Collision-related porphyry Cu deposits formed by input of ultrapotassic melts into the sulfide-rich lower crust

Collision-related porphyry Cu deposits (PCDs) are restricted to previous magmatic arcs, in which sulfide-rich lower crust occurred. Fertile adakite-like porphyries associated with PCDs have higher K₂O contents and K₂O/Na₂O ratios than barren porphyries emplaced in the same arc. The elevated K₂O/Na₂O ratios of fertile porphyries reflect substantial inputs of coeval hydrous, oxidized ultrapotassic melts. Input of such melts could increase the water content and oxygen fugacity of the lower-crust-derived melts, which in turn would promote the breakdown of sulphides in the lower crust and increase the contents of Cu and S in the melts, making them favourable for the formation of large PCDs. The relatively low K₂O contents and K₂O/Na₂O ratios of barren porphyries indicate limited input of ultrapotassic melts; these magmas have low potential to form PCDs. Thus, the input of ultrapotassic melts into the sulfide-rich juvenile lower crust drives the formation of collision-related PCDs.