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

V. N. Vasudev ¹

Affiliations
1 120/45, 3rd Block, 6th Cross, 6th A Main Tyagaraja Nagac Bangalore - 560 028, IN

Abstract

No Abstract.

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V. N. Vasudev ¹, Brian Chadwick ², Allen P. Nutman ³, G. V. Hegde ⁴

Affiliations
1 120/45(A) III Block, Thyagrajanagar, Bangalore 560 028, IN
2 Earth Resources Centre, University, Exeter EX44QE, GB
3 Research School of Earth Sciences, Australian National University, Canberra ACT0200, AU
4 Department of Mines and Geology, Government of Karnataka, 16/3-5 S. P. Complex, Lalbagh Road, Bangalore 560 027, IN

Abstract

The Palkanmardi conglomerate is one of many polymict conglomerates interbedded with greywackes in the NE of the Hutti schist belt. These conglomerates are up to a few metres thick, unsorted and include rounded to sub-angular clasts of granodiorite, granite, vein quartz and rnetabasalt in a matrix of coarse-to medium-grained greywacke. Cross-bedding, convolute bedding and grading are well preserved in the interbedded greywackes in spite of deformation and homfelsing during Late Archaean regional high temperature - low pressure metamorphism. These primary structures and lack of sorting in the conglomerates are consistent with deposition as turbidites and debrites in an unstable submarine environment. This new interpretation contrasts with previous views that the Palkanmardi conglomerate is autoclastic, pyroclastic-detrital or glacio-fluvial. The conglomerate-greywacke sequence occurs low in the lithostratigraphy and is overlain by metabsalts.

A clast of granodiorite in the conglomerate bed at Palkanmardi village has yielded a SHRIMP weighted mean ²⁰⁷Pb/²⁰⁶Pb zircon age of 2576±12 Ma which is interpreted as the magmatic age of the erosional provenance. Moreover, SHRIMP zircon geochronology using a sample from the steep elongate wedge of granodiorite that extends for at least 150 km SE of the schist belt has yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2561±24 Ma and a concordia upper intercept age of 2580±31 Ma. These ages are indistinguishable within their errors and are interpreted as the age of magmatic crystallisation. Combined with the low stratigraphic position of the Palkanmardi conglomerate, the age data imply that basin development (volcanism and sedimentation) in the Hutti belt was not only rapid, but began very late in the Archaean history of this part of the Dharwar craton.

Keywords

Late Archaean, Hutti Schist Belt, SHRIMP Ages, Intra-Arc Basin, Dharwar Craton.

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V. N. Vasudev ¹, C. Naganna ¹

Affiliations
1 Department of Geology, Central College, Bangalore, IN

Abstract

The gold deposits at Hutti situated on the north western fringe of Hutti-Maski Schist belt are of gold-quartz-sulphide reef type. A detailed study has shown three distinct stages of mineralization. Minerals formed during the first stage are rutile, pyrite (with traces of gold) arsenopyrite, specularite, quartz (reef and vein type) and carbonate. Minerals of the second stage are pyrrhotite. chalcopyrite, sphalerite. galena and gold. Minerals of the third stage are pyrite, calcite and quartz found along numerous minor faults (slips) and major faults. Based on the textural and associational criteria, three generations of gold are distinguished. They are: 1. Gold-pyrite association. 2. Gold-aresenopyrite-quarrz association. 3. Gold-sphalerite-quartz association. The study has further shown that only the gold of the second and third generations constitute the lode. Gold is in the native state and no tellurides are present.

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R. Srinivasan ¹, B. L. Sreenivas ¹, V. N. Vasudev ²

Affiliations
1 Geomysore Services, 12, Palace Road, Bangalore 560 052, IN
2 Chitradurga Copper Co., Ali Asker Road, Bangalore 560052, IN

Abstract

No Abstract.

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V. N. Vasudev ¹, R. Srinivasan ²

Affiliations
1 Chitradurga Copper Co. Ltd., 16/1, Ali Asker Road, Bangalore-560052, IN
2 Geomysore Services, 12, Palace Road, Bangalore-560052, IN

Abstract

The vanadium bearing titaniferous magnetite deposits of Karnataka occur within gabbro-anorthosite members, spatially associated with the ultramafic rocks emplaced in the platformal miogeosynclinal regions of the Dharwar schist belts. They are generally conformable, syngenetic magmatic deposits, although instances of magmatic injection are known. Vanadium occurs in magnetite lattice substituting for ferric iron. The vanadium bearing titaniferous magnetites often carry copper and iron sulphide mineralization. The association of titaniferous magnetite with the ultramafic-mafic suite and their tectonic setting suggest, that the ultramafic-mafic sequence of rocks of Dharwar greenstone belts are probably not alpine, but compare better with concentric complexes.

