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Pyrrhotite Fe 1-x S is the main component of platinum group elements (PGE) ores and contains from few tenths of ppm to a few hundred ppm of disseminated Pt. Here we report an investigation of the state of Pt in synthetic pyrrhotite performed by X-ray absorption spectroscopy (XAS) in combination with theoretical spectra modeling. The pyrrhotite crystals were obtained by means of salt flux technique, using an eutectic mixture of alkali metal halides as a transport media. Analysis of the chemical composition of synthesized crystals showed that an increase of the temperature and sulfur fugacity yields higher concentrations of Pt in pyrrhotite. The Pt content reaches 0.6 wt% at the maximum studied temperature and sulfur fugacity ( t = 720°C, log f (S 2 ) = -0.1) in Pt-saturated system. Analysis of Pt L 3 -edge XANES spectra revealed that Pt presents in pyrrhotite in the 4+ and 2+ “formal” oxidation states. Theoretical modeling of XANES and approximation of EXAFS spectra showed that Pt 4+ substitutes for Fe in the crystal lattice of pyrrhotite, whereas Pt 2+ forms PtS-like clusters disseminated in the pyrrhotite matrix. Atoms of isomorphous Pt are surrounded by 6 S atoms at a distance of 2.39±0.02 Å. According to theoretical FDMNES simulations of XANES spectra, in the solid solution state the 2 nd coordination sphere of Pt contains one vacancy in the Fe sublattice within the Fe-layer. The PtS-like clusters can be considered as a quench product. High sulfur fugacity stabilizes the solid solution Pt and prevents the formation of the PtS-like clusters during cooling.

The distortion of the atomic structure around In and Cu atoms in sphalerite ZnS was explored by reverse Monte Carlo (RMC) method applied to the Extended x-ray absorption fine structure (EXAFS) interpretation. Parameters of the local atomic structure (interatomic distances) around dopants in synthetic In- and In-Cu-bearing sphalerites were determined by fitting In and Cu K-edge EXAFS spectra using evolutionary algorithm (EA) of the RMC method. These data were complemented with quantum chemical Density Functional Theory (DFT) calculations and theoretical modeling of XANES spectra. The RMC-EXAFS method showed that the three coordination shells of In-bearing sphalerite are characterized by almost symmetrical Gaussian type peaks, which shapes are close to the one of pure sphalerite. This shape of the peaks is characteristic of symmetrical undisturbed structural environment of In in the sphalerite solid solution which is formed via the charge compensation schemes 3Zn2+↔2In3++□, where □ is a Zn vacancy. However, in case of (In,Cu)-bearing sphalerites formation of solid solution state follows the charge compensation scheme 2Zn2+↔Cu++In3+. In this case some splitting of the RDF peaks are observed, the splitting rate correlates with impurities concentrations. In contrast to In, the local atomic structure around Cu is not symmetric. The 2nd coordination shell around Cu (which consists of 12 metal atoms) can be described by three distinct Gaussian contributions. This splitting of interatomic distances in the 2nd coordination shell points out on the significant distortion of the ZnS crystal structure around Cu. The theoretical calculation of Cu K-edge XANES based on the distorted structural environment near Cu provides better agreement with the experiment than the symmetrical atomic model. According to our DFT calculations, considerable splitting of the 2nd coordination shell (up to 0.1 Å for Cu) occurs in the case of close positions of the impurity atoms or the Zn vacancy. The DFT calculations showed that the geometries with the close arrangement (clustering) of the impurities – In and Cu atoms, or the In atom and a vacancy, are energetically more favorable than the random distribution of the defects. However, as no heavy In atoms were detected in the 2nd shell of Cu, and the 2nd shell of In is almost undisturbed, we conclude that the defects are distributed randomly (or at least not close to each other). The disagreement of RMC-EXAFS fittings with results of DFT calculations, according to which the closest arrangement of dopants is the most stable configuration, can be explained by the presence of other defects of the sphalerite crystal lattice which were not considered in the DFT calculations.

