Enrichment of rubidium and cesium in upper Triassic coals from the Cam Pha coalfield, NE Vietnam: Implications on sediment source and hydrothermal influence

• The late Triassic coals from, Northeastern Vietnam were formed in limnic sedimentary environment. • Coals from the Coc Sau and Duong Huy open-pit mine in Cam Pha Coalfield are promising economic source of 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 provenanceRb and Cs enrichment are influenced by the acidic anaerobic freshwater environments and hydrothermal solution. Research on critical metals in coal has aroused much interest because some coals are highly enriched in critical metals (Ge, Ga, Li, U, Nb, Ta, Zr, Hf, etc.), and consequently their corresponding coal combustion products may provide an alternative source for these elements. As two critical metals that plays important roles in a wide range of modern industries and various technological applications, Rb and Cs has been found enriched in the upper Triassic coal from the Cam Pha coalfield, NE Vietnam. However, the modes of occurrence and enrichment origin of Rb and Cs in the Cam Pha coal remains unclear. Based on the integrated mineralogical and geochemical analyses of the upper Triassic coals from the Duong Huy and Cao Son open-pit mines in the Cam Pha coalfield, this study focuses on investigating the enrichment and economic potential of rubidium and cesium in these coals. The Nos. 13, 14 and 15 coals from the Cam Pha coalfield are characterized by low-medium ash yield, low volatile matter yield and low sulfur content, belonging to anthracite. In comparison with the world hard coals, Rb and Cs is enriched in the No. 14 coal (Rb and Cs up to 142.8 mg/kg and 12.4 mg/kg, respectively) from Cao Son open-pit mine, as well as in non-coal rocks of No.13 coal (Rb and Cs up to 332.0 mg/kg and 26.4 mg/kg, respectively) and in the No. 15 coal from Duong Huy open-pit mine (Rb and Cs up to 128.5 mg/kg and 13.4 mg/kg, respectively). Rb and Cs mainly occur in K-rich aluminosilicates, such as muscovite in the three coal seams. The enrichment of Rb and Cs in coals from the Cam Pha coalfield is primarily ascribed to the terrigenous supply from the feldspar and mica-rich original felsic rocks in surrounding areas, which migrated into the coal basin through weathering processes during the coalification stage. The acidic anaerobic freshwater environments and the hydrothermal activity are also favorable for the enrichment of Rb and Cs in the study area. Rb and Cs are further enriched in coal fly ashes, reaching the industrial or marginal grade and presenting a promising economic potential for recovery of Rb and Cs from these coals.