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

A. K. Rai ¹, Himadri Basu ², S. Zakaulla ¹, K. Umamaheswar ²

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Bangalore-560072, IN
2 Atomic Minerals Directorate for Exploration and Research, Hyderabad-500016, IN

Abstract

Miniature pits of varying size and shape are found in the topmost arenaceous layers of the Gulcheru quartzite. Though pits in sedimentary rocks form in a variety of ways, in clean beach sands they owe their origin to mainly physical processes. Small pits, found in the Gulcheru quartzite, have been formed as a result of interplay of difierent physico-Chemical factors which prevailed in syn- as well as post-Depositional environments. In addition to different agents of physical weathering, which played a vital role in the formation of pits in the Gulcheru quartzite, syn-depositional precipitation of iron oxides and their syn-to post-diagenetic chemical changes are believed to be the main chemical factors.

Keywords

Pits, Ferric-Hydroxide, Gulcheru Quartzite, Cuddapah Basin, Andhra Pradesh.

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Himadri Basu ¹, G. R. Gangadharan ¹, Suresh Kuman ¹, U. P. Sharma ¹, A. K. Rai ¹, Anjan Chaki ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Nagarbhavi, Bangalore 560 072, IN

Abstract

Middle Proterozoic Gulcheru Quartzite marks the onset of sedimentation, after the Eparchaean unconforrnity, in the Cuddapah Basin. In the southwestern margin of the basin it is made up dominantly of orthoquartzites. On the basis of lithology, stratigraphic relationship, bedding characteristics, texture and colour of sediments and sedimentary structures, five lithofacies were recognized within Gulcheru Quartzite. The Gulcheru Quartzite, though dominantly of shallow marine origin, shows imprints of other depositional regimes. Detailed interpretation of the facies and study of their field relationships reveal that pink massive quartzite, the lowermost unit of Gulcheru Quartzite, is of fluvio-aeolian origin. Later marine transgression .led to the development of a moderate to low energy beach, which evolved with time into a barrier: spit complex.

Keywords

Depositional environment, Facies analysis, Fluvio-Aeolian, Barrier-Spit, Gulcheru Quartzite, Cuddapah Basin, Andhra Pradesh.

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E. V. S. S. K. Babu ¹, S. Nayak ¹, A. K. Rai ², P. Krishnamurthy ³, S. N. Kak ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy 'Begumpet, Hyderabad - 500 016, IN
2 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Shillong, Meghalaya, IN
3 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Tatanagar, Jamshedpur, IN

Abstract

Occurrences of discrete selenide phases in the geological environment are rather rare and are confined to a few sulphide deposits and sandstone-type uranium deposits such as those of the Colorado plateau, USA. The occurrence of discrete selenide phases viz., clausthalite (PbSe), ferroselite (FeSe₂) and Co-penroseite (CoNiCuSe₂) are reported for the first time from the Uraniferous Mahadek sandstones of Domiasiat and Wahkyn South, West Khasi Hills District, Meghalaya. The selenide phases are associated with pyritiferous organic matter, pitchblende and other ore minerals such as marcasite and sphalerite.

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Sukanta Dey ¹, A. K. Rai ¹, Anjan Chaki ²

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Nagarabhavi, Bangalore-560072, IN
2 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Begumpet, Hyderabad-500016, IN

Abstract

The basal conglomerate and sandstone all along the northern margin of the Proterozojc Kaladgi basm are highly enriched in K-feldspar ranging m size from sand to pebble They are believed to have undergone K-metasomatism by earlier workers However, the present study shows that K-feldspar grains present in these rocks are predominantly temgenous detrital This is indicated by sympathetic relation between constituent quartz and feldspar in size, presence of fresh rounded microcline with or without authigenic overgrowth, typical arkosic texture and geochemistry of the rocks, and their sharp erosional contact with the underlying granitoids An uplifted, restncted source, dominantly consisting of K-rich granitoids and pegmatites to the north of the basin supplied the immature arkosic detritus possibly under humid and warm climatic condition The testimony for the presence of a highly K-rich source weakens the theory of K-metasomatism in the Kaladgi basin as proposed by some earlier authors.

Keywords

Arkose, Petrography, Geochemistry, Provenance composition and tectonism, Kaladgi Basin, Karnataka.

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Sukanta Dey ¹, A. K. Rai ², Anjan Chaki ¹

Affiliations
1 Department of Atomic Energy, Begumpet, Hyderabad - 500 016, IN
2 Department of Atomic Energy, Nagarabhavi, Bangalore - 560 072, IN

Abstract

Mildly deformed granitoids exposed around Bilgi in the northernmost part of the eastern Dharwar craton are divided into two groups viz. granodiorites and monzogranites. The granodiorites contain microgranular enclaves and amphibolite xenoliths, and show low-Al TTG affinity with high SiO₂ (71-74 %), Na₂O, Y and Sr/Y, moderate to moderately high Mg#, Cr and Ni, low to moderate LILE, and low Nb and Ta. However, compared to similar TTGs from different cratons the Bilgi granodiorites have distinctly higher K₂O, K₂O/Na₂O, Rb and lower REE and Th. The amphibolite xenoliths are characterized by variable enrichment of K₂O, Rb, Ba and Th and depletion of Ti, Zr and P compared to MORB. The microgranular enclaves are quartz diorite to granodiorite in composition with high Mg, Ni and Cr, and compared to MORB, are enriched in LILE and depleted in Ti and Y. The monzogranites, compared to the granodiorites, display higher SiO₂, K₂O and Rb with lower Mg#, although still maintaining the high Na₂O, Ni and Cr and low REE character.

The Bilgi granodiorites are explained as transitional TTGs late synkinematic with respect to regional deformation. Geochemical signatures and regional geological set up suggest that they are probably derived from partial melting of a highly depleted slab material (metabasalt) followed by variable contamination or assimilation of intermediate crustal rocks in a subduction zone set up. Late stage fluid activity on the granodioritic magma is probably responsible for the generation of monzogranites. The amphibolite xenoliths predate the granodiorites and possibly represent fragments of a schist belt carried away by the granitic magma. They are probably island arc basalt derived from mantle source that has been metasomatized by slab-derived fluids. The microgranular enclaves are coeval with the Bilgi granodiorites and also likely to be island arc magmas derived from mantle variably enriched in slab-derived and within-plate components.

Keywords

Transitional TTG, Petrography, Geochemistry, Petrogenesis, Geodynamic Setting, Dharwar Craton.

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