Significant enrichment of Rb and Cs in the Late Triassic coals from the Coc Sau surface mine, Cam Pha Coalfield, Quang Ninh Province, Vietnam

•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.