Characteristics Of Ore-forming Fluid Of Dabu Porphyry Cu-Mo Deposit In Tibet

Dabu porphyry copper-molybdenum deposit, located at the northern margin ofthe Yarlung Zangbo suture zone, is situated in the continental margin of centralGangdese volcanic arc and belongs to Gangdese-Nyainqentanglha fold system.Through the analysis of fluid inclusions’ temperature, H-O isotope and laser Raman,we determined the main alteration and mineralization types and constrained thephysical and chemical conditions of magmatic exsolution fluid. We also identified theprocess of ore-forming fluid evolution and the mechanism of metallogenic materialmigration and precipitation.Based on the field geological and laboratory microscopic observations,combined with the characteristics of mineral alteration, the Dabu deposit has typicalalteration features that are similar to those in other porphyry Cu deposits, mainlyincluding potassium silicatization, sericitization, propylitization and clayization. Themineralization process can be divided into three stages: the early stage, the middlestage and the late stage. The early stage is characterized by quartz-feldspar-chalcopyrite and quartz-biotite-chalcopyrite veins, which is associated withpotassium silicatization; The middle stage is the main mineralization stage dominatedby quartz-molybdenite-pyrite–chalcopyrite veins; The late stage mainly developsquartz-pyrite veins. Depending on the different proportion bubbles,types of inclusionsin the Veins can be divideded into three categories: vapor-rich inclusions, liquid-richinclusions and brine inclusions. And found the phenomenon that the coexistence ofthree types inclusions shows the boiling inclusions characteristics. Laser Raman(LRM) measurement and analysis, The fluid inclusions of Dabu deposit generallycontain CO2in the gas phase composition from the early to late stage, In addition tochalcopyrite, hematite and other opaque daughter minerals. High salinity fluidinclusions also contain halite, sylvine, and apatite transparent daughter minerals. TheδDH2O values vary between5.8‰and7.0‰, while the δ18OH2O values between-87‰and-122‰, indicate that the early fluid of Dabu porphyry deposit isdominated by magmatic water, with little or no meteoric water.The VL-type fluidinclusions of potassium silicatizated veins of Dabu deposit is the record of earliestore-forming fluid, whose homogeneous temperature is up to474℃, with average salinity of about9.2wt%NaCleqv, The salinity of ore-forming fluid is similar withthe early vapor-rich inclusions in early vein of Qulong porphyry deposit. Theore-forming fluid is one NaCl-CO2-H2O system that are high-temperature, moderatesalinity and CO2riched supercritical fluid. With pressure decreasing, the fluidseparated into low salinity vapor-rich fluid and high salinity liquid-rich fluid. Highsalinity liquid-rich fluid has stronger ability to carry metal than the low salinityvapor-rich fluid. Deposition of ore-forming elements is not the result of the decresingof temperature, but the fluid boiling result from the decreasing of pressure.