Geophysical and Geochemical Approaches to Evaluating Rare Earth Element (REE) Potential in the Southeastern U.S.

Science Center Objects

Rare-earth elements (REE) are an essential component of numerous advanced technology applications including high efficiency batteries, emerging energy technologies, and key defense systems. The goal of this study was to assist in the evaluation of the distribution of rare earth element deposits in the southeastern U.S., with a focus on sediment-hosted resources. 

heavy mineral sands

Layered deposit of unconsolidated heavy mineral sands along the shores of Folly Beach, South Carolina.

(Credit: Carleton Bern, USGS. Public domain.)

Science Issue and Relevance

Rare-earth elements (REE) are an essential component of numerous advanced technology applications including high efficiency batteries, emerging energy technologies, and key defense systems. The goal of this study was to assist in the evaluation of the distribution of rare earth element deposits in the southeastern U.S., with a focus on sediment-hosted resources. Minerals housed in sedimentary environments are much easier to extract than those hosted in intrusions, and thus provide a potentially important U.S. resource. Various studies have indicated the presence of such resources in the southeastern U.S. in the form of placer, phosphoritic, and residual (soil/clay) environments, but many questions remain regarding the nature and amount of these resources and how they vary geographically over this large area (Overstreet, 1967; Pirkle et al., 1989; Grosz and Schruben, 1994; Long et al., 2010; Cross and Lassetter, 2011; Foley, 2012). Geophysical, geochemical, and mineralogical approaches were developed and applied using data available at the national scale complemented by targeted field studies at specific sites.

Methods to Address Issue

Geophysical data analyses provide a means of remotely sensing mineral concentrations of interest and mapping their probable spatial extent. When combined with geochemical and mineralogical ground-truth data, they form a powerful tool. We examined how and under what conditions geophysical and geochemical methods can be used for the detection and evaluation of rare earth element mineral deposits in sediment-hosted environments, with an emphasis on using data available at the national scale. The results of this study revealed key information regarding the distribution and composition of heavy mineral sand (placer) deposits in the southeastern U.S. Results were analyzed to determine how these deposits relate to the surrounding geology (especially sedimentary provenance) and have implications for the relative roles of erosion, alluvial processes, and coastal processes. Synthesis efforts contributed to our broader understanding of the processes associated with genesis and evolution of these deposit types.

Geophysical Data Analyses: Objectives were to develop geophysical approaches to rare earth element evaluation, including the analysis and synthesis of existing and new relevant data to evaluate links between geophysical and geochemical properties, rare earth element content and local geologic processes. Regional radiometric and magnetic data are publicly available over the southeastern U.S.; several studies have shown that these methods can highlight certain mineral concentrations in sedimentary environments (Force et al., 1982; Grosz, 1983; Peterson et al., 1986; Grosz et al., 1989; Grosz and Schruben, 1994; Shah et al., 2012; Shah and Harris, 2012). 

Geochemical Data Analyses: Objectives were to conduct geochemical and mineralogical analyses of heavy mineral sand samples from both archives and field efforts. Analysis of new and existing data on the samples was performed to evaluate links between geophysical properties, rare earth element content and local geologic processes. Soil and stream sediment samples have also been collected over much of the region, with analyses performed as part of other USGS research efforts. In some areas rare earth element components have been measured, but in others only proxies were available, suggesting a benefit from further geochemical and mineralogical analysis.

Synthesis - Statistics, Modeling, Integration: Objectives were to synthesize geophysical, geochemical and geological data using statistics and/or other quantitative approaches, and to relate results to geological processes acting on a regional scale. The results provided insights into the formation and preservation of placer deposits that may or may not host rare earth elements.

monazite and xenotime content in the Atlantic coastal plain

Maps showing monazite (left) and xenotime (right) content in coastal plain NURE and NGS estimated using the La concentration in monazite and the Yb concentration in xenotime, after correcting for monazite- and zircon-hosted Yb. For reference, mapped exposures of Cretaceous, Paleogene and Quaternary sediments are identified, but limestone and calcareous units are omitted. The monazite belts identified by Mertie (1953) are shown as dark polygons interior to the coastal plains. Note that no samples exist in southwestern Tennessee in the NURE database and NGS samples for that region lack INAA data. Modified from Figure 8, Bern and others, 2016, doi:10.1016/j.gexplo.2015.12.011.

