Study On The Geological Characteristics And Genesis Of The Pusangguo Cobalt-rich Cu-dominated Polymetallic Deposit In The Gangdese Metallogenic Belt

The Pusanguo deposit is located in the middle part of Gangdese metallogenic belt in Tibet.It is the first large-scale skarn cobalt-rich copper-lead-zinc polymetallic deposit discovered in Gangdese metallogenic belt in recent years.It is also the only cobalt-rich copper-polymetallic deposit in Gangdese metallogenic belt.This deposit has complex useful elements and high grade of copper,lead and zinc.However,the ore-forming intrusion has not been ascertained,and whether there is porphyry-type orebody or not needs further research and exploration.Therefore,it is urgent to dissect the geological characteristics and discuss the genesis of this deposit,which has both important theoretical value and practical significance of guiding exploration.In view of this,based on the existing problems,this paper mainly study geological characteristics,mineralogy and mineral chemistry,geochronology,the characteristics of ore-forming fluid,and isotopic geochemistry to establish the ore-forming model and provide theroretical guidance.This study has mainly made the following progress and achievements:1.The mineralogical and chemical characteristics of the ores and the occurrence state of the cobalt were systematically identified.The orebodies are mainly stratiform or lentoid developed in the skarn contact zone between the acid magmatic rocks and the Takena Formation,forming the polymetallic orebodies.The main minerals are chalcopyrite,sphalerite,galena and pyrite,followed by cobalt-rich minerals,chalcocite,bornite,aikinite,emplectite and hematite.Skarn belongs to the calcareous skarn,including andradite,grossularite,diopside,wollastonite,amphibole,epidote and chlorite.With the evolution of mineralization,the environment of skarn has changed from early high temperature and high oxygen fugacity acidic environment to late low temperature and low oxygen fugacity alkaline environment.The occurrence state of cobalt mainly includes the replacement of Fe,Zn and the independent cobalt minerals.Chalcopyrite is the most important cobalt-bearing mineral in this deposit,and the cobalt-rich minerals are mainly sphalerite,followed by pyrite.2.A systematic study on the geochronology of intermediate acid magmatic rocks related to mineralization in this deposit was carried out for the first time,and the time limit of diagenesis and mineralization was determined.The zircon U-Pb dating results show that the emplacement age of biotite granodiorite is 13.6±0.2 Ma to 14.8±0.4 Ma,and the age of diorite porphyrite intrusion is 13.6±0.1 Ma to 14.6±0.3 Ma,indicating that the age of rock formation is Miocene.The sphalerite Rb-Sr isochron age is13.2±0.7 Ma,and the mineralization age is Miocene.The ore-forming age is coupled with the time of magmatic rocks,indicating that the medium-acid magma intrusion is closely related to mineralization.3.Through the comprehensive study of fluid inclusions and H-O-He-Ar-S-Pb isotopic features,the sources of the fluids,metals and the mechanism of mineral precipitation are discussed.There are five types of fluid inclusions,including W type,S type,C type,L type and V type.In the early skarn stage,the ore-forming fluid is mainly from magmatic water,and the fluid is NaCl-H₂O-CO₂-CH₄ with high temperature,high salinity and high oxygen fugacity.In the late mineralization stage,meteoric water was added,and the fluids were mainly low-temperature and low-salinity NaCl-H₂O.Metallogenic fluids and metals are mainly related to intermediate-acid magmatic rocks with mixed source characteristics of crust-mantle.Temperature reduction,water-rock reaction and fluid boiling mainly lead to the mineral precipitation.4.Based on the geochemical,isotopic and chronological results,the genetic type,and deep dynamics setting of the magmatic rock were preliminarily discussed,and the ore-forming model was established for the first time.All the intermediate-acid rocks have adakitic properties,the copper and lead is mainly related to biotite granodiorite,and the zinc and iron is mainly related to diorite porphyrite.All these rocks are formed in the Miocene post-collisional tectonic setting.The transformation from the regional collision to extension causes the lithosphere of the Indian continent to be disintegrated to form the basic magma melts intruding into the Lhasa terrane and thicken the juvenile lower crust,resulting in the partial melting of the garnet-bearing amphibole faces in thickened lower crust.The adakitic magma flows up to a favorable structural site to form the intrusive rocks and forms a skarn-type cobalt-rich copper-lead-zinc orebody in contact with the Takena Formation.