Post-cretaceous Magmatism And Metallogenesis In Xiongmei Area,Middle Bangong Co-nujiang Metallogenic Belt

Xiongmei area is located in Shenzha County in the middle part of Bangong Co-Nujiang metallogenic belt in Tibet.There are Xiongmei porphyry copper-molybdenum deposit,Shesuo skarn copper polymetallic deposit,Kuga and Sangri skarn copper deposits.In this paper,the age and genetic types of these copper-bearing magmatic rocks have been determined by such as petrography,zircon U-Pb age,isotope and petrogeochemical analysis.(1)It is preliminarily confirmed that it was formed in the post-collision divided into early Cretaceous about 110 Ma(Shesuo,Xiongmei,Kuga)and late Cretaceous 80 Ma(Sangri).(2)The Pb isotopes of the rocks indicate that the magma source areas are all from the crust.The ore-bearing granodiorite porphyry in Xiongmei mining area has high aluminium,calc-alkaline characteristics and is S-type granite formed by argillaceous sedimentary rocks.The ore-bearing rocks in Shesuo,Sangri and Kuga areas are all type ?.Geochemistry shows that these ore-bearing rocks are rich in large ion lithophile elements and depleted in high field strength elements,indicating that subduction sediment components have an important influence on magma formation.At the same time,the mineral fractional crystallization of ore-bearing magma from these crustal sources occurred during the process of uplift and emplacement:the granodiorite in Xiongmei mining area is mainly controlled by the fractional crystallization of biotite,the monzogranite in Shesuo area shows the trend of plagioclase crystallization,the Kuga area shows the trend of potash feldspar crystallization,and the Sangri area shows the trend of amphibole crystallization.(3)The value of sulfur isotopicl in Xiongmei porphyry copper-molybdenum deposit is-2.5-6.1.The sulfur isotope is pyramidal distribution,showing the characteristics of magmatic sulfur.The Pb isotopes compositions of 206Pb/204Pb,207Pb/204Pb,208Pb/204Pb are 18.220?19.005,15.626?15.770,38.510?39.856,which show the characteristics of common Pb.The source of Pb is clearly shown as the upper crust,and the mineral source is consistent with the magmatic source area(4)The age and source area of magmatism in Xiongmei area have obvious characteristics of post-collision stage,during which the Paleo-Tethyan Ocean in Bangong Co-Nujiang belt has been completely closed.During the early Cretaceous,subduction plates broke off,a large number of asthenospheric materials upwelled and participated in magmatic activities,which prompted the melting of upper crustal components and formed ore-bearing magmatic rocks with a large number of metallogenic elements,and formed Xiongmei,Shesuo and Sangri deposits near the surface.During the late Cretaceous,the thicker lithosphere was dismantled under the action of gravity and the thinner lithosphere was subjected to the subduction of the upwelling mantle.The Sangri deposit was formed by melting.