Methane-rich fluid evolution of the Baogutu porphyry Cu–Mo–Au deposit, Xinjiang, NW China

Baogutu is the first porphyry Cu–Mo–Au deposit discovered in Western Junggar, Xinjiang, NW China. The ore-bearing intrusion is a dioritic intrusive complex that includes stage 1 diorites and minor stage 2 diorite porphyries. The stage 1 diorites have produced concentric potassic and propylitic alteration zone and overprinted phyllic alteration and host much of the Cu–Mo–Au mineralization at Baogutu. The Baogutu porphyry Cu–Mo–Au deposit consists of unusually disseminated and minor vein-style mineralization. The fluid evolution occurred in stage 1 diorite from late magmatic stage to hydrothermal stage (stages 1B and 1C) is constructed based on alteration and fluid inclusion analysis by microthermometry, Raman Spectroscopy and Quadrupole Mass Spectrometer.Five fluid types are distinguished and they are L1 (liquid-rich 2-phase), L2 (liquid-vapor 2-phase), V1 (vapor-rich 2-phase), V2 (mono-phase vapor), and H (multi-phase solid). The dominant inclusions rich in reductive CH4 plus H2O while minor inclusions rich in CH4 and CO2. The inclusions in quartz from the late magmatic stage contain the most brine inclusions and less other inclusions with CH4-rich and H2O-rich compositions. The inclusions in quartz from stage 1B contain all fluid types and have CH4-rich and CH4 + H2O compositions with rare CO2. The inclusions in quartz from stage 1C are characterized by more vapor inclusions without brine inclusions and a clear CO2 concentration in a few assemblages with CH4 + CO2 composition.Using microthermometry, we estimate fluid trapping conditions at T > 400 °C and P = 1500 to 3100 bar in late magmatic stage, T = 200 to 400 °C and P = 50 to 320 bar in disseminated quartz in stage 1B, T = 180 to 400 °C and P = 20 to 260 bar in vein quartz in stage 1B, and T = 170 to 400 °C and P = 20 to 230 bar in stage 1C. Such late magmatic conditions are compatible with the fluid evolution as a result of CH4-rich inclusions derived from mantle magma. Such hydrothermal conditions in stages 1B and 1C with small P–T fluctuations are compatible with the dominant disseminated mineralization at Baogutu that indicates a weak fracturing. The overall fluid path at Baogutu is toward lower pressure and small changed temperature, with composition transit of fluid from a high halite CH4-rich system to low halite CH4 + CO2 system. It may lead to the formation of this Cu–Mo–Au deposit.