Mineralogy and elemental geochemistry of Pennsylvanian high-sulfur coals in the Huanghebei Coalfield, Shandong, Northern China: Implications for the enrichment and controlling factors of U

Abstract

Uranium enrichment in coal is of great significance for understanding metallogenic processes and assessing resource potential. Numerous studies have investigated the geochemical behavior and modes of occurrence of U in super-high-organic‑sulfur coals from Southern China. However, the Huanghebei Coalfield in Shandong, a major high‑sulfur coal-bearing region in North China, yet remains largely unidentified. This study investigates the U enrichment and controlling factors of the Pennsylvanian No.11 coal from Huanghebei Coalfield, Shandong, Northern China. Uranium is enriched in the No. 11 coal (up to 89.3 μg/g), where U, Mo, and V are exclusively concentrated in the topmost coal bench underlying the limestone roof. The enrichment of critical elements, including U, Mo, and V, is attributed to infiltration of marine water from the overlying strata during the early diagenetic stage. Uranium in the coal shows both organic and inorganic associations. Notably, a small portion of U occurs in uraninite, which is more easily reduced and precipitated under anoxic conditions. The relationships of Al₂O₃ vs. TiO₂, Al₂O₃/TiO₂-Nb/Yb and the REY distribution patterns indicate that the sediment-source rocks of the No. 11 coal were the North Qinling Orogenic Belt. The predominant minerals identified in the No. 11 coal are NH₄-illite, calcite, quartz, pyrite, bassanite and kaolinite, with minor amounts of dolomite and siderite. The modes of occurrence of NH₄-illite, kaolinite, calcite, pyrite, and quartz, as well as fluorapatite, indicate that the No. 11 coal was likely influenced by multi-stage injection of hydrothermal fluids. Authigenic NH₄-illite is relatively high abundant, probably formed by the interaction of kaolinite with nitrogen released from organic matter during metamorphism induced by igneous intrusion. The δ¹³C_VPDB and δ¹⁸O_VPDB values of epigenetic carbonates in the coals indicate that the carbon and oxygen in the carbonates are primarily derived from hydrothermal fluid activities. The No. 11 coal has potential for resource development and utilization as raw material for U and deserves further attention.