GEOPHYSICAL AND GEOCHEMICAL APPROACHES TO EVALUATING PLACER RARE-EARTH ELEMENT RESOURCES IN THE SOUTHEASTERN U.S. COASTAL PLAIN

We combine geophysical, geochemical and geological data to evaluate rare earth element (REE) potential in placer deposits of the southeastern U.S. coastal plain. Airborne radiometric data collected as part of the National Uranium Resource Evaluation program show high equivalent thorium (eTh) anomalies over Cretaceous and Tertiary sediments from the Cape Fear arch in NC to eastern AL. These anomalies are strongest near the Fall Line (the Piedmont boundary) and decrease sharply with distance from the Piedmont. Over Quaternary areas, eTh anomalies are generally weaker except in localized spots where they can be nearly as high as those near the Piedmont. Prominent eTh anomalies are also observed over major riverbeds, including north of the Cape Fear Arch, where surrounding areas have relatively low values. Comparisons to geochemical and mineralogical data strongly suggest that these eTh anomalies are due to monazite (Th-REE phosphate). To better understand the anomaly sources, we conducted ground geophysical surveys over and analyzed sand samples from sites within the region. The surveys show local variations in eTh anomalies consistent with variations in sample composition and heavy mineral concentration, indicating high-resolution radiometric data as a useful tool for estimating REE content. Regional differences in the heavy-mineral assemblage are reflected in the magnitude of local geophysical survey anomalies, co-minerals, sample geochemistry, gamma emission spectra and magnetic susceptibility measurements. Combined, these data allow broad mapping of coastal plain areas to distinguish where monazite is sourced primarily from high-grade metamorphic rocks vs. igneous rocks. Comparisons to upland Piedmont rocks strongly suggest that away from major fluvial floodplains, the dominant source of the heaviest minerals, including monazite, is the igneous and metamorphic rocks closest to the Fall Line such as the Uchee and Kiokee belts in GA and SC and the Carolina and Eastern slate belts in NC and VA. In contrast, heavy minerals in major fluvial floodplains appear to originate from a wider portion of the Piedmont, resulting in increased monazite in areas such as the James River, VA, which may be sourced from the Petersburg granite and/or the Goochland-Raleigh belt.