Ore-forming process of the Saibagou gold deposit in the Northern Qaidam Orogen: Evidence from fluid inclusions, D-O isotopes and pyrite geochemistry

The northern Qaidam Orogen that hosts over 100 gold deposits is an important gold mineralization belt in Qinghai Province (western China). However, ore-forming processes and genesis of these gold deposits remain controversial. We analyzed the fluid inclusions (FIs), D-O isotopes and pyrite trace element compositions of the Saibagou gold deposit in the Tommorrit-Saibagou area and discussed the genesis of this deposit. These results show that the Saibagou gold ores contain three types of FIs: aqueous, aqueous-CO₂, and pure CO₂ type. The ore-forming fluids in each stage changed from an early medium temperature and low-salinity H₂O-NaCl ± CO₂ system to a late low-temperature and low-salinity H₂O-NaCl system. The D-O isotopes show that the ore-forming fluids are the mixed fluids of metamorphic water and meteoric water. In the main-ore stage, the ore fluids are mainly metamorphic and minor magmatic water sourced, with increasing meteoric water incursion toward the late-ore stage. The Au and other ore-forming elements (such as Ag, Cu, Pb, Zn, Te) are positively correlated and are mainly enriched in pyrite in the stage II. We suggest that the Saibagou is an orogenic gold deposit and is closely related to the Late Silurian northward subduction and hyperplastic orogenesis in Qaidam Block.