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Mesozoic Multiple Magmatic Activities and Molybdenum Mineralization in the Chalukou Ore District, Da Hinggan Mountains
详细信息   
摘要
Located at the northernmost corner of the Deerbugan metallogenic belt,the newly found Chalukou deposit is the largest molybdenum polymetallic deposit ever discovered in Northeast China.It contains 1.24 Mt of Mo(metal),0.26 Mt of Pb-Zn and 2773 t of Ag.As a part of the evaluation project on the molybdenum deposit,mineral exploration around the Chalukou deposit is still under way.During the Mesozoic Yanshanian orogeny,intense extensional tectonic and igneous activities resulted in the intrusion of large-scale acid volcanic rocks and granitoids at Chalukou and its neighboring areas.The Chalukou granite porphyry and rhyolite porphyry stocks were emplaced in the Jurassic volcano-sedimentary sequence controlled by NE-and NW-trending fault zones. Rock-forming minerals of the two porphyry stocks are mainly quartz,microcline,microperthite and plagioclase associated with minor biotite and sericite.Accessory minerals are magnetite,apatite,zircon,sphene and allanite.The two porphyry stocks are petrochemically characterized by relatively high SiO2,K2O,F,Rb,Nb,U,Th,Y and Yb and obviously low CaO,Fe2O3+FeO,MgO,Sr,Ba and REE,with positive ?Nd(t) value.Molybdenum mineralization occurs within the Chalukou porphyry stocks and volcanic rocks as stockwork,veins,veinlets and disseminated blocks.The molybdenum ore consists of pyrite,chalcopyrite,molybdenite,sphalerite, galena,magnetite,pyrrhotite,K-feldspar,fluorite,quartz,sericite and chlorite.As the most important economic molybdenum mineral, molybdenite occurs in the forms of impregnation,radioflake,thin film,aggregated flakes and veinlet.Re-Os isotopic age dating of eight molybdenite separates from the Chalukou deposit yielded an isochron age of 146.9 ± 0.8 Ma and model ages ranging from 146.2 to 149.1Ma with an average value of 147.4Ma.As the Re-Os isochron age is in agreement with field geological evidence,the authors hold that the Re-Os isochron age represents the major ore-forming time of the Chalukou deposit,which corresponded to geodynamical setting of intracontinental extension after the collision of the Siberian plate and North China-Mongolian tract. Hydrothermal alteration at Chalukou shows rough zonation of K feldspathization,greisenization,silicification,and propylitization from the two porphyry stocks outwards.Of these alterations,silicification and greisenization are well developed,and have intimate relationship with molybdenum mineralization.Studies show that all these ore-forming materials,dynamic forces and fluids might have been provided by the Yanshanian granitoid magma.Moreover,the accumulation conditions of the ore-forming materials were offered by the NW-and NE-trending lineament and ring fractured zone as well as Jurassic volcano-sedimentary rocks.A comparative analysis indicates that the Chalukou deposit belongs to high-F porphyry type molybdenum deposit.The Late Jurassic lithosphere stretching and asthenosphere upwelling might have resulted in the melting of the thickened lower crustal material,and then produced molybdenum-rich granitoid magma.When the magma moved along structures developed within the upper crust,ore-bearing fluid derived from condensation fraction of the magma filled the structures and replaced country rocks,thus forming the deposits.Therefore, it is believed that the Chalukou deposit was a product of the Late Jurassic intraplate granitoid magma in the northeastern part of the North China-Mongolian tract.The genetic model and mineral exploration criteria of the Chalukou deposit can also be used as useful tools during the comprehensive evaluation of concealed molybdenum deposits in northern Da Hinggan Mountains.

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