Calciocarbonatite
Category: Plutonic
Type Sövite.
Commons Carbonatite is rare and exotic igneous rock. Despite the high content of alkaline elements, it is not classified as an alkaline rock. Most commonly it occurs in small intrusive complexes alongside alkaline silicate rocks such as nepheline syenites, ijolites, and/or urtites. A rock is classified as carbonatite if it contains more than 50 vol. % primary carbonate minerals in modal composition. Carbonatites can be either plutonic or volcanic rocks. Several types of carbonatites are distinguished based on their carbonate mineral composition (Streckeisen, 1978). Calciocarbonatite (calcite carbonatite) consists primarily of calcite; its coarse-grained variety is known as sövite, while the fine- to medium-grained variety is called alvikite. Dolomite predominates in dolomite carbonatite, which is also referred as beforsite. Ferrocarbonatite contains iron carbonates, whereas natrocarbonatite is composed mainly of sodium, potassium, and calcium carbonates. Natrocarbonatite is sometimes known by the local name lengaite, derived from its type locality—the Oldoinyo Lengai volcano in Tanzania—which is also the only place in the world where this type of carbonatite occurs. Gregoryite and nyerereite are among the principal minerals found in natrocarbonatites. Calciocarbonatite is also a chemically defined variety of carbonatite characterized by a CaO/(CaO+MgO+FeO+Fe2O3+MnO) ratio greater than 0.8. Oxide contents are expressed in weight percent. Natrocarbonatite is classified as an alkaline rock.
Name origin The term carbonatite first appeared in a 1921 paper by Brögger (Brögger, 1921), in which he described carbonatite as an igneous rock from the Fen complex in the Telemark region of Norway. Brögger was the first proponent of the magmatic origin of carbonatites, and the Fen locality became the type locality, a site that is protected today. The magmatic origin of carbonatites was not definitively accepted by the geological community until the eruption of natrocarbonatite lava from the Oldoinyo Lengai volcano in Tanzania was observed in 1960.
Locality Evate, Monapo structure, Nampula Province, Mozambique (sample courtesy of P. Hvožďara).
GPS: 14°52'52.61" S 40°12'59.57" E
Major minerals Calcite, dolomite, fluorapatite, phlogopite, spinel, magnetite, ilmenite, geikielite, olivine, amphibole, diopside.
Accessory minerals Monazite, baddeleyite, zircon, zirconolite, allanite, sulfides (pyrite, pyrrhotite, chalcopyrite), cerianite, thorianite, sulfates (barite, gypsum, celestine, anhydrite) (Hurai et al., 2017; Gajdošová 2016).
Classification Carbonatites are classified according the modal comosition of carbonate minerals. If the modal composition cannot be determined due to the fine-grained nature of the rock, or if the carbonates form a complete Ca-Fe-Mg solid solution, classification is based on whole-rock chemical analysis. Chemical classification is applied only to carbonatites with an SiO2 content up to 20 wt.% (Woolley and Kempe, 1989). This classification is based on the wt.% of MgO, FeO, Fe2O3, MnO, and CaO oxides. The ternary diagram includes fields for calciocarbonatite (CaO), magnesiocarbonatite (MgO), and ferrocarbonatite (FeO + Fe2O3+ MnO). If the SiO2 content exceeds 20 wt.%, the rock is classified as silicocarbonatite. More detailed carbonatite classifications are presented by Harmer and Gittins (1997) and Le Bas (1999). These classifications are not based on the genesis of the rock. Calciocarbonatite is also a chemically defined variety of carbonatite in which the ratio CaO/ (CaO + MgO + FeO + Fe2O3 + MnO) > 0.8. Oxide contents are expressed in wt.%. Calcite predominates in the modal composition of the Evate carbonatite; therefore, it is classified as calciocarbonatite, with a ratio of CaO/ (CaO + MgO + FeO + Fe2O3 + MnO) = 0.92.
Colour White, grey, pink.
Structure Compact, massive.
Granularity Medium-grained (1 mm – 3 mm) to coarse-grained (3 mm – 1 cm).
Texture Phaneritic, with minerals identifiable by the naked eye (apatite, magnetite, olivine, phlogopite, and sulfides).
Alterations No alterations were observed.
