Effect of S on the aqueous and gaseous transport of Cu in porphyry and epithermal systems: Constraints from in situ XAS measurements up to 600 °C and 300 bars

Abstract

The effect of S on the solubility of Cu in high-temperature fluids at conditions relative to porphyry and epithermal ore deposit formation has been investigated conducting in situ X-ray absorption (XAS) and Raman spectroscopy from 325 to 600 °C at 300 bars. The experimental results identify the Cl or S-complexes responsible for Cu transport in high-temperature fluids and provide information on transport in porphyry/epithermal systems and the effect of fluid composition on the transfer of Cu from the porphyry to the epithermal environment. In S-free HCl solutions, Cu solubility is about 10 times higher in high-density (ρ > 0.6 g·cm− 3) versus low-density (ρ < 0.3 g·cm− 3) fluids. During the transition from high- to low-density, Cu speciation evolves from CuCl2− complexes to CuCl(H2O) or CuCl(HCl), depending on HCl concentrations. In sulfur-only solutions Cu solubility is extremely low. The addition of S to Cl-bearing fluids results in a drop of Cu solubility in high- and low-density fluids. Solubilities drop from weight percent levels (in Cl-only solutions) to hundreds of ppm in high-density fluids (containing sulfur). In low-density fluids, Cu concentrations similarly drop, from hundreds of ppm to below detection. The low Cu concentrations in the presence of sulfur preclude the characterization of Cu complexes, except in high-density Cl + S fluids, where CuCl2− is found to remain the dominant specie even for Cl concentrations < 1 wt%. Overall, these results suggest that the presence of S of mixed oxidation state (sulfide S2 −, sulfite S4 + and sulfate S6 +) limits transport of Cu in porphyry and epithermal systems and may even trigger Cu precipitation. Furthermore, these experiments provide evidence that the presence of the S3− ion does not increase Cu solubility in fluids. Thus, if present in such environments, S3− ion could fractionate Cu from Au in porphyry-epithermal environments.