Carbonatites Composition and Formation

Carbonatites are uncommon and strange igneous rocks with more than 50% carbonate minerals by volume and less than 20 wt. % silica (SiO₂). Those with more than 20% silica are known as silicarbonatite.

These rocks can be 1) plutonic/intrusive if they form deep in the Earth’s crust or 2) volcanic/extrusive if they form on the Earth’s surface.

However, some carbonatites are subvolcanic or hypabyssal rocks. They formed close to the Earth’s surface, usually less than 2 km or 1.2 mi deep.

The name carbonatites infers the unusually high carbonate content not seen in most igneous rocks, which are silicic.

Pramatha Nath Bose, an Indian geologist and paleontologist, first described carbonatites in the Lower Narbada Valley of India in 1884. But for many years, geologists couldn’t agree on its igneous origin. Some called them mega xenoliths of sedimentary or igneous limestones.

However, by 1960, an experiment showed calcite could form from the liquidus phase at 0.1 GPa and about 650°C (1202°F). Later, natrocarbonatite lavas in Oldoinyo Lengai, Tanzania, confirmed they were igneous rocks.

Description of carbonatites

Carbonatites are rare and peculiar igneous rocks dominated by carbonates, not silica, as is the norm. They resemble marbles and are indistinguishable without geochemical testing.

According to Nature, most carbonatites are intrusive, coarse-grained textured rocks that form as cumulates. Cumulates form when carbonatitic melts crystallize, and these crystals settle and accumulate.  

However, some are extrusive with a fine-grained or aphanitic texture, while others, the subvolcanic, are fine-grained.

They are peculiar or unusual because:

1. Have more than 50% carbonates and less than 20% silica
2. Carbonatites are highly enriched with rare earth elements  (REE) like lanthanum (La), neodymium (Nd), Cerium (Ce), and praseodymium (Pr).
3. Unusually high elements like barium, caesium, rubidium, strontium, phosphorus, and niobium
4. High in volatiles like carbon dioxide, fluorine, chlorine, and sulfur,
5. They show very high solubilities of many elements considered rare in silicate magmas

The age of carbonatites is Archean to the present. The oldest known emplacement is in Tupertalik in Greenland, about 3.007 Ga, while the youngest is Ol Doinyo Lengai, currently active.

Ancient ones mostly have calcite, while recent ones have sodium carbonate. Reasons include water dissolving and carrying away sodium carbonates. This may be why extrusive ones don’t appear in geologic records.

Carbonatite magmas or melt and lava

Carbonatite magmas or melts form ionic liquids with extremely low viscosity of ≈5 × 10⁻³ poise, near water, and low densities. Viscosity is a measure of resistance to flow.

Therefore, they rise fast at 20-65 m/s and flow tens of meters per second, outrunning people.

Their viscosity is very low because they are low in silica and don’t polymerize like in silicic magma.

Also, according to a research article, carbonatite melts have a high solubility of elements not seen in silicate magmas and the highest capacity to dissolve volatiles like water and halogens at crustal pressure.

Lastly, these melts are probably more alkaline or higher in sodium than the resultant rock. Hydrothermal fluids in magma remove more of it as it cools. This may be why sodium carbonate isn’t present in plutonic carbonatites.

Carbonatite eruption

Carbonatite (natrocarbonatite) eruptions are known only in recorded history in Doinyo Lengai in Tanzania along the East Africa Rift Valley.

These eruptions are characterized by the coolest eruption temperature at 500–600 °C (932- 1112°F) and involve highly fluid, nearly water-like lava.

Most eruptions are effusive eruptions forming lava flows that resemble a pahoehoe. Pahoehoes are basaltic lava flows with a ropy, hummocky, or smooth surface common in Hawaii.

However, others are explosives, ejecting pyroclasts, or volcanic debris into the air. In 2007, it produced a column 3 km high.

Pyroclasts and lava formed quickly undergo alteration and weathering. After a few hours of eruptions, the lava and ash will change from black or dark brown to white. They will then turn grey after a few days and brown after a few weeks.

It appears that the intensity of carbonatite volcanism has increased in recent times. Possible reasons are lower geothermal gradient favoring low-temperature partial melting.

Alternatively, weathering may have eliminated extrusive deposits from the geological records.

Mineral composition

Carbonatite composition is highly variable. These rocks mostly contain carbonate minerals like calcite, dolomite, ankerite, burbankite, strontanite, parisite siderite, and, less often, ancylite. Also, they have lesser amounts of non-carbonate minerals like silicates, phosphate, oxides,  sulfides, spinels, and sulfates like barite and halides.

Silicate amounts vary greatly. Common silicates include olivine, mica (phlogopite, vermiculite, or biotite), clinopyroxenes (diopside and aegirine-augite), amphibole, zircon, natrolite, andradite, allanite, monticellite, etc.

Also, it has silica-undersaturated feldspathoids like sodalite, nepheline, and other feldspathoids.

