First insights into mineralogy, geochemistry, and isotopic signatures of the Upper Triassic high‑sulfur coals from the Thai Nguyen Coal field, NE Vietnam.

This paper represents the first integrated research on the mineralogical, geochemical and stable isotope characteristics of the Upper Triassic high‑sulfur coals from the Khanh Hoa open-pit mine, Thai Nguyen Coalfield, NE Vietnam. Formed in a marine-influenced carbonate succession, the Khanh Hoa coal is characterized by medium ash yield, low volatile matter yield, and high sulfur content. Minerals in the Khanh Hoa coal are predominantly composed of authigenic calcite in the upper portion; and pyrite, quartz, with minor amounts of tobelite ([(NH 4), K] (Si, Al) 4 Al 2 O 10 (OH) 2), gypsum in the middle and lower portions. The high‑sulfur Khanh Hoa coal is enriched in an As-Mo-Sb-U-Cs-P assemblage, which presents both an organic and inorganic mode of occurrence, and is largely ascribed to hydrothermal fluids. The δ 13C values of the Khanh Hoa coal range from −24.77‰ to −22.80‰ (avg. −23.33‰), reflecting a change in sedimentation pattern (from transgressive to high stand system upwards). The δ 15N values of the Khanh Hoa coal range from 2.41‰ to 3.73‰ (avg. 3.30‰), possibly resulted from influenced by seawater influence and a redox environment. The mineralogical, geochemical and isotopic characteristics reflected that the high‑sulfur Khanh Hoa coal was formed/preserved in marine-influenced anoxic environment, with low detrital input, and strong activities of multi-stage hydrothermal solutions. [ABSTRACT FROM AUTHOR]