Magmatism And Ore Genesis Of The Yumugou Porphyry-Skarn Molybdenum-Tungsten Deposit In The East Qinling Orogen,Central China

Molybdenum(Mo),one of strategic mineral resources,is mostly hosted within porphyry Mo deposits.Therefore,the studies on the ore genesis of porphyry Mo deposits have important scientific significance.The Yumugou deposit in the world-class East Qinling Mo metallogenic belt is a newly discovered medium-sized porphyry-skarn Mo-W deposit,with inferred Mo and W resources of more than60,000 tons.The orebodies of this deposit are mainly distributed within the Huangbeiling porphyry and occur at the inner and outer contact zones between the Huangbeiling porphyry and wall rocks.However,published studies mostly focused on the basic geological characteristics of the Yumugou deposit,including geological conditions of mineralization,ore-controlling structures,and so on,which result in its origin along with formation and evolution processes still unclear.In addition,although a few studies have conducted whole-rock,biotite and apatite geochemical studies on the ore-hosting Huangbeiling porphyry,its genetic type is still ambiguous between the I-and S-type granite.Together,these correspondingly hinder the understandings of the plutonic and metallogenetic regularity,which also will not be helpful to guide prospecting and exploration in the Yumugou Mo-W deposit.In view of these scientific issues,we took the Yumugou Mo-W deposit as a research object,and conducted detailed petrography,whole-rock geochemistry,zircon/titanite U-Pb dating,molybdenite Re-Os dating,and zircon/titanite/biotite/muscovite geochemistry on the ore-hosting Huangbeiling porphyry and various skarns in this deposit,with major view to determine the age,genetic type,magma source and evolution processes of the Huangbeiling porphyry and to reveal the ore-forming age,ore-forming material source and hydrothermal evolution and physical-chemical conditions of the Yumugou deposit.In combination with the tectonic evolution history of the Qinling Orogen,we also attempted to construct a dynamic model of magmatism and mineralization for further interpreting the tectonic dynamic background of the Yumugou deposit.The major conclusions of this thesis are drawn as follow:(1)The Huangbeiling porphyry developed two periods of magmatism during the Late Mesozoic,with one represented by the Late Jurassic-Early Cretaceous biotite monzogranite(～143 Ma)and the other by the Early-Middle Cretaceous muscovite monzogranite(～127 Ma).Titanite U-Pb(149-144 Ma)and molybdenite Re-Os(148-147 Ma)dating of skarns suggest that the timing of formation of the Yumugou deposit was constrained at ca.148-146 Ma,which is closely correlated with the early biotite monzogranite in the Huangbeiling porphyry.(2)The Huangbeiling porphyry belongs to high-K calc-alkaline to shoshonite series as well as metaluminous to peraluminous rocks,which is characterized by the enrichment of light rare earth elements(LREEs),the depletion of heavy rare earth elements(HREEs),and the Eu negative anomalies.Moreover,this porphyry is also classified as S-type granite that was generated during transitional period from compression to extension tectonics in the Qinling Orogen.Based on whole-rock Mg#values(13.8-41.2)and zircon Lu-Hf isotopic data(εHf(t)=-21.5 to-14.9),the magma of the Huangbeiling porphyry was mainly derived from reworking ancient crustal components,with minor input of mantle-derived material,which underwent partial melting of crust and fractional crystallization during magma ascend.(3)The formation temperatures of biotite and muscovite monzogranites in the Huangbeiling porphyry were determined in the range of 765-772 ℃ and 670-742 ℃,respectively.These different monzogranites all belong to cold granite and are correlated with subduction or collision tectonics.Furthermore,based on biotite crystallization pressures of biotite monzogranite(0.1-1.3 kbar),the emplacement depths of the Huangbeiling porphyry were constrained at 0.3-4.6 km,with an average value of 2.0 km.(4)The molybdenite in the Yumugou deposit show Re contents varying from 3to 13 ppm,which intervene the corresponding values between the crust-derived(1-10ppm)and crust-mantle mixing(10-100 ppm)molybdenites,implying crust-mantle mixing ore-forming material source.In comparison with magmatic and ore-forming material source of ore-hosting porphyry,skarn-hydrothermal Zn deposit,and hydrothermal vein Pb-Zn-Ag deposit in the Luanchuan region,the whole Luanchuan porphyry-skarn-hydrothermal metallogenic system might share a single source.Moreover,the Yumugou deposit has high oxygen and fluorine fugacity,which promote the enrichment of metallic Mo during the magmatic-hydrothermal evolution.(5)The early biotite monzogranite(157-132 Ma)in the Yumugou deposit was correlated with the post-collisional lithospheric mantle delamination between the North China Craton and the Yangtze Craton as well as the southward intra-continental subduction of the North China Craton,which underwent partial melting of crust and fractional crystallization during magma ascend.Subsequently,this period of magma was migrated and emplaced at shallow crust,with concomitant the migration and enrichment of ore-forming materials to form the Yumugou deposit and Huangbeiling biotite monzogranite.During ca.130-125 Ma,the Qinling Orogen was transformed from compression to extension tectonics,which caused the reduction of crustal pressures and dehydration melting to generate the small-scale muscovite monzogranite unrelated to Mo mineralization in the Huangbeiling region.