Microbean techniques of stable isotope analysis offer great insight into petrogenetic processes because minerals contain more information concerning their origins and subsequent modifications than can easily be obtained by conventional isotopic sampling and analytical techniques. S isotope studies with SHRIMP ion probe have revealed dramatic 34S variations within and among minerals in the Salton Sea geothermal system, identifying sulfides. The isotopic equilibration of sedimentary sulfide and sulfate minerals during prograde metamorphism, and the extent of recycling of sedimentary S by sulfur poor hydrothermal fluids will be examined. From determinations of 34S for aqueous S species the degree of isotopic equilibrium attained between H2S and SO4 in saline hydrothermal fluids and the relative contributions of H2S to hydrothermal fluids from igneous processes and hydrothermal reduction of host rocks evaporites will be evaluated. It is proposed to extend our studies of S isotope systematics to active silicic volcanic-hosted continental geothermal systems, making use of new drill cores and fluid samples from the Valles Caldera and Coso geothermal systems. The proposed study complements our research that are transporting precious as well as base metals. We will continue to use the unsurpassed 30 um spatial resolution of the SHRIMP ion microprobe for studying microscopic 34S variations within and among minerals. Conventional techniques to determine 34S of H2S and SO4 in coeval geothermal fluids and to characterize the bulk S isotope composition of minerals will be used. Study of microscopic 34S zoning patterns in vein minerals will help constrain mechanisms of ore precipitation. Elucidation of the distribution of S isotope geochemistry of sulfur sources in host- rocks and the degrees of isotopic equilibrium reached between minerals and aqueous species at hydrothermal temperatures will allow more accurate modeling of metal transport and deposition in volcanic-hosted hydrothermal systems.
