An experimental study of copper(I)-chloride and copper(I)-acetate complexing in hydrothermal solutions between 50°C and 250° and vapor-saturated pressure

The transport of copper is affected by complexing with ligands such as chloride, bisulphide and acetate. Many copper deposits are formed from hydrothermal waters and brines and in order to understand their formation we need thermodynamic properties of aqueous copper species. The solubility of cuprite experiments in pH-buffered acetate and chloride solutions was determined at 50°C, 150°C and 250°C and water vapor saturated pressures. Chloride and acetate concentrations were varied using NaCl (0.001 m to 2 m) and NaAc (0.1 m to 2 m, where Ac refers to acetate). Measured copper concentrations varied between 0.0001 m and 0.2 m and show systematic increases with increasing temperature and acetate or chloride concentration. Logarithms of the formation constants (log K) of copper complexes were fitted by a simplex function minimization method, then speciation models and uncertainties of fitted log K values were examined by residual maps and speciation plots of experimental solutions. Interpretation of our experimental data shows that CuCl_(aq), CuCl₂^-, CuAc_(aq) and CuAc₂^- were present in our experiments at all temperatures. CuCl₃^2- is an important complex at 50°C, but cannot be detected unequivocally at 150°C and 250°C. This study presents the first experimentally-derived log K values for copper(I) acetate complexes (CuAc_(aq) and CuAc₂^-). The derived log K values of copper(I) chloride complexes (CuCl_(aq) and CuCl^-₂) are similar to recently published experimentally derived values (Xiao et al., 1998). Based on new log K values generated from this study, our calculations indicate that copper transport as acetate complexes is important in acetate-bearing fluids lacking competition of other ligands such as Cl^- and HS^-.