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

S. Ravi ¹, T. K. Rau ¹, N. S. Reddy ¹, S. S. Nayak ¹

Affiliations
1 Geological Survey of India, Southern Region, Hyderabad – 500 068, IN

Abstract

The diamondiferous Upper Proterozoic conglomerates and the associated sandstone beds display sedimentary structures that are suggestive of provenance in the northwest, i.e. towards the Tungabhadra river drainage basin. Multidisciplinary surveys in this region have led to the discovery of a kimberlite body (350 m x 470 m), concealed under alluvial soil spread on the left bank of the Tungabhadra River near Mantralayam. The study area, represented by the granitoids of Peninsular Gneissic Complex with remnants of Gadwal schist belt, presents a domal upwarp that plunges into the river. The domal surface displays intense fracturing with prominent NW-SE/WNW-ESE and subordinate NE-SW trends, which show marked preponderance and dilation on the northern low angle plunge parts. Traverses coupled with extensive stream sediment sampling and the study of specific mineral chemistry is suggestive of presence of kimberlite, which is supported by actual recovery of micro-diamonds from the yellow ground. The kimberlite contains mantle nodules of phlogopite-bearing websterite and siliceous mica-garnet-chrome diopside bearing lherzolite; the selective non-silicification of interstitial chrome diopside in the latter being enigmatic. The silicification of these mantle nodules could be due to the emplacement of kimberlite into a surface water body and subsequent action of silicasaturated groundwater.

Keywords

Kimberlite, Microdiamonds, Mantle Nodules, Tungabhadra River, Kurnool District, Andhra Pradesh.

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S. Ravi ¹, S. V. Satyanarayana ¹

Affiliations
1 Geological Survey of India, Bandlaguda, Hyderabad - 500 068, IN

Abstract

No Abstract.

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T. Ajit Kumar Reddy ¹, M. Sridhar ², S. Ravi ², V. Chakravarthi ², S. Neelakantam ²

Affiliations
1 Geological Survey of India, 3-5-170/G, Narayanguda, Hyderabad - 500 029, IN
2 Geological Survey of India, Hyderabad - 500 068, IN

Abstract

Search for primary source rocks of diamond in close proximity of the well known alluvial gravel workings for diamonds along the palaeochannels of the River Krishna, has resulted in the discovery of a lamproite field comprising several lamproite clan rocks in parts of Krishna and Nalgonda districts of Andhra Pradesh (South India). These rare rock types are defined in this work as 'lamproites', on the basis of their occurrence in a unique tectonic setting and containing one or more of the typomorphic minerals and having the requisite rock geochemical characters of members of the suite as a whole. These bodies with an exotic and diagnostic mineralogy, composition and textures occur in a complex tectonic setting with unique intrusive history. The discovery of this lamproite field is considered very significant from the point of diamond exploration as it is located very close to the famous Golconda Mines along the banks of the River Krishna which yielded many of the celebrated diamonds in the past. This field is christened "Krishna Lamproite Field", in view of the proximity of the lamproite occurrences to the Krishna River having the alluvial placer diamonds. These rocks are emplaced along the eastern margin of the Precambrian Dharwar Craton, at the contact between the northeastern periphery of the Proterozoic Cuddapah Basin and the Archaean crystalline basement rocks, in the form of dykes (usually in clusters) and plugged conduits. The emplacement is controlled by major faultslfractures, trending in NNW-SSE and WNW-ESE directions and occur closely associated with mafic (dolerite) dykes and followed the same channels for their emplacement. The lamproites contain glass, numerous vesicles, and xenoliths of the basement granite-gneiss, dolerite and probably platformal sediments of the Cuddapah Basin, besides exhibiting fragmental and pyroclastic textures supgestive of near-surface emplacement. Radiometric age determinations suggests emplacement of these lamproites took place around 1224±14 Ma, probably during the reactivation of major crustal lineaments coinciding with the uplift of the mid-Proterozoic Nallamalai sediments of the Cuddapah Basin.

Diopside, phlogopite, tetra-ferri phlogopite, richterite, sanidine, apatite and pseudomorphs of leucite, enstatite and olivine are the major primary constituent minerals present. Diopside-leucite-lamproite, phlogopite-leucitelamproite, olivine-lamproite and richterite-phlogopite-lamproites are the variants. The bodies are ultrapotassic (molar K₂O/Na₂O >10), and contain high TiO₂, P₂O₅, K₂O, MgO, Ba, Sr, Zr, Cr, Ni, La and Ce, pointing to crystallisation from typical lamproite magma. The presence of mantle peridotite xenoliths and xenocrystic minerals such as chromediopside, spinel and garnet indicate deep seated mantle derivation of the Krishna lamproites.

Keywords

Lamproites, Petrology And Geochemistry, Emplacement, Major Faults/Fractures, Krishna Lamproite Field, Andhra Pradesh.

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A. R. Nambiar ¹, T. Ajit Kumar Reddy ², M. Sridhar ², S. Ravi ², V. Chakravarthi ², S. Neelakantam ²

Affiliations
1 Geological Survey of India, Op.: Karnataka and Goa, Vasudha Bhavan, Bangalore - 560 078, IN
2 3-5-17Q/G, Narayanguda, Hyderabad - 500 029, IN

Abstract

No Abstract.

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