On The Magmatic Rocks And Ore-forming Fluid In The Jiapigou Gold Deposits, Jilin Province

Jiapigou gold deposits are located in the Northeast margin of North China Plate and mainly output in the Jiapigou granite - greenstone belt, with NW distribution. In this belt there are abundant mineral resources. Studied in detail on geological background, metallogenic epoch, the characteristics of fluid inclusions, the characteristics of geology and geochemistry of granites and dikes which are closely related to the ore deposit:It is shown that the magmatic rocks develop and are mainly granites with different dikes of felstone, lamprophyre, orthophyre. Many samples were collected from the gold deposits districts and the results of chemical composition indicate that the magmatic rocks belong to peraluminous and high-K calc-alkaline series I-type and little S-type, which was formed in the active continental margin. The orthophyre and dolerit belong to peraluminous and high-K calc-alkaline series. During the process of magma evolvemention differentiation degree was high. The trace elements of the magmatic rock are generally rich in large ion lithophile elements (LILE). The rare earth elements (REE) are relatively lower. The distribution pattern of the rare earth tends to the right shape showing clear fractionating of LREE and HREE with no obvious differentiation among HREE. Except for some special samples, generally there is no obvious Eu abnormality while there is weak Ce negative abnormality which explains its weak acid forming condition. On the whole the magmatic rocks in Jiapigou area have very similar distribution patterns for rare earth and trace elements and quite likely the magmatic rock originates from the same magma source. By dating the zircon U-Pb which is closely related to magmatic rock, it comes to the conclusion that the magmatic samples were formed in 166.2±2 Ma. The lead isotopic probably comes from the upper crust, medium and burial metamorphism and orogenic belts.Jiapigou gold deposit exists in the NW ductile-brittle shearing belt. The surrounding rocks of the ore body are mainly Archean amphibolites and gneiss, Mesozoic granites and Mesozoic dikes. The main ore is quartz vein, with a small amount of altered rock type. Ore minerals are mainly pyrite, chalcopyrite, sphalerite , etc. The surrounding rock alterations of ore body are sericite and chlorite, silicides, etc. Mineralization can be divided into five stages: quartz stage, quartz-pyrite stage, pyrite-quartz stage, polymetallic sulfide-quartz stage and quartz-carbonate stage, among which the quartz-pyrite stage is the main ore forming stage.The main ore-forming temperature is 100~250℃, belonging to medium temperature. The salinity is 4 %~10 %, which is in the scope of medium-low salinity. The main gas phase composition of fluid inclusion is CO2, H2O, and then CO, N2 and O2. All these show the mineralization environment is oxygen-free. And there are CH4, C2H2 and C2H4, etc, which shows the magmatic hydrothermalism is mixed with atmospheric precipitation. According to the organic and temperature test of group inclusion, the calculation of density is 0.54~1.04 g/cm3 and ore-forming process occurred in average depths of 1.73~3.70 km, pH 3.68~4.02 and ore-forming fluids Eh range from -0.332~ -0.284. Indicating that the fluid of Jiapigou Gold ore was acidity reduction, formed in the same environment with magmatic rocks that in the study area show some internal connecting links between them. Main metallogenic epoch is later than the granites age in the region, belonging to theYanshanian .