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

I. V. Sastry ¹, T. P. S. Rawat ¹, K. K. Dwivedy ²

Affiliations
1 Atomic Minerals Division Shillong - 793 001, IN
2 Atomic Minerals Division Hyderabad - 560016, IN

Abstract

Basaltic andesites of Precambrian age are reported and described for the first time from Garo Hills, Meghalaya. These volcanics occur as xenoliths within Proterozoic granitoids. Petromineralogical and petrochemkal characters of the volcanics revealed them to be basaltic andesite. The basaltic andesite are observed both as porphyritic and aphyric types.

Full Text

   

Mithilesh Sharma ¹, T. P. S. Rawat ¹, Y. C. Sharma ², B. M. Swarnkar ¹, Jagmer Singh ³

Affiliations
1 Department of Atomic Energy, AMD, New Delhi - 110 066, IN
2 40312, Chand Nagar, New Delhi - 1 10 018, IN
3 407, Pacific Apartment, X/39, Dwarka, New Delhi - 110 045, IN

Abstract

Ground radiometric survey carried out in the early Tertiary Dagshai-Kasauli Formations in the northwestern foothill Himalaya has led to the discovery of uranium mineralised sandstone bands extending for about (a) 500 m, with 5-10 m width, at Tarol, 8 km southwest of Solan and (b) 250 m, with 4-7 m width, at Maltu, 6.5 km southwest of Solan, in the Solan district of Himachal Pradesh. Grab samples from (a) Tarol assayed up to 1.41% eU₃O₈, 1.65% U₃O₈ (β/γ) and <0.005% Tho₂ and (b) Maltu assayed up to 0.24% eU₃O₈, 0.28% U₃O₈ (β/γ) and 2. In both the localities, majority of the samples show disequilibrium in favour of parent (uranium). Uraninite and uranophane are the main uranium minerals, which occur in close association with disseminated pellets of organic matter.

Keywords

Economic Geology, U-Mineralisation, Sandstone Type, Tertiary, Dharamsala, Solan, Himachal Pradesh.

Full Text

   

T. P. S. Rawat ¹, G. B. Joshi ¹, Bhaskar Basu ¹, Nurul Absar ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, R.K. Puram, New Delhi -110 066, IN

Abstract

Black shale type uranium deposits, though of low grade (<0.001 to 0.05% U₃O₈), contain large uranium resources because of their immense volume. The Neoproterozoic-Cambrian Krol-Tal Himalayan sequence covering a cumulative area of about 1000 sq km in five different synclinal basins from Sirmour district, Himachal Pradesh in the west to Nainital district, Uttar Pradesh in east contain such black shale horizons. The uranium mineralisation found in black shale in the Nigalidhar syncline of Himachal Pradesh and its implication of being an indicator for search of such uranium deposits in Himalayas is discussed.

Keywords

Uranium, Black Shale, Proterozoic, Tal Group, Himachal Pradesh.

Full Text

     

U. K. Pandey ¹, D. V. L. N. Sastry ¹, B. K. Pandey ¹, Madhuparna Roy ², T. P. S. Rawat ², Rajeeva Ranjan ³, V. K. Shrivastava ³

Affiliations
1 Atomic Mineral Directorate for Exploration and Research, Geochronology Lab., Begumpet, Hyderabad - 500 016, IN
2 Atomic Mineral Directorate for Exploration and Research, Northern region, New Delhi - 110 066, IN
3 Atomic Mineral Directorate for Exploration and Research, Central Region, Nagpur - 440 001, IN

Abstract

The Dargawan gabbros intrusive into the Moli Subgroup of Bijawar Group, yielded Rb-Sr whole rock isochron age of 1967 ± 140 Ma. Based on the oldest age from overlying Lower Vindhyan (1.6Ga) and the underlying youngest basement ages (2.2 Ga), the time range of Bijawar sedimentation may be assigned as 2.1-1.6 Ga (Paleoproterozoic). Sm-Nd Model ages (T_DM), obtained, for Dargawan gabbros, is c. 2876 - 3145 Ma. High initial ⁸⁷Sr/ ⁸⁶Sr ratio of 0.70451 (higher than the contemporary mantle) and negative ε_Ndi (at 1.9 Ga) value of -1.5 to -4.5, indicate assimilation of Archaean lower crustal component by the enriched mantle source magma at the time of gabbroic intrusion. The dolerite, from Damdama area, which is intrusive into the basement and overlying sediments of Chandrapur Group in the central Indian craton, yielded Rb-Sr internal isochron age of 1641 ± 120 Ma. The high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7098 and ε^Ndi value of -3.5 to -3.7 (at 1.6 Ga) is due to contamination of the mantle source magma with the overlying sediments. These dolerites have younger Sm-Nd Model ages (T_DM) than Dargawan gabbros as c. 2462 - 2675 Ma, which is similar to the age of the Sambalpur granite, from which probably sediments to this part of Chattisgarh basin are derived. Hence mixing of sediments with the Damdama dyke during its emplacement, gives rise to high initial ⁸⁷Sr/⁸⁶Sr and low initial ¹⁴³Nd/¹⁴⁴ ratios for these dykes. The c. 1600 Ma age indicates minimum age of onset of the sedimentation in the Chandrapur Group of Chattisgarh basin. Both the above mafic intrusions might have taken place in an intracratonic rift related (anorogenic) tectonic setting. This study is the first reliable age report on the onset of sedimentation in the Chandrapur Group. The total minimum time span of Chandrapur and Raipur Group may be 1.6 Ga to 1.0 Ga (Mesoproterozoic). The unconformably underlying Shingora Group of rocks of Chhattisgarh Supergroup thus indicates Paleoproterozoic age (older than 1.6 Ga). Most part of the recently classified Chattisgarh Supergroup and Bijawar- Vindhyan sequence are of Mesoproterozoic-Paleoproterozoic age and not of Neoproterozoic-Mesoproterozoic age as considered earlier. Petrographic study of basic dykes from Damdama area (eastern margin of Chattisgarh Supergroup) indicated presence of primary uranium mineral brannerite associated with goethite. This is the evidence of mafic intrusive providing geotherm and helping in scavenging the uranium from the surrounding and later alterations causing remobilisation and reconcentration of pre-existing uranium in host rocks as well as in mafic dyke itself otherwise mafic rocks are poor source of uranium and can not have primary uranium minerals initially. It can be concluded that mafic dykes have role in uranium mineralisation although indirectly.

Keywords

Geochronology, Age of Sedimentation, Uranium Mineralisation, Northern and Central Indian Cratons.

Full Text