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

M. Ram Mohan ¹, D. Srinivasa Sarma ¹, S. N. Charan ¹, V. Balaram ¹, V. B. Rajasekhar ¹, Talat Ahmad ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad-500007, IN
2 Department of Geology, University of Delhi, Delhi-110007, IN

Abstract

Gadag Greenstone Belt (GGB) is the northern continuation of Chitradurga Greenstone Belt (CGB). It consists of a variety of metavolcanic and metasedimentary rocks. Two types of metavolcanic assemblages are found in this terrane (l) the tholeute-calc-alkaline island arc bimodal assemblage and (u) the tholeute-high-Mg basalt assemblage The tholente-calc-alkaline assemblage is exposed in the central and northern parts, whereas the tholentic-high-Mg basaltic assemblage is found in the southwestern part of the belt. Tholente-high-Mg basalts are represented by the coarse-grained amphibolites formed under lower amphibolite facies conditions REE and HFSE data along with major element compositions confirm that these coarse-grained amphibolites are tholentic basalts derived from an intraoceanic island arc setting. The REE patterns are coherent, flat to slightly LREE depleted (La/Yb_n =0.79 to 1.20, La/Sm_n =0.84-0.97, Gd/Yb_n=1.07-1.50) with no Eu anomaly. Relationship between compatible and incompatible elements suggests least effects of alteration and no crustal contamination or fractional crystallization. The mixing calculations indicate that these rocks are derived by partial melting of a depleted mantle source, with source composition in between that of the N-MORB and high-Mg basalts.

Keywords

Geochemistry, Petrogenesis, Amphibolites, Gadag Greenstone Belt.

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S. M. Naqvi , D. Srinivasa Sarma , R. H. Sawkar , M. Ram Mohan , J. G. Rana Prathap , S. Viswanathan ¹

Affiliations
1 Flat B-203, Block-B, United Avenue Apts, South End, 7 1-29, Ameerpet, Hyderabad-500016, IN

Abstract

No Abstract.

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D. Srinivasa Sarma ¹, Janet Muhling ², M. Ram Mohan ¹, Neal McNaughton ³

Affiliations
1 National Geophysical Research lnstitute, Uppal Road, Hyderabad-500007, IN
2 Centre for Microscopy and Microanalysis, University of Western Australia, 35, Stirling Highway, Crawley, 6009, IN
3 Centre for Exploration Targeting, University of Western Australia, 35, Stiriling Highway, Crawley 6009, IN

Abstract

We report here, for the first time, the occurrence of hydrothermal monazite and xenotime in alteration haloes associated with the gold-Mineralized zones of the Hutti Gold Deposit in Karnataka, India. These minerals were identified by systematic scanning of polished thin sections in an SEM using backscattered electron and energy dispersive X-Ray detectors. Most of the grains are very small (<10 μm), although some larger gram (30-40 μm) have also been identified Hydrothermal monazite and xenotime, though volumetrically minor, have been found to be important components in many orogenic gold deposits in Australia, Canada, Brazil and South Africa,as they form a part of the primary ore mineral assemblage, elther in ore-Bearing veins itself or are intergrown wlth ore minerals in wallrock alteration zones. Both monazite and xenotime are excellent geochronometers, and their importance is that they allow precise ages to be obtained for gold mineralization events. The discovery of these minerals in hydrothermally altered rocks from the Hutti gold deposit Dharwar Craton enables us to estimate the age of gold mineralization event(s).

Keywords

Hutti Gold Deposit, Alteration Zones, Hydrothermal Minerals, Monazite, Xenotime, Karnataka.

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S. M. Naqvi ¹, D. Srinivasa Sarma ¹, R. H. Sawkar ², M. Ram Mohan ¹, J. G. Rana Prathap ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN
2 Mineral Sales Private Limited, Hospet - 583 203, IN

Abstract

Acid volcanics found in auriferous greenstone belts of the Dharwar Craton are rhyolites, adakitic rhyohtes and high silica adakites Adakites are compositionally similar to TTG and characterized by high Na^/K^O, depleted MgO, Cr, Ni, Y and Yb. The adakitic melts were most probably generated by the partial melting of the basaltic slab below a mantle wedge LILE and LREE enriched IAB are the dominant volcanic members of the greenstone belts IAB of the greenstone belts were generated from partial melting of the mantle wedge under the influence of slab derived fluids. The wedge derived IAB and slab derived adakites were deformed and metamorphosed to generate fluids responsible for the gold endowment of these belts. Rhyohtes and possibly rhyohtic adakites were generated by the melting of the sialic continental crust forming top of the descending slab. Identification of adakites in auriferous greenstone belts strengthens the genetic link between adakite magmatism, subduction and Neoarchaean gold endowment.

Keywords

Adakites, Dharwar Craton, Gold Greenstone Belt, Karnataka.

