-AsS-S-compositionmaterialwas
a composite of ultra-fine grains of phasesof ideal endmember composition. It may have existed as a gel in the200-300'C experiments,as suggestedby its desiccatedappearanceand porous core region (Figs. 2b and 2c), but
this possibility was not investigatedfurther. FeSr-AsS-Scomposition material appearedto be a quench phase in
Frcunn 2. Backscatteredelectron images.(a) Matted euhedral acicular grains of arsenopyritewith zones of light, medium,the higher temperatureexperiments.
A selectionof EMPA compositionsis given in Table and dark contrast; SA42 (400 "C, 57 d), scale bar is 10 pm. (b)I, and compositionsof arsenopyriteare plotted in Figures Compositionally zoned botryoidal FeSr-AsS-S-compositionma3 and4. With the notableexceptionof FeSr-AsS-S-com- terial; light contrast is As rich, dark contrast is FeS, rich; SA30
(215'C,57 d), scalebar is 100 pm. (c) Oscillatory-zoned
oolitic
position material, the phasespresentgenerally conformedmaterial; SA29 (215 "C, 57 d), scale
FeS,-AsS-S-composition
to the known phase relations (Clark 1960; Kretschmar
bar is 10 pm.
and Scott 1976).All sulfidesand sulfarsenides,with thesingle exception of westerveldite, contained sporadic

t87

FLEET AND MUMIN: Au IN PYRITE, MARCASITE, AND ARSENOPYRITE

1.0a) 1.s

o
(o

'| .3

o

1.2

o
o

1.1

()

1.0

o.7
aa

(I'

E

0.5

o
o,

aa
aa
a
aa

0.7

i

0.4

@.tr

0.3

E

33

32

iron (at%)
1.5synthetic
arsenopyrite

1.4
o

1.3

8

()

1.2

o

o

1.1

o
€

O1(( D

0.1
34

30

o

q ++

I

rE

2
9P

I

synthetic
arsenopyrite

Cg

Sa"

0.2

a

I
I

tt

0.0

b)

Etr
o
@

E

u.6

o

s

o

l

f

synthetic
arsenopyrite

tr

8e

o
a
(tt

=

0.9

o

1.4

'%
o
[- . o34

otiron (at%)
Frcunn 4. Variationof Au with Fe in euhedralgrainsof
syntheticarsenopyrite.
Open symbolsare zonesof light BSE
contrast(cf. Fig. 2a), centeredsymbolsare zonesof medium
contrast,and solid symbolsare zonesof dark contrast;squares
areSA42(400"C,57 d), circlesareSA41(400"C,57 d),crosses
are dataof Wu and Delbove(1989),line is substitutionFe :Au; all dataarecalculated
to theformula(Fe,Au,E)As,.,S,.,.

the maximum Au contentof 0.25 wtVo was significantly
lower
than that in arsenopyritefrom the sameexperiment,9(0.94wt%o).
(l)
0.9o
The two arsenopyriteanalysesin Table I (Fe.,rAs,,rS.r,o.
are representativeof the composition
andFejruAso,.Sruu)
t
!
!r
o
o .'
t '.
a
:
f
products. The total variations
range
of
our
experimentalo.
.
....t l.t
o.7 ?
.t
.
in Fe (31.3-33.4),As (27.2-41.5),and S (26.6-39.6at%o)oaare somewhat greater than those reported for the dry
0.6
Fe-As-S system(Kretschmarand Scott 1976)and nature(Klemm 1965; Kretschmarand Scott 1976; Sharp et al.
U.J
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1985). Most of our syntheticarsenopyritewas deficientin Fe relative to the ideal FeAsS formula (Figs. 3a andgold (at%)4). We observed the expected correlation of S/As ratio
Frcurn 3. Compositionsof synthetic arsenopyrite.The data with bulk composition and temperature,but the inhomoare EMPA results for different growth zones of euhedralgrains
geneity of our products did not permit precise thermomfrom five experiments.Solid circles are results for experiments
etry over the small temperatureinterval investigated inwith bulk compositions close to ideal FeAsS stoichiometry,(400-500 "C; cf. Kretschmarand Scott 1976).Also,sA13 (500'C, 3 d), SA18 (415 "C, 14 d), SA4l (400"C, 57 d), detailarsenopyrite
1, the replacement product of pyrrhotite,
SA42 (400 "C, 57 d), and open circles are results for a bulk
composition intermediatebetween FeAsS and FeS., SA43 (400 containedsignificantly lower As and Au contentsthan the'C, 57 d). Note that almost all arsenopyrite
compositionsare Fe euhedral,heterogeneouslynucleatedarsenopyrite.
deficient (a), and high Au contents are restricted to As-excess
The Au contents of the synthetic arsenopyriteranged
arsenopyrite(b).
(5A41,400'C,57 d; Fig.3b),u p t o 3 . 0 w t V o , 0 . 8 5a t % oa value far in excessof that reported for natural arsenopyrite (1.5 wt%o:Johanet al. 1989) and syntheticarseamounts of invisible gold. Westerveldite had a slightly
nopyrite (1.7 wt%o;Wu and Delbove 1989).The highest
metal-excess composition (Fe > As) and did not contain Au contents were in euhedral grains from experiments
S when crystallized from ternary starting compositions.
with a starting mixture of FeS + As in the proportion of(SAl3, 500 'C, 3d; SAl8, 415'C, 14
Similarly, Fe-rich pyrrhotite had only a trace amount ofideal arsenopyrite
As, and S-rich pynhotite had no detectable As. The maxd; and SA41 and SA42, 400'C, 57 d). High Au contents
imum S content observed in lcillinsite was 4.2 at%o.and correlated only with compositions that were As rich rel(I'at.u

