| The Gangdese metallogenic belt,Tibet,is one of the world-class porphyry-skarnepithermal type copper-polymetallic belts in China.Despite a lot of achievements have gained on the exploration and research of the Gangdese porphyry metallogenic belt,there are some specific scientific issues on metallogeny still unknown.It includes mineralization spatially variation,magmatic-hydrothermal fluid evolution,and ore-related elements migration and precipitation.Based on documented researches,this study conducts geochronology,whole rocks geochemistry and fluid inclusions of Jiama and Lakang'e deposits,and focused research geochemistry of biotite formed under different setting and in different sub-types of porphyry copper system in Gangdese metallogenic blet to discuss the different formation processes of those sub-types,and further study formation mechanism of different mineralization type through detailed geological record,the microscopic identification system and the LA-ICP-MS and EPMA testing methods and techniques.Through study we gained the following awareness:1.Different geochemical magmatic hydrothermal systems have different biotite signature.The magmatic biotites has high Ba O(> 0.1wt%)and Ti O2(> 3wt%),while the hydrothermal biotites has low Ba O and Ti O2.There are two types of hydrothermal biotite in Bangpu deposit,identified in Mo(Cu)mineralization system and in Cu(Mo)mineralization system.The geochemistry of hydrothermal biotite in orebody I and orebody II of Xiongcun deposit are obviously different.2.The biotite of porphyry Cu-Mo deposits(Bangpu,Jiama,Qulong,Lakang'e)formed in collisional stretching setting are Mg-biotite or phlogopite,with relatively high Mg,and low Fe,Mn and Cl.But biotite in porphyry Cu-Au deposit(Xiongcun)formed in island-arc setting are Mg-biotite,close to Fe-biotite,with high low Mg,and relative high Cl,Mn and Fe content.3.Biotite mineral composition can be indicator to hydrothermal fluid temperature,oxygen fugacity,and halogen fugacity ratio of porphyry metallogenic system in Gangdese metallogenic belt.The biotite composition in collisional stretch type porphyry Cu-Mo deposits indicates high oxygen fugacity,log(fHF/fHCl),log(fH2O/fHF)and temperature,but that in island-arc porphyry Cu-Au deposit suggests low oxygen fugacity,log(fHF/fHCl),log(fH2O/fHF)and temperature.4.Also,biotite mineral chemistry can be indicator to petrogenesis and metallogeny.Combined former research data and mineral chemistry of biotite,it shows that the porphyritic intrusions in the eastern of Gangdese metallogenic belt are I-type granite,calc-alkaline orogenic suites,and originated from mixture of crust-mantle source.From Bangpu? Lakange? Jiama? Qulong? Xiongcun,the contamination of crustal material gradually decreased,material of mantle source gradually increased.The F,Cl and oxygen fugacity in magmatic-hydrothermal systems play an important role in distribution and migration of ore-forming elements like Cu,Mo,and Au in the melt or the fluid phase.High oxygen fugacity and F content of magmatic-hydrothermal systems is in favorable to the Mo aggregation and migration.High oxygen fugacity and Cl are in favour to Cu aggregation and migration.Low oxygen fugacity and high Cl is in favour to Au aggregation and migration.The temperature decreasing,pressure decreasing,p H increasing,and oxygen fugacity changing combination cause precipitation of ore-forming elements during the evolution of magmatic-hydrothermal.Magma source and F,Cl content in the magmatic-hydrothermal systems are the important factors of impact different porphyry mineralization types.5.Biotite can be effective indicator to exploration of porphyry deposit.The biotite related to porphyry copper polymetallic mineralization in Gangdese metallogenic belt are Mg-biotite or phlogopite with Mg(Mg/Fe commonly >1),and have high volatile content(F or Cl).Biotites in porphyry Cu-Au deposits are rich in Cl,biotites in porphyry Mo-Cu deposit are rich in F,and biotites in porphyry Cu-Mo deposit are rich in F and Cl. |
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