Mineralogical And Geochemical Characteristics Of Laojunshan Granite In SE Yunnan And Its Implication For Diagenesis And Mineralization

Granite is a widely distributed rock type in the continental crust,which carries a lot of deep-seated information of the earth and provides the source of ore-forming materials for many deposits.Its evolution characteristics are often used to invert the maturity of the continental crust.These important properties make it an important object in geoscience research.Laojunshan area in SE Yunnan is one of the important W-Sn-Zn polymetallic metallogenic areas in China,with strong geological process and favorable metallogenic conditions.The Nanyangtian large-scale tungsten deposit and Dulong super large-scale Sn-Zn polymetallic deposit,as representative deposits closely related to the late Yanshan Laojunshan granite.In this paper,the typical granite(porphyry)samples from Dulong,Nanlao,Nanwenhe and Nanyangtian in Laojunshan area are selected for detailed petrographic observation of mica,feldspar and chlorite in the granite(porphyry).EPMA and LA-ICPMS are used to test the major and minor elements of the above minerals,and the magmatic properties,origin,evolution trend and physicochemical conditions of the granite are discussed The geochemical characteristics of the main rock forming minerals are further analyzed to indicate the formation process of the polymetallic deposits around them.According to the significant differences of mineral types and contents in petrology,the Dulong granite is divided into DL-I and DL-II granites.Petrographic observation shows that there are two phases of muscovite in the Nanwenhe granite,namely NWH-I and NWH-II muscovite.The main achievements of this paper are as follows :(1)Most of biotite belongs to phlogopite,the rest is ferruginous mica;Muscovite is impure dolomite containing albite and chlorite molecules;plagioclase is mainly albite;chlorite is mainly vermicular chlorite(ferrochlorite)of iron rich species.Biotite and muscovite tested in this study are primary mica directly crystallized from magma.(2)Combined with EPMA test results and theoretical calculation,it shows that there are obvious differences in crystallization temperature and pressure among different types of biotite,muscovite and chlorite.Among them,the crystallization temperatures of biotite,muscovite and chlorite are 529-669 ℃,208-503 ℃ and238-255 ℃,respectively;the crystallization pressures of biotite and muscovite are381-522 MPa and 127-477 MPa,respectively;the converted crystallization depths of biotite and muscovite are 14-20 km and 4.62-17.37 km,respectively.The oxygen fugacity of biotite crystals varies from-18.3 to-15.5.(3)Based on the variation of biotite and muscovite element content,this study suggests that the evolution degree of granite porphyry from Nanyangtian→DL-I granite→Nanlao granite→Nanwenhe→DL-II granite is gradually enhanced.With the increase of magma evolution degree,Li,Rb,CS,Cr,Zn,Nb and other elements are enriched in biotite of late evolution,while Ga,Sc and other elements are enriched in muscovite of late evolution;however,W and Sn may be enriched in muscovite of early evolution and muscovite of late evolution;Pb tends to be enriched in alkaline feldspar of late evolution with the increase of magma evolution degree.