iit:l .,.:f:..Tti,;

188

FLEET AND MUMIN: Au IN PYRITE, MARCASITE, AND ARSENOPYRITE

32.6 66.9 04

0

3t 4 61.6 6,7 3400
321 63I

3,7 0

Frcunn 5. Digitally processed
EMPA BSE imageof compositepyrite grain from the Deep Star deposit,Carlin Trend,
EMPA BSE imageof comNevada,showingAs core,broadAs-poormargin,andAu-bearing, Frcunn 6. Digitally processed
zonedarsenian-pyrite-marcasite-arsenoFe-deficientarsenianpyrite rim. Compositions
are reportedas posite,compositionallyatomicpercentfor Fe, S, andAs, andpartsper million for Au. pyrite grainfrom the DeepStardeposit,CarlinTiend,Nevada,arereportedas
overgrowth.Compositions
Notethereplacement
relationships
betweencompositionalzones. showingarsenopyrite
atomicpercentfor As andpartsper million for Au.
ative to the ideal formula of FeAsS (Fig. 3b). Composi- icates and carbonates, as internal growth bands intions showing excess S had only sporadic, low amounts composite pyrite grains
lsimilar to the oscillatory-zonedof Au, which were generally lower than those reported grainsdescribedby Fleet et al. (1989, 1993)and Mumin
below for coexisting arsenianpyrite and marcasite.The et al. (1994)1,in composite,compositionallyzonedarsenhigh Au contentsare restrictedto composition zonesthat ian-pyrite-marcasite-arsenopyrite
grains (Fig. 6), and as
are light (electron rich) in backscatteredelectron images dispersed,heterogeneouslynucleated and relatively ho(BSE; Fig. 2a). There is so much Au in theselight zones mogeneousmicrometer-sizegrains. Our observationson
that it largely accountsfor the BSE contrast observed.
the common associationof Au-rich arsenianpyrite overThe As content of arsenian pyrite ranged up to 9.3 growths on As-poor pyrite (Fig. 5) were generally conwt%o,5.0 at%o.Au was detectablein about one-half of sistent with Arehart et al. (1993). However, the Au-bearthesespot analysesbut generallywas less than 0.1 wt%o; ing arsenian pyrite rims appeared to be in boththe highest value recorded was 0.64 wt7o. Arsenian mar- replacement and overgrowth relationship with As-poor
casite was more As rich than the arsenianpyrite, with a (Au-free) pyrite grains. The detailed micrograph (Fig. 5)
maximum As contentof 16.5wtVo,9.'7atvo.An contents reveals three generationsof pyrite crystallization: an arwere again quite variable, and the maximum value was senian Au-free core region; an Au-poor pyrite that partly
0.35 wt%o.The FeSr-AsS-S-compositionmaterial was replacedthe precursorpyrite and also crystallized by hetalso gold bearing, in variable amounts ranging up to a erogeneous nucleation; and a narrow marginal overmaximum recordedvalue of 0.13 wt%o.
growth (or perhapsreplacement)of Au-bearing arsenian
pyrite. We also observed appreciablyhigher Au contents
Drscussrox
than Arehart et al. (1993) in some ore samples(up toGold-sulfideassociationin Carlin Trend deposits
0.37 wtVo).
The present gold-sulfide ore samples from the DeepNo special precautions were taken to avoid physicalStar deposit consisted of structurally controlled breccia contamination of ore minerals during preparationof polore (Clode 1994; authors' work in progress).They con- ished sections.Smearing of soft ore minerals during metained up to about 30Vo of ultra-fine-grainedpyrite and chanical polishing can introduce significant error into themarcasite, which imparted a black sooty appearanceto quantitative determination of minor amounts of Au andhand samples,and arsenopyrite,and tp to l0%oorpiment Ag by EMPA (e.g.,Remondet al. 1981, 1987).However,* realgar,and assayedbetweenl.ll and5.04 ouncesper this type of interferencewould have been minimal in theton of gold.
present study. Native gold was not present in our ore
Pyrite was disseminated within fragments of host samplesfrom the Deep Star deposit and is relatively unrocks, as vein filling and cement, and was predominately important in Carlin Trend depositsin general (e.g., Bakin the form of As- and Au-poor anhedral to euhedral ken et al. 1989, 1991).Gold filings from capsulematerial
grains. Ten to 25Vaof the pyrite was arsenian,which oc- were present in some polished sections of experimental
curred variously as a replacementand overgrowth of As- products but only in minor amounts.
-poor pyrite (Fig. 5), as a replacementof Fe-bearing silMarcasite was relatively abundant,occurring as an in-

