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

T. C. Devaraju ¹, M. S. Sadashivaiah ¹

Affiliations
1 Department of Geology, Karnatak University, Dharwar, IN

Abstract

The pyroxene-quartz-magnetite rocks occurring in association with the charnockites and other rocks of the Satnur-Halaguru area, which were regarded by many of the early officers of the Mysore Geological Department as members belonging to charnockite series have been reinvestigated. The investigation has brought to light the occurrence in these rocks, of iron-rich minerals like, ferrohypersthene, eulite, hedenbergite, ferroactinolite, iron-rich garnet and the unusual occurrence of pyroxmangite and orthoclase, It is suggested from a detailed study of the field relation, petrology, mineralogy and petrochemistry that the pyroxene-quartz- magnetite rocks form a separate entity, representing metamorphosed iron-rich sediments, which except for their formation under similar metamorphic conditions namely, pyroxene granulite facies, have no relation whatsoever with the intimately associated charnockites of the area.

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T. C. Devaraju ¹, M. S. Sadashivaiah ¹

Affiliations
1 Department of Geology, Karnatak University, Dharwar, IN

Abstract

Orthopyroxene-bearing rocks viz., quartz-norite, cordierite-biotite-hypersthene granulite, leptynite and hypersthene-biotite quartzite containing minerals like hypersthene (aluminous), diopside, brown hornblende, biotite, garnet, cordierite, plagioclase and quartz, occur constituting an integral part of the high-grade metapelites of the area. The field and petrological observations suggest that these orthopyroxene-bearing rocks are variants of the metapelites, derived by the metamorphism of appropriate sediments and have no connection with the associated charnockites of the area.

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T. C. Devaraju ¹, K. S. Anantha Murthy ¹

Affiliations
1 Department of Geology, Karnatak University, Dharwar - 580003, IN

Abstract

Barytes occurs as small, subeconomic, discontinuous lenses and bands interstratified with sericite and fuchsite-bearing quartzite. The mineral has 2V_z=34°, nβ=1.632 and (γ - α)=0.095. There is a close similarity in the powder X-ray data obtained for three samples of the mineral and between these and the ASTM values for BaSO₄. The deposit bears evidence of sedimentary origin; both biochemical processes and volcanic exhalations are inferred to have contributed to the formation of the deposit. The deposit perhaps constitutes the terminal phase of a volcanic cycle.

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T. C. Devaraju ¹, K. S. Anantha Murthy ¹, N. M. Udayashankar ¹

Affiliations
1 Department of Geology, Karnatak University, Dharwad-580003, IN

Abstract

The 'bauxite' of Shivaganga is actually an aluminous clay deposit containing essentially a mixture of halloysite and kaolinite.

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T. C. Devaraju ¹, V. H. Uttangi ¹, J. J. M. M. M. Coolen ²

Affiliations
1 Department of Geology, Karnataka University, Dharwar 580003, IN
2 Institute of Earth Sciences, Free University, De Boelelaan, Amsterdam, NL

Abstract

Hogbomite analysing 59.94% Al₂O₃, 6.01% TiO₂, 0.21% V₂O₃, 21.02%, FeO, 0.14%, MnO, 9.66%, MgO and 1.06% ZnO occurs as a widely distributed mineral phase in the vanadiferrous Fe-Ti ore deposits of Madangere area. It occurs in well developed prismatic crystals, and small scales/rods/patches enclosed in magnetite ore grains, and locally as rims to spinel. While the coarse prisms of hogbomite bear characters of primary mineral phase, the small scales/rods/patches appear to represent exsolved phase and that forming rims to spinel is derived by the alteration of spinel. The mineral is reported for the first time from India.

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T. C. Devaraju ¹, K. S. Anantha Murthy ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580 003, IN

Abstract

Mineralogy of ferroan ankerite with the composition (Fe, Mn)_0.67 (Mg)_0.38 (Ca)_0.93 C₂O₆ is discussed with the aid of X-ray, DTA, and chemical data. The mineral typically occurs in the iron-enriched carbonate bands, which are either closely associated or enclosed within the iron formation. Available evidence suggests syngenetic chemical sedimentary origin for the mineral.

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T. C. Devaraju ¹, S. D. Khanadali ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwar, IN

Abstract

Analysis of 34 bauxite-laterite and 11 precursor granitic gneiss samples, from Paduvari plateau, fer alkali and alkaline earth elements has revealed: (1) the strong depletion of laterites/bauxites in Ca, Na, K, Sr, Li and Rb, with a reduction in Ca by 124 times, Na and K by 55 times, Sr by 38 times, Li and Rb by 7½-8 times, (2) low mobility of Mg and Be to result in a depletion of only 2½ times, (3) the lack of significant or systematic variation in the content of these elements in the laterite-bauxite profile and (4) the retention of Na: K and Li : Rb ratios of the precursor gneiss in the laterite/ bauxite. The study substantiates the general observation that alkali and alkaline earth elements with their low ionic potential (except Be) are thoroughly leached during lateritization.

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T. C. Devaraju ¹, V. H. Uttangji ¹, J. J. M. M. M. Coolen ²

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580003, IN
2 Instituut Voor Aardwetenshappen, Vrije Universiteit, 1007 MC Amsterdam, NL

Abstract

Sheridanite with the formula (Mg_7.97 AI_2.64 Fe"_1.38 Ti_0.01 Mn_0.01) [(Si_5.3 Al_2.7) O₂₀)] (OH)₁₆ occurs as one of the main gangue minerals in the V-Ti-Fe deposits of Madangere. It is found chiefly as late stage pyrogenic mineral and to a very limited extent as secondary mineral after hogbomite.

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T. C. Devaraju ¹, J. J. M. M. M. Coolen ²

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580003, IN
2 Instituut voor Aardwerenschappen, Vrije Universiteit, Postbus 7161,1007 MC Amsterdam, NL

Abstract

Electron microprobe analyses of coexisting phases in a basic scapolite-garnet-hornblende-two-pyroxene granulite from a Sargur supracrustal enclave near Doddakanya are presented. The results of various mineral geothermometers and geobarometers indicate temperature and pressure conditions of 810±20°C and 9.5 ± 0.5 kbar during metamorphic reequilibration. A comparison with other data from granulites of southern India shows that granulite-facies metamorphism of relatively high pressure is a common feature of the granulite region of the Archaean shield.

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T. C. Devaraju ¹, K. S. Anantha Murthy ²

Affiliations
1 Department of Studies in Geology, Karnataka University, Dharwad 580003, IN
2 Department of Geology, Bangalore University, Bangalore 560056, IN

Abstract

Carbonates of the Chiknayakanhalli (CN halli) area include (i) dolomite, (ii) limestone, (iii) ankerite and (iv) calc-schist. Of these, dolomite is the most widely developed rock. chemically the CN halli carbonates cover a wide range from almost pure calcite and dolomite-ankerite to Mg-limestone, shaly limestone and cherty limestone/dolomite/ankerite. In their trace element content, the rocks are typically much depleted in comparison with carbonatites. Available evidence indicates overall primary chemical sedimentary deposition of the carbonates in a barred basin with a gradual rise in salinity, pH, temperature and Mg:Ca ratio of the environment. Evidence of dolomitization is limited to the border zones between limestone and dolomite and there is no recognizable indication of organic activity.

