COPPER, IRON, AND ZINC IN SULFUR-RICH MAGMATIC VOLATILE PHASES AT 500-700 °C AND 100 MPA

Fluid inclusion data from magmatic-hydrothermal ore deposits and from experimental studies note that Cu, Fe, and Zn are soluble in and can be transported by a magmatic volatile phase (MVP), however, the concentrations can vary substantially as a function of temperature, pressure, oxygen fugacity, sulfur fugacity, salinity and acidity. In an attempt to determine under what conditions the solubilities of Cu, Fe, and Zn are maximized in MVPs, we performed experiments in the MVP-intermediate solid solution-pyrrhotite-quartz±bornite system at 500-700°C, 100 MPa and at an oxygen fugacity buffered by Ni-NiO. The MVP was composed initially of NaCl+KCl+HCl+H₂O, with a starting total salinity of 5 wt.% NaCl (equivalent) and HCl set to <2000 µg/g. Experiments were performed with a supercritical fluid (500 and 600°C) and in the vapor+brine region of immiscibility (700°C). Fluid inclusions were trapped in quartz after equilibrating with Cu-Fe sulfides and a gold metal capsule. Synthetic fluid inclusions were analyzed by synchrotron x-ray microprobe at GSECARS of the Advanced Photon Source at Argonne National Laboratory. Fe, Cu, and Zn concentrations in the supercritical fluid at 500°C were found to be ~1500 µg/g, 350 µg/g and 250 µg/g, respectively. The concentrations of Fe, Cu, and Zn in the supercritical fluid at 600°C were ~10,000 µg/g, 800 µg/g, and 350 µg/g, respectively. Fe and Zn concentrations in the vapor at 700°C were ~5000 µg/g and ~350 µg/g, respectively, and did not vary with sulfur fugacity. Cu concentrations in the vapor at 700°C increased from ~300 µg/g to ~2500 µg/g with increasing sulfur fugacity, but never surpassed the concentration of Cu in the brine (~7000 µg/g). Fe, Cu, and Zn are always higher in the brine (~30000, 7000, and 2000 µg/g, respectively) than coexisting vapor or low salinity supercritical fluid illustrating these metals strong affinities for chloride. We hypothesize that Cu, Fe and Zn will complex predominantly with chloride and partition preferentially in the brine whereas Cu can also complex with reduced sulfur species, such as HS^-, and may be substantially enhanced in a vapor or low salinity supercritical fluid in addition to brine. Thus, Cu can be effectively transported in both low- and high- salinity fluids enriched with reduced sulfur species in select subsolidus, hydrothermal systems.