Geology, fluid inclusions, H-O isotope, and hydrothermal zircon U-Pb geochronology of the Daqingshan orogenic gold deposit in Beishan orogenic belt, Xinjiang, NW China

Abstract

The Daqingshan gold deposit, located in the Beishan orogenic belt, northwestern China, is hosted in a Carboniferous greenschist facies metavolcanic-sedimentary sequence. Three periods of metamorphic and deformational events were recognized, corresponding with the three types of quartz veins in the deposit area. Quartz veins formed in the second period (D2) are the main auriferous ones that are strictly controlled by the subsidiary structures of the Kuluke shear zone. Gold mineralization occurs both as quartz veins and as enveloping altered wall rocks. Native gold, pyrite, and arsenopyrite are the primary ore minerals, while quartz and sericite are the main gangue minerals. Wall-rock alteration predominantly consists of silicification, sericitization, arsenopyritization, and pyritization. Systematic fluid inclusion investigation shows that CO2-H2O three-phase inclusions are the dominant type of fluid inclusions in the auriferous quartz veins, suggesting that the ore-forming fluids belong to a CO2-H2O ± CH4 ± N2 system that is particularly rich in CO2. The ore fluids are characterized by medium to high temperature (230~380 °C), low salinity (2~8% NaCleqv.), and medium density (0.8~0.9 g/cm3). Oxygen isotopes (δ18Ofluid = 11.3‰ to 13.9‰) indicate that the ore-forming fluids were most likely derived from metamorphic water. Fluid immiscibility might be the critical mechanism leading to the deposition of gold. The U-Pb dating results of hydrothermal zircons from auriferous quartz veins indicated that the maximum timing of the gold mineralization at Daqingshan was 295 Ma, the Early Permian. Comprehensive studies suggest that the Daqingshan deposit is a typical orogenic gold deposit formed during the subduction-accretionary phase of the Beishan orogenic belt. Significantly, recognizing the ore genesis of the Daqingshan gold deposit has an important implication for deep and regional mining exploration of southern Beishan.