Processing of Zn ore accounts for >95% of production of In - a "critical" metal which is widely used in the high-tech electronics. The main source of In is sphalerite (Zn, Fe)S which also can host industrial concentrations of Au. Here we use X-ray absorption spectroscopy to investigate the coupled chemistry of In and Au in synthetic sphalerite crystals - analogues of natural minerals. The concentrations of In and Au were found to correlate with each other and reached 0.5 wt% in crystals synthesized at 850 °C. Both metals are homogeneously distributed within the sphalerite matrix. However, their positions within the mineral are different. In accord with X-ray absorption near edge structure (XANES) spectroscopy the formal oxidation state of these elements is +3 (In) and +1 (Au). Analysis of extended X-ray absorption fine structure (EXAFS) spectra revealed that In replaces Zn in the structure of sphalerite. The In-ligand distance increases by 0.12 Å and 0.09-0.10 Å for the 1st and 2nd coordination spheres, respectively, in comparison with pure ZnS. The In-S distance in the 3rd coordination sphere is close to the one of pure sphalerite. The In K-edge and Au L3-edge XANES and EXAFS spectra suggest that there is no In-Au clustering. Gold in sphalerite is coordinated with 2.5±0.3 S atoms at Au-S distance of 2.35±0.01 Å in the 1st coordination sphere, whereas distant coordination spheres have disordered nature. Our data suggest that at least two different forms of Au are present in sphalerite. At high Au concentrations (0.03-0.5 wt%) the nanosized Au2S clusters predominate, probably with small admixture of the Au solid solution characterized by higher Au-S distance. Alike Au, the other 1st group metals (Me) Cu and Ag, which often are present in high (tenths ppm to wt%) concentrations in sphalerite, can form nanosized Me-S clusters with only traces (ppm level) of metal in the solid solution state.

Hydrothermal chloride-rich fluids are identified at the late stages of the formation of platinum group element (PGE) deposits in giant layered intrusions, and are considered as the PGEs transport media in Cu(-Mo,Au) porphyry systems. In order to quantify the hydrothermal mobility of Pt we performed an investigation of the speciation of Pt in hydrothermal chloride-bearing fluids and dry melt by means of X-ray absorption spectroscopy (XAS). The experiments consisted in recording the Pt L-3-edge X-ray absorption near edge structure/extended X-ray absorption fine structure (XANES/EXAFS) spectra of Pt-bearing fluids obtained by dissolution of Pt metal in KCl/HCl and CsCl/HCl fluids in the temperature range from 450 to 575 degrees C at pressures from 0.5 to 5 kbar. A spectrum of Pt dissolved in dry CsCl/NaCl/KCl + K2S2O8 melt was recorded at 650 degrees C. The capillary method, when the experimental solution together with Pt((cr) )is sealed inside a silica glass capillary, was used. As was determined from the XANES spectra, in all the experimental systems Pt existed in the +2 oxidation state. Analysis of EXAFS spectra showed that Pt is coordinated by four Cl atoms with Rpt-cl = 2.31 +/- 0.01 angstrom independently of the T-P-compositional parameters. No evidence of the formation of complex with alkali metal cations in the second coordination sphere of Pt was found by the analysis of the EXAFS spectra of concentrated CsCl brines and melt. Our results imply that PtCl42- is the main Pt-Cl complex which predominates in hydrothermal fluids at t > 400 degrees C and fluid density d > 0.3 g.cm(-3). Experimental data obtained for dry melt of alkali metal chlorides suggest that Pt-Cl complexes can dominate Pt speciation in chloride-bearing aluminosilicate melts where Cl exhibits a salt-like atomic arrangement and ionic bonding. The literature data on the Pt solubility constant, Pt-(cr) + 2HCl((aq))degrees + 2Cl(-) = PtCl42- + H-2(aq), are compiled and fitted to the simple density model equation log K-s degrees (PtCl42- ) = 0.973 - 8202. T(K)(-1) - 5.505 . log d (w) + 2223 . log d(w) .T(K)(-1), where d(w) is the pure water density in g.cm(-3). The equation, combined with the extended Debye-Hiickel equation for activity coefficients, can be used to calculate the solubility of Pt up to 1000 degrees C/5 kbar. It accurately predicts the solubility of Pt in concentrated chloride brine (up to 50 wt% NaCl) at parameters of magmatic-hydrothermal transition (800 degrees C/1.4 kbar). At fluid/vapor density below 0.3 g.cm(-3) a neutral complex PtCl2 degrees((aq)) is suggested as the dominant Pt species. Our data demonstrate that Pt is highly mobile in high-temperature oxidized chloride-rich hydrothermal fluids. For example, at 800 degrees C/2 kbar the concentration of Pt can reach a few wt% in the 1 wt% HCl/50 wt% NaCl fluid which is in equilibrium with magnetite-hematite buffer. Once a Cl-reach fluid exsolves from alumuinosilicate melt, Pt follows Cl and enriches the fluid phase where it exists mostly in the form of PtCl42-. Decrease of temperature, acidity, and fluid chlorinity results in precipitation of Pt from the fluid phase.

