Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

N. P. Subrahmanyam ¹, G. V. U. Rao ¹

Affiliations
1 Metallurgy Division, Bhabha Atomic Research Centre, Trombay, IN

Abstract

Lenticular veins of carbonatite are located in the pheripheral Erinpura granites E of Alkalic Mer pluton in Mundwara igneous complex, Sirohi District, Rajasthan. These veins vary in length from 20' to 200' and are composed of discontinuous lenses arranged en echelon. Calcite is the principal mineral of these veins and the associated accessory minerals are ankerite, hematite, magnetite, aegirine, apatite, feldspar, quartz and baryte. Frothy and fluidal textures as well as chilled marginal and fenitisation effects are observed in these veins. Martitised magnetite and chalcedony indicate low temperature of formation whereas the vesicular nature of these veins indicates low confining pressures (may be as low as I bar). These veins.are having high Sr, Ba and RE contents. The calcites from these are enriched in Sr, Ba, RE, Mo, Mg and P compared to their sedimentary counterparts. Thus, these veins are alvikites formed under pneumatolytic-hydrothermal conditions.

Lenticular veins are not reported by earlier workers and the Mundwara veins seem to be unique in this respect.

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N. P. Subrahmanyam ¹, G. V. D. Rao ¹

Affiliations
1 Ore Dressing Section, Metallurgy Group, Bhabha Atomic Research Centre, III, Sarojini Devi Road, Secunderabad 500003., IN

Abstract

Alvikite veins, genetically related to Mer alkali pluton arc located in the peripheral Erinpura granites of Mundwara Igneous Complex. These veins vary in length from 7 to 70 metres and are lenticular with discontinuous lenses arranged en echelon., Fluidal, pegmatitic, porphyritic, vesicular and frothy textures, chilled marginal effects, fenitization and carbonatization are observed. Their mineralogy and textures indicate that they are pneumatolytic hydrothermal veins.

Trace element content (in ppm) is Y-53, La-163, Ce-74, Sc-18, Rb-4, Ta-5, Th-5, U-3, Sr-997, Ba-684, Pb-36, V-26. Cr-15, Co-6 and Cu-4. The mean values and the range of trace element content (in %) of pure calcites of these veins are, Sr-0.605(0.71-0.41); Ba-0.059 (0.07-0.03); Ce-O.038 (0.047-0.033); La-0.029 (0.034-0.021); Y-0.0069 (0,0081-0.005); Mn-O.4 (0.47-0.32); Mg-0.05 (0.14-nil); P-0.05 (0.19-nil) ; Sr, Ba, Mn and Mg contents are well within the range of values reported for other carbonatites, though they are on the lower side of the average values. Lower values may be due to the hydrothermal nature of these veins.

Mer Pluton occupying a small 2 Sq. Km. area, presents a complete suite of differentiated rock series, pyroxenite, banded gabbro, mela gabbro, theralite, essexite, nepheline syenite, sodalite syenite and the parent chilled basalt, basanite. High oxygen fugacity conditions of formation of this rock suite are indicated by the crystallization of magnetite in considerable quantities from the earliest formed pyroxenite, conversion of titanomagnetite to titanohematite, ilmenite to titanohematite and rutile in the chilled basalt, and extensive maghemitization of magnetite in almost all other rock types. Petrochemical studies indicate that the carbonated basanitic magma, following the calc-alkaline trend of fractional crystallization, has given rise to the miascitic suite of Mer pluton. It is inferred that the carbonatites are formed from tile residual carbothermal liquids. Na, Fe-rich vapour phase, in equilibrium with other phases in carbothermal solutions has fenitized the surrounding rocks.

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N. P. Subrahmanyam ¹, G. V. U. Rao ¹

Affiliations
1 Ore Dressing Section, Bhabha Atomic Research Centre, 1-10-153/156, Begumpet, Hyderabad 500016, IN

Abstract

Rutile concentrate produced by the Indian Rare Earths Limited from Kerala beach sands analyses 1.5% Nb₂O₅ and 0.13% Ta₂O₅. Incident light microscopy and scanning electron microscopy have revealed that niobium and tantalum are camouflaging Ti in this rutile. Exsolved acicular inclusions of tapiolite are observed in a few grains. Preliminary experiments to obtain Nb-Ta rich fraction from this rutile by physical methods have not given encouraging results, but it is observed that these elements get relatively enriched in the residual titanium dust, which accumulates as a waste product in the titanium pilot plant at Nuclear Fuel Complex, Hyderabad. Concentrates containing up to 31% Nb₂O₅ and 3.8% Ta₂O₅ could be obtained from this residual dust after elimination of carbon.

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N. P. Subrahmanyam ¹, N. K. Rao ¹, D. Narasimhan ¹, G. V. U. Rao ¹, N. K. Jaggi ², K. R. P. M. Rao ²

Affiliations
1 Ore Dressing Section, Bhabha Atomic Research Centre, Begumpet, Hyderabad - 500 016, IN
2 Nuclear Physics Division, B. A. R. C., Bombay - 400085, IN

Abstract

The study shows that the alteration of ilmenite has taken place according to the two stage model proposed by Grey and Reid (1975). Identification of the intermediate altered compound, pseudorutile, was found difficult in earlier studies by optical and X-ray diffraction methods, because of the occurrence of this phase in altered ilmenite in very fine grain size (30 Å), its poor crystallinity and coincidence of many of the diffraction lines with those of other phases. The present study has shown that the Mossbauer technique is more suitable in the identification and estimation of this phase.

The relative amount of pseudorutile increases progressively in the magnetic fractions in the following order: 0.30 - 0.35 amps (5%), 0.25 - 0.30 amps (40%), 0-0.25 amps (75%) and 0.35 - 0.50amps (95%), indicating that the magnetic susceptibility of the grains is increasing initially with progressive pseudorutile formation, and decreasing in the later stages of alteration. Reflectivity and hardness are also increasing up to the stage of pseudorutile formation and decreasing in the later stages.

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N. P. Subrahmanyam ¹, C. Leelanandam ²

Affiliations
1 Ore Dressing Section, Bhabha Atomic Research Centre, AMD Complex, Begumpet, Hyderabad 500 016, IN
2 Department of Geology, Osmania University, Hyderabad 500 007, IN

Abstract

The Toa layered suite ranges from feldspalhic peridotites to gabbros and gabbroic anorthosites. This suite differs from the Mer and MusaIa plutons in having coarse-grained cumulophyric rocks (with ubiquitous olivine) and scanty feldspathoidal dikes.

Observed features are traceable to the high H₂O content and high H₂O/CO₂ ratio of the parental liquid. High H₂O content has reduced the viscosity of the magma by many orders of magnitude and facilitated rapid crystallization and settling of crystals leading to the formation of coarse-grained cumulophyric suite of rocks. With its selective reactivity to Si-O-Si bridges, H₂O has depressed the activities of the saturated components (ab, or and qz) in the magma and led to the crystallization of considerable amounts of olivine from the silica-undersaturated components and consequently of feldspars (instead of feldspathoids). The depression of the liquidus of albite has facilitated the crystallization of more calcic feldspars leading to the formation of gabbroic anorthosites. A model of pulsed emplacement of magma is proposed to account (1) for the differing H₂O and CO₂ contents of the parental liquids of the Musala, Mer and Toa plutons, and (2) for the three different trends of differentiation in the three plutons which gave rise to an amazing variety of rock types in the Mundwara Complex.

Keywords

Alkaline Rocks, Toa Pluton, Mundwara Complex, Rajasthan.

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