Geology, Geochemistry And Genesis Of The Chaima Gold Deposit In The Fengxian County, Shaanxi Province

The Chaima gold deposit is located on the west of the super-large Baguamiaogold deposit, which is a representative deposit in the Fengxian-Taibai oreconcentration. In this paper, we take the deposit as our research object by using themethods of metallogenic characteristics, systematically fluid inclusion measuring, thegas-liquid phase composition, stable isotope and metallogenic chronology andcontrasting with the Baguamiao and Simaoling gold deposit, we got the mainconclusions as follow:1) The gold mineralization could be divided into three stages: the quartz-carbonate stage, the quartz-carbonate-sulfide stage and the quartz stage, in which thequartz-carbonate-sulfide stage is the main. Studies on fluid inclusions of each stageshow that the ore-forming fluids belong to the intermediate-low salinity system withintermediate-low temperature and density. The mineralization fluid of the main stagebelongs to the Ca2+-Na+-Cl--SO42--H2O system. The gas phase is mainly H2O and CO2,rich in organic gases such as CO and CH4. The temperature and salinity are reductedfrom the early stage to the late, which is same to the content of the Na+and Cl-.However, the content of the Ca2+, F-and the organic gases are increased in theore-forming process. The diversity of fluid inclusion types in the main stage isresulted from the fluid immiscibility.2) The mineragraphy research suggests that the features of pyrites in wall rocksand stages of mineralization are different. The mineral arisen sequence in the mainstage indicates that the ore-forming depth was epizonal, the environment wasreductive and the oxygen fugacity declined in the process of mineralization. Thecomposition of pyrites in the main stage imply that wall rock may provide somematerial to the mineralization and the ore-forming process may have relationship withthe magmatic water.3) Stable isotope studies, including H, O, C and S, indicate that the ore-forming fluid was composed of magmatic, metamorphic water and meteoric water. The richCO2fluid was a kind of mixed hydrothermal solution. The sulfur isotopic compositionof the ores is similar to the wall rock, which shows that sulfur in this stage has stratasource and influence from magmatic sulfur was relatively small. By Contrast with theBaguamiao and Simaoling gold deposits, the differences between tectonic setting andsurrounding rocks result in atmospheric water in different proportion of the three golddeposit ore-forming fluid, and the degree of contamination between surrounding rocksand mineralization fluid is also different.4) The Sm-Nd isotopic dating is carried out on the calcite and dolomite from themain stage, yielding an isochron age of203.2±1.6Ma，showing that the depositformed in Late Triassic.5) Combined with the previous research on the gold deposits, magmatite andtectonic movement, we suggest that the Chaima gold deposit is an orogenic golddeposit related to magmatic water formed after the collision of Qinling in lateIndosinian.