Geological Characteristics And Ore-forming Fluid Evolution Of Xiejiagou Gold Deposit In The Jiaodong Peninsula

Jiaodong peninsula is the largest gold ore concentration area of China, and gold reserves in this district account for a quarter of China. Gold Deposits in northwestern Jiaodong peninsula is controlled by three main faults, Sanshandao fault, Jiaojia fault and Zhaoping fault, compose three main gold metallogenic belts.Xiejiagou gold deposit is located between Jiaojia fault zone and Zhaoping fault zone, Linglong granite is its host rock. Fault structure and intermediate-basic dikes develop well in Xiejiagou mining area, and ore body is mainly located in the strongly-altered fault zone. Ore minerals of Xiejiagou gold deposit mainly include pyrite, chalcopyrite, galena, sphalerite, native gold, electrum and a small quantity of hessite and argentite. Wall rock alteration include potash feldspathization, silicification, pyritization, sericitization, chloritization and carbonatization. Silicification, pyritization and sericitization are widely distributed, and along with the gold mineralization period. Based on field observation and microscopic study, four hydrothermal stages can be recognized:quartz-coarse grained pyrite stage (stage I), quartz-medium fine grained pyrite-chalcopyrite stage (stage II), quartz-polymetallic sulfides stage (stageⅢ), and carbonate-quartz stage(stageⅣ), and quartz-medium fine grained pyrite-chalcopyrite stage (stageⅡ) and quartz-polymetallic sulfides stage (stageⅢ) are the main ore-forming stages. Ore microscopy research and scanning electron microscopy-energy spectrum analysis shows gold minerals of Xiejiagou gold deposit include native gold and electrum, and mainly electrum. Pyrite, galena, chalcopyrite and quartz are the main gold-bearing minerals. Native gold and electrum are closed related to metal sulfides (pyrite, galena, chalcopyrite), especially pyrite. In addition, native gold and electrum intergrow with hessite and argentite.Fluid inclusions in gold-bearing quartz vein can be divided into four types:single-phase fluid inclusion, gas-liquid two-phase fluid inclusion, CO2-bearing three-phase inclusion, and daughter mineral-bearing fluid inclusion. Based on microscopic study, microscopic temperature measurement, and Laser Raman spectrum composition determination, this paper argues that the ore-forming fluid of Xiejiagou gold deposit evolves continuously:In the early mineralization, ore-forming fluid is composed by CO2-rich, medium-high temperature (308-377℃), narrow salinity range low salinity (6.88～8.55%) fluid; In the main metallogenic period, ore-forming fluid is composed by medium-low temperature (226～331℃), wide salinity medium-low saltinity (4.87～10.29%) fluid, content of CO2 is reduced gradually, and in the meantime some CH4 jion in the ore-forming fluid. Immiscible fluid inclusion assemblage are recognized in the quartz of main metallogenic period, indicate fluid immiscibility is happened during the main metallogenic period. Combined with previous hydrogen and oxygen isotope data, this paper argues ore-forming fluid of Xiejiagou gold deposit is high temperature and high pressure fluid originates from magma. During the main metallogenic, ore-forming fluid flowing into fault zone and mixing with meteoric water. Cooling and pressure change cause fluid immiscibility. Fluid immiscibility result in ore-forming fluid composition change. Fluid immiscibility caused by change of pressure and temperature lead to the gold precipitation at fault zone.