1265

The Canadian MineralogistVol. 38, pp. 1265-1281 (2000)

CHEMICAL SPECIATION OF GOLD IN ARSENOPYRITE

LOUIS J. CABRI§

Canada Centre for Mineral and Energy Technology, 555 Booth Street, Ottawa, Ontario K1A 0G1, Canada

MATTHEW NEWVILLE§

Consortium for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, U.S.A.

ROBERT A. GORDON AND E. DARYL CROZIER

Department of Physics, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada, and PNC–CAT,Sector 20, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.

STEPHEN R. SUTTON

Consortium for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, andDepartment of Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637, U.S.A.

GREG MCMAHON¶

Canada Centre for Mineral and Energy Technology, 568 Booth Street, Ottawa, Ontario K1A 0G1, Canada

DE-TONG JIANG†

Department of Physics, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada, and PNC-CAT,Sector 20, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.

ABSTRACT

The speciation of Au in gold-bearing arsenopyrite (FeAsS) from four gold deposits (Olympiada, Sentachan, São Bento andSheba) was determined by micro-X-ray absorption near-edge structure (XANES) on grains well characterized microscopicallyand by electron-microprobe and secondary-ion mass spectrometry analyses and images. “Invisible” gold in arsenopyrite occursin two apparently mutually exclusive chemical forms: chemically bound and elemental. Arsenopyrite from the Sentachan, SãoBento and Sheba deposits contains chemically bound gold. With comparable constituent electronegativities and a white-linefeature in the XANES indicating unoccupied Au 5d-states, but absorption-edge positions comparable to Au1+ species, the bond-ing is interpreted as being covalent rather than ionic. The invisible gold in arsenopyrite from the Olympiada deposit, on the otherhand, occurs as very small particles of Au0, probably less than a few nanometers in diameter. Micro-XANES data for theOlympiada and Sentachan arsenopyrite support earlier results obtained by 197Au Mössbauer spectroscopy on arsenopyrite con-centrates. In some arsenopyrite crystals, the gold concentration is closely related to growth zoning. This feature represents con-ditions during crystallization and does not correlate with the chemical form of the gold. Similarly, selenium, where present,correlates with gold in some deposits and not in others, irrespective of the gold speciation. The finding of two types of invisiblegold in arsenopyrite from different deposits has beneficial implications for extractive metallurgy.

Keywords: gold, arsenopyrite, micro-XANES, valence, X-ray microprobe, SIMS, EPMA.

§ E-mail addresses: lcabri@sympatico.ca, newville@cars1.uchicago.edu¶ Present address: Fibics Incorporated, 556 Booth Street, Suite 200, Ottawa, Ontario K1A 0G1, Canada.† Present address: Canadian Light Source, University of Saskatchewan, 107 North Road, Saskatoon, Saskatchewan S7N 5C6,

Canada.

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1266 THE CANADIAN MINERALOGIST

INTRODUCTION

It is only in the last two decades that detailed char-acterization of submicroscopic or “invisible” gold insulfide minerals became of interest for scientific andeconomic reasons, following the discovery of large low-grade orebodies that can in some cases be mined byheap-leaching techniques. Some of these refractory goldores occur in sediment-hosted gold deposits, typified bythose in the area of Carlin, Nevada (Seabrook 1989).However, refractory gold ores also are found in manydeposits worldwide, in different geological environ-ments and ages (Boyle 1979). Early investigations intounderstanding how gold occurs in sulfides, beginningwith those of Bürg (1930), have been reviewed by Cabriet al. (1989). The question of the chemical form of “in-visible” gold in sulfide and sulfarsenide minerals hasalso attracted much interest in the last 12 years follow-ing the report of chemically bound Au in sulfides(Marion et al. 1986, Wagner et al. 1986). The term “in-visible” arises as one cannot distinguish by conventionalmicroscopy between gold chemically bound in the min-eral and submicroscopic inclusions. The ore genesis andmetallurgical implications for the presence of invisiblegold in minerals such as arsenopyrite are potentiallyvery significant (Mumin et al. 1994), and experimentaltechniques to determine the valence of gold in sulfidesalso are of current interest (Li et al. 1995, Simon et al.1999, Wagner et al. 1994). Experimental evidence, how-ever, remains controversial because of low concentra-tions of gold, near the limits of resolution forconventional laboratory techniques, and its non-uniformdistribution in sulfide minerals.

REVIEW OF RECENT WORK

Among many papers on Au in arsenopyrite (FeAsS)since 1989, the following are relevant. On the basis ofelectron-probe microanalysis (EPMA), Johan et al.(1989) discussed substitution mechanisms in arsenopy-rite from the Châtelet and Villeranges deposits (France).They proposed a substitution mechanism 2As[Fe] ⇔(Au,Sb) + Fe, where As[Fe] is As on an Fe site. On thebasis of the existence of Fe3+ in a non-polar state in thearsenopyrite structure (Buerger 1936), they suggestedthe presence of Au3+. Marcoux et al. (1989) mapped thedistribution of Au in an arsenopyrite grain using EPMAin the Au-rich arsenopyrite from these two French de-posits and also showed an inverse correlation betweenAu and Fe. Wu & Delbove (1989) reported a negativecorrelation between levels of Fe and Au in syntheticarsenopyrite, suggesting substitution of Fe by Au. Theirsynthetic arsenopyrite crystals were found to be zoned,with a Au-rich rim and a Au-poor core. Wu et al. (1990)presented a 197Au Mössbauer spectrum of synthetic ar-senopyrite with peaks corresponding to a dominantchemically combined Au form (at ~+3.5 mm/s), as wellas a minor peak representing Au0. In reviewing the rela-tionship between crystal morphology and Au content forarsenopyrite crystals from some French deposits, theynoted that the smaller, needle-shaped crystals are morelikely to be Au carriers than larger pyramidal crystals,concluding that rapid crystallization might be a factorto promote Au-bearing arsenopyrite. Cook & Chrys-soulis (1990), in reviewing analyses of sulfide mineralsby secondary ion mass spectrometry (SIMS), concludedthat Au is found preferentially in the arsenopyrite struc-

