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

Abstract

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 °C, 500 and 1000 bar. The concentration of H2S varied from 0.08 to 0.4 m [mol⋅(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°CuHS = −3.99 ± 0.07 (350 °C/500 bar)−4.61 ± 0.09 (450 °C/500 bar), −4.26 ± 0.11 (450 °C/1000 bar);0.5 Cu2S(cr) + 0.5 H2S(aq) + HS− = Cu(HS)2− log K°Cu(HS)2- = −0.85 ± 0.08 (350 °C/500 bar)−0.65 ± 0.11 (450 °C/1000 bar);0.5 Cu2S(cr) + HCl(aq) = CuCl(aq) + 0.5 H2S(aq) log K°CuCl = −3.11 ± 0.70 (450 °C/1000 bar);0.5 Cu2S(cr) + HCl(aq) + Cl− = CuCl2− + 0.5 H2S(aq) log K°CuCl2- = 0.33 ± 0.19 (450 °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-Hückel 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 Cu-HS-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 °C and from Psat. to 2000 bar, and up to 1000 °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 (ρH2O < 0.35 g⋅cm−3) up to 700 °C. The obtained thermodynamic data imply that CuCl2− predominates in high-density ore-forming fluids (ρH2O > 0.35 g⋅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 near-neutral). However, their concentration in the presence of Cu-bearing sulfides is low (< 0.5 ppm at 500 °C/1500 bar) and decreases with temperature decreasing.