Study Of Deposit Geology And Ore-forming Fluid In 460 Gold Mine, Beishan Area, Gansu Province

The 460 gold deposit occur in the suture zone of Siberian plate and Kazakstan plate, adjacent to the Hongshishan-Heiyingshan terrane. The emergence stratums of mining area are shallow metamorphic volcaniclastic rocks of the Lower Carboniferous Baishan Formation. The ore-control and-host structure are mainly controlled by the NE-trending fault. The magmatic rocks of mining area are mainly granodiorite, in batholiths, stock output.Ore bodies of 460 gold deposit are mainly controlled by the EW and NE fracture and the ore bodies occur as veinlet in the fracture or fissure of granodiorite. The type of ore host is quartz vein. Ore minerals are pyrite, limonite, chalcopyrite, covellite, and trace amounts of bornite, chalcocite, etc; gangue minerals are quartz, chlorite, hornblende, epidote, sericite, etc. Gold mainly occurs as gold tellurides, with sylvanite and krennerite as the dominant minerals.The texture of ores include crystalline-granular texture, pressure texture, intersertal texture, etc. The structure of ores contain veinlets, banded, massive, disseminated, etc. The main wall-rock are pyritization, carbonation, epidotization, sericitization and chloritization, which are the typical alteration assemblage of low sulphidation. The metallogenic process from early to late can be divided into the quartz-pyrite stage, the quartz-sulfide stage and the quartz-carbonate stage.There are lots of fluid inclusions in quartz and calcite. Petrographic observations indicate that the types of fluid inclusions are two phase(liquid and gas) aqueous inclusions(W type) and daughter-crystal-bearing inclusions(S type). The aqueous inclusions are distributed in each stage, vary in shape, including rounded, oval, irregular, etc. Most aqueous inclusions are 5~15μm in size, with bubbles accounting for 10 vol %~30vol%. The temperature results shows that from early stage to late stage, the homogeneous temperatures of fluid inclusions are 202.4~348.3°C, 135.9~282.1°C, 104.8~165.8°C respectively. The salinities are 1.4%~10.98% Na Cl.eqv, 2.07%~13.51% Na Cl.eqv and 0.35%~ 8.14% Na Cl.eqv respectively. Ore-forming fluid shows the characteristics of the low temperature and low salinity. The Metallogenic environment belongs to hypabyssal environment. hydrogen and oxygen isotope analysis show that the ore-forming fluid is not a single source, which has the characteristics of magmatic mixed with meteoric. Fluid mixing effect is the main reason for the precipitation of Au, Te ore-forming elements.According to the study of zircon U-Pb age, the rock diagenetic age of 460 gold deposit is 340~341Ma. Based on the regional gold deposit formation age and the magma intrusion activities time has obvious coupling relationship, with the study of the geological and fluid inclusion features, we conclude that the 460 deposit is an epithermal telluride-containing gold deposit, which is formed in the middle-late Hercynian stage.