A spectrophotometric study of aqueous copper(I)-chloride complexes in LiCl solutions between 100 °C and 250 °C

Copper transport and deposition in highly saline hydrothermal fluids are controlled by the stability of copper(I) complexes with ligands such as chloride and hydrosulphide. However, our understanding of the behavior of copper(I) chloride complexes at elevated temperatures and in highly saline brines is limited by the conditions of existing experimental studies where the maximum chloride concentration is 2 m. This paper presents the results of a study of copper(I) chloride complexes at much higher chloride concentrations, 1.5 m to 9.1 m, using a UV spectrophotometric method. The UV spectra of copper(I)-bearing LiCl solutions were measured at temperatures between 100 °C and 250 °C at vapor-saturated pressures and quantitative interpretation of the spectra shows that CuCl₂^- , CuCl₃^2-, and CuCl₄^3-, were present in the experimental solutions. The fitted logarithms of formation constants (log K ) for CuCl₂^- are in good agreement with the previous results of solubility experiments reported by Xiao et al. (1998) and Liu et al. (2001) The log K values for CuCl₃^2- also agree with those of Liu et al. (2001) and theoretical estimates of Sverjensky et al. (1997). This study presents the first experimentally determined formation constants for CuCl₄^3-, at temperatures greater than 25 °C, and indicates that this complex predominates at chloride concentrations greater than 5 m. Based on the new log K values generated from this study, the calculated chalcopyrite solubility in NaCl solutions indicates that in addition to cooling, fluid mixing (dilution of saline fluids) may be an important factor controlling the deposition of copper minerals from hydrothermal solutions. 2002 Elsevier Science Ltd.