Study On Geochemical Characteristics And Genetic Mechanism Of Badaguan Copper-Molybdenum Deposit In Inner Mongolia, Ne China

The Badaguan copper-molybdenum deposit is a typical medium scale porphyry deposit in the middle of Derbugan metallogenic belt, in Inner Mongolia, China. Derbugan metallogenic belt situated in the southeast side of the Mongolia Okhotsk suture zone, which belongs to Eerguna block. Based on the systematic field investigation and laboratory study, the auther choosed sufficient samples from orebearing rock-bodies, ore minerals and gangue minerals of the Badaguan Cu-Mo deposit to conduct geochemical analyses including rock geochemistry, geochronology, isotopic geochemistry and fluid inclusion. Finally the paper discussed mineralization mechanism and main ore controlling factors.Whole-rock geochemical data indicated that the ore-bearing rocks were complex plutons from homologous magama which emplaced in multiple stages. The ore-bearing rocks were I-type granites and belonged to calc-alkaline rock series with moderate levels of potassium, which originated from volcanic arc settings. Combined with an analysis of regional tectonic background and Sr-Nd-Pb isotope data, the author hold that the ore-bearing rocks were derived from partial melting of mantle wedge which overlain on oceanic plate during the Mongolia-Okhotsk Ocean plate southward subduction, and mixed with some ancient lower crust components during the magmatic evolution process.The result of LA-ICP-MS zircon U-Pb dating showed that the emplacement age of quartz diorite was 236.5±3.5Ma. The Re-Os dating of molybdenite gave a weighted average model age of 226.7±2.4Ma, which indicated that the deposit was formed in the early Late Triassic. The mineralization processes occurred after mineralizing plutons emplaced. During the emplacement and mineralization, this region suffered a strong tectonic deformation under the background of the activity of Derbugan faults, and provided channels and space for the flowing of ore-forming fluid and the precipitation of ore-forming materials. The Ca, Mg, Na, Ta, K and ∑REE in ore-bearing rock gradually decreased, and the content of Si, Pb increased during the main mineralization process.Fluid inclusion analyses suggested that ore-forming fluid belongs to a Na ClH2O±CO2 system, with moderate-high temperature, moderate-low salinity, and with the characteristics of magmatic hydrothermal water and partly mixed meteoric water. The mainly temperature of early stage ranges from 340°C to 400°C, and the salinity ranges from 9%Na Cleqv to 14%Na Cleqv, the environment of magmatic hydrothermal was high oxygen fugacity, and it was helpful to mineralize that the reduction of sulfur during hydrothermal magnetite formation. The mainly temperature of middle stage ranges from 300°C to 360°C, and the salinity ranges from 5%Na Cleqv to 9%Na Cleqv, the main reason of the precipitation of Cu, Mo maybe initiated by fluid boiling. The mainly temperature of late stage ranges from 240°C to 300°C, and the salinity ranges from 1%Na Cleqv to 5%Na Cleqv, galena, sphalerite and hessite formed in this stage. Oreforming fluid density is low overall, and the mineralization occurred in the depth of 5~8km. It is noteworthy that the C-H-O-S isotope and the first time found of hessite both indicated the mantle origin of ore-forming materials.Comprehensive study reveals that the Badaguan Cu-Mo deposit formed after magmatic activity on Eerguna block in volcanic arc settings. The Cu, Mo and other elements migrated with the hydrothermal fluids originated from magmatic activity, combined with sulfur from mantle, enriched and mineralized finally. Structural fracture, boiling of ore-forming fluid, oxygen fugacity changes is the main ore controlling factors.