Chloride-bearing fluids are widespread in the Earth's interior from low-temperature subsurface conditions to deep lithosphere. The concentration of chloride salts varies from diluted aqueous solutions to concentrated brines and anhydrous (dry) chloride melts beneath volcanoes. Here we report an investigation of the state of Au in hydrothermal chloride fluids and anhydrous melts by means of in situ X-ray absorption spectroscopy combined with ab initio molecular dynamics simulations and thermodynamic modeling. The experiments included registration of Au L 3 -edge X-ray absorption near edge structure/extended X-ray absorption fine structure spectra of Au-bearing fluids in the temperature range from 350 to 575 °C at pressures of 150-4500 bar. Spectra of Au dissolved in dry CsCl/NaCl/KCl + K 2 S 2 O 8 melt were recorded at 650 °C. It was found that Au is coordinated by two Cl atoms (R Au-Cl = 2.25-2.28 Å). The alkali metal atoms (Me) were detected in the distant coordination sphere of Au at R Au-Me = 3.3-4.1 Å. The alkali metal cations in the vicinity of Au-Cl complex partly compensate the positive charge located on Au and, by this way, affect the Au-Cl distance. An increase of the fluid pressure causes expansion of the second coordination sphere composed of the alkali metal cations, which leads to the increase of the positive Au charge and results in slight contraction of the first coordination sphere of Au. Accordingly, the transport of Au in high-temperature chloride-bearing natural ore-forming fluids of moderate to high densities (>0.3 g·cm -3 ) can be explicitly described by the formation of the AuCl 2 - at any salt concentration from low-salinity fluids to hydrosaline liquids and anhydrous melts. In general, this means that the hydrothermal fluid chemistry simplifies with increasing temperature.

Pyrite (FeS2) is a typical container of Pt in ores of magmatic and hydrothermal origin and in some carbonrich ores of sedimentary-diagenetic origin. Knowledge of the state of Pt disseminated in the matrix of pyrite, including local atomic environment (type of atoms in the nearest and distant coordination shells, coordination numbers, interatomic distances) and oxidation state, is necessary for physical-chemical modeling of platinum group element mineralization and for the improvement of Pt ore extraction and processing technologies. Here we report results of an investigation of local atomic structure of synthetic Pt-bearing pyrites by means of X-ray absorption spectroscopy (XAS). Synthesis experiments, performed at 580° and 590°C in a Pt-saturated system by means of salt-flux method, yielded crystals of pyrite with concentrations of Pt up to 4 wt %. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) showed that the distribution of Pt within the pyrite grains is of zonal character, but within the distinct zones Pt is distributed homogeneously. Negative correlation between the concentrations of Pt and Fe was observed in the synthesized pyrite grains. The slope of the correlation line corresponds to the formation of the solid solution in the Pt-Fe-S system and/or to the formation of PtS2. The XAS experiments revealed the existence of two forms of Pt in pyrite. The main form is the solid solution Pt(IV), which isomorphically substitutes for Fe. The Pt-S distance in pyrite is ~0.1 Å longer than that of Fe-S in pure pyrite. The distortion of the pyrite crystal structure disappears at R >2.5 Å. The second Ptrich form was identified by means of high-resolution transmission electron microscopy (HRTEM) as nanosized inclusions of PtS2. Heating experiments with in situ registration of X-ray absorption spectra resulted in partial decomposition (dissolution) of PtS2 nanosized inclusions with the formation of the solid solution (Fe1–xPtx)S2. Therefore, the PtS2 nanosized particles can be considered as a quench product. Our data demonstrate that both Pt solid solution and PtS2 nanosized inclusions (at high Pt content) can exist in natural Pt-bearing pyrites.