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B. P. Radhakrishna ¹, T. C. Devaraju ¹, B. Mahabaleswar ¹

Affiliations
1 Geological Society of India, IN

Abstract

No Abstract.

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T. C. Devaraju ¹, K. Laajoki ²

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580003, IN
2 Department of Geology, University of Oulu, Linnanamaa, Oulu 57, FI

Abstract

The iron-formations of the high-grade metamorphic terrain of Southern Karnataka comprise both Mn-poor (av. 0.3% MnO) and Mn-rich (av, 7.4% MnO) types. The two types are distributed throughout the 1273km² area studied. Each type occurs constituting a large proportion of an individual body and within each body, complex variations from one type to the other, both along and across the strike, might exist. Despite their high-grade metamorphism the rocks have preserved primary sedimentary banding; there are, however, no relics of original textures or minerals. A very striking mineralogical feature of these iron-formations is the common occurrence of pyroxenes and garnets as major mineral phases in addition to magnetite and quartz. Mn-rich types are typically enriched in garnet whereas the Mn-poor types are generally almost devoid of it or contain' a small proportions of it. Pyroxenoids appear in the Mn-rich iron-formation analysing over 6% MnO. The other minerals of restricted occurrence include ferroactinolite, grunerite, Ksfeldspar (orthoclase-microcline), plagioclase (An 20-25), olivine, biotite and chlorite. Apatite is the most common accessory; zircon is scarce.

A detailed electronprobe microanalysis has revealed; (1) the low MnO content of less than 2%,1%,0.4% and 0.03% respectively, of garnet (70-81% almandine), orthopyroxene (ferrohypersthene-eulite ; 30-22 En), clinopyroxene (augite-ferroaugite to saliteferrosalite) and magnetite in the Mn-poor iron-formation, (2) the high MnO content of 10-20.5%, 2.5 to 15%, 2.25·7.5 and 0.2-0.3% respectively of garnet (almandine 30-54 - spessartite 39-49), orthopyroxene (manganoan ferrohypersthene-rnanganoan eulite), clinopyroxene (manganoan augite-ferroaugite to manganoan salite-ferrosalite) and magnetite in the Ma-rich iron-formation, (3) the occurrence of pyroxenoids varying in character from pyroxferroite to pyroxmangite with the Fe: Mn ratios either just below or above 1 : 1, (4) the common occurrence of (exsolution) intergrowths of one pyroxene in the other, (5) the occurrence of Mn-poor magnetite (≤0.03%MnO) carrying abundant exsolutions of hercynite and Mn-poor (around 1% MnO), ilmenite in the Mn-poor iron-formation and (6) magnetites (around 0.3% MnO) carrying Mn-rich ilmenite (MnO 15-30%) exsolutions in the Mn-rich iron-formation. An empirical study of the Mg-Fe-Mn distribution in the co-existing pyroxene and garnet has shown that these have crystallised in equilibrium metamorphic conditions corresponding to granulite facies. Estimations based on better suited pyroxene-garnet compositions occurring in the associated granulite facies rocks have indicated the formation of ironformations of the area at temperatures of 720-820°C and pressures of 7-11 Kbars.

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T. C. Devaraju ¹, K. S. Anantha Murthy ², S. D. Khanadali ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580003, IN
2 Department of Geology, Bangalore University, Jnanabharati, Bangalore 560056, IN

Abstract

The iron-formation of the Chiknayakanhalli (CN Halli) greenstone belt constitutes the youngest lithological unit of the late Archaean supracrustal geosynclinal sequence (3 to 2.4 b.y. old) which unconformably overlies the older (3 b.y.) granitic gneisses. They are consistently located in the core parts of the synclinal fold structures and occur over-lying either carbonates or chlorite schists. There are four main sedimentary facies, viz., oxide - (OF), mixed oxide-silicate - (MOSF), mixed oxide.silicate·carbonate-(MOSCF), . and carbonate-facies-(CF) constituting the iron-formation, All the facies types are strongly banded on metre to micron scale and frequently show small to large scale fold structures of both depositional and tectonic origin. While the lighter bands account for 43 to 57 per cent of the rocks and contain either silica (chert to quartz) and lor silicate (grunerite), the dark bands are chiefly made up of hematite, magnetite and goethite in OF, magnetite in MOSF, siderite and magnetite in MOSCF and ankerite-ferroan ankerite in CP. Although some of hematite and goethite could be interpreted as secondary after magnetite, a good proportion of these and the bulk of magnetite are syngenetic sedimentary. Grunerite appears to have been derived by the metamorphism of the precursor minnesotaite. The striking simplicity in chemical composition and distinctly low minor and trace element contents .are in accordance with the formation of eN Halli Iron-Formation through the process of inorganic chemical deposition. The carbonates underlying the iron-formation show upward iron enrichment and the iron-formation is interpreted as terminal phases of chemical sedimentary carbonate deposition in an essentially 'quiet' and' closed' environment.

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S. D. Khanadali , T. C. Devaraju ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 3, IN

Abstract

The different litho-units of the weathering profile of Paduvan Plateau show clear gradation contacts and contain relics of nearly all the minerals which are native to the underlying granitic gneiss. Mineralogically, the latent profile is dominated by gibbsite with kaolinite compared to lithomarge and goethite-hematite in the ferruginous laterite horizons. There is evidence of development of gibbsite both by the direct alteration of feldspar of the precursor rock and by the desilication of kaolinite. Four stages of transformation represented by altered gneiss, lithomarge, laterite and aluminous laterite bauxite are maintained in the weathering profile. As compared with the basalt derived laterite profiles, the Paduvari occurrence is characterised by relatively strong depletion of soda and potash and enrichment of ferrides. The field, petrographic mineralogical and geochemical data provide enough evidence of formation of Paduvan laterite profile by the in-situ chemical weathering of the underlying granitic gneiss and the aluminous laterite-bauxite by the 'in-situ chemical reworking' of laterite.

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D. B. Nadagouda ¹, R. S. Hanagodimath ¹, T. C. Devaraju ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580 003, IN

Abstract

The banded Iron-formation, corresponding mostly to oxide facies, exposed in the south-western parts of Chitradurga belt has been studied with respect to their field, petrographic, mineralogical and petrochemical aspects. It is concluded that, these iron-formations are essentially the results of chemical precipitation of iron and silica derived from continental weathering of basic rocks under anaerobic, quiet, below-wave base marine conditions. The banding is attributed to cyclic changes in climatic conditions.

