Carbonatites
Definitions and classifications
The rocks now known as carbonatites were originally described by Bose (1884) from the Lower Narbada Valley of India, but it was not until the investigations of Högbohm (1895) at Alnö, Sweden, and of Brøgger (1921) at Fen, in Norway, that a magmatic origin was postulated for the carbonate-bearing rocks found in these alkaline complexes. Not everyone agreed with this concept; in particular, the highly influential petrologists Reginald Daly (1933) and James Shand (1943) remained adamant that these "igneous limestones" were merely megaxenoliths of sedimentary material. This petrological divide remained until the seminal experimental work of Wyllie & Tuttle (1960), who showed that calcite could crystallize as a liquidus phase at temperatures as low as ∼650°C at 0.1 GPa. This study sounded the death knell of the limestone syntexis hypothesis (Shand 1943) for the genesis of undersaturated alkaline rocks, and the work ushered in a decade of renewed interest in carbonatites in general (Heinrich 1966, Tuttle & Gittins 1966), highlighted by the discovery of the natrocarbonatite lavas at Oldoinyo Lengai, Tanzania (Guest 1956, Dawson 1962).Carbonatites are defined in the IUGS system of classification as: "igneous rocks composed of more than 50 modal per cent primary (i.e., magmatic) carbonate (sensu lato) and containing less than 20 wt.% SiO2" (Le Maitre 2002).
Depending on the predominant carbonate mineral, a carbonatite is referred to as a calcite carbonatite, dolomite carbonatite, or ferrocarbonatite, where the main carbonate is iron-rich (Fig.1 a). If more than one carbonate mineral is present, the carbonates are named in order of increasing modal concentrations. For example, a calcite-dolomite carbonatite is composed predominately of dolomite. If nonessential minerals (e.g. biotite) are present, this can be reflected in the name as biotite-calcite carbonatite.
Where the modal classification cannot be applied, the IUGS chemical classification may be used (Fig.1 b). This classification, based on wt.% ratios subdivide carbonatites into calciocarbonatites, magnesiocarbonatites, and ferrocarbonatites. For calciocarbonatites, the ratio of CaO/(CaO + MgO + FeO + Fe2O3 + MnO) is greater than 0.8. The remaining carbonatites are subdivided into magnesiocarbonatite [MgO > (FeO + Fe2O3 + MnO)] and ferrocarbonatite [MgO < (FeO + Fe2O3 + MnO)] (Woolley and Kempe 1989; Le Maitre 2002). If the SiO2 content of the rock exceeds 20%, it is referred to as silicocarbonatite. A natrocarbonatite is a special variety of carbonatite consisting mainly of Na-K-Ca carbonates, such as nyerereite and gregoryite, known from Ol Doinyo Lengai volcano (Tanzania).
A refinement to the IUGS chemical classification based on molar proportions, proposed by Gittins and Harmer (1997), introduces the term ferruginous calciocarbonatites (Fig.1 b). The boundary separating calciocarbonatites from magnesiocarbonatites and ferruginous calciocarbonatites is set at 0.75, above which carbonatites contain more than 50% calcite on a molar basis. Although not universally accepted, Gittins and Harmer’s classification is commonly used in studies of carbonatite-hosted ore deposits (e.g. Trofanenko et al. 2016).
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Fig.1 Carbonatite classifications according to (a) IUGS based on wt.% (Le Maitre 2002) and (b) Gittins and Harmer (1997) based on molar proportions. C/CMF is the molar ratio of CaO/[CaO + MgO + FeO* + MnO]; FeO* expressed as molar FeO if both FeO and Fe2O3 are determined. From Simandl, G. J., & Paradis, S. (2018).
A mineralogical-genetic classification of carbonatites was proposed by Mitchell (2005). His benchmark paper points to pitfalls of the IUGS classification and subdivides carbonatites into primary carbonatites and carbothermal residua. The term carbohydrothermal carbonatite is defined by Woolley and Kjarsgaard (2008b) as carbonatite which precipitated at subsolidus temperatures from a mixed CO2-H2O fluid that can be either CO2-rich (i.e. carbothermal), or H2O-rich (i.e. hydrothermal).
Origin of carbonatites
There are currently three main hypotheses explaining the origin of carbonatite melts:(1) immiscible separation of parental carbonated silicate magmas at crustal or mantle pressures.
(2) crystal fractionation of parental carbonated silicate magmas such as olivine melilitites or kamafugites.
(3) low-degree partial melting of carbonated mantle peridotite below 70 km depth.
Hypotheses invoking or supporting a possible derivation of carbonatites from the Earth’s crust, or from the Earth’s mantle with some crustal contribution, have also been proposed. Furthermore, a recent study based on boron isotopes of carbonatites worldwide suggests that, although most carbonatites may originate in the upper mantle, younger carbonatites (< 300 My) probably involve at least partial subducted crustal component (Hulett et al. 2016).
