GEOPHYSICAL IMAGING OF REE-BEARING MINERALS HOSTED IN SOUTHEASTERN U.S. PLACER DEPOSITS

Rare earth element (REE)-bearing minerals have been observed in southeastern U.S. rocks and sediments but usually in small quantities. REE minerals that are less susceptible to alteration, such as monazite and xenotime, can become concentrated through fluvial and marine processes, forming constituents of heavy mineral concentrations or placer deposits. Other dense minerals in these deposits include ilmenite, rutile, zircon, and staurolite, which in some areas occur in economic quantities and are actively being explored or extracted. Our goal is to gain an understanding of how REE concentrations vary throughout the southeastern U.S. within heavy mineral assemblages. To evaluate such a large region we combine geophysical, geochemical, mineralogical, and geological approaches.

Radiometric (gamma-ray count and spectrometry for K, U, and Th) and magnetic surveys show anomalies associated with heavy-mineral sand deposits. Radiometric coverage over the southeastern U.S. includes gamma spectrometry data collected as part of the NURE program, but flight line spacing may be as wide as 9-10 km. Total count gamma surveys were also conducted during the 1970’s and 1980’s with much denser flight line spacing. While total count surveys do not provide relative estimates of K, U, and Th, they may be combined with the NURE, geological, and geochemical data to infer such quantities. Aeromagnetic surveys cover most of the southeastern U.S., but these surveys show little evidence of heavy mineral deposits. This is probably because signals from sedimentary sources are often subtle, and may be attenuated for typical flight heights of 160 m or more and measurement spacing of ~40 m or more. Nonetheless, shipboard and ground surveys do indicate magnetic anomalies in areas where heavy mineral concentrations are present. The interpretation of geophysical anomalies must take into account regional variations in heavy mineral assemblages, which depend locally on the composition of source rocks and the history of fluvial and marine processes. By combining geophysical survey data and corresponding sample geochemical and mineralogical data in areas where heavy mineral deposits are present, such variations can be estimated. These data in turn provide insights into how potential REE resources might vary throughout the southeastern U.S.