The Geochemistry And Genesis Of Pusangguo Skarn Copper Polymetallic Deposit In Tibet

This paper mainly focuse on the Pusangguo copper polymetallic deposit in Namulin area,Tibet.Based on the study of the deposit geology,age of igneous rocks,skarn mineralogy,fluid inclusion,isotopic geochemistry,the author analyses the geochronology of igneous rocks and ore doposit,geochemistry and petrogenesis of igneous rocks,characteristics of ore-forming fluid and source of ore-forming materials,explores the formation mechanism and initially establishes a model of Pusangguo deposit.Pusangguo deposit is located in the middle of Gangdese metallogenic belt,the northern edge of the South Gangdese.The stratum in Pusangguo deposit are mainly carbonate rocks and volcanic rocks of the Lower Cretaceous Takena Formation.The zircon U-Pb age of the volcanic rocks is 99.5±2.4 Ma.Intusive rocks in Pusangguo deposit include diorite porphyry and granodiorite,both of which are high-K calc-alkaline and wek peraluminous,have similar characteristics of rare earth and trace elements.They show affinity of adakite and the source region characateristics of garnet-amphibole phase.The zircon U-Pb age of diorite porphyry is15.75±0.25 Ma,and the zircon U-Pb age of granodiorite ranges from 13 Ma to 15 Ma?age of inclusion is 15.15±0.42 Ma?.The values of(⁸⁷Sr/⁸⁶Sr)_i of diorite porphyry are0.70613⁰.70751,¹⁴³Nd/¹⁴⁴Nd are 0.51232-0.51248,and?Nd?t?are-5.9^-2.8;The values of(⁸⁷Sr/⁸⁶Sr)_i of granodiorite are 0.70613⁰.70751,¹⁴³Nd/¹⁴⁴Nd are 0.51232-0.51248,and?Nd?t?are-5.9^-2.8.The Sr,Nd,Pb isotopic characteristics indicate the crust-mantle mixed source.Diorite porphyry has positive?Hf?t?values?0.4³.0?and lower model ages?794⁹38 Ma?,which shows the source of partial melting of ancient crust,having the involution of a lot of mantle magma.Granodiorite has negative?Hf?t?values?-9.7^-2.3?and a higher model ages?1090¹502 Ma?,which shows a source of partial melting of ancient crust,having the involution of few mantle magma..This is significantly different with diorite porphyry.In the contact zone of diorite porphyry and carbonate wall rock,skarn,marble and hornfels are developed.Layering ore body distributes in garnet-actinolite skarn belt,with typical characteristics of contact metasomatic skarn type.Skarn minerals are garnet,actinolite,diopside,tremolite,idocrase,phlogopite,epidote and chlorite;metal minerals are chalcopyrite,sphalerite,square galena,pyrite and magnetite.Mineralization stage can be divided into skarn period?dry and wet skarn stage?,quartz-sulfide phase?quartz-pyrite stage and quartz-calcite-sulfide stage?and supergene oxidation stage.Chemical compositons of garnet and sulfide show that the mineralization environment,from early to late,changes from oxidation to partial restore.PH conditions transform from acidic to alkaline.Ar-Ar isotopic age of mica samples of wet skarn stage is 14.98±0.26 Ma,belong to post-collison mineralization events,approximately accounting for the mineralization age of Pusangguo deposit.The study of fluid inclusion in Pusangguo reveal the characteristics of ore-forming fluids.From skarn stages to quartz sulfide stages,the ore-forming fluid evolve gradually from middle-high temperature,middle-low salinity to low temperature,low salinity,reflecting a large number of meteoric are involved in ore-forming fluids at the late period.This is in line with the characteristics of hydrogen and oxygen isotopes.This type fluid system is always located at the top of porphyry copper deposits,suggesting possible development of porphyry copper deposit in deep.Lead and sulfur isotope characteristics show that the source of ore-forming materials are mainly magmatic hydrothermal.Finally,we build up a metallogenic model of the Pusangguo copper polymetallic deposit.