Prediction Of Tin Mineralization In Tengchong Terrain Constrained By Study Of Regional And Ore-forming Magmatic Rocks

Tengchong terrain is located in the western part of Yunnan Province,the southeast margin of Qinghai-Tibet Plateau and the southwest corner of "Sanjiang" area.Magmatic rocks are widely distributed in this region,and granite-related hydrothermal deposits are particularly developed,especially the tin deposits.In this study,several late Cretaceous-early Eocene magmatic rocks in Tengchong terrain were sampled,and zircon U-Pb dating,zircon trace elements and zircon Lu-Hf isotope tests were carried out on the granites at seven samples,including Pomanshao,Lushan,Lishupo,Xingba,Lushui,Mengdian and Mengnong.As the most representative tin deposits and the only two large tin deposits in Tengchong terrain,Xiaolonghe and Lailishan tin deposits have great significance for the regional tin mineralization.Based on the data which published by predecessors and the results of data analysis in this study,the regional and mining rocks were analyzed by petrology means to trace source rocks and explain the rock properties.Combined with zircon Hf isotope mapping,the favorable conditions for tin mineralization in Tengchong terraine were analyzed to determine the prospecting direction.Through observe hand specimens and sections,samples in this study include granite,biotite-dolomite granite,biotite granite and diorite,etc.Zircon U-Pb test shows these samples can be divided into ~ 91.2 Ma(early late Cretaceous),75.6 ~ 71.3 Ma(late late Cretaceous)and 61.1 ~ 53 Ma(Paleocene to early Eocene).Combined with previous studies,granitic rocks in have negative zircons ?Hf value as a range of-51.23 to 1.49,and its Hf isotopic model age to depleted mantle is much older than its U-Pb age,shows magma is formed by ancient basement remelting.The two-stage Hf model ages of cretaceous rocks are concentrated between 2000 Ma to 900 Ma,and the two-stage Hf model ages of Paleocene to early Eocene rocks are concentrated between 1900 Ma to 600 Ma,both of which are from the remelting of the ancient crust of the Proterozoic.Hf isotope mapping shows Tengchong terrain distributes widely negative zircons ?Hf value in NW to SE ribbon area,and zircons ?Hf value gradually rose to NE and SW areas.In some edges of SW of Tengchong terrain zircons ?Hf value can be positive.Xiaolonghe granite is related to the subduction collision between Tengchong and Sunda plate.Lailishan granite was formed during the main collision stage of subduction collision between the Indian plate and the Eurasian plate along the Yarlung-Zangbu River suture zone.The oreforming rocks are composed of quartz,potassium feldspar,plagioclase and biotite,and amphibole can be seen in all of them,showing the characteristics of I-type granite.Both of them are high fractionated granites with characterized by high-potassium peraluminous granite,w(TFe O)/w(Mg O)between 4 to 16,high value of 10000Ga/Al,La and Ce contents remain constant and Y increases as the Rb contents increasing,have amphibole etc.Both of them located in the region of ?Hf value<-6,means the magma were formed by remelting lower crust of the Proterozoic.And zircon trace element characteristics show that Lailishan granite has a low oxygen fugacity.The above contents are favorable for Tin deposits and the main target of prospecting Tin deposits in Tengchong terrain.Combined with zircon Hf isotope mapping,Xingba biotite granite(XB14-48)has similar petrology characteristics,magmatic source,isotope composition and oxygen fugidity to the Xiaolonghe and Lailishan ore-forming rocks,suggesting that it has good metallogenic potential.