Ore-forming Fluids Evolution Of The Baoxingchangorphyry Cu-Mo Depositin Western Yunnan

The Baoxingchang Cu-Mo deposit,tectonically occurring in the east-central partof the Jinshajiang-Ailaoshan suture zone, is one of the typical alkali-rich porphyrydeposits in the Sanjiang metallogenic belt.Base on the systematic petrologicalresearch, mineral paragenetic sequence and the ore stages were determined, and themicrothermometry,composition of the fluid inclusions and the H-O isotopes wereanalyzed,also the soruce and evolution of the ore-forming fluids,and metallogenicmechanism of copper-molybdenum resources were discussed in this study.1.Three main types of hydrothermal veins have been identified: A-type, B-typeand D-type veins based on its cutting relationship and petrological observations.Quartz, K-feldspar, biotite, less chalcopyrite and pyrite are the major mineralassemblages in A-type vein. B-type veins having cut through the A-type veins.Mineral assemblages are mainly quartz, molybdenite, chalcopyrite and pyrite. D-typeveins cutting through the earlier ore veins. Quartz, chlorite, pyrite and lesschalcopyrite are the main minerals occurred in the veins.2. The results of the microthermometry and composition analysis of thefluidinclusions show that the ore-forming fluid is characterized by medium-hightemperature, medium-high salinity and CO2-rich.Ore-forming fluids of the early stageisNaCl-H2O-CO2system. The classification of the fluid inclusion at this stagecontains vapor-rich water inclusions, CO2-rich inclusions and daughter mineralbearinginclusions. whose homogenization temperatures vary from380℃to400℃,and salinity ranging from3.3%to16.34%NaCleqand45.64%to52.89%NaCleq.Thetypes of fluid inclusions of the main mineralization stage the same as the early stage.whose homogenization temperatures vary from300℃to360℃, and salinity rangingfrom1.73%to11.23%NaCleqand34.24%to52.04%NaCleq. The types of fluidinclusions of the late mineralization stage contains liquid-rich inclusions. whosehomogenization temperatures vary from260℃to300℃, and salinity ranging from2.4%to10.74%NaCleq.3. The H-O isotopes suggest that the ore-forming fluids in the early stage aremainly magmatic fluids.With the develop of the mineralization,the water of theore-forming fluids was increased. The ore-forming fluids in the late stage are mainlymeteoric water.4.In the main mineralization stage,there may be a sudden drop in pressure duringthe migration-mineralization process, which leads to the escape of CO2and othervolatilegases, accompanying by the increase of pH value, causing the precipitationwith Cu-Mo metallic mineral.