EARTH MRI IN SOUTH CAROLINA: AN INTEGRATIVE APPROACH FOR LOCATING CRITICAL MINERALS IN THE UPPER COASTAL PLAIN OF SOUTH CAROLINA

REE-bearing minerals (REM) monazite and xenotime are found in Piedmont crystalline rocks and Atlantic Coastal Plain sediments in South Carolina (SC). In the SC Upper Coastal Plain (UCP), heavy mineral sand deposits (HMS) have been explored for critical commodities REE, Ti and Zr since the 1950s. Published studies comparing high-resolution aeroradiometric surveys with borehole logs and geochemical data have indicated a potential for undiscovered HMS in the UCP.

In 2019, Earth MRI funded an SC Geological Survey mapping project in the UCP to target REE, Ti, and Zr. High-resolution airborne geophysical surveys, geochemistry, advanced mineralogy, and U/Pb zircon geochronology supported mapping. A handheld gamma meter provided ground-truth for the geophysics and guided sample selection. Radiometric equivalent thorium (eTh), which can be used as a proxy for monazite in parts of the UCP, was used to locate potential REM. Piedmont crystalline rocks above the Fall Line and UCP sediments were sampled and analyzed with whole-rock XRF, EPMA, LA-ICP-MS, and Tescan Integrated Mineral Analyzer (TIMA-X) particle mapping.

Mapping produced four 1:24K geologic maps and a 10 quadrangle 1:100K compilation. Exposed units are Cretaceous ­– Quaternary, with Eocene units predominant. eTh variations measured in outcrop validated the airborne radiometric data. Paleocene and Eocene units have the highest eTh values in the SC UCP, whereas Late Paleozoic metagranites have the highest eTh values in the Piedmont. Heavy mineral (HM) fractions (>2.9 g/cc) of samples accounted for 0.10-2.7% of the mass. The HM splits of Eocene samples average 8% monazite, 0.79% xenotime, 33% ilmenite, 16% rutile, and 17% zircon. Paleocene sample HMs average 7% monazite, 0.10% xenotime, 19% ilmenite, 16% rutile, and 20% zircon. Metagranite HMs average 0.40% monazite, 1.69% xenotime, 5% ilmenite, 8% rutile and <1% zircon. One sample of metagranite yields as much as 8% xenotime. Total REE + Y ranges from 27.7 ppm in the Piedmont to 773.5 ppm in the Eocene. Particle mapping of UCP samples shows that REM, Ti-, Zr-, and Th-bearing grains can be easily separated using gravity methods. Additional work is needed to determine whether these deposits are economic and will include U/Pb zircon geochronology to develop a source-to-sink model of HMS transport across South Carolina.