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Overview

scientist studying regolith exposures
USGS scientist Robert Ayuso studying exposures of light-to-middle rare earth element-enriched regolith developed on the Stewartsville pluton, near Hardy, Virginia. Photograph by Nora Foley, USGS.

Why is USGS doing this research?

Finding domestic economic sources of rare earth elements will help reduce U.S. dependence on foreign sources. Regolith is the geologic setting of ion adsorption clay rare earth element deposits. These deposits are of increasing interest to mining industries, as evidenced by newly filed U.S. patents that cover novel separation and extraction methods for deriving rare earth elements from clays. The ion adsorption clay deposits of South China currently supply virtually all heavy rare earth elements, yttrium, and approximately 30% of Chinese light rare earth elements reaching global markets. In addition, these ores can be mined and processed at significant cost savings over other rare earth element deposit types (such as carbonatites, Bokan Mountain type ores), and their low thorium and uranium contents streamline waste disposal.

The main criteria required to form this promising deposit type are

  1. deep weathering of granitic rocks and
  2. source rock that is capable of forming rare earth element-enriched regolith.

Because source granites for the South China heavy rare earth element deposits are deeply weathered, little is known about necessary granite characteristics that are likely to yield a heavy rare earth element-enriched regolith. To find economic rare earth element ion-adsorption clay deposits in the U.S., we must determine the very specific source rock types and geochemical parameters necessary for the mass transfer, accumulation, and, most importantly, the retention of adsorbed heavy rare earth elements and yttrium in regolith.

There are very few assessment techniques to determine the regional distribution, extent, thickness, grades, and tonnages of rare earth element-enriched regolith in the southeast U.S. Also, little is known about the environmental effects of mining these deposits, or the effects of the application of modern environmentally sustainable mining technologies to these deposits.

How is our research addressing this issue?

The Southeastern United States contains numerous granites of the type that contain high concentrations of rare earth elements.

  • We are conducting studies of granites of the southeastern U.S.:
    • to understand and assess likely chemical and physical processes that can lead to the enrichment and retention of rare earth elements, especially heavy rare earth elements, in regolith deposits,
    • to characterize the minerals in which heavy rare earth elements reside in regolith, and
    • to identify the geochemical parameters under which heavy rare earth elements are retained as adsorbed ions.
  • We expect to establish the full range of granitic suites of highly prospective source rocks for rare earth element deposits in the southeastern U.S., especially heavy rare earth element deposits, including favorable geochemical and mineralogical characteristics.
  • We plan to develop criteria and methodologies to delineate the occurrence of rare earth element-clay resources and define characteristics that relate to sustainable mining of rare earth element clay deposits, especially those containing a high proportion of heavy rare earth elements.

Some potential outcomes of this research:

  • An advancement in heavy rare earth element resource identification and assessment of adsorbed high value heavy rare earth element and yttrium in regolith.
  • The discovery of profiles with economic grades of heavy rare earth elements among the granites in the southeastern U.S.
scientists discussing soil sampling
USGS scientists Robert Ayuso and Mark Carter discussing best methods to sample soils overlying the rare earth element-enriched anorogenic granite at Suck Mountain, Virginia. Photograph by Nora Foley, USGS.
scientists sampling soil
USGS scientists Robert Ayuso and Arthur Merschat collecting surface samples in soils overlying Striped Rock Pluton, Virginia. Photograph by Nora Foley, USGS.
representative soil profile
Representative soil profile overlying coarse-grained amphibole-biotite granite of the Carboniferous age Liberty Hill pluton, South Carolina. Two prominent layers of the B-horizon shows an upper reddish colored clay-rich zone above a more white/tan colored clay-rich zone which extends down to tan-colored saprolite just above unweathered granite, which is extracted as quarry stone. This photo was taken at the Coral Grey Quarry (accessed by permission). Photograph by Bernard Hubbard, USGS.

