The Granitic Magmatism And Its Significance Of Mineralization In Gejiu?Yunnan

Yunnan Gejiu is located in the west of the South China block.The geotectonic position is located in the juncture of the Pacific tectonic domain and the Tethyan tectonic domain.Magmatic activity in the area is very strong,forming a series of granitoid.Gejiu area were exposed two categories of biotite granitoid,respectively,the western medium-coarse grained biotite granite and the eastern medium-fine grained biotite granite.This paper systematically studies the petrology,geochemistry and geochronology of the two types of granites.And analyses the diagenetic age,type and tectonic setting of the granite,discusses the magmatic source and formation mechanism.Two types of granite have no obvious difference in structure and mineral composition,which are composed of quartz,feldspar,plagioclase and biotite.However,from the medium-coarse grained biotite granite to the medium-fine grained biotite granite,the compositon of quartz,feldspar gradually increased,and the composition of biotite,plagioclase gradually decreased.The zircon LA-ICP-MS U-Pb age of the medium-coarse grained is(82.08±0.56)Ma.Combining with the other age data of the equal-grained biotite granite in the area,two kinds of granite'diagenetic age is concentrated in 80~82Ma.Apparently they are formed at the same time.In other words,the equal-granid biotite granite is production of Yanshanian magmatism.Geochemically,the two kinds of granite are peraluminous and rich in Si,K,and depleted in Fe,Mg,Ti.They are characterized by calc-alkaline.At the same time,the medium-coarse grained biotite granite compared to the medium-coarse grained biotite granite,it is more acid and more depleted in K,alkali,Al.The two types of granite has similar characteristics of trace elemental composition.They are both rich in Rb,Th,U and Hf,and depleted in Ba,Sr,P,Ti and so on.The medium-coarse grained biotite granite' chondrite-normalized REE patterns is the right type,reflected the characteristics of LREE is comparatively rich;The medium-fine grained biotite granite' chondrite-normallized REE patterns is the V type,reflected the fractionation of LEE and HREE is not obvious.Meanwhile it has significant tetrad effect.The medium-coarse grained biotite granite shows moderate negative Eu anomalies.The medium-coarse grained biotite granite shows strongly negative Eu anomalies,indicating its differentiation is obvious.Two types of granite' whole rock zircon saturation temperature calculation results were more than 800 ?,which are belong to high temperature granite.The equal-grained biotite granite is A-type granite which differentiation has taken place.The granite is caused by the partial melting of low maturity of mud sandy clastic rock formed which was crust origin.The evolution of the two kinds of granite in mineralogy,geochemistry and chronology was obvious,and they originated from the same magma reservoir.There is a lot of evidence suggesting the granite formed in the extension tectonic environment after collisional orogeny period.The coarse-fine grained biotite granite is closely related to tin mineralization.Its characteristics which are presented as low oxygen fugacity,highly differentiation and the interaction between fluid and melt have important instructions for forming large Sn polymetallic deposit.