1. Introduction
Carbonatites are igneous rocks, which consist of more than 50 vol.% of primary magmatic carbonates and less than 20 wt.% of SiO
2 [
1]. A wide range of deposits of strategically important mineral materials, such as rare (Nb, Ta, Zr, Li et al.), rare earth (REE+Y) and radioactive elements, as well as the apatite, and fluorite deposits and others, are associated with carbonatite complexes. A great scientific and practical interest in ore-bearing carbonatite complexes today has been caused by the accumulation of a considerable amount of information about their geological structure and mineral composition. Therefore, the study of carbonatite complexes, especially, mineralogy and ore formation processes, is important in fundamental science and in exploring the production process. In this paper, we present new mineralogical data on the Sevathur Nb-Ba-REE minerals because little information on many of these minerals is available in the literature, and also this will help to determine the history of crystallization and subsequent alteration of the Sevathur minerals.
The Sevathur carbonatite complex is the first discovered carbonatite from Tamil Nadu province in 1966 by the State Geology Survey [
2] (also referred to as Koratti) and was confirmed by Deans in 1968 (Deans, unpublished report, Overseas Geological Survey, England). Earlier, the carbonatite was considered as banded limestone during prospecting for vermiculite deposits found within pyroxenite bordering carbonatite outcrop. In 1968, Semenov recognized pyrochlore from the Sevathur carbonatite, and Borodin delineated pyrochlore-rich zones in 1969. Afterward, detailed geological prospecting of the Sevathur carbonatite and adjacent areas was carried out by the United Nations Development Program as part of the Tamil Nadu Mineral Development project. In later years, eight more carbonatite occurrences were reported from this region, all of which are located along the same NE-trending lineament [
3], and intrusive into Archean Peninsular gneisses.
The Sevathur carbonatites contain high concentrations of apatite, monazite, magnetite, pyrochlore and vermiculite. The pyrochlore contains 24% of U
3O
8 [
2] and occurred within early generation carbonatite, unlike most other carbonatite complexes in the Indian subcontinent. There are about 360 tons of Nb
2O and 1.2 million tons of vermiculite reserves in the Sevathur carbonatites [
4].
In this paper, we present new data on the mineralogy of dolomite carbonatites of the Sevathur complex. Also, we discuss the post carbonatite emplacement hydrothermal processes those are responsible for the formation of some minerals.
2. Geological Setting
The Sevathur carbonatite complex is located near the same named village that is situated 9 km SSW of Tirupattur (Tamil Nadu, India). The age (
207Pb/
204Pb) of the Sevathur ankeritic carbonatites obtained by Schleicher et al. [
5] is 801 ± 11 Ma. This age relates to the end of the carbonatitic magmatism because the ankeritic carbonatites are the youngest stage of carbonatite formation.
There are a large number of Proterozoic carbonatite complexes (Sevathur, Samalpatti, Jogipatti et al.) in southern India belonging to Precambrian (~1600–600 Ma) alkaline magmatism within the Eastern Ghats Mobile Belt (EGMB). They are emplaced within the Precambrian granulite terrains (Southern Ghats Terrain, SGT) along NE–SW trending fault systems [
3,
4,
6].
In the Sevathur complex, dolomite carbonatite constitutes the main carbonatite exposure with a length of 2 km and an approximate width of 200 m in the central part (
Figure 1).
The outcrop of the carbonatite is arcuate in form (strikes from N60°E–S60°W through to N–S to E–W) and shows an inward dip of about 60°. In addition to this major outcrop of carbonatite, there are few thin dikes of calcite and ankerite carbonatite; the largest being 400 m long and striking N5–E–S5–W. On the northern and western sides of the complex, the carbonatite is in contact with pyroxenite while at the southern end it is surrounded by granitic gneiss. Dolomite carbonatite shows presence of many xenoliths, which include protolith syenite, gneiss and pyroxenite. Magmatic banding is conspicuous in some parts of the outcrops. Along such banding sodic amphibole, sodic pyroxene, phlogopite, apatite, magnetite and pyrochlore are common minerals (
Figure 2a).
In banded outcrops of carbonatite, some bands are particularly rich in magnetite, apatite and monazite (
Figure 2b). Effects of fenitization upon the surrounding host rocks are widespread, such as the conversion of pyroxenite to vermiculite (in pyroxenite). "Contact fenite" formed at the immediate contact between carbonatite and peninsular gneiss contains aegirine- and orthoclase-rich zones while outcrops of syenite show incipient development of Na-pyroxene.
