Copper complexation and solubility in high-temperature hydrothermal fluids: A combined study by Raman, X-ray fluorescence, and X-ray absorption spectroscopies and ab initio molecular dynamics simulations

Abstract

Data for the solubility of CuS (reacting to Cu2S), Cu, and bornite + chalcopyrite + pyrite (reacting to Cu-Fe-S solid solution) in H2O + NaCl fluids were determined in situ using synchrotron-radiation X-ray fluorescence (SR-XRF) spectroscopy. The aqueous Cu concentrations ranged between 25 ppm at 500 °C, 320 MPa, 0.5 m NaCl and 760 ppm at 500 °C, 310 MPa, 4.78 m NaCl, increased with temperature along an isochore and with NaCl molality, and decreased with pressure. The X-ray absorption near edge structure (XANES) spectra of the fluid from dissolution of CuS or Cu in H2O + NaCl at 500 °C are nearly identical with published spectra of CuCl2−(aq), but differ significantly from reported spectra of Cu(I) in Cl-free hydrosulfide solutions.Raman spectra of H2O + HCl ± NaCl fluids reacted with CuS or, for comparison, metallic Cu were measured at temperatures to 600 °C and pressures to 2 GPa to test if this technique can provide additional information on the complexation of Cu(I) and on the solubility of copper in hydrothermal fluids. These spectra showed that CuCl2−(aq) was the most abundant Cu(I) species at all conditions. In addition, CuCl32−(aq) was observed at high HCl concentrations, but the Raman spectra provided no convincing evidence for Cu(I) complexation with H2S or S3− (HS−(aq) was below detection in these acidic fluids). Generally, there was an increase in the sum of the integrated intensities of the bands assigned to Cu complexes with increasing HCl concentration, and a decrease if sulfide was present. At all fluid compositions, the intensity of the Raman bands from CuCl stretching vibrations decreased with increasing pressure at constant temperature for single-phase fluids, without formation of additional bands. Based on ab initio modeling, the complexes CuCl2−(aq), Cu(HS)2−(aq), and Cu(HS)Cl−(aq) are not distinguishable by Raman spectroscopy, but the stretching vibration of Cu(I) complexes with H2S should occur at significantly lower wavenumbers.Overall, the results indicate that Cu(I) is transported predominantly as CuCl2−(aq) in reducing sulfur-free or H2S ± S3−-bearing chloridic hydrothermal fluids. The decrease in the Cu solubility in H2O + HCl ± NaCl ± H2S fluids with increasing pressure without a detectable change in speciation is caused by decrease in the formation constant of CuCl2−(aq) with pressure. Changes in the copper speciation and depressurization can be ruled out as causes for hydrothermal copper ore formation at high fluid densities above the critical density. At this condition, copper ore may precipitate by dilution, cooling in the presence of H2S, increase in pH, and/or an increase in the H2S activity.