“SOURCE TO SINK” TI-ZR-REE PLACER DEPOSITS IN U.S. ATLANTIC COASTAL PLAIN SEDIMENTS OF SOUTH CAROLINA

Ti-Zr-REE placer deposits, also known as heavy mineral sands (HMS), are the primary global sources of critical commodities Ti and Zr and are of renewed interest for REEs. Shallow (< 1 m) concentrations of heavy minerals can be imaged using high-resolution airborne radiometric data by highlighting Th in monazite, which is commonly observed within the heavy mineral assemblage. We use Earth MRI airborne magnetic and radiometric survey data collected over Atlantic Coastal Plain sediments in South Carolina to better understand the distribution of heavy mineral concentrations and their relation to formation age, geomorphology, etc. Geologic maps, sample geochemistry, and outcrop radiometric measurements are used to ground-truth interpretations. The survey data extend from Piedmont crystalline rock in the NW to the Atlantic Ocean in the SE, providing continuous “source to sink” coverage of Cretaceous to Holocene sediments.

Radiometric Th is generally highest over Cretaceous and Paleogene formations in the NW (the Upper Coastal Plain or UCP), drops sharply across the Orangeburg Scarp (a Pliocene paleoshoreline) into the Middle Coastal Plain and remains low except along riverbanks, and is variable further SE over Quaternary formations (the Lower Coastal Plain or LCP). Samples show higher concentrations of monazite within heavy mineral separates from the older units, indicating composition as the likely source of the contrast. In the LCP, high Th is generally observed over sand-rich paleoshorelines, especially in areas with multiple episodes of shoreline activity or combined coastal and tidal activity. In the UCP the geomorphology is fluvial-dominated and sands and clays are generally interbedded, but we find that locally, Th is highest within formations with bedding that suggests a history of rapid sea level changes, especially the upper Cretaceous Sawdust Landing and upper middle Eocene Orangeburg District beds, also showing the importance of reworking. Th is also elevated over fluvial terraces and eolian sediments of various ages. Interestingly, in the UCP, Th is generally higher over a broad region interpreted to overlie high-grade metamorphic rock of the Kiokee Belt and an area extending SE from this Belt just south of the Congaree River, but this distribution is not yet understood.