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[Display omitted] • The late Triassic coals from, Northeastern Vietnam were formed in limnic sedimentary environment. • Coals from the Coc Sau open-pit mine in Cam Pha Coalfield are promising economic source of critical elements (Rb and Cs). • High enriched Rb and Cs in the Coc Sau coals most likely occur in K-rich clay minerals, e.g., muscovite. • Rb and Cs enrichment are originated from terrigenous influx from felsic-intermediate sediment provenance. The current study investigates the mineralogical and geochemical characteristics of the Late Triassic coals from the Coc Sau open-pit mine, Cam Pha Coalfield (Quang Ninh Province, Northeastern Vietnam) using Powder X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), inductively coupled plasma atomic-emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). Coals from Coc Sau open-pit mine are characterized by medium ash yield (avg. 29.7%), low sulfur content (avg. 0.17%), and low moisture content (avg. 1.8%). The volatile matter content (avg. 8.4%) indicates a semianthracite rank. The crystalline phases identified in Coc Sau coals and host rocks (roof, floor, parting) are primarily quartz, kaolinite and muscovite, with small proportions of ankerite, clinochlore, anatase, calcite, and tobelite. Illite, pyrite, and siderite were occasionally detected in individual samples. In addition, several minerals below the XRD detection limit, such as barite, apatite, zircon, xenotime, and florencite were observed by SEM. The low sulfur concentrations, low Sr/Ba ratio and relatively high ash yield of these coals indicate a continental lake basin sedimentary environment with a relatively strong supply of terrigenous detritus. Quartz, kaolinite, and muscovite were most likely derived from the terrigenous source region surrounding the basin, and the other minerals are mainly of authigenic origin. Relative to worldwide coals, the Coc Sau coals and host rocks are highly enriched in Rb and Cs, which can be a potential source for Rb and Cs recovery during coal utilization and combustion. Rubidium and Cs enriched in the coals are positively correlated with Al and K, indicating that they most likely occur in potassium-rich clay minerals (e.g., muscovite), and originate from weathered felsic terrigenous detritus. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Upper Triassic coal from the Nui Hong open-pit mine is high-organic-sulfur coal. • The Nui Hong coal is promising economic source for strategic metal recovery. • Highly elevated Cs in coal most likely occurs in K-rich tobelite and illite. • Elevated Mo, Sb, U, Ge, and Tl are mainly associated with organic matter in coal. • Enrichment of strategic metals in Nui Hong coal is due to hydrothermal activities. 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. [ABSTRACT FROM AUTHOR]