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Unconventional Resources of Rare Elements: The Bearing of Source and Process on the Genesis of Residual Deposits

SE USA and China maps
The Southeastern United States has extensive suites of granitic rocks that are deeply weathered and show mobility of rare earth elements (REE) (Foley et al., 2013).

Our initial studies were aimed at identifying the potential for rare earth element ion-adsorption clay type deposits in weathered granite rocks of the southeastern United States. Residual deposits, including laterites, bauxites, clays, weathered crusts, and soils, are unconventional resources for many rare earth elements that are, in general, much less well-understood compared to more conventional rare element resources.

Study location map
Study locations in the Southeastern USA.

We identified plutons and prospective regolith (layer of unconsolidated rocky material covering bedrock) sites in the southeastern United States with similar geologic attributes to those of the South China ion-adsorption clay deposits -- attributes such as rock type, whole-rock chemistry, topographic setting, and accessibility. We focused on two major belts of weathered igneous rocks, selected due to their whole rock chemistries and locations:

  • Neoproterozoic (1,000 – 542 million years) age plutons and related rocks of the Blue Ridge and Piedmont physiographic provinces that have high contents of Ga, F, Nb, Sn, Ta, Y, and Zr, and enriched rare earth elements including the Robertson River (Virginia), Stewartsville (Virginia), and Striped Rock (southwestern Virginia) plutons),
  • Hercynian (upper Paleozoic, 350-250 million years) age plutons of the Piedmont province with larger volumes of weathered rock and less-enriched REE compositions (including the Petersburg (Virginia), Liberty Hill (South Carolina), Newberry (South Carolina), Pacolet (South Carolina), Elberton (Georgia), and Sparta (Georgia) plutons).

We made significant progress in understanding the ion adsorption clay deposit type. We determined that weathering processes and prospective source rocks necessary for the development of light-medium rare earth element ion-exchange deposits do occur in granite-derived regolith of the U.S. Piedmont. Our work proved that Neoproterozoic anorogenic (A-type) granites of the southeastern U.S. are prospective source rocks for granite-derived regolith highly enriched in light-medium rare earth elements. More importantly, we identified bedrock-soil profiles enriched in light-medium rare earth elements at grades averaging ~1000 ppm rare earth elements, which are comparable to those currently being mined in South China (500-3000 ppm rare earth elements).

granite petrogenesis diagrams
Southeast United States plutons having high contents of Ga, F, Nb, Sn, Ta, Y, Zr, and rare earth elements (ΣREE: 300~1500 ppm) include the Robertson River batholith, and the Stewartsville, Striped Rock, Suck Mountain, and Beech plutons. Granite petrogenesis diagrams show that the compositional ranges of large igneous suites of the Southeast U.S. generally match those typical for A-type or highly fractionated I-type granitic rocks and are comparable to granites of China (oval outline) that host economic rare earth element ion-adsorption clay (REE IAC) deposits. Data source: Foley and Ayuso, 2015; Ayuso and Foley, 2016. View full size plots. (756 KB).

Many new and as-yet-unresolved questions regarding the occurrence, distribution and genesis of REE ion-exchange deposits were identified during this project; hence the need for continued research.

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Publications - Journal Articles and Reports

world map showing bauxite deposit locations
World map showing distribution of bauxite deposits (sources of gallium). This is Fig. 1 from Compilation of gallium resource data for bauxite deposits: U.S. Geological Survey Open-File Report 2013–1272.

