矿床地球化学国家重点实验室知识库

The discovery, using short wavelength infrared radiation (SWIR), that some low-sulfidation epithermal gold deposits are associated with hydrothermal alteration involving ammonium minerals, particularly buddingtonite ((NH₄)AlSi₃O₈), has led to the suggestion that the gold may have been transported as a complex involving ammonia (NH₃). The speciation of gold in ammonia-rich solutions, however, has received little attention from experimentalists and, consequently, it is not known whether gold forms stable complexes with NH₃ and whether such species could contribute significantly to the transport of gold in epithermal ore-forming systems. To resolve this issue, we have conducted experiments to investigate the speciation and solubility of gold in ammonia-bearing solutions at temperatures of 225 and 250 °C under vapour-saturated water pressure with oxygen fugacity buffered by the assemblage magnetite-hematite. Based on the results of these experiments, gold does form stable species with ammonia at elevated temperature. The gold is interpreted to have been dissolved predominantly as Au(NH₃)OH⁰ in solutions with a NH₃⁰ concentration of 0.005 ∼ 0.49 m and a pH(T) of 3.29 ∼ 7.47. This species formed via the reaction, Au _(s) + NH_{3 (aq)} + H₂O = Au(NH₃)OH_(aq) + 1/2 H_{2 (g)} The logarithms of the equilibrium constant for this reaction are −8.95 ± 0.84 and −7.71 ± 0.85 for 225 and 250 °C, respectively. In order to determine whether this species is important for the transport of gold in epithermal systems, we modeled the speciation of gold in a fluid at conditions typical of those epithermal gold depositing fluids. This modeling shows that the solubility of gold as AuHS⁰ and Au(HS)₂^- is much higher than that as Au(NH₃)OH⁰ except in alkaline ammonia-bearing fluids. The association of gold mineralization with hydrothermal alteration involving the formation of ammonium-bearing minerals in the deposits is therefore probably unrelated to the transport of gold as a Au-NH₃ complex but simply reflects the high concentration of NH₄⁺ in the ore-forming fluid.