Ore Genesis And Tectonic Setting Of The Banduo Lead-Zinc Deposit In Somdo Area,Tibet

The Banduo Pb–Zn deposit in the Somdo area of Tibet is a newly discovered medium-sized deposit hosted by the Late Triassic granodiorite in the central Gangdese.It is of great significance for the determination of the ore-forming period of the Gangdese metallogenic belt and the regional mineralization theory.Due to the short discovery time,high altitude and low degree of exploration,the theoretical research is very weak.Therefore,based on the study of geological background of the deposit,a combination of the U–Pb dating,zircon Hf isotope dating,sulfide Rb–Sr dating,fluid inclusions microthermometry,Laser Raman spectroscopy,whole-rock geochemistry and H–O–S–Pb isotope test was completed to detemine the ore genesis,mineralization age and tectonic setting of the deposit.The results show that the Banduo Pb-Zn deposit belongs to the mesotherm hydrothermal vein-type with close space-time and genetic relationship with granodiorite,and experienced three metallogenic stages:quartz–arsenopyrite–pyrite–pyrrhotite stage?I?,quartz–pyrite–sphalerite–chalcopyrite–galena stage?II?and quartz–calcite stage?III?.The weighted-mean U–Pb ages of zircon from the granodiorite are203.8±1.7 Ma?MSWD=0.53?and 206.1±0.4 Ma?MSWD=0.098?,the weighted-mean Rb–Sr isochron age of sulfides from the Pb–Zn mineralization?202.7±2.3 Ma;MSWD=1.3?indicates the crystallization age of granodiorite crystallization and associated polymetallic mineralization is the Late Triassic,which means Banduo Pb–Zn deposit is the oldest deposit in the Gangdese belt.Fluid inclusions microthermometry,Laser Raman spectroscopy and H–O isotope test show the ore-forming fluid of the deposit is NaCl–H₂O system.Fluid inclusions are composed of vapor-liquid two-phase inclusions with a small amount of pure gas inclusions in the earlier mineralization stage and vapor-liquid two-phase inclusions in the mid and late stage,and there is fluid boiling phenomenon.In the middle stages,the fluid composition is meteoric water.The major ore-forming stage's fluid is characterized by medium temperature?266.3°C average?,low salinity?3.39%NaCl eqv everage?and low density?0.79 g/cm³ average?.The S-Pb-Sr isotope characteristics of metal sulfides indicate that the ore-forming materials are mainly derived from the mantle and the crust.The weakly altered granodiorite in the deposit is characterized by SiO₂=66.78–67.85wt.%,K₂O=2.57–3.37 wt.%,Al₂O₃=13.53–15.15 wt.%,and Mg#=45.5–51.3.It is enriched in LREE and LILE,depleted in HREE and HFSE,and exhibits a negative Eu anomaly.They have positive and negative?_Hf?t?values?-2.5–1.7?and model ages?TC DM?of 1.14–1.40 Ga.Hf isotopes and whole-rock geochemistry indicate the source of the Late Triassic granodiorite likely consisted of a mixture of mantle and ancient lower-crust material,which underwent variable degrees of fractional crystallization.Combined with regional data,the Banduo Pb–Zn deposit and the Late Triassic granodiorite likely formed as a result of the northward subduction of the Neo-Tethys Ocean.Through the research and determination of the metallogenic age and tectonic setting of the Banduo Pb–Zn deposit,it can be concluded that the Late Triassic magmatism and associated mineralization in the Gangdese were resulted from the northward subduction of the Neo-Tethys Ocean.On the other hand,this extends the age of granitoid-hosted mineralization to the Late Triassic.Therefore,the non-ferrous metallogenic potential of the Late Triassic granite in the Gangdese metallogenic belt cannot be ignored,and should be listed as an important ore-prospecting direction.