2015

Bern, C.R., Thompson, A., and Chadwick, O.A., 2015, Quantification of colloidal and aqueous element transfer in soils: The dual-phase mass balance model: Geochimica et Cosmochimica Acta, 151, 15 February 2015, p. 1-18. doi: 10.1016/j.gca.2014.12.008

Foley, N. and Ayuso, R., 2015, REE enrichment in granite-derived regolith deposits of the Southeastern United States: Prospective source rocks and accumulation processes, In: Simandl, G.J. and Neetz, M., (Eds.), Symposium on Strategic and Critical Materials Proceedings, November 13-14, 2015, Victoria, British Columbia, British Columbia Ministry of Energy and Mines, British Columbia Geological Survey Paper 2015-3, pp. 131-138. View Foley Symposium Paper. [PDF file, 2.9 MB]

Foley, N., Ayuso, R., Hubbard, B., Bern C., and Shah, A., 2015, Geochemical and Mineralogical Characteristics of REE in Granite-Derived Regolith of the Southeastern United States, in Proceeding of the 13th Biennial Society for Geology Applied to Ore Depoists (SGA) Meeting, 24-27 August 2015, Nancy, France; Proceedings, Vol. 2, p. 725-728. View Foley 2015 SGA extended abstract.

2014

Foley, N.K., Ayuso, R.A., Bern, C.R., Hubbard, B.E., and Vazquez, J.A., 2014, REE Distribution and Mobility in Regolith Formed on Granite Bedrock of the Southeast United States: Acta Geologica Sinica (English Edition), Special Issue: Meeting Abstracts: 14th Quadrennial International Association on the Genesis of Ore Deposits Symposium, August 19-22, 2014, Kunming, China, 88 (s2), p. 428-430. doi: 10.1111/1755-6724.12373_8

Schulte, R.F., and Foley, N.K., 2014, Compilation of gallium resource data for bauxite deposits: U.S. Geological Survey Open-File Report 2013–1272, 14 p., 3 separate tables, http://pubs.usgs.gov/of/2013/1272/.

2013

Ayuso, R., Tucker, R., Peters, S., Foley, N., Jackson, J., Robinson, S., and Bove, M., 2013, Preliminary radiogenic isotope study on the origin of the Khanneshin carbonatite complex, Helmand Province, Afghanistan: Journal of Geochemical Exploration, 133, October 2013, p. 6-14. doi: 10.1016/j.gexplo.2013.06.012

Cosenza, A., Ayuso, R.A., Foley, N., Albanese, S., Lima, A., Messina, A., and De Vivo, B., 2012, The geogenic vs anthropogenic source of harmful element anomalies in the eastern-central sector of the Peloritani Mountains (Sicily, Italy): Rendiconti Online Della Soceità Geologica Italiana, 21(Supplement 1), p. 265-267, http://rendiconti.socgeol.it/244/fulltext.html?ida=826.

Foley, N.K. , and Ayuso, R.A., 2013, Rare earth element mobility in high-alumina altered metavolcanic deposits, South Carolina, USA: Journal of Geochemical Exploration, 133, October 2013, p. 50–67. doi: 10.1016/j.gexplo.2013.03.008

Foley, N.K., De Vivo, Benedetto, and Salminen, Reijo, 2013, Rare Earth Elements: The role of geology, exploration, and analytical geochemistry in ensuring diverse sources of supply and a globally sustainable resource: Journal of Geochemical Exploration, 133, p. 1-5, http://dx.doi.org/10.1016/j.gexplo.2013.08.001.

Foley, Nora, and Jaskula, Brian, 2013, Gallium—A smart metal: U.S. Geological Survey Fact Sheet 2013–3006, 2 p., http://pubs.usgs.gov/fs/2013/3006/.

2012

Cosenza, A., Ayuso, R.A., Foley, N., Albanese, S., Lima, A., Messina, A., and De Vivo, B., 2012, The geogenic vs anthropogenic source of harmful element anomalies in the eastern-central sector of the Peloritani Mountains (Sicily, Italy): Rendiconti Online Della Soceità Geologica Italiana, 21(Supplement 1), p. 265-267, http://rendiconti.socgeol.it/244/fulltext.html?ida=826.