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Kauko Laajoki ¹, T. C. Devaraju ²

Affiliations
1 Department of Geology, University of Oulu, Linnamaa, 90570 Oulu, FI
2 Department of Studies in Geology, Karnatak University, Dharwad 580003, IN

Abstract

Electron probe microanalytical data is given for Fe-chlorite (31% FeO) and grunerite (31.4% FeO) occurring in the mixed oxide-silicate facies iron-formation and for ankerite (av. 17.5% FeO), siderite (av. 44,2% FeO) and stilpnomelane (31.6% FeO, 5.9% Al₂O_3, 2% K₂O) in the carbonate iron-formation of Chiknayakanhalli Schist belt. While Fe-rich chlorite, ankerite and siderite appear to represent recrystallized diagenetic and primary sedimentary phases of the same minerals, grunerite has formed from metamorphic alteration of minnesotaite and stilpnomelane from a montmorillonite (nontronite?)-like precursor. The minerals identified in the iron-formation fit into the overall greenschist to low amphibolite facies metamorphism witnessed by the schists.

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S. D. Khanadali ¹, T. C. Devaraju ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580003, IN

Abstract

The laterite-bauxite profile of Paduvari plateau, which bears evidence of having been derived by the in-situ chemical weathering of essentially early Precambrian granodiorite-tonalite gneisses, shows: (1) The overall predominance of gibbsite in the entire profile except in the lithomarge zone, (2) the abundance of kaolinite in the lithomarge zone and enrichment of goethite/hematite in the ferruginous laterite crust, (3) the general downward increase of kaolinite and upward increase of gibbsite, (4) the consistent occurrence of allophane, in minor amounts, in the entire profile, (5) evidence of formation of gibbsite both by direct alteration of feldspars of the precursor rock aqd by desilication of kaolinite, (6) occurrence of nearly all the minerals, which were present in the precursor rock, as resistant relics, (7) occurrence of zircon of the same shape and size as those in the precursor rock, and (8) the high crystallinity and the more common tight framework-type of microtexture of bauxites and aluminous laterites suggesting the formation of these deposits under extremely good leaching conditions, high above the water table.

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Ali Khoshroo ¹, T. C. Devaraju ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580003, IN

Abstract

Clays are localized in their occurrence to the sheared and deeply weathered rhyolite-rhyodacite). These contain illite in association with minor proportion of neoforrned chlorite and a large proportion of relic quartz. XRD, IR and thermal properties of Mujjur illite correspond to pure illite. The clays are characterized by higher alkalies (K₂0 4.7%), with correspondingly low PCE value of 1420°C, absence of swelling minerals and very low CEC and dye adsorption.

Keywords

Mineralogy, Economic Geology, Non-Metals, Gadag, Karnataka.

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A. G. Ugarkar ¹, T. C. Devaraju ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580003, Karnataka, IN

Abstract

Gadag greenstone belt, which has a history of ancient gold mining, hosts three distinct auriferous zones viz., Western Zone, Middle Zone and Eastern Zone distributed both in metavolcanic and metasedimentary suites. The paper presents ore mineralogy of the Western Auriferous Zone which is hosted by pillowed and sheared metavolcanic rocks. Based on the relative abundance of quartz veins and sulphide minerals this zone could be classified into Type-A ore and Type-B ore.

Type-A ore comprises white quartz veins (30-90 cms thick) displaying evidence of early deformations. They contain chloritic/micaceous leafy aggregates and traces of arsenopyrite, pyrite, chalcopyrite and sphalerite ± scheelite; gold occurs in native form in vein quartz.

Type-B ore comprises mainly chlorite-muscovite/sericite-ankerite-monazite sulphide schist with subordinate vein quartz. Associated sulphides are arsenopyrite, pyrite, pyrrhotite, chalcopyrite, sphalerite and galena in the orderof decreasing abundance. Scheelite is seen occasionally. Gold occurs as: (i) native gold in silicate and quartz gangue, (ii) native gold inclusions and microfracture fillings in arsenopyrite and (iii) in combined state in arsenopyrite. Arsenopyrite could serve as a main pathfinder mineral for exploration, mining as well as processing of Type-B ore.

Mineralogical characteristics, wall-rock alterations and structural features related to Type-A ore, Type-B ore and host rocks characterise the gold mineralisation in the Western Auriferous Zone as structure controlled hydrothermal epigenetic lode.

Keywords

Gold, Sulphides, Ore Mineralogy, Greenstone Belt, Gadag, Karnataka.

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T. C. Devaraju ¹, Ali Khoshroo ², A. G. UGARKAR ¹

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580003, IN
2 Aerodynamic Research Center, P.O. Box 19584/449, Tehran, IR

Abstract

A dozen clay deposits occurring in the districts of Belgaum, Dharwad and Chitradurga of Karnataka State have been studied. ln Dharwad district, while the clay deposits derived from greywackes/phyllites are dominated by kaolinite, those derived from the acid metavolcanics contain illite as the sole clay mineral. In Belgaum and Chitradurgadistricts the deposits are kaolinitic, characterised by poor crystallinity and near absence of illite.

Chemically, the clay deposits studied have relatively high silica and low titania. The transformation of precursor rocks to clay deposits which was brought about by insitu chemical weathering and subsequent reworking of the weathered products under tropical climatic conditions was accompanied by enrichment of Al, Zr and Cr and removal of Sit Fe. Ni, Cu and Zn. Presence of relicts of nearly all the major and accessory minerals native to the precursor rocks, in all the litho-units of clay profiles is in agreement with such an interpretation.

Keywords

Economic Geology, Clay, Geochemistry, Mineralogy Chitradurga, Dharwad, Belgaum, Karnataka.

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K. R. Jayaraj ¹, S. D. Khanadali ¹, T. C. Devaraju ¹, B. Spiering ²

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580 003, IN
2 Mineralogisch-Petrologisches Institut, Universitat Bonn, Poppelsdorfer Schlos, 5300 Bonn 1, DE

Abstract

Hogbomite is of widespread occurrence as an accessory and minor mineral in the V-Ti-Fe ore deposits of Ubrani, Tavarekere, Masanikere, Magyatahalli and Devaranarsipur in Shimoga district and Mulemane in Uttar Kannada district. Both primary and secondary hogbomite are found. The mineral shows considerable variation in chemistry. The analytical data indicate that hogbomite structure is relatively more open than that of coexisting magnetite/ilmenite and can admit a variety of substitutions.

Keywords

Hogbomite, Electron Microprobe Analysis, Iron Ore Deposits, Karnataka.

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S. Gehar ¹, K. Laajoki ¹, T. C. Devaraju ²

Affiliations
1 Dept. of Geology, University of Oulu, FI
2 Dept. of Studies in Geology, Karnatak University, Dharwad-580 003, IN

Abstract

No Abstract.

