Mafic and ultramafic magmatism and associated mineralization in the Dharwar craton,southern India |
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| Authors: | T C Devaraju R P Viljoen R H Sawkar T L Sudhakara |
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| Institution: | (1) Rajamangala, Haliyal Road, Saptapur, Dharwad, 580 001, India;(2) School of Geosciences, University of Witwatersrand, Johannesburg, South Africa;(3) MSPL Limited, Baldota Enclave, Abheraj Baldota Road, Hospet, 583 203, India;(4) Department of Applied Geology, Kuvempu University, Jnanasahyadri, Shankaraghatta, Shimoga, 577 451, India |
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| 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. |
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| Keywords: | Mafic magmatism Mafic dykes Kimberlite-lamproite Mineralization Dharwar craton |
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