Enrichment of strategic metals in the Upper Triassic coal from the Nui Hong open-pit mine, Thai Nguyen Coalfield, NE Vietnam

Research on coal-hosted strategic metal ore deposits is an important subject in coal geology and mineral deposit worldwide. However, the related geological characteristics of coals from Vietnam have seldom been investigated, let alone the strategic metals in coals. This paper investigates the mineralogical and geochemical characteristics of the Upper Triassic coal from the Nui Hong open-pit mine, Thai Nguyen Coalfield, NE Vietnam, with emphasis on the enrichment of strategic metals in coal. The Nui Hong coal is low-volatile bituminous with high sulfur organic content. Minerals in the Nui Hong coal is predominantly composed of authigenic quartz, tobelite, pyrite, and calcite, with a small proportion of sphalerite, gypsum and fluorapatite in some coal bench samples. Mo-U-V-Se and Ge-W-Sb-Cs-Tl(-U) strategic metal element assemblages along with As-P toxic element association is significantly enriched in the Nui Hong coal, pointing to a significant economic benefit for Cs, Mo, U, and Ge recovery from the Nui Hong coal. The Ge enrichment in the Nui Hong coal provides a new perspective on prospecting of coal-hosted Ge deposit in high-rank coal. Cesium mostly occurs in K-rich aluminosilicate minerals (tobelite and/or illite), while Mo, Sb, U, Ge, and Tl are predominantly associated with organic matter in the Nui Hong coal. Multistage hydrothermal activities occurring in the marine-influenced S-rich, Fe-poor and clastic-starved anoxic-euxinic coal-forming environment are responsible for the significant enrichment of strategic metal elements in the upper Triassic coals from the Nui Hong open-pit mine, Thai Nguyen Province.