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T. C. Devaraju ¹, T. A. Halkoaho ², K. Laajoki ², M. Shreerama ¹, G. Subbarao ¹

Affiliations
1 Depaftment of Studies in Geology, Karnataka University, Dharwad - 580 003, IN
2 Department of Geosciences and Astronomy, University of Oulu, Linnanmaa, SF 90570, Oulu, FI

Abstract

Electron Microprobe studies on BIF from Kudremukh area have indicated the presence of Ag-Bi- and Pb-Tellurides in association with other sulphides

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T. C. Devaraju ¹, T. T. Alapieti ², A. A. Halkoaho ², K. A. Jayaraj ¹, S. D. Khanadali ¹

Affiliations
1 Dept of Studies in Geology, Karnatak University, Dharwad 580 003, IN
2 Dept of Geology, University of Oulu, Linnanmaa, SF 905 070, Oulu, FI

Abstract

No Abstract.

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T. C. Devaraju ¹, K. Laajoki ², H. Makkonen ²

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad - 580 003, IN
2 Institute of Geosciences and Astronomy, Department of Geology, University of Oulu, Linnanmaa, SF-90570, FI

Abstract

Analcite with the chemical formula close to Na₁₅Al₁₆Si₃₂O₉₆ occurs as one of the important groundmass minerals of some of the peralkaline Neoproterozoic dykes in Southern Karnataka. Observations suggest its formation upon emplacement, at low temperature, from a soda-enriched and silica deficient residual melt, after intratelluric preemplacement crystallization of K-feldspar and alkali pyroxene phenocrysts.

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E. C. Hansen ¹, R. J. Stern ², T. C. Devaraju ³, B. Mahabaleswar ⁴, P. J. Kenny ¹

Affiliations
1 Department of Geology, Hope College, Holland, MI 49422-9000, US
2 Program in Geosciences, The University of Texas at Dallas, P.O. Box 830688, Richardsoni TX 750083-0688, US
3 Department of Studies in Geology, Kamataka University, Dharwad 580 003, IN
4 Department of Geology, Bangalore University, Jnana Bharathi, Bangalore 560 056, IN

Abstract

The Late Archaean regional amphibolite facies to granulite facies progression in southern Karnataka contains two textural varieties of charnockite (orthopyroxene-bearing quartzofeldspathic rocks). Banded chamockites typified by the banded pyroxene gneisses in the vicinity of Halaguru are thought to be older than the "incipient" coarse-grained charnockitic alteration of am.phibolite facies gneisses, as at Kabbaldurga, and similar alteration of the banded charnockites in many places south of there.

Whole-rock Rb-Sr isochrons were obtained for banded chamockites from the Chillapura quarry near Halaguru, and for incipient charnockite in amphibolite facies gneiss from a quarry near Honganuru, just east of Chamarajnagar. Large multi-layered samples and individual layers (light versus dark layers) were analysed separately in an attempt to discriminate older and younger metamorphic ages. For both quarries, all data yielded good isochrons with ages near 2.5 Ga, or terminal Archaean (2.50 ± 0.05 Ga with initial ⁸⁷Sr/⁸⁶Sr = 0.70500 ± 0.00009 for Chillapura and 2.54 ± 0.17 Ga, 0.7088 ± 0.0016 for Honganuru). More than one age of charnockitic metamorphism based on isotope systematics of samples collected from single quarries is not evident.

The high initial Sr ratios for both localities suggest, however, that the country rocks in southern Kamataka had a protracted crustal history prior to 2.5 Ga ago. Moreover, when the mean isotopic data of both quarries are plotted on the same diagram, "a regional isochron" close to 3.0 Ga with low initial Sr ratio results. This plausibly corresponds to a crustal accretion age, in accord with the interpretaiton of previous whole-rock isotope studies of this region based on combining data from widely-separated quarries. A widespread high-grade metamorphic event coinciding with, or following shortly after, crustal accretion remains an open possibility.

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T. C. Devaraju ¹, R. H. Sawkar ², R. P. Viljoen ³, T. L. Sudhakara ², M. G. Omkesh Murthy ², S. Vijaya Kumar ², R. G. Vijaya Vikram ²

Affiliations
1 Rajamangala', Saptapur, Haliyal Road, Dharwad - 580 001, IN
2 Ramgad Minerals and Mining Pvt. Ltd., Baldota Enclave, Hospet - 583 203, IN
3 School of Geosciences, University of Witwatersrand, Johannesburg, ZA

Abstract

The ultramafic-mafic rocks of the Archaean Holenarasipur greenstone belts of the western Dharwar craton in southern India has been explored for PGE and base metal mineralization. This initiative was prompted by the overall favourable geological setting and the presence of anomalies of Pt (10-27 ppb) and Pd (8-21 ppb) in several of the stream sediment samples collected from different parts of the belt. The exploration programme consisted of field examination and analysis of over 276 samples collected from a range of mafic-ultramafic rocks in four main exploration blocks of the belt viz. Yedegondanahalli, Bantaratalalu, Yenneholeranganabetta and Bettadasatenahalli. The investigation has revealed weak mineralization with 10 to 111 ppb Pt, 16 to 166 ppb Pd and 40 to 277 ppb of (Pt+Pd). Anomalies were located in serpentinite, amphibolite and sill/dyke-like bodies of hornblendite. Two small patches of sulphide mineralisation located in the belt, in the Yedegondanahalli and Yenneholeranganabetta blocks, are Cu dominated (>1.0 wt% Cu) and analyze up to 0.3 wt% Ni, 0.09 wt% Co, 40 ppm Ag and 0.5 ppm Au.

Keywords

Ultramafic-Mafic Rocks, PGE-Base Metal Exploration, Holenarasipur Belt, Dharwar Craton.

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T. C. Devaraju

Abstract

No Abstract.

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K. T. Vidyadharan ¹, T. C. Devaraju ²

Affiliations
1 Flat No-310, Block-'B', Maharaja Residency, Balmatta, Mangalore-575001, IN
2 'Rajamangala', Saptapur, Haliyal Road, Dharwad-580001, IN

Abstract

No Abstract.

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S. Balakrishnan ¹, R. Madusudanan ², S. Bhattacharjee ³, S. T. Narahari ³, R. Ananthanarayana ³, T. C. Devaraju ⁴

Affiliations
1 Geological Survey of India, Bangalore - 560 078, IN
2 Geological Survey of India, Bangalore - 560078, IN
3 Petrology Division, GSI, Hyderabad - 500 078, IN
4 'Rajamangala', Saptapur, Haliyal Road, Dharwad - 580001, IN

Abstract

No Abstract.

