Geochemistry of Hazardous Trace Elements in Yili Coal and Their Geological Controls: Evidence from Mineralogy and Carbonate C-O Isotopes

Coal can enrich hazardous trace elements in specific depositional environments, and their distribution and modes of occurrence are controlled by multiple geological factors. This study investigated the geochemistry of hazardous trace elements in the Middle Jurassic Xishanyao Formation coals of the Piliqing Open-Pit Mine, Yili Coalfield, using proximate, mineralogical (XRD, SEM-EDS), and geochemical (ICP-MS, carbonate C-O isotope) analyses. The coals were ultra-low-ash and low-sulfur subbituminous, with a whole-coal weighted-average CaO content of 1.95%. Most hazardous trace elements were not enriched relative to world coal averages: F, Cr, Ni, Co, As, Sr, and Th were near global values; whereas Se, Cd, Mo, Tl, Pb, U, and Hg were depleted. Vertically, F, V, Cr, Hg, Pb, Tl, Th, and U were higher in the roof and floor strata than in the coal seams, indicating terrigenous detrital input as the primary control, whereas Sr was governed by carbonate and aluminosilicate minerals. The only pronounced enrichment was the anomalously high V (212 μg/g) in sample C407, which possibly recorded locally reducing conditions at the base of the No. 4 seam; however, this single-sample observation precludes a firm syngenetic interpretation. Carbonate C-O isotopes showed that the calcite veins precipitated from coal-altering epigenetic hydrothermal fluids that carried an admixed organic-matter-derived carbon component. These fluids delivered mainly Ca, Mn, and Sr, without adding harmful elements to the coal. This signature reflects a dominantly syngenetic geochemical pattern that was locally overprinted by non-oxidizing hydrothermal fluids, in which weakly oxidizing, shallow-water conditions were unfavorable for the reducing enrichment of U, Mo, Re, and Se. In contrast to the coal-hosted uranium deposits of the Southern Yili Basin margin, Piliqing coals showed no evidence of post-depositional oxidized-fluid infiltration, indicating limited environmental risk.