Minerals of the Fe-As-S system are ubiquitous components of Au ores in many deposits of hydrothermal origin, including world-class volcanogenic massive sulfide, low-temperature epithermal and mesothermal ones. The “invisible” (or refractory) form of Au is present in all types of hydrothermal ores and in many cases predominates. Knowledge of the “invisible” Au chemical state (local atomic environment/structural position, electronic structure and oxidation state) is a fundamental problem crucial for understanding conditions of the ore formation, is necessary for physical-chemical modeling of hydrothermal Au mineralization, and will help to create more efficient technologies of ore concentration and Au extraction. We report investigation of the “invisible” Au chemical state in synthetic analogues of natural minerals: pyrite (FeS2), arsenopyrite (FeAsS), and löllingite (FeAs2). The compounds were synthesized using hydrothermal (pyrite) and salt flux techniques (all the minerals) and studied using X-ray absorption fine structure spectroscopy (XAFS) in high energy resolution fluorescence detection (HERFD) mode in combination with first-principles quantum chemical calculations. The concentration of the “invisible” Au in the synthesized löllingite (800±300 ppm) was much higher compared to arsenopyrite (23±14 ppm), whereas the lowest Au content with zonal Au concentration profiles was observed in crystals of salt flux pyrite. The concentration of Au in the hydrothermal pyrite (40-90 ppm) is independent on sulfur fugacity and probably reflects the maximum Au solubility at the experimental P/T parameters (450 °C, 1 kbar). It is shown that Au replaces Fe in structure of löllingite, arsenopyrite, and hydrothermal pyrite. The nearest-neighbors Au-ligand distance increases by 0.14 Å (pyrite), 0.16 Å (löllingite), and 0.23 (As), 0.13 (S) Å for arsenopyrite relative to Fe-ligand distance in pure compounds. Distortion of the local atomic structure around the Au is negatively correlated with the distance and disappears at r > ~4 Å. The chemically bound Au is stable only in hydrothermal pyrite, whereas the pyrite synthesized in the absence of hydrothermal fluid contains only Au°. Heating (metamorphism) of hydrothermal pyrite results in decomposition of the chemically bound Au with formation of Au° nuggets which coarsen with temperature. Depending on chemical composition of the host mineral Au can play a role either of cation or anion: the Bader atomic partial charges of Au decrease in the order of pyrite (+0.4 e) > arsenopyrite (0) > löllingite (-0.4 e). Our results suggest that other noble metals (platinum group elements, Ag) can form the chemically bound refractory admixture in base metal sulfides/chalcogenides. The chemical state, as well as the concentration, of this form of noble metals can differ depending on the composition of the host mineral and the ore history.

The oxidation state of hydrothermal fluids, which form economic deposits of noble metals, varies in wide limits – from oxidized ones typical for porphyry mineralization to reduced which formed volcanogenic massive sulfide deposits. Sulfur-bearing species, along with chloride, are the most important ligands that form stable aqueous complexes with Au and determine Au concentration in natural ore-generating fluids. Depending on the f(O2) value, in high-temperature fluids (t > 300 °C) the dominant forms of sulfur are sulfides (H2S, HS-), sulfites (SO2, HSO3-, SO32-), sulfates (HSO4-, SO42-), and the radical species (S3-). Here we report an investigation of Au complexation in high-temperature sulfide-bearing fluids of contrasting oxidation states. The solubility of Au was measured in “oxidized” sulfide fluid (H2S/SO42- buffer controls the Red/Ox state) at 450 °C, 1000 bar, and compared with the Au solubility in “reduced” sulfide systems (H2S/HS- predominate) reported in the literature. The measured values of the Au solubility matches best the model of the formation of Au(HS)2- at near-neutral to weakly acidic pH, and AuHS° in acidic solutions. The solubility constants have been determined for the reactions, Au(cr) + H2S°(aq) + HS- = Au(HS)2- + 0.5 H2(g) log KAu(HS)2- = -0.9 ± 0.1 , Au(cr) + H2S°(aq) = AuHS°(aq) + 0.5 H2(g) log KAuHS = -6.5 ± 0.1 . The average value of log KAu(HS)2- = -1.3 ± 0.5 was calculated for 450 °C (P = 500 – 1500 bar) using all the available Au solubility constants obtained in both “reduced” and “oxidized” sulfide systems. The local atomic environment of Au in high-temperature hydrothermal fluids has been studied using X-ray absorption fine structure spectroscopy (XAFS) in high energy resolution fluorescence detection (HERFD) mode in combination