Rare Earth Element Accumulation Processes Resulting in High-Value Metal Enrichments in Regolith

Science Center Objects

The Southeastern United States contains numerous granites of the type that contain high concentrations of rare earth elements. We are conducting studies of these granites to advance rare earth element (REE) resource identification and assessment by resulting in an improved understanding of the fundamental source rock types, modes and occurrences, and geochemical parameters necessary for the mass transfer, accumulation, and retention of adsorbed high value REEs, heavy-REEs (HREE), and yttrium (Y) in regolith.

Scientist studying regolith

USGS scientist Robert Ayuso studying exposures of light-to-middle rare earth element-enriched regolith developed on the Stewartsville pluton, near Hardy, Virginia. (Credit: Nora Foley, USGS. Public domain.)

Science Issue and Relevance

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.

soil profile over granite

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). (Credit: Bernard Hubbard, USGS. Public domain.)

Methodology to Address 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 collecting soil samples

USGS scientists Robert Ayuso and Arthur Merschat collecting surface samples in soils overlying Striped Rock Pluton, Virginia. (Credit: Nora Foley, USGS. Public domain.)

Scientist discussion near lake

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. (Credit: Nora Foley, USGS. Public domain.)

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

bauxite deposit world map

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. (Public domain.)

study location map

Unconventional Resources of Rare Elements Project - study locations in the Southeastern USA. (Public domain.)

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. 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.

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 ppmrare earth elements, which are comparable to those currently being mined in South China (500-3000 ppmrare 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.  (Public domain.)