SOMMAIRE

Nous avons déterminé le mode d’incorporation de l’or dans l’arsénopyrite (FeAsS) provenant de quatre gisements d’or(Olympiada, Sentachan, São Bento et Sheba) par microstructure fine du seuil d’absorption X (XANES), cette analyse étanteffectuée sur des grains bien caractérisés par microscopie, par analyses à la microsonde électronique et par spectrométrie demasse des ions secondaires (analyses et images). Dans l’arsénopyrite, l’or dit “invisible” est présent sous deux formes qui semblentmutuellement exclusives: chimiquement lié et en particules élémentaires. L’arsénopyrite des gisements de Sentachan, São Bentoet Sheba contient l’or chimiquement lié. Avec des valeurs comparables de l’électronégativité des constituents et une bandeblanche dans les spectres XANES indiquant des atomes d’or aux orbites 5d non occupés, en même temps qu’un seuil d’absorptioncomparable à celui de l’espèce Au1+, le schéma de liaisons serait plutôt covalent qu’ionique. En revanche, l’or invisible dugisement d’Olympiada se présente sous forme d’infimes particules de Au0, d’un diamètre probablement inférieur à quelquesnanomètres. Les données obtenues de concentrés d’arsénopyrite des gisements Olympiada et Sentachan par microanalyse XANESconfirment les résultats obtenus antérieurement par spectroscopie de Mössbauer de l’isotope 197Au. Dans certains cristaux, lateneur en or semble étroitement liée à la zonation des cristaux. Cet attribut résulte de conditions spécifiques au cours de lacristallisation, et ne montre pas de corrélation avec la forme chimique de l’or. De même, le sélénium, où il est présent, montre unecorrélation avec l’or dans certains gisements et non dans d’autres, quel que soit le mode d’incorporation de l’or. La découverte dedeux modes d’incorporation de l’or invisible dans l’arsénopyrite a des implications bénéfiques en métallurgie de l’extraction.

(Traduit par la Rédaction)

Mots-clés: or, arsenopyrite, microanalyse XANES, valence, microsonde de rayons X, spectrométrie de masse des ions secondaires(SIMS), analyse à la microsonde électronique (EPMA).

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CHEMICAL SPECIATION OF GOLD IN ARSENOPYRITE 1267

ture and may concentrate significantly in the finer grain-size, where two generations of arsenopyrite are present.However, there are exceptions to this correlation.Aylmore et al. (1993) used atomic location by channel-ing-enhanced microanalysis (ALCHEMI) to determinethe location of Au within synthetic arsenopyrite. Afterconfirming by electron diffraction that the basic struc-ture of arsenopyrite is monoclinic P21/c (Fuess et al.1987), they examined the partitioning of Au betweenthe Fe site and the (As + S) sites along the [101] zoneaxis. They found no significant correlation, concludingthat a high percentage of the gold occurs either as fineinclusions or is located in a lattice defect or interstitialsite. The purpose of our study was to use a number ofdifferent techniques, including EPMA, SIMS and mi-cro-X-ray absorption near-edge structure (XANES), tocharacterize arsenopyrite in order to understand how theAu is contained within the minerals. It should be notedthat this is the first report of micro-XANES studies doneon a gold-bearing mineral. The recent work of Simon etal. (1999) was done using a large beam on a concen-trate of arsenian pyrite grains.

Arehart et al. (1993) quoted unpublished X-ray pho-toelectron spectroscopic (XPS) measurements to sug-gest that gold enters the arsenopyrite structure in anoxidized form as Au3+. Li et al. (1995) also reportedresults of XPS analyses of a mixed pyrite–arsenopyritesample from the Jinya gold mine (China). They stated:“The inter-peak distance and relative intensity ratio arebasically identical to the experimental values for Au0.There is no doubt they are 4f7/2 and 4f5/2 of gold, andthe measured values for 4f7/2 are conspicuously lowerthan the 84.0 eV of Au0, being between 82.5 and 83.5eV, respectively. As can be seen from the standard datapresently used for XPS analysis, values of electron bind-ing energy for gold in the gold compounds listed are allabove 84.0 eV. Measured data for gold-bearing com-pounds are conspicuously lower than binding energydata for other gold compounds in which the gold isknown to be Au1+, Au3+ and Au0 (Fig. 2), thereby indi-cating that the gold in these compounds has neither zerovalence nor has it lost its valence electron (showing aspositive valence) but is in a state of having acquired avalence electron or sharing a valence electron with otherelements. Consequently, we propose that gold in thecrystal lattices of these compounds exists in a negative-valence state.” This unusual assignment has not beenconfirmed, and it is conceivable that the reported lowereV values might be due to sample charging (D.J.Vaughan, pers. commun., 1997). In their study of ad-sorption and deposition of Au on pyrite surfaces,Mycroft et al. (1995) noted the need for careful inter-pretation of the XPS binding energy data. They showedthat Au 4f7/2 binding energy is a function of not onlyoxidation state but also size of the Au particles.

