Geochemical Sampling and Regional Interpretation (Illinois and Appalachian Basin) of High Alumina Underclay Deposits in the Central and Eastern U.S. (2020-2022)

The Maryland Geological Survey, in conjunction with other state geological surveys and the U.S. Geological Survey (USGS), examined the critical mineral potential of aluminum-rich, kaolinitic clays associated with coal beds in Pennsylvanian-aged strata. These clays have historically been utilized for brick-making in Maryland (Watts et al., 1922) and are called “underclays” because they often occur beneath coal beds. Recent studies suggest that Pennsylvanian underclays contain elevated concentrations of lithium (Li), aluminum (Al), and rare earth elements (REEs) (Rozelle et al., 2021).

Underclays have been sampled by eight state geological surveys throughout the Appalachian plateaus physiographic province of Maryland, Pennsylvania, West Virginia, and Ohio; as well as a portion of Kentucky, through the Illinois Basin and into Iowa. Geochemical analysis was completed by the USGS and published by the West Virginia Geological Survey in 2024.

Map of Pennsylvanian-aged strata where high-alumina underclays are being sampled. Locations of 15 sites sampled by geologists from the Maryland Geological Survey are shown in red.

Map showing starred locations of claystone with elevated Li in the Allegheny Formation

Stratigraphic column of Pennsylvanian units, including high-alumina clays in MD, PA, and WV

Left: Results show elevated Li (>300 parts per million) in claystone from the Lower and Middle Kittanning coal intervals of the Allegheny Formation (starred locations).
Right: Pennsylvanian stratigraphy, including major high-alumina clays identified in Maryland, Pennsylvania, and West Virginia (Brezinski and Kollar, 2011).

Geochemistry Samples

Trace and major element content for 120 underclay samples from 15 locations in or near Garrett County, Maryland can be found below. Preliminary results show elevated Li (>300 parts per million) in claystone from the Lower and Middle Kittanning coal intervals of the Allegheny Formation.

Underclay Samples (zip, 116 KB - includes .csv & .shp)

References

West Virginia Geological and Economic Survey final report incorporating data from MGS: Regional Investigation Of Rare Earth Element Enriched Underclay Deposits In The Central And Eastern United States

Brezinski, D.K., and Kollar, A.D., 2011, Appalachian Pennsylvanian climatic events and their congruent biotic responses, in Ruffolo, R.M., and Ciampaglio, C.N., eds., From the Shield to the Sea: Geological Field Trips from the 2011 Joint Meeting of the GSA Northeastern and North-Central Sections: Geological Society of America Field Guide 20, p. 45–60, doi:10.1130/2011.0020(03).

Rozelle, P. L., Feineman, M. D., White, T. S., Crescenzo, N. A., Kump, L. R., Larson, A. R., & Pisupati, S. V. (2021). The Mercer Clay in Pennsylvania as a Polymetallic Mineral Resource: Review and Update. Mining, Metallurgy and Exploration. https://doi.org/10.1007/s42461-021-00452-5

Watts, A.S., Schurecht, H.G., Swartz, C.K., and Hall, G.M., 1922. The fire clays of Maryland. Maryland Geological Survey, Volume 11, p. 291-480.

Contact Information
Christopher P. Williams, Program Chief, Coastal and Environmental
(410) 554-5559 | christopherp.williams@maryland.gov
Rebecca Kavage Adams, Geologist
(410) 554-5553 | rebecca.adams@maryland.gov