Concentration of Gold(I) Thiosulfate Complex Ions on the Surface of Alumina Gel and their Change in Chemical State: Preliminary Experiment in the Elucidation of the Formation Mechanism of Epithermal Gold Deposits

Abstract

AbstractIn order to elucidate the formation mechanism of low‐sulfidation epithermal gold deposit, the adsorption of [Au(S2O3)2]3− (a model compound for gold(I) complex ion) on alumina gel (a model compound for the aluminum‐bearing minerals) and change in chemical state of [Au(S2O3)2]3− after adsorption on the surface of alumina gel were investigated as a basic model experiment. In the pH range from 4 to 6, the amount of [Au(S2O3)2]3− adsorbed on alumina gel decreased with increasing pH and decreased drastically between pH 6 and 7, and then approached zero above pH 8 at 30°C. At 60°C, the amount of gold adsorbed above pH 7 was enhanced compared with that at 30°C. This adsorption tendency indicates that [Au(S2O3)2]3− is mainly adsorbed by electrostatic interaction between negative charges of [Au(S2O3)2]3− and positive charges of alumina gel because of its isoelectric point around pH 9. The chemical state of gold after adsorption of [Au(S2O3)2]3− on alumina gel was examined using X‐ray absorption near edge structure (XANES). The result showed that [Au(S2O3)2]3− was spontaneously reduced to elemental gold even in the absence of specific reducing agents after adsorption on alumina gel. This reduction reaction might occur by two steps: (i) disproportionation of the adsorbed [Au(S2O3)2]3− at the surface of alumina gel, and (ii) spontaneous reduction of the resulting gold(III) complex ions on the surface of alumina gel. The experimental results suggest that aluminum plays an important role in the concentration of gold(I) complex ions and subsequent reduction of gold during the formation of low‐sulfidation epithermal gold deposits.