The Evolution Of Ore-forming Fluid Of The Pulang Porphyry Copper Polymetallic Deposit In The Northwest Yunnan Province,China

As one of few giant porphyry deposits relating to subduction of the Paleo-Tethyan ocean basin in Tethyan tectonic domain,the Pulang PCD,in the northwest of Yunan Province,has its unique value of research.Great amout of copper and molybdenum has widely mineralized within the composite intrusions emplaced in Indosinian,while the copper and the molybdenum are obviously separated and precipitate.To deeply explore the reasons why metals have been mineralized differently,scanning electron microscope-cathodoluminescence?SEM-CL?was applied in this thesis to identify quartz generations contained in different stages of veins.This thesis tries to conduct the research through the lens of hydrothermal evolution by using microthermometry and laser raman microprobe of fluid inclusions including S isotope analysis at the same time.Quartz veins in porphyry copper deposits record the physiochemical evolution of fluids in subvolcanic magmatic-hydrothermal systems.According to the relations of crosscut and the constitution of metal sulfides,this thesis identifies four types of veins that are closely related to mineralization of copper and molybdenum,referring to classical A,B and D veins in PCDs,which could be divided to Cu-stage and Mo-stage.Specifically,these four types of veins are Vein1:quratz+copper-bearing veinlets,Vein2:quratz+molybdenite±chalcopyrite±pyrite veins,Vein3:quartz+chalcopyrite+pyrite±pyrrhotite viens,and Vein4:quartz+massive pyrite-chalcopyrite±calcite±chalcocite veins.According to the intensity of the SEM-CL,this thesis demonstrates five stages of quratz with petrographic observation including Q1,Q2a,Q2b,Q2c and Q3 in Pulang PCD,and concludes that the ore-forming fluid has the multi-stage evolution.In stage Q1,the quartz is accompanied with little copper mineralization precipitated firstly.Q2 consists of three successive stages of quratz.The Q2a quartz is brightest.The Q2b always coexists with molybdenite,which indicates that molybdenum mineralization happens in this stage,while copper largely precipitates in stages Q2a,Q2c and Q3.According to the analysis of microthermometry and laser raman microprobe,there are obvious variations of physiochemical property of ore-forming fluid in multiple stages.The Q2b ore-forming fluid,exsolving from deeper magma chamber and boiling at upper crust,controls the beginning and the end of the precipitation of molybdenite.As an incompatible element,molybdenum enriches into recidual melt during the fractional crystallization of magma.Then it is transported by lately exsolved fluid.The degassing of CO₂ destroys the condition of low pH and oxygen fugacity,which makes mineralization of molybdenum only happens in a short window period,resulting in temporal and spatial separation of copper and molybdenum.The analytic result of metal sulfide?³⁴S indicates that?³⁴S fractionation is periodically balanced.Combining with the appearance of two homogeneous temperature members in one salinity range of the FIs from Q2c to Q3,the fluid mixture is concluded as mixing in some substance of the stratum,as well in the mid-late stage of ore-forming fluid evolution.