3. Materials and Methods
Samples were collected from the central part of the massif during several field work explorations. Petrographic investigations of polished thin sections were carried out under a polarization microscope (OLYMPUS BX-51, OLYMPUS Co., Tokyo, Japan) in Geological Institute, Siberian Branch of the Russian Academy of Sciences (Ulan-Ude, Russia). Photos of photos of key moments are taken on a digital camera (MicroPublisher 3.3 RTV, Qlmaging, Surrey, Canada). The rock textures and chemical compositions of the minerals were studied by polished rock samples under a scanning electron microscope with energy dispersive X-ray spectroscopy (LEO 1430VP EM, LEO Carl Zeiss SMT Ltd, Germany) with an Oxford Inca Energy 350 spectrometer, Oxford Instruments, Great Britain). The operation conditions for energy dispersive X-ray spectroscopy: 20 kV beam energy, 0.4 nA beam current, and 50 s spectrum live acquisition time. The results were tested against synthetic and natural minerals: SiO2 (O,Si), BaF2 (F,Ba), NaAlSi3O8 (Na), MgCaSi2O6 (Mg,Ca), Al2O3 (Al), Ca2P2O7 (P), KAlSi3O8 (K), LaP5O14 (La), CeP5O14 (Ce), PrP5O14 (Pr), NdP5O14 (Nd), Cr met. (Cr), Mn met. (Mn) and Fe met. (Fe). Instrument error for REEs was less than 0.50 wt.%. Matrix correction was performed with the XPP algorithm as part of the built-in Inca Energy software. The polished rocks samples were analyzed at the Analytical center of mineralogical, geochemical and isotope Studies at the Geological Institute, Siberian Branch of the Russian Academy of Sciences (Ulan-Ude, Russia)
Chemical compositions of the minerals were also studied by electron microprobe (JEOL JXA-8100, JEOL Ltd., Japan). The operation conditions were: WDS mode, 20 kV, 15 nA, 1–2 µm beam diameter. For some minerals, we used a beam current of 10 nA and an acceleration voltage of 15 kV; for Fe–Ti oxides −20 nA and 15 kV; for monazite, −40 nA and 20 kV; and, for apatite, −10 nA and 20 kV. The peak counting time was 16 s for major and 30–60 s for minor elements. Natural minerals and synthetic phases were used as standards (element, detection limits in ppm): SiO2 (Si, 158), rutile (Ti, 120), LiNbO3 (Nb, 142), Sr silicate glass (Sr, 442), F-apatite (Ca, 115; P, 387; F, 477), hematite (Fe, 148), CePO4 (Ce, 236), LaPO4 (La, 272), BaSO4 (S, 178), NdPO4 (Nd, 362), Cl-apatite (Cl, 74), and PrPO4 (Pr, 401). The electron microprobe studies were carried out at the Analytical Center for Multi-Elemental and Isotope Research Siberian Branch, Russian Academy of Science (Novosibirsk, Russia).
4. Results
The carbonatite outcrop mainly consists of dolomite carbonatite cut by a small number of ankerite carbonatite dikes. Calcite carbonatite are rare as thin veins cutting the dolomite carbonatite. The dolomite carbonatite has primarily medium- to coarse-grained structures and porphyritic textures. The porphyritic carbonatite contains phenocrysts of dolomite located within a matrix of equidimensional dolomite grains. There are laths of dolomite in such rocks. The dolomite carbonatites have rhythmic banding between medium- and fine-grained types. Banding is due to the presence of silicate minerals (for example, phlogopite, aegirine and Na-amphibole) along with magnetite, apatite, pyrochlore, monazite and zircon. In addition, there is leushite in the dolomite carbonatite [
2]. Also, sulfides (pyrrhotite, pyrite, galena, and chalcopyrite) are present in accessory amounts. Pyrochlore is usually coarse-grained and occurs both as disseminated grains and in bands. It is enriched by UO
2 and is metamict [
7].
The ankerite is usually unaltered in ankeritic carbonatite; however, some grains have undergone oxidation at the crystal margins or along the cleavage planes. Some phlogopite and amphibole grains are altered to chlorite. The contact between ankeritic carbonatite and the dolomitic one is prominent, with replacement of dolomitic carbonatite by later ankeritic fraction. Chlorite in contact zones is formed from amphibole andphlogopite. Such features are similar to the observed one along the contact between dolomitic and ankeritic carbonatites at Newania [
8].
Dolomite (CaMg(CO
3)
2) (
Figure 3 and
Figure 4) is the main carbonate mineral in all analyzed samples of dolomite carbonatite. It has low amounts of iron and manganese, and its content of strontium is less than in calcite (
Table 1) (
Figure 5).
Calcite (CaCO
3) formed later than dolomite and is interstitial between dolomite grains (
Figure 3 and
Figure 4). It contains SrO, MgO and low amounts of FeO and MnO; sometimes it contains BaO (
Table 1). Using the diagram of Goldsmith [
9] we obtained that the sub-solidus temperature of formation of calcite-dolomite is 600–650 °C. An average overall composition of the mixed carbonate was calculated by multiple spots on SEM/EDS (FeO 1.48–1.52, MnO 0.63–0.66, MgO 3.82–4.63, CaO 44.68–44.13, SrO 1.65–2.08).
Calcite contains exsolved dolomite (
Figure 6). It also contains small inclusions of norsethite, baryte, strontianite, barytocalcite, benstonite and calcioburbankite (
Figure 6).