The existence of both visible native gold and chemi-cally bound gold in sulfide and sulfarsenide mineralshas been reported for many mesothermal gold deposits

and may be regarded as the rule (Oberthür et al. 1997).Oberthür et al. (1997) also reported that arsenopyritecrystals from different deposits in the Ashanti gold belt(Ghana) have an Au-rich rim and an Au-poor core. Onthe basis of spot analyses by secondary ion mass spec-trometry with a beam 30 �m in diameter, they noted aninhomogeneous Au distribution in crystals greater than50 �m in diameter. Fleet & Mumin (1997) provided areview of earlier work on arsenopyrite and suggestedthat invisible Au in arsenopyrite from sediment-hosteddeposits represents Au removed from ore fluids bychemisorption at As-rich, Fe-deficient surface sites andincorporated as a metastable solid-solution. They con-cluded that “the oxidation state of invisible gold (Au0,Au1+) remains uncertain because the chemisorption pro-cess is intrinsically non-systematic in terms of crystal-chemical parameters and does not result in definitiveatomic substitution trends.” In a study of invisible goldfrom the Campbell mine (Ontario), Tarnocai et al.(1997) also reviewed and discussed earlier results. Theyreported that (As + S)atomic is relatively constant at ~66at.%, whereas the (As/S)atomic value shows an increasein Au-rich rims. They concluded that Au substitutes forFe, not As. They also suggested that such variations inAs and S, together with Au content, would be highlyunlikely if Au were to occur as very fine inclusions(Au0). They concluded that the Au is likely structurallybound in arsenopyrite in their samples.

Genkin et al. (1998) studied arsenopyrite from fourSiberian gold deposits using a variety of techniques (oremicroscopy, instrumental neutron-activation analysis(INAA), SIMS, EPMA and 197Au Mössbauer spectros-copy). They suggested that the gold-bearing arsenopy-rite in the Olympiada deposit formed contemporane-ously with halos of retrograde alteration controlled byshear zones, with deposition from a fluid through fluid–rock interactions; the native gold was considered to havedeposited later. Genkin et al. (1998) stated that Au oc-curs chemically bound in arsenopyrite as well as in theform of submicroscopic Au0 in the four deposits con-sidered, with chemically bound Au dominant in threeof the four deposits. It is difficult to deduce the mannerin which gold is bonded to its neighbors (or to evenknow what these neighbors are) on the basis of 197AuMössbauer spectra. However, Genkin et al. (1998)stated that the “…. invisible gold cannot be bonded ineither the linear two-fold coordination typical for themonovalent, or the planar fourfold coordination typicalfor the trivalent valence state (Bartunik et al. 1970, Par-ish 1982). The most appealing interpretation is that thegold replaces iron in both arsenopyrite and pyrite. Instoichiometric arsenopyrite, it would then be surroundedby a distorted octahedron of three As and three S atomsas nearest neighbors, but the actual number and arrange-ment of neighbors may vary owing to deviations fromstoichiometry that are common in arsenopyrite(Bortnikov 1993) and possibly lead to disorder in thehost lattice. …… The absence of a substantial quadru-

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1268 THE CANADIAN MINERALOGIST

pole splitting of the Mössbauer line of chemically boundinvisible gold in arsenopyrite and pyrite is to be ex-pected if the gold occupies Fe sites, which have a rathersymmetrical environment of six nearest As and S neigh-bors. If the gold replaces As in the [AsS]3– molecularunits, the absence of a substantial quadrupole splittingis difficult to understand. Moreover, the magnitude ofthe isomer shift of Au in arsenopyrite appears to con-firm the replacement of iron (Friedl et al. 1995).”

Experimental studies on deposition of Au from so-lutions have produced conflicting results, with somesuggesting deposition of Au3+ and Au1+ on sulfide sur-faces followed by reduction to Au0 (Jean & Bancroft1985), and others that electrochemical deposition of Auon sulfide and sulfarsenide surfaces is important indeposition from solutions with the sulfides acting ascathodes in galvanic cells (Möller & Kerstein 1994). Onthe other hand, Cardile et al. (1993) showed that sig-nificant Au0 is absent on sulfide substrates in their ex-periments. The gold on sulfide was interpreted to bepresent as Au1+ on the basis of comparison ofMössbauer data on the substrates with spectra for Au2S.

More recently, XANES and extended X-ray absorp-tion fine structure (EXAFS) analyses on Au were madeof arsenian pyrite [Fe(S,As)2] concentrates from theTwin Creeks deposit, northwestern Nevada (Simon etal. 1999). A beam of 15 � 1 mm produced mixed spec-tra; each spectrum was fitted with a linear combinationof three reference spectra. These authors concluded thatAu is present in the arsenian pyrite as compounds ofAu1+ (with coordination numbers 2 and 4) together withmetallic Au0, and that the arsenic is present as As1–. TheXANES spectra that show significant Au1+ in twofoldcoordination are reported to be similar to Au2S, suggest-ing that Au1+ is linearly bonded to two sulfide ligands.Thus the literature on Au in arsenopyrite between 1989and 1999 indicates a broad agreement that Au correlatesinversely with Fe, but there is a wide range of interpre-tations on the Au speciation, ranging from Au1–, Au0,Au1+ and Au3+. Thus the speciation of gold in arsenopy-rite is not well understood, in spite of intense studiesover the last 12 or so years.