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T. C. Devaraju ¹, T. T. Alapieti ², R. J. Kaukonen ³, T. L. Sudhakara ³

Affiliations
1 Rajamangala', Saptapur, Haliyal Rd., Dharwad-580 00 1, IN
2 Institute of Geosciences, University of Oulu, P.O. Box 3000, SF-9057, Oulu -57, FI
3 Institute of Geosciences, University of Oulu, P.O. Box 3000, SF-9057, Oulu-57, FI

Abstract

Ultramafic complexes containing chromite-bearing units occur scattered over a large area of the Western Dharwar Craton (WDC). They are broadly separated into two groups; the older complexes, forming a part of the oldest known Sargur Group and occurring as dismembered enclaves within the Peninsular Gneissic Complex, and the younger, forming part of greenstone belt sequence having emplaced in the early stages of basin formation. In this study, chromitebearing units of the ultramafic complexes of the older group forming the Nuggihalli belt and Rangapura-Shivani complex and the younger group forming the ultramafic complexes of Channagiri, Shankaraghatta and Usgao, have been selectively examined. While Shankaraghatta ultramafite contains sparse disseminations, the other complexes include in addition mm-cm scale bands and lenses\pods of chromite; commercially workable deposits of chromite are confined only to the Nuggihalli belt. Further, whereas PGE mineralization of possible commercial value is recognized in the Channagiri complex, the Shankaraghatta complex encloses Ni-Au-PGE mineralization.

Almost 200 spot analyses of chromite grains occurring in the five different ultramafic complexes of WDC have been obtained. Although it is the most resistant mineral of the rocks, in the intensely altered Channagiri complex it shows deep alteration whereas in the other less altered complexes the alterations are localized to outer zones and margins of the mineral. The least altered Cr-spinel composition (Spl) with X_Mg0.01-0.656, Xcr 0.553-0.955 and X, 0.014-0.215 is on the whole in the range of aluminian chromite, but, it shows significant variations from one cornplex to the other and even within the limits of a single complex, related to primary magmatic processes. An interesting finding is the presence of occasional relics of titanium chromite in the PGE mineralized Channagiri complex.

In addition to common alteration of Spl to ferrian chromite (SpII) (with A1₂O₃:0.47-6.46 %; MgO: 1.4-4.196; Cr₂0,: 36.1-54.9 %; Fe20,: 11.6-23.1 %), alteration of the mineral to chromian magnetite and magnetite (with A1₂0₃,: 0.01-0.84 %; MgO: 0.01-0.07 %; Cr₂O₃,: 1.17-28.81 %; Fe₂O₃,: 34.7-63.6 %) is also recorded. These alterations, as well as near total conversion of olivine and pyroxenes into antigorite and chlorite, were brought about by pervasive low-grade regional metamorphism which accompanied penetrative fluid action and deformation. The alteration comprised of two way diffusion of elements. While the formation of ferrian chrornite was associated with quantitative outward migration of Al, Mg and very moderate Cr into the surrounding and inward migration of iron from the surrounding and oxidation of iron, the conversion of chromite into Cr-magnetite/magnetite was brought about by quantitative outward migration of Cr, A1 and Mg into the surrounding and inward diffusion of iron.

As has been the case with many of the examples studied world over, the Cr-spinels of WDC too have chemical characteristics which largely overlap with those of both layered and ophiolitic/podiform examples and do not permit their characterization exclusively as layered or podiform types.

Keywords

Cr-Spinels, Aluminian chromite, Ferrian chromite, Western Dharwar Craton, Karnataka.

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T. C. Devaraju ¹, T. T. Alapieti ², R. J. Kaukonen ², T. L. Sudhakara ³

Affiliations
1 'Rajamangala', Saptapur, Haliyal Road, Dharwad - 580 001, IN
2 Institute of Geosciences, University of Oulu, PO Box 3000, FIN-90014, Oulu, FI
3 MSPL Limited, Nehru Co-Operative Colony, Hospet - 583 203, IN

Abstract

The Channagiri mafic-Ultramafic complex constitutes the main lithounit of Hegdale Gudda Formation of the Shimoga Supracrustal Belt. Mapping at a scale of 1 2000 coupled with detailed petrological examination of the Platinum Group Element (PGE) mineralized 3 5 km x 0 3 km. Hanumalapur segment of the complex has revealed that it is composed of outer gabbroic zone and central 100-150 m ultramafic zone. While V-Ti magnetite seams occupy the outer boundary of the ultramafic zone, several small isolated lenses/pods of anorthositic variation occur within the ultramafic zone itself. The complex is affected by penetrative deformation, hydrothermal alteration and low-grade metamorphism, which together have caused complete replacement of olivine and pyroxenes by chlorite, amphibole, epidote and carbonate. The gabbro forming outer zone is in the range of quartz gabbro-melagabbro and is strongly banded/foliated. The ultramafite in the central zone is a complex mix of the dominant fine (chromite-chlorite Schist) and subordinate coarse grained units. The magnetite occupying the western interface zone is the typically coarse grained V-Ti variety whereas that on the eastern side includes narrow chromiferous lenses. PGE data obtained for the different units of Hanumalapur segment has revealed that only the central fine-grained ultramafite and the chromiferous lenses in the eastern magnetite seam host PGE mineralization of commercial interest with Pt+Pd values ranging from 1 5 to 6 ppm, the other lithologies of the segment as well as those constituting the remaining segments of Channagiri ultramafic complex and Hegdale Gudda Formation have not yielded positive anomalies of PGE mineralization. The governing control of chromite over the separation of PGE in the very early stages of magmatic crystallization is inferred taking into account the close association of PGE with the chromite-bearing units. The recorded textural relationships suggest that the PGEs remained virtually immobile during later penetrative alterations of the host rock.

Geochemically, the PGE mineralized ultramafic units of Hanumalapur segment carry signatures of basaltic komatiite to Fe-Rich tholerite and in terms of Pd/Ir Ni/Cu, Pd/Pt Ni/Cu and Ni/Pd Cu/Pt ratios they compare with layered intrusions.

The chromite controlled PGE mineralization of Hanumalapura roughly resembles the UG2 of the Bushveld complex and a part of the SJ reef of the Penikat Detailed logging of a 252 m deep drill core has revealed the existence of three ore reefs of 20, 30 and 35cm thickness, analyzing 37,50 and 40 ppm Pt+Pd respectively located within a 5 m zone and 5 lower grade reefs of 035 m to 10 m thickness analyzing 1 3 to 1 8 ppm Pt+Pd (and 07 to 1 8% Cu), located in a 50 m wide zone, just above the main ore zone. The PGE reefs of Hanumalapur segment include both Pt- and Pd rich ones but the latter dominate.

Keywords

Channagiri Mafic-Ultramafic Complex, PGE Distribution, Chromite-Controlled, Hanumalapur Segment, Shimoga Belt, Karnataka.

