Abstract:

The recently discovered Jiama super large copper-polymetallic deposit is located in the middle section of Gangdese metallogenic belt, Tibet. Systematic studies including petrography, microthermometry, laser Raman microprobe and SEM/EDS were carried out on the primary fluid inclusions in metallogentic granite and major ore bodies. The analytical results show that dominant fluid inclusions from skarn ore related to metallogensis include liquid-rich, gas-rich two-phase and daughter-minerals bearing polyphase types. Homogenization temperatures of the fluid inclusions range from 225 ℃ to 500 ℃. The average of the salinities of ore-forming fluid is 36.2% NaCleq and the densities vary from 0.89 to 0.98 g/cm³. The ore-forming fluids belong to NaCl-H2O type characterized by high-moderate temperature, high salinity, low density and strong reduction. Besides H₂O, the gas compositions are immiscible system rich in CH₄, H₂S, CO₂ and N₂ etc. The occurrence of organic matter, such as CH₄, C₂H₄ and C₃H₆, suggests that the ore-forming materials were formed in a reduced environment. Based on estimation of mineralization pressure and the relationship between pressure and depth in fracture zones, the mineralization depth is considered to be at 2.2-8.3 km. The analytical results of hydrogen and oxygen isotopic elements show that ore-forming fluids are mainly derived from magmatic water mixed with meteoric water later. The coexistence of daughter-minerals bearing multi-phase inclusions and those liquid-and gas-rich inclusions with different filling degrees, together with their similar homogenization temperatures and much different salinities, indicates that ore-forming fluids had experienced boiling.

Key words: copper-polymetallic deposit, fluid inclusion, mineralization process, ore-forming fluids, ore deposits, Jiama, Tibet