Carbonatite and Alkaline Intrusion-Related Rare Earth

Element Deposits─A Deposit Model

By Philip L. Verplanck and Bradley S. Van Gosen

Open-File Report 2011–1256

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

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U.S. Geological Survey

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U.S. Geological Survey, Reston, Virginia: 2011

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Verplanck, P.L., and Van Gosen, B.S., 2011, Carbonatite and alkaline intrusion-related rare earth element

deposits─A deposit model: U.S. Geological Survey Open-File Report 2011–1256, 6 p. 

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1 

Carbonatite and Alkaline Intrusion-Related Rare Earth

Element Deposits─A Deposit Model

By Philip L. Verplanck and Bradley S. Van Gosen

The rare earth elements (REEs) are not as rare in nature as their name implies, but economic 

deposits with these elements are not common and few deposits have been large producers. In the past 

25 years, demand for REEs has increased dramatically because of their wide and diverse use in hightechnology applications. Yet, presently the global production and supply of REEs come from only a 

few sources. China produces more than 95 percent of the world’s supply of REEs. Because of China’s 

decision to restrict exports of these elements, the price of REEs has increased and industrial countries 

are concerned about supply shortages (Tse, 2011). As a result, understanding the distribution and 

origin of REE deposits, and identifying and quantifying our nation’s REE resources have become 

priorities.

Carbonatite and alkaline intrusive complexes, as well as their weathering products, are the 

primary sources of REEs (Long and others, 2010). The general mineral deposit model summarized 

here is part of an effort by the U.S. Geological Survey’s Mineral Resources Program to update existing 

models and develop new descriptive mineral deposit models to supplement previously published 

models for use in mineral-resource and mineral-environmental assessments. We are discussing 

carbonatite and alkaline intrusion-related REE deposits together because of their spatial association, 

common enrichment in incompatible elements, and similarities in genesis. A wide variety of 

commodities have been exploited from carbonatites and alkaline igneous rocks, such as rare earth 

elements, niobium, phosphate, titanium, vermiculite, barite, fluorite, copper, calcite, and zirconium. 

Other enrichments include manganese, strontium, tantalum, thorium, vanadium, and uranium. 

The REEs are defined as the elements from lanthanum to lutetium (atomic numbers 57 to 71) 

and yttrium (atomic number 39). Yttrium is typically included as a REE with the “lanthanides” 

(lanthanum to lutetium) because of its similarity in chemical properties and applications. The elements 

from lanthanum to gadolinium are referred to as light REEs (LREEs); the elements from terbium to 

lutetium are referred to as heavy REEs (HREEs). Overall, the REEs have similar geochemical 

properties, because they all form stable 3+ ions of similar size. Slight differences in their geochemical 

behavior occur because, with increasing atomic number, there is a steady decrease in ionic size. 

Cerium and europium can exist in oxidation states other than 3+

, with cerium also occurring as 4+ and 

europium as 2+. Yttrium only occurs as 3+ and has an ionic radius similar to holmium (1.019 compared 

to 1.015 angstroms, respectively); thus, yttrium is grouped with the HREEs. Although the geochemical 

properties of REEs are similar, their metallurgical, chemical, catalytic, electrical, magnetic, and optical 

properties vary, and these unique properties and differences have lead to their prominence in a variety 

of emerging technologies. 

Whereas no REE deposit models have been published, some deposits are described in the 

literature. In particular, extensive work has been done on the petrology of carbonatites and alkaline 

intrusive complexes. Understanding the petrology and petrogenesis of carbonatites and alkaline 
2 

intrusive complexes is essential in unraveling the complexities of REE deposits associated with these 

rock types. General descriptions of a variety of types of REE deposits can be found in Neary and 

Highley (1984), Mariano (1989a, b), Wall and Mariano (1996), Castor and Hedrick (2006), Castor 

(2008), and Long and others (2010). Orris and Grauch (2002) provide a worldwide compilation of data 

on rare earth mines, deposits, and occurrences. Berger and others (2009) compiled tonnage and grade 

data for niobium and REE-bearing carbonatite deposits. Bell (1989) and chapters within provide an 

excellent geologic framework for carbonatites; Fitton and Upton (1987) and chapters within provide an 

excellent framework for alkaline intrusive complexes.

Alkaline rocks form an expansive category of igneous rocks. Using a broad definition, alkaline 

rocks are deficient in SiO2 relative to Na2O, K2O, and CaO (Winter, 2001). Two of the most important 

subclasses of alkaline rocks with respect to REE deposits are carbonatites and peralkaline rocks. 

Carbonatites are defined by the International Union of Geological Sciences (IUGS) system of igneous 

rock classification as having more than 50 modal percent primary carbonate minerals, such as calcite, 

dolomite, and ankerite, and less than 20 percent SiO2 (Le Maitre, 2002). Most identified carbonatites 

are intrusive bodies, but a few extrusive examples are known, most prominently an active carbonatite 

volcano in northern Tanzania (Oldoinyo Lengai volcano; Woolley and Church, 2005). Recent work 

has shown that carbonatites can be quite diverse and likely originate from multiple processes 

(Woolley, 2003; Mitchell, 2005). Alkaline intrusive rocks also contain elevated concentrations of 

REEs; these rock types tend to be spatially associated with carbonatites, but not in all examples. 

