Petrography and Geochemistry of Ore-Bearing Granites and Pegmatites from Bastar Craton, Central India

The Himalayan foredeep basin formed on the Paleozoic through early Cenozoic Tethyan continental margin of the Indian Plate [1,2]. The Cretaceous-Paleocene sedimentary basin (Tansen Basin), located in the southern part of the Lesser Himalaya, deposited on an eroded surface of Precambrian basement of the Lesser Himalaya [3,4]. The foreland basin sequences of the Nepal Himalaya are mostly continental deposits (some marine beds) developed first in a passive-margin setting [1] succeeded by detritus from the collision of the Indian plate with the Asian plate about ca.60-50Ma [5]. Although some recent studies have suggested that the initial India-Asia collision took place at ~34Ma [6-9], whereas Najman et al. [10] suggests the terminal collision of India with Asia took place at 54Ma.

The regional geological setting of the Bastar craton is available in various publications [8-11]. The oldest lithologies in the area are granitic gneisses of the Sukma Group [12], which formed at 2636-2528Ma [13]. The gneisses contain metamorphosed enclaves of mafic and ultramafic rocks represented by hornblende schists and tremolite-actinolite-talc-chlorite schists (Figure 1). The gneisses are overlain by the sheared, ferruginised, and silicified metasedimentary sequence of the Bengpal Group, as well as amphibolites [14]. Paleoproterozoic granites, namely, the Paliam-, Darba-, Katekalyan-, and Kawadgaon-granites, have intruded the Sukma Gneisses and metasedimentary rocks of the Bengpal Group. The Kawadgaon granite yielded an Rb-Sr, whole-rock isochron age of 2497±152Ma (error at 2 sigma level), with an initial 87Sr/86Sr ratio of 0.7142 [14]. The granite of the Kawadgaon appears to be older than granites of the Paliam and Darba areas in the southeastern part of the Bastar craton (2308±48Ma) [10]. Furthermore, in the eastern part of the Bastar craton, granite of similar age (2301±53Ma) [15] is reported from Cholanguda. Still younger felsic phases (i.e., leucogranite) are known from the Barsur (2101±323Ma) [13] and Pujariguda (2110±47Ma) [15] granites in the Bastar craton. It is thus apparent that the Bastar craton has witnessed various pulses of granitic magmatism. The granitic activity in the Bastar craton over the period from 2300-2100Ma [14] is also considered to be a major tectonothermal event in the central Indian Tectonic Zone [16].

Figure 1:Location of Bastar craton in relation to inset map of India. (b) Location of Bastar-Malkanjgiri pegmatite belt (BMPB) in a part of the Bastar craton. (c) Geological map of BMPB of the Bastar craton, central India [8-12].

Field observations

The granites (~2.5-2.1Ga) in Bastar craton occurs in the form of low lying hillocks, mounds and also flat country rocks. They are intruded by pegmatites of various dimensions. Around Kawadgaon, a distinct vertical zonation is noted in between rare metal and tin pegmatites (Figure 2,3). The pegmatites occur within the granite either as veins or as fracture fillings (Figure 4a). Generally, the granite and associated pegmatite aureoles locally show physical continuity (Figure 4b). Gradation of textural and mineralogical features between the two is common. Fractionation from granite into progressively more evolved and zoned pegmatite types in the aureoles is evident in the field (Figure 4c-4e). These features suggest that the pegmatites are late magmatic injections. Unzoned pegmatite veins exhibit ramifying and intersection relationship, with younger ones dislocating the older ones. Similarly, quartz veins/stringers display ramifying, branching and criss-cross patterns and also solution breccias. Metabasics, which occur within the ore-bearing granites, are also intruded by rare metal pegmatites (Figure 4f). Aplitic dykes that consist of quartz, feldspar (microcline and sodic plagioclase), zoisite, and epidote are of variable dimensions and are present within the granite.

Figure 2:Geological map of Kawadgaon area.

Figure 3:Geological cross section along A-B and C-D (Figure 1) showing disposition of rare metal and tin pegmatites in the Kawadgaon area [5].

Figure 4:Field photographs from in and around Kawadgaon area.

Figure 4a: Unzoned pegmatite occurring as veins within biotite granite, some pegmatites also occur as fracture fillings. Note intersection relationships of unzoned pegmatite veins, with younger ones (mostly filling fractures) dislocating the older ones.

Figure 4b: Pegmatitic granite.