Petrographic characteristics It is a medium- to coarse-grained rock; therefore, it can also be termed sövite. The groundmass of the calciocarbonatite consists of carbonates—calcite and dolomite. Apatite, phlogopite, magnetite, and sulfides are also visible by the naked eye. Dolomite forms exsolutions within the calcite that are observable only under the microscope.
Usage Carbonatites contain minerals that can be enriched with elements of significant economic potential, such as REEs, Nb, U, Ta, Ba, Sr, V, Th, Zr, and Cu (Winter, 2010), as well as Fe, F, Pt, and PGEs (platinum-group elements). Other elements with economic potential include Zn, Ti, Y, Mo, Mn, and S. They are also increasingly exploited for phosphate production and can serve as sources of micas and vermiculite. Significant carbonatite deposits currently being mined include the Bayan Obo complex in Inner Mongolia (minerals enriched in REEs and Nb); the Oka complex (minerals enriched in Nb) and Saint Honoré (minerals enriched in Nb-Ta) in Quebec; Iron Hill in Colorado (minerals enriched in Fe); Magnet Cove in Arkansas (minerals enriched in Ti); Mountain Pass in California (minerals enriched in REEs and Cu); the Jacupiranga deposit in Brazil (mined for phosphates); the Kovdor complex in Russia (mined for Fe ore, micas, and vermiculite); and Mount Weld (minerals enriched in REEs) and Mud Tank (mined for vermiculite and zircon) in Australia.
Literature Brøgger, W. C., 1921: Die Eruptivgesteine des Kristianiagebietes: IV. Das Fengebiet in Telemark, Norwegen. Norske Vindenskapsakademi i Oslo Skrifter I Mat-Nat Klasse 1920, 9, Kristiania, 408 pp. Gajdošová, M., 2016: Termometria a mineralógia karbonatitov z ložiska Evate v Mozambiku. (Diplomová práca). Univerzita Komenského v Bratislave. 94 pp. (in Slovak). Harmer, R. E., Gittins, J., 1997: The origins of dolomitic carbonatites: field and experimental constraints. J. Afr. Earth Sci. 25, 5-28. Hurai, V., Paquette, J.-L., Huraiová, M., Slobodník, M., Hvožďara, P., Siegfried, P., Gajdošová, M., Milovská, S., 2017: New insights into the origin of the Evate apatite-iron oxide-carbonate deposit, Northeastern Mozambique, constrained by mineralogy, textures, thermochronometry, and fluid inclusions. Ore Geology Reviews, 80, 1072-1091. Le Bas, M. J., 1999: Sövite and alvikite: two chemically distinct calciocarbonatites C1 and C2. South Africa. Journal of Geology, 102, 109-121. Mourão, C., Mata J., Doucelance, R., Madeira, J., da Silveira, Silva, L.C., Moreira, M., 2010,: Quaternary extrusive calciocarbonatite volcanism on Brava Island (Cape Verde): A nephelinite-carbonatite immiscibility product. Journal of African Earth Sciences, 56, 59-74. Streckeisen, A., 1978: IUGS Subcommision on the Systematics of Igneous Rocks. Classification and nomenclature of volcanic rocks, lamprophyres, carbonatites and melilite rocks. Recommendatiins and suggestions. Neues Jahrbuch für Mineralogie. Stuttgard. Abhandlungen. 143, 1-14. Winter, J. D., 2010: An Introduction to Igneous and Metamorphic Petrology. Second Edition. Prentice Hall, New Jersey, 702 pp. Woolley, A. R., Kempe, D. R. C., 1989: Carbonatites: Nomenclature, average chemical composition, and element distribution. In Bell, K. (ed.), Carbonatites, Genesis and Evolution. Unwin Hyman. London, 1-14. Zaitsev, A.N., Williams, C.T., Jeffries, T.E., Strekopytov, S., Moutte, J., Ivashchenkova, O.V., Spratt, J., Petrov, S.V., Wall, F., Seltmann, R., Borozdin, P., 2014: Rare earth elements in phoscorites and carbonatites of the Devonian Kola Alkaline Province, Russia: Examples from Kovdor, Khibina, Vuoriyarvi and Turiy Mys complexes. Ore Geology Reviews, 61, 204-225.
