Critical Mineral Resources in Heavy Mineral Sands of the U.S. Atlantic Coastal Plain

Science Center Objects

In many parts of the southeastern U.S., dark-colored sands can be seen at beaches or beneath soil. These sands contain titanium, zirconium, and rare earth elements, which are considered critical mineral resources. Such sands are present in areas from the coast to a hundred miles or more inland beneath soil within the Atlantic Coastal Plain Province.  In some locales they are concentrated enough to be mined and are referred to as placer or heavy mineral sand deposits. These deposits are important because of the simplicity of extracting sands, the ease with which mined areas are remediated, and the vast potential resource area.

The USGS is working to better understand geologic controls on the location, formation, and composition of these deposits. This involves a source-to-sink approach considering sources of sediments in the upland rocks of the Piedmont, transport, deposition, and reworking within the coastal plain, and delivery to and continued reworking offshore. Results will inform local communities about the presence of these resources, which are used in applications ranging from advanced electronics to medical devices to renewable energy.

heavy minerals sands

Heavy mineral sands at Folly Beach, SC.

(Credit: Anjana Shah, USGS. Public domain.)

Science Issue and Relevance

Critical minerals from heavy mineral sands in the southeastern U.S. are important for a variety of uses. The primary commodities are titanium, zirconium, and rare earth elements (Van Gosen and others, 2014). Rare earth elements (REEs) are needed for cell phones, supermagnets, wind turbines, solar panels and rechargeable batteries (Van Gosen and others, 2017). Titanium, because of its remarkable strength, low weight, and resistance to corrosion, is used for aircraft, shipbuilding, geothermal applications, and medical devices such as heart valves and artificial joints, as well as for white pigment in paints (Woodruff and others, 2017). Zirconium, which is resistant to both heat and corrosion, is used in furnaces, ceramics, fiber-optic components, and other applications (Jones and others, 2017). 

spiral density separator

Spiral density separator in a placer deposit processing plant.

(Credit: Anjana Shah, USGS. Public domain.)

Heavy mineral sand deposits are formed by wind and water. Hard rock is eroded by rivers and wind, or if near the coast, also by waves, tides, and coastal currents. The sediments derived from these rocks are transported, deposited, and then reworked by the same processes. This reworking causes them to become sorted by density, size, and shape. This sorting can concentrate denser and heavier mineral grains such as ilmenite, leucoxene, and rutile (containing titanium), monazite and xenotime (containing REEs), and zircon (containing zirconium), creating economic deposits. 

Heavy mineral deposits have several attractive advantages over other types of mineral deposits. 1) Extraction is relatively simple, requiring only physical methods to separate out heavy minerals such as density, magnetic and electrical methods, rather than chemical methods that can introduce toxicity, 2) Remediation is also relatively simple because corresponding restoration methods are also physical, and 3) the potential resource in the southeastern U.S. is vast, extending from New Jersey to northern Florida.  

While such deposits have been mined for decades in the southeastern U.S., numerous scientific questions remain: 

  • What are the geologic controls, from source to sink, on the distribution and composition of these deposits? Where are heavy mineral sand deposits likely to be located? 

  • How does the heavy mineral assemblage vary over the eastern seaboard? Broad regional variations have been documented onshore; do they translate to the offshore environment? 

  • How can geophysical and geochemical exploration tools be developed for placer deposit exploration? 

Methodology to Address Issue

Geophysics: Geophysical methods help us to address the question “where are the deposits likely to be found?” These methods facilitate imaging of certain rock properties using sensors that are completely passive; results facilitate geologic mapping over large areas. Radiometric methods, which involve measuring natural, low-level radioactivity, highlight variations in potassium, thorium, and uranium within the upper one meter of Earth's surface. These data, which may be collected from air on while on the ground, can delineate shallow heavy mineral sand deposits (Force and others, 1982; Shah and others, 2017). Magnetic data have been used to infer heavy mineral deposits in the offshore environment, mostly when collected from boats so the sensors are close to the deposits (Mudge and Teakle, 2003; Shah and others, 2012).

New airborne radiometric data are being collected over South Carolina and other areas in collaboration with the Earth MRI effort (Day and others, 2019). Comparisons between these data and geologic, geochemical, geomorphologic, stratigraphic, and other data are helping us better understand the relations between geologic features and the presence of heavy mineral sand deposits. In the offshore environment, we are using recently collected data to compare magnetic anomalies with sample data to better understand these more enigmatic deposits.  

Contact for Geophysics: A. Shah 

potassium data along Santee River

High-resolution airborne radiometric potassium anomalies and lidar data collected near the Santee River in South Carolina.

(Credit: Anjana Shah, USGS. Public domain.)

inside geophysical aircraft

The inside of a geophysical survey aircraft.

(Credit: Anjana Shah, USGS. Public domain.)

