Mineralogy and geochemistry of the late Permian super-high-organic-sulfur coal from the Naliang mine, Xian’an coalfield in Guangxi province, China, with emphasis on the enrichment of the critical elemental assemblage of V-Se-Mo-U

Abstract Super-high-organic-sulfur (SHOS) coals are limited in their utilization because they are rich in sulfur and other toxic trace elements. SHOS coals are generally rich in V, Se, Mo, U, Re, rare earth elements and yttrium (REY). This paper investigates the causes of elemental enrichment in the Naliang SHOS coals (Xian’an Coalfield, Guangxi Province, China) and analyzes geological controls on elemental/mineralogical anomalies in coal and associated rock samples. Compared with world hard coal averages, the Naliang coals are rich in V, and Se, highly rich in U, and unusually rich in Mo. Minerals in the Naliang coals include quartz, kaolinite, illite, mixed-layer illite, pyrite, calcite, dolomite, gypsum, anhydrite, albite, heulandite, and diaspore. Minor iron sulfate minerals, Fe(Al, Si)-oxysulfate minerals, and ammonia illite were observed under the scanning electron microscope with an energy dispersive spectrometer. Based on geochemical indicators such as Al 2 O 3 /TiO 2 ratios and REY distribution patterns, the source material compositions of Naliang coals are felsic igneous rocks from the Yunkai Upland. The ratio of Sr/Ba is > 1, the high concentration of B (67.9 µg/g-429 µg/g), and the presence of fossil foraminifera provide strong evidence that the Naliang coals were affected by seawater. The vein-filled occurrences of quartz, pyrite, calcite, and kaolinite, along with the similar distribution of elemental assemblage of V- Mo-U and Se-Hg, indicate that the Naliang coals may have been subjected to multi-stage hydrothermal fluid injections. The discovery of hydrothermal-origin pyrite offers new evidence for sulfur origins in SHOS coals.