HEAVY MINERAL SANDS IN CRETACEOUS TO NEOGENE SEDIMENTS OF SOUTH CAROLINA REVEALED BY AIRBORNE RADIOMETRIC DATA

The South Carolina Upper Coastal Plain, which is highly prospective for Ti-Zr-REE heavy mineral sand (HMS) deposits, is of strong interest because of previous studies showing elevated REE-bearing monazite and xenotime within the heavy mineral assemblage. This region comprises fluvial, marine, and eolian sediments of Cretaceous to Pliocene age. There is limited information, however, regarding relations between HMS favorability and depositional settings, geomorphology, or formation age. Furthermore, differences in facies are difficult to discern in most areas, unlike Quaternary sediments closer to the Atlantic coast where distinct dependencies on grain size were previously observed.

We use Earth MRI airborne radiometric data collected in 2020, previous and new geologic maps, and ground truth sampling to elucidate features and/or formations likely to host HMS concentrations. Numerous studies have shown a direct correlation between radiometric Th and HMS concentrations due to the presence of monazite, indicating use as a proxy for HMS presence. Visual and statistical comparisons between the airborne data and geologic maps show radiometric Th is elevated over 1) Cretaceous units, representing undivided fluvial sediments and the deltaic/marginal marine Late 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. Ground truth observations show that each of these units/features is associated with opaque mineral sands except for the Cretaceous fluvial units, which are typically clay-rich units containing kaolin or other aluminosilicates/phyllosilicates. The source of elevated Th in these fluvial units is unknown.

Of the marine deposits, those associated with elevated Th also show layering between sands and clays. This suggests a scenario where sea-level changes resulted in increased exposure to coastal and marine processes. These processes would have led to the reworking and sorting of sediments to concentrate heavy minerals. In contrast, the generally massive-bedded units may represent extensive primary deposition of sediments without substantial reworking. We examine links between observed Th, grain size variation, sea-level changes, and sample data to explore these possibilities.