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G. V. Ananta Iyer ¹, V. N. Vasudev ²

Affiliations
1 Inorganic & Physical Chemistry Dept., Indian Institute of Science, Bangalore 560012, IN
2 Chitradurga Copper Co. Ltd., 16/1, Ali Asker Road, Bangalore 560052, IN

Abstract

Geochemical data on Archaean goldfields metavolcanics (2.7 b. yrs) have been plotted on major and minor element discriminant diagrams to define the magma type and to formulate the tectonic environment model for the origin of these ancient volcanic rocks. The meta basalts of Kolar and Hutti (Karnataka, South India), Norseman (Eastern Goldfields, Western Australia). Steynsdorp (Barberton, South Africa) and Yellowknife (Canada), plot within the field of mid-ocean ridge basalts (MORB). The marginal basin basalts (MBB) of South Sandwich Islands of Scotia Arc Region of Phanerozoic age also plot within the field of MORB. Unlike the depleted MORB, the MBB are enriched in certain large ion lithophile (LIL) elements with higher Rb/Sr, Ba/Sr and lower K/Ba ratios. Further, in contrast to MORB, they exhibit slightly enriched LREE patterns. The LIL element enriched goldfield volcanics of Kolar, Hutti, Norseman, Steynsdorp and Yellowknife, with their slightly enriched LREE patterns, compare more favourably with MBB than with MORB. Based on geochemical features and geological setting, marginal basin model is proposed as the most favourable tectonic environment for the generation of gold bearing Archaean greenstone belts of Kolar and Hutti.

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I. V. Chernyshev ¹, Yu. G. Safonov ¹, B. P. Radhakrishna ², B. Krishna Rao ³, V. N. Vasudev ², M. Deb ⁴

Affiliations
1 Institute of Geology of Ore Deposits (IGEM) Academy of Sciences, 35, Staromonetnii Street, Moscow, USSR, RU
2 Chitradurga Copper Co., IN
3 Mineralogical Inst., University of Mysore, Manasa Gangotri, Mysore 6, IN
4 Dept. of Geology, University of Delhi, Delhi 7, IN

Abstract

The isotope composition of leads from ore deposits in general reflects the age of ore mineralisation and its genetical features. Two examples have been given to illustrate a direct connection of ore lead isotope composition with the source ore. Calculations of the model age from lead isotope data form the basis for genetic reconstructions of Precambrian deposits. Radical improvement in the accuracy of mass spectrometric isotope lead analysis has made possible the employment of two-stage and in some cases more complicated models for genetic reconstructions. The Penehanga Bay lead-zinc deposit and gold-silver deposits of South Dakota have been studied using high accuracy mass spectrometer. Both the deposits show that ore formation took place one to two billion years after craton formation, and lead present in ores originated at least from two different Sources.

Keeping this background of experience, fifteen galena samples from five sulphide and gold-sulphide deposits of the Indian Shield have been selected and determination of lead isotope composition has been carried out by the Soviet authors in the USSR using recently developed M.I.1320 Mass Spectrometer. The isotopic ratios of galena from Ingaldhal copper deposit are the most primitive among the analysed ores of the Indian Shield. The results of lead isotopic determination have been interpreted in terms of the general theory of 'plumbotectonics '.

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A. V. Murali ¹, P. B. Pawaskar ¹, G. R. Reddy ¹, K. V. Subbarao ², V. N. Vasudev ³, M. Sankar Das ¹

Affiliations
1 Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay-400085, IN
2 Geology Department, Indian lnstitute of Technology, Powai, Bombay-400 076, IN
3 Chitradurga Copper Company, 16, Ali Asker Road, Bangalore-560052, IN

Abstract

No Abstract.

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V. N. Vasudev ¹, S. Jayaram ²

Affiliations
1 Chitradurga Copper Company Limited, 16/1, Ali Asker Road, Bangalore 560052, IN
2 Department of Mines & Geology, Race Course Road, Bangalore 1, IN

Abstract

No Abstract.

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Yu. G. Safonov ¹, B. P. Radhakrishna ², B. Krishna Rao ³, V. N. Vasudev ⁴, K. Krishnam Raju ⁵, L. P. Nosik ¹, Y. N. Pashkov ¹

Affiliations
1 Institute of Geology of ore deposits (IGEM), Academy of Sciences, 35, Staromonetnii Street, Moscow, RU
2 Geological Society of India, Bangalore 560053, IN
3 Department of Geology, University of Mysore, Manasagangotri, Mysore 570005, IN
4 Department of Mines & Geology, Bangalore 560001, IN
5 Hutti Gold Mines, Raichur District, IN

Abstract

No Abstract.

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G. V. Ananta Iyer ¹, V. N. Vasudev ², S. Jayaram ²

Affiliations
1 Inorganic & Physical Chemistry Dept., Indian Institute of Science, Bangalore 560012, IN
2 Department of Mines & Geology, Bangalore 560001, IN

Abstract

No Abstract.

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A. G. Menon ¹, V. S. Venkatasubramanian ¹, V. N. Vasudev ², G. V. Ananta Iyer ¹

Affiliations
1 Department of Physics, Indian Institute of Science, Bangalore 560012, IN
2 Department of Mines & Geology, Bangalore 560001, IN

Abstract

The δ³⁴S values of pyrrhotite and arsenopyrite samples from the gold bearing quartz-sulphide reefs of Hutti have been measured. A narrow range of mantle-meteoritic values is observed indicating a magmatic-hydrothermal origin of the ores. An attempt is made to evaluate the solution parameters by the Ohmoto procedure, as outlined in Parts I and II.

