Geochemistry of middle jurassic coal-bearing strata from the Xingmei Mine, Xinjiang, and the origin of localized barium enrichment.

Abstract

Research on the geochemical characteristics of coal-bearing strata in the Yanqi Coalfield, Xinjiang, remains limited. This study investigates the No. 8- 2 coal seam and its adjacent strata in the Xingmei Mine, which is located in the Yanqi Coalfield, using mineralogical and geochemical methods to characterize its composition, constrain provenance, and clarify the causes of localized barium enrichment. The coal is characterized by low ash and sulfur, high volatile matter, and is dominated by quartz, kaolinite, and pyrite. Trace element analysis reveals that Ba is slightly enriched in the coal samples, whereas it is enriched in the surrounding roof, parting, and floor samples. Provenance analysis suggests detrital input mainly from the central and south Tianshan region. Paleoenvironmental proxies reveal a humid-arid-humid climate shift from roof to coal to floor, with redox transitions from dysoxic to weakly reducing-oxidizing, returning to dysoxic conditions, under freshwater and terrigenous input. Elevated Ba concentrations are observed in samples collected near the coal-roof/floor interfaces, including XM-5P (15,826 µg/g) and XM-13 (3,071 µg/g), hosted in barite. Barite likely formed under freshwater conditions, high ash content, transitional contact zones, and Ba²⁺ supply from sediment provenance, with SO₄²⁻ most likely derived from acid rain.