In the present paper, we report on a multifaceted in-vestigation into the chemistry and speciation of invis-ible gold in five samples of arsenopyrite from fourmesothermal deposits: the Olympiada and Sentachandeposits in Siberia, Russia (Genkin et al. 1998), the SãoBento mine in Minas Gerais, Brazil (Steele et al. 2000)and the Sheba mine in the Transvaal, South Africa(Schouwstra & De Villiers 1988, Cabri et al. 1989). Thepresent study involves a detailed characterization of thearsenopyrite grains by traditional methods such asEPMA, and by more recently applied methods such asSIMS and micro-XANES. Because gold may be hetero-geneously speciated, we considered it important to usea spatially resolved, species-sensitive analytical tech-nique such as micro-XANES, in an attempt to obtainsingle-species data. This new study on individual grains

of arsenopyrite, together with the XANES data onarsenian pyrite concentrates (Simon et al. 1999), pro-vide a comparison of how invisible gold occurs in thetwo most important minerals present in refractory sul-fide ores.

SAMPLES AND TECHNIQUES

OF THEIR CHARACTERIZATION

The specific samples examined in this investigationwere: (1) Olympiada sample O–4, grains 47, 61, 62 plusten additional randomly chosen grains; sample O–5,grains 1 and 29, (2) Sentachan sample, grain 11, (3) SãoBento sample, grain SPB–1, and (4) Sheba mine sampleRPS–353, grain RPS–4. In addition, a grain ofaurostibite (AuSb2) from Olympiada sample O–4 wasexamined using XANES. All the arsenopyrite samples(in the form of concentrates) had been mounted in pol-ished sections and previously studied by ore micros-copy, SEM, EPMA, 197Au Mössbauer spectroscopy andSIMS (Genkin et al. 1998). At different stages, somepolished sections were lightly re-polished in order toreduce the SIMS craters.

In all cases, a preliminary examination was made atCANMET using reflected light microscopy to selectsuitable grains for analysis. The EPMA were done bywavelength-dispersion spectrometry at CANMET/MMSL with a JEOL–733 electron microanalyzer, oper-ated at 20 kV and with a beam current of 20 nA (cupreading), using the following X-ray lines and standards:FeK� and SK� (pyrite), CoK� (synthetic CoAsS),AsK� (synthetic FeAs2), NiK� and SbK� (syntheticNiSb), ClK� (synthetic NaCl), PK� (apatite) and AuL�(metal). The beam current was increased to 100 nA andthe counting time was 100 s for the Au analyses, in or-der to lower the minimum detection level (MDL) to0.029 wt.%. From five to eight spot analyses (1–2 �m)were done on each grain (n in Table 1). Corrections tothe raw data were made using Armstrong’s ZAF pro-gram.

The SIMS quantitative analyses and direct ion im-ages were obtained at CANMET/MTL using proceduresas described in Cabri & McMahon (1995) and inMcMahon & Cabri (1998).

The XANES analyses were performed at the PNC–CAT and GSE–CARS undulator beamlines, Sectors 20and 13, respectively, at the Advanced Photon Source,Argonne National Laboratory (Heald et al. 1999, 2000,Newville et al. 1999, in prep., Sutton & Rivers 1999).

Monochromatic X-rays were focused (3 �m � 3 �mspot, PNC; 10 �m � 10 �m spot, CARS, penetrationdepth ~5 �m) on the grains using a Kirkpatrick–Baezmirror pair (Eng et al. 1998). The samples were mountedon an XY translation stage to allow the focused beam tobe positioned on any part of the sample. Fluorescent X-rays were detected using a multi-element germaniumdetector at the PNC–CAT beamline and using an Ox-ford WDX–600 wavelength-dispersion spectrometer at

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CHEMICAL SPECIATION OF GOLD IN ARSENOPYRITE 1269

the GSE–CARS beamline (Rivers & Sutton 1995, Smith& Rivers 1995). The germanium detector has the ad-vantage of collecting the full fluorescence energy-spec-trum and allowing multiple regions in that spectrum tobe monitored during a scan, but it has a worse energyresolution (200 eV versus 20 eV) than the spectrometer.The spectrometer has the advantage of higher resolu-tion of energy, allowing discrimination against AsK�.

RESULTS

Composition of the arsenopyrite grains studied

The arsenopyrite grains were re-analyzed by EPMA,specifically within the same areas where the X-rayswere focused for the XANES data. Four of the grainsare shown in Figure 1, with the location of each EPMA

indicated by X or numbers with respect to the still vis-ible SIMS craters. The mean compositions for six grainsare given in Table 1. The complete dataset may be ob-tained from the Depository of Unpublished Data, CISTI,National Research Council of Canada, Ottawa, OntarioK1A 0S2. All the arsenopyrite grains have S in excessof As and are metal-rich, with M:[S,As] ranging from1.006:1.994 for Sheba grain RPS–4 to 1.0244:1.984 forOlympiada grain 47.

Post-XANES SIMS ion images were taken ofOlympiada (O–4) grains 47, 61 and 62 to show the spa-tial distribution of Au as well as the anticorrelation withSe (Figs. 2, 3). Two types of Au distribution are seen.On the one hand, the Au is closely related to the well-developed rhombdohedral growth-zones in arsenopyrite(Fig. 2, top), and on the other, the Au does not appear tobe related to any obvious growth-zones, but rather oc-

FIG. 1. Back-scattered electron images of four arsenopyrite grains with locations of EPMA indicated by an X or number.Remains of the SIMS sputter craters may be seen in three of the grains, even after a light re-polish.

