Coupled Solubility of Cu and Ag in Chloride-Bearing Hydrothermal Fluids (350–650°С, 1000–1500 bar)

The coupled solubility of Cu(cr) and Ag(cr) was measured in acidic chloride solutions at 350°C/1000 bar, 450°C/1000 bar, and 653°C/1450 bar in a wide range of chloride concentrations (0.02m HCl + (0–15m) NaCl). The experiments were performed using the autoclave (350, 450°C) and ampoule (653°C) techniques, the dissolved metal concentrations were measured after quenching the experimental system. The equilibrium constants of the reaction combining the dominant forms of transport of the metals are determined: $${\text{C}}{{{\text{u}}}_{{\left( {{\text{cr}}} \right)}}} + {\text{AgCl}}_{2}^{ - } = {\text{A}}{{{\text{g}}}_{{\left( {{\text{cr}}} \right)}}} + {\text{CuCl}}_{2}^{ - }\,\,\,\,\,\,\,K_{{{\text{(Cu-Ag)}}}}^{^\circ }\,{\text{.}}$$ The calculated reaction constant is independent of chloride concentration. It is known that $${\text{AgCl}}_{2}^{ - }$$ dominates in the studied chloride concentration range. Therefore, the main copper form of occurrence is $${\text{CuCl}}_{2}^{ - }$$ in all experimental fluids with chloride content up to 47 wt %, which is close to the NaCl saturation limit. The constant of the coupled Cu and Ag solubility was determined for the experimental PT parameters as log $$K_{{{\text{(Cu-Ag)}}}}^{^\circ }$$ = 2.65 ± 0.20 (350°С/1000 bar), 2.28 ± 0.10 (450°С/1000 bar), and 1.49 ± 0.34 (653°С/1450 bar). These data, together with values from the literature for temperatures from 200 to 900°C and pressure up to 2000 bar, were fitted to the density model equation: log $$K_{{{\text{(Cu-Ag)}}}}^{^\circ }$$ = 1.066 + 1.108 × 103T(K)–1 + 3.585 log d(w) – 1.443log d(w) × 103 T(K)–1, where d(w) is the pure water density. According to these data, copper is much more soluble in chloride solutions compared to silver, but the difference in the solubility decreases with the increase in temperature. Reliable literature data on the silver solubility constant allow that of copper solubility to be calculated: $${\text{C}}{{{\text{u}}}_{{\left( {{\text{cr}}} \right)}}} + {\text{HC}}{{{\text{l}}}_{{({\text{aq}})}}}{\text{ + C}}{{{\text{l}}}^{ - }} = {\text{CuCl}}_{2}^{ - }{\text{ + 0}}{\text{.5}}{{{\text{H}}}_{{{\text{2}}\left( {{\text{aq}}} \right)}}}\,\,\,\,\,\,\,K_{{{\text{(Cu)}}}}^{^\circ },$$ log $$K_{{{\text{(Cu)}}}}^{^\circ }$$ = 1.39 ± 0.20 (350°С, 1000 bar), 1.91 ± 0.10 (450°С, 1000 bar), and 2.06 ± 0.34 (653°С, 1450 bar). The new values of $$K_{{{\text{(Cu)}}}}^{^\circ }$$ are combined with reliable literature data to calculate the density-model parameters. The obtained density-model equation can be used to calculate the copper solubility constant up to 800°C and pressures up to 2000 bar: log $$K_{{{\text{(Cu)}}}}^{^\circ }$$ = 6.889 – 3.298 × 103T(K)–1 + 8.694log d(w) – 4.807log d(w) × 103T(K)–1. The solubility of chalcopyrite in the system with pyrite–hematite–magnetite and K-feldspar–muscovite–quartz mineral buffer assemblages is discussed.