Abstract:Gold is transported in high-temperature chloride-bearing hydrothermal fluids in the form of AuCl 2 − . The stability of this complex has been extensively studied, but there is still considerable disagreement between available experimental data on the temperature region 300-500 • C. To solve this problem, we measured the solubility of gold in HCl/NaCl fluids (NaCl concentration varied from 0.1 to 3 mol·(kg·H 2 O) −1 ) at 450 • C and pressures from 500 to 1500 bar (1 bar = 10 5 Pa). The experiments were performed using a batch autoclave method at contrasting redox conditions: in reduced experiments hydrogen was added to the autoclave, and in oxidized experiments the redox state was controlled by the aqueous SO 2 /SO 3 buffer. Hydrogen pressure in the autoclaves was measured after the experiments in the reduced system. The gold solubility constant, Au (cr) + HCl • (aq)

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Pyrrhotite Fe 1-x S is the main sulphide component of platinum group elements (PGEs) ores where it commonly contains from a few tenths to a few dozen 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 modelling. 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 sulphur fugacity yields higher concentrations of Pt in pyrrhotite. The Pt content reaches 0.6 wt% at the maximum achieved temperature and sulphur 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 modelling of XANES and interpretation 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 4+ are surrounded by 6 S atoms at a distance of 2.39±0.02 Å. According to theoretical FDMNES simulations, the 2 nd coordination sphere of the solid solution Pt contains one vacancy in the Fe sublattice within the Fe-layer. The Pt 2+ S-like clusters can be considered as a quench product. High sulphur fugacity stabilizes the solid solution Pt and prevents the formation of the PtS-like clusters during cooling. The maximum content of the solid solution Pt in pyrrhotite is ca. 50 times lower than in pyrite and can be approximated by a straight line in the log C(Pt) vs. 1/T coordinates, it increases from 1 ppm at 350°C to 3 wt% at 900°C.

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