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Goa, North Kanara |
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Goa, India |
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Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Goa and North Kanara belt of iron and manganese deposits are distributed within and define a NNW trending belt that is around 2 km wide and extends for 95 km through Goa, passing into south-western Maharashtra State to the north, and north-western Karnataka State in the south, close to the Arabian Sea coast of western India.
The iron ores ar hosted by a sequence that has been correlated with the Neoarchaean Dharwar Supergroup supracrustal rocks which overlie and are infolded into a Mesoarchaean basement in the Western Dharwar craton. The 500 x 300 km, NNW-SSE elongated Western Dharwar craton is bounded to the west by the Arabian Sea; to the east by the linear NNW west zone of tectonic activity occupied by the 2.5 to 2.3 Ma Closepet Granite complex separating it from the Eastern Dhawar craton; immediately to the north of Goa by the blanket of Cretaceous to Tertiary Deccan basalts; and to the south by the transition to the granulite facies charnockite-khondalite rocks that occupy the tip of the Indian Peninsular.
The basement is represented by 3.4 to 2.9 Ma granitic (tonalite to tronhjemite) gneisses and granodiorites of the Peninsular Gneiss with scattered enclaves of the older (Mesoarchaean) Sargur supracrustals, largely composed of high grade mafic to ultramafic rocks occuring as amphibolites and mafic granulites, with lesser magnetite-quartzites (or other iron formations) calc-silicates and minor quartzites containing sillimanite, kyanite and fuchsite. Some authors suggest the Goa deposits are developed within the Sargur supracrustals, rather than the Dharwar Supergroup.
The Neoarchaean Dharwar Supergroup supracrustals have changing facies from east to west, as follows:
i). An amphibolite facies sequence of paragneiss-metaarkose interbedded with calcsilicates and quartzites with some banded iron formations. These rocks appear to be structurally (thrust faulted) separated (east side up) from the next belt to the west.
ii). A 450 km long NNW-SSE trending belt of greenschist facies rocks commencing with a quartz pebble conglomerate unit at the base, overlain by a sequence of metabasic rocks with intercalated quartzite and schist, discordantly followed by a thicker succession of basic and minor acid volcanic rocks with intercalated alternating banded ferruginous quartzite and accompanying phyllite. These are in turn succeeded by phyllites and metapelitic schists and greywackes, including conglomerates, ferruginous quartzites and volcanic rocks.
iii). a basin like area to the west of the southern section of the previous belt, with a succession that commences with a quartz pebble conglomerate, overlain by meta-basaltic rocks, intercalated with local crossbedded quartzites and conglomerates, overlain by banded ferruginous cherts interbedded with phyllites, then more metabasaltic rocks and interbedded phyllites, followed by conglomerates passing laterally into phyllites and crossbedded quartzites.
iv). The most extensive belt to the west is also generally of greenschist facies, extends NNW-SSE and includes the Goa - North Kanara district towards its north-western margin. It comprises basalt, andesite and dacitic volcanic rocks, greywackes conglomerates and iron formations. In the Goa district komatiites have been reported from the sequence. Just to the south, the sequence have been intruded by a 2.7 Ga batholith.
The iron deposits are found within stratified ferruginous/iron formation layers that have undergone substantial tectonic deformation and have been de-silicified and upgraded above the water table. The primary iron formations contain 19 to 36% Fe and are associated with a pink phyllitic unit and decrease in grade and quality from north to south. The banded hematite-magnetite quartzites that comprise the iron formations occur as lensoid bodies within the pink phyllite, while manganese bearing cherts and quartzites are present as narrow bands.
Below the water table only primary iron formation is apparently found, while immediately above there is a de-silicified zone composed of 'blue dust' which is an un-cemented powder of hematite and little magnetite with 63 to 67% Fe. This 'blue dust' passes through a thin transition zone of 'biscuity ore' that has been loosely cemented, but retains a banding, with 59 to 62% Fe, into an overlying interval of massive, hard cemented cherry red to steel grey ore with around 63% Fe. Overlying this there is a zone of hard lateritised ore containing boulders of hematite-magnetite and hydrated oxides of iron (goethite and limonites) and alumina which averages 45 to 50% Fe, although the boulders may contain up to 70% Fe.
Samples of ore assay as follows (Krishnaswamy & Sinha, 1988):
- Lumpy ore - 57 to 61% Fe, 1.45 to 3.05% SiO2, 5.4 to 7.75% Al2O3, 0.13 to 0.33% Mn, 0.18 to 0.25% TiO2, up to 0.03%S, up to 0.05% P.
- Powdery ore - 61.75 to 65% Fe, 1.5 to 3.0% SiO2, 2.1 to 5.75% Al2O3, 0.1 to 0.6% Mn, up to 0.25% TiO2, up to 0.02%S, up to 0.04% P.
The Goa - North Kanara belt appears to comprise a string of more than 50 relatively small deposits, each of a few million tonnes up to the largest of around 125 Mt, that occupy the crests and slopes of hills and mounds. It has been observed that the highest hills and steepest slopes have the best deposits in terms of size and tenor. The banded hematite quartzite rocks are generally confined to pink phyllite horizon. Ore minerals principally are haematite with smaller occurrences of magnetite, limonite and goethite.
Current (2006) significant mines includes Codli (a multipit operation) and Sonshi of Sesa Goa Ltd, which together in 2005 produced 4 Mt of ore (3.2 Mt of fines and 0.8 Mt of lump). The Bicholm mine was re-opened in 2015. Production from these mines from 2015 will allow the company to utilise its full annual production allowance of 5.5 Mt, as approved by the Goa government.
In 1988 (Krishnaswamy & Sinha, 1988) it was estimated that the Goa - North Kanara district had cumulative reserves of around 890 Mt of ore.
In 1965 the total production from Goa was 6.58 Mt of ore. In 1984, some 13.4 Mt of fines and lump ore were produced from the district, while in 2004 the capacity was of the order of 7.5 Mt per annum.
The most recent source geological information used to prepare this decription was dated: 2006.
This description is a summary from published sources, the chief of which are listed below.
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