Collision‐related porphyry Cu deposits formed by input of ultrapotassic melts into the sulfide‐rich lower crust
Yuanchuan Zheng, Yang Shen, Lu Wang, William L. Griffin, Zengqian Hou
Abstract
Abstract 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 K 2 O contents and K 2 O/Na 2 O ratios than barren porphyries emplaced in the same arc. The elevated K 2 O/Na 2 O 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 K 2 O contents and K 2 O/Na 2 O 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.