FLEET AND MUMIN: Au IN PYRITE, MARCASITE. AND ARSENOPYRITE

189

0.10
CarlinTrend

0.09

pyrite-marcasitearsenopyrite
-o

0.08
0.07
o.1

(I'

R
fs

C)

-9E

o
o
(U

Ec)
oc',
o . 0 o,40

50
sulfur (at%)

oa

o

U.UO

0.05
0.04

i

o

O

I
o
oo
.o

0.01

60

70

r=-0'85

o

0.03

0.00
29
30

arsenianpyrite

0.02

it

10

DeepStardeposit

o

30

31

t:

33

34

iron (at%)
Frcunn8. Inversecorrelation
pyriteofAu andFein arsenian
and marcasitefrom
Deep

the
Stardeposit Line is substitution
Frcunr 7. Compositionsof arsenianpyrite and marcasiteand
Fe : Au (sampleDS17).
arsenopyritefrom Carlin Tiend ores. Circles are As content andsmall squaresare Au content.Note the continuity of EMpA compositions from FeS, to FeAsS. Also, almost all compositionsare
Fe deficient, with As contents plotting above the FeSr-FeAsS Synthetic arsenopyriteand arsenian pyrite andline. Figure includes data for compositionally zoned arsenian- marcasite
pyrite-marcasite-arsenopyrite
grainsbut not the resultsfor sample
Crystallization of pyrite, marcasite,and arsenopyriteis
DSlT of Fisure 8.

well known to be sluggish and reaction-path dependent
at low temperature(e.g., Murowchick and Barnes 1986;
Schoonenand Barnes1991;Wu and Delbove 1989).Pretergrowth with both As-poor and arsenian pyrite. Arse- vious and present studies on natural assemblageshave
nopyrite was also abundant as isolated euhedral grains revealed widespread metastablepersistenceof solid soand overgrowths to pyrite and, less commonly, over- lutions along the FeS,-FeAsSjoin (Fig. 7; also,Fig. 9 ofgrowths and internal zones of the composite, composi- Fleet et al. 1989). Kretschmarand Scott (1976) wereforced to accept that most of the characteristiccompositionally zoned arsenian-pyrite-marcasite-arsenopyrite
intional zoning in natural arsenopyriteis a nonequilibrium
tergrowths(Fig. 6).
The compositions of pyrite, arsenian pyrite and mar- feature reflecting the kinetics of mineral growth and local
casite, and arsenopyritefrom the Carlin Tiend gold de- fluctuationsin the activity ratio of S to As species.Thereposits extended from FeS, to near FeAsS (Fig. 7). The fore, we did not set out in the present experimental inrange in As content of arsenianpyrite exceededthat re- vestigation to establish the stable phase relations in the
ported for pyrite from other gold deposits (Fleet et al. Fe-As-S system at low temperature,anticipating experi1989) and the present experiments.The arsenianpyrite mental difficulties and suspectingthat such stable phase
was also markedly Fe deficient and generally had elevat- relations may have little relevance to the natural goldsulfide ore assemblages.Clearly, had the experiments
ed amounts of Au (Fig. 8). On the other hand, the arsebeen reversed,the product phaseslikely would have cornopyrite was restricted to compositionsmore S rich thanrespondedto thoseofthe dry systeminvestigatedat highthe ideal stoichiometry, consistent with low-temperatureer temperatureby Clark (1960) and Kretschmarand Scottcrystallization (cf. Kretschmar and Scott 1976), and was(r916).
relatively Au poor (Fig. 7), consistent with the present
Thus, we merely attempted to reproduce the naturalobservationsfor synthetic S-rich arsenopyrite.The comassemblagesand phase compositions of Au-bearing
positions intermediate between arsenianpyrite and arseFe-As-S minerals.The experimentswere designedto limnopyrite (about 57-40 atVoS; Fig. 7) were all from rel- it chemical communication
within the chargesand to proatively broad and seemingly homogeneousinterior zones mote surface-controlledand topotaxial replacement
reof the composite,compositionally zoned arsenian-pyrite- actions. We used a low fluid/solid ratio, the starting
marcasite-arsenopyriteintergrowths (Fig. 6). However, materials were not ground and mixed, and the experitheseinterior zones were still too nuurowfor meaningful ments were not intemrpted for further grinding and mixXRD analysis with conventional X-ray sources, and, ing to promote homogenizationof the product phases.
therefore, the possibility that the intermediate composiThe presentresults for the euhedral arsenopyritecrystions represent two-phase mixtures has not been tals (acicular morphology, compositional zonation, wide
excluded.
variation in S/As ratio and in Au content within sinsle

