Petrogenesis Of Adakites And Its Potential For Porphyry Copper Mineralization In Gangdese Belt,Tibet

The Gangdese porphyry Cu belt in the southern Tibet is an important part of the Tethyan metallogenic domain.The porphyry Cu deposits in this belt were all developed in Miocene and Jurassic,while fertile intrusions have geochemical affinities with adakite.However,the adakitic magmas were also formed in other periods but are typically lack alteration and associated mineralization.Since the generation of porphyry Cu deposits is closely associated with the nature of fertile intrusions,the comparative study of fertile porphyries and barren porphyries is the key to find out the critical factors of porphyry mineralization.In this paper,the adakites in Gangdese belt are the main research subject.Based on the study of the petrogenesis of adakites,we further explore the coupling relationship among the nature of magmatic source regions,magmatic evolution process,regional tectonics and the porphyry mineralization.Through the study of the collision-related fertile porphyries and barren porphyries in the Gangdese belt,it is found that the collision-related porphyry Cu deposits are formed by a combination of nature of magmatic source regions,magmatic evolution process and regional tectonics.Partial melting of the ancient lower crust is not benefit to the development of porphyry Cu deposits,whereas only the magmas derived from a juvenile lower crust enriched in accumulated sulfides and hydrous minerals replenished by melts and fluids during subduction of oceanic slabs,that can produce water-and Curich oxidized adakitic magmas,have the potential to develop porphyry Cu deposits.Thus,collision-related porphyry Cu deposits all occur in the middle-east segment of the Gangdese belt containing large amounts of juvenile materials,whereas the barren intrusions located in the western and easternmost parts composed of ancient crust.Porphyry copper deposits are commonly developed in thickened crust.In the Gangdese belt,although there are differences in thickening mechanism between the east and west segment,the overall crust had been significantly thickened in Eocene.However,regional compressive stress is not benefit to large parent magma chamber formation,which results in the lack of fluids and melts replenishment in shallow porphyries.Thus,the juvenile lower crust-derived adakitic magmas developed in the early stage of Indian-Asian collision are commonly lack alteration and associated mineralization.In contrast,relatively large magma chamber can well develop under extension setting in Miocene,which in turn favorable to the formation of large and giant porphyry Cu deposits.Small size porphyry deposits can develop during the transitional period of compression and extension setting.Ore-forming metal series of the collision-related porphyry Cu deposits are mainly influenced by the magmatic processes.Pb–Zn mineralization in the Cu–Mo deposits could mainly be associated with the assimilation of the fertile magmas by metapelites in the upper crust,while highly fractional crystallization of the associated fertile magmas is benefit for enrichment of W.In the Gangdese belt,the Late Cretaceous adakites were derived from partial melting of oceanic slab triggered by subduction of the Neo-Tethyan mid-ocean ridge.Although they have high water content and a significant contribution of mantle components and also experienced high degree of fractional crystallization,the obviously lower oxygen fugacity implies that the Late Cretaceous adakites have weak potential for porphyry Cu mineralization.