Hydrosulphide complexing of Au(I) in hydrothermal solutions from 150-400 degrees C and 500-1500bar

The solubility of gold has been measured in aqueous sulphide solutions at temperatures between 150 degrees C and 500 degrees C and pressures of 500-1500 bar over a wide range of pH and total dissolved sulphur concentrations. The solubilities ranged from 0.002-1 mg/kg (1 x 10(-8) to 5 X 10(-6) m) in experiments with low total sulphur and acid pH, and from 2-108 mg/kg (1 x 10(-5) to 5 X 10(-4) m) in solutions with high total reduced sulphur concentrations and near neutral pH. The solubilities generally increased with increasing temperature, pH, and total dissolved sulphur. At near neutral pH, an inverse correlation between solubility and pressure was observed, whereas in acid pH solutions, above 150 degrees C, increasing pressure also increased the solubility. In near neutral pH solutions a solubility maximum was observed. This maximum is due to the species Au(HS)(2)(-). However, with increasing temperature, in accordance with the shift of pK(1) of H2S towards more alkaline pH, the maximum solubility also shifts to higher pH-values and consequently, at high temperatures the species stable at lower pH will dominate. It has been unambiguously proven that over a wide range of temperatures and pressures in reduced sulphur-containing hydrothermal solutions of low pH, the stoichiometry of the dominant Au(I)-hydrosulphide complex, is AuHS0. High temperature and high pressure equilibrium constants for the formation of the Au(I)-hydrosulphide complexes, AuHS0, and Au(HS)(2)(-), pertaining to the equilibria Au-(s) + H2S = AuHS0 + 1/2H(2(g)) (1) and Au-(s) + H2S + HS-- = Au(HS)(2)(-) + 1/2H(2(g)), (2) have been calculated. The nonlinear least squares fitted equilibrium constant for reaction (1) varies from log K-(1) = -6.81 at 150 degrees C/500 bar to a maximum of -5.90 at 200 degrees C/1500 bar and decreases again at higher temperatures (-7.83 at 400 degrees C/500 bars). For reaction (2), a similar variation occurs: log K-(2) = -1.45 at 150 degrees C/500 bar to -1.03 at 250 degrees C/500 bar and -1.75 at 400 degrees C/1500 bar. The thermodynamic functions for the Au(I)-hydrosulphide formation reactions and the cumulative and stepwise formation constants were derived after transforming the above reactions into isocoulombic form. The equilibrium constants for the uncharged complex, AuHS0, show that this species plays an important role in the transport and deposition of gold in ore depositing environments which are characterised by low pH fluids.