Reduced porphyry copper-gold deposits: A new variation on an old theme

Abstract

It is a widely accepted fact that porphyry Cu-Au deposits form from highly oxidized ore fluids. Despite this assertion, several otherwise typical porphyry Cu-Au deposits exhibit clear evidence of having formed from relatively reduced ore fluids. These “reduced” porphyry Cu-Au deposits lack primary hematite, magnetite, and sulfate minerals (i.e., anhydrite), but contain abundant hypogene pyrrhotite, commonly have carbonic-rich ore fluids with substantial CH₄, and are associated with ilmenite-bearing, reduced I-type granitoids. Based on a synthesis of theoretical, experimental, and field data, a variation on the classic porphyry Cu model is proposed to explain the formation of reduced porphyry Cu-Au deposits and their relatively Cu-poor but Au-rich nature. The proposed reduced porphyry Cu-Au model does not contradict the current understanding of porphyry Cu-Au formation. Rather, it adds yet another variation on the theme of porphyry Cu-Au genesis.