with ab initio molecular dynamics (AIMD) and Reverse Monte Carlo (RMC) simulations. Interpretation of Au L3-edge EXAFS spectra showed that, independently of the Red/Ox (sulfide or sulfide/sulfate systems) and PT – conditions (350 – 450 °C, 500 bar) two S atoms are located in the first coordination shell of Au at 2.29±0.02Å. Comparison of the experimental spectra with those simulated by means of AIMD revealed that EXAFS spectroscopy is not sensitive to the presence of light atoms like S in a distant coordination shell of Au. However, theoretical calculations indicated that the shape of Au L3-edge HERFD-XANES spectra depends upon the composition of the distant coordination shell and, therefore, can be used to discriminate between Au(HS)2-, Au-(HS)-S3- and, probably, other complexes with distant-coordination-shell anions. Experimental Au L3-edge HERFD-XANES spectra are identical for all studied PT- and Red/Ox-parameters. These results allowed us to conclude that Au(HS)2- complex predominates Au speciation in weakly acidic to weakly alkaline pH independently from the oxidation state of the fluid. Besides that, XAFS experiment demonstrated that the formation of mixed Au-HS-Cl complex can be neglected. With increasing pressure (to nkbar) or decreasing temperature (to < 300 °C), due to increasing of concentration of S species in intermediate oxidation states, formation of the Au-HS complexes can be accompanied by the formation of other species with (hydro)sulfite, thiosulfate, (hydro)polysulfide, and sulfur radicals, which would enhance the hydrothermal Au mobility. Stability of these complexes needs further experimental and theoretical examination.

Geological processes leading to formation of sulfide ores often result in precipitation of gold-bearing sulfides which can contain high concentrations of this metal in “invisible” (or ”refractory”) state. Covellite (CuS) is ubiquitous mineral in many types of the ore deposits, and numerous studies of the natural ores show that covellite can contain high concentrations of Au. At the same time, Au-bearing covellite withstands cooling in contrast to other minerals of the Cu-Fe-S system (chalcocite, bornite, chalcopyrite), where Au exsolves at low temperatures. This makes covellite a convenient model system for investigation of the chemical state (local environment and valence) of the “invisible” Au in copper-sulfide ores (copper-porphyry, epithermal, volcanogenic massive sulfide, SEDEX deposits). Therefore, it is necessary to determine the location of Au in the covellite matrix as it will have important implications for the methods employed by mineral processing industry to extract Au from sulfide ores. Here we investigate the chemical state of Cu and Au in synthetic covellite containing up to 0.3 wt.% of Au in the “invisible” state. The covellite crystals were synthesized by hydrothermal and salt flux methods. Formation of the chemically bound Au is indicated by strong dependence of the concentration of Au in covellite on the sulfur fugacity in the experimental system (d(log C(Au))/d(log f(S2)) ∼ 0.65). The Au concentration of covellite grows with increasing temperature from 400 to 450 °C, whereas further temperature increase to 500 °C has only minor effect. The synthesized minerals were studied using X-ray absorption fine structure spectroscopy (XAFS) in high energy resolution fluorescence detection (HERFD) mode. Ab initio simulations of Cu K edge XANES spectra show that the Cu oxidation state in two structural positions in covellite (tetrahedral and triangular coordination with S atoms) is identical: the total loss of electronic charge for the 3d shell is ∼ 0.3 for both positions of Cu. This result is confirmed by theoretical analysis of electron density performed using quantum theory of atoms in molecules (QTAIM). Modeling of the Au L3 edge EXAFS/XANES spectra showed that Au in covellite exists in the form of the isomorphous solid solution formed by substitution for Cu atoms in triangular coordination with the Me-S distance in the first coordination shell increased by 0.18 Å relative to the pure CuS structure. The “formal” oxidation state of Au in covellite is +1. The Bader partial atomic charge for Au in covellite is lower than the charge of Cu (+0.2 e vs. +0.5 e) indicating that the degree of covalency for the Au-bearing covellite is higher than that of pure CuS. The analysis of electronic density of states shows that this structural position of Au results in strong interactions between hybridized Au s,p,d, S p, and Cu p,d orbitals. Such chemical bonding of Au to S and Cu can result in the formation of Au-bearing solid solution with other minerals in the Cu-Fe-S system.