On the other hand, oxides include magnetite, zirconia (baddeleyite), rutile, pyrochlore, perovskite, hematite, ilmenite, and hematite, while phosphates are apatite and monazites.

Lastly, sulfides are pyrite, galena, pyrrhotite, and sphalerite, while halides include fluorite, halite, or sylvite.

1. Carbonatite subtypes or varieties

• Calcite carbonatites or calciocarbonatites:  It is dominated by calcite (former names sövite (coarse-grained) and alvikite (fine-grained).
• Dolomite carbonatite or magnesiocarbonatites: Dolomite is the dominant carbonate, initially called rauhaugite or beforsite.
• Ferrocarbonatite: It is dominated by iron-carbonate like ankerite or siderite.
• Siderite calcite carbonatite or ferruginous calciocarbonatite: It is dominated by calcite and then siderite, an iron-rich carbonate.

2. Natrocarbonatite

Natrocarbonatite is a peculiar alkaline carbonatite with a carbonate complex of sodium, potassium, and calcium (Na, K, and Ca). Gregoryite (Na₂, K₂,Ca)CO₃) and nyerereite (Na₂Ca(CO₃) are the dominant carbonates.

Natrocarbonatite melt likely forms from the extreme fractional crystallization of sodium-rich olivine melilitite.

Lastly, sodium and potassium carbonates in natrocarbonatite are unstable on the earth’s surface and weather quickly.

Naming

Carbonatite names can include prefixes of dominant minerals like calcite or dolomite. If several, follow the order of their dominance and place a hyphen between the minerals. Examples are calcite, dolomite, or dolomite-calcite carbonatite.

Also, you can use the name of a non-essential mineral like biotite or olivine prefix. If it has a dominant carbonate, start with the non-essential mineral, then the dominant carbonate(s) mineral hyphen between the non-essential mineral and the dominant carbonate(s).

For instance, olivine-calcite carbonatite means that calcite is the dominant carbonate mineral. However, the said rock also has olivine, a non-essential mineral.

How do carbonatites form?

Isotopic evidence and association with kimberlite, lamproites, and diamonds prove that the origin of carbonatites is mantle. They don’t form from melting marble or limestones by intruding magma.

However, they may interact by enriched lithospheric mantle but only to a small extent. The lithospheric mantle is the rigid upper part of the mantle that, together with the crust, makes the lithosphere layer.

There are three ways in which carbonatites form which are:

• Extremely low-degree partial melting from carbonate-rich mantle peridotites, i.e., only slight, about 0.01%.
• Silicate-carbonatite liquid immiscibility like oil and water, where carbonatite melts, segregates form from parental alkaline silicate. It then precipitates and forms carbonatite cumulates. Carbonatites associated with silica undersaturated alkaline rocks probably formed this way.
• Unusual or extreme fractional crystallization from parent alkaline silica-undersaturated silicate magmas. Such rocks include olivine melilitites, nephelinites, or lamprophyres

Carbonatite melt source rocks are mostly primary, i.e., were present from the Earth’s formation.

However, some may form from partial melting of subducted carbonate rocks, especially the younger ones.

Lastly, alkalis (Na and K) play a role in forming these melts.

How do carbonatites occur and associated rocks?

Most carbonatites are intrusive or plutonic, subvolcanic. However, some are extrusive, including pyroclasts, tuff-rings, stratocones, and lava flows.

Extrusive ones are uncommon because they quickly erode. Thus, they aren’t preserved in geological records, not necessarily because they don’t occur.  

Most carbonatite bodies coexist with alkaline igneous rocks, forming relatively small (<25km²) alkaline-carbonatite complexes. Such complexes can host multiple intrusions, including planar or dike swarms of both rocks.

Some complexes are concentrically arranged and become poorer in silica towards the core. Phalaborwa, South Africa, is a concentrically zoned complex.

In other complexes, carbonatite can also zone itself concentrically with distinct zones. For instance, it can have older outer calcite carbonate, followed by dolomite carbonate, and a younger core with siderite or carbonatite.

Mount Weld and Mud Tank in Australia is a good example. It has multistage cylindrical bodies with distinct phases.

Carbonatites are associated with alkaline silicate rocks like nephelinites, nepheline syenite, phonolites, ijolites, and urtites in alkaline-carbonatite complexes. Less commonly associated rocks are lamprophyres, pyroxenites, peridotites, and melilitites.   

These rocks are mostly silica-undersaturated alkaline ultramafic rocks or evolved ones with feldspathoids.

Also, some carbonatites occur as isolated volcanic necks, pipes (diatreme), dikes or dike rings, sills, cone sheets, veins, lopoliths, breccia (pyroclastic deposits), plugs, or small plutons (stocks).

Where are carbonatites found or deposits?

Carbonatites occur on all continents, including small ones like Antarctica. Most occur on stable continental intraplates or cratons and are associated with continental rift systems like the Rhine Valley and East Africa.

