Geological And Geochemical Characteristics Of Porphyry Molybdenum Deposit, East Qinling Gold Heap City And Metallogenic Dynamics Background,

The East Qinling Mo deposit belt located in southern margin of the North China Craton which with a great amount of Mesozoic granites.The detailed geological and geochemical characteristic as well as LA-ICP-MS zircon U-Pb dating of the Jinduicheng super large porphyry Mo deposit,Huashan pluton,Laoniushan pluton,and the Heyu pluton have been carried out in this thesis.The Jinduicheng granitic porphyry,Huashan pluton,Heyu pluton as well as the Laoniushan pluton have the similar geochemical features.They are all belong to high K cal-alkaline granites with high SiO₂,Al₂O₃ and K₂O+Na₂O contents.These plutons are I-type granites which enriched in large ion lithophile elements(LILE),depleted in high field strength elements(HFSE),with obvious fractionation between LREE and HREE,and low Y,Yb contents(Y＜18μg/g,Yb＜2μg/g).The characteristics mentioned above indicate that, the granites were formed by partial melting of the thickened lower crust with the fractionation or residence of Plagioclase+hornblende assemblage.The LA-ICP-MS zircon U-Pb ages of the Jinduicheng granitic porphyry,Huashan pluton, Laoniushan pluton and the Heyu pluton are 141.5±1.5Ma,133.8±1.1Ma,146.1±4.1Ma, 134.5±1.5Ma,respectively.They all have low I_Sr,negativeε_Nd(t) andε_Hf(t) values,and depleted in radiogenic lead.Zircon Hf isotopic characteristics show that,they are derived from the accretionary crust formed in about 2.0Ga～1.8Ga.But the inhomogenous Hf isotopic compositions of the Jinduicheng granitic porphyry suggest that the primitive magma has been mixed by the mantle materials.The ores and pyrites share the similar Chondrite-normalized REE patterns with the granitic porphyry in the Jinduicheng Mo deposit.Mo has consistently added to the country rocks in the processes of alteration and mineralization.All of the characteristics above indicate that the ore-bearing hydrothermal fluids are derived from the porphyry but not the country rocks.The lead,sulfur,carbon,hydrogen-oxygen and Helium-argon isotopic characteristics show that,the metallogenetic materiels and fluids of Jinduicheng deposit have the same sources with the porphyry,which were all derived from the mantle in the early-stage with addition of the meteoric water in the late-stage.The diagenesis and mineralization of the East Qinling Mo deposits occurred in the same dynamic setting.The formation of the plutons is associated with A-type subduction of the Qinling Orogen in Mesozoic.The granites and Mo deposits were formed in the transition stage geodynamic setting from compression to extension in Jurassic-Cretaceous(about 140Ma).During this process,the conditions of decompression and geothermal increasing and result in partial melting of the thickend lower crust(2.1Ga～1.7Ga) in the North China Craton. The magma generated by the partial melting formed the collisional transformation-type granites.But nearby the fault,the basaltic magma derived from asthenospher upwelling and underplating induced the partial melting of the lower crust(2.1Ga～1.7Ga),the magma formed by the partial melting were mixing with the basaltic magma and forming the Jinduicheng granitic porphyry and porphyry Mo deposit.