Geochemical characteristics and paleoenvironmental significance of the Xishanyao Formation coal in the eastern Junggar Basin

The Junggar Basin is an important coal-bearing basin in northwestern China. The coalfield in the east of the Junggar basin is an important part of the large Xinjiang coal base, and contains rich coal resources. In this study, we undertook a multi-proxy study evaluating trace elements, macerals and vitrinite maximum reflectance from coal seams B₀, B₁ and B₂ of the middle Jurassic Xishanyao Formation to characterize paleoredox and paleosalinity conditions of coal-forming swamp and atmospheric oxygen level and paleoclimate during coal-forming period in the eastern Junggar Basin. 220 coal samples were selected from coal seams B₀, B₁ and B₂ in order to determine trace elements. Moreover, macerals and vitrinite maximum reflectance of 16 coal samples was investigated. Compared with average values for world low-rank coals, the coal seams B₀, B₁ and B₂ of the middle Jurassic Xishanyao Formation in the eastern Junggar Basin are slightly enriched in Sr, in addition, the coal seam B₀ is also slightly enriched in Co. The analysis of Ni/Co-V/Cr and Ni/Co-Mo diagrams indicates that swamp was in oxic and dysoxic condition, which infers that the water in swamp has strong activity and high free oxygen content. According to B/Ga ratio analysis, the paleosalinity of swamp gradually increases from coal seam B₀ to coal seam B₁ and then to coal seam B₂. The high paleosalinity of swamp during coal-forming period of coal seam B₂ may be caused by an increase in the evaporation of water, which also indicates that the paleoclimate was relatively dry and hot during this period. The Sr/Cu ratios in coal seams B₀, B₁ and B₂ reveal that the paleoclimate experienced a gradual transition from relatively warm and humid condition to relatively dry and hot condition from bottom to top during the Xishanyao Formation coal-forming period. The inertinite content varies from 40.4 to 57.2 vol.% with an average of 48.5 vol.% in coal seams B₀, B₁, and B₂. Inferred atmospheric oxygen concentration in the middle Jurassic, as estimated from inertinite contents, were ～27.7%, which is much higher than the minimum required for sustained combustion.