Chalcocite Cu2S solubility in aqueous sulfide and chloride fluids. Thermodynamic properties of copper(I) aqueous species and copper transport in hydrothermal systems

The most important Cu reserves, including ores of porphyry deposits, were formed by Cl- and S-bearing hydrothermal fluids transporting and depositing metals within the magmatic-hydrothermal ore systems. In order to determine the state of Cu in the fluids, we measured the solubility of chalcocite Cu2S at 350 and 450 degrees C, 500 and 1000 bar. The concentration of H2S varied from 0.08 to 0.4 m [mol center dot(kg H2O)(-1)], NaOH - from 0 to 0.2 m, NaCl + HCl - from 0.01 to 4 m. The solubility of chalcocite can be described by the reactions: 0.5 Cu2S(cr) + 0.5 H2S(aq) = CuHS(aq) log K degrees(CuHS) = -3.99 +/- 0.07 (350 degrees C/500 bar) -4.61 +/- 0.09 (450 degrees C/500 bar), -4.26 +/- 0.11 (450 degrees C/1000 bar); 0.5 Cu2S(cr) + 0.5 H2S(aq) + HS- = Cu(HS)(2)(-) log K degrees Cu(HS)(2)- = -0.85 +/- 0.08 (350 degrees C/500 bar) -0.65 +/- 0.11 (450 degrees C/1000 bar); 0.5 Cu2S(cr) + HCl(aq) = CuCl(aq) + 0.5 H2S(aq) log K degrees CuCl = -3.11 +/- 0.70 (450 degrees C/1000 bar); 0.5 Cu2S(cr) + HCl(aq) + Cl- = CuCl2- + 0.5 H2S(aq) log K degrees CuCl2- = 0.33 +/- 0.19 (450 degrees C/1000 bar) These results, together with data of Rubtsova et al. (2023) on the coupled solubility of Cu-(cr) and Ag in chloride fluids, demonstrated that a simple extended Debye-Huckel equation describe activity coefficients of charged species at any NaCl concentration up to the solubility limit at near- and supercritical PT-parameters; activity coefficients of neutral species can be calculated with Setchenov parameter equal to zero. The Cu2S(cr) solubility in sulfide-chloride-bearing fluids is consistent with the formation of Cu-Cl complexes which implies that mixed CuHS-Cl complexes can be neglected. The experimental values pooled with the reliable literature data were fitted to the HKF (Helgeson-Kirkham-Flowers) model equation of state (EoS). The obtained HKF EoS parameters allow to predict thermodynamic properties of CuHS(aq), Cu(HS)(2)(-), and CuCl(aq) from 25 to 700 degrees C and from P-sat. to 2000 bar, and up to 1000 degrees C/5000 bar for CuCl2-. The AD (Akinfiev and Diamond) EoS parameters were derived for CuCl(aq). The AD EoS can be used to calculate the CuCl( aq) Gibbs free energy in the low-density fluids (rho(H2O) < 0.35 g center dot cm(-3)) up to 700 degrees C. The obtained thermodynamic data imply that CuCl2- predominates in high-density ore-forming fluids (rho(H2O) > 0.35 g center dot cm(-3)) of total chloride content >0.1 wt% up to the NaCl saturation limit. The hydrosulfide complexes are the main forms of Cu occurrence in the fluid with concentration of dissolved chlorides <0.1 wt% equilibrated with silicate mineral assemblages (e.g., K-feldspar-muscovite-quartz, pH is nearneutral). However, their concentration in the presence of Cu-bearing sulfides is low (< 0.5 ppm at 500 degrees C/1500 bar) and decreases with temperature decreasing.