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Yu. G. Safonov ¹, A. D. Genkin ¹, V. N. Vasudev ², B. Krishna Rao ³, G. V. Anantha Iyer ⁴

Affiliations
1 Institute of Geology of Ore Deposits, Academy of Science USSR, Moscow, RU
2 Department of Mines of Geology, Bangalore 560001, IN
3 Department of Geology, University of Mysore, Mysore 570005, IN
4 Indian Institute of Science, Bangalore 560012, IN

Abstract

Twenty-five ore minerals have been identified from a study of ore-bearing samples from different reefs of the Kolar Goldfields. The mineral association includes fifteen minerals being reported for the first time: altaite, hessite hedleyite, tzumoite, volynskyite gudmundite, ullmannite, brethauptite, tetrahedrite, hawleyite, cotunite, electrum and native bismuth. Besides, two new minerals, as yet unnamed, have been determined through electronprobe - Pb₃Te₉Cl₄S₂ and PbTeCI₂ They occur along with cotunite (PbCI₂) and altaite.

The study of time relations of different ore and vein mineral assemblages in the Kolar gold-quartz reefs has revealed six stages of mineralization: 1) Early quartz, 2) quartz-feldspar (Pegmatite), 3) scheelite, 4) early sulphides with gold, 5) sulphide-gold-tellurides (with quartz) and 6) quartz-carbonate. Fluid inclusions indicate that the mineral association of quartz-feldspar stage has developed under high TP-conditions of 250-300°C and 1.8 and 3.5 kb. Isotope composition of sulphur in the sulphides indicate the juvenile nature of sulphur. Available data suggest that the quartz of first stage is of metasomatic origin, derived possibly from host amphibolites; gold-sulphide and gold-telluride mineralization took place through hydrothermal processes.

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V. N. Vasudev ¹

Affiliations
1 Department of Mines & Geology, Bangalore, IN

Abstract

No Abstract.

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G. V. Anantha Iyer ¹, V. N. Vasudev ²

Affiliations
1 Indian Institute of Science, Bangalore 560012, IN
2 Department of Mines and Geology, Bangalore 560001, IN

Abstract

Massive volcanic-hosted sulphide deposits, within the Jogimardi volcanic suite (JVS) are Cu-rich with minor Zn and Pb and form an important early phase in the evolution of the late Archaean Chitradurga belt. The deposits are concordant stratabound lenses in the bedded mafic tuffs emplaced as submarine volcaniclastites. The JVS is an integral part of the Chitradurga antiform flanked by synclines on either side. The stratiform massive and vein-type sulphide deposits of the JVS are confined to the eastern limb of the antiform.

The volcanic rock succession in the JVS consists of 85% tholeiitic basa1ts; 12% low silica variolitic andesites and 3% of rhyolites. The basalts are quartz-normative tholeiites, not known to occur in MORB environment. Unlike those of MORB, in their major, minor and REE geochemistry, the basalts related to massive sulphide deposits are almost identical to back-arc basalts generated in an ensialic marginal basin environment.

The thermal and chemical environment of ore formation are determined based on fluid inclusions data and δ³⁴S pyrite values of individual sulphide phases. These data indicatce that the ores were deposited under anoxic chemical environment at a temperature of 350°C within a pH range of 5 to 7 corresponding to proximal facies conditions. The stratiform sulphide ore body in the area is located in the vicioity of a fracture controlled stringer/stock-work ore zone - 'a hydrothermal discharge vent' in the pillowed basalts of tholeiitic composition which confirms the proximal nature of the deposit.

Uranogenic and thorogenic lead isotope studies on the galenas of Ingladhal stratiform sulphide bodies indicate that they are not of mantle origin but derived from the basement gneisses, dated at 3 b.y.

Geologic setting, preservation of quench plagioclase textures in the pillowed basalts and volcanogenic synsedimentary nature of the stratiform sulphide deposits indicate that the sulphides were deposited initially in the JVS prior to folding and regional greenschist facies metamorphism.

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V. N. Vasudev ¹, Ananda Deb Mukherjee ²

Affiliations
1 Department of Mines and Geology, Bangalore 560 001, IN
2 Department of Geological Sciences, Jadhavpur University, Calcutta, IN

Abstract

Stratiform pyrrhotitic sulphide ores occur at Ganacharpura which forms a part of mafic volcanic and volcano-sedimentary terrain of the Archaean Kolar greenstone belt. The ores are interbanded with graphite-quartz-sericite schist and are metamorphosed and deformed along with the associated rock types of the area. Dominant ore mineral is pyrrhotite while chalcopyrite, pyrite, sphalerite, galena, magnetite and mackinawite are present in subordinate amounts. The mineral chemistry of the co-existing sulphide phases indicate equilibration at 587°C and 4.98 Kbar total pressure. The fugacities of sulphur and oxygen, reflected in various sulphide assemblages varied considerably, but the fluid composition during metamorphism was essentially internally controlled instead of being externally imposed. The co-existing pyrrhotite, pyrite and magnetite indicate log f_s2= -2.4 and log f_o2= -17.8 during culmination of metamorphism. It is suggested that the ores are of volcano-sedimentary type and are later modified by mobilisation/remobilisation during diastrophism. The arc-forming metals and sulphur have probably been derived from volcanic exhalations.

