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

Chanchal Sarbajna ¹, P. Krishnamurthy ¹, A. K. Paul ¹, D. C. Banerjee ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad - 500 016, IN

Abstract

Detailed mineralogical and chemical studies by EPMA on the Nb-Ta minerals from the rare metal pegmatites of southern Karnataka have revealed wide compositional variations, and the presence of some new minerals like tapiolite and microlite, hitherto unreported from these pegmatites. Back scattered electron (BSE) images have indicated complex zoning patterns such as oscillatory, patchy and/or their combinations, associated with replacement of columbite-tantalite by microlite, manifested by the variations of Nb₂O₅, Ta₂O₅, FeO and MnO. Such features, although known from a few rare meral pegmatites of the world, are being recorded here for the first time in India. Oscillatory zones in columbite-tantalites are manifested as alternating dark and light grey coloured bands, of which darker bands are Nb-rich (30-300 μM thick) and lighter bands are Ta-rich (125-450 μm thick). The wider zones may, in turn, consist of a group of very fine sub-zones (1-50 μm) of slightly varying composition: Patchy zoned crystals exhibit corroded remnants of early formed columbite-tantalite (with 39.73-44.02% Ta₂O₅), surrounded by later formed zones which are enriched in Ta₂O₅ (up to 50.99 %) in columbite-tantalite leading to the formation of microlite containing up to 73.4% Ta₂O₅. The zoning of the columbite-tantalites has been attributed to periodic changes in the composition of the major components such as Nb, Ta, Fe and Mn in the pegmatite fluid system, apparently influenced by the late stage fractionation of volatiles. This ultimately generated fluids rich in Ta and Na, resulting in resorption and replacement with patchy zoning of early formed, zoned columbite-tantalites.

Keywords

Columbite-Tantalite, Rare Metal Pegmatite, Zoning, Ore Mineralogy, Southern Karnataka.

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Chanchal Sarbajna ¹, R. P. Sinha ¹, P. Krishnamurthy ¹, K. V. G. Krishna ², R. Viswanathan ¹, D. C. Banerjee ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad - 500 016, IN
2 Atomic Minerals Directorate for Exploration and Research, Nagarbhavi, Bangalore - 560072, IN

Abstract

White coloured alkali beryl from the pegmatites of Mariagalla-Allapatna in Mandya district, Karnataka contains distinctly higher alkali elements (Na, Li, Rb, Ca) and relatively low beryllium oxide (c. 11 % BeO) compared to normal beryl. Beryl occurs mostly between graphic intermediate zone and quartz core in the complexly zoned, spodumene-bearing Li-Cs-Ta (LCT) type pegmatites in this area. The average value (n = 7) for alkali beryl from this area are Na₂O 1.26%, Li₂O 0.6%, Cs₂O, 0.74%, K₂O 0.07 and Rb₂O 0.08%. Cs content in some of the beryls are higher than Na and hence such beryls may be termed as a new Na-Li-Cs type X-ray powder diffraction data for the beryl (a_o = 9.2765-9.339 Å, c_o = 9.2079-9.2487 Å) reveal differences in crystal structure with respect to normal beryls from Royalstone, Massachusetts, USA (a_o = 9.215Å, c_o = 9.192Å) and compares well with the reported alkali beryl from Arizona, USA (a_o = 9.30Å, c_o = 9.20Å) Their refractive index (N_ω) ranges from 1.588 to 1.592 and the specific gravity varies from 2.74 to 2.79.

Keywords

Economic Geology, Geochemistry, Mineralogy, Rare Metal Pegmatites, Beryl, Marlagalla, Karnataka.

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Chanchal Sarbajna ¹, P. Krishnamurthy ¹

Affiliations
1 Atomic Minerals Division, Hyderabad - 500 016, IN

Abstract

The Allapatna granite has been found to be a low-calcic (CaO-0.85%), peraluminous (A/C+N+K ratio = 1.04) and potassic type (K₂O=4.75%), enriched in Rb(362), Ga(25), Cs(3), Nb( 19), U(l3), Ta(18), Th(38), Pb(55) and strongly depleted in Ba(333). Such abundances together with low K/Rb ( 118.9), Ba/Rb (0.92) and Th/U (2.92), and high Rb/Sr (3.45) ratios than normal granite indicate their highly fractionated and fertile character and strongly suggests that the Li-Cs-Ta (LeT) rich rare metal bearing pegmatites of Marlagalla and nearby areas may have been derived from such a parental granitic magma.

Keywords

Granite, Pegmatites, Rare Metals, Allapatna, Karnataka.

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P. Krishnamurthy ¹, Chanchal Sarbajna ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Hyderabad-500 016, IN

Abstract

A ring-like structure is defined by a late Archaean granite around Chandra. It is a fertile granite with a parental status akin to the rare metal pegmatite-hosted deposits of Li-Be-Nb and Ta around Allapatna-Marlagalla. This observation has important implications for new exploration targets for rare metal pegmatites in southern Karnataka and also geological evolution of the terrain as a whole.

Keywords

Petrology, Ring Structure, Fertile Granite, Karnataka.

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S. Viswanathan ¹, O. P. Somani ², Chanchal Sarbajna ², R. P. Sinha ²

Affiliations
1 Flat B-203, Block-B, United Ave Apts,South End, 7-1-29, Ameerpet, Hyderabad - 500 016, IN
2 Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, West Block-7, R K Puram, New Delhi - 110 066, IN

Abstract

No Abstract.

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Chanchal Sarbajna ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad - 500 016, IN

Abstract

No Abstract.

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