The Ore-forming Processes Of Molybdenum Polymetallic Deposits To The West Side Of Mangling Pluton, North Qinling

North Qinling belt is one part of the East Qinling Mo polymetallic ore belt. The area to the west side of Mangling pluton is an important ore district in North Qinling belt, however, systematic study on granite petrogensis and metallogeny is insufficient. Based on the detailed field investigation and previous researches, this paper selected three typical Mo polymetallic deposits, i.e. Mahe, Nantai and Gaogou and their related granite plutons to study. By geochronology and geochemistry as well as regional geology this paper will discuss the mineralization of Mo polymetallic deposits, the sources of granites and Mo polymetallic deposits, and the temporal and spatial evolution of granitic constraint on the Mo mineralization.Nantai Mo polymetallic deposit is hosted in the granitic porphyry and the contact zones between the marble of the Kuanping group and the granitic porphyry, and the deposit developed typical mineralization of porphyry-type and skarn-type. Mahe and Gaogou Mo polymetallic deposits mainly hosted in biotite quartz schist of the subsets Kuanping group. Ore body is predominantly exhibit as lamellar, stratoid and lenticular, and controlled by magmatic rocks and strata.Zircon U-Pb LA-ICPMS dating for the Nantai, Taoguanping, Xigou and Gaogou granitoid plutons from the mines to the west side of Mangling pluton yield151±Ma,157±Ma,153±Ma and150±3Ma respectively, indicating that they were formed at late Jurassic.Granites from this area are characterized by high in SiO2(SiO2=68.12%～77.38%) and alkali (Na20+K2O=7.94%～10.78%), showing that they are metaluminous or weakly peraluminous and high-K calc-alkaline or shoshonite series. These granites are riched in K、Pb and depleted in R、Ba、 La、Ce、Sr、P、Ti, and have a large variation in zircon Hf compositions with εHf(t)=-31.2--0.8, suggest that the source of granites involved relatively complicated component, they are derived from an ancient continental crust such as the Archaean Taihua Group, Qinling Group and Kuanping Group, and mixed with the juvenile component.Re-Os isotopic dating for the molybdenites samples from the mines of the three Mo polymetallic deposits yield149±2Ma,148±2Ma,149±3Ma, respectively. Both the ages of granites and metallogenic are the same within the errors, suggesting that the Mo polymetallic deposits in this area were formed at late Jurassic, which at the same period with the main porphyry-skarn type Mo deposits of the southern margin of North China Block.Fluid inclusions in quartz phenocrysts and quartz-molybdenite veins from the Nantai Mo polymetallic deposit can be divided into four types, i.e., L-type and V-type aqueous fluid inclusions,C-type three-phases CO2-bearing inclusions and S-type multi-phase inclusions containing daughter minerals. Homogenization temperatures of fluid inclusions in the early stage of quartz phenocrysts can be divided into three intervals, i.e.,340-400℃,220～280℃and140-200℃, with salinity of57.9%～>73.96%,30.06%～38.01%,3.39%～18.55%, respectively.The main mineralization stage in quartz-molybdenite veins were280～380℃,200～240℃and120～180℃,with salinities of ranging from0.43%to12.85%wt%NaCl.eqv. The late stage in non-mineralization quartz veins has only homogenization temperatures of120～240℃, with salinities of ranging from1.91%to10.73%wt%NaCl.eqv. Coexistence of two and more types of fluid inclusions in the three homogenization temperatures intervals of the principal mineralization stage, with contrasting salinities are homogenized to divergent phases at similar temperature. It suggests that boiling had already occurred several times during the ore-forming evolution and may be the dominated factor for metal precipitation. It is comparable to the Qiushuwan Mo(Cu) deposit in North Qinling.The sulfur isotope of Mo polymetallic deposits in this area was predominantly derived from magma, mixed with a small amount of sulfur from strata. The ore-forming metals chiefly come from the lower crust, but mixed with a small amount of mantle components. The hydrogen and oxygen isotopic composition in quartz suggests that the ore-forming fluid was principally derived from the mixture of magma water and meteoric water.The metallogenic age of Mo polymetallic deposits to the west side of Mangling pluton in the North Qinling belt concentrated in149to146Ma, indicating that their geodynamic setting are the tectonic regime changing its main stress from NS-trending to EW-trending.