The solubility of gold in hydrogen sulfide gas: An experimental study

The solubility of gold in H2S gas has been investigated at temperatures of 300, 350 and 400 °C and pressures up to 230 bars. Experimentally determined values of the solubility of Au are 0.4–1.4 ppb at 300 °C, 1–8 ppb at 350 °C and 8.6–95 ppb at 400 °C. Owing to a positive dependence of the logarithm of the fugacity of gold on the logarithm of the fugacity of H2S, it is proposed that the solubility of Au can be attributed to formation of a solvated gaseous sulfide or bisulfide complex through reactions of the type:equation(A)Au(s)+(n+1)·H2S(g)=AuS·(H2S)n(g)+H2(g)Au(s)+(n+1)·H2S(g)=AuS·(H2S)n(g)+H2(g)orequation(B)Au(g)+(n+1)·H2S(g)=AuHS·(H2S)n(g)+12H2(g)which are fH2fH2-dependent; n is a statistical solvation number. If the redox potential is buffered by S/H2S (the case in our study), the corresponding reactions are:equation(C)Au(s)+n·H2S(g)+S(l)=AuS·(H2S)n(g)Au(s)+n·H2S(g)+S(l)=AuS·(H2S)n(g)or equation(D)Au(s)+(n+12)·H2S(g)+12S(l)=AuHS·(H2S)n(g)Values of n for these latter reactions were calculated to be 1.8 and 1.2 at 300 °C, 1.7 and 1.0 at 350 °C, 2.2 and 1.7 at 400 °C, respectively. The equilibrium constants for reactions (3) and  increase with temperature and have values of log K = −11.1 ± 0.2 at 300 °C (both reactions), −10.7 ± 0.3 and −10.5 ± 0.3 at 350 °C, and −10.6 ± 0.2 and −10.4 ± 0.2 at 400 °C, respectively. At the conditions of this study, the solubility of gold in H2S gas is relatively high, and the results obtained provide strong evidence that H2S can play an important role in the vapor transport of gold and the formation of hydrothermal gold deposits.