Primary rare-earth element (REE) deposits are associated with carbonatites, alkali igneous complexes, granites, pegmatites, and migmatites. Intensive,
in situ lateritic weathering of rocks enriched in REE and rocks containing relatively normal REE can concentrate REE into secondary “supergene” deposits. REE are mobilized from the near surface and concentrated as authigenic REE-bearing minerals, colloids, phosphates, and as REE adsorbed on clay minerals (ion-adsorption) or adsorbed on iron and manganese oxides in deeper soil horizons. Lateritic REE deposits were formed by weathering of carbonatites, hydrothermally altered tuffaceous sandstone, granites, and alkaline granites. Examples include Jianghua, China with 0.012 Mt @ 0.035% total rare earth oxides or TREO); Mt. Weld, Australia (23.94 Mt @ 7.9% rare earth oxides or REO) Araxa, Brazil (6.34 Mt @ 5% TREO); Tantalus, Madagascar (1.5 Mt @ 0.8% REO); and Ngulla, Tanzania (40 Mt @ 4.07% REO). Other deposits of this type include Stromberg in Australia, Madeira in Brazil, and Mabounie in Gabon, Some of these deposits are of lower REE grade than hard-rock deposits such as Mountain Pass, CA (13.6 Mt @8.24% TREO), Bokan Mt., AK (30 Mt @0.08% TREO), and Bear Lodge Mtns.,WY (10.7 Mt @3.6% TREO), but others are equivalent in REE grade. In lateritic deposits, mineralization is near-surface (generally <100 m), is amenable to low-cost open-pit mining, and REE commonly are recoverable by low-cost leach technology, rather than complex and more expensive mineral-specific processing required for many hard-rock REE deposits.
Domestic U.S. sources of supergene, lateritic REE deposits may be present in the southeastern United States. NURE stream sediment data shows large areas enriched in REE (Dy = 60-500 ppm, La = 200-5200 ppm, Y = 30-700 ppm, Ce = 200-9700 ppm, Sm = 100-4560 ppm, Yb = 10-300 ppm and background total values of these REE are less than 30 ppm) in the Blue Ridge and Piedmont metamorphic terranes. The highest REE in these areas are associated with deeply weathered sillimanite, sericite and muscovite schists. Intensive lateritic weathering has produced large, economic deposits of bauxite and kaolin in Cretaceous and Paleocene sedimentary rocks of Georgia’s Upper Coastal Plain, and anomalous REE suggest these rocks may also be promising for supergene, lateritic REE deposits.
