The Yanshanian Granitic Magmatism Of The Zhashui-Shanyang Ore Cluster District In The South Qinling And The Mineralization Potential Evaluation

Based on integrating and analysis of the previous work of fundamental geology and Yanshanian magmatism characteristics for the Zhashui-Shanyang ore cluster district in the South Qinling Orogen, this study conduct petrology, geochemistry, in situ zircon U-Pb geochronology, Lu-Hf isotope and electronic probe measurements to discuss the physical-chemical conditions of the rock formation, petrogenesis, material source and geodynamic setting for the Yanshanian plutons in the district. Combining the published geochemical and Sr-Nd-Pb-Hf data, we explored the regularities of magmatism and evaluated metallogenetic potentiality of the Yanshanian plutons in the Zhashui-Shanyang ore cluster district.The Yanshanian plutons in the Zhashui-Shanyang ore cluster district are all high K cal-alkaline to shoshonite series characterized by high silicon, alkali and medium aluminum, enrichement of LREE and LILE, and depletion of LREE and HREE, with high Sr and low Yb, Y contents (Yb<1.9?g/g, Y<20?g/g), which demonstrate that the fractionation or residence of garnet phase in their source region.The study of zircon U-Pb chronology shows that magmatism of the Zhashui-Shanyang ore cluster district mainly occurred in 139.0Ma?149.1Ma, including Xiaohekou pluton 141.3Ma, Yuanzijie pluton 141.7Ma, Xiaguanfang pluton 139.1Ma, Shuangyuangou pluton 140.1Ma,142.1Ma, Tudigou pluton 140.7Ma,141.2Ma,141.8Ma, Baishagou pluton 139.0Ma, 141.6Ma,144.1 Ma, and Chigou ?-? plutons 140.8Ma,149.1 Ma,149.0Ma,144.2Ma, 142.3Ma,141.9Ma and 142.1Ma, respectively. The Yanshanian magmatism can be subdivided into stage ? (156Ma?129 Ma) and stage ? (117Ma?104 Ma), the former of which are mainly distributed in the South Qinling, and the latter are distributed in the southern margin of the North China Craton (NCC).The published Sr-Nd-Pb-Hf isotope data of the Yanshanian granites in the QinlingOrogen and the new zircon Lu-Hf isotope data in the Zhashui-Shanyang ore cluster district in this study imply participation of mantle materials with different proportion and decreasing mixing degree of the crust-mantle magma for the Yanshanian magmatic system from the south to the north unit. The Yanshanian granites of both South Qinling and North Qinling could have been sourced from the South Qinling basement. The magmatic source of southern margin of the NCC granites has relatively complex composition, which mainly includes the crystalline basement of the NCC.The Yanshanian granites in the Qinling Orogen occurred in the same geodynamic setting, the transition stage from compression to extension in Late Jurassic-Early Cretaceous (about 140Ma). During this process, thinning of lithosphere and uprising of ore-forming substance from asthenosphere, which jointly caused interaction between the crust and the mantle, sped melting of the thickened lower crustal material and then resulted in grantitic magma. When the magma moved along structures developed within the upper crust, ore-bearing fluid derived from condensation fraction of the magma filled the structures and replaced country rock, thus forming the porphyry-skarn deposits.The Yanshanian granites in the Zhashui-Shanyang ore cluster district, which have the average diagenetic temperature of 508? to 779?, diagenetic pressure of 0.52kbar to 4.95kbar, water content of 3.21% to 6.35%, lg(fO2) value of -19.37 to -14.99 and ?FMQ value of -1.41 to 2.21, are coincide with the ore-bearing intrusions of the typical porphyry-skarn deposit on the tectonic background, rock types, wall rock alteration, and mineralization characteristics. Ore-forming element abundances (such as Cu, Mo and Ag) are significantly higher than the mean values of the granties in the world and the South Qinling, additionally, the water content is generally higher than the theoretical value (>4.0%) of the ore-bearing rocks of the typical porphyry deposit, which demonstrate that the Zhashui-Shanyang ore cluster district has the advantageous geological-geochemical conditions of forming porphyry deposit. However, the wide range of the magmatic volatile matter and oxygen fugacity are unfavourable for large-scale mineralization. Therefore, it is difficult to form the large-scale porphyry-skarn deposit in the Zhashui-Shanyang ore cluster district, but still has a great potentiality to form the medium-small size porphyry-skarn type copper-polymetallic deposits, which is of vital significance for mineral exploration in the area.