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B. Chadwick ¹, V. N. Vasudev ², S. Jayaram ³

Affiliations
1 Department of Geology, University, Exeter EX4 4QE, GB
2 Department of Mines and Geology, Government of Karnataka, Gandhinagar, Bellary 583 101, IN
3 Department of Mines and Geology, Government of Karnataka, Bangalore 560 001, IN

Abstract

The Dharwar Supergroup in the Shimoga basin east of Bhadravathi has been subdivided into seven formations on the basis of sedimentary structures and facies. The lowest is dominated by metabasites including metalavas and tuffs, metagabbros with seams of titaniferous magnetite, serpentinites and talc schists. A range of sedimentary rocks including phyllitic greywackes, quartzites, dolomitic limestones, conglomerates and local banded ferruginous cherts characterise the formations overlying the metabasites. These formations include local metabasites, and the youngest formation contains a thick suite of metarhyolites. The Dharwar lithostratigraphy is characterised by rapid lateral variations in thickness and facies which are the consequence of its original unstable alluvial and relatively shallow marine depositional environments. The instability is marked by slump structures in the phyllitic greywackes and by debris flows at many levels in the stratigraphy. The Dharwar rocks were deposited on a basement of homogeneous granodiorite s.l. and foliated multiphase quartzo-feldspathic orthogneisses. Erosion of elevated parts of the basement and the Dharwar cover provided detritus for alluvial fans, shallow marine quartzites and more distal phyllitic greywackes. Correlation of the lowest formations in tbe Bababudan basin immediately to the south shows that the Dharwar Supergroup becomes progressively younger from south to north in this part of Karnataka.

The structure is characterised by large upright synclines in the Dharwar cover with intervening domal areas of basement. Many of the cover-basement contacts are faults, but some are interpreted as rotated and steepened unconformities. Flushing of water and CO₂ through much of the basement and cover took place during deformation. Buoyancy forces related to heating at deep crustal levels and concomitant compressional forces, generated perhaps in a transpressive oblique-slip regime, may account for the structure of the cover and its basement. Comparable tectonothermal instability, but with trans-tensile components, may have characterised the Dharwar volcanic and depositional phases.

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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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B. M. Ravindra ¹, B. Krishna Rao ², V. N. Vasudev ³

Affiliations
1 Geology Wing, Tungabhadra Project (CADA), Munirabad 583233, IN
2 Geology Department, University of Mysore, Mysore 570006, IN
3 Department of Mines and Geology, Mineral Wing, Bellary 583101, IN

Abstract

Low-grade copper sulphide mineralisation around Kalyadi is localised in an early Precambrian volcanosedimentary suite, characterised by cyclic sedimentational features. The rocks have been intruded by ultramafics, granite and dolerite. The supracrustals have been subjected to three phases of deformation and low-grade metamorphism.

The Fe-Cu sulphide mineralisation is stratiform and stratabound, represented by pyrite. chalcopyrite and pyrrhotite. The salient features of tbe mineralisation suggest that the sulphide ore material derived from volcanic exhalatives was deposited along with their volcanosedimentary host rocks syngenetically about 3.0 b.y. ago. The mineralisation is essentially a sulphide facies of iron-formation. Banded ferruginous quartzite also exists in the area, suggesting consanguineous sulphide-oxide facies relationship. The ore has been deformed and metamorphosed along with the host rocks resulting in mobilisation and moderate enrichment.

Keywords

Economic Geology, Metals, Ore Petrology, Copper, Hassan District, Karnataka.

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B. Chadwick ¹, V. N. Vasudev ², B. Krishna Rao ³, G. V. Hegde ²

Affiliations
1 Earth Resources Centre, University, Exeter EX44QE,, GB
2 Department of Mines and Geology, Government of Karnataka, 16/3-5 S.P. Complex, Lalbagh Road, Bangalore 560027, IN
3 Department of Geology, University of Mysore, Manasa Gangotri, Mysore 576 006, IN

Abstract

The Dharwar sedimentary and volcanic rocks adjacent to their basement granodiorites and gneisses in the Honnali dome can be interpreted in terms of three principal lithostratigraphic units. Each unit represents a distinct stage of basin development. The lowest unit is formed by the mainly shallow marine metabasite-orthoquartzite association of the Bababudan Group. It marks an early extensional phase of basin development. The group in the Honnali area has been subdivided into the Kudrekonda and Kalva Rangan Durga Formations. The second unit comprises mainly shallow marine sedimentary rocks, including polymict conglomerates and limestones, with intercalations of basic to acid volcanic rocks. The lithological association indicates unstable depositional and volcanic conditions with variable uplift and subsidence during the second stage of basin development. The unit is equivalent to the lower part of the Chitradurga Group and it has heen subdivided into the Musinhal, Adrihalli, Aleshpur, Medur and Daginkatte Formations. The acid volcanic rocks forming the Daginkatte Formation were formerly regarded as rhyolitic flows, but they are reinterpreted as ash flow deposits on the grounds of their textures. The third unit corresponds to the upper part of the Chitradurga Group. It begins with the thin, but persistent Basavapatna Formation of banded ferruginous cherts and interbedded carbonaceous phyllites. This formation is overlain by the re-defined Ranibennur Formation of greywackes and local volcanic intercalations. Whilst a marine setting with initial widespread quiescent conditions is indicated by the third unit, its significance is uncertain because of the lack of data from the large, but poorly exposed tract of greywackes.

The Dharwar rocks are dominated by ductile structures in the form of south or southwesterly verging inclined folds with superimposed, steep north-trending folds. In contrast, the basement rocks in the Honnali dome deformed cataclastically on myriad fractures and retrograde shear zones. The dome has a faulted southern boundary, whilst a steepened unconfonnity forms the remaining parts of the boundary. The contrasting structures in the cover and the basement may be modelled in terms of crustal shortening propagating from the north ornortheast on listric faults dipping northeast through the basement into a deep detachment. Later strike-slip displacements on steep NS faults and shear zones modified the south-verging structures. Alternatively, the structure may be interpreted in terms of jostling of basement blocks within a zone of transpression related to sub-horizontal displacements on NS faults and shear zones.

