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

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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B. Krishna Rao ¹, M. Hanuma Prasad ¹

Affiliations
1 Department of Geology, University of Mysore, Mysore 570006, IN

Abstract

Chemical composition of the phyllites of the Copper Mountain region of the late Archaean Sandur Schist belt in the eastern block of the Dharwar craton indicates that the fine-grained clastic sediments from which they were formed, were derived from a provenance comprising -60% felsic, - 30% mafic and 10% ultramafic rocks. High Zr/Nb and Zr/Y ratios and lack of strong -ve Eu anomalies suggest that the felsic component of the provenance was dominantly made up of tonalites-trondhjemites; granites and granodiorites were very subordinate constituents. The sediments appear to have accumulated in a continental island are environment. Rarity of granitic constituents in the pre-Dharwar provenance of Sandur basin contrasts with the abundance of such constituents in the provenance for the fine-grained clastic sediments of basins in the western block of the Dharwar craton. The continental crust of the eastern block was apparently less evolved during the pre-Dharwar time as compared to that in the western block of the Dharwar craton.

Keywords

Geochemistry, Phyllites, Copper Mountain, Sandur Schist Belt, Karnataka.

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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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M. K. Devarajan ¹, M. Hanuma Prasad ¹, A. V. Keshava Prasad ¹, M. K. Soni ¹

Affiliations
1 Operation M.P. II, Geological Survey of India, P.O. Garha, Jabalpur-482003, IN

Abstract

Gold Mineralization in the eastern part of the Late Archaean Mahakoshal Belt is observed in sulphidic chert associated with banded ferruginous chert (BFC) and in quartz-carbonate-sulphide veins and quartz-carbonate veins emplaced into argillites, greywackes, carbonaceous phyllite and BFC. In the western part of the Mahakoshal belt gold mineralization is noticed in quartz-carbonate veins intruded along the lithological contacts between metabasalts and dolomitic limestone, and in quartz veins/quartz porphyry dykes within dolomitic limestone. Bulk of the gold mineralization is associated with quartzcarbonate and quartz-carbonate-sulphide veins, emplaced during the initial phase of deformation of the Mahakoshal Belt.

Keywords

Economic Geology, Gold, Mahakoshal Belt, Son-Narmada Lineament, Madhya Pradesh.

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B. Krishna Rao ¹, M. S. Sethumadhav ¹, M. Hanuma Prasad ², T. D. Mahabaleshwar ¹, Ashok V. Rao ¹, H. Albert Gilg ³

Affiliations
1 Department of studies in Geology, University of Mysore, Manasagangotri, Mysore - 570 006, IN
2 Geological survey of India, Sanjivani Nagar, Garha Road, Jabalpur - 482 003, IN
3 Institute for Mineralogy and Geochemistry, Technical University of Munich, Lichtenberg str, D-85747, Garching, Munich, DE

Abstract

Vein-type magnesite deposit of Doddakanya area is situated in the southern part of the Archaean Dharwar craton. Magnesite is confined to a c.3.3 Ga old ultramafic body which constitutes a tectonically dismembered part of a layered ultramafic-mafic complex emplaced into supracrustal rocks of the Sargur Group. The magnesite-bearing ultramafic body is composed of dunite with minor harzburgite and exhibits varying degrees of serpentinization and weathering. About 50 vol.% of the ultramafic body is serpentinized and the intensity of serpentinization decreases with depth. The serpentine minerals are represented essentially by lizardite/chrysotile exhibiting pseudomorphic mesh-texture. Serpentinization of the ultramafic body took place within 6 km of the Earth's surface at low temperatures (<200°C) and essentially involved volume expansion (isochemical) process. During weathering, the relict olivine of the serpentinized ultramafic body was pseudomorphously replaced by a mixture of amorphous material (with variable amounts of Fe, Mg, Si and volatiles) and minor ferric oxyhydroxides and secondary silica. The Mg mobilized during weathering participated in the formation of magnesite in tensional fractures developed during the late- to post-serpentinization event. Oxygen isotope data  (δ¹⁸O = av. 30.6‰ SMOW) indicate the precipitation of magnesite at ambient temperatures and carbon isotope values (δ¹³C = av. -2.6‰ PDB) suggest the derivation of carbon from a biogenic soil source dominated by C4 plants. The latter constrains the timing of the magnesite formation to a period not older than 8 - 7 Ma.

