ASSESSMENT OF CRITICAL RAW MATERIALS (CRM) IN PENNSYLVANIAN-AGE COALS AND ASSOCIATED SEDIMENTS OF THE CENTRAL APPALACHIAN BASIN, EASTERN U.S.A.

Virginia Tech leads a regional team participating in the U.S. Department of Energy’s (DOE) Carbon Ore, Rare Earth and Critical Minerals (CORE-CM) initiative to assess unconventional resources of strategic minerals in the Central Appalachian Coal Basin (CAB). Rare earth elements (REE) and other critical raw materials (CRM) such as gallium, lithium and zirconium are known to occur in elevated concentrations in the Pennsylvanian-age coal strata and related waste materials, yet the mineral comportment, distribution and economic viability remains uncertain.

Working with industry partners, the team gathered over 650 samples for laboratory analysis and scanned 720 samples using portable x-ray fluorescence (pXRF) equipment. pXRF provides the means for rapid analysis of CRM content at close spacings in drill core and in-situ channel samples spanning coal seams, floor and roof strata. The comparison of pXRF data with laboratory results, geologic data, and downhole spectral gamma logs can provide input to depositional models for CRM resource evaluations.

Our findings show that pXRF is capable of accurately measuring low concentrations of many CRM with high confidence (Ba, Ga, K, Nb, Rb, Sr, Th, Y), whereas for others (La, Ce, Co, Nd, Sc, Ti, V) the detection limits are very high or spectral interferences result in increased uncertainties. pXRF is thus unable to directly assess total REE content (∑REE+Y+Sc). However, laboratory results for coal underclay materials show mean values for Y (36 ppm) and Th (18 ppm) that compare well with pXRF, 33 ppm and 21 ppm, respectively. The lab results show Y is highly correlated (R² = 0.801) with heavy REE (∑HREE-Y), while Y+Th is correlated (R² = 0.707) with ∑REE (-Y-Sc). When applied to the larger pXRF dataset, these correlations provide useful proxies for estimating ∑REE+Y_pXRF in the underclays, ranging from 129 ppm to 766 ppm, averaging 293 ppm, which is comparable with ∑REE+Y_LAB (317 ppm).

Enrichments of Th, Y, Ga (29 ppm) and Zr (222 ppm) may reflect the presence of resistant detrital heavy minerals such as monazite, xenotime, and zircon in the Al-rich underclay matrix. Several of the profiled coal seams and wall rocks contained thin volcanic ash layers up to 4-5 inches in thickness. The extent to which heavy minerals, clay and ash layers serve as sources of CRM will be subjects for future studies.