The solubility of gold in H2O-H2S vapour at elevated temperature and pressure

This experimental study sheds light on the complexation of gold in reduced sulphur-bearing vapour, specifically, in H2O-H2S gas mixtures. The solubility of gold was determined in experiments at temperatures of 300, 350 and 365 °C and reached 2.2, 6.6 and 6.3 μg/kg, respectively. The density of the vapour varied from 0.02 to 0.22 g/cm3, the mole fraction of H2S varied from 0.03 to 0.96, and the pressure in the cell reached 263 bar. Statistically significant correlations of the amount of gold dissolved in the fluid with the fugacity of H2O and H2S permit the experimental data to be fitted to a solvation/hydration model. According to this model, the solubility of gold in H2O-H2S gas mixtures is controlled by the formation of sulphide or bisulphide species solvated by H2S or H2O molecules. Formation of gold sulphide species is favoured statistically over gold bisulphide species and thus the gold is interpreted to dissolve according to reactions of the form:(A1)Au(s)+(n+1)H2S(g)=AuS·(H2S)n(g)+H2(g)(A2)Au(s)+H2S(g)+mH2O(g)=AuS·(H2O)m(g)+H2(g)Equilibrium constants for Reaction (A1) and the corresponding solvation numbers (KA1 and n) were evaluated from the study of Zezin et al. (2007). The equilibrium constants as well as the hydration numbers for Reaction (A2) (KA2 and m) were adjusted simultaneously by a custom-designed optimization algorithm and were tested statistically. The resulting values of log KA2 and m are −15.3 and 2.3 at 300 and 350 °C and −15.1 and 2.2 at 365 °C, respectively. Using the calculated stoichiometry and stability of Reactions (A1) and (A2), it is now possible to quantitatively evaluate the contribution of reduced sulphur species to the transport of gold in aqueous vapour at temperatures up to 365 °C. This information will find application in modelling gold ore-forming processes in vapour-bearing magmatic hydrothermal systems, notably those of epithermal environments.