Rare earth element occurrence and distribution within the largest U.S. coal resource: Geochemical variability of powder river Basin Coals, Wyoming and Montana

Abstract

Unconventional geologic sources of rare earth elements, such as those preserved within coal-bearing strata, represent a potential domestic supply of critical minerals. Here we present major and trace element geochemical data from both the coal bed and stratabounding sediments. Representative samples were collected from the Wyodak-Anderson Coal Zone in U.S. Powder River Basin (WY, MT) and then analyzed using inductively coupled plasma mass spectroscopy and optical emission spectroscopy. These chemical analyses show that coal samples collected from multiple locations across the basin host concentrations of rare earth elements plus yttrium above average coal values for the United States. Additionally, these subbituminous coals are anomalously concentrated in CaO relative to coals from other U.S. basins. Total concentrations of rare earth elements and yttrium are highly variable when sampled at different depths within coal beds but show consistent relative enrichment at their margins. Sample locations within the basin and their proximity to bounding highlands appear to show predictable trends in rare earth element variability indicative of geochemical sorting mechanisms through geologic time. This contribution not only indicates an unconventional source of rare earth elements plus yttrium for active coal mines in the Powder River Basin, but also presents pathways for future work to grow our current understanding of this potential resource.