Early sulfide saturation is not detrimental to porphyry Cu-Au formation

Ore-forming magmas are commonly considered to have been unusually metal rich. Because Cu and Au are strongly chalcophile, early sulfide saturation has been regarded as detrimental to porphyry Cu-Au mineralization. Here we demonstrate, based on amphibole-rich cumulate xenoliths and amphibole megacrysts from the Tongling porphyry(-skarn) Cu-Au mining district in southeastern China, that this view is not necessarily correct. Age data combined with petrological and geochemical evidence suggest that the mineralizing magmas at Tongling underwent significant fractional crystallization of amphibole, clinopyroxene, and magmatic sulfides in the middle to lower crust. The fact that the silicate melts nevertheless were able to produce substantial porphyry(-skarn) Cu-Au deposits implies that the formation of metal-rich cumulates at depth was not detrimental to their fertility. On the contrary, the common association of porphyry Cu (Au, Mo) deposits with high-Sr/Y magmas suggests that amphibole fractionation at depth even promotes the mineralization potential, despite the likely loss of metals.