Characteristics and geological significance of inclusions in searlesite from the Fengcheng Formation in the Junggar Basin, China.

The sodium-rich dolomite and oil shale in the Fengcheng Formation of the Junggar Basin are important oil and gas reservoirs and major source rocks. Correctly understanding the genesis of siliceous boron sodium rocks is of great significance for identifying the origin of sodium-rich dolomite and predicting high-quality dolomite reservoirs. In order to investigate the genesis of the dolomite and siliceous rocks in the Fengcheng Formation, this study selected 60 core samples from the formation and prepared thin sections. By observing the petrographic characteristics of siliceous boron sodium rocks and their fluid inclusions, as well as measuring the homogenization temperature and salinity of the inclusions, the genesis of siliceous boron sodium rocks and their significance for the host rocks are explored. The results show that: (1) blocky and vein-like siliceous boron sodium rocks were found in the mudstone, dolomite, and siliceous rocks of the Fengcheng Formation, with fluid inclusions including oil inclusions and aqueous inclusions of gas-liquid two-phase (O-type), pure liquid (PO-type), gas-liquid two-phase fluid (W-type), and pure liquid fluid (PW-type); (2) the homogenization temperature peak of blocky siliceous boron sodium rocks exhibits a decrease from 108.8~129.4℃ in the deep section to 72.8~89.8℃ in the shallow section, while that of vein-like siliceous boron sodium rocks ranges from 50.4~86.4℃ in the deep section to 54.2~66.4℃ in the shallow section; (3) the salinity of W-type fluid inclusions in blocky siliceous boron sodium rocks varies from 8.41% to 18.22% without a distinct concentration range, while that of vein-like siliceous boron sodium rocks ranges from 6.88% to 15.37%, indicating that the fluids during the sedimentation of the rocks are high salinity fluids. Based on the petrographic and fluid inclusion studies of the siliceous boron sodium rocks in the Fengcheng Formation of the Junggar Basin, along with previous geochemical research on siliceous rocks in the study area, it is concluded that the host rocks of siliceous boron sodium rocks are deposited by the mixing of medium-low temperature hydrothermal fluids with alkaline salt lake water under an alkaline salt lake background.