Magmatic Controls on Metal Endowments of Porphyry Cu-Au Deposits

Abstract

Abstract This overview illustrates the processes controlling magma fertility in the formation of porphyry Cu-Au deposits. Magma fertility means all magmatic parameters (e.g., metal and volatile contents, magma and fluid volumes) that might result in higher amounts of metals, which are exsolvable from the magma. Mantle source processes seem to play a fundamental role in the enrichment of primary melts with H2O, S, and Cl, all essential ingredients to form porphyry deposits, but do not have a particular role in Cu enrichment. Cu-rich porphyry Cu-Au deposits (i.e., with Au/Cu ~4 X 10–6) are associated with large magmatic volumes accumulated in the lower thick crust of continental arcs during long-lived periods of compression in a synsubduction environment. Mineralization occurs after such accumulations have reached significant volumes and is the result of the transfer of hydrous magmas from deep to shallower crustal levels, probably favored by tectonic stress changes. Au-rich porphyry Cu-Au deposits (i.e., with Au/Cu ~80 X 10–6) are associated with magmatic systems that have evolved at overall shallower crustal levels and for this reason can be found in geodynamic settings characterized by thinner crust (e.g., island arcs with intermediate crust thickness) and/or in variably extensional settings occurring above a slab-metasomatized mantle (postsubduction setting, extensional synsubduction setting). The six largest Au-rich porphyry Cu-Au deposits (>~1,300 tonnes Au) are associated with variably alkaline magmas, which are typical of postsubduction and/or extensional settings, suggesting a petrogenetic control on the Au-rich nature of these deposits.