A few carbonatites occur on continental margins and are linked to orogens or plate separation. Examples are in Pakistan along Himalayan thrust belts. However, there is a likelihood they were emplaced before orogenesis.

Only three cases of carbonatites are known on oceanic islands, i.e., the Kerguelen Islands, Fuerteventura in the Canary Islands, and Santiago in Cape Verde. Their origin could be remnants of continental crust as the African plate drifted.

There are over 500 known locations or occurrences with carbonatite deposits, with more than half of the deposits in Africa.

African deposits occur as individuals on crustal domes. Some are clustered on large swells, some up to 1000 kilometers across. Most are concentrated on the East African Rift.

Some places where carbonatites occur include Canada, especially Fennoscandia, Quebec, Eastern Africa, Southern Africa, India, Russia, New Zealand, Germany, etc.

Locations with carbonatites in the US include Mountain Pass in California, Elk Creek in Nebraska, Bearpaw Mountains in Montana, and Gem Park and Iron Hill in Colorado. Others are the Magnet Cove igneous complex in Arkansas and the Avon Volcanic District in Missouri.

In Canada, they occur in the Rocky Mountain Rare Metal Belt in British Colombia and Oka and Saint-Honoré in Quebec, Canada.

Notable places outside North America with carbonatite deposits include Ol Doinyo Lengai in Tanzania, Jacupiranga and Minas Gerais in Brazil, Mud Tank and Mount Weld in Australia, and Palabora Complex in South Africa.

Others are Newania and Amba Dongar in India, Fen Complex in Norway, Kovdor and Vischnevogorsk in Russia, Ayopaya in Bolivia, and Cerro Impacto in Venezuela.

Fenitization

Fenitization is the process by which hydrothermal fluids from alkaline or carbonatitic magma released during cooling percolate into and alter the composition of surrounding country rock.

This intense sodium or alkali metasomatism process produces rocks called fenite with alkali-rich minerals.

Some products of fenitization include aegirine, nepheline, alkali feldspar, alkali amphiboles like arfvedsonite or glaucophane, phlogopite, and carbonates. This can occur with hematite, phosphates, iron, and titanium oxides.

Lastly, if fenitization occurs in the mantle, it possibly generates kimberlite or lamproites.

Carbonatite significance

Carbonatites and associated alkaline rocks are a source of economically valuable metals and mineral ores like niobium (pyrochlore ore), magnetite (iron ore), thorium, vanadium, apatite (phosphorus), uranium, and fluorite.

Also, some have titanium, barite (barium ore), tantalum, zirconia, vermiculite, and unusual concentrations of rare earth elements (REE), including the highest lanthanide known in rocks.

Some of the minerals with REE are fluorocarbonates like bastnäsite (most exploited) and synchysite parisite, phosphates like monazite and apatite, and hydrated carbonates like ancylite.

These valuable minerals, metals, and REE may be from primary magma or deposited by hydrothermal fluids. Some occur as laterites derived from carbonates.

Examples of carbonatites where REEs are mined are Maoniuping, Dalucao, and Bayan Obo (largest REEs worldwide) in China and Mountain Pass in California, USA, Jacupiranga, Brazil, and Mount Weld in Western Australia.

On the other hand, niobium is Araxá and Tapira Complex in Minas Gerais state, Brazil, Oka and Niobec Niobium in Quebec, Canada, and Catalão (Brazil)

Palabora Complex South Africa has copper ores, mica, vermiculate and apatite. Also, it has small amounts of linnaeite (cobalt source), baddeleyite (zirconium and hafnium source), and gold, silver, platinum, and nickel byproducts.

Apatite, magnetite and REE are mined in Kola Peninsula of Russia. Also, apatite occurs in Siilinjärvi in Finland

Lastly, Uzbekistan has small diamonds, while Ipanema in Brazil has vermiculite.

Frequently Asked Questions (FAQs)

References

• Neukirchen, F., & Ries, G. (2020). The world of mineral deposits: A beginner’s Guide to Economic Geology. Springer.
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• Philpotts, A. R., & Ague, J. J. (2022). Principles of igneous and Metamorphic Petrology (3rd ed.). Cambridge University Press.
• Winter, J. D. (2014). Principles of igneous and metamorphic petrology. Pearson Education.
• Yaxley, G. M., Anenburg, M., Tappe, S., Decree, S., & Guzmics, T. (2022). Carbonatites: Classification, sources, evolution, and emplacement. Annual Review of Earth and Planetary Sciences, 50(1), 261–293. https://doi.org/10.1146/annurev-earth-032320-104243
• Simandl, G. J., & Paradis, S. (2018). Carbonatites: Related ore deposits, resources, footprint, and Exploration Methods. Applied Earth Science, 127(4), 123–152. https://doi.org/10.1080/25726838.2018.1516935
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• Randive, K., & Meshram, T. (2020). An overview of the carbonatites from the Indian subcontinent. Open Geosciences, 12(1), 85–116. https://doi.org/10.1515/geo-2020-0007