Carbonatites vs Pegmatites: Two Families of Rocks Rich in Rare Earth Elements
Among the many geological settings capable of concentrating rare earth elements, two rock families stand out in particular: carbonatites and pegmatites. Although both can host these elements, their formation processes and their mineralogical and geochemical characteristics differ considerably. These distinctive features shape the nature of the minerals present and the distribution of rare earths within each rock type.
Understanding how carbonatites and pegmatites form, the rare earth-bearing minerals they contain and their geochemical differences offers a clearer picture of the variety of geological settings examined in mineral exploration.
Carbonatites: Mantle-Derived Igneous Rocks Rich in Light Rare Earths
Carbonatites are the world's main source of light rare earth elements. Their mineral makeup and mode of formation set them apart from almost every other known igneous rock.
A Rare, Deep-Seated Igneous Rock with a Carbonate Composition
What makes carbonatites unusual is largely the nature of the minerals they are made of. While most igneous rocks are dominated by silicate minerals, carbonatites consist mainly of carbonate minerals. By definition, a carbonatite contains more than 50% by volume of primary carbonate minerals, most often calcite and/or dolomite [1] [2]. Carbonatitic magmas are therefore among the most silica-poor magmas known [3].
On a global scale, carbonatites remain relatively uncommon. According to some geological compilations, roughly 330 occurrences have been documented [4]. They are most often associated with continental rift zones [4] [1], that is, regions where the Earth's crust stretches and thins, making it easier for deep magmas to rise. This link with rifts matters because it reflects the deep origin of these rocks: the parent magmas of carbonatites, along with some of the elements they concentrate, including rare earths, are derived from mantle sources, in other words from the Earth's mantle lying beneath the crust [1].
The exact way carbonatites form nevertheless remains an active area of research [2]. Several mechanisms may be involved, including partial melting of the mantle, the separation of carbonate liquids from silica-undersaturated alkaline silicate magmas, or the fractionation of carbonated silicate magmas [3]. Even though continental rifts are the most common setting, some carbonatites are also found in orogenic environments, linked to mountain building [1]. This range of settings and mechanisms explains why carbonatites cannot be reduced to a single formation model: they form a rare and complex family of rocks that is still being studied to better understand its origin and mineral potential.
Why Do Carbonatites Concentrate Light Rare Earths?
The ability of carbonatites to concentrate light rare earths largely comes down to how certain elements behave in an evolving magma. In geochemistry, the term incompatible elements refers to elements that are not easily taken up by the first minerals to crystallize as the magma cools. Instead of being quickly incorporated into those minerals, they stay in the residual liquid, where their concentration gradually rises. Light rare earth elements, or LREE, such as lanthanum (La), cerium (Ce), praseodymium (Pr) and neodymium (Nd), belong to this group.
In carbonatites, this behaviour is especially pronounced. LREE tend to partition preferentially into carbonatitic magmas [5]. As crystallization proceeds, alkalis and light rare earths therefore build up in the residual liquids rather than in the earliest minerals [5]. In many carbonatites, the fractionation of minerals such as calcite or ferroan dolomite can then drive these liquids toward mobile alkaline melts or brines, from which primary rare earth carbonates may form [5].
This process helps explain why carbonatites can reach the highest rare earth concentrations of any igneous rock [1], sometimes up to several weight percent of rare earth oxides [4]. That said, it would be wrong to conclude that every carbonatite offers the same potential. Their mineralogical and chemical composition varies according to several factors, including the origin of the magma, liquid immiscibility, fractional crystallization and post-magmatic alteration [3]. Some carbonatites are thus strongly enriched in rare earths, while others lack the mineralogical, geochemical or economic characteristics required for mining [1] [3].
Rare Earth-Bearing Minerals in Carbonatites
In carbonatites, rare earths are mainly hosted by fluorocarbonates such as bastnäsite, as well as by phosphates like monazite and apatite [1]. Bastnäsite is particularly important for light rare earths, notably lanthanum, cerium, praseodymium and neodymium. This mineralogy partly explains why carbonatites have been, for several decades, a major global source of niobium and light rare earths [1]. Their economic potential nonetheless remains selective: although several hundred carbonatites are known worldwide, only a small number are mined for rare earths.
