Characteristic And Evolution Of Ore-forming Fluids From Jiama Polymetallic Copper Deposit, Mozhugongka County, Tibet

Jiama polymetallic copper deposit locates in east of Gangdese volcano - magmatic arc. The main orebodies are hosted in stratiform structural zone between limestone of the upper Jurassic and sandy slate, chert of the low Cretaceous, and are controlled by sub-fold produced regional thrust nappe structure and ore field gliding nappe structures. The length is about 3400m with the strike NWW and over 2000m with the dip NNE. The orebodies are blindsemi-blind overall. The morphology of orebodies are stratiform,beded,lenticular. The stages for the formation of the deposit are magmatic–hydrothermal transitional stage, postmagmatic hydrothermal stage, prograde stage, retrograde stage and quartz- sulfide-calcite stage.The study on homogenization and the characteristics of melt inclusions and fliud inclusions in Jiama Polymetallic Copper Deposit show that the porphyry body's formation temperature is 730-1080℃, the exsolution pressure of fluid is 59.1 MPa. The temperature of five mineralization stages is 170-540℃, the salinity concentrate in 15%-50%,and the density is 0.9233-1.0805g/cm3. The ore-forming fluids is mainly NaCl-H2O system and NaCl-CO2-H2O system. Ore pressure is less than 37.3MPa, mineralized mainly above 0.6519km, it is a high temperatured,hypabyssal and magmatic-hydrothermal depositThe ratio of ion compositions, gas compositions, illustrated diagrams and the analysis of oxygen and hydrogen isotope, indicate that the ore-forming fluids of Jiama Polymetallic Copper Deposit is origined from magma, with the mixing of precipitate water in later period. The formation and evolution of ore-forming fluids experienced exsolution of magma, phase separation of supercritical fluid, decompression and boiling, the filling or metasomatism between magmatic hydrotherm-volatile and carbonate wall rocks. The metallogenic elements selectively enter in the volatile and transport. Decreasing temperature,pH,fO2,fS2 and increasing Eh are the favourable conditions for the deposition of sulfides such as Cu,Au,Mo in the Ore-forming fluids. The decompression,boiling,and mixing cause the deposition of metallogenic elements finally. The porphyry magma experienced contamination of upper earth crust and earth mantle. The deposit genesis is related to magmatic-hydrothermal mineralization in the Unified Porphyry- Skarn-Epithermal Ore-forming system.