Geochemical variations of major, trace, rare earth elements in some Gondwana and Eocene coals of India with a comparison of their germanium, lithium, and mercury content

Abstract

The geochemistry of major, trace, rare earth elements (REEs), with special reference to Ge, Li, and Hg in selected Gondwana and Eocene coals, has been studied. Major oxide and trace element ratios have been utilised to compare the tectonic setting, provenance of source rocks, and paleoweathering conditions that prevailed the during formation of these coals. The Gondwana coals have a higher mean mercury and ash content (244.5 μg/kg and 17.2 %, respectively) than the Eocene coals (142.1 μg/kg, 8.9 % respectively). The major oxides, SiO2, Fe2O3, and Al2O3, in Gondwana and Eocene coals, are enriched relative to the upper continental crust (UCC), Chinese and American coals. The UCC normalized trace element concentration coefficients of the Gondwana and Eocene coals show enrichment in Ge, Se, Th, Co, Mo, Sn, W, and Li. The mean Ge and Li concentrations (mg/kg) in Gondwana (106.7, 154.7) and Eocene (120.0, 252.6) exceed the corresponding values in world coal (2.2, and 12). The Rajmahal coals have the highest mean Ge concentration (168.8 mg/kg) among the Gondwana coals while Eocene coals from East Jaintia Hills have the highest mean value (343.7 mg/kg). The Gondwana coals have a higher mean As concentration (3.5 mg/kg) in comparison to the Eocene coals (1.9 mg/kg). The mean of the rare earth elements in Gondwana (24.1 mg/kg) is higher than that in Eocene coals (11.3 mg/kg) and these values are less than the World coals (68.5 mg/kg). Both the Gondwana and Eocene coals are enriched in light rare earth elements (LREEs). However, among the Eocene coals, the Meghalayan coals show REE enrichment in comparison to Assam coals. Both the Gondwana and Eocene coals were formed in warm and humid climates under oxic conditions with moderate weathering of the source rocks.