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1270 THE CANADIAN MINERALOGIST

FIG. 2. SIMS ion images of two arsenopyrite grains from Olympiada O–4. The field ofview for the images is 62.5 �m in diameter and is entirely within arsenopyrite for theupper two images. The Au is concentrated in the central parts of grain 47, closely fol-lowing growth zones in rhombdohedral crystals with no Au in their centers. However,Au is also absent along the rim of the grain, and Se is concentrated in the Au-free zones.The two lower images represent the edges of grain 61 (seen at top and left of the lowerimages), showing again anticorrelation of Se and Au.

* Based on a total of 3 atoms:(Fe,Co,Ni,Au)x(As,Sb)ySz.

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CHEMICAL SPECIATION OF GOLD IN ARSENOPYRITE 1271

curs as Au-rich patches and lenticular zones (Fig. 2,lower, and Fig. 3, top). The Au concentration inOlympiada grain 47 was also mapped by X-ray fluores-cence (Fig. 4), showing essentially the same spatial dis-tribution related to growth zones in the SIMS ion image(Fig. 2), but at lower resolution. The grain from SãoBento (SPB–1, Fig. 3) shows that the Au is closely re-lated to growth zones, as was shown for several othergrains of arsenopyrite from the same deposit (Steele etal. 2000). In contrast to the Olympiada grains, however,Se correlates positively with Au. The Au distribution inarsenopyrite from the Sheba mine shows a spatial rela-tionship to the elongate morphology of the grain(Fig. 5). In this case, no Se was detected, and Ni has anirregular distribution, with some areas of high Ni in Au-poor zones. Grain 11 from the Sentachan deposit has aheterogeneous distribution of Au compared to the S dis-tribution (Fig. 6), with no Se detected.

FIG. 3. SIMS ion images of arsenopyrite grain 62 from Olympiada O–4 (top) and for anarsenopyrite grain from the São Bento deposit (lower). The field of view for the imagesis 62.5 �m in diameter and is entirely within arsenopyrite for all the images. The upperimages show again anticorrelation of Se and Au, whereas the lower images show corre-lation of Se and Au.

FIG. 4. X-ray-fluorescence map of the entire arsenopyritegrain 47 (Olympiada O–4), taken prior to the SIMS ionimage shown in Figure 2 (for part of that grain). A 6 � 7�m beam was used in 5-�m steps in x–y direction, collect-ing the Au fluorescence for 2 s. The incident energy wasset to 12.0 keV, well above the Au edge.

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1272 THE CANADIAN MINERALOGIST

FIG. 5. SIMS ion images of an arsenopyrite grain from the Sheba mine. The field of viewfor the images is 62.5 �m in diameter and is entirely within arsenopyrite for all theimages. Black areas in the S image represent small non-sulfide mineral inclusions. NoSe was detected. The Au is concentrated along the length of the elongate grain, whereasthe Ni is distributed irregularly, with some of the Ni concentrated in Au-poor zones.

FIG. 6. SIMS ion images of arsenopyrite grain 11 from Sentachan. The field of view forthe images is 62.5 �m in diameter and is entirely within arsenopyrite for both images.The black areas in the S image represent small non-sulfide mineral inclusions. No Sewas detected. The Au distribution is heterogeneous.

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CHEMICAL SPECIATION OF GOLD IN ARSENOPYRITE 1273

XANES data for arsenopyrite

The XANES spectra were collected on three occa-sions, over an interval of several months. Data collectedfor Au LIII XANES edge shifts and white line positionsare given in Table 2. Edge shifts were determined fromthe peak of the first derivative of the spectra, with un-certainties based on the energy step-size and naturalwidth of the absorption edge. We believe that the edgeshift for arsenopyrite grain 47 (O–4, April data) is a littlehigh, possibly because of the influence of the As back-ground. In the case of the edge shift for the São Bentoarsenopyrite, the 2.2 value may also be high because ofthe lower Au:As ratio (Table 1; see below, Fig. 13).

The proximity in energy of the AuL� and AsK� fluo-rescent lines presents a challenge in determining thechemical form of “invisible” gold with X-rays. The Kabsorption edge of arsenic is 51 eV below that of thegold LIII edge. X-rays incident with sufficient energy toproduce AuL� fluorescent photons are also sufficientlyenergetic to produce fluorescence from arsenic, andfrom other elements (Fe, Ni, Cu) with lower absorption-edge energies, as shown in the energy spectra from themulti-element detector (Fig. 7). At 10,543 eV, the AsK�fluorescence is centered 842 eV above the AuL� fluo-rescence (9,711 eV), but has a tail into the gold regionof the spectrum, giving a background to the gold signal.

Using the multi-element detector, the gold signal(with background) as a function of position is shown inFigure 8 for the Sheba arsenopyrite grain. The gold dis-tribution, not surprisingly, is non-uniform, with a largergold signal along the upper edge of the grain and in aspike in the left half. Compared to the SIMS region, the

edge region has 22% higher fluorescence signal, and thespike is 60% higher. The As fluorescence distribution(not shown) is similar, but more uniform, and showsonly a 7% increase at the edge and no spike. The upperenergy spectrum (A) shown in Figure 7 comes from thespike on the left, and the lower spectrum (B), from theregion in the upper right, as indicated in Figure 8.XANES spectra for these two regions are shown in Fig-ure 9, where a gold signal originating from absorption

FIG. 7. X-ray-fluorescence spectra as a function of energy for regions A (upper curve) andB (lower) in the Sheba grain (Fig. 5). The lower curve was displaced from the upper bydividing by 5.