Photomicrographs
The panalotriomorphic texture of the carbonatite is composed primarily of calcite (Cal) with dolomite (Dol) exsolutions—along with olivine (Ol) and apatite (Ap). Under plane-polarized light, the calcite is transparent and non-pleochroic, displaying excellent rhombohedral cleavage (visible to the right of the apatite). Under crossed polars, it exhibits high-order pastel interference colors, corresponding to its high birefringence. Twinning lamellae—formed under elevated pressure and highlighted by vivid colors—are also clearly visible under crossed polars. The apatite appears black under crossed polars (to the left of the olivine) because the section is perpendicular to the crystallographic axis z, causing it to appear isotropic. Other sections of apatite display very low-order, dark gray interference colors. Rare accessory baddeleyite (ZrO2) appears brown under plane-polarized light; under crossed polars, it exhibits high-order interference colors that are masked by its intrinsic color. It is notable for its distinctively high relief.
Normative composition
The carbonatite from the Evate locality is a plutonic, SiO2-undersaturated rock. This silica undersaturation is reflected in the presence of normative olivine - ol, nepheline - ne, leucite - lc, and kaliophilite - kp. The high CaO content translates into a high content of normative calcite - cc. Excess CO2 remaining after calcite formation was assigned to normative magnesite - mgs; this phase was selected because the rock contains microscopically observable dolomite exsolutions. SO3 content was assigned to normative anhydrite - anh. The rock has a very low Al2O3 content, which was fully consumed during the formation of normative orthoclase - or and albite - ab, leaving no Al2O3 available to form normative anorthite - an. Excess Na2O remaining after albite formation was allocated to normative acmite - ac, while the remaining Fe2O3 was assigned to normative magnetite - mt. The high P2O5 concentration is reflected in a high content of normative apatite - ap. The normative composition mirrors the modal composition of the rock, which consists predominantly of calcite and is rich in apatite. Among silicate minerals, olivine is the most common constituent, while non-silicate minerals include magnetite and ilmenite, and, less frequently, geikielite.
Normative minerals
SiO2
TiO2
ZrO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
F
S
CO2
Total
Molar proportion of normative mineral
Molecular mass of normative mineral
Weight % of normative mineral
Oxide
(wt. %)
0.46
0.01
0.04
1.86
0.72
0.05
2.15
54.04
0.08
0.01
9.93
3.05
32.57
104.97
Molecular
weight
60.08
79.88
101.96
159.69
71.85
70.94
40.31
56.08
61.98
94.20
141.95
80.06
44.01
Molecular
proportion
0.0077
0.0001
0.0004
0.0116
0.0107
0.0007
0.0533
0.9636
0.0013
0.0001
0.0700
0.0381
0.7401
ap
0.2332
0.0700
0.0700
328.68
22.99
anh
0.0381
0.0381
0.0381
136.13
5.19
il
0.0001
0.0001
0.0001
151.75
0.02
cc
0.6924
0.6924
0.6924
100.09
69.30
mgs
0.0477
0.0477
0.0477
84.31
4.02
or´
0.0006
0.0001
0.0001
0.0001
556.67
0.06
ab´
0.0017
0.0003
0.0003
0.0003
524.46
0.15
ac
0.0040
0.0010
0.0010
0.0010
462.02
0.46
mt
0.0106
0.0106
0.0106
231.54
2.45
zvyšky
0.0000
0.0056
hy´
0.0056
0.0000
0.0056
0.0056
100.39
0.57
ol´
0.0028
0.0000
0.0056
0.0028
140.69
0.40
ne
0.0006
0.0003
0.0003
0.0003
284.11
0.08
lc´
0.0004
0.0001
0.0001
0.0001
436.50
0.05
ol
0.0028
0.0000
0.0056
0.0028
140.69
0.40
lc
0.00006
0.00002
0.00002
0.00002
436.50
0.01
kp
0.0002
0.0001
0.0001
0.0001
316.33
0.03
D: 0.0044
D1: 0.0015
D2: 0.0015
D3: 0.0004
D4: 0.0002
D5: 0.0002
D6: 0.0002
Mg/(Mg+Fe2+): 1.000
Total of normative wt. % 104.97
Comment The total Fe content, reported as Fe2O3 in the chemical analysis, was partitioned between FeO (30%) and Fe2O3 (70%). Normative anhydrite– anh was calculated based on a 1:1 molar ratio of SO3 to CaO. Normative anhydrite was used because the rock contains modal gypsum. Since the normative composition does not account for hydrous minerals such as gypsum (CaSO4 . 2H2O), normative anhydrite (CaSO4, alebo CaO.SO3) had to be used. The SiO2 undersaturation of the rock was balanced by the formation of normative kaliophilite – kp and leucite – LC, calculated from the final deficit, D6.
