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Re-Os Isotopic Age Dating of Molybdenite Separates from the Chaobuleng Skarn Iron-Polymetallic Deposit,Dong Ujimqin Banner,Inner Mongolia

NIE Fengjun1) ZHANG Wanyi1) DU Andao2) JIANG Sihong1) LIU Yan1)1) Institute of Mineral Resources,CAGS,Beijing 100037;2) National Research Center of Geoanalysis,Beijing 100037  
Located in the easternmost part of the Chaganobo-Aoyoute-Chaobuleng Early Paleozoic tectono-magmatic belt on the southern edge of the Siberian plate,the Chaobuleng deposit is a medium-size skarn iron-polymetallic deposit occurring in central-eastern Inner Mongolia.Systematic drilling results show that the deposit contains metal reserves of Fe 1.9 Mt,Bi 17×104 t,Ag 222 t,Sn 0.61×104 t,Cu 3.06×104 t,Au 1.68 t,Pb 9518 t and W 3979 t.The unique geology of the deposit has attracted the attention of ore geologists from China and abroad.During the Mesozoic Yanshanian orogeny,intensive tectonic and igneous activities resulted in the large-scale granitoid plutonism and relevant contact metasomatism in Chaobuleng and its neighboring areas.The Chaobuleng gabbro and granitoid stocks were emplaced in the Middle Devonian sedimentary sequence of the Taerbagete Group,controlled by the NE-trending fault zone.A large number of skarn blocks are located along the contact zone between these two intrusive stocks and their carbonate wall rocks.Although the gabbro and granite stocks show obvious differences in textures and mineral assemblages,both of them belong to alkaline-rich calc-alkaline igneous series.Iron-polymetallic mineralization occurs exclusively along the contact zone between the Mesozoic intrusive stocks and the Middle Devonian sedimentary sequences in the forms of pillars,cylinders,stratoid lenses and layers.The main metallic minerals of the iron-polymetallic ore from Chaobuleng are magnetite,pyrite,chalcopyrite,molybdenite,sphalerite,pyrrhotite,scheelite,galena,arsenopyrite,marcasite,bornite,bismutinite,galenobismutite,and native bismuth.The gangue mineral assemblage consists mainly of diopside,amphibole,grossular,andradite,almandine,vesuvianite,tremolite,actinolite,plagioclase,epidote,quartz,biotite,sericite,chlorite and calcite.Re-Os isotopic age dating of four molybdenite separates from the No.Ⅰ mineralized zone gives an isochron age of(140.7 ± 1.8) Ma,and the model ages range from 139.0 to 140.9 Ma with an average value of 139.5 Ma.As the Re-Os isochron age is in agreement with the field geological evidence,and the molybdenite has co-existing relations with chalcopyrite,pyrrhotite and pyrite,the authors hold that the Re-Os isochron age represents the major ore-forming time of the skarn iron-polymetallic mineralization occurring in the Chaobuleng deposit and its neighboring areas.Combined with field geological observations and petrological evidence,it is suggested that the ore-forming materials might have been derived from a mixed source of depleted mantle-and crustal-derived magma and related fluids.It is also considered that the Early Cretaceous intra-plate granitoid plutonism and the relevant iron-polymetallic mineralization were well developed in the Chaobuleng mineralized district and its neighboring areas after the Late Jurassic amalgamation of the North China massif and the Siberia plate.Therefore,the Chaobuleng mineralized district and its neighboring areas have a great potential for Mesozoic Yanshanian skarn iron-polymetallic deposits.
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