INSIGHTS INTO CRITICAL MINERAL OCCURRENCE IN APPALACHIA USING THE GEOCHEMISTRY OF UNDERCLAYS

Aluminum-rich clays and claystones associated with coal horizons are found throughout northern and central West Virginia, and adjacent areas. These clays (variably named as underclays, fireclays, tonsteins, Bolivar clays) have the potential to be an under-appreciated low-grade, large-volume, critical mineral resource that offers several advantages including: large volumes that are structurally simple and laterally continuous; deposits that are at or near the surface and easily exploited by open-pit mining; and locations in areas that have long histories of mining and abundant infrastructure. Past studies show that alumina (Al₂O₃) is routinely above 20% in these clay layers and up to 40%, and layers can have mean thicknesses up to 9 m in areas of existing and past coal mining. New and unpublished geochemical data indicate that some clay layers host anomalously high (>300 ppm) ion-exchangeable rare earth element (REE) concentrations. Similar clays are thought to host high lithium (Li) concentrations, and the presence of elevated aluminum makes elevated concentrations of gallium and indium possible.

The origins of these clays (volcanic versus pedogenic) are unresolved, and there has been negligible modern study of the economic potential of clays in West Virginia or their geological origin. The potential for understanding critical metal occurrences in West Virginia was hampered by a lack of modern geochemical data and understanding of the distribution of metals in different lithologies and stratigraphic positions. Current work includes locating deposits and determining the stratigraphic and lithological intervals with the greatest promise of hosting these elevated critical mineral resources. A more advanced understanding will require in situ lithofacies characterization with semi-quantitative elemental analysis. We have started this work by collecting a stratigraphically and spatially representative sample suite (over 270 samples from the Appalachian Basin were geochemically analyzed) for assaying in order to provide the data necessary to estimate the economic potential of this resource as well as understand its geological origin.