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

D. K. Paul ¹, J. H. Crocket ², T. A. K. Reddy ³, N. C. Pant ⁴

Affiliations
1 Department of Geology, Presidency College, College Street, Calcutta - 700 073, IN
2 School of Geography and Geology, McMaster University, Hamilton, Ontario, L8S 4K1, CA
3 Geological Survey of India, Bandlaguda, Hyderabad - 500 068, IN
4 Geological Survey of India, Nh5p, Nit, Faridabad, IN

Abstract

A suite of potassic ultramafic rocks occurs in the Krishna district in the southern Indian craton along with clusters of kimberlite. The principal minerals include olivine, pyroxene, amphibole and phlogopite. Compositional variation exists in the phlogopite of the different occurrences of the region. Mg number varies from 0.37 to 0.56 and the groundmass phlogopites are enriched in Fe and Ti and poor in Al compared to the phenocrysts. Major and trace element data indicate enrichment of LILE and LREE (La/Yb = 24 - 104; Gd/Yb = 3.8 - 8.02). Ba contents vary from 132 to 5327 ppm, which is rather low compared to literature values for lamproite. However, the abundances of Zr (> 500 ppm), Sr (> 1000 ppm) and La (>200 ppm) are similar to those in the lamproite. The potassic ultramafic rocks have variable but high U/Pb (2-37, average 17.2) and Th/U (4.6-10.7) ratios. Concentrations of Platinum Group Elements (PGE) are quite variable with Ir, Pt and Pd varying by >9, 21 and 15 times respectively over the suite. Significant features include differences in fractionation between PGE and enrichment of Cu relative to the mantle source region. Some of the samples are not ultrapotassic and peralkaline. However, combining their mineralogical and geochemical properties, these rocks of the Krishna region may be considered as a member of the lamproite clan. There is no evidence of significant crustal contamination, but fractional crystallisation of olivine and perovskite is suggested from the geochemical relations. Available isotopic age data indicate that the ca. 1220 Ma old Krishna lamproite preceded kimberlite emplacement in the region by about 160 million years. Partial melting of a mantle metasomatically enriched in Ti and Fe would yield the Krishna lamproite magma. Amphibole contributed to the melt, but the HREE abundance data preclude garnet involvement. The depth of melt formation was likely to be shallower than that of kimberlites.

Keywords

Lamproite, Proterozoic, Krishna District, Southern India, Platinum Group Elements, Petrogenesis.

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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.

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K. R. P. Rao ¹, T. A. K. Reddy ¹, K. V. S. Rao ¹, K. S. B. Rao ¹, N. V. Rao ¹

Affiliations
1 Geological Survey of India, Operation: Andhra Pradesh, Hyderabad 500 068, IN

Abstract

Apart from the well known occurrence of kimberlites in south India at Wajrakarur in Anantapur district, kimberlites were located by Geological Survey of India (GSI), during 1984-85 around Maddur and Narayanpet areas in Mahabubnagar district of Andhra Pradesh. Landsat imagery and photo-interpretation studies followed by intense geological traverses including stream sediment surveys, launched during 1994-96, led to the discovery of 20 additional kimberlites in the western part of Mahabubnagar district, Andhra Pradesh and in the adjoining Gulbarga district, Karnataka, thus reporting kimberlite for the first time from the state of Karnataka. A total of 30 kimberlites have been reported in this field named as "Narayan pet Kimberlite Field" (NKF).

The kimberlites are emplaced along E-W and NW-SE trending major faults and associated NE-SW tear faults in the Peninsular Gneissic Complex carrying enclaves of Dharwar greenstones of Gadwal schist belt. These kimberiites occur both as small bodies of a few metres across and also as dykes with a strike length of about 2 km. The characteristic mineral assemblages and textures indicate that these bodies correspond to hypabyssal facies or ischolar_main zone kimberlites. The indicator mineral suite present in these rocks are olivine, chrome diopside, magnesian ilmenite, chrome spinel, ai1d rare pyrope garnet Mantle xenoliths are rare, nevertheless spinel Iherzolite; abundantly found in two kimberlites.

All the kimberlites of NKF are depleted in SiO₂ (∼ 35%), potassic (K₂O >Na₂O) and magnesium rich (MgO >20%). The REE abundance of these kimherlites is consistent with their derivation by partial melting of mantle and subsequent fractional crystallisation of the melt. The similarity of petrological and geochemical characters of these kimberlites indicate that all the kimberlites of NKF are derived from similar source in the upper mantle.

Preliminary processing and testing of some of the kimberlite bodies have not yielded any diamonds so far. However, the reported stray diamond finds in the Krishna and Bhima River gravels indicate the presence of diamondiferous kimberlites further south and southwest of the area.

Keywords

Economic Geology, Geochemistry, Kimberlite, Andhra Pradesh, Karnataka.

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