The characteristics and evolution of the ore-forming fluids in the Xiaojianshan gold deposit, eastern Tianshan Mountains

The Xiaojianshan gold deposit is located at the southern margin of the Kanggur ductile shear zone in the eastern Tianshan Mountains. The fluid characteristics and genetic types of ore deposits are poorly understood.Low greenschist facies metamorphism is widely developed in the mining area. The deposit is composed of many steeply inclined altered rock ore bodies with an average grade of Au 3.11×10-6~24.99×10-6, striking between 100°~120°. The metallogenic process can be divided into three stages: ①pyrite-magnetite-chlorite-sericite-quartz stage, ②pyrite-chalcopyrite-gold-quartz-chlorite stage, and ③quartz-calcite-low sulfide stage. Based on the study of the fluid inclusions in quartz veins of different stages by petrography, microthermometry and H-O isotope, it appears that CO2-bearing fluid inclusions and aqueous two-phase fluid inclusions existed in the Xiaojianshan gold deposit inearly to late stages. CO2-bearing fluid inclusions decreased gradually and the ratio of gas to liquid for aqueous two-phase fluid inclusions was smaller. From early to late stages, microthermometry results show that the average homogenization temperature of fluid inclusions was 216.9~396.4℃, 183.1~319.2℃ and 145.1~220.8℃; salinity was 1.40%~10.11%NaCleq, 1.91%~11.22%NaCleq, 1.63%~6.74%NaCleq. The ore forming fluids were medium-low temperature and low salinity NaCl-H2O-CO2 type solutions and the evolution was from medium temperature and salinity to low temperature and salinity. H-O isotopic composition indicates that δDV-SMOW values -22.550‰ and δ18Owater values 9.44‰ in the early stage of mineralization indicating the genesis of the metamorphic water. In the late stage of mineralization, δDV-SMOW values were -41.913‰~-34.796‰ and δ18Owater values were 1.99‰~3.98‰, with an average of -37.413‰ and 2.99‰ respectively indicating that fluids were mixed water and closed to metamorphic water. These results indicate that the source of ore-forming fluids was mainly metamorphic water, and there may be a mixture of magmatic and atmospheric water in the metallogenic process. The ore-forming fluids show the evolution from metamorphic to mixed water. Comprehensive analysis shows that the genetic type of Xiaojianshan gold deposit is orogenic gold deposit. Its metallogenic model is that the metamorphic fluid generated in the early ductile shear deformation process extracts mineralizing materials from rocks during migration, forms gold-bearing ore-forming fluid, and crystallizes at fissures such as mylonite facies, leading to the initial enrichment of gold. In the late stages, with the rapid uplift of the crust, the geological body changes from ductile deformation to brittle-ductile and brittle deformation, accompanied by the intrusion of granite veins, the metamorphic fluid leaches and extracts gold and other metallogenic materials from the rock in the course of migration, forming ore-bearing fluids. Ore-forming materials in the fluid are unloaded and precipitated in favorable areas where fissures or faults develop to form gold orebodies with the mixing of magmatic water and atmospheric precipitation, as well as the reduction of depth and pressure. © 2021, Science Press. All right reserved.