Stability of AuCl2− from 25 to 1000 °C at Pressures to 5000 bar and Consequences for Hydrothermal Gold Mobilization

Gold is transported in high-temperature chloride-bearing hydrothermal fluids in the form of AuCl2−. The stability of this complex has been extensively studied, but there is still considerable disagreement between available experimental data on the temperature region 300–500 °C. To solve this problem, we measured the solubility of gold in HCl/NaCl fluids (NaCl concentration varied from 0.1 to 3 mol·(kg·H2O)−1) at 450 °C and pressures from 500 to 1500 bar (1 bar = 105 Pa). The experiments were performed using a batch autoclave method at contrasting redox conditions: in reduced experiments hydrogen was added to the autoclave, and in oxidized experiments the redox state was controlled by the aqueous SO2/SO3 buffer. Hydrogen pressure in the autoclaves was measured after the experiments in the reduced system. The gold solubility constant, Au(cr) + HCl°(aq) + Cl− = AuCl2− + 0.5 H2°(aq), was determined for the experimental T-P parameters as log Ks° = −4.77 ± 0.07 (500 bar), −5.11 ± 0.08 (1000 bar), and −5.43 ± 0.09 (1500 bar). These data, together with values from the literature for temperatures from 25 to 1000 °C, were fitted to the simple equation log Ks° = 4.302 − 7304∙T(K)−1 − 4.77∙log d(w) + 11080∙(log d(w))∙T(K)−1 − 6.94 × 106∙(log d(w)) T(K)−2, where d(w) is the pure water density. This equation can be used together with the extended Debye–Huckel equation for activity coefficients to calculate gold solubility at pressures up to 5000 bar at fluid chlorinities at least up to 30 wt %. The speciation of gold in natural chloride-bearing fluids is discussed.