International audience; The solubility of (i) Au(cr), (ii) mixtures of crystalline Ag2S Ag3AuS2, and (iii) Ag3AuS2 (uytenbogaardtite) AgAuS (petrovskaite) was measured at 25 °C, 1 bar, whereas the solubility of AgAuS(cr) was measured at 91 250 °C and Psat. These measurements were performed in aqueous sulphide solutions containing ?0.012 0.12 m of total reduced sulfur. At 25 °C sealed glass ampoules were employed, and Ti autoclaves allowing sampling of experimental solutions were used at higher temperatures. It was found that under investigated experimental conditions AgAuS dissolves incongruently with the formation of Ag3AuS2. The values of the following equilibrium reaction constants were determined: Au+HS0(aq)+HS=Au(HS)2-+0.5H(i)K=K AgAuS+HS+0.5HS0(aq)=1.5AgS+Au(HS)2-(ii)K 1.5AgAuS+HS+0.5HS0(aq)=0.5AgAuS+Au(HS)2-(iii)K It was found that at 25 °C, 1 bar log K(i) = ?5.62 ± 0.14, log K(ii) = ?4.49 ± 0.26 and log K(iii) = ?4.03 ± 0.15. Solubility data collected at higher temperatures resulted in the following values for log K(iii): ?2.12 ± 0.25, ?1.32 ± 0.12, and ?1.04 ± 0.08 at 91, 150, and 250 °C, respectively (the latter value corresponds to the reaction 1.97 AgAuS+HS+0.5HS0(aq)=0.735AgAuS+Au(HS)2- because Ag3AuS2(cr) undergoes a phase transition at 183 °C). These values are in good agreement with log K(iii) = ?2.81 ± 0.10 obtained at 80 °C by Zotov et al. (Zotov, A.V., Baranova, N.N., Bannykh, L.N., 1996. Solubility of the gold sulfides Au2S and AgAuS in solutions containing hydrogen sulfide at 25 80 °C and pressures of 1 and 500 bar. Geochem. Int.34, 216 221). The values of K(ii) and K(iii) determined in the present study were combined with the thermodynamic properties of Ag Au sulphides reported by Osadchii and Rappo (Osadchii, E.G., Rappo, O.A., 2004. Determination of standard thermodynamic properties of sulfides in the Ag Au S system by means of a solid-state galvanic cell. Am. Miner.89, 1405 1410), and thermodynamic properties for aqueous species and hydrogen from SUPCRT92 database to generate the values of K. The results of the three experimental series performed at 25 °C, Psat (involving metallic gold (i) and Ag Au sulphides (ii) and (iii)) yield the average value of log K=-5.38±0.25.LogK values derived at higher temperatures are ?3.85 ± 0.20, ?3.13 ± 0.29, ?2.22 ± 0.20, and ?1.96 ± 0.25 at 80, 91, 150 and 250 °C, respectively.At 150 and 250 °C these logK values are in close agreement with the data of Shenberger and Barnes (Shenberger, D.M., Barnes, H.L., 1989.Solubility of gold in aqueous solutions from 150 to 350 °C. Geochim.Cosmochim.Acta53, 269 278).The logK value obtained at 25 °C, 1 bar combined with Au2S(cr) solubility products (AuS+3HS+H=2Au(HS)2- and AuS+HS+H=2AuHS0(aq)) measured at 25 °C by Renders and Seward (Renders, P.J., Seward, T.M., 1989.The stability of hydrosulphido- and sulphido-complexes of Au(I) and Ag(I) at 25 °C. Geochim.Cosmochim.Acta53, 245 253) yields ?fG(298.15K)o(AuS)=19.41±1.43kJ/mol and log K(AuHS0) = ?10.98 ± 0.30 for the reaction Au+HS0(aq)=AuHS0(aq)+0.5HK.