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C. Srikantappa ¹, B. N. Shankar ², V. N. Vasudev ³

Affiliations
1 Department of Geology, Manasagangotri, University of Mysore, Mysore 570 006, IN
2 Department of Mines and Geology, Krishnamurthypuram, Mysore, 570 004, IN
3 Department of Mines and Geology, Lalbagh Road, Bangalore 560 004, IN

Abstract

Ultramafic rocks of the Karighatta schist belt display spinifex texture and komatiitic chemistry indicating their volcanic origin.

Keywords

Paidotitic Komatiite, Karighatta Schist Belt, Karnataka.

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B. Krishna Rao ¹, M. Hanuma Prasad ¹, V. N. Vasudev ², M. S. Sethumadhav ¹

Affiliations
1 Department of Geology, University of Mysore, Manasagangotri, Mysore-570006, IN
2 Department of Mines and Geology, 16, S.P.Complex, Lalbagh Raod, Bangalore-560027, IN

Abstract

Magnesite mineralization in the Copper Mountain areaofthe Sandurschist belt occurs scattered in a 5 km long and 50 to 200 m wide grunerite schist unit of Late Archaean age. Magnesite occurs mostly as less than a cm to about 50 cm thick parallel to subparallel veins. The veins are confined to gently dipping joints and tensional fractures transecting the steeply dipping bedding and schistosity of the host rock. Magnesite is cryptocrystalline and is associated with minor amounts of dolomite/magnesian calcite, calcite, cryptocrystalline silica and iron oxyhydroxides. Magnesite veins are essentially of cavity filling origin. Field setting, rock and mineral chemistry and stable isotope data indicate that magnesite was precipitated at ambienttemperatures from descending meteoric waters enriched in biogenic and atmospheric CO₂ and Mg²⁺, the latter derived from the upper weathered portion of the host grunerite schist.

Keywords

Magnesite, Economic Geology, Weathering, Sandur Schist Belt, Karnataka.

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Brian Chadwick ¹, V. N. Vasudev ², Nazeer Ahmed ³

Affiliations
1 Earth Resources Centre, University of Exeter, Exeter EX4 4QE, GB
2 120/45(A), 3rd Block, Tyagarajanagar, Bangalore - 560 028, IN
3 Department of Mines and Geology, 16/3-5, S.P. Complex, Bangalore - 560 027, IN

Abstract

Six formations are defined in a new lithostratigraphy of the volcanic and sedimentary rocks of the schist belt. The formations (Yeshwantnagar, Deogiri, Raman Mala, Donimalai, Taluru, Vibhuti Gudda) young consistently northeast, except for a part of the Taluru Formation in the northeast of the belt. We propose the new term Sandur Group to include these formations. Their combined thickness of c.35 km is in part an effect of thrust thickening. The Sandur Group was deposited in mainly shallow marine environments in a setting comparable with that of unstable mixed-mode basins. An incomplete, upright, synclinal sheath fold dominates the structure in the east of the schist belt, and another incomplete synclinal sheath fold occurs in the west, its hinge areas and northeastern limb having been cut out by a steepened thrust (the Sandur valley discontinuity). The margins of the belt were intruded by syntectonic, multipulse granites with magmatic and crystal-plastic solid-state linear and planar fabrics and the granite emplacement outlasted deformation. HT/LP metamorphism of the schist belt was contemporaneous with deformation and granite emplacement.

The granites are an integral part of the Late Archaean polyphase granite complex in eastern Karnataka and contiguous parts of Andhra Pradesh and Tamil Nadu, for which we propose the term Dharwar batholith. Published isotopic age data show that its western part includes anatectic granites and relies of Peninsular Gneiss (>2900 Ma), but the eastern part is dominated by juvenile granites. The batholith accreted onto the Archaean foreland to the west comprising Late Archaean marginal basins of the Dharwar Supergroup (Kudremukh. Bababudan-Ranibennur, Chitradurga-Gadag) and their basement of Peninsular Gneiss (>3000 Ma) during Late Archaean plate convergence with NE-SW shortening and sinistral transcurrent displacements. The steep linear belts (Kolar, Ramagiri, Kushtagi. etc.) and irregular tracts (Sandur, Hutti) of volcanic and sedimentary rocks ineastem Karnataka formed as intraarc basins above the evolving Dharwar batholith.

Keywords

Crustal Evolution, Stratigraphy, Structure, Intrusive Granites, Sandur Belt, Karnataka.

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A. P. Nutman ¹, Brian Chadwick ², B. Krishna Rao ³, V. N. Vasudev ⁴

Affiliations
1 Research School of Earth Sciences, Australian National University, A.C.T. 0200, AU
2 Earth Resources Centre, University of Exeter, Exeter EX4 4QE, GB
3 Department of Geology, University of Mysore, Mysore - 570 006, IN
4 120/45(A), III Block, Thyagarajanagar, Bangalore - 560 028, IN

Abstract

SHRIMP U/Pb dating of zircon grains from a schistose acid volcanic rock from the Daginkatte Formation in the Chitradurga Group of the Late Archaean Dhwar Supergroup in western Karnataka has yielded a precise concordant age of 2614 ± 8 Ma which indicates the time of melt crystallisation. In contrast, zircons from similar schistose acid volcanic rocks in the Vibhuti Gudda Formation of the Sandur Group in the Sandur schist belt of eastern Karnataka yielded less precise concordia intercept ages of 2658±14 and 2691±18 Ma.

