Geological characteristics and genetic mechanism of the Xiaojiashan tungsten deposit in the Barkol district, Xinjiang

The Xiaojiashan tungsten deposit, located at the southern edge of the central area of east Junggar, is one of the metallic deposits in Bogda-Harlik tectonic belt. The structural position of the deposit is in the Harlik double anticline and the tectonic line is mainly EW-striking. The regional strata are mainly composed of 1st and 2nd lithologic member of the 1st Subformation of Middle Devonian Dananhu Formation (D₂d₁¹, D₂d₁²). The mining area has sporadic quartz diorite, moyite and a small number of intermediate-acid granodiorite veins. The orebodies occur in a late Hercian granite intrusion near the metamorphic crystal tuff which consists of the 2nd lithologic member of the 1st Dananhu Subgroup in Middle Devonian (D₂d₁²), whose type is wolframite ores of quartz vein, and wolframite is the main useful metal. Tungsten-bearing quartz veins are divided into gray wolframite-quartz vein and white quartz vein. Quadrupole mass spectrometry reveals that fluid inclusions contain major gas phase contents of CO₂, H₂O and a little variable positive ion component of Ca²⁺. In addition, there is a little variable components of Ar, C₂H₆ in gas phase. Meanwhile, fluid inclusions contain major liquid phase contents of Cl^-, Na⁺ and a little variable positive ion component of Ca²⁺. It can be inferred that the ore-forming fluid of the Xiaojiashan wolframite deposit is characterized by a CO₂-rich, low salinity, and H₂O-CO₂-NaCl system. The origin of the Xiaojiashan wolframite deposit can be considered as a high-temperature hydrothermal quartz vein type deposit controlled by fissure structures. Wolframite-bearing ore veins are distributed mainly over fissure structures. A portion of tungsten must have been sourced from migration and enrichment of mantle hydrothermal intrusion in wall rock and strata. Therefore, W-bearing complexes ore-forming fluid decomposed in the migration process of W, and promoted the precipitation of W in the Xiaojiashan deposit.