Alkaline rocks can be further classified based on their chemistry. Peralkaline rocks are one subset of 

alkaline rocks, defined by (Na2O + K2O)/(Al2O3)>1, and they commonly are enriched in REEs. 

Mineral deposits associated with carbonatites account for the greatest production of REEs. 

Important deposits are the Bayan Obo deposit in Inner Mongolia (China), and the Sulphide Queen 

carbonatite of the Mountain Pass district, California. Until the late 1980s, the Mountain Pass mine was 

the largest global producer of REEs, subsequently supplanted by the Bayan Obo deposit, which is 

currently the largest producer of REEs. Carbonatite REE deposits are extremely enriched in light 

REEs, particularly lanthanum, cerium, and neodymium. The origin of the Bayan Obo deposit has been 

debated, but recent work by Yang and others (2011) demonstrates that the deposit fits into the 

carbonatite class of REE deposits.

Although there are no alkaline intrusion-related deposits currently being mined for REEs, many 

are in various stages of exploration because these peralkaline igneous rocks contain relatively high 

concentrations of heavy REEs. Deposits of interest include those at Bokan Mountain, southeastern 

Alaska; Thor Lake, Strange Lake and Kipawa Lake in Canada; Kola Peninsula, Russia; and 

Ilímaussaq, Greenland. 

Common characteristics of carbonatite and alkaline intrusion-related REE deposits are: (1) they 

tend to be enriched in high field strength elements (REEs, Y, Nb, Zr, and U); (2) REE-rich carbonatites 

tend to be enriched in the LREEs; (3) REE-rich alkaline intrusion-related deposits have variable REE 

enrichments, but are not as enriched in LREEs as carbonatites; (4) REE deposits can contain a variety 

of ore minerals that are usually either REE-bearing carbonates, phosphates, or fluorates; (5) significant 

REE-bearing ore minerals include bastnäsite, monazite, and xenotime; (6) ore minerals can be coarsely 

crystalline to fine grained and can have complex replacement textures; (7) the origin of the REEs is 

commonly crystallization through magmatic processes, but economic enrichment of REEs may also be 

because of precipitation of minerals from a magmatic hydrothermal solution or redistribute of 

magmatic REEs by the hydrothermal fluid; (8) many carbonatite occurrences include breccias; and (9) 

the intrusive bodies generally have alteration halos derived from alkali-rich fluids (fenitization) that 

migrated into and reacted with the surrounding rock. 
3 

Carbonatites and peralkaline igneous rocks associated with REE deposits tend to occur within 

stable continental tectonic units, in areas defined as shields, cratons, and crystalline blocks; they are 

generally associated with intracontinental rift and fault systems (Berger and others, 2009). These 

igneous rocks formed from the cooling of silica-undersaturated, alkaline magmas, which were derived 

by small degrees of partial melting of rocks in the Earth’s mantle. The evolution of these initial mantle 

melts to form REE mineral deposits is not fully understood. Although carbonatites and alkaline 

intrusions are spatially associated, a definitive genetic association is not agreed upon. As summarized 

by Winter (2001), petrogenetic discussions focus on these questions: 

Are carbonatite and alkali magmas developed separately by partial melting within the mantle or do 

carbonatites evolve from parental alkali magmas?

If carbonatites evolve from parental alkali magmas, then is it by fractional crystallization or by liquid 

immiscibility? At what depths does this occur? 

If carbonatites are derived directly from a carbonatite magma, then what is the nature of the parental 

carbonatite magma? 

Regardless of the petrogenetic debates, at some point carbonatite magma is generated, which 

contains anomalous concentrations of REEs and other incompatible elements. The source of REEs is 

the initial magma, and with decreasing pressure and temperature the magma evolves, which leads to 

further enrichments in REEs. Crystallization is one process that drives the evolution of the magma. 

The REE mineralization can be associated with primary magmatic minerals, as has been interpreted for 

the Mountain Pass carbonatite (Mariano, 1989a); however, more commonly, the REE mineralization is 

associated with fluid phases expelled from the carbonatite magmas. The carbonatites can vary laterally 

and vertically from calcic to dolomitic to iron-rich across a district and within a single carbonatite 

mass, and there will be associated major variations in accessory minerals. Following primary 

crystallization, alteration by magmatic, or less commonly meteoric, fluids can produce complex 

assemblages of minerals, accounting for some of the mineralogical and geochemical variation between 

carbonatite units (Castor, 2008). Carbonatite masses and dikes of differing mineralogy and chemistry 

within a single district can represent separate intrusive episodes. 

The principal REE-bearing minerals associated with carbonatites are fluocarbonates 

(bastnäsite, parasite, and synchysite), hydrated carbonates (ancylite), and phosphates (monazite and 

apatite) (Zaitsev and others, 1998). Other REE phases are less common, such as britholite and 

burbankite. Because most carbonatites experienced complex petrogenetic evolution, which included 

hydrothermal overprinting of primary magmatic phases, the resulting mineralogical and textural 

characteristics can be quite varied and intricate, and thus difficult to summarize. For this reason, 

limited published information is available regarding paragenesis, zoning patterns, and structure. 