FLEET AND MUMIN: AU IN PYRITE, MARCASITE' AND ARSENOPYRITE

190
0.5

zonedarsenopyrite
Le Chatelet& Villeranges
0.4

o

G'
0.3
o\

tt
o\

g

\o
tr

E

8, o.z oo
0.1

t\

'\+oo
tt

\o
o

tr

I

are very similar to colloform pyrite of other paragenesis.
Both reaction kinetics and solution conditions must exert
considerableinfluence on the precipitation of phases,like
pyrite, that nucleatewith difficulty. Clearly, it is not possible to simulate the natural conditions (time scale,variation in fluid conditions, etc.) precisely in the laboratory.
The presentexperimentshave reproducedthe full range
of S/As ratio for arsenianpyrite reported by Fleet et al.
(1989). Howeveq we have not reproducedthe virtual continuum of FeSr-FeAsScompositions presently reported
for the zonal grains from the Deep Star depositconsistingof pyrite, arsenian pyrite, marcasite, and arsenopyrite(Fie. 7).

Uptake of As and Au by sulfide minerals
Arsenian pyrite and arsenopyrite are both important
hosts for invisible gold in nature and in laboratory syn0.0 r-amountsof invisible gold in pyrite andiE
2 . 5 thesis. Significant
1.0
2.O
0.0
marcasite correlate only with As-rich compositions andvacanciesin metalsite(at%)
in arsenopyriteonly with As-excesscompositions (Fleet
from the Chdtelet et al. 1993;presentFig. 3b; Wu and Delbove 1989).Also,
Frcuno 9. Compositionof arsenopyrite
and Villerangesgold deposits,CentralMassif,France.Squares arsenopyrite, and panicularly Au-bearing arsenopyrite,
areLe Chdtelet,solid for growthzoneA and openfor growth tends to be Fe deficient in nature (e.g., Klemm 1965;
solidfor growthzoneC andopen Kretschmarand Scott 1976; Sharp et al. 1985; present
zoneB; circlesareVilleranges,
for growthzoneD. DataarefromJohanet al. (1989)recalculated Fig. 7) and in laboratory synthesis (Wu and Delbove
lineswerevisuallyfitted. 1989; Fig. 3a), as does Au-bearing arsenianpyrite andto theformula(Fe,Au,tr)(As,Sb),=-S,=.imarcasite(Fig. 8; Table 1). Thus, invisible gold in sulfide
minerals is associatedwith both anomalous As content
grains and from one zone to another, deficiency in Fe) and Fe deficiency. The structural accommodationof theare comparableto those of Wu and Delbove (1989), who metal deficiency,the oxidation stateof cations and ligand
experimentedat 500 'C using a starting mixture of FerO, atoms, and the physical and chemical state of Au are all* As + S and an appreciablyhigher fluid/solid ratio. Our important and troublesomequestionsin the crystal chemcompositionally zoned euhedral arsenopyritebroadly re- istry of chalcogenides(compoundsof metals with S, Se,
produced arsenopyrite from gold deposits as well, al- and Te) and pnictides (compoundsof metals with B As'
though the Au contents and variation in composition are Sb, and Bi) and the geochemistryof Au.
Hypothetically, Fe deficiency in arsenian pyrite andhigher than commonly observedin nature.
Arsenian pyrite and marcasiteformed readily at 300- marcasite and arsenopyrite could be accommodatedby
400'C, but only in limited amountsand,apparently,only either nonstoichiometryor cationic As. Nonstoichiometryby replacement of pyrite and pyrrhotite. We previously is very extensive in transition-metal chalcogenidesbeobserved Au-bearing arsenian pyrite as a replacement longing to the NiAs-CdI, structure series (e.g., Jeannin
product of preexisting pure pyrite from an epithermal 1970; and pyrrhotite). Therefore, it does not seem unreagold deposit, where the parageneticsequencewas con- sonable to expect nonstoichiometry in pyrite andfirmed by the fracture-controlled spatial distribution of marcasiteas well, particularly where the chalcogenligand
the arsenian pyrite (Fleet and Chryssoulis, unpublished (S, Se, Te) is partially substitutedby a pnictogen(B As,
results). Also, the Au-bearing arsenianrims of fine-scale Sb, Bi) and in light of the wealth of recent EMPA data
pyrite from the Deep Star deposit, Carlin Trend, appear for arsenianpyrite and marcasiteand arsenopyrite.
Johan et al. (1989) reported some remarkableelement
to be partly replacementin nature. However, arsenianpyrite elsewhere(e.g., Fleet et al. 1989) generallyappears conelations for individual Au-rich growth zones of arto have formed by crystal growth. To this extent, we have senopyrite from the Ch0telet and Villeranges deposits,
not reproduced the complex pyrite microstructures re- Massif Central, France,which they interpretedby invokported by Fleer et al. (1989, 1993) but, perhaps,would ing complex solid solutions of the type (Fe,Au,As,Sb)
not have expectedto do so without having some mech- As,*"S,.,, and a substitutionmechanismof 2As : Au (oranism in the experimental procedure that simulated epi- Sb) + Fe. Although As is formally more electropositive
are 1.8-1.9 for Fe, 2.1 fot
sodic fluctuation in fluid composition.The colloform tex- than Au (electronegativitiestures and oscillatory zonation that characterize some As, 2.3 for Au, and 2.6 for S; Batsanov1968) and As3*
natural arsenianpyrite is reproducedby the FeSr-AsS-S- is similar in size to Fe3* (effective ionic- radii for octacomposition material, albeit for more As- and S-rich hedral coordinationare 0.58 and 0.65 A, respectively;
compositions than in nature, and these growth textures Shannon 1976), the octahedralcation site of arsenopyriteo\TFLEET AND MUMIN: Au IN PYRITE. MARCASITE, AND ARSENOPYRITE