Figure 4c-4e: Zoned rare metal pegmatite showing 2-3 distinct zones, namely, Quartz Core (QC), Perthite Zone (PZ) and Intermediate Zone (IZ). Fourth zone, that is, wall/border zone is not visible in the photo. In IZ, quartz, perthitic and non-perthitic microcline, and muscovite are present.

Figure 4f: Geological map of a rare metal pegmatite emplaced in metabasic rock occurring within the 2.5Ga ore-bearing granite.

Petrography and mineralogy

The granites are mesocratic to leucocratic, and medium to coarse-grained. Under the microscope, the granites exhibit mostly hypidiomorphic granular texture (Figure 5a) and, at places, myrmekitic and granophyric textures, besides graphic intergrowth of quartz and microcline. The granites comprise chiefly of quartz, microcline, perthite, albite-oligoclase, with muscovite, biotite (Figure 5b,5c), hornblende (rare), sericite, and chlorite as accessories, besides xenotime, monazite, zircon, rutile, sphene, ilmenite, magnetite, zoisite and clinozoisite. Quartz shows signs of strain. Dominance of primary muscovite over biotite (Figure 5b) in granites suggests them to be of peraluminous nature. Laths of primary muscovite, at places, show distinct contact with the perthite and microcline. It exhibits high order interference colour. Biotite laths are brownish and contain innumerable pleochroic haloes around tiny monazite grains. Chloritisation of biotite is also noted commonly (Figure 5c). Occasionally, granites show postcrystallisation modification of feldspar and mica suggesting that they have been influenced by younger secondary processes (Figure 5d). These incipient modifications are of hydrothermal origin due to late phase metasomatism and attendant mineralization of rare metal and /or rare earths. At places, yellowish monazite grains are also present (Figure 5d). Pegmatites show evidences of greisenisation and albitisation. Greisenised pegmatites show aggregates of muscovite, topaz, and quartz. Albitisation is marked by the development of cleavelandite and sugary albite. In both the cases, tiny grains of albite occasionally show polysynthetic twinning. Occasionally, relict patches of perthite are preserved within the sugary albite.

Figure 5:Microphotographs of ore-bearing granites showing:

Figure 5a: Hypidiomorphic granular texture.

Figure 5b: Two-mica type nature with muscovite (M) and biotite (B), with a dominance of M over B.

Figure 5c: Alteration of biotite in to chlorite.

Figure 5d: Monazite (Mz) grains with muscovite (M) and Incipient Modification (IMF) of its constituent minerals. IMF may be linked with hydrothermal activity due to late phase metasomatism responsible for kaolinisation and/albitisation and attendant mineralization. Note yellow-coloured, Euhedral Monazite (Mz) crystal with dotted brownish pigmentation lines.

Abundance of ore minerals in granites is less. Various ore minerals observed are beryl, columbite-tantalite, monazite, xenotime, and ilmenite. Beryl occurs as tiny euhedral crystals, occasionally measuring up to 5cm long. Other ore minerals are small in grain size and have been noted in heavy mineral concentrates recovered from granites after their crushing and heavy media liquid separation, followed by isodynamic magnetic separation. In pegmatites derived from these granites, diversified mineral assemblage has been noted. The ore minerals observed are columbite-tantalite, ixiolite, pseudo-ixiolite, wodginite, microlite, fersmite, euxenite, aeschynite, beryl, cassiterite, monazite, xenotime, zircon, ilmenite, triplite, and magnetite (all identified by x-ray diffraction studies) [17,18]. Among these, columbite-tantalite, ixiolite, wodginite, beryl, and cassiterite are widespread, and are commercially exploitable [18]. In fact, so far, from Kawadgaon and adjoining areas about 150,000 tonnes of RMRE-bearing ores have already been mined, and still more than this quantity of the ore is available for mining. Associated gangue minerals are represented by quartz, microcline, perthite, albite (and its variants cleavelandite and sugary albite), muscovite, biotite, sericite, almandite, illite, and kaolinite. Microcline perthite and microcline are the main feldspar. The albitised pegmatites contain grey, white, and earthy albite. Due to pervasive albitisation, microcline perthite has been almost completely replaced by albite. It is milky white with ash grey tinge and occurs as cleavelandite and/or equigranular saccharoidal aggregates of sugary albite.