Zircons from high-strain grey gneisses which appear to be the oldest recognisable component of the Late Archaean granite complex adjacent to the Sandur schist belt have an imprecise age of 2719 ± 40 Ma. Zircons from the youngest recognisable granite adjacent to the belt have also yielded an imprecise, but younger, age of 2570 ± 62 Ma. The ages show that granite emplacement and deformation adjacent to the Sandur schist belt took pIace in a period of c.150 Ma.

The imprecise ages of zircon in the acid volcanic rocks in the Sandur Group and the adjacent granites are related to Neoproterozoic loss of lead which may have been an effect of either weathering or a regional thermal event. We favour the latter in the light of the record of Pan-African thermal effects in the east and south of southern Peninsular India.

Keywords

Geochronology, Zircon, Acid Volcanics, Sandur Schist Belt, Karnataka.

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V. N. Vasudev ¹

Affiliations
1 Bangalore, IN

Abstract

No Abstract.

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V. N. Vasudev ¹

Affiliations
1 120/45(A), 3rd Block, T.R. Nagar, Bangalore 560 052, IN

Abstract

No Abstract.

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M. Hanuma Prasad ¹, B. Krishna Rao ², V. N. Vasudev ¹, B. Basavalingu ¹

Affiliations
1 Department of studies in Geology, University of Mysore, Mysore 570 006, IN
2 Department of Mines and Geology, 16, S.P. Complex, Lalbagh Road, Bangalore 570 027, IN

Abstract

Compositions of co-existing mineral pairs, viz., (a) cordierite- gedrite- garnet- biotite, (b) staurolite-biotite-garnet-muscovite-chlorite and (c) andalusite-staurolite-biotite-gamet in metapelites, and (d) amphibole-plagioclase pair in metabasalts of the Sandur schist belt in the eastern block of the Dharwar craton, Southern India, yield metamorphic temperatures and pressures in the range of 550°-600° C and 4-5.2 kbar, respectively. Textural relationships of the minerals in the above mineral assemblages suggest the development of cordierite, gedrite, garnet, staurolite and biotite during the main-fabric forming event (regional metamorphism). Increase in the grade of metamorphism from greenschist to amphibolite facies associated with the main deformational event is observed from the central to the peripheral portions of the belt; this can be attributed to the steep thermal gradient produced by the syntectonic granitoids. Andalusite post-dates all the other metamorphic minerals and its development outlasts the main deformational episode in the belt. Andalu∼ite is a product of thermal metamorphism, superimposed on regional metamorphism and its growth coincides with the upliftment of the terrain during the emplacement of Iate- to post-kinematic granitoids, which, under comparatively low pressure regime (∼3.75 kbar), maintained earlier peak metamorphic temperatures.

Keywords

Metamorphic Petrology, Sandur Schist Belt, Kamataka.

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M. Hanuma Prasad ¹, B. Krishna Rao ¹, V. N. Vasudev ², R. Srinivasan ³, V. Balaram ³

Affiliations
1 Department of Geology, University of Mysore, Manasagangotri, Mysore - 570 006, IN
2 Mineral Resources Consultant, 120/45(A), 3^rdBlock, TR Nagar, Bangalore - 560 028, IN
3 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

In the late Archaean Sandur supracrustal belt of the Dharwar craton, Southem India, bimodal (mafic-felsic) volcanic rocks are encountered in its eastern region (Copper Mountain region). The mafic volcanic rocks are represented by Al-depleted picritic basalt and tholeliitic basalt. Pieritic basalt has a HREE - depleted pattern. Tholeliitic basalt exhibits two types of REE patterns; (a) slightly depleted - to flat - LREE and unfractionated HREE patterns, and (b) LREE - enriched and HREEdepleted patterns. The felsic volcanic rocks are Na-Rhyolites, which show calc-alkaline affinity and LREE - enriched and HREE-depleted chondrite-normalised patterns. Geochemical signatures show that the picritic basalt owes its origin to melting of a mantle diapir at depths around 100 km. The LREE - depleted tholeiites are not genetically related to the picritic basalt. They were derived through high degree partial melting of a depleted shallow mantle source. The internal variations observed in the LREE - depleted tholeiitic suite are due to derivation of magma through different degrees of melting of the same mantle source. REE-based petrogenetic modelling shows that the LREE - depleted and LREE - enriched tholeiites are not related to a common parent magma. The LREE - enriched tholeiites were generated from a mantle source, which has been metasomatized by subduction related melt/fluid phase. REE chemistry shows that the mafic and felsic volcanic rocks, though spatially and temporally associated, are not genetically related. The HREE depleted rhyolites were derived by partial melting of a garnet - amphibole bearing source, possibly a subducting oceanic crust. Feild, geochemical and petrogenetic aspects suggest that the magmatic rocks of the study area evolved in an active plate margin environment.

Keywords

Geochemistry, Meauics, Late Archaean Sandur Belt, Kamataka.

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V. N. Vasudev ¹

Affiliations
1 Bangalore, IN

Abstract

No Abstract.