However, regardless of their mode of formation, most researchers agree that alka-lis (Na and K) play an important role in the genesis of calcite and dolomite car-bonatites, and ferrocarbonatite intrusions. The importance of alkalis in the genesis of carbonatites is consistent with studies of low-temperature (< 600°C) natro-carbonatitic lavas from Ol Doinyo Lengai, Tanzania that contain 38-40 wt.% combined Na2O and K2O, 4.5 wt.% F, 5.7 wt.% Cl, approximately 15 wt.% Ca, and less than 1 wt.% combined Mg and Fe. Petrographic and geochemical evi-dence from extrusive carbonatites, as well as evidence from intrusive carbonatites suggests that calcite- and dolomite-rich carbonatites are residues or cumulates de-rived from alkali-bearing (moderately alkaline) melts.
Tectonic setting
Most carbonatites and alkaline-carbonatite complexes are emplaced in continen-tal (88% cratonic, 10.5% non-cratonic) settings (Fig.2) in Archean and Proterozoic rocks, or in Phanerozoic rocks underlain by a Precambrian basement..jpg)
Fig.2: Main global occurrences of carbonatites and carbonatite related REE de-posits in the world. From Liu, Y., & Hou, Z. (2017).
Carbonatites form in extensional tectonic settings, along major linear trends re-lated to large-scale intra-plate fracture zones, in association with doming features (crustal arching), or in relation to slab windows in subducting plates. The link be-tween these tectonic features and intense magmatic activity means that many car-bonatites are also temporally and spatially related to large igneous provinces. Carbonatites in orogenic settings are sometimes referred to as post-collisional (Chakmouradian et al. 2008). This is an unfortunate term because carbonatites that are found in orogenic settings may have been emplaced before a transition from extensional to compressional tectonic regimes, or during post-orogenic ex-tensional relaxation and collapse prior to dynamo-thermal metamorphic climax.
Carbonatites are identified in three oceanic island regions: (1) The Canary Is-lands; (2) The Cape Verde Islands, and (3) The Kerguelen Islands; all of which are located of the African continent. However, it is possible that these islands are underlain by remnants of continental lithosphere stranded during drifting of the African plate.
Carbonatite-associated igneous rocks
Almost all carbonatites are associated with alkaline complexes. Worldwide, only 24% of carbonatite rocks are not part of alkaline-carbonatite complexes. A num-ber of distinct carboatite-silicate rock associations occur that include melilitite-sövite, nephelinite-sövite, pyroxenite-sövite, and olivine-rich ultrabasites-dolomitic carbonatite. The relationships between carbonatites and their associat-ed silicate rocks are complex and are still not completely understood. Whether both melts were generated from the same parental magma, or whether both were generated independently of one another, still remains one of the fundamental problems in carbonatite petrogenesis.Phoscorites: Phoscorites are magnetite, olivine, apatite rocks usually associated with carbonatites (Le Maitre 2002) and ultramafic rocks of alkaline-carbonatite complexes. In some cases, there is gradation between ultramafic rocks and phoscorite.
The definition presented by Le Maitre (2002): "a magnetite, olivine, apatite rock usually associated with carbonatites", is very restrictive because olivine commonly retrogrades into pyroxene, amphibole, and serpentine. A much broader defini-tion and classification of phoscorites are anchored in Russian literature (e.g. Yegorov 1993; Krasnova et al. 2004) and proposes that phoscorite should be re-defined as a "plutonic ultramafic rock comprising magnetite, apatite, and one of the silicates, forsterite, diopside, or phlogopite". The term phoscorite is a mnemonic, derived originally from the name of the Phosphate Development Corporation and refers to magnetite-olivine-apatite rocks ringing the Loolekop carbonatite body of the Phalaborwa Complex, South Africa.
Alkali metasomatism
Most intrusive carbonatites, alkaline-carbonatite complexes, and many agpaitic and miaskitic alkaline intrusions are surrounded by country rock affected by intrusion related metasomatism. Metasomatism is defined as: "a solid state process by which the chemical composition of a rock is altered in a pervasive manner and which involves the introduction and/or removal of chemical components as a result of the interaction of the rock with fluids".The alkali metasomatism that characterizes most of carbonatites complexes is known as fenitisation or fenitisation-type metasomatism. Fenitisation-type meta-somatism commonly consists of desilication accompanied by the addition of Na, K, Fe3+, ± Ca, ± Al to the host rock that surrounds carbonatites or carbonatite-alkaline complexes. Other elements that may be introduced into country rock by fenitisation-type metasomatism are Ba, Nb, Sr, Sc, Rb, Zn, V and in some cases REE, and Nb. Such metasomatism may manifest itself by the development of Na- and K-amphiboles, aegirine-augite, K-feldspar, albite, perthite, mesoperthite, an-tiperthite, nepheline, and pale brown mica, and albite (Fig.3).
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Fig.3: Schematic representation of bi-metasomatic fenitisation type interaction between carbonatite melt and related fluids with country rock. Direction of migration of elements is indicated by arrows. Minerals commonly observed in country rock affected by fenitisation type metasomatism are listed. From Simandl, G. J., & Paradis, S. (2018).
