Molybdenum Isotopic Composition Of Molybdenite From Eastern Qinling

Eastern Qinling is the biggest molybdenum(Mo)ore belt in china and it contains about 8 million tons of Mo.The mechanism of mineralization,Mo source and the influence of hydrothermal fluid alteration have long been under great debate.These ore deposits mainly formed during late Triassic,early Cretaceous and late Cretaceous,and the mineralization in these three period are probably related to the collision between Yangtze Plate and North China Plate,the back-arc extension caused by subduction of Iznagy Plate to Pacific plate,and massive lithospheric thinning and delamination in eastern China,respectively.With the development of metal stable isotope measurements,Mo isotope has been widely used to study the formation of ore deposits.Recently,Mo isotopic fractionation in many geological processes has been recognized.In mineralization processes,the different coordination structures of Mo in various mineral phases could lead to Mo isotopic fractionation.There are three types of Mo isotopic fractionation pattern in magmatic hydrothermal process which all lead to heavy Mo isotopes enrichment in the mineral phases.Besides,the crystal type of molybdenite,crystallization temperature and the mineralization age of the molybdenite could influence the Mo isotopic composition of molybdenite.Therefore,Mo isotope analysis can help to understand the Mo sources,distinguish the genetic types and evaluate the mineralization temperature and batches of the ore deposit.In this study,we have chosen several typical Mo ore deposits located in Nannihu,Sandaozhuang,Shanggusi,Nangou,Ruyang(donggou)and Yechangping for Mo isotope analysis.The ore-forming types include porphyry,quartz vein,Pegmatite and skarn.Nannihu and Sandaozhuang granites are considered as peraluminous and the mineralization age is 144.3Ma.Shanggusi granite is characterized by high alkaline contents,considered as weak per-aluminous and the mineralization age is 123Ma.Nangou granite is characterized by high silica,low titanium,high alkaline and high aluminum contents and the mineralization age is 107?103Ma.Ruyang(donggou)granite is characterized by high silica,high alkaline,low iron and low magnesium contents,and belongs to aluminous and per-aluminous rocks and the mineralization age is 116?115Ma.Yechangping granite is moyite which has experienced multiple alteration processes and the mineralization age is 145Ma.We have analyzed the crystal types,Mo contents and Mo isotopic compositions of molybdenite from eastern Qinling using X-ray diffraction(XRD),inductively coupled plasma mass spectrometry(ICP-MS)and multiple collector inductively coupled plasma mass spectrometry(MC-ICP-MS),respectively.Then based on these results,we try to investigate the factors which could influence the Mo isotopic composition of molybdenite.Several major findings are as follows:1.The mean Mo isotopic composition of molybdenite from eastern Qinling is lighter than the Mo isotopic compositions of the continental crust.When combined with the study of major and trace elements and other isotopic systems of the molybdenite,we can approximately estimate that the source of ore-forming material comes from the lower crust.The large range of Mo isotopic composition of molybdenite may be influenced by other factors.2.We found that the crystal type(2H and 3R),crystallization temperature and the mineralization age of the molybdenite have no significant relationship with Mo isotopic composition of the ore deposit.3.We found that the variation of the Mo isotopic composition of molybdenite is mainly due to the influence of the veins in the alteration process.With the invasion of the hydrothermal veins,the Mo migrate between the rock and fluids,which might have led to the Mo isotopic fractionation.