Beyond rare earths, carbonatites may also be associated with niobium, thorium, phosphates, iron, fluorite, copper, vanadium, titanium, uranium or calcite [1] [2]. Thorium deserves special mention, since it is both a naturally radioactive element and a resource being studied for certain advanced nuclear pathways. This variety of commodities highlights the co-production potential of carbonatites, but it is also a reminder that each deposit must be assessed on the basis of its mineralogy, possible by-products and processing requirements.
Pegmatites: Coarse-Grained Granitic Rocks with a Variable Geochemical Profile
Unlike carbonatites, pegmatites are generally tied to the evolution of granitic magmas, and therefore to silicate magmas. They stand out for their very large crystals and for their capacity to concentrate certain rare elements during the final stages of crystallization. They can thus contain rare earths, but their geochemical profile is often more variable than that of carbonatites.
Formation and Characteristics of Granitic Pegmatites
A granitic pegmatite is an igneous rock characterized by generally large crystals, composed mainly of quartz, feldspars and micas. Its formation is often linked to the late stages of granitic magma evolution, although some pegmatites may also result from the partial melting of crustal rocks [8].
As a magma cools, fractional crystallization leads to the gradual formation and separation of certain minerals. Elements that do not fit easily into these early minerals, known as incompatible elements, can then accumulate in the residual liquid. This process contributes to the enrichment of some pegmatites in rare elements, including rare earths [8].
Granitic pegmatites display several geological features:
Large crystals: linked to the particular crystallization conditions of pegmatitic magmas;
A variable mineral composition: minerals may be unevenly distributed, sometimes forming distinct zones within a single pegmatite;
Diverse accessory minerals: some can concentrate rare elements, notably niobium, tantalum, yttrium or rare earths, depending on the type of pegmatite.
In some systems, hydrothermal fluids also come into play during the final stages of formation. They can locally modify the composition of the rock and promote the redistribution or precipitation of rare earth-bearing minerals [12].
So, despite shared characteristics, pegmatites can show very different compositions and mineral concentrations depending on their origin and geological history.
Rare Earth-Bearing Minerals in Pegmatites and Their Geochemical Profile
The geochemical profile of pegmatites varies widely depending on their family and formation conditions. Some pegmatites may be enriched in yttrium (Y) and heavy rare earths (HREE), notably dysprosium (Dy) and terbium (Tb), while others are dominated by light rare earths (LREE). These differences are largely linked to the nature of the accessory minerals present and to the evolution of the magmatic system [8].
Some pegmatites, particularly among the rare-element-enriched families, may contain minerals that carry yttrium and heavy rare earths, such as xenotime and gadolinite [8]. Xenotime is an yttrium phosphate often associated with heavy rare earths, whereas gadolinite is a silicate that can also concentrate yttrium and certain heavy elements. In some rare-element pegmatites, rare earth-bearing minerals may occur alongside niobium- and tantalum-bearing minerals. These associations differ from one pegmatite family to another and add to the diversity of their geochemical profiles [8].
This mineralogical diversity also affects the potential of pegmatites as sources of rare earths. Unlike the large carbonatites mined for these elements, their economic importance generally remains more limited and depends on the specific characteristics of each deposit [10].
Carbonatites and Pegmatites: A Geochemical and Economic Comparison
Table 1: Carbonatites vs granitic pegmatites for rare earth elements
| Criterion | Carbonatites | Granitic pegmatites |
|---|---|---|
| Rock type | Igneous rocks dominated by carbonate minerals such as calcite and dolomite | Generally granitic igneous rocks, dominated by quartz, feldspars and micas |
| Geological origin | Mantle-derived magmas, often associated with continental rift zones | Mainly associated with evolved granitic magmas, with other possible origins, including partial melting of crustal rocks. |
| Rare earth profile | Generally dominated by light rare earths, notably La, Ce, Pr and Nd | More variable; some pegmatites may be enriched in yttrium and heavy rare earths |
| Host minerals | Mainly bastnäsite, monazite and apatite | Accessory minerals such as monazite, xenotime or apatite, depending on the type of pegmatite |
| Internal variability | Varies with magma evolution and post-magmatic alteration, but often organized around LREE enrichment | Highly variable from one body to another, and sometimes within the same pegmatite |
| Economic role | Major global source of light rare earths and niobium | Generally limited economic interest for rare earths, but variable potential for other rare elements |
What Role Do Carbonatites and Pegmatites Play in Rare Earth Supply?
