Study On The Characteristics Of Ore-forming Fluid And Genesis Of Dachang Tongkeng Tin Polymetallic Deposit,Guangxi

The Dachang Tongkeng deposit that located in the middle of Danchi tin polymetallic metallogenic belt,northwest Guangxi,is one of the famous super-large tin polymetallic deposits in the world.The Tongkeng deposit is composed of the upper tin polymetallic orebodies and the newly discovered deep skarn type zinc-copper orebodies.The predecessors have done a lot of research on the mineralization characteristics of the upper tin polymetallic orebody,but the genesis of the layered tin polymetallic orebody is quite different,and there are few studies on the deep zinc-copper orebody.Based on the detailed field geological survey and ore mineralogy study,the fluid inclusions of different metallogenic stage in tin polymetallic orebody and zinc-copper orebody have been tested microscopically in this paper.According to the studies of Pb isotope and He-Ar isotope,the characteristics and evolution of the ore-forming fluids,the source of ore-forming materials and the genesis of the deposits are discussed,which provides new information for further determining the metallogenic mechanism of the deposit.The conclusions are summarized as following:(1)H2O-NaCl type,H2O-NaCl-CO2 type and a small amount of pure CO2 type fluid inclusions were observed in the tin polymetallic orebodies and zinc-copper orebodies.Among them,CO2-bearing inclusion are obviously developed in the main metallogenic stage.In addition,a small amount of the halite-bearing daughter mineral inclusions are found in the zinc-copper orebodies,but such inclusions are not found in the tin polymetallic orebodies.(2)Microscopic tests show that the temperature and salinity of ore-forming fluids in tin polymetallic orebodies and zinc-copper orebodies decrease gradually from early to late,but the fluid density tends to increase.In general,the temperature of ore-forming fluid is decreased from the deep zinc-copper ore(320?400?)near the granite batholith to the upper tin polymetallic ore(260?380?)away from the granite batholith,and the salinity is also decreased(5?9 eq.wt%NaCl?3?7eq.wt%NaCl).(3)Estimation of thermodynamic parameters shows that the ore-forming fluids of the upper tin polymetallic ore are generally characterized by weak acidity(5.69?6.87),relative reduction-weak oxidation(-0.4526?0.1814 V),high oxygen fugacity(10-32.53?10-24.11)and low sulfur fugacity(10-13.56?10-7.78).The ore-forming fluids of the deep zinc-copper ore are generally characterized by neutral-weak acidity(5.79?7.17),reduction(-0.5903?0.1255 V),low oxygen flugacity(10-4.72?10-30.25)and high sulfur fugacity(10-0.86?10-6.34).(4)The boiling of CO2-rich fluids in early metallogenic stage and the mixing of fluids in late metallogenic stage may be important factors leading to the precipitation of cassiterite.The boiling of CO2-rich ore-forming fluids is an important mechanism of zinc-copper mineral precipitation.(5)The 3He/4He ratio of primary sedimentary pyrite fluid inclusions in the Devonian Liujiang formation siliceous rock is 1.47 Ra.It is suggested that mantle fluids were obviously added during Devonian sedimentation and diagenesis and there are submarine hydrothermal exhalative sedimentation in Devonian.The 1He/1He ratio of deep zinc-copper ore ore-forming fluids is 0.84?1.85 Ra,and the 40Ar/36Ar ratio is 339.3?350.7.It indicates that the ore-forming fluids of zinc-copper orebody are the result of atmospheric precipitation through deep crustal circulation and mixing with deep magmatic fluids and mantle fluids.The ore-forming mantle fluids originate from inheritance and transformation of submarine hydrothermal exhalative sedimentation.(6)Devonian submarine hydrothermal exhalative sedimentation events exist in Tongkeng mining area and provide mantle-derived materials for mineralization.The superposition of late Yanshanian mineralization is the main event of deposit formation.The superposition of multiphase and multistage mineralization resulted in the accumulation of huge amounts of tin polymetallic elements in Tongkeng mining area.