Tectonomagmatic Evolution And Gold Metallogeny In South Beishan Mountain, Northwest China

The study area, south Beishan Mountain area, is one of the important gold clusters in northwest China. Heretofore, three middle-scale gold deposits and several tens of small-scale gold deposits have been discovered, explored and mined. Located on the intergrated part of the Tarim plate and Kazakhstan plate, there are widely exposed Precambrian and Paleozoic strata, complex deep faults and shear zones and intensive magmatic activities in south Beishan Mountain. Although some research on the metallogenesis of individual gold deposit, i.e. Xiaoxigong gold deposit, have been done in the study area, systematic and comprehensive work on the gold metallogenesis and its relationship to the regional tectonic and magmatic events have not been developed. Started from the systematic summarize of regional metallogenic geological settings, three middle-scaled gold deposits, i.e. Shijinpo, Xinlaojinchang and Xiaoxigong gold deposits have been selected for anatomizing in detail. For the three representative gold deposits, combined with detailed field geologic survey and abundant experimental analysis, comprehensive work have been done involved their geological characteristics, the nature and evolution of ore-forming fluid, the origin of ore-forming materials and the ore-forming ages. Base on the detailed research on typical gold deposits and compared with other gold deposits, these gold deposits have been divided into three types. Furthermore, the main ore-controlled factors inclued the strata, magmatic rocks and tectonic elements have been analyzed, and the relationship between tectonic-magmatic evolution and gold metallogeny has been discussed.The Shijinpo gold deposit is located in the margin of Shijinpo composite granitic batholith and occurs as Au-bearing quartz veins. SHRIMP dating of zircon seleted from the Shijinpo porphyritic granite suggested that it is formed in early Devonian. Accroding to detailed petrologic features and geochemical data, it is indicated that the host porphyritic granite and their monzonitic enclaves were generated by mixing of mantle-derived basic magma and its induced crustal felsic magma in local tensile enviroment during the accretion of Huaniushan block. The ore-forming fluid is CO₂-H₂O-NaCl-CH₄ system with low- to intermediate-temperature (90～342℃) and low-salinity (mostly <10%NaCleq) which contain elevated CO2 and small CH4. Fluid immiscibility is common. Combined with the oxygen and hydrogen isotope features, it is concluded that the ore-forming fluids derived from the magmatic fluid and meteoric water, which mixing aroused gold deposition. The sulfur and lead isotopic character of ore minerals and wall rocks suggested that the ore-forming materials would be of deep origin and derived from the Shijinpo pluton.The Xinlaojinchang gold deposit occurs in lower-Permian Zhesi group volcanics and volcanic arenite. Study of geochemistry features indicated that the Zhesi group volcanics derived from melting partly of depleted mantle with rudimental garnet in fast tensile environment and has few hybridization with crustal metarials. It is formed in inner active continental margin or back-arc basin during the early Permian subduction of south Beishan limited ocean crust. The ore-forming fluids of Xinlaojinchang gold deposit is CO2-H2O-NaCl±CH4 or CO2-H2O-NaSO4±CH4 system with low temperature (100³00 ℃) and low salinity (mostly <10%NaCleq). The study of oxygen and hydrogen isotope indicate that the ore-forming fluid mainly come from magmatic water in Xinjinchang mineralized zone and meteoric water in Laojinchang. Fluid mixing is the main mechanism to bring in the gold precipitation. Tracing of sulfur and lead isotope suggest that the ore-forming materials come from the Zhesi group volcanics and volcanic arenite. According to the geological features, it is deduced that the gold mineralization of Xinlaojinchang district followed closely the magmatic activity which formed the Zhesi group volcanics and the ore-forming age is about in early Permian.The Xiaoxigong gold deposit is the one related to ductile shear zone, whose orebody occures in brittle-ductile transition zone and brittle crushed zone that both are continuous evolutionary parts in one identical shear zone. Study of fluid inclusions in quartz from the Xiaoxigong gold deposit indicated that the ore-forming fluids is CO₂-H₂O-CH₄-NaCl system with low- to intermediate- temperature (133⁴00℃) and low- to intermediate- salinity which contain elevated CO₂. Fluid immiscibility is obvious. The study of oxygen and hydrogen isotope indicate that the ore-forming fluids are multiple sources, which included magmatic water, metamorphic water and meteoric water. The ore-forming materials come from upper subgroup of Xijianshan group Archaean metamorphics and Hercynian metamorphic remelted granite. The ore-forming age of the Xiaoxigong deposit is 267²84Ma ago. It is reckoned that the Hercynian granitoids play an important role in the metallogenic process.Based on the host rocks and geological features, gold deposits in south Beishan area can be classified into three types: (1) pluton-related gold deposits, such as Shijinpo, Mojindong and Huaniushan; (2) hosted by volcanics gold deposits, such as Xinlaojinchang, Beijin and Jinchanggou; (3) hosted by metamophics gold deposits, such as Xiaoxigong, Jinmiaogou and Xiaowannanshan. Proterozoic and Archaean strata and late Paleozoic marine volcanic-sedimentary rocks exposed in the south Beishan Mountain compose of the source bed of gold deposits. The distribution of these gold deposits and the orebodies thereinto are controlled by degressively ranked structure. Large-scale gold metallogenesis is triggered by tectonomagmatic events during late Paleozoic bidirectional subduction of south Beishan ocean crust. Based on the regional metallogenic condition, rules and ore-controlled factors in south Beishan Mountain, four prospective targets are outlined: (1) Periphery area of Xiaoxigong gold district; (2) Baidunzi-Shibandun area; (3) Shijinpo-Huaniushan area and (4) Laojinchang-Gubaoquan area.