CONVENTIONAL RARE EARTH ELEMENT DEPOSITS OF THE EASTERN UNITED STATES: RESOURCE POTENTIAL IN A GLOBAL CONTEXT

World reserves of most rare earth elements (REEs) are thought to be sufficient to meet forecasts of world consumption well into the 21st century [1]; however, their global distribution varies widely [2]. Currently, China produces more than 42% of the world’s mine production of REEs [1]. Four types of conventional REE deposits now supply most of the REEs that are vital to modern life [2]. Carbonatite deposits (type 1) are known in the West and Central United States (U.S.). The Northeast and East-Central regions have REE resources in alkaline igneous deposits (type 2). The Southeast region has REE resources in placer deposits of heavy minerals (type 3) and in occurrences of REE ion adsorption clays (IAC) in granite-derived regolith (type 4) [3].

Three of these deposit types mainly produce light REEs (LREEs): Bastnäsite [(M)(CO₃)F; M=LREE, Ca] deposits in alkaline igneous rocks and carbonatites of China and the U.S. together constitute the largest proportion of the world's economic REE resources. Monazite [(M)PO₄; M= LREE, Th]-bearing sands in Australia, Brazil, China, India, Malaysia, South Africa, Sri Lanka, Thailand, and the U.S. comprise the second largest segment. Loparite [(LREE,Ca,Sr)(Ti,Nb)O₃] is mined from mineral deposits in layered alkaline rocks of the Kola Peninsula, Russia. The sources of virtually all heavy REEs, yttrium, and a large proportion of LREEs are regolith-hosted IAC REE deposits of the type (4) found in China and Southeast Asia.

Knowledge of tectonic setting, rock associations, deposit morphology, and deposit genesis has led to the discovery of many conventional REE deposits. Developments on this front are expected to result in further discoveries as processes that aid in the formation of mineable deposits are better understood. To advance our knowledge base there is a need for fundamental studies, both experimental and field-based types, aimed at quantifying geological and geochemical processes that promote redistribution, enrichment, and fractionation of REEs.

[1] USGS, 2022, U.S. Geol. Survey, https://doi.org/10.3133/mcs2022: 202.

[2] Foley & Ayuso (in press) Ch. 1, Rare Earth Industry Status and Prospects, Springer-Nature: 1-54.

[3] Foley & Ayuso (2015) British Columbia Geol. Survey Paper 2015-3: 131-138.