t91

seemsquite unsuited to cationic As, which prefers a py- charge resulting from the local As-rich, Fe-deficientsites
ramidal coordinationwith S (e.g., AsS., AsAsS, etc.). or excited surface statesor depositedby sorption and reAlso, cationic As in arsenopyritewould result in As3*- duction (e.g.,Mycroft et al. 1995).However,such small
As'z-bonds,which seemenergeticallyunfavorable.Bond clusters would surely be revealed routinely as defects inresonance,which is often invoked to explain the valence high-resolutionTEM images of auriferousarsenianpyrite
matching required for As-As interactions in sulfarsenide and marcasiteand arsenopyrite(cf. Bakken et al. 1991).
compounds,would significantly modify the crystal-chemWe had anticipatedthat the elementcorrelationsfor theical properties of the cationic arsenic species.Moreover, natural and synthetic arsenian pyrite and marcasite andSb and Au in the ChAtelet and Villeranges arsenopyrite arsenopyritewould reveal the oxidation state of the inare essentiallymutually exclusive in a manner analogous visible gold (i.e., Auo, Aul*, etc.). Unfortunately,alto As and Ag in tetrahedrite(e.g., Fig. 8 of Mumin et al. though the amount of invisible gold is generally always
1994). The EMPA results of Johanet al. (1989) are pres- less than the excessof As or the deficiencyof Fe (Figs.
ently recalculated to a nonstoichiometric formula 3, 4, and 7-9), consistentlinear trends are not evident.
[(Fe,Au,nXAs,Sb),*,S,,",where n is a cation vacancy], We note that the correspondenceof the Wu and Delbove
yielding element correlations that indicate a substitution (1989) results for syntheticarsenopyritewith the Fe :mechanismof the type 2Fe + Au : 3n Gig. 9).
Au substitution line (cf. Fig. a) may be an artifact of the
The high Au contents of the Ch6telet arsenopyriteare limited data presentedin this earlier study.
restrictedto As-excesscompositions:The EMPA locaThe EMPA resultsof Johanet al. (1989) for the ChAtions for growth zone A (Fig. 9) that have no observable telet and Villeranges arsenopyrite suggest a closer apAu all have compositionswith S/As > 1.0. However, proach to local equilibration for individual growth zones
growth zone D of the Villeranges arsenopyritedoes have than the presentexperimentalproducts. The four growth
Au-rich compositionswith S/As > 1.0,althoughthe over- zones analyzed(Fig. 9) each reveal the samesubstitution
all correlation of increasing Au with increasing As is mechanism 2Fe + Au : 3!, but this is not helpful inmaintained.
assigningthe oxidation stateof Au. The Chdteletand VilConsiderationof the oxidation state of the cations and lerangesarsenopyritesamples,in fact, show an increaseligand atoms in arsenianpyrite and marcasite and arse- in invisible gold with decreasingmetal-site vacancies,
nopyrite is beyond the scopeof the presentpaper,requir- which is the oppositeof the trends for arsenianpyrite anding new theoretical calculations and chemical spectro- marcasitefrom Carlin Trend and syntheticarsenianpyrite
scopic measurements(e.g., microbeam XPS) to study the and marcasiteand arsenopyrite.
