Thio complexes of gold and the transport of gold in hydrothermal ore solutions |
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| Authors: | TM Seward |
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| Institution: | Chemistry Division, D.S.I.R., Private Bag, Petone, New Zealand |
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| Abstract: | The solubility of gold in aqueous sulphide solutions has been determined from pH20°C ≈ 4 to pH20°C ≈ 9.5 in the presence of a pyrite-pyrrhotite redox buffer at temperatures from 160 to 300°C and 1000 bar pressure. Maximum solubilities were obtained in the neutral region of pH as, for example, with mNaHS = 0.15 m, pH20°C = 5.96, T = 309°C, P = 1000 bar where a gold solubility of 225 mg/kg was obtained. It was concluded that three thio gold complexes contributed to the solubility. The complex Au2(HS)2S2? predominated in alkaline solution, the Au(HS)2? complex occurred in the neutral pH region, and in the acid pH region, it was concluded with less certainty that the Au(HS)° complex was present. Formation constants calculated forAu2(HS)2S2? and Au (HS)2? emphasize their high stability. In the temperature range from 175 to 250°C, values of pβ for Au2(HS)2S2? vary from ?53.0 to 47.9 (±1.6) and from ?23.1 to ?19.5 ( ± 1.5) for Au(HS)2?. Equilibrium constante for the dissolution reactions, and 2Au° + H2S + 2H8? ? Au(HS)2? + H2 vary from pKm = +2.4 to +2.55 (±0.10) for Au2(HS)2S2? and from pKn = + 1.29 to + 1.19 (±0.10) for Au(HS)2? over the temperature range 175 to 250°C. Enthalpies of these dissolution reactions were calculated to be ΔHm° = ?5.2 ±2.0 kcal/mol and ΔHn° = +1.7 ±2.0 kcal/mol respectively. It was concluded that gold is probably transported in hydrothermal ore solutions as both thio and chloro complexes and may be deposited in response to changes in temperature, pressure, pH, oxidation potential of the system and total sulphur concentration. |
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