GEOPHYSICAL APPROACHES TO EVALUATING SEDIMENT-HOSTED RARE EARTH ELEMENT POTENTIAL IN THE SOUTHEASTERN U.S

Placer mineral deposits generally contain relatively small, if any, amounts of rare earth element (REE) minerals, but these minerals are often mixed with other resources that are present in economic quantities. These REE minerals are usually easier and less costly to extract than intrusive-hosted counterparts since they often require only physical separation from other unconsolidated sediments. The southeastern U.S. Coastal Plain is known to contain REE- bearing monazite and xenotime in combination with economic quantities of other minerals such as ilmenite, rutile, zircon, and staurolite. However, questions remain regarding the quantity and geographic variability of REE potential. Because this region is very large, analyses of data available at a national scale such as airborne geophysical surveys, stream sediment, and soil geochemical databases are critical components of REE resource evaluation.

Radiometric (gamma-ray spectrometry) and magnetic surveys provide key geophysical imaging tools. Radiometric data often show regional Th anomalies over sedimentary units associated with elevated monazite within ~1 m of Earth’s surface. Simple analyses show direct correspondences between radiometric Th anomalies and stream sediment geochemical variations. Most REE-bearing minerals will not generate a measurable magnetic anomaly, but other components of the heavy mineral suite such as magnetite, maghemite, and hematite may generate a detectable anomaly if the sensor is maintained close enough to the magnetic source. Such elevations may be difficult to achieve during airborne surveys; nonetheless, an advantage is that magnetic surveys can image sources at greater depths than radiometric methods if the source is strong enough.

To better understand the deposits that airborne geophysical anomalies may represent we conducted ground radiometric and magnetic surveys over sites containing heavy mineral concentrations. Data were collected in Cretaceous through Holocene sedimentary environments in North and South Carolina; auger and drill hole samples were also obtained. Initial results show both radiometric and magnetic anomalies over areas with elevated heavy mineral concentrations, suggesting potential for these methods as an REE evaluation tool.