Carbonatite / Clays

Weathered carbonatite rare earths sit in the deep weathered cap that forms when a carbonatite weathers in a tropical climate. At Mount Weld in Western Australia, weathering removed calcium, magnesium, silicon and potassium, which raised the rare earth content of what was left (Verplanck and others, USGS, 2025). Recovering them depends on secondary minerals, which at some deposits are extremely fine-grained.

How weathered carbonatite deposits form

The USGS sorts carbonatite rare earth deposits into three groups by process: primary magmatic, hydrothermal-metasomatic and supergene. The supergene group forms when deep weathering in tropical environments turns the top of a carbonatite into a rare earth-enriched laterite (USGS Professional Paper 1802-O). Thick laterites of this kind can form in humid tropical climates with moderate to high rainfall.

As of 2017, the USGS knew of no laterite-hosted rare earth deposit in the U.S. (USGS). The USGS deposit model counts carbonatites and alkaline intrusive complexes, along with their weathering products, as the primary sources of rare earths (USGS Open-File Report 2011-1256).

Carbonatite laterite and ionic adsorption clay

Both form by tropical weathering but hold their rare earths differently. In south China's ionic adsorption clays, rare earths leached from granite become weakly fixed onto clay minerals and can be easily extracted with weak acids. In weathered carbonatite, the USGS gives monazite, rhabdophane, crandallite-group minerals and cerianite as examples of the laterite's rare earth minerals, all secondary phases (USGS).

At Mount Weld, secondary phosphate minerals are the main hosts, with monazite dominant (Verplanck and others). Announcing its proposed purchase of Meteoric Resources in October 2026, Lynas Rare Earths said the deal could diversify its resource base to two bodies, "one hard rock and one ionic clay" (Lynas and Meteoric). Lynas mines Mount Weld (Lynas).

Which rare earths weathered carbonatite holds

Carbonatite rare earth deposits are strongly enriched in the light rare earths, particularly lanthanum, cerium and neodymium (USGS). At Araxá in Brazil, St George Mining says its August 2026 resource gives exposure to the magnet metals neodymium and praseodymium (St George). Cerium and lanthanum still make up most of the rare earth content there (St George annual report, September 2026).

In 2017 the USGS reported that more than 90% of world niobium supply came from laterites over the Barreiro carbonatite near Araxá (USGS).

Where weathered carbonatite rare earths are found

The USGS names Araxá and Catalão I in Brazil and Mount Weld as examples of residual deposits more than 100 meters thick (USGS).

Brazil. St George interprets Araxá as residual enrichment from prolonged tropical weathering, with rare earths mainly in monazite and niobium in pyrochlore (St George, June 2026). Araxá's mineralized saprolite runs from surface to more than 150 meters deep in places (St George, August 2026). Área Zero, in the northern part of the same Barreiro complex, formed by supergene enrichment of carbonatitic rock (Oliveira and Leite Júnior, Geociências, 2020).

At Poços de Caldas, a 1991 study inferred that the weathered host rock of the Morro do Ferro thorium and rare earth deposit was originally a carbonatite (Waber, SKB). The district's development projects are a different type: Viridis calls Colossus an ionic adsorption clay project (Viridis, July 2026), and Lynas calls Caldeira an ionic clay deposit (Lynas).

Russia. At Tomtor in Yakutia, the thickest weathering crust formed over the carbonatite core, and the richest ores fill depressions in that crust. The study's authors interpret those ores as hydrothermal-sedimentary, with a likely role for biological concentration of rare earth phosphates (Lazareva and others, Russian Geology and Geophysics, 2015). At Chuktukon in Krasnoyarsk Territory, the ore is fine pyrochlore-monazite material in laterite weathering crusts over carbonatite (Kuzmin and others, 2018).

South Africa and Australia. Zandkopsdrift in the Northern Cape is a carbonatite pipe with a lateritic cap more than 80 meters thick in places, its rare earths held mostly by very fine monazite, according to a 2016 conference poster (Le Bras and others, HZDR). At Mick Well in Western Australia, Kingfisher Mining said in November 2024 that it had found both hard rock and clay rare earth mineralization tied to carbonatites (Kingfisher). Kingfisher completed the sale of 12 Gascoyne exploration licenses, including the Mick Well tenements, to a Dreadnought Resources subsidiary on March 31, 2026 (Kingfisher; tenement list).

Why processing weathered carbonatite is hard

At Tomtor, monazite in the richest ores occurs as particles of about 50 nanometers (Lazareva and others). Chuktukon ore is mostly ultrafine particles made of nanoscale aggregates, and earlier studies found it could not be concentrated by traditional methods (Kuzmin and others).

Mount Weld makes a concentrate first, and St George's test work at Araxá follows the same route. In the June 2026 quarter, Lynas said ore variation and the switch from a movable crusher to a fixed one affected Mount Weld concentrate quality, and that feedstock in turn affected cracking and leaching productivity in Kalgoorlie and Kuantan (Lynas).

At Araxá, St George's flotation tests on about five metric tons of saprolite sent a rare earth stream into the niobium flotation tailings. St George plans to test pilot plant tailings of this kind for an intermediate such as mixed rare earth carbonate or oxalate (St George, September 2026).

REEx wrote in August 2026 that metallurgy is the decisive question for Araxá (REEx coverage). In July it noted that recoveries, processing costs and separation efficiency decide whether a project succeeds (REEx coverage). It had earlier asked how complex monazite and pyrochlore processing would be at scale (REEx coverage).

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