Research On Fluid Geochemical Characteristics And Metallogenic Mechanism Of The Shuangwang Gold Deposit In Shanxi, China

The Shuangwang gold deposit is an important large gold deposit occurs in the West Qinling area. It is located in the South Qinling gold metallogenic belt between the Shangzhou-Danfeng fault and Fengxian-Zhen'an fault. Ore-bearing breccia bodies are distributed in the Xinghongpu Formation of the middle Devonian. Ore bodies do not have obvious boundary in breccia bodies. Ore-bearing breccia's cement is ankerite and quartz. Based on a description of geological characteristics of the Shuangwang gold deposit and discussion on the origin of its auriferous breccia body, this paper deals emphatically with the fluid inclusions and the carbon, hydrogen and oxygen isotopes of the ores. Shuangwang gold deposit is formed in hypabyssal depth, medium-low temperature and high pressure. The ore-forming fluid's main gas composition contains H₂O, CO₂ and N₂,and its liquid contains much Mg²⁺, Ca²⁺, K⁺, SO₄^2-, Na⁺, Cl^- and some HCO₃^-. The temperature of mineralizing fluid is between 93.6 and 431.7℃, which is medium-high temperature hydrothermal; the average salinity is 8.76%; the density is 0.60 to 1.07g/ cm³; and the depth is 2.95 to 9.11 km. The fluid of main mineralizing stage is the system of Mg2+ and Cl- , but late mineralizing stage is the system of Ca2+ and Cl-, which can be concluded that the fluid evolution caused for this kind of change. According to ions evolution, we can find that the stage of ankerite filling and cementing is main gold enrichment period. Mineralization degree in main stage is 900g/L, and in late stage it is 481g/L. Mineralization degree and gold grade are positive correlation. The fluid's pH-value reduced from 4.46 to 4.89; Eh-value is between -0.092 and 0.063, that reducibility was decreasing;Oxygen fugacity abruptly decline in late mineralization stage. Ore-forming fluid is a compound with the mixture of magmatic-water, construction-water and organic-water. The element of gold transfer in the form of [Au (HS)₂]^-. The reason why fluid unloads gold to form deposit is pyrite precipitating and fluid boiling with the change of temperature, pH-value and Eh-value.