Mineralogy and Provenance: One of the most fundamental questions regarding heavy mineral sands is “where did the sediments come from?" The provenance of a placer deposit is directly tied to its mineralogy; for example, heavy mineral sands derived from high-grade metamorphic rocks tend to be richer in rutile and monazite, while those from low- to medium-grade metamorphic rocks typically have lower rutile and monazite but higher titanite and staurolite (Force 1991).   

grain map

Grain map showing minerals detected.

(Credit: Christopher Holm-Denoma, USGS. Public domain.)

One approach to determining provenance is through geochronologic methods. Knowing the age of a sediment grain can facilitate determining its likely region of origin. These methods have traditionally involved U-Pb dating on zircon grains, but more recent work dating monazite grains shows more specific information (e.g. Hietpas and others, 2011). This is because zircons, which are formed as primary minerals in igneous rocks or high temperature melts, are very resistant to destruction by low grade metamorphic and sedimentary processes; a zircon grain may be recycled multiple times throughout Earth’s history. In contrast, monazite can form in response to more subtle temperature or pressure events. In the southeastern U.S., many zircon grains date to the the voluminous igneous and metamorphic rocks created during the Grenville orogeny, while monazite grains may reflect the later Taconic, Acadian, or Alleghenian orogenies. The effects of these later events are more variable over the region, so monazite can provide a more distinctive provenance signature (Hietpas and others, 2011).  

For this effort we will be conducting multi-mineral geochronometer studies (e.g. zircon and monazite geochronology from the same sample) on both existing and new samples collected both onshore and offshore. Geochemical and mineralogical data will also be collected where needed. Information regarding local and regional variations in the likely provenance and heavy mineral assemblage will help us to better understand long-range sediment transport, recycling, and deposit formation processes. 

Contact for Mineralogy and Provenance: Chris Holm-Denoma 

References/Further Reading 

Day, W.C., 2019, The Earth Mapping Resources Initiative (Earth MRI)—Mapping the Nation’s critical mineral resources (ver.  1.1,  March 2019): U.S. Geological Survey Fact Sheet 2019–3007, 2 p., https://doi.org/10.3133/fs20193007

Force, E.R., 1991, Geology of Titanium-Mineral Deposits: Geological Society of America Special Paper 259, 112 p., doi:10.1130/SPE259-p1. 

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. 

Hietpas, J., Samson, S., and Moecher, D., 2011, A direct comparison of the ages of detrital monazite versus detrital zircon in Appalachian foreland basin sandstones: Searching for the record of Phanerozoic orogenic events, Earth and Planetary Science Letters, v. 310, p. 488-497. 

Jones, J.V., III, Piatak, N.M., and Bedinger, G.M., 2017, Zirconium and hafnium, chap. V of Schulz, K.J., DeYoung, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., Critical mineral resources of the United States—Economic and environmental geology and prospects for future supply: U.S. Geological Survey Professional Paper 1802, p. V1–V26, https://doi.org/10.3133/pp1802V 

Mudge, S., and Teakle, M., 2003, Geophysical exploration for heavy-mineral sands near Mindarie, South Australia: Australian Society of Exploration Geophysicists (ASEG) Extended Abstracts 2003, no. 3, p. 249–255. 

Shah, A.K., Bern, C.R., Van Gosen, B.S., Daniels, D.L., Benzel, W.M., Budahn, J.R., Ellefsen, K.J., Karst, A. and Davis, R., 2017. Rare earth mineral potential in the southeastern US Coastal Plain from integrated geophysical, geochemical, and geological approaches. GSA Bulletin, 129(9-10), pp. 1140-1157. 

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 plate. 

Shah, A.K., Pratt, T.L., Horton, J.W., Howard, S., and Harris, M.S.,2019, New airborne magnetic and radiometric data over the Charleston, South Carolina, area reveal subsurface structures and variations in Quaternary sedimentary processes, Abstract GP33B-0745m presented at he 2019 AGU Fall Meeting, San Francisco, CA, 9-13 Dec., https://agu.confex.com/agu/fm19/meetingapp.cgi/Paper/564049.   

Shah, A.K., Vogt, P.R., 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, v. 303, pp. 26-41. 

Van Gosen, B.S., Fey, D.L., Shah, A.K., Verplanck, P.L., and Hoefen, T.M., 2014, Deposit model for heavy-mineral sands in coastal environments: U.S. Geological Survey Scientific Investigations Report 2010–5070–L, 51 p., https://doi.org/10.3133/sir20105070L

Van Gosen, B.S., Verplanck, P.L., Seal, R.R., II, Long, K.R., and Gambogi, Joseph, 2017, Rare-earth elements, chap. O of Schulz, K.J., DeYoung, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., Critical mineral resources of the United States—Economic and environmental geology and prospects for future supply: U.S. Geological Survey Professional Paper 1802, p. O1–O31, https://doi.org/10.3133/pp1802O.

Woodruff, L.G., Bedinger, G.M., and Piatak, N.M., 2017, Titanium, chap. T of Schulz, K.J., DeYoung, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., Critical mineral resources of the United States—Economic and environmental geology and prospects for future supply: U.S. Geological Survey Professional Paper 1802, p. T1–T23, https://doi.org/10.3133/ pp1802T

 

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