The Porphyry Molybdenum-Copper Deposit At Bangpu,Tibet:Fluid Evolution And Mineralization

The porphyry Mo （Cu） deposit at Bangpu, locating in the east section of Gangdese,Tibet, belongs to Gangdese-Nyainqentanglha compound volcanic magmatic arc. Withstudies on fluid and melt inclusions, Hf and He-Ar isotope compositions, this paperreports types of alteration and mineralization, the physical and chemical environmentfor magmatic exsolution, the fluid evolution history as well as the mechanism ofmetal transportation and precipitation.Lu-Hf isotope analysis of zircon grains from monzogranite porphyry and biotitemonzonitic granite suggests that their parental magmas were mainly derived from ayoung mantle and partly mixed with old continental crust rock during evolution. Byusing melt inclusions microthermometry we unravel the evolution of magmaticsystem happened from850℃to1223℃and fluid exsolution occurred at bothmagmatic chamber and hypergene emplacement porphyry. Coexistence of B20Hinclusions and silicate melt inclusions provides robust evidence of this process atpressure between150and200Mpa, corresponding to depths between5.6and7.4km.Bangpu deposit displays a consistent alteration-mineralization pattern that comprises,centrally from the bottom upward, of potassic-silicic, sericitic, propylitic andadvanced argillic. According to the veins types and crosscutting relationships, themineralization at Bangpu can be divided into three stages: Pre-stage （Ⅰ）, main-stage（Ⅱ） and post-stage （Ⅲ）. Main-stage needs further subdivision as Mo mineralizationstage （Ⅱ₁） and Cu mineralization stage （Ⅱ₂）. Fluid inclusions at Bangpu can bedistinguished as types B15, B20H, B35, B60and B80based on the phases present atroom temperature （The letter “B” denotes “bubble”, and the number indicates theaverage volume percent occupied by the bubble in inclusions of that type. The letter“H” refers to the presence of halite as a daughter mineral）. Fluid inclusionsmicrothermometry analysis gave the entire temperatures of mineralization rangingfrom240℃to419℃and pressure from38to116MPa, corresponding to depths from3.1and4.3km. B20H and B35inclusions dominate in Ⅱ₁stage with wide range ofhomogenization temperatures （240℃⁴05℃） and pressure from38to85Mpa. They formed molybdenum veins and quartz-molybdenum veins with potassic-silicic andsericitic alteration. In Ⅱ₂stage, fluid inclusions are mainly B60and B80types withhomogenization temperatures from293℃to419℃and pressure from32to84Mpa.They formed quartz-chalcopyrite±pyrite veins and chalcopyrite veins with silicic andpropylitic alteration.We conclude that the CO₂-bearing, aqueous fluids at Bangpu are consist of crust andmantle fluids with the former dominant, and lack of meteoric water involvementduring the whole mineralization. Isotope studies indicate metals at Bangpu theirmagmatic source. Pressure fluctuations, low pH and high S-content were probably themain driving force for Mo （Cu） sulfide deposition.The porphyry Mo （Cu） deposit at Bangpu, consequence of mineralization explosionduring the extension environment after India-Asia continental collision, shares manycommons with porphyry Cu deposit but has a closer relationship between magma andmetals precipitation.