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S. M. Naqvi ¹, B. L. Narayana ¹, P. Rama Rao ¹, S. M. Ahmad ¹, B. Uday Raj ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN

Abstract

The lowermost member of the Javanahalli schist belt consists of meta-arkoses/ paragneisses with interbedded epidote-quartzite and calc-silicate bands. These meta-arkoses exhibit relict current-bedding, arkosic texture and grain-to-grain variation in anorthite content within the same sample. Plagioclase is subordinate to K-feldspar and is relatively more weathered and sericitized. Orthoclase predominates over microcline in the total K-feldspar content and the quartz/feldspar ratio is generally very low. The meta-arkoses appear to be first-cycle sediments.

The composition of these paragneisses is unusual for Archaean metasediments. They are enriched in K, Si, Fe, Mg, Th, Rb, Ni, Cr and Co and depleted in Na, AI, Sr and often in Ca. The meta-arkoses of Javanahalli schist belt may be the product of strong and/or rapid chemical disintegration of Archaean tonalitic gneisses and basic and ultrabasic rocks. all of which were prominent components of the continental crust at the time the Javanahalli belt formed. If the meta-arkoses have not undergone major metasomatism, then their high abundance of K and Rb may indicate intense chemical leaching of these tonalitic source areas. Most of the saIt of the primitive oceans might have been derived from this intense leaching of the ancient cratonic tonalite gneisses.

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V. N. Vasudev ¹, S. M. Naqvi ², M. Shukla ³, B. Uday Raj ²

Affiliations
1 Department of Mines and Geology, Race Course Road, Bangalore, IN
2 National Geophysical Research Institute, Hyderabad, IN
3 Birbal Sahni Institute of Palaeobotany, Lucknow, IN

Abstract

Stromatolites of stratifera type showing development of pseudocolumnar structures occur both in chert as well as dolomites near the Kumsi Manganese Mines in the Shimoga schist belt. This report ot stromatolites in the Shimoga belt further confirms widespread biogenic activity prior to 2.6 Ga in the Dharwar Craton.

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R. Srinivasan ¹, S. M. Naqvi ¹, B. Uday Raj ¹, D. V. Subba Rao ¹, V. Balaram ¹, T. Gnaneshwar Rao ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Turbidite greywackes of the northwestern part of the Chitradurga Schist Belt constitute a part of >2.6 Ga Cbitradurga Group of Dharwar Supergroup. They consist of a detrital assemblage of mono- and poly-crystalline quartz, microcline and plagioclase feldspar. and volcanic rock fragments. Quartz content and Na₂O/K₂O ratios show their quartz intermediate character. In this respect and also in their high FeO(t)+ MgO content they are similar to other Archaean greywackes. The TiO₂ content. K₂O/Na₂O, Al₂O₃/CaO+Na₂O and Al₂O₃/SiO₂ ratios plotted against Fe₂O₃(t)+MgO, although assign an island arc environment for these greywackes, do not unequivocally discriminate between continental and oceanic island arc setting. The Chitradurga greywackes are highly enriched in Zr, Cr and Ni indicating a mixed felsic and mafic source. The contribution from contemporary volcanism in the basin could be significant. The QFL proportions indicate a dissected arc and/or recycled orogenic nature of the provenance. The chemical index of alteration varying from 58 to 63 and Al₂O₃/Na₂O ratio less than 6, indicate relatively unweathered nature of source rock and chemical immaturity of the sediment respectively. The REE patterns show wen-defined negative europium anomalies which reflect granitoid upper crust in the provenance during the Archaean.

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B. Uday Raj ¹, S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad 500007, IN

Abstract

Relicts of sedimentary precursors in the Peninsular Gneisses around Melukote-Katteri area ofDharwarCraton are recognised on the basis of field, petrological and geochemical data. The Melukote gneiss has interbeds of carbonates, quartzites. iron-formations and para-amphibolites with a concordant planar fabric. These gneisses are devoid of mafic-ultramafic xenoliths which are one of the essential characteristics ofthe orthogneiss component of Peninsular Gneiss (PG), The Melukote paragneiss (MG) contains interstitial calcite, relict arkosic texture, subrounded to round quartz, plagioclase and zircon. Two generations of quartz and plagioclase are conspicuous in the MG. The MG are high SiO₂ and low Al₂O₃ Cr and Ni are significantly enriched - up to 334 ppm and 73 ppm respectively. The Rb and Sr contents are lower than the typical tonalite-trondhjemite gneisses (TTG). The high HREE patterns of MG and very low negative Eu anomalies are distinct from those of TTG in the surrounding areas. These data indicate that MG represent first cycle immature sediment derived from a heterogenous provenance.

Keywords

Petrology, Geochemistry, Peninsular Gneiss, Melukote, Dharwar Craton.

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P. V. Sunder Raju ¹, S. Nirmal Charan ¹, D. V. Subba Rao ¹, B. Uday Raj ¹, S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Gold mineralisation occurs at Chikkasiddavanahalli (C S Hall!) in Archaean volcano-Sedimentary sequence of the Ingaldhal Formation, in Chiradurga Group, Dharwar craton. The C S Hal1i ridges and low mounds are about 1 2 km long and 100 in wide with N-S strike and vertical to steep easterly dips comprising mixed sulphide/oxide facies banded iron formations (MSOBIF) and associated ferruginous phyllites, metavolcanics and minor carbonates. Several horizons of BIF's occur as parallel to subparallel linear brands and extend over a strike length of 20-40 km with widths ranging from 2-25 m. These BIF s are highly sheared, deformed and metamorphosed to upper greenschist facies. The shears in BIF's are occupied by several syn-Tectonic quartz carbonate veins of various dimensions and found to be favourable loci for Gold-Sulphide mineralisation. Petrological studies show major mineral constituents with pyrite, arsenopyrite and subordinate grunerite, hornblende, chlorite, muscovite, ankerite, calcite, magnesian siderite and quartz. Most gold deposits world over are hosted in second order and subsidiary shear zones. The primary gold in BIFs is enriched with intense fracturing controlled by major shear systems extending from Gadag in the north and Doduguni in the south Active convergent margin settings related to accretionary processes and fluid activity are suggested to have led to Epigenetic gold mineralisation in BIFs at C S Halli area with an average gold grade of 2 08 g/t.

Keywords

Au Mineralisation, Structural Controls, Shear Zones, C S Halli, Chitradurga Schist Belt, Karnataka.

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