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Jayasree Thomas ¹, M. S. Riyazulla ¹, S. Subramanian ², V. N. Vasudev ³, C. S. Gundewar ⁴

Affiliations
1 Indian Bureau of Mines, G G Palaya, Bangalore-560022, IN
2 Department of Metallurgy, Indian Institute of Science, Bangalore-560012, IN
3 Australian Indian Resources, SBI Colony,7th Main, 111 Block, Bangalore-560034, IN
4 Indian Bureau of Mines, Indira Bhavan, Civil lines, Nagpur-400001, IN

Abstract

Cobalt mineralisation in the Kalyadi copper-Cobalt deposit in the Hassan district, Karnataka, is mostly in sulphidic banded, feldspathic cherty quartzite. Electron microprobe study conducted on sulphide concentrates from this deposit revealed that cobalt up to 3.21 wt% is present exclusively in pyrite grains, whereas it is absent in chalcopyrite and arsenopyrite. From this, it infers that pyrite crystallized first, followed by chalcopyrite and arsenopyrite with most cobalt present in the mineralizing system replacing Fe in the paragenetically early-Formed pyrite,thereby rendering the system deficient in cobalt during later crystallization of chalcopyrite and arsenopyrite.

Keywords

Pyrite, Cobalt, Electron Microprobe Study, Kalyadi, Karnataka.

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Brian Chadwick ¹, V. N. Vasudev ², G. V. Hegde ³, A. P. Nutman ⁴

Affiliations
1 17 St Mary's Park, Ottery St Mary EX 11 IJA, GB
2 Australian Indian Resources Group Companies, 125/45, 3rd Block, 6th Cross, 6th Main, Tyagarajanagar, Bangalore-560 028, IN
3 Department of Mines and Geology, Government of Karnataka, "Khanija Bhavan", Race Course Road, Bangalore 560001, IN
4 Research School of Earth Sciences, Australian National University, Canberra ACT 0200, AU

Abstract

Neoarchaean granites adjacent to the Chitradurga schist belt were emplaced in the inner margin of the foreland in the context of the Neoarchaean oblique convergent setting of the Dharwar craton. Two previously unreported granites, one 50 km and the other 80 km NW of Chitradurga town, and a mylonitised granite in the hanging wall of a duplex in the NW of the schist belt yielded SHRIMP U/Pb zircon emplacement ages of 2648±40 Ma, 2598±19 Ma, and ca. 2600 Ma, respectively, the large errors being due to radiogenic Pb loss during an unidentified Neoproterozoic event. Some discrete zircon grains and xenocrystic cores yielded ≥3000 Ma ages that were derived from older rocks during anatexis or emplacement. The granites NW of Chitradurga town were emplaced as steep sheets trending NW·SE. The Chitradurga granite has a similar form, bifurcating N of Chitradurga town into two separate, steeply dipping, NW-SE sheets. Magmatic-and solid-state fabrics in these granites show that emplacement took place during, but was outlasted by, sinistral and dextral strike-Parallel shear. Emplacement of the granite above the hanging wall of the duplex in the NW of the schist belt was outlasted by top-SW displacement.

The shapes of the granites and their emplacement in relation to the structure of the Ranibennur and Chitradurga schist belts in the west of the craton are modelled as a mid-Crustal part of a craton-Wide imbricate fold-Thrust belt. The relationships show that whereas some Neoarchaean granites in the craton were emplaced prior to, or during, SW-vergent thrust thickening, most granites and related plutonic suites in the foreland and accretionary complex were emplaced later as multipulse injections in steep NW-SE sheets or wedges during orogen-parallel. sinistral and dextral shear. Steep highstrain zones in the foreland and accretionary complex are interpreted as listric structures that ischolar_main into an attachment at a depth of ca. 18-20km in accord with the depth of the boundary between upper and lower crust placed at ca. 23 km from seismic reflection data published in 1979 and in more recent studies. The new structural observations and zircon dating, combined with published isotopic age data, show that the inner margin of the foreland in the west of the craton and the outer margin of the accretionary complex in the east are linked in a diffuse, steeply dipping, Orogen-parallel boundary zone at least 200 km wide.

Keywords

Neoarchaean, Dharwar Craton, Fold-Thrust Belt, Transpressive Orogen, Oblique Convergence, Attachment, Chitradurga Schist Belt, Karnataka.

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V. N. Vasudev ¹

Affiliations
1 125/45(A), III Block, Thyagarajanagar, Bangalore - 560 028, IN

Abstract

No Abstract.

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Brian Chadwick ¹, V. N. Vasudev ², G. V. Hegde ³

Affiliations
1 Pennsylvania Close, Exeter EX4 6DJ, GB
2 Geomysore Services (India) Pvt. Ltd., 120/45(A), III Block, 6^th Cross, 6^th Main, Thyagarajanagar, Bangalore - 560 028, IN
3 Department of Mines and Geology, Government of Karnataka, "Khanija Bhavana", Race Course Road, Bangalore - 560001, IN

Abstract

The Late Archaean Dharwar Supergroup in the Chitradurga schist belt NW of Tungabhadra river is dominated by low-grade metabasalts, polymict conglomerates, ferruginous cherts and greywackes with intercalations of dolomitic limestone and felsic volcanic rocks. The belt was thickened as the result of overturning on large-scale folds and stacking on reverse faults in a hinterland-dipping duplex, herein called the Gadag duplex. Gold mineralisation is confined to, and was controlled by, the duplex structure. In the Late Archaean oblique convergent setting of the Dharwar craton, the duplex dips NE away from the foreland formed by the western part of the craton and into the accretionary complex of granites (Dharwar batholith) and intra-arc schist belts comprising the eastern part.

