The solubility of copper in high-temperature magmatic vapors: A quest for the significance of various chloride and sulfide complexes

Abstract We conducted experiments to determine the effect of various chemical components (NaCl, KCl, HCl, FeCl 2 , H 2 S, SO 2 ) on the solubility of Cu in single phase aqueous vapors at 1000 °C and 150 MPa. The experiments were conducted in Au 97 Cu 3 alloy capsules buffering Cu activities at 0.01. The volatile phase was sampled at run conditions by the entrapment of synthetic fluid inclusions in quartz. To test if the volatile phase had reached equilibrium before the isolation of the inclusions by fracture healing, we trapped two inclusion generations, one in an initially prefractured chip and another in a quartz chip that was fractured in situ during the experiments. The synthetic fluid inclusions were subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry. In pure water, the apparent solubility of Cu is below the limits of detection of 6 μg/g, showing the low stability of hydroxy Cu complexes at our experimental conditions. The presence of alkali chlorides supports modest Cu solubility likely in the form of NaCuCl 2 and KCuCl 2 complexes. In the H 2 O–H 2 S (+SiO 2 and Au 97 Cu 3 ) system at an f H 2 S of 10.4 MPa the apparent solubility of Cu is lower by a factor of ∼5 than that in a S-free 0.5 m NaCl solution, showing that copper hydrosulfide complexes are only moderately stable at these conditions. Addition of 4.7 mol% of sulfur to the H 2 O–NaCl system at an f O 2 of 0.4 log units below the Ni–NiO buffer, yielding dominantly H 2 S species, results in only a moderate increase in apparent Cu solubility, which diminishes in the presence of HCl. The addition of KCl results in a strong increase of apparent Cu solubility in the presence of H 2 S. The solubility of Cu increases with the fugacity of oxygen in both the H 2 O–NaCl and the H 2 O–S–NaCl system following an approximately fourth root relationship as expected based on the stoichiometry of the involved redox reactions. Replacement of NaCl by FeCl 2 exerted only a minor effect on the Cu solubility. Results of our experiments, combined with thermochemical data obtained by ab initio quantum chemical calculations, suggest dissolution of Cu dominantly as Na(/K)CuCl 2 , Na(/K)Cu(HS) 2, H 2 SCuHS, and Na(/K)ClCuHS, the relative abundance of which are dictated by the H 2 S/total chloride and HCl/alkali chloride ratios.