fine-scale arsenian-pyrite-marcasite-arsenopyrite
gold-depositsysinterWe suggestthat in sediment-hosted
growths effectively. Superficially, it does appearthat the tems, Au is removed from ore fluids largely by chemicovalently bonded sulfarsenide atom pairs have a re- sorption on the As-rich, Fe-deficient growth surfacesof
markable capacity for controlling oxidation state in py- pyrite, marcasite,and arsenopyrite.The active sitesfixing
rite-marcasite-arsenopyrite
materials.
Au would be formed by soft Lewis bases(e.g.,As, AsS),In respectto the physical state of invisible gold in ar- and vacanciesin nearest-neighborcation sites would persenianpyrite and marcasiteand arsenopyrite,there is little mit the Au atoms to customize their local atomic envidirect evidence favoring colloidal gold to the exclusion ronment. Thus, the solid solution of Au could be accomof lattice-boundAu (cf. Bakkenet al. 1989, 1991).In rhe panied by point defects, and its nearest-neighborabsenceof the routine observation of nanometer-scale, coordination in the bulk sulfides need not necessarilybe
evenly dispersedgold particles, the present element cor- octahedral. However, we note that Cu, a metal with arelations are quite compelling evidence favoring lattice- somewhat similar coordination chemistry to Au, doesbound Au. Invisible gold correlateswith As content in form a pyrite-structurephase at high pressure(King andboth natural and synthetic arsenianpyrite and marcasite Prewitt 1979).We emphasizethat the hypothesizedchemand, in high amounts,with excessAs in most natural and isorption processis not restricted to crystal growth. The
all syntheticarsenopyrite(e.g.,Fleet et al. 1993;present presenceof arsenianpyrite and marcasiteas replacement
Figs. 3b,'7, and 9; presentTable 1), and with Fe defi- phasesin both natural and laboratory products suggests
ciency in natural and synthetic arsenianpyrite and mar- that in these casesthe hypothesizedchemisorption must
casite and arsenopyrite(presentFigs. 7-9; presenrTable also be accompaniedby coupled replacementby As, Au,
1; Johanet al. 1989).Also, the light (Au-rich) zonesin and vacancies.
the BSE imagesof the presentsyntheticarsenopyrite(Fig.
The chemisorbing surface would be conditioned dif2a) have fairly uniform contrast, indicating an even spa- ferently for different solution conditions, establishingtial distribution of the heavy Au atoms. These consistent only a local equilibrium with the Au-bearing fluid. Thus,
observationsare incompatible with the incorporation of the incorporation of invisible gold would be intrinsicallyinvisible gold as nanometer-scaleparticles during crystal nonsystematicin terms of crystal-chemical parameters,
growth. It is possible that very small (ingstrom-scale) depending only on the solution conditions imparting aclusters of Au atoms, perhaps stabilized in the ore fluid minimum excessof As and deficiency of Fe to the active
as clustercompounds(e.g.,Hall and Mingos 1984),could surface. This type of control on the chemisorption probe attractedto the growth surfacesof sulfides by surface cess,coupled with the tendencyof the covalently bonded

