15 Tosdal Dilles Cooke Elements

From Source to Sinks in Auriferous Magmatic-Hydrothermal Porphyry and Epithermal Deposits Richard M. Tosdal1, John H. Dilles2 and David R. Cooke3 1811-5209/09/0005-0289$2.50   DOI: 10.2113/gselements.5.5.289

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uriferous porphyry Cu deposits are restricted to convergent plate settings, whereas epithermal precious metal deposits form at extending convergent plate settings and in rifts. Both deposit types are linked to magma carrying metals and ligands that rises to form an upper-crustal magma chamber. Magma convection and fractionation lead to volatile exsolution and collection in the apical parts of the chamber, from which exsolved hydrothermal fluid ascends to form either a porphyry Cu–Au deposit associated with stocks and dykes generally at 2–5 km depth, or an epithermal deposit associated with coeval volcanic rocks at depths of 1.4 kb or 5 km depth) of relatively low density, with ~5 wt% dissolved solids, dominated by salts. The ascending and depressurizing parent fluid intersects the broad field of fluid immiscibility between 750°C and the critical point of a dilute salty fluid water at 374°C, leading to the separation of a small quantity of high-density brine from a larger volume of low-density vapour (Williams-Jones and Heinrich 2005). Gold, being dominantly transported as a chloride complex at high temperature (Williams-Jones et al. 2009 this issue), is precipitated along with Cu–Fe sulfides in the porphyry environment at around 400°C as a result of decreasing solubility, during cooling, of metal chloride complexes in the presence of H2S (Hemley and Hunt 1992). Precipitation of Au and other metals is enhanced by fluid–rock reaction, changes in pH and fluid speciation, brine–vapour unmixing (so-called boiling) and, in a few cases, potentially by local fluid mixing (Beane and Titley 1981; Seedorff et al. 2005). With cooling to ~400°C, most SO2 in the hydrothermal fluid disproportionates via reaction with water in the rising fluid to form H2SO4 and H2S, which promotes sulfide precipE lements

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Gold-bearing porphyry Cu deposits Porphyry Cu deposits with average Au contents of >0.4 gram (g)/metric ton (t) are defined as gold rich (Sillitoe 2000). In general, Au contents are