THE ROLE OF SULFUR IN THE FORMATION OF PORPHYRY ORE DEPOSITS

Cu- and Au-rich porphyry ore deposits contain substantial mineralization in cupolas above a solidified igneous magma (granite, s.l.). The metals are transported from the magma to the cupola by a supercritical fluid, low-salinity vapor, or high-salinity brine. Reduced sulfur (S^-2) is required to form sulfide minerals, but the role of reduced or oxidized sulfur (+4 or +6) in the transport of metals to the cupola is contentious. Experiments were performed at 500-700 °C and 50-100 MPa to determine the equilibrium concentrations of Cu, Au, Fe, Mn, and Zn in vapors, brines, and supercritical fluids as a function of total chloride (Cl), oxygen (O₂) and sulfur (S₂) fugacity. Cu concentrations in brine and vapor ranged from 800 to 27000 μg/g and 40 to 1000 μg/g, respectively. Cu was strongly influenced by the total Cl of the fluid and was always greater in coexisting brine than vapor. Cu concentrations in supercritical fluids, 5 wt.% NaCl equivalent, were 120-3300 μg/g. Maximum Au concentrations were 152, 18, and 16 μg/g in brine, vapor, and supercritical fluid, respectively. Au behaved similarly to Cu as concentrations increased with increasing Cl and temperature, but was more affected by temperature changes than Cu. Variations in O₂, S₂, H₂S, and SO₂ fugacities that spanned orders of magnitude had little observable impact on Cu, Au, Fe, Mn, and Zn concentrations in the brine, vapor, or supercritical fluid. Thus, sulfur is not an effective ligand during the formation of porphyry deposits. We hypothesize that the temperature and Cl content of a hydrothermal fluid are the critical factors controlling the potential of a fluid to carry metals in the porphyry environment. Reduced sulfur (e.g., as H₂S) is required for the precipitation of sulfide minerals (e.g., chalcopyrite) by: CuCl + FeCl₂ + 2 H₂S → CuFeS₂ + 3 HCl + 0.5 H₂. Additionally, our experiments indicate that substantial Au (up to 5500 μg/g) can be incorporated into the crystal structures of bornite and chalcopyrite which may explain why Au is often found within sulfide minerals. Sulfur is likely sourced from the magma and transported with the metals by the hydrothermal fluid, with mineral formation induced by changes in temperature, oxidation, and/or acidity.