Keywords

Economic Geology, Archaean Ultramafic Rocks, Vein-Type Magnesite, Serpentinization, C and O Isotopes, Supergene Origin, Doddakanya, Karnataka.

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M. Hanuma Prasad ¹, Asfar Hakim ¹, B. Krishna Rao ²

Affiliations
1 GSI Colony, Sanjivani Nagar, Jabalpur - 482 003, IN
2 Department of Geology, University of Mysore, Mysore - 570 006, IN

Abstract

The Bundelkhand Granitic Complex of central India contains inclusions of Archaean volcanic and sedimentary rocks and orthogneisses, some of which are 15 km by 6 km in size, Earlier workers considered that these inclusions belong to two distinct lithostratigraphic formations separated in time by a migmatization event. Our new data suggest that the migmatization event post-dated all of the volcanic and sedimentary rocks and hence the two-fold stratigraphic subdivision is not viable. Further, intrusion of orthogneisses resulted in an increase in grade of metamorphism in some volcanic and sedimentary rocks specifically near the contact.

Keywords

Structural Geology, Orthogneisses, Volcanic and Sedimentary Rocks, Bundelkhand Granitic Complex, Madhya Pradesh.

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Abhinaba Roy ¹, M. Hanuma Prasad ¹, S. A. Chore ¹, L. L. Viswakarma ²

Affiliations
1 Geological Survey of India, Seminary Hills, Nagpur - 400 006, IN
2 Geological Survey of India, 0p.MP-I, Bhopal - 462016, IN

Abstract

No Abstract.

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Abhinaba Roy ¹, M. K. Devarajan ¹, M. Hanuma Prasad ¹

Affiliations
1 Geological Survey of India, Central Region, PO. Seminary Hills, Nagpur - 440 006, IN

Abstract

A prominent E-W trending, sub-vertical to steep south dipping, ductile shear zone occurs along the southern margin of the Palaeoproterozoic Mahakoshal supracrustal belt of Central India. Shear zone structures have been documented from the supracrustal rocks as well as from the intrusive granites (s.l.) from an area near Singrauli. The supracrustal belt away from the shear zone displays a set of subhorizontal upright folds on E-W to ENE-WSW striking axial planes. The intensity of deformation increases southward; the folds become tight to isoclinal with slight overturning towards the north while the fold hinges arc rotated from the subhorizontal position to sub-vertical or down-dip reclined attitude. Mylonitic foliation within the shear zone is sub-parallel to the axial plane schistosity of the dominant fold-set outside the shear zone. The granitic rocks show both magmatic and solid-state fabrics which are part of a continuum of emplacement and deformation in the shear zone. In the initial stages of solid-state deformation, which gave rise to protomylonites, coarse K-feldspar porphyroclasts behavcd in a brittle manner in an overall bulk ductile deformation in the matrix. With increase in intensity of deformation accompanied by the introduction of fluids, the deformation mechanism in feldspar changed from brittle to ductile. Mylonitic foliation is invariably associated with a pervasive stretching lineation. Shear-sense indicators suggest oblique-slip movement with a large reverse-slip component and a small sinistral strike-slip component. The geometry of the folds and their sequence of development are similar in the rocks within and outside the shear zone. Concordance between the granite plutons and the supracrustal rocks is due to syntectonic emplacement. Low pressure-medium temperature metamorphism in the Mahakoshal supracrustal belt contemporaneous with granite intrusion suggests depth of emplacement about 10- 12 km. The bulk kinematic model involves N-S to NNESSW subhorizontal compression over which was superimposed in the southern part a sinistral shearing movement during progressive transpressional deformation.

Keywords

Ductile Shear Zone, Mylonites, Syntectonic Granite, Shear-Sense Indicators, Mahakoshal Group, Palaeoproterozoic, Central India.

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