Ore Genesis And Metallogenic Background Of Biliya Valley Lead–Zinc Polymetallic Deposit On The Western Slope Of Great Xing'an Range

The Biliya valley Cu?Ag?–Pb–Zn deposit,located in Derbugan metallogenic belt,western slope of Great Xing'an Range,which was discovered in resent years.In order to further explore the genetic type of mineral deposit and metallogenic background,this paper studies the deposit geology,fluid inclusions,chronology,element and isotope geochemistry systematically on the basis of previous studies.The main results achieved in this paper are as follows.The study of the geological characteristics of the mining area and the deposit reveals that,in the study area,the main outcropped geological bodies are the volcanic rocks of Tamulangou Formation and Manketouebo Formation.Andesitic porphyrite is closely associated with the ore bodies.The ore bodies are mainly controlled by a series of NW faults that formed during the sinistral shearing of the Derbugan deep fault.The deposit is composed of 87 ore bodies?including 16 main orebodies?which are vein like,and the ore bodies are mainly hosted in volcanic rocks of Tamulangou Formation and Manketouebo Formation.The ore minerals are mainly pyrite,sphalerite,galena,chalcopyrite,secondly tetrahedrite,the silver bearing mineral is argentite.The types of wall rock alterations are silicification,carbonation,sericitization,illicization,propylitization?chloritization?and flouritization.From the center of ore bodies to both sides,the alteration zones are defined as follows:silicification–carbonation–flouritization zone,sericitization–illicization zone and propylitization?chloritization?zone.The process of mineralization can be classified into three main stages:?1?quartz+pyrite+sphalerite stage???;?2?quartz+pyrite+sphalerite+galena+argentite+chalcopyrite±tetrahedrite stage???;?3?quartz+pyrite+calcite+fluorite±opal stage???.Systematically study on the fluid inclusions?FIs?of quartz and sphalerite shows that the FIs in quartz of the early stage???are composed of liquid–rich?WL type?and H₂O–CO₂?C type?types,their homogenization temperature,w?NaCl?,and density vary from 188?to 254?,1.83%to 4.80%,0.81g/cm³ to 0.92g/cm³,respectively,and the fluid in this stage generally belongs to the H₂O–NaCl–CO₂ system with medium–low temperature and low salinity.The FIs in quartz and sphalerite of the main stage???are composed of liquid–rich?WL type?type,their homogenization temperature,w?NaCl?,and density vary from 160?to 188?,3.71%to 7.17%,0.92g/cm³ to 0.96g/cm³,respectively,and the fluid in this stage generally belongs to the H₂O–NaCl–CH₄ system with low temperature and medium–low salinity.The FIs in quartz of the late stage???are composed of liquid–rich?WL type?and liquid?L type?types,their homogenization temperature,w?NaCl?,and density vary from 130?to 165?,1.22%to 3.55%,0.93g/cm³to 0.95g/cm³,respectively,and the fluid in this stage generally belongs to the H₂O–NaCl system with low temperature and low salinity.As the temperature decreases,the ore–forming fluid gradually evolved from H₂O–NaCl–CO₂ system to H₂O–NaCl system.Hydrogen–oxygen isotopic study of quartz fluid inclusions of altered minerals shows that,the early stage of ore–forming fluid shows some characteristics of mixture fluid of meteoric water with magma water.With the increase of the mixing proportion of meteoric water,fluid of mineralization in the main and late stages gradually evolved to be dominated by meteoric water.The Pb–Sr isotope study of single sulfide minerals reveals that metallogenic material is derived from crust–mantle mixing,and it may come from the young lower crust with MORB property,mixed with mantle material.Study on isotopic chronology of the host rock?rhyolite?,andesitic porphyrite and ore sulfides reveals that zircon U–Pb dating result from the host rock?rhyolite?is163.7±2.0Ma.Zircon U–Pb dating result from Andesitic porphyrite closely associated with the ore bodies is 133.9±0.9Ma.Metal sulfide Rb–Sr dating result suggests that the Rb–Sr isochron age of the Biliya Valley deposit is 131.3±2.4Ma.It reveals that there are two magmatic events of Middle Jurassic and early Cretaceous in the late Mesozoic.In a word,the mineralization age of the deposit is early cretaceous,closed to the thermal event of the associated subvolcanic rocks?andesite porphyrite?in the mining area.Geochemical characteristics of the host rock?rhyolite?and associated subvolcanic rocks?andesitic porphyrite?reveal that the host rock?rhyolite?is rich in large ion lithophile elements?LILE?such as Rb,K et al and light rare–earth elements?LREE?,in addition,it is depleted in high field strength elements?HFSE?such as Nb,Ta,Ti et al and heavy rare–earth elements?HREE?,indicating that the rocks were formed in an island arc or in the environment of magma arc in the continental margin.The diagenetic material is derived from mixture of crust and mantle.Andesitic porphyrite is rich in large ion lithophile elements?LILE?and light rare–earth elements?LREE?,relatively it is depleted in high field strength elements?HFSE?and heavy rare–earth elements?HREE?.In addition,andesitic porphyrite is rich in K and poor in Mg.It indicates that the rocks were formed in continental margin arc volcanic rocks.It was likely to be originated from crustal melting induced by lower crustal delamination coupled with deep process of asthenosphere upwelling.The rocks were formed by partial melting of the Mesoproterozoic lower crust with minor input of depleted mantle materials.The above characteristics indicate the the Biliya Valley copper?silver?lead zinc deposit is a low–sulfidation epithermal deposit,which has the basic characteristics of low–sulfidation epithermal deposit at home and abroad.The Great Xing'an Range was in a compression environment under the influence of the subduction of the paleo–pacific plate in Jurassic,and the host rock rhyolite was formed in this period.During the period of late Jurassic–Early Cretaceous,due to the change of subduction direction of Pacific plate,the tectonic environment of Great Xing'an Range transformed from compression to extension.In the early Cretaceous,the delamination of the thickened crust induced the upwelling of the asthenosphere,the upwelling asthenosphere increased the thermal gradient and caused the partial melting of the thickened lower crust,as well as provide a potential heat resource for the widespread intermediate–acid magmatism in the area region.The Biliya Valley deposit was formed under the background of the subduction of Pacific plate,the mineralization occurred in the post–subduction extensional tectonic environment.