HIGH-RESOLUTION AIRBORNE RADIOMETRIC DATA IN COASTAL SOUTH CAROLINA REVEAL VARIATIONS IN HEAVY MINERAL SAND DEPOSITS AND PROVIDE INSIGHTS INTO COASTAL SEDIMENT DELIVERY SYSTEMS

Heavy mineral sand deposits in the southeastern U.S. contain critical mineral resources of Ti (in ilmenite and rutile), Zr (in zircon) and REE (in monazite). Such deposits are of key interest because of their presence over a vast area and the ease with which minerals are extracted and mined areas are remediated. Most exploration information is obtained through shallow drilling and sometimes regional-scale geophysical studies. We present new aeroradiometric data of unprecedented resolution over coastal plain sediments (400 m line spacing), covering a 125 x 110 km² area near Charleston, SC. This method uses gamma spectrometry to image variations in K, U and Th within the upper 50-100 cm, effectively providing remote geochemical data, even in vegetated areas. Comparisons to previous sample data show that elevated Th primarily represents monazite contained in shallow heavy mineral concentrations. The airborne data show striking contrasts over sands vs. clays with linear, highly localized Th highs within sand units, especially near river mouths or beach ridges where crosscut features suggest multiple episodes of reworking. For estuarine clays, Th is lower and more distributed, suggesting the lower energy environment did not locally concentrate heavy minerals. U is linearly correlated with Th over beaches but shows deviations from that trend in areas containing phosphorite deposits. These deviations occur mostly in river valleys and areas once mined for phosphate. Elevated K, attributed to potassium feldspar and/or mica, is observed over the Santee River, the Princess Anne Fm. (last interglacial highstand) and younger sediments, and urban areas. K decreases as formation age increases, consistent with removal by leaching. “Immature” sediments in younger formations and the Santee River thus indicate the importance of sediment delivery to the shore via river systems with origins in the metamorphic/igneous Piedmont Province, longshore transport, and reworking along the modern shoreline.