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S. N. Charan ¹, E. V. S. S. K. Babu ¹, S. M. Naqvi ¹, J. G. Rana Prathap ¹, M. Ram Mohan ¹, D. Srinivasa Sarma ¹

Affiliations
1 National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad - 500 606, IN

Abstract

The major part of the Peninsular Gneiss in Dharwar craton is made up of Trondjhemite-Tonalite-Granodiorite (TTG) emplaced at different periods ranging from 3.60 to 2.50 Ga. The sodic-silicic magma precursors of these rocks have geochemical features characteristic of partial melting of hydrated basalt. In these TTGs, enclaves of amphibolites (± garnet) are abundant. These restites are considered to be the residue of a basaltic crust after its partial melting. A detailed study of these (residue) enclaves reveals textures formed due to the process of partial melting. Major, trace and REE analysis of these residue enclaves and the melt TTGs and microprobe analysis of the coexisting minerals show partitioning of REE and HFSE between the precursor melt of TTGs and the upper amphibolite facies residues. Formation of garnetiferous amphibolites with biotite, Cpx and plagioclase consequent to melting, has squeezed the original MORB type of basaltic crust and given rise to the TTGs, depleted in Y, Yb, K₂O, MgO, FeO, TiO₂ and enriched in La, Th, U, Zr and Hf. Coevally during the process of melting, the hydrated basalt was depleted in Na₂O, Al₂O₃, LREE, Th, U and enriched in K₂O, MgO, Nb, Ti, Yb, Y, Sc, Ni, Cr and Co. Mineral chemistry of co-existing garnet-biotite and amphiboleplagioclase in these amphibolitic (restite) enclaves indicates an average temperature of 700 ± 50 °C and pressure of 5 ± 1 Kbar. These data are inferred to indicate that during the garnet stability field metamorphism, effective fractionation of HREE and HFSE has taken place between the restites having Fe-Mg silicates, ilmenites and the extracted melt generated from the MORB type of hydrated basalt. These results are strongly substantiated by the reported melting experiments on hydrated basalts.

Keywords

Fractionation, HFSE, REE, TTG, Restite, Dharwar Craton.

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S. M. Naqvi ¹, C. Manikyamba ¹, T. Gnaneshwar Rao ¹, D. V. Subba Rao ¹, M. Ram Mohan ¹, D. Srinivasa Sarma ¹

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

Abstract

Metavolcanics of 20 km wide Sultanpura volcanic block of the Neoarchaean Sandur (greenstone) schist belt are divided into tholeiitic basalts, high Mg basalts, Al-depleted and Al-undepleted komatiitic ultramatic schists. Metabasalts are metamorphosed to amphibolite facies, but still preserve their pillow structures. Ultramasic komatiitic rocks are transformed to actinolite-tremolitc schists with no recognizable original textures or mineralogy. Mctabasalts and ultramafic komatiitic schists are interbedded with thin layers of sulphidic banded iron formations, argillaceous carbonate rocks and carbon phyllites that are interprcied as metamorphosed pclagic sediments of the deep ocean. No terrigenous sediments are found in Sultanpura block indicating that eruption of these submarine volcanic rocks took place in the deeper part of the ocean, away from the western and eastern shelf parts of the Sandur belt, where terrigenous sediments are abundant. SuItanpura block in its west and east is discordantly boundcd by thrust, subduction complex and shclf lhcies sedimcnts. These observations are interpreted to indicate that Sultanpura block is a telescoped prolo-oceanic part between the two shelves and island arc complexes. MgO of melavolcanic rocks varies from 6 to 30%, with a gap between 16 to 22%. Al₂O₃/TiO₂ shows characteristic variation for tholeiites (10-15), high Mg basalts (13-21), Al-unclepleted ultramafic komatiitic schists (9-23) and Al-depleted ultramafic komatiitic schists (11-20). CaO/Al₂O₃, of tholciites and high Mg basalts is ∼1, whereas for the ultramafic komatiitic schists, this ratio exhibits a range between 0.5 to 2, as a consequence of CaO mobility.

REE, HFSE and ¹⁴³Nd/¹⁴⁴Nda ta from Sultanpura volcanic rocks vary between CHUR (Chondrite Uniform Rcservoir), primitive mantle and depleted manilc, but appear to be derived from primitive mantle and have been probably contaminated by continental crust. Although the abundance of REE varics from 2 to 12 chondrite, the patterns are smooth and flat with small negative or positive Eu anomalies as artifacts of alteration. Generally positive, but in few samples negative Nb anomalies are also found, with (Ce/Sm)_N, and (Gd/Yb)_N, being near chondritic. Ti/V, Ti/Zr, Zr/Y, Sc/Y, Nb/La, Nb/Th, Nb/U, MgO/TiO₂, MgO/FeO and Al₂O₃/TiO₂, also for many samples are ncar chondritic, εNd=+0.8649±0.00024 resembling CHUR. ThMb, NblU and some other ratios are near to those of Ontong Java and Gorgana plateaus (0.80 Ga) and the tholeiite-komatiitic sequence found in 2.7 Ga Southern volcanic zone, Abitibi belt of Canada and 2.1 Ga Birimian belt of West Africa. Collectively, thcsc data indicate that a mantle plume. derived from an enriched mantle, possibly played an important role in the oceanic volcanic sequence of Sultanpura block. Some of the HFSE follow the olivine control line, whereas other elements following the olivine control line define a narrow array tube. Formation of such array tubes on the plots of some HFSE elements and their ratios, and the scatters of HFSE/REE ratios, probably suggest dynamic melting of the plume during ascent. Entrenchment, mixing of Archaean ocean ridge basalts (AORB), crustal contamination and subduction of such a plume-fed slab may have generated the compositional heterogeneities observed in the Sultanpura metavolcanic rocks.

Keywords

Archaean Plumes, Greenstone Belts, Geochemistry, Tectonic Evolution, Sultanpura Volcanic Block, Sandur Schist Belt, Dharwar Craton.

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M. Ram Mohan ¹, Sita Bora ²

Affiliations
1 NGRI-CSIR, Hyderabad, IN
2 Kumaun University, Nainital, IN

Abstract

No Abstract.

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