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T. C. Devaraju ¹, Hannu Huhma ², T. L. Sudharara ³, R. J. Kaukonen ⁴, T. T Alapieti ⁴

Affiliations
1 Rajamangala, Saptapui, Hahyal Road, Dharwad - 580 001, IN
2 Geological Survey of Finland, PO Box 96, FI-02151, Espoo, FI
3 MSPL Limited, Baldota Enclave, Hospet - 583 203, IN
4 Department ol Geosciences, University of Oulu, PO Box 3000, FI-90014, Oulu, FI

Abstract

Gianitoids comprising the northern block of Western Dharwar Craton (WDC), enclosed within the 225 km long and 40 km wide transect corridor stretching from East of Gadag to the Goa coast, have been examined in the field, mapped and interpreted on the basis of petrological, geochemical and Sm-Nd isotopic data. The rocks constitute 43% of the corridor lithology and include 5 large plutons of batholithic dimensions and two Stock-Like bodies. Those are classified as Anmod Ghat trondhjemite(AGT), Chandranath granite(CNG), Dudhsagar granite(DSG) Ramnagar migmatite (RNM), Ramnagar granite gneiss(RNGn), Ramanagar porphyntic granite(RNPG), Annigen-Majjigudda granite(AMG) and Hatalgen-Naregal migmatite(HNM) Of these, only RNM and HNM show the typical polyphase migmatitic character, the remaining are lather homogeneous with only localized occurrence of tectomcally emplacedmahc rocks and pegmatiteaphte. Modal mineral compositions coupled with geochemical parameters especially the combination of K, Rb, Sr, Sr/Ba, La_N/Y_N and Eu/Eu^* values, serve as the distinctive features of the granitoids. The granitoids and greenstone belts of the corridor show overall structural conformity and evidence of Low-Grade metamorphism in the range of Greenschistlow-Amphibohte facies. There is no obvious evidence of their diapinc emplacement and contact metamorphism the granitoids bear essentially a basement relation towards the associated greenstone belts. The results of Sm-Nd isotopic analyses have indicated existence of mainly three groups viz , (I) the older trondhjemite (AGT) - Tonahte (RNM) - Monzogranite (HNM) providing model ages (T_DM) of around 3 3 Ga, (ii) a suite of granitoids viz CNG, DSG, RNGn and RNPG with T_DM between 2 96 and 2 83 Ga and (in) younger granite viz AMG, dating 2 68 Ga The isotopic data suggests that the protohth of AGT was different from that of CNG, RNGn and RNPG Higher soda and Na₂O K₂0 ratio of 1 1 or more and marked depletion in REE of all the younger granitoids of the tiansect are distinctive chemical features as compared to Closepet Granite, Hyderabad Gianite and other younger K rich granites in the Eastern Dharwar Craton The geochemical characteristics recorded place the granitoids of the corridor closer to those constituting Peninsular Gneissic Complex in the Southern part of WDC.

Various chemical parameters suggest derivation of the transect granitoids from predominantly igneous protohths and their emplacement in syncolhsional volcanic arc regime In addition, their lithogeochemical features suggest that they constitute separate plutons AGT is related to a 3 3-3 4 Ga old crust forming event RNM and HNM are products of reworking of old crust whereas the remaining granitoids of the corridor appear to be plume related.

Keywords

Geochemistry, Granitoids, Gadag-Goa Transect, Model Sm-Nd Isotopic Data Igneous Protoliths, Postorogenic, Syn-Colhsional Regime, Western Dharwar Craton.

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T. C. Devaraju ¹

Affiliations
1 'Rajamangala' Saptapur, Haliyal Road, Dharwad 580 001, IN

Abstract

No Abstract.

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T. C. Devaraju ¹, R. J. Kaukonen ², T. L. Sudhakara ¹, T. T. Alapieti ²

Affiliations
1 Department of Applied Geology, Kuvempu University, Jnanasahyadri Shankaraghatta, Shimoga- 577 45 1, IN
2 Institute of Geosciences, University of Oulu, PO Box 3000, Oulu, FI

Abstract

The polyphase migmatite gneiss exposed in the vicinity of Naregal contains unusual ultramafic enclaves with the modal composition of 50% tremolite, 21% olivine, 16% phlogopite, 10% Cr-magnetite(with 5 6 wt% Cr₂O₃) and 2% anthophyllite. Geochemically the enclaves resemble 'Barberton komatite' in respect of high MgO, Ni, Cr and Low Al₂O₃, and TiO₂, and one of 'Wajrakarur kimberlite' plug rocks with reference to'high K₂O, Rb Zr and Ba. The finding suggests the possible existence of pristine crust in the northern part of the Western Dharwar Craton.

Keywords

Ultramafic Enclaves, Polyphase Gneiss, Fresh Olivine, Phlogopite, Pristine Crust, Naregal Karnataka.

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T. C. Devaraju ¹, K. Gowd Reddy ²

Affiliations
1 'Rajamangala', Saptapur, Dharwad - 580001, IN
2 Nirmal Nagar, Dharwad - 580003, IN

Abstract

No Abstract.

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T. C. Devaraju ¹, T. T. Alapieti ², R. J. Kaukonen ²

Affiliations
1 Department of Applied Geology, Kuvempu University, Jnanasahyadri, Shankaraghatta, Shimoga - 577 451, IN
2 institute of Geosciences, University of Oulu, P.O.Box 3000, Oulu, FI

Abstract

SEM-EDS study of outcrop samples from the PGE mineralized, layered Hanumalapura segment of the Channagiri Mafic-Ultramafic complex, has revealed that they contain a variety of Platinum-Group mineral inclusions. Among more than 25 different PGM's distinguished, sperrylite, stibiopalladinite, hollingworthite, keithconnite, mertieite II, laurite, ruarsite together with a suite of Pt and Pd alloys, constitute the more important carriers of PGE in the ore reefs. From the close association of PGM's with chromite-bearing host rocks, it is inferred that crystallization of chromite had the governing control over the concentration of PGE.

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V. Balaram ¹, T. C. devaraju ²

Affiliations
1 National Geophysical Research Institute Hyderabad - 500 007, IN
2 Department of Applied Geology, Kuvempu University, Shankaraghatta - 577 451, IN

Abstract

No Abstract.

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T. C. Devaraju ¹, T. T. Alapieti ², R. J. Kaukonen ²

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad - 580 003, IN
2 Institute of Geosciences, University of Oulu, P.O.Box 3000, FI

Abstract

Almost Thirty Individual Lenses of Ultramatites Occur Within about 150 Km²of the Basement Granitords of the Peninsular Gneiss forming Southeastern flanks of the Shimoga Supracrustal Belt. They form Small elongated Bodies (<50 M X l Km) which are most Commonly Oriented Parallel to the General Northwesterly Strike of the Basement Granitoids.

The Ultramafites fall Mainly in the Range of Chromitite-dunite-peridotite-pyroxenite. the Original Textures as well as Mineralogy of the Rocks are Considerably Modified by Pervasive Deuteric and Low-grade Metamorphic Alterations. Chlorite (and/or Antigorite) and Fe-cr Oxides are the Ubiquitous Minerals. Amphibole is abundant only in Hornblendite. Carbonate, Epidote, talc, Magnetite and Sphene Occur in Minor Amounts. The Re1ics of the Original Olivine and Pyroxenes are very Rare.