The Gadag duplex comprises four thrust slices. Its foot wall block comprises metabasalts and greywackes of the Dharwar Supergroup and underlying, variably mylonitised granites s.l. with magmatic-state and mylonite fabrics indicating top-NW shear sense. The supergroup in the foot wall block is arched over domes of these granites to link with the lowest metabasalts and greywackes in the eastern margin of the Ranibennur schist belt. Lit-par-lit injections in the metabasalts suggest the plutonic rocks are younger than the Dharwar Supergroup, i.e. they are probably Late Archaean, not pre-2900 Ma basement to the schist belt. Arching of the foot wall block is attributed to NE-SW shortening broadly contemporaneous with, or soon after, thrust stacking in the Gadag duplex.

The hanging wall block comprises mylonitised granites s.l. with top-SW shear sense indicators. The roof thrust south of Gadag dips gently NE, but steepens further SE. Lit-par-Ut injections in metabasalts adjacent to the roof thrust indicate a tectoni sed intrusive contact like that in the foot wall block. Large-scale gentle folding of the shallow part of the roof thrust is linked to sinistral displacement along the steepened thrust. Plutonic rocks in the hanging and foot wall blocks are tentatively correlated with granites in the western margin of the Dharwar batholith.

Keywords

Late Archaean, Duplex Structure, Oblique Convergence, Gold Mineralization, Chitradurga Schist Belt, Dharwar Craton, Karnataka.

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V. N. Vasudev ¹

Affiliations
1 120145 (A), III Block Thyagarajnagar Bangaiore - 560 028, IN

Abstract

No Abstract.

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Brian Chadwick ¹, G. V. Hegde ², Allen P. Nutman ³, V. N. Vasudev ⁴

Affiliations
1 12 Pennsylvania Closc, Exeter EX4 6DJ, GB
2 Department of Mines and Geology, Govt. of Karnataka, Khanija Bhavana, Race Course Road, Bangalore 560 001, IN
3 Research School of Earth Sciences, Australian National University, Canberra ACT 0200, AU
4 Australian-Indian Resources Pvt. Ltd., 120/45(A) 111 Block, Thyagarajanagar, Bangalore 560 028, IN

Abstract

The predominantly syenitic Koppal pluton was emplaccd in the western flank of the Dharwar batholith which forms the eastern half of the Dharwar craton. Previous work has shown that the batholith accreted against an older continental foreland reprcsented by the western half of the craton during Late Archacan oblique convergence. A sample of syenitc has yielded a SHRIMP U/Pb zircon age of 2528±9 Ma which is interpreted as the timc of emplacement at a late stage in the accretion of the batholith. Its age and composition make the Koppal pluton a unique feature of the Dharwar craton. Melt commingling of syenitc with other intcrrnediate to basic magmas is indicated by commmon mafic enclaves, disrupted mafic sheets and larger netveined bodies rich in hornblende and pyroxene. The arc setting of the Koppal pluton is consistent with syenitic plutons in other Late Archaean terrains.

The pluton has an elliptical outcrop trending NE-SW with an upright, asymmetrical funnel shape indicated by its magmatic-state LS fabrics and diffuse compositional banding. Banding is a function of variations in microclinc, hornblende and clinopyroxene abundances. Thc pluton was ernplaced broadly parallel to diffuse magmatic banding in the host granites which lies oblique to trends of the steep NW-SE belts of plutonic rocks that characterise the Dharwar batholith as a whole. This oblique orientation of magmatic banding has been interpreted previously as the result of emplacement when magmatic pressure exceeded regional compressive stress during sinistral displacement on steep shear zones trending NW-SE. Emplacement of the Koppal pluton is interpreted in a similar way.

Keywords

Late Archaean, Syenitc, SHRIMP U/Pb Age, Koppal, Dharwar Craton.

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Brian Chadwick ¹, G. V. Hegde ², V. N. Vasudev ³

Affiliations
1 Pennsylvania Close, Exeter EX4 6DJ, GB
2 Department of Mines and Geology, Government of Karnataka, 49 Khanija Bhavan, Race Course Road, Bangalore - 560 001, IN
3 120/45(A), 111 Block, Thyagarajanagar, Bangalore - 560 028, IN

Abstract

No Abstract.

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R. H. Sawkar ¹, V. N. Vasudev ²

Affiliations
1 Geological Society of India, Bangalore - 560 019, IN
2 Geomysore Services (India) Pvt. Ltd., Bangalore, IN

Abstract

No Abstract.

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V. N. Vasudev ¹

Affiliations
1 Geological Society of India, Bangalore, IN

Abstract

No Abstract.

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V. N. Vasudev

Abstract

No Abstract.

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V. N. Vasudev ¹

Affiliations
1 Bangalore, IN

Abstract

No Abstract.

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V. N. Vasudev

Abstract

No Abstract.

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B. P. Radhakrishna ¹, V. N. Vasudev ¹

Affiliations
1 Geological Society of India, 16 Ali Asker Road, Bangalore 560052, IN

Abstract

No Abstract.

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