Chemical composition
Carbonatites, as unique rocks, are characterized by low SiO2 content, high volatile content, and elevated levels of Ba, Sr, and light rare earth elements (LREE). Among the major oxides, the calciocarbonatite from Evate (sample F 59/96) exhibits the highest CaO (54.11 wt. %) and MgO (2.15 wt. %) contents. The SiO2 content and MnO contents are very low, 0.46 wt. % and 0.05 wt. %, respectively. The LOI content (volatile components— H2O, CO2, F, and Cl) is relatively high (29.7 wt. %), and the S content reaches up to 1.22 wt. %. While P2O5 content is typically low in common carbonatites (up to 1 wt. %), the Evate calciocarbonatite shows elevated P2O5 levels (9.93 wt. %), making it a source of phosphate ore. Regarding trace elements, it has high Sr (4142 ppm) and Ba (367 ppm) contents. Elevated trace element levels are also observed for Y (70 ppm), V (8 ppm), Zr (13 ppm), Ni (20 ppm), and Th (20 ppm). Contents of metallic elements such as Pb (16 ppm), Zn (3 ppm), Cu (6 ppm), Co (4 ppm), and Au (1 ppm) are slightly elevated. Among the REEs, the light rare earth elements show higher concentrations: La (251 ppm), Ce (613 ppm), Pr (80 ppm), Nd (357 ppm), and Sm (59 ppm). The content of heavy rare earth elements (HREE) is lower: Eu (13 ppm), Gd (41 ppm), Tb (3 ppm), Dy (15 ppm), Ho (2 ppm), Er (4 ppm), Tm (0.6 ppm), Yb (3 ppm), Lu (0.5 ppm) (Gajdošová 2016).
Evate, Mozambikque - sample F 59/96
SiO2
0.46TiO2
0.01Al2O3
0.04Fe2O3
2.65MnO
0.05MgO
2.15CaO
54.04Na2O
0.08K2O
0.01P2O5
9.93LOI
29.7CO2
32.57SO3
3.05Total
99.12Mg(Mg/Fe2+)
0.62A/CNK
<0A/NK
0.28
The CO2 content was derived from the total C content, and the SO3 content was calculated from the S content. The total sum includes only the loss on ignition (LOI) (Gajdošová 2016, PhD thesis in Slovak with an English abstract).
Brava Island, Cape Verde - CY-95, extrusive calciocarbonatite
SiO2
2.03TiO2
0.18Al2O3
0.82Fe2O3
5.06MnO
0.96MgO
1.00CaO
50.04Na2O
0.20K2O
0.13P2O5
0.90LOI
37.21F
1.10Total
99.64Mg(Mg/Fe2+)
0.28A/CNK
<0A/NK
1.75
Mourão et al., 2010.
Brava Island, Cape Verde - CY-139, intrusive calciocarbonatite
SiO2
0.29TiO2
<0.001Al2O3
0.09Fe2O3
1.88MnO
0.53MgO
2.07CaO
51.01Na2O
0.07K2O
<0.01P2O5
0.37LOI
42.68F
0.14Total
99.13Mg(Mg/Fe2+)
0.69A/CNK
<0A/NK
0.71
Mourão et al., 2010.
Kovdor, Kola Alkaline Province, Kola Peninsula, Russia - sample 57/249
SiO2
1.38TiO2
0.03Al2O3
0.52Fe2O3
8.56MnO
0.16MgO
5.07CaO
45.68Na2O
0.09K2O
0.28P2O5
0.16CO2
37.09Total
99.02Mg(Mg/Fe2+)
0.54A/CNK
0.01A/NK
1.15
Zaitsev et al., 2014.