RARE EARTH ELEMENT AND CRITICAL MATERIAL OCCURRENCE AND DISTRIBUTION IN POWDER RIVER BASIN COAL CORE, WYOMING

A broader recognition of coal as an alternative resource for critical metals has led to a surge in interest in occurrences within U.S. coals. Review of the USGS’s COALQUAL database, in-house coal data, and research on fly ashes by the U.S. Department of Energy (DOE) and the University of Wyoming Center for Economic Geology Research (CEGR) has identified certain coals that contain concentrations of rare earth elements (REE) above U.S. mean values that may represent ore targets for critical minerals. Within the REE concentrated coals, several other critical material (CM) species also occur in high concentration (e.g., Al, Ba, Mg, Sr, Ti, and Zr). Evaluation of coal fired power station waste streams, in comparison to USGS data and other research, has identified Wyoming’s Powder River Basin (PRB) as an area of interest for REE and CM research. Nevertheless, the PRB has remained one of the least characterized major coal basins in terms of trace element chemistry and distribution. Despite the fact that the PRB is the largest U.S. coal stock in terms of power station fuel use and coal reserves, detailed trace element data for the PRB are largely lacking.

Here, we evaluate a PRB coal core at high resolution for major and trace element concentrations to identify locations of REE and CM enrichment. We identify locations that contain the highest REE and CM concentrations, and make inferences about the geological and geochemical processes that may underlie these enrichment horizons. We also describe the potential utility of proxy major and trace elemental analyses in identifying REE and CM enriched portions of PRB coals in order to circumvent high volume geochemical analysis during future exploration. In this study REE enrichments associated with both volcanogenic input and humic bound clay were identified. Moreover, total concentrations of REE vary considerably within the coal seam. Both total REE and individual REE species vary in relative abundance, indicating geochemical sorting. For example, total REE concentration (ash basis) varies from less than 100.6 to over 2510 ppm, while in the same sample set critical REE (Nd, Eu, Tb, Dy, Er, and Y) range from 26.1 to 647.7 ppm and the percent of critical REEs ranges from 18.74 to 71.15%. CM species also vary within PRB coals. For example, Ti varies from 0.12 to over 1.08% and Sr from 1440 to over 10000 ppm.