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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]