ION-ADSORPTION REGOLITH-HOSTED REE DEPOSITS IN SOUTH CAROLINA: PHASE I RESULTS FROM AN EARTH MRI GEOCHEMICAL RECONNAISSANCE PROJECT

Ion adsorption regolith-hosted REE deposits (IA-REE) produce 20% of REEs worldwide, including 95% of HREEs. IA-REE develop in humid, subtropical regions, where laterization of igneous rocks produces clay minerals that adsorb REEs from their host minerals, producing thick, clay-rich saprolite and soil profiles. Economic IA-REE containing light REEs (LREE) are typically formed in-situ, whereas heavy REE (HREE)-bearing IA-REE form where regolith-derived soils are concentrated through erosion-transport and deposition.

NeoAcadian and Alleghanian granite regolith in the Southeastern Appalachian Piedmont is a potential source of IA-REE. Earth MRI funded a cooperative effort between the NC and SC Geological Surveys, and VA Energy for geochemical reconnaissance and to evaluate the applicability of the IA-REE deposit model to the Southern Appalachian Piedmont. Phase I of this project involved pluton selection, sample area delineation, and preliminary surficial sampling. Target plutons and sample areas were chosen using a combination of geophysical methods, soil maps, and geomorphology. Th/K ratios and lateritic indices calculated from high-resolution radiometric surveys suggested that the Liberty Hill, Pageland, and Winnsboro granite plutons are targets for further study. DEM-derived slope and curvature maps and NRCS soil maps were evaluated to select sample areas. Surface and near-surface (<1m) sampling involved soil augering, a gamma scintillometer (γ), and a portable X-Ray Fluorescence analyzer (pXRF) with REE-mode. Th was used as an indicator for in-situ LREEs, and Sc and Y for HREEs, where instrument detection was limited. Comparisons of aeroradiometric Th with γ- and pXRF-Th in granitic soils show that while variability exists, the three methods are correlative. γ- and pXRF-Th values range from 15-20 ppm but are locally > 50 ppm in soils that develop from fine-crystalline granites. pXRF results for La, Ce, Pr, Nd were commonly below LOD; however, Y values reached 30 ppm.

Phase II and III will target areas having significant regolith thickness to describe changes in REE values in vertical profile, and will use X-Ray Diffraction and Near Infrared Spectroscopy to determine clay mineral speciation and characterize the abundance of REEs in regolith deposits.