Study On Genesis Of Jiangligou W-(Cu-Mo)Deposit,West Qinling Orogenic Belt,NW China:Constraints From Geological,Geochronology And Geochemical Evidence

Tungsten metal is an indispensable and irreplaceable important metal raw material for the development of modern electronics,energy,national defense and other high-tech industries.Because of its important national defense and high-tech industrial value,scarcity,irreplaceability,and non-uniformity of global deposits,tungsten has been listed as a "critical metal" by countries in Europe,the USA,Japan,and the United Kingdom for strategic reserves.Tungsten,as an important rare metal in China,can only be mined for 23 years,which is not optimistic.The W deposits in China are mainly located in the South China metallogenic belt and the Jiangnan paleocontinent porphyry skarn metallogenic belt.In recent years,a series of Triassic porphyry-skarn Cu-Au-W-Mo deposits have been discovered in the West Qinling Orogen Belt(WQB),which has become a region with great potential for searching for Triassic deposits.The Jiangligou skarn W-(Cu-Mo)deposit is the only skarn tungsten deposit of medium size,thus,it is used as a typical deposit to carry out geological characteristics of the deposit,ore-forming fluid system,skarn mineralogy and mineral precipitation process research.Finally,the genesis model of the deposit is established with the goal to provide theoretical support and practical guidance for finding similar deposits under comparable geological conditions in the region.The closure of the Paleo-Tethys Ocean in the Triassic period along the current Mianlue-Animaqin suture zone marks the completion of the final collage of the North China Block(NCB)and South China Block(SCB).Extensively Triassic magmatism has developed in the Qinling orogenic belt during the subduction,collision and post-collision,which provides favorable conditions for mineralization in this area.LA-ICP-MS zircon U-Pb dating results of the Jiangligou granitic complex(JGC)show that:the three lithofacies biotite monzonite granite,granite porphyry and fine-grained granite have weighted average ages of 229.1±1.8Ma(MSWD=0.82),222.5±1.4Ma(MSWD=0.41)and 217.1±1.8Ma(MSWD=1.4),respectively,indicating that the rock was crystallized in the Late Triassic.Geochemically,they are high silicon(SiO2=72.67%?77.84%),alkali-rich(K2O+Na2O=7.67%?8.75%),weak peraluminous(A/CNK=1.01?1.06),high potassium calcium to shoshonite series rock and belong to the highly differentiated I-type granite.The zircon U-Pb weighted average age of the mafic microgranular enclave(MME)hosted in the JGC is 225.9±1.7Ma(MSWD=0.85),which formed at the same time as the JGC suggested a mixed origin.The JGC and MME high ?Hf(t)variation with TDM of 0.68-1.26Ga(JGC)and 0.79-0.92(MME)indicate a mixed source of Meso-Neoproterozoic lithospheric mantle and juvenile crust.The U-Pb weighted average age of quartz diorite zircon in the mining area is 244.9±2.7Ma(MSWD=2.4),positive ?Hf(t)value and low initial indicator(87Sr/86Sr)suggested a subcontinental lithospheric mantle.Geochemical characteristics show the characteristics of subduction island-related arc magmatic rocks,which are active continental margin arc magmatism in the background of the Early Triassic subduction.The original magma of the JGC is a mixed magma formed by mixing the partially melted lithospheric mantle with the central Neoproterozoic crust,and then forms the JGC by strong crystallization differentiation.The Re-Os isochron age of molybdenite in the ore is 217±1Ma(MSWD=1.4),which represents the ore-forming age,indicating that the metallogeny is closely related to the latter two stages of granite,and the deposit was formed in the Late Triassic.The composition of skarn minerals such as garnet and pyroxene in the deposit indicate that the deposit is an oxidation-reduction transitional type skarn tungsten deposit.The Nb-Ta element decoupling of granite porphyry and fine-grained granite are affected by hydrothermal fluid,and the Nb/Ta ratio is extremely low,suggesting a close relationship with mineralization.The majority ore rocks of the deposit is skarn type ore.Research on the characteristics of rare earth trace elements of scheelite grains which precipitated during different mineralization stages,indicates that scheelite can be divided into four generations,and each generation can be further divided into different types.The precipitation process of scheelite in each generation is closely associated with processes of skarn formation.The difference in the chondrite-normalized REE patterns,Mo content and some other trace elements in scheelite are the results of the syn-crystallized minerals causing fractionation of HREE,difference in REE composition and redox state of the ore-forming fluids.Scheelite batch crystallization simulation calculations suggest that the variation of REE within second and third generation of scheelite,can be the result of continuous crystallization from a fluid with fluctuating redox properties.Combine the research results above,this paper established the precipitation process of scheelite in each generation.The fluid inclusions in the deposit are mainly gas-liquid two-phase inclusions.The gas phase components in the gas-rich inclusions in the oxide stage and Mo-Cu sulfide stage contain CO2+CH4±N2,which may indicate the process of mixing between magmatic water and organic water which has equivalent with the organic bedrock or the interaction of ore-forming fluid with carbonaceous surrounding rocks and rocks containing NH4+.This process leads to an increase of pH and a decrease in temperature,which facilitate the precipitation of scheelite and further cooling to trigger the formation of sulfide ores.This is also consistent with the H-O isotope results of quartz in ore-forming veins.Based on the research above and the regional stratigraphic overthrust,fold belts development,volcanic rock and magmatic rock research,it is believed that the Jiangligou granitic complex is intruded under collision to the post-collision background,when South China Block Under subduction northward along the Animaqin-Mianlue suture in the Middle-Late Triassic.The roll-back of the subducted Paleo-Tethys ocean slab triggered the upwelling of the asthenosphere in the lower part of the area,resulting in the partial melting of the lithospheric mantle,which further induces partial melting of the lower crust.The mantle-and crust-derived magma mixed in a mixed magma chamber,and after intense crystallization differentiation.The highly differentiated granitic magma batch extraction(229Ma-223Ma-217Ma)from the mixed chamber and intruded below the Jiangligou anticline,and mineralization occurs at the contact zone between the marble rock and calcareous siltstone.Finally,based on the above knowledge and research results,the genesis model of Jiangligou skarn W-(Cu-Mo)deposit has been established.