PLACER-TYPE RARE EARTH ELEMENT DEPOSITS (Invited Presentation)

Ancient and modern placer deposits formed in alluvial and coastal environments have been significant sources of the rare earth elements (REEs). The REE-bearing minerals in placer-type deposits are primarily monazite [(Ce,La,Nd,Th)PO₄] and sometimes xenotime (YPO₄), both high-density (heavy) minerals that accumulate with the suite of heavy minerals. Monazite has been extracted from heavy-mineral placers as a byproduct of associated industrial minerals, such as Ti oxide minerals (ilmenite, rutile), zircon, sillimanite, garnet, staurolite, and others. Xenotime has been produced from some alluvial placers as a byproduct of tin (cassiterite) placer mining.

Placers can be classified as eluvial, alluvial, eolian, beach, and fossil (paleo) deposit types. Monazite-bearing placer-type deposits can occur in residual weathering zones, beaches, rivers and streams, dunes, and offshore areas. The detritus of sand, silt, clays, and heavy minerals are sourced by the erosion of crystalline rocks, mainly igneous rocks and moderate- to high-grade metamorphic rocks. In fluvial settings, slope is an important factor in the concentration of heavy minerals. In coastal settings, waves, currents, tides, and wind sort and concentrate mineral grains based on size and density.

Placer deposits containing monazite are known on all continents. In the past, byproduct monazite has been recovered from placers in Australia, Brazil, India, Malaysia, Thailand, China, New Zealand, Sri Lanka, Indonesia, Zaire, Korea, and the United States. More recently, monazite has been recovered from coastal and alluvial placers in India, Malaysia, Sri Lanka, Thailand, and Brazil. India’s southern coasts are mined for industrial minerals and monazite as a coproduct, used for its REEs and its Th, to be used as a nuclear fuel source.

Monazite- or xenotime-rich placers offer advantages as sources of REEs. Ancient and modern deposits of heavy-mineral sands formed in coastal settings can be as much as about 1-km wide and more than 5-km long. The monazite or xenotime occur with other heavy minerals of industrial value, and they are durable and often the densest mineral within the placer, thereby easily separated mechanically. Thus, monazite or xenotime can be recovered with little additional processing from placers along with the economic industrial minerals.