Thermodynamic, petrologic, geochemical, and isotopic constraints on metal (Re, Pt, Pd, Ir, Ru, Os, and other metals) mobility during serpentinization in ultramafic-hosted hydrothermal circulation systems
Understanding metal mobility during hydrothermal alteration of ultramafic rocks bears importance to a number of outstanding problems in the earth sciences, such as quantifying hydrothermal fluxes of trace-metals into or out of the ocean, quantifying the solubilities and transport processes of metals during subduction and mantle metasomatism, and finally, understanding the genesis of base metal ore deposits. The hypothesis that is tested here is that serpentinizing hydrothermal fluids can result in significant leaching of metals from ultramafic lithologies. A three-year study, in which theoretical and observational data are combined to investigate the geochemical behavior of trace metals during the serpentinization of ultramafic lithologies (at temperatures <400 o), is proposed. Particular emphasis is placed on obtaining high quality data for the platinum group elements (PGEs) and other chalcophile elements. The mobility and transport pathways of trace metals during hydrothermal alteration of ultramafic rocks are investigated using a continuous harzburgite to serpentinite transition outcrop in northern California. Detailed field mapping, petrography, and isotopic (Os, Sr, and O) and geochemical data are combined to estimate the degree of metal mobilization, metamorphic temperatures, and water-rock ratios during serpentinization. Relative metal mobilities are determined by direct comparison of the metal contents between serpentinized harzburgites and fresh harzburgites and order-of-magnitude absolute estimates of metal solubilities are calculated using estimates of the water-rock ratio, as constrained by bulk-rock Os, Sr, or O isotopic systematics. The fO2, pH, and major- and minor-element composition and speciation in the serpentinizing hydrothermal fluids are inferred by combining thermodynamic modeling of heterogeneous aqueous equilibria with field-based constraints on equilibrium solid phases. The theoretical solubilities and speciations of trace metals, particularly of Re and the PGEs, are also calculated using appropriate association constants for various complexes, if known. This integration of observation and theory provides a better understanding of what properties of hydrothermal fluids (e.g., salinity, fO2, pH, and temperature) facilitate metal transport.
