The Study Of The Metallogenic Characteristics And Genesis Of The Bianjiadayuan Pb-Zn-Ag Deposit In Inner Mongolia, China

Located in the southern section of the Daxing’anling metallogenic belt, theBianjiadayuan Pb-Zn-Ag deposit is a typical hydrothermal vein-type polymetallicdeposit. Through systematic syudy of geological and geochemical characteristics ofthe Bianjiadayuan Pb-Zn-Ag deposit, conclusions concerning the mineral composition,the occurance of silver, the genesis and mineralization mechanism of this deposit havebeen drawn as follows:(1) In order to determine the diagenetic age of the deposit, the LA-ICP-MS U-Pbage study was conducted on the zircon in the gabbro and quartz porphyry associatedwith the mineralization. The results are133.19Ma and140Ma for the gabbro andquartz porphyry, respectively, indicating that the intrusive rocks resulted from themagmatism in early Cretaceous period. The petrological and geochemical studiesshow that the Bianjiadayuan quartz porphyry are characterized by high K calc-alkllinemagama, strongly peraluminous, on the other hand the characteristics of trace elementalso show that the quartz porphyry of Bianjiadayuan are similar to the typical A1within-plate anorogenic granite, indicating that the magma came from lithosphericmantle and resided in tensional tectonic setting.(2) On the basis of detailed field geological survey and systematic laboratorystudy including microscopic identification and electron analysis, the mineralcomposition and the occurance of silver has been identified. The metalliferousminerals of Bianjiadayuan Pb-Zn-Ag deposit are mainly consist of arsenopyrite，pyrite，galena，chalcopyrite，sphalerite，pyrrhotite，falkmanite and so on. Silverminerals are mainly consist of pyrrhotite，freibergite tetrahedrite and. There threeforms of silver in the ores, Which are independent silver, the silver of microscopicinclusions and ospmophous silver.(3) Through the fluid inclusion microthermometry, Laser Raman spectroscopicand hydrogen oxygen isotopes studies suggest that the main types of fluid inclusionsin the Bianjiadayuan Pb-Zn-Ag deposit are only three, which are vapor-rich phaseinclusions, liquid-rich phase inclusions and pure liquid phase inclusions. The study of fluid inclusions shows that the ore-forming fluid is middle in temperature，low-to-middle in salinity, low density, low-to-middle in pressure and medium-lowdepth. The gas composition of fluid inclusions are mainly H2O, CO2and CH4. Thestudy of hydrogen oxygen isotopes show that the fluid inclusions came from themixing of magma water and meteoric water.(4) The results of stable isotopic geochemistry show that the carbon of minera-lization hydrothermal are mainly come from the magma, and the later hydrothermalore fluids show the characteristics of mixture fluids of meteoric water with magmaticwater as the meteoric water add. The sulfur and lead isotopic compositions of depositshow that the sulfur is mainly come from the magmatic source. The lead isotopic insulfides indicate that the mineralization may be induced by the magmatism, and theorigin of lead in the deposit was mainly from mantle source as well as little from crustsource.(5) The history of regional tectonic evolution showed that the later of Yanshanianis the transformation of the tectonic regime from NS to nearly EW directions, and thisperiod is also considered to be a peak of mineralization and magmatism of thesouthern Daxing’anling. Based on the ore-forming age analysis of typical in themineralization on the belt and the geological characteristics of BianjiadayuanPb-Zn-Ag deposit, we concluded that the Bianjiandayuan Pb-Zn-Ag deposit is likelyformed in the late Yanshannian, has a closely relationship with quartz porphyry.