Ore-forming Fluids Evolution Of The Yangshan Gold Belt In The Western Qinling Belt,Central China

The Yangshan gold belt is the largest independent gold deposit with huge proved gold resources/reserves. Base on the systematic petrological research, the ore stages and mineral paragenetic sequence were determined, and the microthermometry, composition of the fluid inclusions and the C-H-O isotopes were analyzed, also the origin and evolution of the ore-forming fluids, and the migration and enrichment mechanism of gold resources were discussed in this study.1. According to the petrological observations of the mineralized veinlets and ore samples, the gold mineralization process could be divided into four stages:lean-ore quartz veins stage (Ⅰ), quartz-sulfides stage (Ⅱ), quartz-stibnite stage (Ⅲ), and quartz-calcite veins stage (Ⅳ).2. The results of the microthermometry and composition analysis of the fluid inclusions show that the ore-forming fluid is characterized by medium-low temperature (120～320℃), low salinity (<10.0wt.%NaCl eqv) and CO2-rich. Ore-forming fluids of stage Ⅰ is H2O-NaCl-CO2system, with a lot of CO2-rich inclusions, whose homogenization temperatures vary from260℃to320℃, and salinity between0.62%and4.87%NaCl eqv. The fluid inclusions of stage Ⅱ are CO2-bearing and H2O-solution inclusions, with the homogenization temperatures clustering around220℃～280℃, and the salinity clustering around6%～10%NaCl eqv. The CO2/H2O ratio in stage Ⅱ is obviously lower than that in stage Ⅰ, which probably indicates that the volatile components (CO2) escaped from the ore-forming fluids. All the fluid inclusions of stage Ⅲ and Ⅳ are H2O-solution, with homogenization temperatures clustering around160℃～240℃and120℃～180℃respectively, and salinities clustering around5%～7%NaCl eqv and2%～7%NaCl eqv, respectively.3. The C-H-O isotopes suggest that the ore-forming fluids in the stage Ⅰ and Ⅱ are mainly metamorphic fluids (δD values between-78‰and-56‰; δ18OW-sMOW values between5.94‰and13.11‰). Meanwhile the ore-forming fluids in the stage Ⅲ and Ⅳ are metamorphic fluids which were mixed with meteoric water (δD values between-82‰and-61‰; δ18OW-SMOW values between4.484‰and6.45‰). There is almost no variation of the δ13CCO2values from stage Ⅰ to Ⅳ (range between-2.5‰and-4.1‰), which also shows the characteristics of mixed origin of the ore-forming fluids.4. Based on the results of the microthermometry and composition analysis of the fluid inclusions and the C-H-O isotopes, the migration of gold is mainly in the form of sulfur complexes, and there may be a sudden drop in pressure during the migration-mineralization process, which leads to the escape of CO2and other volatile gases, accompanying by the increase of pH value, causing the decomposition of the sulfur complexes and finally the precipitation with arsenopyrite and pyrite. During the late ore stage, with the addition of meteoric water, the fluid mixing causes the precipitation of sulfur complexes, which may also be the main reason for the present of visible gold.