Experimental Determination of the Behavior of Gold(I) in Sulfide Fluids at Acidic pH

The solubility of mixed sulfides AgAuS + Ag3AuS2 (+Ag2S) at 110, 175, and 220°C and the saturated vapor pressure was experimentally studied in the acidic pH region depending on the concentration of H2S (0.05 – 3.2m, mol(kg H2O)–1). It was found that the main forms of Au are complexes AuHS(aq) and AuHS(H2S)(aq). The equilibrium constant of the reaction AuHS(aq) + 3 H2S(aq) = AuHS(H2S)3(aq) was determined: logK° = –1.03 ± 0.36 at 110°C, 1.26 ± 0.33 at 175°C, and 1.02 ± 0.59 at 220°C. The values of the standard thermodynamic properties and parameters of the Helgeson–Kirkham–Flowers model for the AuHS(H2S)3(aq) complex are calculated. The stability of the AgHS(H2S)3(aq) complex is significantly lower than that of its Au analogue. The AuHS(H2S)3(aq) complex can transport Au in the temperature range of 150–300°C at a high concentration of sulfide sulfur >0.3m (1 wt %) in the acidic pH region (e.g., at high sulfidation epithermal deposits).