(Credit: Carleton Bern, USGS. )

References Cited

Cross, A. and Lassetter, W.L. 2011, Stratigraphic modeling for concealed phosphate deposits in Virginia’s Coastal Plain: U.S. Geological Survey Mineral Resources External Research Program Final Technical Report for Agreement G10AP00054.

Duval, J.S., Carson, J.M., Holman, P.B., and Darnley, A.G., 2005, Terrestrial radioactivity and gamma-ray exposure in the United States and Canada: U.S. Geological Survey Open-File Report 2005-1413, https://doi.org/10.3133/ofr20051413.

Foley, N., 2012, Behavior of REE in high-alumina alteration zones formed by weathering of felsic volcanic rocks [abs]: The 22nd V.M. Goldschmidt Conference, Montreal, Canada: Mineralogical Magazine, 76(6) 1712. Available at https://goldschmidtabstracts.info//abstracts/abstractView?id=2012001192.

Force, E.R., Grosz, A.E., Loferski, P.J., and Maybin, A.H., 1982, Aeroradioactivity maps in heavy-mineral exploration—Charleston, South Carolina, area: U.S. Geological Survey Professional Paper 1218, 19 p., 2 plates, https://doi.org/10.3133/pp1218.

Grosz, A.E., 1983, Application of total count aeroradiometric maps to the exploration for heavy-mineral deposits in the coastal plain of Virginia: U.S. Geological Survey Professional Paper 1263, scale 1:250,000, 20 p., https://doi.org/10.3133/pp1218.

Grosz, A.E., Cathcart, J.B., Macke, D.L., Knapp, M.S., Schmidt, Walter, and Scott, T.M., 1989, Geologic interpretation of the gamma-ray aeroradiometric maps of central and northern Florida: U.S. Geological Survey Professional Paper 1461, 48 p., https://doi.org/10.3133/pp1461.

Grosz, A.E., and Schruben, P.G., 1994, NURE Geochemical and geophysical surveys - defining prospective terranes for United States placer exploration: U.S. Geological Survey Bulletin 2097, 9 p., https://doi.org/10.3133/b2097.

Long, K.R., Van Gosen, B.S., Foley, N.K., and Cordier, Daniel, 2010, The principal rare earth elements deposits of the United States—A summary of domestic deposits and a global perspective: U.S. Geological Survey Scientific Investigations Report 2010-5220, 96 p., https://doi.org/10.3133/sir20105220.

Overstreet, W.C., 1967, The Geologic Occurrence of Monazite: U.S. Geological Survey Professional Paper 530, 327 p., https://doi.org/10.3133/pp530.

Peterson, C.D., Komar, P.D., and Scheidegger, K.F., 1986, Distribution, geometry, and origin of heavy mineral placer deposits on Oregon beaches: Journal of Sedimentary Petrology, 56 (1), p. 67-77, doi:10.1306/212F8882-2B24-11D7-8648000102C1865D.

Pirkle, F.L., Pirkle, E.C., Pirkle, W.A., Dicks, S.E., Jones, D.S., and Mallard, E.A., 1989, Altama heavy mineral deposits in southeastern Georgia: Economic Geology, 84 (2), p. 425-433, doi:10.2113/gsecongeo.84.2.425.

Shah, A.K., Vogt, P., Rosenbaum, J.G., Newell, W., Cronin, T.M., Willard, D.A., Hagen, R.A., Brozena, J., and Hofstra, A., 2012, Shipboard magnetic field "noise" reveals shallow heavy mineral sediment concentrations in Chesapeake Bay: Marine Geology, Volumes 303–306, 15 March 2012, p. 26-41, doi:10.1016/j.margeo.2012.02.006.

Shah, A.K., and Harris, M.S., 2012, Shipboard surveys track magnetic sources in marine sediments—Geophysical studies of the Stono and North Edisto Inlets near Charleston, South Carolina: U.S. Geological Survey Open-File Report 2012–1112, 1 poster, https://doi.org/10.3133/ofr20121112.

U.S. Geological Survey, 2004, The National Geochemical Survey: Database and Documentation: U.S. Geological Survey Open-File Report 2004-1001, https://doi.org/10.3133/ofr20041001.

 

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