Cu solubility and complexation in high-temperature aqueous fluids

Understanding of the genesis of hydrothermal copper ore deposits requires robust knowledge of the transport properties of Cu in aqueous fluids over a wide range of pressure and temperature. The aim of this study is to combine experimental and computational techniques to investigate the Cu solubility and complexation in sulfur-bearing fluids at high temperatures and pressures. Synthetic CuS crystals were equilibrated in aqueous NaCl ± HCl solutions at 500 and 600 °C and pressures up to 650 MPa using modified hydrothermal diamond-anvil cells [2]. Dissolved Cu concentrations, determined from confocal synchrotron-radiation X-ray fluorescence (XRF) spectrometry at equilibrium conditions, ranged between 25 ppm at 500 °C, 320 MPa and 338 ppm at 600 °C, 445 MPa. Cu solubility increased with temperature along an isochore and decreased with pressure at constant temperature. Chlorine-richer fluids yielded higher dissolved Cu contents. Cu complexation was studied by in-situ X-ray near-edge absorption structure (XANES) and Raman spectroscopy at comparable conditions to the XRF experiments. The XANES spectra are in good agreement with CuCl2– complexes in solution [3], whereas the Raman spectra clearly show a different shape than expected for purely Cu–Cl bands [4]. We tentatively interpret those as mixed Cu–Cl–HS/H2S complexes, which will be verified by ab-initio molecular dynamics simulations with subsequent quasi-normal mode analysis