Geochemically, the Ultramafites include High-cr, High-mg, High-a1 and High-fe Types. of the Four, High-cr and -Mg types Account for the Largest Proportion, whereas High-A1 and Fe-types Constitute only Local Variations. the overall High Magnesia (25.6 Wt%) and Alumina (9.5%), Iow Lime (2.8 %) and very Low Alkalis (< 0.1%) are Distinctive Major Element Geochemical Features. Marked Enrichment of Ni (1317 Pprn) in High-Cr and -Mg Types, Ti (3.9%) and V (1176 ppm) in High-fe Type and Zr (291 Ppm) in High-A1 Type are among Distinctive Trace Element Geochemical Features. Low REE (3.2 - 18.5 Ppm) and Nearly Flat Chondrite Normalized Patterns Characterize High-Cr and -Mg Types. the High-A1 Type has Contrastingly High REE (141.6 Ppm) and Markedly Enriched in LREE.

The High-Cr and -Mg Types Bear Geochemical Signatures of Derivation from Higher Degree of Partial Melting of Garnet-lherzolite Layer of the Mantle. Mixing of Primitive and Evolved Magmas is Indicated by the Composite Association of High-Cr, -Mg, and -Fe Units Noted in some of the Ultramafic Bodies. the High-a1 Type with Strong Enrichment in Ree and Zr Bears Evidence of Derivation from Segregation of Late Stage Residual Liquids. Kornatiitic Bulk Composition of the Parental Magma is Indicated by some of the High-Cr and -Mg Ulyarnafites. the Chromite-dunlte-peridotite-pyroxentite Association Strongly Suggests that the Ultramafite Lenses of the Area Constitute Parts of a Tectonically Emplaced Layered Body.

Analysis of as many as 27 Samples of Ultramafites, Covering all the important Petrographic Variations has not Revealed either Anomalies in the Content of Pt and Pd or Possible Potential of PGE minerailzation. Samples from the layered Ultrarnafite body to the East of Rangapura, Containing Variable Proportions of Chromite Bands, However, have Yielded Significantly High Os, Ir, Ru and Rh Values (the Total of these Ranges from 130 to 240 Ppb). these Samples also Contain Minute Platinum Group Mineral (pgm) Inclusions, Mainly in the Range of Ladrite-irarsile and less Frequently those Corresponding to Erlichmanite, Osarsite, Irarsite and Bowieite. the Pge Mineral Inclusions are Early formed Phases of Deep Seated Crystallization which became Entrapped in Later Crystallized Chrornite and Silicates. the Ubiquitous Hydrothermal and Low-grade Metamorphic Alterations have not Apparently Caused Significant Migration of Pge.

Keywords

Ultramafic lenses, Geochemistry, PGE and PGM, Shimoga Schist Belt, Karnataka.

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T. C. Devaraju ¹, T. T. Alapieti ², R. J. Kaukonen ²

Affiliations
1 Department of Applied Geology, Kuvempu University, Jnanasahyadri, Shankaraghatta, Shimoga - 577 45 1, IN
2 Institute of Geosciences, University of Oulu, P.O.Box 3000, FI

Abstract

Association of Ni and PGE with the already known gold mineralization is recognized in about 5 m wide sulphide zone in the ultramafic rock exposed between Bhadra Reservoir and northern border of Kuvempu University Campus. The mineralization is picked up over a strike length of almost 3 km. The mineralized ultramafite, which is a metadunite, contains about 94% serpentine, 3% sulphides, 1% magnesite and 0.5% Fe-Cr oxide. Millerite and pentlandite are the main carriers of Ni. Gold varies from native gold to electrum. Melonite greatly dominates michenerite, merenskyite and Pd-Ti-Bi-Sb alloy among Pd minerals identified. Pt is present both as sperrylite and as irarsite. Au as well as Pd-Pt minerals occur as minute inclusions of 1 to 10 microns in size. About 50% of these inclusions are in sulphides, 10% in silicates and the remaining are located along the contacts between the sulphides and silicates.

The ultramafite contains isolated pods and patches of chlorite-Rock which are contrastingly enriched in Al, Fe, Ti, Mn, REE, Y, Th, U, Zn and Zr and depleted in Si, Mg, Ni, Cr, As and Bi. These chlorite pods are interpreted as products of late stage residual liquid of ultramafic magma.

The Shankaraghatta ultramafite is not a layered intrusion. The primary economic target in this case is Au and Ni and not PGE as in stratabound deposits. Here the distribution of Ni, Au and PGE is controlled by sulphides. Concentration of the metals in sulphide melt by liquid immiscibility is suggested. Very high MgO and contrastingly low Al₂O₃ and TiO₂ of the mineralized metadunite of Shankaraghatta suggest its formation from ultramafic magma derived from high degree (20-25%) of partial melting of fertile mantle associated with plume activity.

Keywords

Late Archaean, Metadunite, Ni-Au-PGE Mineralization, Millerite, Pentlandite, Melonite, Liquid Immiscibility, Shimoga Schist Belt, Karnataka.

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K. T. Vidyadharan ¹, S. Paranthaman ¹, K. N. Rao ², T. C. Devaraju ³, T. T. Alapieti ⁴, R. J. Kaukonen ⁴

Affiliations
1 Geological Survey of India, Bangalore, IN
2 Geological Survey of India, Hyderabad, IN
3 Department of Studies in Geology, Karnatak University, Dharwad - 580 003, IN
4 Institute of Geosciences, University of Oulu, P 0 Box 3000, FI

Abstract

No Abstract.

Full Text

   

T. C. Devaraju ¹

Affiliations
1 Department of Studies in Geology Karnatak University Dharwad - 580 003, IN

Abstract

No Abstract.

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T. C. Devaraju ¹, R. P. Viljoen ², R. H. Sawkar ³, T. L. Sudhakara ⁴

Affiliations
1 'Rajamangala', Haliyal Road, Saptapur, Dharwad - 580 001, IN
2 School of Geosciences, University of Witwatersrand, Johannesburg, ZA
3 MSPL Limited, 'Baldota Enclave', Abheraj Baldota Road, Hospet - 583 203, IN
4 Department of Applied Geology, Kuvempu University, Jnanasahyadri, Shankaraghatta, Shimoga - 577 451, IN

Abstract

Evidence of mafic and ultramafic magmatism exists in many parts of the Dharwar craton which is divided into two blocks, the West Dharwar Craton (WDC) and the East Dharwar Craton (EDC). The mafic-ultramafic rocks occur in supracrustal/greenstone belts and in numerous enclaves and slivers in the WDC. The oldest recorded maficultramafic rocks, which are mainly komatiitic in nature, are preserved in the Sargur Group which is more than 3.3-3.4 Ga old, the youngest being manifested by 63-76 Ma old mafic dyke magmatism, possibly related to Deccan volcanism.

