Geological Characteristics And Genesis Of The Dongshengmiao Polymetallic-pyrite Deposit, Inner Mongolia

The Dongshengmiao polymetallic-pyrite deposit is located in the western part of northern margin of North China Craton?NCC?,the northern part of Inner Mongolia geoaxis,and the secondary rift basin of the Langshan-Baiyun Obo rift zone,which belongs to the Mesoproterozoic Langshan-Zhartaishan polymetallic metallogenic belt.There are also occurred some large and super-large polymetallic deposits,such as Huogeqi,Tanyaokou and Jiashengpan.The metallogenic geological environment in the metallogenic belt.Based on the systematic field geological survey of this area,the study on geochemical characteristics,petrographic and mineralogical facies of wall rock and ore,and S isotope and electron probe microanalysis of typical metallic sulfides,the metallogenic geological characteristics and geochemical characteristics of the deposit are fully discussed in this paper.Based on the previous research results,the genesis of the deposit is discussed,and the metallogenic model is established.The results obtained in this paper are as follows:1.The main exposed strata are Langshan Group and Cretaceous Lisangou Formation.The structure is dominated by NE-trending faults and folds.Indosinian dimica-monzonitic granite and a large number of granite-porphyry and metaquartz-albite-porphyry dikes are developed in the northwest of the area.Sericite-graphite schist and graphite-bearing dolomite marble of the second to the fourth member of the Langshan Group are closely related to mineralization,and NE-trending faults and folds are the main ore-controlling factors.2.The original rocks of sericite graphite schist and graphite-bearing dolomite marble were restored,which are wall rock of the deposit.The results show that the two original rocks are sandy carbonaceous mudstone,carbonaceous sandy mudstone,sandy mudstone and carbonaceous dolomite,sediment-bearing dolomite,dolomite,respectively.3.Ore bodies mainly occurred as stratiform,stratoid,and minor lenticular and vein-like,which hosted in sericite graphite schist and graphite-bearing dolomite marble of the second rock formation of the Langshan Group.The occurrence of ore bodies is consistent with the strata and always folds with the strata.Ore minerals are mainly composed of pyrite,pyrrhotite,sphalerite and galena,and minor chalcopyrite;gangue minerals mainly consist of quartz,dolomite,calcite and graphite.Ore types are mainly composed of graphite schist and graphite-bearing dolomite marble;wall rock alteration is weak,with only local carbonation,silicification,sericitization.4.The trace and REE compositions of ores and wall rock show that trace elements of sericite graphite schist and graphite-bearing dolomite marble are characterized by enrichment of Ba,U and depletion of high field strength elements such as Th,Sr,P and Ti,reflecting the rapid accumulation characteristics of shallow marine sediments.The total REE varies from 83.1 to 194.99 ppm,8.31 to 31.29 ppm,and the value of LREE/HREE is 9.01.The average values of?Ce are 1.06 and 1.03,respectively.The anomalous characteristics are not obvious,which can be inferred as anoxicreductive sedimentary environment.The REE composition characteristics of ore and wall rock are basically the same.It is presumed that the ore-forming material comes from the wall rock,which is consistent with the forming environment of the wall rock.5.According to the results of electron probe microanalysis,the composition characteristics of sphalerite,pyrrhotite and pyrite are determined.The results show that sphalerite posseses high Fe content?8.93%⁹.52%?,and Zn/Fe ratio varies from 5.7 to 8.6,with an average value of 6.95,which is similar to sphalerite?Zn/Fe=7?from the genesis of submarine sedimentary exhalation.The atomic content of pyrrhotite is range from 46.20%to 46.49%,belonging to monoclinic pyrrhotite,and its molecular formula is approximately Fe₆S₇^Fe₇S₈.The Co/Ni ratio of pyrite is greater than and less than 1,which is consistent of the genetic characteristics of sedimentary exhalation.In addition,the previous studies on sulfur isotope of ore also showed that sulfur originated mainly from seawater sulfate and minor from magma.According to the studies mentioned above,we concluded that the Dongshengmiao polymetallic-pyrite deposit belongs to sedimentary exhalation deposit of SMS type affected by submarine volcanic activities in the same period,and the tectonic setting of metallogenesis is passive continental margin environment.