HEAVY MINERAL SANDS IN THE SOUTH CAROLINA UPPER COASTAL PLAIN

Heavy mineral sands (HMS), which contain critical mineral resources Ti, Zr, and REE, have been mined from U.S. Atlantic Coastal Plain strata for decades. Most operations have been in Pliocene and younger sedimentary formations, leaving relatively little known about the favorability of older strata. Here we present analyses of HMS occurrences using 2019-2020 airborne radiometric data, old and new geologic mapping, and new sample data over an area from the South Carolina Piedmont to the Atlantic Coast. Goals include a better understanding of HMS formation processes and prospectivity, especially in Cretaceous and Paleogene sediments.

Airborne radiometric data (gamma spectrometry for K, Th, and U) have been used to better understand the composition of shallow (< 1 m) sediments and to image concentrations of HMS in sedimentary environments. Higher Th values are associated with heavy minerals due to monazite in the heavy mineral assemblage, and K typically represents feldspar and/or mica, indicating the presence of immature minerals.

In Pliocene and younger sediments of the South Carolina coastal plain, the highest Th values and corresponding heavy minerals are usually observed within paleoshorelines (comprising mostly sand) and flood plains of modern rivers with headwaters in the Piedmont. Cretaceous and Paleogene sediments, however, generally do not exhibit unique geomorphic expressions of sands vs. mud (such as paleoshorelines) so facies dependencies are less clear. Statistical comparisons between the airborne data, geologic mapping, and sample observations show that radiometric Th reflects HMS concentrations over 1) the deltaic marine upper Cretaceous Sawdust Landing Fm., 2) The marine lower to middle Eocene Congaree Fm. and upper middle Eocene Orangeburg District beds, 3) Fluvial terraces of various ages, and 4) Eolian sediments of various ages. We also find elevated Th over older Cretaceous units representing undivided fluvial sediments, but HMS were not observed in these regions; the source of the Th is not clear. The marine units with HMS (1 and 2) are typically interlayered sands and clays, suggesting sea level changes that resulted in increased exposure to coastal and marine processes. These processes would have reworked and sorted sediments, concentrating heavy minerals and increasing prospectivity.