Earth Elements in Western Canadian Coals

Abstract

Rare earth elements (REE) were determined in an age series, Lower Cretaceous to Eocene, of seven western Canadian coals using instrumental neutron activation analysis. The coals are from three tectonic belts, namely, Insular (Suquash, Comox, and Nanaimo), Intennontane (Tulameen and Hat Creek), and Northeast Rocky Mountain (Quintette and Bullmoose). Rank ranges from lignile-subbi-tuminous B C (Hat Creek) to medium-volatile bituminous (Quintette and Bull-moose). Concentrations of LREEs and HREEs are highest in Quintette (e.g., 32.3 ppm Ce) and Suquash (e.g., 0.47 ppm Lu) coals respectively and lowest in the Nanaimo (e.g., 8.7ppm Ce and 0.11 ppm Lu) and Comox coals. Chondrile-normal-ized patterns show enrichment of the LREE compared with HREE. In addition, an Eu anomaly occurs in all but the Nanaimo and Comox deposits. Shale-normalized patterns show little differentiation with respect to shale in the Intennontane and Northeast Rocky Mountain coals, however, HREE enrichments occur in the Insular Belt deposits. High correlation coefficients with specific ash constituents in the coal indicate that the REE are associated with clay minerals, heavy minerals, and calcite. Preferential occurrence of the HREE or LREE in different mineral phases occurs in, for example, the Comox and Tulameen deposits. Vertical distributions of the REE primarily reflect variations in ash content of the coal-bearing sections, however, some variations do occur. In the Tulameen deposit the LREE are progressively more depleted toward the base of the section, both in coal and interbedded shale. Considerable variability exists in REE concentrations and shale-normalized patterns from coals worldwide. In general Cretaceous coals appear to be more HREE enriched than Tertiary or Carboniferous coals. REE concentrations in the Canadian coals examined do not reflect age or rank of the deposits. Rather, source rock variations, modified by weathering, and depositional/diagenetic histories of the deposits, are thought to control the REE variations observed.