Search

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.

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.