Abstract:

Most porphyry Cu deposits in the world occur in magmatic arc settings and are formed in association with calc-alkaline arc magmas related to subduction of oceanic-slab. This paper reviews some significant porphyry Cu deposits in China, including (1) the Mid-Miocene Gangdese porphyry Cu belt in Tibet, (2) the Himalayan Yulong porphyry Cu belt in east Tibet, (3) the Mid-Jurassic Dexing porphyry Cu ore-field and (4) porphyry Cu deposits in the Middle-Lower Yangtze metallogenic belt in east China(MLYMB）. These porphyry Cu deposits show broad similarity with those in the worldwide arc settings in many aspects, such as mineralization style, alteration zonation, sulfide assemblage, and ore-forming fluid system. However, they occur in a variety of continental settings from intra-continental environments during the late and post-collisional periods followed continent-continent collision to intra-plate environments related to post-orogenic and anorogenic processes. They are usually associated with high-K calc-alkaline and shoshonotic magmas that are not related to oceanic-slab subduction processes. Main host porphyry phases are granodioritic (in east China) and monzogranitic (in Tibet), and show geochemical affinity with adakites. However, these host rocks are characterized by high K₂O contents (1.2%~8.5%) and low ε_Nd(t)(-10~+3), distinguishing them from adakites derived from partial melting of subducted oceanic-slab. These adakitic magmas were most likely derived from the thickened mafic lower crust source, involving various accounts of juvenile mantle components. The upwelling of asthenospheric mantle to heat the juvenile lower crust or the delamination of a thickened high-density mafic lower-crust are regarded to be major deep lithospheric processes for generating adakitc magmas in continental settings. The key factors led to adakitic magmas fertile are most likely crust/mantle interaction processes on the base of thickened lower crust in continental settings rather than oceanic-slab dehydration process for arc magmas. It seems that the fertility of continental magmas mainly depends on contribution of mantle metals to a juenvile lower-crustal source and to relevant hydrous highly-oxidized adakitic magmas generated during breakdown of amphibole in a mafic source. Extensional, transtensional, and transpressional tectonic regimes in continental settings are thought to not permit adakitic magmas to undergo the MASH processing like arc magmas, but favor the sufficiently voluminous evolved magmas ascent upwards to upper crust levels. The strike-slip faulting systems and associated pull-apart basins, orogen-transverse normal faults and their intersection with other faults, and pre-existing lineaments and structural weakness in continental settings provide optimal conditions for focused flow and emplacement, thus controlling the spatial-temporal localization of the Cu-bearing felsic stocks.

Key words: porphyry Cu deposits, ore-bearing porphyoic rock, post-collission, later collission, magmatic origin, geodynamic setting, continental environment, China