COPPER AND GOLD PARTITIONING BETWEEN BRINE AND VAPOR IN SULFUR-RICH MAGMATIC-HYDROTHERMAL ENVIRONMENTS AT 700 °C AND 100 MPA

This study examined the partitioning of copper and gold between sulfur-rich vapors and brines at conditions that are representative of porphyry ore deposits. Experiments were conducted at 700 °C and 100 MPa, with oxygen fugacity buffered by either Ni-NiO (NNO) or MnO-Mn₃O₄ (MM) and sulfur fugacity buffered by either pyrrhotite + chalcopyrite (po-bearing product) or bornite + pyrrhotite + chalcopyrite (bn-bearing product). Gold capsules were loaded with one of the sulfide mineral assemblages, a quartz core to trap fluid inclusions, and an aqueous solution of 5 wt.% NaCl equivalent that contained NaCl + KCl + HCl with molar NaCl/KCl and NaCl/HCl ratios of 1 and 100, respectively. Low salinity vapor and high salinity brine fluid inclusions were trapped in quartz and their salinities were estimated using microthermometry, and verified against expected salinities from the NaCl-H₂O system. Na, K, Fe, Cu, and Au concentrations from 73 brine and 39 vapor synthetic aqueous fluid inclusions were determined by using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Concentrations obtained from LA-ICP-MS analyses were compared to concentrations obtained previously for the same inclusions via X-ray fluorescence (XRF) synchrotron microprobe at the Advanced Photon Source at Argonne National Laboratory. Nernst-style partition coefficients between brine and vapor for a given element (D_x^b/v) were determined as a function of O₂, S₂, H₂S, and SO₂ fugacities. D_x^b/v is defined as the concentration of an element in the brine divided by the concentration in the vapor. Over the entire range of sulfur fugacity and oxygen fugacity examined, D_Cu^b/v ranged from 1.6 to 4.2, and generally increased with increasing oxygen fugacity. Gold behaved similarly as D_Au^b/v increased from 5.0 to 7.8 with increasing O₂ fugacity.  D_x^b/v for copper and gold were broadly unchanged or decreased slightly with increasing S₂ fugacity. These results suggest that copper and gold partitioned into brine relative to vapor at all conditions studied, but D_x^b/v decreased with decreasing O₂ or increasing S₂ fugacity. Our data further support that chloride complexation dominates the transport of Au, Cu, and Fe in most porphyry ore systems.