192

FLEET AND MUMIN: AU IN PYRITE, MARCASITE, AND ARSENOPYRITE

district, Nevada In D.M. Hausen,W. Petruk, R D Hagni, and A. Vassiliou, Eds., ProcessMineralogy XI: Characterizationof metallurgicaland recyclableproducts,p 13-23. The Minerals, Metals, and Materials
Society, Warrendale,Pennsylvania.
Batsanov,S S (1968) The concept of electronegativity Conclusionsand
prospects RussianChemical Reviews, 37, 332-351.
Berger,B R., and Bagby, W.C. (1991) The geology and origin of Carlintype gold deposits In R.P Foster,Ed, Gold metallogenyand exploration, p.210-248 Blackie,Glasgow.
Cabri, L.J , Chryssoulis,S.L, De Villiers, J.PR, LaFlamme,J.H.G, andBuseck, PR. (1989) The nature of "invisible" gold in arsenopyrite
CanadianMineralogist, 27, 353-362
Cathelineau,M., Boiron, M -C., Holliger, P, Marion, P, and Denis, M(1989) Gold in arsenopyrites:Crystal chemistry, location and state,physical and chemical conditions. Economic Geology Monograph 6,
328-341.
Christensen,O D (1993) Carlin trend geologicaloverview. In O.D Christensen,Ed, Gold depositsof the Carlin Trend, Nevada,p. 12-26. Sociery of Economic GeologistsGuidebook Series 18, Denver.
Clark, L.A (1960) The Fe-As-S system:Phaserelationsand applications
EconomicGeology,55, 1345-1381
Ctode, C H (1994) Geology of the Deep Star gold deposit,EurekaCounty,
Nevada NortheasternNevada Section of A.I.M E Spring Field Trip
Abstract Volume
Cook, N.J., and Chryssoulis, S L (1990) Concentrationsof "invisible"
gold in the corlmon sulfides.CanadianMineralogist, 28, l-16
Fleet, M.8., Maclean, PJ., and Barbier, J (1989) Oscillatory-zonedAs-bearing pyrite from strata-boundand stratiform gold deposits:An indicator of ore fluid evolution. Economic Geology Monograph 6, 356362.
Fleet, M.E , Chryssoulis,S L , Maclean, PJ , Davidson,R., and Weisener,
C.G (1993) Arsenian pyrite fom gold deposits:Au and As distribution
investigatedby SIMS and EMB and color staining and surface oxidation by XPS and LIMS CanadianMineralogist, 31, 1-1'7
Hall, K P, and Mingos, D M P (1984) Homo- and heteronuclearclustercompoundsof gold In S.J Lippard, Ed., Progressin inorganic chemistry, vol. 32, p.237-325. Wiley, New York.
Jeannin,Y.P (1970) Recentprogressin the investigationof nonstoichiometry. In A. Rabenau,Ed, Problems of nonstoichiometty,p. 71-129.
North-Holland, Amsterdam.
M. (1989)Ars6nopyriteauriJohan,Z, Marcoux,E, and Bonnemaison,
fbre: Mode de substitutionde Au dansla structurede FeAsS.Academie
de Sciences(Paris)ComptesRendus,308, 185-191.
King, H E, Jr, and Prewitt, C T. (1979) Structureand symmetry of CuS'
(pyrite strucnrre) American Mineralogist, 64, 1265-121I.
Klemm, D.D (1965) Synthesenund Analysenin den Dreiecksdiagrammen
FeAsS-CoAsS-NiAsSund FeS,-CoS,-NiS' NeuesJahrbuchfilr Mineralogie Abhandlungen,103, 205-255
Kretschmar,U., and Scott, S.D (1976) Phaserelations involving arsenopyrite in the systemFe-As-S and their application CanadianMineralogist, 14,364-386
AcrNowr-BncMENTs
Kuranti, G. (1941) Synthetic study of gold-containingpyrite. Chemical
Abstracts.35.3563.
We thank Y Thibeau and Shao Yue for assistancewith EMPA; G.B
Maclean, PJ (1991) The characterizationof pyrite from gold deposits
Arehart,Z. Johan,B M. Bakken, an unnamedreferee,and D.W. Mogk for
Ph D thesis,University of Westem Ontario, London, Ontario, Canada
helpful reviews of earlier versions of the manuscript; Newmont Gold
Maclean, PJ, and Fleet, M.E (1989) Detrital pyrite in the WitwatersrandCompany,Carlin, for provision of samplesand geologicalinformation ongold fields of South Africa: Evidence from truncatedgrowth banding
the Deep Star mine and other gold depositsalong the Carlin Trend; andEconomic Geology, 84, 20O8-2Ol1
the Nahrral Sciencesand Engineering ResearchCouncil of Canada for
Mao, Shui He (1991) Occurrenceand distribution of invisible gold in afinancial support
Carlin-type gold deposit in China American Mineralogist, 16, 19641972.
REnnRENcns crrEDMichel, D, Giuliani,G., Olivo, G R., and Marini, O J (1994)As growthbanding and the presenceof Au in pyrites from the Santa Rita gold
Arehart, G.B., Chryssoulis,S.L., and Kesler,S.E. (1993) Gold and arsenicvein deposit hosted in ProterozoicmetasedimentsGoii4sState,Brazil
in iron sulfides from sediment-hosteddisseminatedgold deposits:ImEconomic Geology, 89, 193-200.
plications for depositionalprocessesEconomic Geology, 88, 171-185
Bakken, B M , Hochella, M.F, Jr, Marshall, A.E, and Tumer A M (1989) Mumin, A.H., Fleet, M.E, and Chryssoulis,S.L. (1994) Gold mineralization in As-rich mesothermalgold ores of the Bogosu-Presteamining
High-resolution microscopy of gold in unoxidized ore from the Carlin
district of the Ashanti Gold Belt, Ghana:Remobilizationof "invisible"mine, Nevada EconomicGeology,84, lll-119.
gold Mineralium Deposita,29, 445-460
Bakken,B M, Fleming,R.H, and Hochella,M.F, Jr. (1991)High-resoMurowchick, J B , and Barnes, H L (1986) Marcasiteprecipitation fromlution microscopy of auriferous pyrite from the Post deposit, Carlin

