Geological Characteristics And Ore Genesis Of Wulanbaiqi Pb-Zn-Ag Polymetalic Deposit In Inner Mongolia

Located at West Ujimqin Banner,Inner Mongolia,the Wulanbaiqi Pb-Zn-Ag polymetallic deposit is tectonically situated at the suture zone between the Hegenshan ophiolite and Solon-Xar Moron deep fault.The area has favorable metallogenic geological conditions and abundant copper,lead,zinc and silver mineral resources.Based on collecting regional geological data and detailed field geological survey,comprehensive study of metallogenic geological conditions,geological characteristics and ore genesis of the Wulanbaiqi Pb-Zn-Ag polymetallic deposit.The analysis of zircon U-Pb isotopic shows that the zircon U-Pb dating result of the rhyolitic crystal fragment tuff is 129.39±0.78 Ma,the rhyolite is 131.98±0.89 Ma,the granite is 136.37±0.57 Ma,all belonging to Early Cretaceous,all zircons have shown the magmatic origin,measuring age on behalf the rock formation age.The geochemistry of major elements indicates that the volcanic rocks in the mining area are characterized by high-SiO₂,belonging to intermediate-acid rocks,quasi-aluminous to peraluminous rocks,and high-potassium calc-alkaline series to shoshonite series rocks.The trace elements geochemistry are characterized evidently by enrichment Th,U,K and LREE,mediate depletion of HSFE like Ta,Nb,P,Ti,Sr and HREE.The study of tectonic environment shows that the rocks in the mining area was formed in the continental margin arc environment or island arc setting environment under the condition of plate subduction,probably related to the subduction of the Pacific plate to the Eurasian plate.The deposit,mainly occurring in the rhyolitic crystal fragment tuff of Lower Cretaceous Baiyingaolao Formation,is characterized by the stratoid bed.The main metallic minerals are composed of sphalerite,galena,chalcopyrite and some silver-bearing sulfide,minority are pyrite and arsenopyrite.The main textures of the ore are subhedral or xenomorphic granula texture,unmixing of solid solution texture,replacement remnant texture and so on.The metallization has close relationship with such wall-rock alteration as silicification,chloritization,sericitization.Ore-forming process of the deposit can be divided into three stages,including arsenopyrite-pyrite stage,multi-metal sulfide stages and sericite-chlorite stages.The microthermometry of fluid inclusions of the Wulanbaiqi Pb-Zn-Ag polymetallic deposit indicates that the ore-forming fluid belongs to the low-medium temperature,low density,low salinity fluid system.Based on estimating the ore-forming pressure and metallogenic depth,it is concluded that the deposit is formed under the condition of low pressure and shallow depth.The analysis of S and Pb isotopes indicates that the ore-forming materials of the Wulanbaiqi Pb-Zn-Ag polymetallic deposit should come from upper mantle with commingling of the crust.Sulfur isotope did not reach the equilibrium of sulfur isotope fractionation,reflecting the characteristics of rapid precipitation of ore-forming materials.This paper concludes that the deposit is one of the vein filling,mesothermal-epithermal polymetallic deposit,controlled by the Late Yanshanian intermediate-acidic volcanic-subvolcanic rocks and faults.