Carbonatite

Carbonatite is an igneous rock made mostly of carbonate minerals. It is the host rock: the rare earths sit in minerals inside it, most often bastnaesite. Carbonatites have the highest rare earth concentrations of any igneous rock, and they have been the world's main source of light rare earths since the 1960s (USGS Professional Paper 1802-O).

What carbonatite is

The International Union of Geological Sciences classifies carbonatite as an igneous rock with more than 50 modal percent primary carbonate minerals and less than 20 percent silica (USGS Open-File Report 2011-1256). The carbonate is usually calcite and dolomite. It forms from carbonate-rich magma that comes from the mantle, though geologists still debate how that magma forms (USGS PP 1802-O).

Most carbonatites are intrusive. They cool underground as stocks, dikes, veins, tabular bodies and irregular masses.

A few are extrusive. The best-known is Ol Doinyo Lengai in northern Tanzania, an active carbonatite volcano (USGS OFR 2011-1256). It is the only volcano known to have erupted carbonatite lava in historical time, according to the USGS Hawaiian Volcano Observatory.

Why carbonatite concentrates rare earths

As carbonatite magma crystallizes, rare earths either collect in primary rare earth minerals or build up in the late-stage melts and fluids (USGS PP 1802-O). The enrichment favors the light rare earths, particularly lanthanum, cerium and neodymium (USGS OFR 2011-1256).

Depth may decide whether a carbonatite becomes ore. A 2026 Nature Communications study found that magma emplaced deeper than about 10 kilometers keeps rare earths mobile enough to concentrate, while shallow magma traps them early in apatite, REEx reported.

The rare earth minerals carbonatite hosts

The principal rare earth minerals in carbonatites are the fluorocarbonates bastnaesite, parisite and synchysite, the hydrated carbonate ancylite, and the phosphate monazite. Bastnaesite is typically the most important (USGS PP 1802-O). USGS describes three ways the ore forms:

• Magmatic. At Mountain Pass in California, bastnaesite and parisite are interpreted to have crystallized directly from the magma, the only known carbonatite where that is thought to have happened.
• Hydrothermal. At Bayan Obo in China, late fluids formed the ore.
• Weathering. Deep tropical weathering can turn carbonatite into thick rare earth-enriched laterite, as at Mount Weld in Australia and Araxá in Brazil. See carbonatite clays.

Across 58 explored rare earth-niobium carbonatite deposits, the median grade was 0.61 percent total rare earth oxide (USGS PP 1802-O, citing Berger and others, 2009).

Where carbonatite is found

Only 527 carbonatites were known worldwide as of 2008, about 40 percent of them in Africa (USGS PP 1802-O, citing Woolley and Kjarsgaard).

Bayan Obo, a mass of mixed iron deposits and carbonatite in Inner Mongolia, is the world's largest rare earth deposit. USGS listed Maoniuping and Daluxiang in Sichuan, and Weishan, as other carbonatite deposits producing rare earths in China in 2016.

Mountain Pass's Sulphide Queen carbonatite is thought to be the largest rare earth resource in the United States (USGS PP 1802-O). Mount Weld, mined by Lynas Rare Earths, is a carbonatite pipe about 3 kilometers across (Lynas, 2020).

Carbonatite matters for niobium too. USGS reported in 2017, citing 2011 data, that more than 90 percent of world niobium came from laterites over the Barreiro carbonatite near Araxá. The buried Elk Creek carbonatite in Nebraska is the largest identified U.S. niobium resource (USGS PP 1802-O).

Not every carbonatite deposit is light rare earth ore. At Lofdal in Namibia, heavy rare earth mineralization is tied to alteration around carbonatite dikes, and the project is focused on dysprosium, terbium and yttrium, according to Namibia Critical Metals. Carbonatite ores rich in heavy rare earths are treated as a separate ore type, carbonatite-xenotime.

Carbonatite projects in development

• Ngualla, Tanzania. Hosted in the Ngualla Carbonatite, a volcanic pipe about 1 billion years old (Mining Technology, 2016).
• Songwe Hill, Malawi. Mkango Resources' carbonatite complex, where synchysite and apatite are the main rare earth minerals (NS Energy, 2023). Mkango announced an updated definitive feasibility study in March 2026 (Engineering News).
• Kangankunde, Malawi. Lindian Resources' carbonatite, where the mineralization is mainly monazite (Mining Technology). Lindian had Stage 1 financing in place and began step-out drilling in December 2025 (REEx).
• Fen, Norway. Saga Rare Earths is developing a project in the Fen Carbonatite Complex, where Rare Earths Norway separately controls much of the field (REEx, September 2026).

How carbonatite ore is processed

No two rare earth ores are alike, so each new mine needs its own process and extensive testing (USGS SIR 2010-5220). At Mountain Pass, MP Materials crushes and grinds the ore and recovers bastnaesite by froth flotation, according to its 2025 annual report. It then roasts the concentrate in a rotary kiln before leaching, purifying and separating it. Songwe Hill's 2026 feasibility study budgets for a mill, flotation and hydrometallurgy plants (Engineering News).

Thorium is the main problem. Carbonatite ores are commonly radioactive, mostly from thorium, and handling that waste is costly and regulated (USGS PP 1802-O; SIR 2010-5220). On the plus side, carbonatite deposits hold few sulfide minerals, and their carbonate neutralizes acid, so acid-mine drainage is minimal (USGS PP 1802-O).

Frequently asked questions