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

Anil Kumar ¹, R. Srinivasan ¹, K. Gopalan ¹, D. J. Patil ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, IN

Abstract

Field observations, trace element and isotopic results are reported for rocks classified as carbonatites from Vinjamur area in the Nellore schist belt. These carbonate rocks occur either inter-bedded with metasedimentary quartzites and pelites or in association with calc-alkaline rather than alkaline volcanic rocks. No fenitised contacts are seen. Their Nb and REE contents are too low for carbonatites. Samples free from baryte veins have less than 150 ppm of Sr with an isotopic ratio greater than 0.7100 δ¹³C_PDB range from -1.58 to - 0.56 ‰ and δ¹⁸O_SMOW from +14.00 to +16‰ , All these features, individually and collectively, show that the carbonate rocks of Vinjamur are most probably sedimentary and not igneous in origin.

Keywords

Carbonatite, Nellore Schist Belt, Andhra Pradesh.

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B. V. Oleinikov ¹, A. F. Korobeinikov ¹, O. V. Koroleva ¹, P. Krishnamurthy ², Anil Kumar ³, K. Gopalan ³

Affiliations
1 Yakut Institute of Geological Sciences, Siberian Branch, Russian Academy of Science, Yakutsk - 677 891, RU
2 Atomic Minerals Division, Hyderabad-500 016, IN
3 National Geophysical Research Institute, Hyderabad-500 007, IN

Abstract

Sixty nine (69) samples of Deccan basalt flows from the Western Ghats representing the Wai, Lonavala and Kalsubai subgroups average 3.7 ppb of gold. Gold content shows limited variation from 3.0 to 5.8 ppb in 62 samples whereas they range from 1.4 to 2.7 ppb in 7 samples. Flows of the Wai subgroup show the maximum variation in gold abundances (1.4 - 5.8 ppb) and which may be related to the presence of primitive uncontaminated basalts (Ambenali)- lowest value-crust-contaminated basalts (Poladpur) and basalts derived from enriched-mantle sources (Mahabaleshwar). Gold appears to be insensitive to crystal fractionation or differentiation as basalts of different types such as the picritic, porphyritic, aphyric and giant plagioclase-bearing types found within a subgroup or formation have similar avcrages (3.6-3.8 ppb).

Deccan Tholciitic basalts contain higher amounts of gold than dome of the occean island (Hawaii) tholciitic basalts island are (Kurile-Kamchatka) tholciites, but contain values similar to those found in the Columbia River basalt province.

Keywords

Gold, Deccan Basalts, Western Ghats, Economic Geology.

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Richa Sahu ¹, Anil Kumar ², K. V. Subbarao ¹, J. N. Walsh ³, T. K. Biswal ¹

Affiliations
1 Indian Institute of Technology, Mumbai - 400 076., IN
2 National Geophysical ,Research Laboratory, Hyderabad - 500 007., IN
3 Royal Holloway, University of London, Egham, Surrey TW20 OEX, GB

Abstract

No Abstract.

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Anil Kumar ¹, K. Gopalan ¹, K. R. P. Rao ², S. S. Nayak ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Geological Survey of India, Bandlaguda, Hyderabad - 500 068, IN

Abstract

Acid-Leached phlogopite Rb-Sr isochron ages of the Kotakonda and Mudalbid kimberlites in the Narayanpet kimberlite field northwest of the Cuddapah basin, south India are 1085±14 and 1099±12 Ma, respectively. These concordant ages are much younger than the recently reported K-Ar and ⁴⁰Ar-³⁹Ar ages of 1363±48 and 1402±5 Ma, respectively for the former kimberlite, and support our earlier conclusion that the south Indian kimberlites erupted episodically close to 1090 Ma ago. Three lamproites-one each from Ramannapeta at the northeastern margin and Chelima and Zangamarajupalle within the Cuddapah basin-give distinctly different Rb-Sr ages of 1224±14, 1354±17 and 1070±22 Ma, respectively. While the Rb-Sr age of the Ramannapeta lamproite is also younger than an earlier reported K-Ar age of 1381±18 Ma, it is the same as that of many spatially close alkaline complexes like at Elchuru and Kunavaram. The ages for the Chelima and Zangamarajupalle rocks are only tentative, as these rocks contain a large secondary carbonate component with its Sr isotopic composition genetically unrelated to that of their phlogopites.

Keywords

Kimberlites, Lamproites, Narayanpet Kimberlite Field, Rb-Sr Ages, Dharwar Craton.

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