RARE EARTH AND CRITICAL ELEMENT RELATIONSHIPS TO COAL DEPOSIT LITHOLOGIES IN SOUTHWEST WYOMING AND NORTHWEST COLORADO

Rare earth elements (REE) and critical minerals (CM) are vital components in a wide variety of applications, not only in the renewable energy sector but also in military, telecommunication, and other civilian uses. The association of REE and CM with coal deposits is widely known but not thoroughly understood. While individual coal samples or beds themselves might not host significant concentrations of REE, recent studies indicate temporal relationships among some REE with coal beds and their immediately adjacent non-coal bounding strata, including interbeds. These strata most commonly include clays, carbonaceous shales, and ash beds, but other related strata are possible in different coal-bearing settings. Variations in REE and CM concentrations and their direct or indirect association with coal beds are not always predictable. This study presents recent findings from studying different associations between coal beds and adjacent rocks to identify possible relationships between REE-CM concentrations and the coals and adjacent rocks.

For this study we collected and analyzed several dozen samples from the U.S. Geological Survey’s Core Research Center (CRC). Additionally, a small number of coal mine outcrops and their adjacent non-coal rocks were included. Most samples are of Cretaceous age. Previous work has shown a potential link between coals in the Upper Cretaceous Williams Fork Formation and REE occurrences in the Yampa ash bed in northwest Colorado. While data are uncommon and sparse, this possible link may suggest REE or CM enrichment from leaching or reworking of the ash bed. Trace element geochemistry, whole rock data, and microscopic examinations are used here to explore these relationships and guide future studies.