Carbonatites and pegmatites illustrate two geological settings in which rare earth elements become concentrated. The former have been a major source of light rare earths for several decades [1]. The latter show more variable compositions and can, in some cases, be enriched in heavy rare earths. Their economic contribution, however, generally remains more modest [8] [10].
Both families belong to a broader range of deposits that also includes alkaline igneous complexes, ion-adsorption clays and heavy mineral sands [11]. For mineral exploration, this diversity is a reminder that a deposit's potential does not depend solely on its rare earth grade. Mineralogy, co-production opportunities and processing conditions also play a significant role. Understanding these characteristics makes it easier to evaluate geological resources and to guide exploration strategies for critical minerals.
To keep up with developments in this field, subscribe to our newsletter and follow us on LinkedIn.
References
[1]: Verplanck, Philip L., et al. "Rare Earth Element Ore Geology of Carbonatites." Rare Earth and Critical Elements in Ore Deposits, edited by Philip L. Verplanck and Murray W. Hitzman, Reviews in Economic Geology, vol. 18, Society of Economic Geologists, 2016, pp. 5–32. https://pubs.usgs.gov/publication/70138176.
[2]: Cullers, Robert L. "Carbonatites." Research Starters: Earth and Atmospheric Sciences, EBSCO Information Services, 2022. https://www.ebsco.com/research-starters/earth-and-atmospheric-sciences/carbonatites.
[3]: Yaxley, Gregory M., et al. "Evolution of Carbonatite Magmas in the Upper Mantle and Crust." Elements, vol. 17, no. 5, 2021, pp. 315–320. https://doi.org/10.2138/GSELEMENTS.17.5.315.
[4]: Bank, Tracy, et al. "Geology of Rare Earth Deposits." NETL Rare Earth Elements Technical Team Meeting, National Energy Technology Laboratory, U.S. Department of Energy, 27 June 2016, presentation. https://netl.doe.gov/projects/files/GeologyofRareEarthDeposits_062716.pdf.
[5]: Anenburg, Michael, et al. "Formation of Rare Earth Deposits in Carbonatites." Elements, vol. 17, no. 5, 2021, pp. 327–332. https://doi.org/10.2138/GSELEMENTS.17.5.327.
[8]: Černý, Petr, and T. Scott Ercit. "The Classification of Granitic Pegmatites Revisited." The Canadian Mineralogist, vol. 43, no. 6, 2005, pp. 2005–2026. https://doi.org/10.2113/gscanmin.43.6.2005.
[10]: Long, Keith R., et al. The Principal Rare Earth Elements Deposits of the United States: A Summary of Domestic Deposits and a Global Perspective. U.S. Geological Survey, Scientific Investigations Report 2010–5220, 2010. https://pubs.usgs.gov/sir/2010/5220/.
[11]: Foley, Nora K., and Robert A. Ayuso. "Conventional Rare Earth Element Mineral Deposits—The Global Landscape." Rare Earth Metals and Minerals Industries: Status and Prospects, Springer, 2024, pp. 17–56. https://www.usgs.gov/publications/conventional-rare-earth-element-mineral-deposits-global-landscape.
[12]: Gagnon, Joel E., et al. "The Composition and Origin of Hydrothermal Fluids in a NYF-Type Granitic Pegmatite, South Platte District, Colorado: Evidence from LA–ICP–MS Analysis of Fluorite- and Quartz-Hosted Fluid Inclusions." The Canadian Mineralogist, vol. 42, no. 5, 2004, pp. 1331–1355. https://doi.org/10.2113/gscanmin.42.5.1331.
[13]: Smith, David B., et al. Geochemical and Mineralogical Maps, with Interpretation, for Soils of the Conterminous United States. U.S. Geological Survey, Scientific Investigations Report. https://pubs.usgs.gov/sir/2017/5118/sir20175118_element.php?el=39.
