Magmatic and structural controls on the tonnage and metal associations of collision-related porphyry copper deposits in southern Tibet

Porphyry Cu deposits (PCDs) in continental collision settings are new targets for modern mineral exploration, and the relationships among locations, tectonic environments, tonnages, and metal associations are not yet understood. The Gangdese PCD belt formed in a typical continental collision setting between the Indian and Asian plates. In the present study, new elemental and isotopic data from fertile porphyries (Cu–Mo, Cu–Mo–Pb–Zn, and Cu–W–Mo) and barren coeval and pre-ore porphyries of the Gangdese PCD belt were combined with previously published data to provide new insights into the metallogenesis of PCDs in continental collision settings. Factors that contributed to the formation of collision-related PCDs in the Lhasa terrane, Tibet, include the nature of the magmatic sources, magmatic evolution during ascent through the crust, and regional tectonics. These PCDs were not formed by partial melting of ancient lower crust. Instead, attributed to the subduction of Neo-Tethyan oceanic slab, arc magmas and slab-derived fluids resulted in the accumulation of sulfides and hydrous minerals at the base of the continental crust. Partial melting of this refertilized juvenile lower crust produced oxidized water- and Cu-rich magmas with the potential to produce large PCDs, although only under favorable regional tectonic conditions. Regional compressional stress is not conducive to the formation of large magma chambers, so the collision-related adakite-like magmas that developed during the early stages of India–Asia collision commonly lack alteration and associated mineralization. In contrast, relatively large magma chambers can develop in extensional settings within the continental collision orogen, which facilitates the formation of large and giant PCDs. Small Cu–W–Mo deposits formed during the transition between compressional and extensional settings. Thus, the regional tectonic setting and structural framework of the host orogen influenced the tonnage of PCDs. The metal associations of collision-related PCDs in the Lhasa terrane were controlled mainly by the extent of assimilation of the upper crust and the degree of fractional crystallization of the ore-forming magmas. Lead–zinc mineralization in Cu–Mo deposits is attributed to the assimilation of metapelites by fertile magmas in the upper crust, and highly fractionated fertile magmas are required to produce W mineralization.