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

B. Krishna Rao ¹, P. N. Satish ¹, M. S. Sethumadhav ¹

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

Abstract

Bauxite-bearing laterite of Boknur-Navge plateau, Belgaum district, Karnataka, exhibits textural, mineralogical and geochemical features of both syngenetic and epigenetic origin. While the syngenetic features of the lateritic residuum developed during a major episode of residual weatheringof'the source rock (Deccan basalt), the epigenetic features originated during an episode of secondary alteration of the pre-existing lateritic material.

Residual weathering of basalt took place in three stages and culminated in the formation of a well-differentiated lateritic residuum consisting of vermicular laterite and massive (textureless and pisolitic) bauxite. Lateritic weathering, under the activity of slightly acidic meteoric waters, involved leaching of ∼90% SiO₂, ∼60% Fe₂O₃ and almost all the K, Na, Ca and Mg from the weathered zone leading to the relative enrichment of Al. Part of the dissolved Si, Fe and AI of the circulating waters were subjected to local migration and reprecipitation, resulting in the segregation of AI-rich (bauxite) and Fe-Si-rich (laterite) units within the laterite profile.

Secondary products derived from the alteration of the bauxite and laterite show zonat variation in their mode of occurrence, textures, mineralogy and chemical composition. During the secondary alteration, the bauxite and laterite of the ∼10 m thick bauxite-bearing laterite zone were subjected to 1) silication and ferrification at the upper horizon, 2) silication at the intermediate horizon, and 3) minordesilication at the lower horizon. Among the three types of alterations, silication is widespread. The 'late-or post-bauxite' episode of alteration of the lateritic residuum was probably brought about by the action of highly acidic meteoric waters charged with Si.

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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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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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S. M. Moosavirad ¹, M. R. Janardhana ², M. S. Sethumadhav ¹, K. N. Prakash Narasimha ¹

Affiliations
1 Department of Studies in Earth Science, University of Mysore, Manasagangotri, Mysore - 570 006, IN
2 Department of Earth Science and Resource Management, Yuvaraja's College, University of Mysore, Mysore - 570 005, IN

Abstract

Lower Jurassic sandstones of Shemshak Formation of Kerman basin, central Iran were analyzed for major and select trace elements to infer their provenance, palaeoweathering of source rocks and tectonic setting. Average modal framework components (Qt: F: L = 67.25: 2.41: 30.48) and chemical composition of the sandstones classify them as litharenites. The sandstones are quartz-rich (∼67% quartz; 75.34 wt.% SiO₂) and derived from a recycled orogen composed of quartzose sedimentary rocks. Average CIA, PIA and CIW values (69%, 76% and 80%, respectively) indicate moderate to intense chemical weathering of the source material. The inferred index of weathering/alteration is the sum total of intensities of weathering witnessed by the lithocomponents during atleast two cycles of sedimentation involving (1) chemical weathering of the source rocks («ultimate» granodiorite source and «proximal» quartzose sedimentary source), (2) chemical weathering during fluvial transport of the detritus, (3) chemical weathering of the detritus in depocenters, and (4) chemical weathering during diagenesis. Sandstones exhibit moderate maturity and were deposited under humid climatic conditions. Plots of the chemical analyses data on tectonic setting discrimination diagrams indicate active continental margin setting, which is in agreement with the tectonic evolutionary history of the Central Iran during Jurassic period.

Keywords

Sandstones, Lower Jurassic, Provenance, Palaeoweathering, Tectonic Setting, Shemshak Formation, Kerman Basin, Central Iran.

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