LIII

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1274 THE CANADIAN MINERALOGIST

FIG. 8. Spatial dependence of the intensity of the gold region of the X-ray-fluorescence spectrum for an arsenopyrite grain fromthe Sheba sample. The region previously examined by SIMS (shown in Fig. 5, which contains 1,182 ppm Au) is outlined. Tworegions (A, B) for which the X-ray spectra are displayed in Figure 7 are indicated.

FIG. 9. XANES spectra from regions A and B of the Sheba grain. The arsenic backgroundXANES was obtained from a gold-deficient region of an arsenopyrite grain from theOlympiada sample and found to be consistent with As XANES from the As region ofthe Sheba sample’s X-ray spectrum.

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CHEMICAL SPECIATION OF GOLD IN ARSENOPYRITE 1275

at the Au LIII edge (~11,919 eV) is clearly discerniblefrom the arsenic background arising from absorption atthe arsenic K-edge (~11,868 eV). The XANES for re-gion A (the spike) is an average of 28 scans, whereasthat for region B is the result of just one scan (counting5 seconds per point). The peak in the XANES evidentjust above the Au edge is a “white line” that arises froman appreciable number of unfilled gold 5d-states andsuggests chemically bound Au. We also recognize thatthe white line at the arsenic K-edge indicates the pres-ence of unfilled As 4p-states.

XANES spectra for arsenopyrite from Olympiada

Genkin et al. (1998) determined by ore microscopythat native gold and aurostibite are both present in thesample from Olympiada (O–4), together with arsenopy-rite. The arsenopyrite also contains invisible Au, over awide range, from 69 ppb to 2,298 ppm Au. 197AuMössbauer data on an arsenopyrite concentrate from thissample showed that chemically bound gold amounts to45 ± 2% relative [ascribed to aurostibite, with an iso-mer shift (IS) = +2.39 ± 0.08] compared to 55 ± 2%relative for metallic Au (Genkin et al. 1998). The SIMSanalyses of individual arsenopyrite grains (then consid-ered to represent mostly chemically bound gold), how-ever, could not be correlated with the 197Au Mössbauerspectroscopy, in terms of the proportion of Au attrib-uted to chemically bound Au in arsenopyrite versus thatattributed to gold chemically bound in aurostibite. TheXANES for grain 47 from this sample, however, showno white line, and yet the Au in this grain is closely re-lated to growth zones (Fig. 2, top). Though the polishedsection had been lightly re-polished to reduce the SIMS

crater, some of the sputtered Au may have contributedto the metallic Au spectrum. Therefore, an additionalseven randomly selected grains of arsenopyrite from thissample were scanned with WDS, but the Au XANESspectra once again show no white line (six spectrashown in Fig. 10). Following this result, a series ofXANES scans were done at several points across grain47; the composite of the scans is shown in Figure 11.Though noisy, the data are fairly similar, and none ofthe spectra show much evidence of a white line. Theedge positions and lack of an intense white line below E= 11,940 eV are consistent with the presence of metal-lic Au. Following this, grain 62 from the same samplewas measured and gave a similar result (Fig. 11). Anarsenopyrite concentrate from Olympiada sample O–5contains a larger proportion of metallic gold (79 ± 9%),as determined by 197Au Mössbauer spectroscopy (Genkinet al. 1998). However, in this case, no native gold oraurostibite was found by ore microscopy or SEM ex-amination. Several Au XANES scans of arsenopyritegrains 29 (Fig. 12) and 1 from this sample showed againthat the gold is metallic. Therefore all XANES measure-ments on arsenopyrite in the two Olympiada samplesare consistent with metallic gold. No chemically boundgold was detected in the Olympiada arsenopyrite.

XANES spectra on Sentachanarsenopyrite and aurostibite

In order to find additional evidence of metallic Auversus chemically bound Au in arsenopyrite, XANESwere measured in arsenopyrite grain 11 from Sentachan(Genkin et al. 1998). 197Au Mössbauer spectroscopy ofan arsenopyrite concentrate from this sample indicated

FIG. 10. Au XANES spectra taken from six randomly selected grains of arsenopyrite insample Olympiada O–4, together with spectrum of metallic gold (foil) as a dashed line.

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1276 THE CANADIAN MINERALOGIST

that >95% of the Au was chemically bound, with an IStypical for sulfides and sulfarsenides (Friedl et al. 1992,1995). For the XANES experiment, two locations justoutside the SIMS crater were analyzed, and both spec-tra show a white line (Fig. 12). In addition, aurostibitein Olympiada sample O–4 was analyzed, and the AuXANES spectrum also shows the expected white line(Fig. 12), clearly different from all the spectra of arse-nopyrite in this sample (Fig. 11).

Comparison of XANES spectra withreference standards

In Figure 13, we show data from the WDX–600spectrometer (with higher resolution of energy than themulti-element detector) for three samples, together withreference XANES spectra for Au foil, Au2S and AuCl3(in sealed ampoule) taken in transmission (foil) or fluo-rescence mode (sulfide, chloride) with ionization cham-

FIG. 11. A composite with Au XANES scans of three arsenopyrite grains from samplesO–4 and O–5 (Olympiada). The spectrum for grain 47 is noisier because it representsthe average of 25 scans on the grain from different points, with variable As concentra-tions.

FIG. 12. Au XANES scans of arsenopyrite grain 11 (Sentachan) and of aurostibite(Olympiada O–4). The Sentachan spectrum is the average of 12 scans, and shows AsK-edge XAFS below 11900 eV.

