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

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.