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.

Full Text

     

S. S. Nayak ¹, C. V. Kasi Viswanathan ¹, T. A. K. Reddy ¹, B. K. Nagaraja Rao ¹

Affiliations
1 Geological Survey of India, IN

Abstract

Rocks of kimberlitic affinity hitherto not recorded are recognized near Maddur, Mahaboobnagar district, Andhra Pradesh. These are identified as olivine lamproites based on the paucity of garnet and picro-ilmenite. However, major element data have similarity with lamproitcs of Western Australia. Petrography is similar to pipe No.5 of Wajrakarur. K₂O is, however, less. The kimberlitic rocks are located in the area intersected by NE-SW (Major) and NW-SE (Intermediate) lineaments and form the northern cluster of kimberlites in Andhra Pradesh. This new location of a kimberlitic province has widened the scope for diamond prospecting in Andhra Pradesh.

Full Text

   

D. K. Paul ¹, S. S. Nayak ², N. C. Pant ³

Affiliations
1 Department of Geology, Presidency College, College Street, Kolkata - 700 073, IN
2 Geological Survey of India, Training Institute, Bandlaguda, Hyderabad - 500 068, IN
3 Geological Survey of India, NH5P, NIT, Faridabad 121 001, Haryana, IN

Abstract

Potassic, ultrapotassic, ultramafic rocks occur mostly as dykes and few diatremes in the southern and central Indian cratons. The ultramafic diatreme in Majhgawan in central India is the only diamond-Producing primary source at the present time. Several occurrences in southern India also contain diamond but are not being commercially exploited. Available isotopic age data on these rocks suggest a Proterozoic age (Table I), a feature that is distinctive compared to other ages on similar rocks barring the Premier pipe in southern Africa.

We present petrographic, mineralogical and geochemical data on carefully selected samples from several localities. Mineralogically there are differences in the content of olivine, Phlogopite, perovskite and spinel among the localities. In common with occurrences elsewhere, the olivines show a range from 83 to 93 mole percent. The macrocrystal olivines are more magnesian. Among the phlogopites, a two-Fold division on the basis of Mg number is discernible. Most of the ilmenites are distinctly less magnesian than the southern African kimberlites. Geochemically the samples show a wide variation in major element abundances. Despite geographical separation and petrographic differences, Zr and Hf maintain a perfect positive correlation. In common with similar rocks, all the samples studied show an enrichment of both compatible and incompatible elements. Compared to established and accepted kimberlites of southern Africa and lamproites of Western Australia, the Indian samples show general similarity but also divergences in geochemical parameters. This is believed to suggest a transitional nature of the Indian potassic, ultramafic rocks between kimberlites and lamproites Petrogenesis of the occurrences is discussed.

Keywords

Indian Kimberlites, Review, Petrology, Geochemistry, Geochronology.

Full Text

     

M. Sridhar ¹, V. S. Chowdhary ¹, S. S. Nayak ¹, Re Augustine ¹

Affiliations
1 Geological Survey of India, Hyderabad - 560 068, IN

Abstract

Three kimberlite pipes are reported from Siddanpalli village in Gadwal area, Mahbubnagar district, Andhra Pradesh, bringing to light a new kimberlite cluster. Significance of the find lies in the fact that these kimberlite pipes are located in the Krishna river basin well known for its alluvial diamond mining activity since historic times. These pipes are being studied for their diamond potential.

Full Text

   

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.

Full Text