Geochemistry of the Permian Oil Shale in the Northern Bogda Mountain, Junggar Basin, Northwest China: Implications for Weathering, Provenance, and Tectonic Setting

The northern Bogda Mountain is located in the southern margin of the Junggar Basin in northwest China. Based on inorganic geochemistry of oil shale samples of the Middle Permian Lucaogou Formation, the depositional environment, weathering degree, the provenance, and tectonic background of the northern Bogda Mountain were determined. The oil shale in the study area was formed in the deep lake environment. The major and trace element ratios suggest that the oil shale was deposited under semi-humid to humid/warm conditions and a brackish water to salt water setting with strong reduction. The chemical index of alteration values indicates that the oil shale experienced weak to moderate weathering conditions. Geochemical data suggest that the parent rocks of oil shale in the Sangonghe area, the Wujiawan area, and the Dongdalongkou area were siliceous rocks from the North Tianshan Mountain, basalts from the Kelameili Mountain and siliceous rocks mixed with basalts from the North Tianshan Mountain, and the Kelameili Mountain, respectively. According to the parent rock types and the thickness distribution of oil shale, the sediment overspread of ancient rivers from the Kelameili Mountain was larger than that from the North Tianshan Mountains. Combined with the tectonic and sedimentary evolution, it can be inferred that the North Tianshan Mountain area and the Kelameili Mountain area were characterized by a continental collision island arc environment during the Carboniferous period. The differences of weathering and provenance led to the differences of nutrient lead phosphorus (P) between different regions of the ancient lake, which thus led to the lateral heterogeneity of oil shale in its thickness and quality.