The Diagenesis And Metallogenesis Of Sinongduo Epithermal Ag-Pb-Zn Deposit In Gangdese Belt,Tibet

The Gangdese belt preserved and recorded the tectonic-magmaticmetallogenesis information since the Cenozoic Tethys oceanic subduction to India-Asia continental collision,and formed a series of Cu,Pb,Zn,Au,Ag,Mo and other polymetallic deposits related to intermediate acid volcanic-intrusive rocks in this belt.Numerous remarkable studies have been carried out on the porphyry-skarn deposits in the early stage,but little study focused on the epithermal deposit which associate with porphyry metallogenic system.The Sinongduo deposit was the first low sulfide epithermal Ag-Pb-Zn deposit in Gangdese belt,detailed metallogenesis studies on this deposit studies could improve the regional metallogenic theory,furthermore,it provides a best reference pattern for the new ideas about developing regional prospecting and relationship analysis between Linzizong volcanic rocks and mineralization.The Sinongduo Ag-Pb-Zn deposit is located in West Gangdese back-arc fault uplift belt.Controlled by radial fracture structure,ore bodies are occured in the Qundian volcano rock in the Dianzhong formation volcanic rocks of Linzizong group,includes the hydrothermal vein type lead zinc silver ore body in fault structure,the concealed explosive breccia type lead zinc silver ore body at the front of the fault,and the independent silver ore body on the top of the fault.The ore minerals are mainly composed of pyrite,galena,sphalerite and independent silver minerals;The gangue mineral is low-temperature alteration mineral assemblage with quartz+ cryptocrystalline siliceous minerals(chalcedony,Jasper,fiorite)+adularia+illite+ sericite+carbonate etc,the ore structure are mainly presented as massive,vein-veinnet,breccia,which show a open space environment;three main types of the ore texture formed by crystallization,metasomatism and exsolution.The volcanic-intrusive rocks in Sinongduo area were formed in Paleocene 65 ~ 62 Ma and Eocene 12 Ma,which reflected the different dynamic background respectively.The volcanic rocks were belong to high potassium peraluminous rocks,trace element composition indicates that the rocks have the arc volcanoes geochemical characteristics,Sr-Nd isotope constrainted the mixing magma source with mantle and crustal derived melts.The geochemical characteristics of granite porphyry show adakite affinity,and related to the circulation of residual oceanic crust and partial melting of lower crust.The 40Ar-39 Ar data of altered illite limited the deposit metallogenic age are 65~62Ma,consistent with the eruption of volcanic rocks,suggested the Sinongduo deposit formed in continental marginal arc background of the ocean-land conversion system.The trace element composition of the ore bearing quartz vein inherited the volcanic rocks geochemical characteristics.The S and Pb isotopes of the metal sulfides reveal the source of magma S and upper crust Pb,and show obvious homologous evolution relationship with the volcanic rocks of the Dianzhong formation.The Si-H-O isotope in quartz veins shows that the precipitation is the dominant source of fluid,and a strong water/rock reaction is occurred by geothermal driving.The ore-forming fluid is characterized by low temperature(200°C ~270°C)and low salinity(2.5%~3.7% Na Cleqv)with high concentrationH2 O,CO2,and little reducing gas like CH4,N2,CO,The ionic composition is mainly Ca2+-Na+-K+-Cl--SO42-,and contains a small amount of F-,Br-,belonging to the simple H2O-NaCl system.Additionally,the ore-forming fluid Ph value is between 5.592~5.77,the average value is 5.719;the metallogenic pressure between 130.3 bar~239.6 bar,average 213.7bar,witch show the nature of low sulphidation epithermal deposits with low temperature,low salinity,low pressure and near neutral reduction fluid.The metallogenic characteristics and H-O isotopic results indicate that the fluid boiling induced by the explosive decompression is the important factors causing mineral deposition.Combining with the geological phenomenon and the results of this study,this paper provide various evidence and pointed out that Sinongduo Ag-Pb-Zn deposit has the metallogenic characteristics of low sulfide epithermal deposit.At the same time,analysed the mechanism of the diagenesis and mineralization.Basis on the regional data analyses,we suggest that the Sinongduo deposit has a close genetic relationship with Linzizong Group volcanic rocks,and was coupling with those skarn and hydrothermal vein type deposits in northern margin of Gangdese metallogenic belt.With the metallogenic model and geochemical survey results,we have conducted metallogenic prediction for Sinongduo district and realized the Au ore bodies in the Linzizong volcanic rocks.This achievement proposed new idea and methods of regional prospecting.