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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 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, 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, 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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