CRITICAL MINERAL ENRICHMENT, MINERALOGY, AND MOBILITY IN PENNSYLVANIAN COAL MEASURES: AN EARTH MRI CASE STUDY FROM WEST VIRGINIA

The U.S. Geological Survey’s (USGS) Earth Mapping Resources Initiative (Earth MRI) works to identify mineralized areas or deposits across the United States that may host critical minerals to be utilized as a domestic resource. Aluminum-rich clays associated with coal horizons in Pennsylvanian-age strata occur throughout the Appalachian Plateau Province, the Central Appalachian Basin, and the Illinois Basin and have the potential to be low-grade, large-volume, critical mineral feedstocks. Past studies show that alumina is routinely 20-40% in these clay layers, and preliminary geochemical data indicate that some clay layers host anomalously high (>300 ppm) rare earth element (REE) concentrations in clay-rich mine roof and floor samples. However, understanding of the distribution of metals in varying lithologies and stratigraphic positions is limited by data density and a lack of modern geochemical data. Led by the West Virginia Geological and Economic Survey and including the participation of eight state geological surveys, a multi-year, regional reconnaissance study collected over one thousand stratigraphic and spatially representative underclay samples in the Appalachian and Illinois Basins. The samples were collected from both core and outcrops and were analyzed by the USGS. Initial results of the geochemical study showed ranges of total REEs from 1,292 ppm to 53 ppm in the Appalachian Basin and 1,205 ppm to 8 ppm in the Illinois Basin. Sampling in West Virginia targeted clay-rich intervals in the Allegheny Formation; total REEs ranged from 924 ppm to 90 ppm. The aluminum concentrations in the samples ranged from 176,000 ppm to 20,900 ppm and lithium concentrations ranged from 1,000 ppm to 12 ppm. REE enrichment was examined by tau plots, which suggest phosphates and/or Al-rich phases exhibit primary controls on REE enrichment, while XRD confirmed that the clays’ mineralogical content did not hold any REE associated minerals. More sampling is needed to determine the future extraction potential of these underclays.