In the Sargur Group, ultramafics rocks greatly dominate over mafic lithological units. Both extrusive and intrusive varieties, the latter in the form of differentiated layered complexes, occur. Mafic volcanics exists in all the greenstone belts of the eastern block and in the Bababudan and Western Ghats belts of the western block. In addition to the Sargur Group where stratigraphic sequences are unclear, mafic magmatism is recorded in three different formations of the Bababudan Group and two sub-divisions of the Shimoga and Chitradurga Groups where basaltic flows are conspicuous. In the well studied greenstone belts of Kolar and Hutti in the EDC, three to four different Formations of mafic volcanic rocks have been mapped. Isotopic dating has indicated that while mafic magmatism in the greenstone belts of the EDC covers only a short time span of between 2.65 to 2.75 Ga, those in the Dharwar Supergroup of the WDC cover a much longer time span from 3.35 to 2.5 Ga.

Mafic dyke magmatism has taken place repeatedly from 2.45 Ga to about 1.0 Ga, but, the peak of emplacement was between 1.8 and 1.4 Ga when the densely developed swarms on the western and south western portions of the Cuddapah Basin and in the central part of Karnataka, were intruded. Emplacement of potassic ultramafic magma in the form of kimberlite-lamproite which is confined to the EDC, is a later magmatic event that took place between 1.4 Ga and 0.8 Ga.

From a mineralization perspective, mafic magmatism of the supracrustal groups of the WDC and the greenstone belts of the EDC are the most important. V-Ti-magnetite bands constitute the most common deposit type recorded in the mafic-ultramafic complexes of the Sargur Group with commercially exploitable chromite deposits occurring in a number of belts. PGE mineralization of possible commercial value has so far been recorded in a single mafic-ultramafic complex, while copper-nickel mineralization occurs at certain localities in the Sargur and Chitradurga Groups. Gold mineralization hosted by mafic (occasionally ultramafic) rocks has been noted in many of the old workings located in supracrustal groups of rocks in the WDC and in the greenstone belts of EDC. Economically exploitable mineralization, however, occurs mainly in the greenstone belts of the Kolar, Ramagiri-Penkacherla and Hutti-Maski and along the eastern margin of the Chitradurga belt, where it is associated with a major N-S striking thrust zone separating the WDC from the EDC. Gold deposits of the eastern greenstone belts are comparable to those of the younger greenstone belts of Canada, Zimbabwe and Australia where the mineralization is associated with quartz carbonate veins often in iron-rich metabasic rocks. The gold was emplaced as hydrothermal fluids, derived from early komatiitic and tholeiitic magmas, and injected into suitable dilatent structures.

The other common type of mineralization associated with the ultramafic rocks of the Sargur Group and supracrustal belts, particularly of the WDC, are asbestos and soapstone, related to autometamorphism/metasomatism. Ruby/Sapphire deposits occur in places at the contacts of ultramafic rocks with the Peninsular Gneiss, and are related to contact metamorphism and metasomatism. Mineable magnesite deposits related to low-temperature hydrothermal/lateritic alteration exist in the zone of weathering, particularly in the more olivine-rich rocks. Recent spurt in diamond exploration is offering promise of discovering economically workable diamondiferous kimberlite/lamproite intrusions in the EDC.

Keywords

Mafic Magmatism, Mafic Dykes, Kimberlite-Lamproite, Mineralization, Dharwar Craton.

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T. C. Devaraju ¹, T. L. Sudhakara ², R. J. Kaukonen ³, R. P. Viljoen ⁴, T. T. Alapieti ³, S. A. Ahmed ², S. Sivakumar ⁵

Affiliations
1 'Rajamangala' Saptapur, Haliyal Road, Dharwad - 580 001, IN
2 Department of Applied Geology, Kuvempu University, Jnanasahyadri, Shankaraghatta - 577 451, IN
3 Institute of Geosciences, University of Oulu, P.O.Box 3000, FIN-90014, Oulu, FI
4 School of Geosciences, University of Witwatersrand, Johannesburg, ZA
5 Ashapura Minechem Ltd., 'Sita Kunj', # 4-119C, Kunjibettu, Udupi - 576 102, IN

Abstract

Late Archaean Supracrustals of the Goa-Dharwar sector (GDS) are composed of a thick sequence of greywacke sequence with narrow intercalations of quartzite, BIF and carbonates. Mafic volcanics occupy the base of the sequence. The greywackes are predominantly tuffacious containing chlorite-sericite and hornblende. Arkosic variations containing biotite dominate the western part of the sector. Fine-grained variations occur as isolated narrow lenses within other types of greywackes. The conglomeratic greywackes are localized along the western and the eastern margins of the sector. All of the greywackes are all typically immature containing coarser clasts of mostly plagioclase (18-23%) and quartz (32-34%). Lithic fragments of felsic volcanic rocks are common. The matrix is dominated by mafic material. Biotite and amphibole are related to metamorphic recrystallization. Chlorite, sericite, epidote, carbonate and chert are products of the interplay of diagenesis and low-grade metamorphism. Fe-Ti oxide, sphene, apatite and zircon are usual accessories. But for slight enrichment in K₂O, the major element chemistry of the GDS greywackes is similar to the chemistry of Late Archaean greywackes. They also compare in respect of V, Co, Hf contents, K₂O/Na₂O, SiO₂/Al₂O₃, Na₂O/Al₂O₃, Ba/Rb, Th/U, La/Th, Sm/Nd ratios, steep REE patterns with distinct LREE enrichment and HREE depletion. The GDS greywackes however are distinctly enriched in Rb, Ba, Sr, Th, U, Cu, Zr, Ce/Ce^* and depleted in Cr, Ni, and Zn. The conglomeritic and biotite bearing verities contain considerable proportions of clasts derived from the basement tonalitic/granitic terrain. The common tuffacious greywackes containing hornblende and biotite-sericite however include only volcanic clasts and bear evidence of derivation from submarine weathering of predominantly felsic volcanics erupted on a large scale to form a magmatic arc in the later stages of geosynclinal deposition. Geochemical data suggest that the GDS greywackes were laid down in progressively changing basin geometry from a passive to active continental margin and island arc setting.

Keywords

Greywackes, Felsic Volcanic Source, Continental Island Arc, Submarine Weathering, Goa-Dharwar Sector.

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T. C. Devaraju ¹, T. T. Alapieti ², K. S. Ananthamurthy ³

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad - 580 003, IN
2 Institute of Geosciences and Astronomy, University of Oulu, Linnanmaa, SF-90570, Oulu, FI
3 Department of Environmental Sciences, Bangalore University, Jnanabharati, Bangalore - 560 056, IN

Abstract

No Abstract.

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T. C. Devaraju ¹, T. T. Alapieti ²

Affiliations
1 Department of studies in Geology, Karnataka University, Dharwad - 580003, IN
2 Department of Geology, Institute of Geoscience and Astronomy, University of Oulu, Linnanmaa SF-90570, Oulu, FI

Abstract

No Abstract.

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