Geochemistry of ore-bearing granites

The average geochemical data of various granitic bodies reveals SiO2 71.79-75.01%, Al2O3 12.71-14.78%, total (Na2O+K2O) 7.93- 8.66%, CaO 0.65-1.00%, TiO2 0.15-0.39%, MnO 0.02-0.27%, P2O5 0.08-0.26%, Rb 283-425ppm, Zr 136-271ppm, Nb 30-44ppm, Y 18-58ppm, Ba 456-679ppm and Sr 72-121ppm (Table 1). Most of the granites are high-K type, with a lesser number of samples falling in the low-K to medium-K types. Chemically, although the granites show a transition from adamellite to granite, with most of them having a K2O/Na2O ratio>1, the bulk of granites fall in the compositional field for granite. The granites are highly peraluminous (A/CNK ratio>1; 1.05 - 1.25), and distinctly enriched in Rb,Y,Nb,Pb and depleted in Sr and Ba. In terms of the molar A/ CNK vs. A/NK plots [19,20], the granite falls in the field for S-type granites, which is further corroborated by the values of alumina saturation index (ASI, molar A/(C+N+K). The available geochemical data indicates that the peraluminous pluton was appreciably influenced by feldspar fractionation [10,18].

Table 1:Average composition (Oxides in Wt.% and trace elements in ppm) of ore-bearing granitic bodies of Darba, Paliam, Metapal [10] and Kawadgaon [18] areas of Bastar craton, central India.

The concentrations of large ion lithophile elements in the granites (average) are 283-425ppm Rb, 72-121ppm Sr, and 456- 679ppm Ba. When compared with the abundances of Rb (170ppm), Sr (100ppm), and Ba (400ppm) in the average low Ca-granite [21,22], the granites show enrichment in Rb, and depletion in Sr, a fact further supported by spider plots of granites (Figure 6) [23]. The granites have K/Rb ratios of 114-142 and Rb/Sr ratio of 3.1- 6.32 (Table 1).

Figure 6:Primordial mantle normalized spidergram of ore-bearing granites from Kawadgaon. Normalising values after Wood et al. [23].

Mineralogical data reveals that the granites are mostly a twomica (biotite and muscovite) type. The key geochemical features of the granites are their sub-alkaline and strongly peraluminous (A/ CNK ratio=1.05 to 3.17; av. 1.27) nature. Also, the granites have a high K/Na ratio and high silica, total alkalies and Rb contents. These mineralogical and geochemical characteristics of the granites suggest a crustal origin [24,25]. The available high initial 87Sr/86Sr isotopic ratio (0.7142) of the granite [14] also indicates that it has been derived from the reworking of the older crustal rocks. Interestingly, mol. CaO/(MgO+FeOt) vs. mol. Al2O3/(MgO+FeOt) plots [26] of granites also suggest the involvement of metapelite and metagreywacke sources in the generation of the magmas. These inferences are coherent with the fact that, similar to the granites of Bastar craton, the biotite-granite and muscovite-biotite granites are also known to be purely crustal-derived granites that are closely associated with the melting of crustal metasedimentary and/or TTG (trondhjemite, tonalite, granodiorite) suite of rocks [27].

Petromineralogical and geochemical studies on ore-bearing granites and pegmatites of the Bastar craton, central India leads to the following conclusions.

1. The ore-bearing granites comprise chiefly of quartz, microcline, perthite, albite-oligoclase, with muscovite, biotite and (secondary) hydrated muscovite, sericite, and chlorite as accessories. In addition, monazite, xenotime, zircon, rutile, sphene, ilmenite, magnetite, zoisite, and clinozoisite are also present. The dominance of primary muscovite over biotite suggests that the granites are peraluminous.

2. Ore minerals associated with pegmatites are columbitetantalite, ixiolite, pseudo-ixiolite, wodginite, microlite, fersmite, euxenite, aeschynite, beryl, cassiterite, monazite, xenotime, zircon, ilmenite, triplite, and magnetite

3. Chemically, the bulk of granites fall in the compositional field for granite. The granites are highly peraluminous, S-type, and distinctly enriched in Rb,Y,Nb,Pb and depleted in Sr and Ba.

4. The high initial 87Sr/86Sr isotopic ratios and mol. CaO/ (MgO+FeOt) vs. mol. Al2O3/(MgO+FeOt) plots indicate that the granitic bodies were derived from the reworking of the older crustal rocks involving metapelite and metagreywacke.

5. A major part of the granites were formed from fractionated melt, whereas further fractionation of melt led to the formation of pegmatites and attendant rare element mineralization.

I thank Prof. Vinod K. Singh for his kind invitation to contribute for edited book ‘Earth Crust Evolution: From Precambrian to Recent’; to an anonymous reviewer for insightful reviews and useful suggestions; and to M/s Archaeology & Anthropology Publishers for very prompt processing of manuscript and printing in most professional way.

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