RARE EARTH ELEMENT AND NIOBIUM ENRICHMENTS IN CARBONATITES

For nearly 50 years, carbonatites have been the primary source of niobium and rare earth elements (REEs), in particular the light REEs, including La, Ce, Pr, and Nd. Carbonatites are a relatively rare type of igneous rock composed of greater than 50 modal % primary carbonate minerals, primarily calcite and/or dolomite. They contain the highest concentrations of REEs of any igneous rocks. Although there are more than 500 known carbonatites in the world, currently the only four carbonatites being mined for REEs are Bayan Obo, Maoniuping, and Dalucao deposits in China, and the laterite deposit overlying a carbonatite at Mount Weld in Western Australia. In addition to REEs, carbonatite-related deposits are the primary source of Nb, with the Araxá deposit, a carbonatite-derived laterite in Minas Gerais state, Brazil, being the dominant producer. Other commodities sometimes produced from carbonatite-related deposits include phosphates, fluorite, copper, vanadium, titanium, and calcite.

Types of ores in carbonatites include those formed as primary magmatic minerals, from late magmatic hydrothermal fluids, and by enrichment in weathered horizons. The principle REE-bearing mineral phases include fluorocarbonates (bastnäsite, parisite, and synchysite), hydrated carbonates (ancylite), and phosphates (monazite and apatite). The dominant REE ore mineral exploited at most mines is bastnäsite, which can be coarse grained and contains approximately 75 wt.% RE₂O₃. Pyrochlore [(Ca,Na)₂Nb₂O₆(OH,F)] is the primary Nb ore mineral. At the Araxá deposit, bariopyrochlore is mined from the lateritic zone.

Carbonatites are primarily associated with continental rifting, but some carbonatites are associated with orogenic activity. Processes responsible for REE enrichment include fractional crystallization of the carbonatitic magma, enrichment of REEs in orthomagmatic or hydrothermal fluids and subsequent precipitation or subsolidus metasomatic redistribution of REEs, and breakdown of primary carbonatitic mineral phases by chemical weathering and sequestration of REEs in secondary minerals. Although there is debate on how carbonatite magmas are generated, the parental magma and REEs are clearly derived from mantle sources.