Ore Genesis And Tectonic Setting Of Shajingou Gold Deposit In Helong Of Yanbian Area, Jilin Province

The Shajingou gold deposit is located at the joint part between the northeast edge of North China Platform and the southeastern Xing’an-Mongolia Orogenic Belt, and belongs to the southeastern Jiapigou-Haigou metallogenic belt in Jilin Province. It has a long discover history, but the exploration work and theoretical research were slightly conducted. Based on analyzing data of regional and mine district geology, field geological survey, some researches are completed, especially on metallogenic geological conditions, geological characteristics, physical-chemical conditions as well as diagenetic and metallogenic age of the Shajingou gold deposit. According to comprehensive study and combined with comparing the same type of deposits in this region, the ore genesis and metallogenic tectonic setting of Shajingou gold deposit has been identified.Besides the Quaternary the main strata cropped out in Shajingou mine are the Cretaceous Dalazi formation, and the major magmatic rocks includes Indosinian granite and Yanshanian diorite. Gold ore bodies, in this deposit, occur within the Yanshanian diorite intrusion, is characterized by ore-bearing quartz veins and controlled by the Sanshui Ping-Chongshan NW-trending faults. The main ore bodies can be divided into quartz vein type and altered rock type.The main metallic minerals are composed of native gold, pyrite, pyrrhotite and chalcopyrite,and the native gold are allotriomorphic granular associated with polymetallic sulphide. The textures of metallic minerals in the gold ore mainly include idiomorphic-allotriomorphic granular, replacement remnant texture, erosion replacement texture, crushing texture, interstitial texture and so on. Ore structures mainly include vein structure, disseminated structure, veinlet and taxitic structure. Wall-rock alterations include potassic alteration, silicification, chloritization, epidotization, sericitization and carbonatization, among which the silicification is closely related to the gold mineralization. According to intersections relationship of different veins, ore texture and ore structure, mineral assemblages, mineral intergrowth relationship, as well as wall-rock alteration, hydrothermal metallogenesis of the Shajingou gold deposit can be divided into four mineralization stages, namely, quartz-potash feldspar stage, quartz-pyrite stage, polymetallic sulfide stage and carbonate stage. The polymetallic sulfide stage is the major mineralization stage.The fluid inclusion petrography, microthermometry and component analysis results show that the three major types of fluid inclusions in quartz grains related to mineralization are present in this deposit:liquid-rich phase(L-type), gas-rich phase(V-type) and CO2-bearing three phase(C-type). Fluid inclusions in the early mineralization stage(quartz-potash feldspar stage) are mainly liquid-rich phase and gas-rich phase, while the major mineralization stage(polymetallic sulfide stage) are multiform inclusions including liquid-rich, gas-rich and CO2-bearing three phase inclusions. However, only liquid-rich fluid inclusions can be observed in the late mineralization stage(carbonate stage). From early to late metallogenic stage, the homogeneous temperature, salinity and pressure of fluid inclusions decrease gradually. Shajingou gold deposit was formed in high-medium temperature, medium-low salinity and moderate depths, accompanying widely fluid boiling from the early to main metallogenic stage which may played an very important role in gold element precipitation. The ore-forming fluid belongs to the Na Cl-H2O-CO2±CH4 system. Comparative study shows that the Shajingou gold deposit is similar, in ore-forming condition, geological characteristics and ore genesis, to those gold deposit(such as Haigou gold deposit, Jiapigou gold deposit and so on) in northeastern margin of North China Platform. It can ben concluded that the Shajingou gold deposit belongs to the intrusion-related mesothermal gold deposit.The isotope dating results show that the weighted mean U-Pb age of zircons from the metallogenic diorite is(171.7±0.9Ma) and the Re-Os isochron age of pyrite associated with the gold mineralization is 168 Ma, which indicate that both the diagenesis and mineralization took place in the middle Jurassic epoch(early Yanshanian). The geochemistry of major elements, trace elements and REE of magmatic rock indicate that the metallogenic diorite belongs to the metaluminous rich-sodium sub alkaline series. Meanwhile, the rocks are featured by rich LREE and LREE/HREE ratios, as well as positive Eu anomalies. It also featured by relative enrichment of large-ion lithophile elements, and high field strength elements show a significant loss. Combined with the analysis of trace element ratio diagram and discrimination diagrams of tectonic setting, a conclusion can be drawn in this paper that the metallogenic diorite of Shajingou gold deposit were derived from the melted enriched mantle caused by fluid metasomatism in subduction zone under the circumstance of the subduction of Pacific Plate.