Similar to carbonatites, the parental magma for alkaline intrusive complexes that host REE 

mineralization is generated as a partial melt of metasomatized mantle material; likewise, this initial 

melt is the source of the REEs. How and where these magmas evolve is not well understood. In 

general, REE deposits associated with peralkaline intrusive complexes fall into two categories: (1) 

deposits in peralkaline, layered complexes, and (2) deposits in veins or dikes associated with 

peralkaline intrusions. Examples of the first category include Thor Lake and Ilímaussaq. An example 

within the United States of the second type is Bokan Mountain. The REE-rich alkaline complexes 

associated with large, layered complexes display evidence of magmatic evolution by fractional 

crystallization and magma chamber processes including the formation of roof and floor cumulates. 

Rare earth element mineralization is associated with individual layers which are rich in REE-bearing 

mineral phases such as eudialyte (Salvi and Williams-Jones, 2005). At Bokan Mountain, Thompson 
4 

and others (1982) and Thompson (1988) invoke fractional crystallization of an already evolved magma 

at shallow crustal levels to explain the observed lithologies. Rare earth element bearing minerals are 

identified in veins or dikes in the groundmass of larger silicate minerals (Warner and Barker, 1989). A 

variety of REE-bearing minerals are associated with alkaline intrusion-related REE deposits, in part 

because secondary processes tend to overprint the primary mineralogy. Important REE-bearing 

mineral phases include apatite, eudialyte, loparite, gittinsite, xenotime, gadolinite, monazite, 

bastnäsite, kainosite, mosandrite, britholite, allanite, and zircon. 

A common feature of most carbonatite and alkaline intrusion-related deposits is that the 

surrounding rocks have been hydrothermally altered because of alkali metasomatism. This style of 

alteration is known as fenitization, so-named because it was first described at the Fen alkaline complex 

in southern Norway. Alkali-rich fluids are released into the country rock from the crystallizing magma, 

and reactions convert the host rock minerals to an assemblage of alkali-bearing minerals. Fenites can 

be classified as sodium or potassium rich. Sodium-rich fenites are characterized by the presence of 

alkaline feldspar with alkali amphibole or sodic pyroxene, and potassium-rich fenites contain 

potassium feldspar (Le Bas, 2008). 

Gamma-ray (radiometric), magnetic, gravity, and remote sensing surveys have been used 

successfully to map REE deposits and in regional reconnaissance surveys to identify targets for more 

detailed geologic mapping and geochemical sampling (Thomas and others, 2010). Because of the 

physical properties of the suite of incompatible elements associated with these deposits, including 

thorium, uranium, tantalum, niobium, titanium, and zirconium, geophysical techniques provide an 

excellent tool for exploration. Aerial gamma-ray surveys measure the naturally emitted gamma 

radiation derived from three radioelements (potassium, uranium, and thorium) which reside in soils 

and rocks within the upper 30 centimeters of the Earth’s surface. Regardless of their form or type, 

REE-bearing deposits contain anomalous concentrations of thorium and uranium, often associated with 

potassic alteration. The radioactivity inherent to these deposits make them well suited for gamma-ray 

surveys. Magnetic and gravity surveys can be used to locate carbonatites and alkaline intrusive 

complexes. Magnetic and gravity anomalies produced by carbonatites and alkaline rocks are because 

of the magnetic and density characteristics of the intrusions, not the REE mineralization. There are 

several notable examples of successfully using airborne magnetic and(or) ground- or airborne-based 

gravity surveys to locate REE-bearing carbonatites or alkaline intrusions. The Mount Weld 

carbonatite, Australia, was discovered during the interpretation of a regional aeromagnetic survey; 

subsequent detailed surveys showed the concentric zonation in its magnetic pattern (Gunn and Dentith, 

1997). In Quebec, the Oka carbonatite-alkaline complex and the St-Honoré carbonatite complex each 

produce strong gravity anomaly highs, well above the surrounding area at Oka (Gold and others, 1967; 

Vallée and Dubac, 1970). 

The environmental issue raised most often in the development of REE deposits is the elevated 

radioactivity of the ore material. The magmatic differentiation processes that concentrate the REEs 

also form minerals containing other incompatible elements, such as thorium and uranium. Thus, 

anomalous values of thorium and uranium, in particular thorium, are common in carbonatites and 

REE-bearing alkaline intrusions. For this reason, thorium-rich areas of REE deposits are typically 

treated as a hazardous material for special handling and treatment, as is the waste in streams generated 

from processing these rocks. In contrast to the majority of uranium-bearing minerals, the thoriumbearing minerals are generally resistant to chemical breakdown, forming relatively insoluble detrital 

minerals. Thus, thorium-bearing minerals typically contribute only trace concentrations of thorium 

(about 1 part per billion) to natural waters (pH 5 to 9). Thorium is insoluble in most natural waters.
5 

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