Comments on the geochemistry of rare-earth elements (La, Ce, Sm, Eu, Tb, Yb, Lu) with examples from coals of north Asia (Siberia, Russian far East, North China, Mongolia, and Kazakhstan)

The geochemistry of the REE (La, Ce, Sm, Eu, Tb, Yb, and Lu) is reviewed with supporting data from coals of North Asia. The 7189 samples from North Asia included in this study represent all coal ranks (lignites, sub bituminous coals, bituminous coals and anthracites) as well as coals formed under different geotectonic regimes and in different sedimentary conditions. These coals are characterized by higher contents of the REE as compared to the coal Clarke. The distribution characteristics and accumulations of the REE have been studied in several coal deposits and basins. Within some basins with the near-background average contents of the REE isolated coal beds enriched in lanthanides have been observed. In coal basins and deposits the relative enrichment in the lanthanides are observed in the fields or zones adjacent to the source area of the terrigenous material. Accumulation of high REE contents in coal deposits can be attributed to the presence of rocks in the source area containing high concentrations of REE and also alkaline or acidic volcanic ash contemporaneous with the peat accumulation in the basin. Transformation of volcanic ashes under the aggressive hydro-environment of a peat bog results in the mobilization and redeposition of the REE adjacent to the altered volcanic ashes (tonsteins) with the formation of high concentrations of REE in the coal beds. The formation of the REE anomalies may also be related to hydrothermal processes. The modes of occurrence of the REE change during the coalification process. Brown coals with a low degree of coalification are dominated by the organic modes of occurrence of the REE, whereas in hard coals and anthracites authigenic mineral forms dominate.