Geologic Trends in Rare Earth Element Abundance in Cretaceous Coal-Bearing Strata of the Uinta Region, Utah and Western Colorado

Cretaceous strata of the Uinta Region of Utah and western Colorado have been an important domestic coal resource. These coal-bearing units, such as the Naturita Sandstone, Ferron Sandstone, Blackhawk Formation and Mesaverde Group, have also been the historic focus of prolific sedimentary research.

A clean energy future in the US is reliant on identifying domestic sources of critical minerals, and particularly rare earth elements (REE). REE-enrichment in coal and coal adjacent strata offers a potential domestic mining resource that could be leveraged in existing coal mining operations for further mineral processing, however the geologic trends of REE-enrichment in coal bearing strata have been relatively unknown to date.

A two-year comprehensive geochemical and stratigraphic study of Cretaceous coastal plain deposits across Utah and western Colorado reveal that REE-enrichment, defined as >200 ppm, is largely found in mudstone deposits adjacent to coal seams and within coal zones. Coal seams themselves are typically not REE-enriched, with a few notable exceptions. This study utilizes >8000 portable X-ray fluorescence (pXRF) and >220 inductively coupled plasma mass spectrometry (ICP-MS) analyses to quantify REE abundance in coals and coal-adjacent strata, collected in stratigraphic and geographic context. Lithologic, stratigraphic and geographic trends in REE-enrichment suggest that both paleodepositional and diagenetic processes control REE-enrichment in coal-bearing strata throughout the region. This study highlights the non-fuel resource mining potential of coal within a changing energy geology landscape.