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CHEMICAL SPECIATION OF GOLD IN ARSENOPYRITE 1277

bers. The latter represent Au0, Au1+ and Au3+, respec-tively (Pantelouris et al. 1995, Wagner et al. 1994). TheXANES spectrum measured from a region 30 �m indiameter with high Au fluorescence signal in an arse-nopyrite grain from the Olympiada O–4 sample stronglyresembles the spectrum for elemental gold, as observedin the other Olympiada data above (Figs. 10, 11). Con-sideration of quadrupole splitting (QS) and IS for Au2S(Wagner et al. 1994) puts this binary compound almostmidway between Au1+ and Au3+, but because theXANES has no white line, few, if any, 5d-states areunoccupied. The XANES spectra from both the Shebaand São Bento samples clearly exhibit a peak at the Auabsorption edge (white line), a feature indicating chemi-cally bound gold, as did the XANES for arsenopyritefrom the Sentachan sample (Fig. 12) and the aurostibite(Fig. 12). Although the Au absorption edges for theSheba and São Bento spectra are at a comparable en-ergy to that of Au2S, the differences in the XANES spec-tral shapes clearly show that the Au-bearing phases inthese two samples are not dominated by Au2S (Fig. 12,Table 2).

DISCUSSION AND CONCLUSIONS

The Au XANES spectra of arsenopyrite from threedeposits (Sheba, São Bento and Sentachan) show whitelines at the Au absorption edge. The Au XANES micro-resolution data on a grain of Sentachan arsenopyrite arein agreement with the 197Au Mössbauer bulk data on anarsenopyrite concentrate from the same sample, indicat-ing >95% chemically bound Au in arsenopyrite (Genkinet al. 1998). However, Au XANES spectra of arsenopy-

rite grains from two well-characterized samples fromthe Olympiada deposit (O–4 and O–5) are distinctlydifferent, showing no white line, consistent with metal-lic gold (and with features also typical of metallic goldabove the edge).

SIMS ion and XRF images of the grain with highestAu content indicate that the Au is closely related togrowth zoning (Figs. 2, 4), whereas images of other ar-senopyrite grains from Olympiada show that the Auoccurs in lenticular zones (Fig. 2) and patches (Fig. 3).São Bento and Sheba arsenopyrite shows chemicallybound Au related to growth zoning (Figs. 3, 5), whereasan arsenopyrite grain from Sentachan, which also has awhite line, shows chemically bound Au with a hetero-geneous distribution (Fig. 6). The chemically boundgold in these three deposits is not aurostibite, becauseno aurostibite was found. We conclude that the spatialdistribution of Au in arsenopyrite is unrelated to thespeciation of Au. Correlation or anti-correlation of Auwith Se is also unrelated to the speciation of Au, sug-gesting that the Se replacement for S is independent ofthe incorporation of Au in arsenopyrite.

To understand the XANES results for arsenopyritefrom Olympiada, which indicate Au0, it is necessary toreview the earlier Mössbauer and SIMS data (Genkin etal. 1998). They ascribed the +2.39 ± 0.08 mm/s IS forsample O–4 to aurostibite, whereas assignment of aweak component at +2.00 ± 0.64 mm/s IS for sampleO–5 is more problematic. There is no mineralogicalevidence that the ill-defined shoulder in the Mössbauerspectrum of O–5 at +2.00 ± 0.64 mm/s (indicatingchemically bound Au) represents aurostibite. However,it is possible that this isomer shift should indeed also be

FIG. 13. Gold XANES spectra from three arsenopyrite grains and reference samples. Spec-tra have been displaced vertically for clarity. The ratio of the arsenic background togold signal for the São Bento sample (150 ppm Au) is comparable to that of the data inFigure 9, taken with the Ge detector, whereas the ratios for the Olympiada and Shebadata, shown here for the spectrometer, are a factor of 10 better.

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1278 THE CANADIAN MINERALOGIST

considered to be due to aurostibite, which has an IS of+2.33 ± 0.02 mm/s (Friedl et al. 1992). Fire assays ofan arsenopyrite concentrate of sample O–5 averaged93.9 ppm, and 39 SIMS analyses of individual grainsshowed that the arsenopyrite contains an average of 77ppm Au, representing 82% of the total Au (Genkin etal. 1998). Thus the invisible Au content of 82% (ob-tained by SIMS analyses) is very close to the 79 ± 9%of the Au assigned as metallic from the Mössbauer data.It was also suggested by Genkin et al. (1998) that thismetallic Au may in part represent submicroscopic gold,in particles as small as 2 nm, which yield Mössbauerspectra that are practically identical to pure gold(Santucci et al. 1996, Stievano et al. 1998).

The XANES data with white lines, which provideevidence of chemical bonding of Au in arsenopyrite,may be interpreted in several ways (e.g., Simon et al.1999). The absorption edge positions for the arsenopy-rite samples occur near that for Au1+ compounds. Whereelectronegativities (EN) are comparable in a compound,one can expect a high degree of covalent interactionsrather than ionic attractions (Pauling 1960). Such a caseexists in gold-bearing arsenopyrite, where the EN of Au,As and S differ by no more than 0.4.

