THE PORPHYRY MO(CU) 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.

By using melt inclusions microthermometry we unravel the evolution of magmatic system happened from 850 to 1223°C and fluid exsolution occurred at both magmatic chamber and hypergene emplacement porphyry. Coexistence of B20H inclusions and silicate melt inclusions provides robust evidence of this process at pressure between 150 and 200Mpa, corresponding to depths between 5.6 and 7.4km.

According to the veins types and crosscutting relationships, the mineralization 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 mineralization stage(II₁) and Cu mineralization stage(II₂). Fluid inclusions at Bangpu can be distinguished as types B15, B20H, B35, B60 and B80 based on the phases present at room temperature (The letter “B” denotes “bubble”, and the number indicates the average 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 inclusions microthermometry analysis gave the entire temperatures of mineralization ranging from 240 to 419°C and pressure from 38 to 116 MPa, corresponding to depths from 3.1 and 4.3km. B20H and B35 inclusions dominate in II₁ stage with wide range of homogenization temperatures(240~405°C) and pressure from 38 to 85Mpa. They formed molybdenum veins and quartz-molybdenum veins with potassic-silicic and sericitic alteration. In II₂stage, fluid inclusions are mainly B60 and B80 types with homogenization temperatures from 293 to 419°C and pressure from 32 to 84Mpa. They formed quartz-chalcopyrite±pyrite veins and chalcopyrite veins with silicic and propylitic alteration.

We conclude that the CO₂-bearing, aqueous fluids at Bangpu are consist of crust and mantle fluids with the former dominant, and lack of meteoric water involvement during the whole mineralization. Isotope studies indicate metals at Bangpu their magmatic source. Pressure fluctuations, low pH and high S-content were probably the main driving force for Mo(Cu) sulfide deposition.