The extent and intensity of metasomatism related to carbonatites and alkaline-carbonatite complexes depends on large number of parameters including (1) Chemical composition, temperature, and pH of the fluids; (2) Chemical and min-eralogical composition of country rock (protolith); (3) Permeability and porosity of the country rock; (4) Temperature gradient between the source of fluids and the country rock, (5) Fluid/rock ratio; (6) Duration of fluid movement.
Morphology and geometry of alkaline-carbonatite complexes
Carbonatites can occur as volcanics or intrusive bodies. The carbonatite phase usually comes late in an intrusive series, after the alkaline silicate magmas. Many carbonatites, however, do not have associated silicate rocks. Carbonatite com-plexes are generally < 25 km2, and are composite, with multiple intrusions of both silicate and carbonatite magma. Exposed intrusive carbonatites include small plugs, cone sheets, and occasional ring-dikes. Planar dikes or dike swarms of both silicate rocks and carbonatites commonly cut the entire intrusive complex.The classic carbonatite model (Fig.4 a) proposed by Garson and Smith (1958) was popularised by Heinrich (1980) and Bowden (1985) and is still in use. This model fits many complexes from Eastern Africa carbonatites complexes, and elsewhere:
In a typical sequence, shallow early ijolite and/or nepheline syenite plugs are fol-lowed by carbonatites that cut the earlier silicate complex. Sovites (typically with over 90% calcite) are the most common type of carbonatite in these complexes and may represent the only carbonatite at a locality. The later manifestations of igneous activity in many complexes is the emplacement of dikes or cone sheets of iron-rich carbonatites, collectively called ferrocarbonatite. An almost universal characteristic of carbonatite complexes is the presence of a distinctive metaso-matic aureole in which the wall rocks (most commonly quartzo-feldspathic gneiss) has been converted to aegirine-rich and alkali amphibole-rich rocks, and in some cases to K-feldspar-rich rocks. The metasomatic rocks are commonly called fenites.
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Fig.4: Morphology of carbonatite complexes as proposed by: (a) Garson and Smith (1958); (b) Le Bas (1987); and (c) Slightly modified from Sage and Wat-kinson (1991) to show convex and concave nature of ring dikes and cone sheets, respectively. From Simandl, G. J., & Paradis, S. (2018).
More recent models (Fig.4 b-c) have been proposed by Le Bas (1977, 1987), and Sage and Watkinson (1991). The model of Le Bas (1987) displays well the age relationships between lithological units and highlights fenitisation type overprints. The model produced by Sage and Watkinson (1991) displays a limited number of relative to the Garson and Smith (1958) model; however, it better depicts the relationship between the volcanic edifice and crater facies. No model depicts all the possible rock associations encountered in alkaline carbonatite complexes or is universally applicable. At deep erosion levels, carbonatites are commonly spatially associated with ultramafic rocks. At moderate levels, they are spatially associated with pyroxenites and jacupirangites, and with ijolites and nepheline syenites at progressively shallower levels (Garson and Smith 1958)..jpg)
Ferrocarbonatite (calcite, ankerite, siderite, iron oxides and iron silicates) from the Ice River Complex of British Columbia. From James St. John
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Calciocarbonatite (sövite) from Hot Spring County, central Arkansas, USA. From James St. John
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Calciocarbonatite dikes from Firesand River Carbonatite Complex, Wawa Lake East roadcut, Ontario, Canada. From James St. John
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Calciocarbonatite (sövite), dominated by the mineral calcite (whitish to very light grayish) and dark magnetite. Magnet Cove Carbonatite, Arkansas, USA. From James St. John
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Calcite-carbonatite (sövite) from the type locality. Søve, Fen Complex, Norway. From Sand Atlas
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Phoscorite with magnetite (black) and albite (white). kovdor, Russia. From École des Mines de Saint-Étienne
Bibliography
• Bell, K., Kjarsgaard, B. A., & Simonetti, A. (1998). Carbonatites-into the twenty-first century. Journal of Petrology, 39(11-12), 1839-1845.
• Krasnova, N. I., Petrov, T. G., Balaganskaya, E. G., Garcia, D., Moutte, J., Zai-tsev, A. N., & Wall, F. (2004). Introduction to phoscorites: occurrence, composi-tion, nomenclature and petrogenesis. In Phoscorites and carbonatites from mantle to mine: the Key example of the Kola Alkaline Province (Vol. 10, pp. 45-74). Mineralogical Society London.
• Liu, Y., & Hou, Z. (2017). A synthesis of mineralization styles with an integrated genetic model of carbonatite-syenite-hosted REE deposits in the Cenozoic Mian-ning-Dechang REE metallogenic belt, the eastern Tibetan Plateau, southwestern China. Journal of Asian Earth Sciences, 137, 35-79. Mitchell, R. H. (2005). Carbonatites and carbonatites and carbonatites. The Ca-nadian Mineralogist, 43(6), 2049-2068.
• Simandl, G. J., & Paradis, S. (2018). Carbonatites: related ore deposits, re-sources, footprint, and exploration methods. Applied Earth Science, 127(4), 123-152.