A better understanding of the electronic environmentaround gold in a mineral such as arsenopyrite might thenbe obtained by comparison with XANES and Möss-bauer data done on intermetallic Au compounds. In theseries of AuM2 compounds, where M = Te, Sb, Sn, In,Ga, and Al (Sham et al. 1979, Bzowski et al. 1995),there is a d to s electron transfer, creating holes in the d-shell, but overall charge neutrality is largely maintainedby s-electrons shifting from the ligands to the Au. TheLIII XANES spectra see mainly the d holes, and it ismainly the s-electrons that cause the Mössbauer shift.There is a linear correlation between the white-line ar-eas at the Au LIII edge (�A3) or the change in number of5d holes at the Au site (�ht) (obtained by XANES mea-surements) and the IS with respect to metallic Au(Table 3). Thus the IS increases proportionally to thenumber of 5d holes (or white-line areas �A3), and thiswas explained as being due to Au forming hybridizedbonds with its ligands (Sham et al. 1979). This effectreduces the number of 5d electrons (from 10 in Au0 in5d10 6s1 configuration), and the holes thus created giverise to the white line. Similar results showing a whiteline was observed for broad-beam XANES on Au forconcentrates of arsenian pyrite (Simon et al. 1999). Thenear-edge Au LIII peak was described as a continuumresonance (Bianconi 1988) due to bound states (with Au5d character) at Au impurity sites. Simon et al. (1999)concluded that Au1+ could be concentrated in randomlydistributed layers with a structure of marcasite or arse-nopyrite, and that the Au1+ is probably incorporated intothe arsenian pyrite structure by adsorption during crys-tal growth.

In the case of Au in arsenopyrite, there is some vari-ability of the mean IS with respect to a Au:Pt source,

but the IS cluster around +3.5 mm/s (Friedl et al. 1995),or +4.7 mm/s with respect to metallic gold. This lattervalue is closest to that for AuIn2 on the IS scale(Table 3), such that one might view the bonding in ar-senopyrite to be similar to that of the Au intermetalliccompounds. It is of interest to note that following thereasoning above, one would expect a smaller white linefor aurostibite than for arsenopyrite since the IS withrespect to Au are +3.56 and +4.77 mm/s, respectively.Indeed, that was found to be the case (Fig. 12).

To describe gold chemically bound in the arsenopy-rite structure in ionic terms as either Au3+ (5d8) or asAu1+ (5d10) appears to be inappropriate, since the posi-tion of the absorption edge rules out the former, and thepeak due to unoccupied 5d-states rules out the latter, inagreement with Mössbauer data (Wagner et al. 1994).It is also relevant to refer to Figure 6 (QS versus IS forAu compounds) in Yang et al. (1998) [derived fromWagner et al. (1994), to which was added the range ofchemically bound Au in sulfides]. The range (in the re-gion 1.0 < QS < 1.3 mm/s) lies well outside that of Au1+

and Au3+ compounds. In describing the valence of theconstituent (chemically bound) elements in the gold-bearing arsenopyrite, where comparable electronegativi-ties indicate significant covalent interactions, one shouldnot, in a strict sense, assign an ionic valence. We con-clude, therefore, that a better though less satisfying de-scription is to say that the gold is bound covalently.

By combining spatially selective micro-XANES andcomplementary analytical techniques, we have alsoshown that the invisible submicroscopic gold in arse-nopyrite from four mesothermal gold deposits occurs inone of two ways: covalently or elementally, and the datasuggest the gold speciation to be deposit-specific. Thefinding of two types of invisible gold was also reportedfor arsenian pyrite (Simon et al. 1999). However, thedata for pyrite may be viewed as resulting from bulkanalyses, whereas the present data on arsenopyrite per-tain to individual grains. The distinction is important,as the pyrite data may come from two different popula-tions of grains. We did not observe coexistence of metal-lic Au and chemically combined Au in our microbeam

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CHEMICAL SPECIATION OF GOLD IN ARSENOPYRITE 1279

study of individual arsenopyrite grains. This findingsuggests that the two forms of Au are mutually exclu-sive in the case of arsenopyrite, with direct implicationson their genesis.

Further, finding two types of invisible gold in arse-nopyrite is of importance to mineral processing andextractive metallurgy. Instead of using an empirical ap-proach to metallurgical tests, extraction techniques canbe improved and results better understood because it islikely that the invisible gold occurring as nanometer-size Au0 will behave differently to the chemically boundAu. Important questions remain to be determined bymicro-XANES on arsenopyrite from many gold depos-its from different settings, well-characterized by othermethods: how common is the occurrence of each type,and do other types also exist? In addition, X-ray absorp-tion fine structure (XAFS) data can be used to deter-mine the coordination, bond length and identities ofneighboring atoms.

ACKNOWLEDGEMENTS

The Natural Sciences and Engineering ResearchCouncil of Canada supported this work financiallythrough an operating grant and a Major Facilities Ac-cess grant. Experiments at the Advanced Photon Source,Argonne National Laboratory were also supported bythe United States Department of Energy, Basic EnergySciences, under grants to the Pacific Northwest Con-sortium (PNC) [grant DE–FG03–97ER45628] and theConsortium for Advanced Radiation Sources (CARS)[grant DE–FG02–94ER14466] Collaborative AccessTeams and the Advanced Photon Source (contract W–31–109–Eng–38). The National Science Foundationunder contract EAR–9317772 also supported experi-ments at CARS. L.J.C. is grateful to J.H.G. Laflamme(CANMET) for the EPMA, to Professor F.E. Wagner(Munich) for numerous helpful discussions, and for re-search support from CANMET/MMSL and the NRCanSynchrotron Committee. Finally, we are grateful for thehelpful comments of referee F.E. Wagner, AssociateEditor N.J. Cook and Editor R.F. Martin.

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Received April 5, 2000, revised manuscript accepted Septem-ber 17, 2000.

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