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Abstracts for Posters and Presentations

2016

Ayuso, R. and Foley, N., 2016, Prospective source rocks and accumulation processes for deposits of the REE Ion Adsorption Clay-type, Southeastern United States: Goldschmidt Abstracts, 2016, p. 131. View Ayuso 2016 Goldschmidt abstract. [PDF file, 91 KB]

2015

Bern, C.R., and Foley, Nora, 2015, High proportions of ion-adsorbed REEs on the Liberty Hill pluton, South Carolina, a possible link to hydrothermal alteration: Geological Society of America Abstracts with Programs. Vol. 47, No. 7, p. 814. View Bern 2015 GSA abstract.

Foley, N., and Ayuso, R., 2015, Rare earth elements in clays: a domestic resource for the future?: Geological Society of America Abstracts with Programs. Vol. 47, No. 7, p. 814. View Foley 2015 GSA abstract.

2014

Ayuso, R., and Foley, N., 2014, Weathering-related mobility of rare earth elements in high-alumina altered metavolcanic deposits, South Carolina, USA: Geological Society of America Abstracts with Programs, Vol. 46, No. 3, p. 78. View Ayuso 2014 GSA abstract.

Bern, Carleton, and Foley, N.K., 2014, Critical Zone weathering and your smartphone: Understanding how mineral decomposition and colloid redistribution can generate rare earth element deposits: American Geophysical Union 2014 Fall Meeting, 15-19 December, San Francisco, CA. View Bern 2014 AGU abstract.

Bern, C.R., and Foley, Nora, 2014, The role of topography in generating economic concentrations of ion-exchangeable rare earth elements: Geological Society of America Abstracts with Programs, Vol. 46, No. 3, p. 91. View Bern GSA 2014 abstract..

Foley, Nora, Ayuso, Robert, Bern, C.R., Hubbard, B.E., Shah, A.K., and Vazquez, J.A., 2014, REE distribution and mobility in residual deposits associated with altered granites: A review and comparison of global data-sets in the search for REE-clay deposits outside of China: Geological Society of America Abstracts with Programs, Vol. 46, No. 3, p. 91. View Foley 2014 GSA abstract.

Hubbard, B.E., and Foley, Nora, 2014, Planned remote sensing and laboratory spectral characterization of residual clay-hosted ion-adsorption type rare earth element (REE) deposits associated with deeply weathered laterite and bauxite soils: Geological Society of America Abstracts with Programs, Vol. 46, No. 3, p. 91. View Hubbard abstract.

2013

Bern, C.R., and Shah, A.K., 2013, Geochemical and mineralogical aspects of heavy mineral sands as potential REE resources in the southeast U.S.: Geological Society of America Abstracts with Programs, Vol. 45, No. 7, p. 277. View Bern and Shah 2013 GSA abstract.

Bern, C.R., Thompson, A., and Chadwick, O.A., 2013, Physical erosion beneath the surface: quantifying colloidal gains and losses in soil: Geological Society of America Abstracts with Programs, Vol. 45, No. 7, p. 408. View Bern 2013 GSA abstract.

Cosenza, A., Ayuso, R., Foley, N., Albanese, S., Lima, A., Messina, A., and De Vivo, B., 2013, The origin of geochemical anomalies in top soils of eastern-central Peloritani Mountains (Sicily, Italy): Goldschmidt Conference 2013, Mineralogical Magazine, 77(5), p. 920. doi: 10.1180/minmag.2013.077.5.3

Foley, N.K., De Vivo, Benedetto, Salminen, Reijo, editors, 2013, Rare Earth Elements: The role of geology, exploration, and analytical geochemistry in ensuring diverse sources of supply and a globally sustainable resource: Special Issue, Journal of Geochemical Exploration, Volume 133, p. 1-214.

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Partners

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Project Contact

Nora Foley
Phone: 703-648-6179
Email: nfoley@usgs.gov
Eastern Mineral and Environmental Resources Science Center

Mineral Resources Program Science Priority

The use of firm, trade, and brand names is for identification purposes only and does not constitute endorsement by the U.S. government.

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