Norsethite (BaMg(CO
3)
2) is found as small (up to 50 µm) exolutions within calcite (
Figure 6b). It contains CaO and FeO, and in some cases SrO (
Table 1).
Strontianite (SrCO
3) associates with barytocalcite (
Figure 6). It contains CaO, BaO and sometimes MgO, FeO (
Table 1).
Benstonite (Ba
6Ca
6Mg(CO
3)
13) forms small (up to 10 µm,
Figure 6) inclusions within calcite. It is characterized by varying concentrations of BaO, CaO, MgO and SrO (
Table 1). The benstonite compositional variation from Sevathur carbonatite can be expressed: (Ba
3.8–5.16Sr
0–1.09Mg
0–1.87)(Ca
2.53–6Mg
0–3.47)(Mg
0.54–1.78Fe
0–0.31)[CO
3]
13. There are negative correlations between barium and calcium, calcium and magnesium, strontium and magnesium (correlation coefficients −0.83, −0.71 and −0.72, respectively). Such values in Sevathur benstonite mean that there is a strong negative correlation between these elements, that is, if the concentrations of barium, calcium and strontium increase, then the concentrations of calcium, magnesium will decrease (
Figure 7).
Calcioburbankite (Na
3(Ca,REE,Sr)
3(CO
3)
5) forms small (up to 30 µm,
Figure 6) inclusions within calcite and associates with monazite, benstonite, norsethite and baryte. It is characterized with varying concentrations of Na
2O, BaO, SrO, CaO, and TR
2O
3. The Sevathur calcioburbankite formulae can be expressed as: (Na
0.69–2.33Ca
0.67–2.09)(Sr
0.59–1.31LREE
0.8–1.28Ba
0.17–0.39)[CO
3]
5. A positive correlation between BaO and SrO; a negative correlation between TR
2O
3 and Na
2O, CaO + SrO + BaO and Na
2O+LREE are presented on
Figure 8. The correlation coefficient of the Sevathur calcioburbankite is close to 1 (+0.68), then there is a positive correlation between BaO and SrO. The correlation coefficients between LREE and Na
2O, CaO + SrO + BaO and Na
2O + LREE (−0.38 and −0.56 respectively) indicate a weak negative correlation between these variables and, also, a low dependence.
Baryte (BaSO
4) forms two generations. The first (baryte I) is presented by fine inclusions in calcite (
Figure 6). The second generation (baryte II), forms rims and microcracks in calcite. The composition of these generations do not differ. It contains SrO (
Table 2).
Fluorapatite (Ca
5(PO
4)
3F) forms prismatic crystals in dolomite (
Figure 3,
Figure 4, and
Figure 9). There are inclusions of calcite, phlogopite and pyrochlore (
Figure 9) in fluorapatite. It is enriched in strontium (
Table 3).
Monazite-(Ce) (CePO
4) is embedded in calcite and dolomite, along with burbankite (
Figure 6e). It also forms rims around apatite (
Figure 9) and contains CaO (
Table 3).
Amphibole (Na(NaCa)Mg
5Si
8O
22(OH)
2) is high magnesian (19.70–23.48 wt.% MgO) richterite with Na
2O (up to 5.88 wt.%) and CaO (6.18–9.02 wt.%) (
Table 4). It forms single grains in dolomite and associates with apatite and magnetite (
Figure 4).
Phlogopite (KMg
3(AlSi
3O
10)(OH)
2) grains are enclosed in dolomite (
Figure 3,
Figure 6a and
Figure 9c). It contains F, FeO and enriched in MgO (
Table 5). Low concentration of K
2O in it is compensated by the kinoshitalite (up to 4.33 wt.% BaO) component (
Table 5). Compositional variations of Si, Al, K and Ba indicate that the major exchange-reaction is Ba + Al ↔ K + Si. Structural formula is given in
Table 5.
Similar barium-rich micas are typical of some kimberlites [
10] and also occurred in some carbonatite complexes, for example, in the Guli massif from the Maymecha–Kotuy province [
11] and in the Chuktukon massif from the Chadobets upland in Russia [
12], the Palabora Carbonatite Complex, South Africa [
13] and in the metasomatized mantle xenoliths [
14].
Magnetite (Fe
2+Fe
3+2O
4) occurs in two generations (
Figure 4 and
Figure 10). The first generation magnetite (magnetite I) occurs as isometric grains within calcite and dolomite. It is in solid solution with ilmenite (50.12–52.41 wt.%. TiO
2, 20.18–21.02 wt.%. MnO, 1.64–0.98 wt.%. MgO, 23.92–26.23 wt.%. FeO). It contains inclusions of amphibole, apatite and dolomite. Magnetite (II) of the second generation forms microveinlets in apatite and amphibole. However, the different generations of magnetite do not show a difference in their composition.
Pyrochlore is present in dolomite along with apatite, and it also sometimes contains inclusions of apatite (
Figure 3 and
Figure 9). Pyrochlore is a hydropyrochlore (H
2O,□)
2Nb
2(O,OH)
6(H
2O) [
15]. It has high concentrations of uranium and barium (
Table 6).