As-S interactionsto act as an oxidation-statebuffer, readily accountsfor the lack of systematiclinear atomic substitution trends. Thus, we conclude that invisible gold inarsenianpyrite and marcasiteand arsenopyriterepresents
Au removed from ore fluids by chemisorptionat As-rich,
Fe-deficient surface sites and incorporated in the solids
in metastablesolid solution. It follows that the metastable
crystallizationof FeS,-FeAsSsolid solutions(Fig. 7) reflects a control by surface-chemicalprocessesalso.
This study is another step in the understandingof the
distribution, chemical state, and manner of incorporationof invisible gold into arsenianpyrite and marcasite andarsenopyrite.Previous studieshave documentedthe common occurrenceof oscillatory-zoned auriferous arsenian
pyrite in sediment-hostedgold deposits, shown that theanomalousAs contentis held in metastablesolid solution.
and demonstratedthat the invisible gold is spatially associatedwith local enrichmentin As (e.g., Fleet et al.
1989,1993; Bakken et al. 1991;Mumin et al. 1994).We
have demonstratedthat invisible gold correlatesstrongly
with As enrichmentand Fe deficiency in both pyrite-marcasite and arsenopyrite.This correlation favors a chemisorption process for the incorporation of Au into theseminerals and makes dominant control by entrapment of
colloidal gold less likely. However, HRTEM studies are
required-toestablishthat colloidal gold particles less than
20-50 A in size do not quantitatively account for theinvisible gold fraction of auriferous pyrite.
The oxidation state of the proposed lattice-bound invisible gold remains uncertain. Indeed, experimental determination of the precisechemical stateof trace amountsof Au in pyrite and marcasite may continue to be very
difficult. Therefore,meaningful future studiesmay be restricted to As-rich growth zones of arsenopyrite,whichhave Au contents of up to 3.0 wt%o.
Finally, we have not shown that Fe deficiency is an
essentialrequirement for the chemisorption of Au onto
growth surfacesof arsenianpyrite and marcasiteand arsenopyrite.We report only that Au contentcorrelateswith
Fe deficiency and thus suggestthat active growth surfaces
are both As rich and Fe deflcient.

FLEET AND MUMIN: Au IN PYRITE, MARCASITE, AND ARSENOPYRITE
hydrothermalsolutions Geochimicaet CosmochimicaActa. 50. 26152630
Mycroft, J.R , Bancroft, G M, Mclntyre, N.S., and Lorimer, J.W. (1995)
Spontaneousdeposition of gold on pyrite from solutions containing
Au(III) and Au(l) chlorides: Part I. A surface study Geochimica et
CosmochimicaActa, 59, 3351-3365.
Radtke, A S (1985) Geology of the Carlin gold deposit,Nevada, 124 p
U S Geological Survey ProfessionalPaper1267.
RemondG, Holloway, PH., and Le Gressus,C. (1981) Electronspectroscopy and microscopyfor studying surfacechangesof mechanically
preparedpyrite and quartz.ScanningElectron Microscopy, 1,483-492
RemondG , Cesbron,F, Tiaxel,K, Campbetl,J L., andCabri,L J (1987)
Electron microprobe analysis and proton induced X-ray specuomery
applied to trace element analysis in sulfides: Problems and prospects
ScanningMicroscopy, 1, 1017-1037
Robinson,B W, and Graham,J (1992) Advancesin electronmicroprobe
trace-elementanalysis. Journal of Computer-AssistedMicroscopy, 4,263-265
Rota, J.C. (1993) Geology and relatedstudiesar rhe Gold euany deposit,
Nevada. In O.D. Christensen,Ed , Gold depositsof the Carlin Trend,
Nevada, p 67-78 Society of Economic GeologistsGuidebook Series
18, Denver.
Schoonen,M.A A., and Barnes,H L (1991) Mechanismsforming pyrite
and marcasitefrom solution: III. Hydrothermal processes.Geochimica
et cosmochimica Acta. 55. 3491-3504.

t93

Shannon,R.D (1976) Revisedeffective ionic radii and systematicstudies
of interatomic distancesin halides and chalcogenidesActa Crystallographica,432,'751-161
Sharp,Z D, Essene,E J, and Kelly, W.C. (1985)A re-examinationof thearsenopyritegeothermometer:Pressureconsiderationsand applications
to natural assemblages.
CanadianMineralogist, 23, 517-534.
Tossel,J A, Vaughan,D.J., and Burdett,J K. (1981) Pyrite, marcasite,and
arsenopyriterype minerals: Crystal chemical and structural principles
Physics and Chemrstry of Minerals, 7, 17'7-184.
Wagner,EE., Sawicki, J.A, Friedl, J , Mandarino, J A., Hanis, D C , andCabri, L J (1994; 'r4u Miissbauerstudy of the gold-silver ditellurides
sylvanite, krennerite and calaverite. Canadian Mineralogist, 32, 189201.
Wells, J D, and Mullens, TE. (1973) Gold-bearingarsenianpyrite determined by microprobe analysis,Cortez and Cadin gold mines, Nevada
Economic Geology, 68, 187-201
Wijeyesekera,S D , and Hoffmann, R. (1983) Marcasitesand arsenopyrites: Structure,bonding, and electricalproperties InorganicChemistry,
22,328'7-3300.
Wu, X., and Delbove, F (1989) Hydrothermal synthesisof gold-bearing
arsenopyrite.Economic Geology, 84, 2029-2032.
MeNuscnrprRECETVED
Mancs 5. 1996
MrNuscnrm ACCEpTED
SesreMsen17. 1996