Gold(I) complexing in aqueous sulphide solutions to 500°C at 500 bar

Abstract

The solubility of gold has been measured in aqueous sulphide solutions from 100 to 500°C at 500 bar in order to determine the stability and stoichiometry of sulphide complexes of gold(I) in hydrothermal solutions. The experiments were carried out in a flow-through system. The solubilities, measured as total dissolved gold, were in the range 3.6 × 10 −8 to 6.65 × 10 −4 mol kg −1 (0.007–131 mg kg −1), in solutions of total reduced sulphur between 0.0164 and 0.133 mol kg −1, total chloride between 0.000 and 0.240 mol kg −1, total sodium between 0.000 and 0.200 mol kg −1, total dissolved hydrogen between 1.63 × 10 −5 and 5.43 × 10 −4 mol kg −1 and a corresponding pH T, p of 1.5 to 9.8. A non-linear least squares treatment of the data demonstrates that the solubility of gold in aqueous sulphide solutions is accurately described by the reactions Au ( s ) + H 2 S ( aq ) = AuHS ( aq ) + 0 .5H 2 ( g ) K s,100 Au ( s ) + H 2 S ( aq ) + HS – = Au ( HS ) 2 – + 0 .5H 2 ( g ) K s,110 where AuHS(aq) is the dominant species in acidic solutions and Au(HS) 2 − under neutral pH conditions. With increasing temperature, the stability field of Au(HS) 2 − shifts to more alkaline pH in accordance with the shift of the first ionisation constant of H 2S(aq). In addition, AuOH(aq) was found to be an important species in acidic solutions at t > 400°C. The equilibrium solubility constant to form AuHS(aq) was found to increase from log K s,100 = −6.20 ± 0.11 at 200°C to maximum of −5.63 ± 0.07 to 0.11 at 350°C and then decrease to −6.25 ± 0.19 at 500°C. The solubility constant to form Au(HS) 2 − increases with increasing temperature from log K s,110 = −2.33 ± 0.14 at 100°C to −1.27 ± 0.07 at 250°C and then decrease to −1.73 ± 0.25 at 500°C. From these measurements, the equilibrium stepwise and cumulative formation constants were calculated. The complex formation at 25°C is characterised by an exothermic enthalpy and small positive entropy, both of which are consistent with the predominantly covalent interaction between Au + and HS −. With increasing temperature, the cumulative formation reactions become endothermic and are accompanied with large positive entropy of reaction, indicating greater electrostatic interaction. The aqueous speciation of gold(I) is very sensitive to fluid composition and temperature. In low-temperature geothermal fluids, gold(I) sulphide complexes predominate whereas at higher temperatures, gold(I) hydroxide and chloride complexes may also play a role in hydrothermal gold transport in the Earth’s crust.