Rare earth elements in bituminous coals and underclays of the Sydney Basin, Nova Scotia: Element sites, distribution, mineralogy

Abstract

Bituminous coals of the Sydney Basin (Nova Scotia, Canada) contain some 30 ppm of rare earth elements (REE) dispersed in illite clay and macerals or concentrated in fossil apatite and resistate minerals (monazite, florencite, xenotime, zircon). For high-ash coals, the bulk of REE are derived from suspended river sediments. Low ash bituminous coals contain some heavy REE complexed with the organic phase or adsorbed by authigenic clays and derived largely from dissolved river load. The concentration of ΣREE in coal ash (72 to 483 ppm) is similar to that of roof and seat clays except for slight local enrichments in heavy REE. Individual seams have diverse REE distribution patterns that reflect the depositional and diagenetic mineralogy, or reveal enrichment at seam margins by organic complexing or fossil apatite uptake. Chondrite-normalized profiles range from light-REE enriched, to heavy-REE enriched. Highly pyritized bands yield ash low in REE with flat profiles. Clay-rich bands yield ash with distribution patterns similar to shales with light-REE enrichment, a weak negative Eu anomaly and a profile slope correlated to Al content. Rare earth element partitioning between organic and mineral phases or between coals and associated strata, differs over the coalfield, reflecting the combined effects of sedimentation, water chemistry, ion-exchange and chelation. Coal seam correlation using REE distribution is limited by geochemical facies. Distribution patterns must be interpreted with caution because of analytical problems inherent to coal.