Gold and Copper Speciation and Partitioning in S- and Cl- bearing Melt-Vapor-Brine Systems

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AbstractEAR-0125805Candela and PiccoliUniversity of MarylandExperiments will be performed to determine the speciation and partitioning of gold and copper in felsic melts + vapor or brine (starting composition of KCl+NaCl+HCl+H2O) assemblages, some with added sulfur, at 800 degrees C covering a range of pressures near 100 MPa. Experiments performed in sulfide-saturated systems (e.g., pyrrhotite-intermediate solid solution assemblage) will be performed to determine copper and gold partitioning in vapor/melt, brine/melt and vapor+brine/melt assemblages. This effort complements experiments on crystal-melt partitioning of Au and Cu among magnetite and biotite, and rhyolite melt. Taken together with existing experimental data, the proposed research will yield a preliminary model for the behavior of Cu and Au in melt-crystal-vapor-brine systems that can be used to test hypotheses for the origin of porphyry and related Au-Cu deposits. The melt-volatile phase experiments proposed here are in high demand given that the relative roles of brine versus vapor and sulfur versus chloride-bearing ligands are not clear in light of the pioneering analytical work on natural fluid inclusions by both Bodnar (1995) and Audetat et al. (1998). These workers have suggested that both sulfur-bearing species and the vapor phase may be important in transporting these Cu and Au in ore-forming environments. Our study is designed to provide basic thermodynamic data to determine the controls on copper and gold partitioning in magmatic-hydrothermal systems and to clarify transport mechanisms for these economically important metals.