Complexation of Cu+ in Hydrothermal NaCl Brines:: Ab initio molecular dynamics and energetics

Chloride complexation of Cu+ controls the solubility of copper(l) oxide and sulfide ore minerals in hydrothermal and diagenetic fluids. Solubility measurements and optical spectra of high temperature CuCl solutions have been interpreted as indicating the formation of CuCl CuCl2-, CuCl43- and CuCl43- complexes. However, no other monovalent cation forms tri- and tetrachloro complexes. EXAFS spectra of high temperature Cu-Cl solutions, moreover, appear to show only CuCl and CuCl2- complexes at T > 100 degrees C. To reconcile these results, I investigated the nature and stability of Cu-Cl complexes using ab initio cluster calculations and ab initio (Car-Parrinello) molecular dynamics simulations for CuCl-NaCl-H2O systems at 25 to 450 degrees C. Ab initio molecular dynamic simulations of I m CuCl in a 4 m Cl solution give a stable CuCl3- complex at 25 degrees C over 4 ps but show that the third Cl- is weakly bound. When the temperature is increased along the liquid-vapour saturation curve to 125 degrees C, the CuCl32- complex dissociates into CuCl2- and Cl-; only CuCl2- forms at 325 degrees C and 1 kbar. Even in a 15.6 m Cl brine at 450 degrees C, only the CuCl2- complex forms over a 4 ps simulation run. Cluster calculations with a static dielectric continuum solvation field (COSMO) were used in an attempt directly estimate free energies of complex formation in aqueous solution. Consistent with the MD simulations, the CuCl32- complex is slightly stable at 25 degrees C but decreases in stability with decreasing dielectric constant (epsilon). The CuCl42- complex is predicted to be unstable at 25 degrees C and becomes increasingly unstable with decreasing dielectric constant. In hydrothermal fluids (epsilon < 30) both the CuCl32- and CuCl43- complexes are unstable to dissociation into CuCl2- and Cl-. The results obtained here are at odds with recent equations of state that predict CuCl32- and CuCl43- complexes are the predominant species in hydrothermal brines. In contrast, I predict that only CuCl2- complexes will be significant at T > 125 degrees C, even in NaCl-saturated brines. The high-temperature (T > 125 degrees C) optical spectra of CuCl solutions and solubility measurements of Cu minerals in Cl-brines need to be reinterpreted in terms of only the CuCl and CuCl2- complexes. (c) 2006 Elsevier Inc. All rights reserved.