Although rare earth elements are relatively abundant in the Earth’s crust, they are rarely concentrated into mineable ore deposits. The estimated average concentration of the rare earth elements in the Earth’s crust, which ranges from around 150 to 220 parts per million, exceeds that of many other metals that are mined on an industrial scale, such as copper (55 parts per million) and zinc (70 parts per million). Unlike most commercially mined base and precious metals, however, rare earth elements are rarely concentrated into mineable ore deposits.
The principal concentrations of rare earth elements are associated with uncommon varieties of igneous rocks, namely alkaline rocks and carbonatites. Potentially useful concentrations of REE-bearing minerals are also found in placer deposits, residual deposits formed from deep weathering of igneous rocks, pegmatites, iron-oxide copper-gold deposits, and marine phosphates.
Alkaline igneous rocks form from cooling of magmas derived by small degrees of partial melting of rocks in the Earth’s mantle. The formation of alkaline rocks is complex and not fully understood but can be thought of as a geologic process that extracts and concentrates those elements that do not fit into the structure of the common rock-forming minerals. The resulting alkaline magmas are rare and unusually enriched in elements such as zirconium, niobium, strontium, barium, lithium, and the rare earth elements.
When these magmas ascend into the Earth’s crust, their chemical composition undergoes further changes in response to variations in pressure, temperature, and composition of surrounding rocks. The result is an astonishing diversity of rock types that are variably enriched in economic elements, including the rare earth elements. The mineral deposits associated with these rocks are likewise quite diverse and awkward to classify, in that the distinctive features of these deposits and their rarity can result in classifications that have only one or a few known examples.
Current mineral-processing practice is capable of sequential separation of multiple mineral phases but it is not always cost effective to do so. When elements of interest are found in two or more mineral phases, each requiring a different extraction technology, mineral processing is relatively costly. Many rare earth elements deposits contain two or more rare earth elements−bearing phases. Therefore, rare earth elements deposits in which the rare earth elements are largely concentrated in a single mineral phase have a competitive advantage.
To date, REE production has largely come from single-mineral-phase deposits, such as Bayan Obo (bastnasite), Mountain Pass (bastnasite), and heavy-mineral placers (monazite). Rare earth elements−bearing minerals, once separated, contain as many as 14 individual rare earth elements (lanthanides and yttrium) that must be further separated and refined. The complexity of extracting and refining rare earth elements is illustrated in the Rare Earth Chemistry section below. Unlike metal sulfides, which are chemically simple compounds, REE-bearing minerals are quite complex. Rare earth elements, are typically extracted and refined through dozens of chemical processes to separate the different rare earth elements and remove impurities.
Source: USGS; The Principal Rare Earth Elements Deposits of the United States—A Summary of Domestic Deposits and a Global Perspective