The iron formations of Karnataka occur in different tectonic environments namely, low grade metamorphic belts, high grade metamorphic belts and cratonic basins. The iron formations which occur in high grade metamorphic belts have often attracted the attention of earlier workers, because of their intimate association with charnockites. Some earlier investigators considered a genetic relationship between the iron formations and charnockites.
They were thought of as gravitative differentiates of a charnockitic magma and as interesting additions to the charnockite series. However, later investigators have shown that the iron formations are formed by the metamorphism of iron-rich sediments and have no genetic relationship whatsoever, to the associated charnockites.
Distribution of Iron Formations:
The Karnataka craton is the oldest part of the Indian shield, with rocks in the age range of 3,800 to 2,500 m.y. A generalised geological map of the Karnataka craton is appended (Figure 7.1).
The supracrustal rocks which are older than 3,000 m.y. are called Sargur supracrustals (older schist belts or high grade metamorphic belts) and supracrustal rocks younger than 3,000 m.y. Dharwar supracrustals (younger schist belts or low grade metamorphic belts).
The Sargur supracrustal rocks are mostly exposed in the southern parts of the craton and are metamorphosed to upper amphibolite to granulite grade. They are found as enclaves in the tonalite-trondhjernitic gneisses which are called peninsular gneisses. Clastic siliceous sediments are absent from these belts and they have high mafic/sedimentary rock proportions.
The Dharwar supracrustal rocks which are exposed in the northern parts of the craton are metamorphosed to greenschist fades, have very low mafic/sediments ratio, and contain considerable amounts of clastic siliceous sediments exhibiting primary sedimentary features. An unconformable relationship between the peninsular gneiss and the low grade metamorphic belts is envisaged.
There is no unanimity of opinion on the stratigraphy and structure of these two metamorphic belts.
These could be summarized into three views:
1. The Dharwar supracrustal rocks represent a group of rocks younger than the peninsular gneisses and these gneisses carry enclaves of older Sargur supracrustals.
2. Together, the Dharwars and Sargurs represent one super-group older than the Peninsular gneiss, and
3. They represent belts formed at different periods covering a span of at least 1,000 m.y. (3,500-2,500), some of which are older and some younger than the gneisses which are themselves poly-phase and polymetamorphic.
The Sargurs, Dharwars and associated gneisses show three phases of deformation and identical fold geometries thus, making it difficult to separate the various supracrustals and gneissic groups on the grounds of structural parameters alone.
The cause of the general unity of structural features through space and time has been explained through:
1. Prevailing E-W compressive directions through most of the early Precambrian; and
2. Also recognising late Archaean shear systems, which caused buckling and refolding of the earlier fold belts making all the linear elements parallel to the direction of shear.
However, the above problem concerning these supracrustals and gneisses can only be resolved by more detailed mapping of critical areas, in the field disposition, ore finds. The iron formations in the Sargur supracrustal belt are found extensively in Satnur, Halagur, Kanakapura and Sivasamudram areas.
The iron formations in these areas occur as continuous bands and as intercalations in the associated gneisses and charnockites. They trend between N and NNW and dip east at angles varying from 35° to 60°. The rocks show distinct banding which is due to the occurrence of thin discontinuous ribbons and laminae of quartz alternating with laminae rich in dark minerals.
While the width of the intercalations is less than 10 metres, the width of continuous bands is more than 30 metres and their strike length ranges from 0.25 to 4 km. The iron formations are fine to coarse-grained rocks with black to brown colour. The distribution of iron formations in these areas is shown in Figure 7.2 and various iron ore mines in the state are given in Figures 7.3 to 7.7.
The texture of the iron formations varies from schistose to xenoblastic. There is rapid variation in the proportion, distribution and grain size of the minerals. All the minerals are anhedral and contain abundant inclusions. The essential minerals in the iron formations are magnetite, orthopyroxenes, clinopyroxenes, amphiboles, garnet, pyroxferroite, lamellar pyroxenes, and quartz.
Chemical Characterization of Iron Formations:
The iron formations have variable SiO2 contents ranging from 40.61 per cent. The total Fe (FeO + Fe2O3) ranges from 28-51 per cent. FeO predominates over Fe2O3. Another important chemical characteristic of the iron formation is the overall high content of MnO (avg. 3.17 per cent).
A comparison of the average major element composition of the iron formations of the present study with those of the well-known iron formations, namely Algoma and Lake Superior types, iron formations of greenstone belts and the international iron formation standard, brings out distinct differences. The iron formations have low SiO2 and FeO contents and higher MgO, Al2O3, MnO and CaO contents when compared to Algoma, Superior and the international iron formation standard.
Trace element usefulness of studies in igneous rocks has been well established, but in sedimentary rocks their application has been sporadic, or even less in the field of iron formations, where it has been introduced quite late.
The Ba content in the majority of samples is around 10 ppm, whereas the Co, Cr, Cu content ranges from 20-100 ppm. The Ni content is very low and ranges from 1-5 ppm. The proportions of Sr, Zn, Rb, Zr and V range between 20-100 ppm.
A comparison of the average trace element composition with those of Algoma, Superior, iron formations of greenstone belts and international iron formation standard, reveals that these iron formations have low contents of Co, Cr and Cu when compared with the Algoma, Superior and iron formations of greenstone belts, although they have the same proportions when compared with the international standard. The Ni content, of the iron formations of the present study is very low when compared to all other types.
The rare earth elements (REE) in sediments and particularly in iron formations of the Precambrian have been used for establishing the nature of the source rock, process of formation, environment of deposition and the nature of precursor mineralogy.
The total REE contents of the iron formations vary from 10.8 to 32.93. The REE concentration of the iron formations of high grade terrain of Karnataka is consistent with the REE concentration of Archaean iron formations elsewhere.
The behaviour of Ce is also considered to be time dependent. Definite anamolous behaviour of Ce is observed in Proterozoic iron formations when compared with iron formations of the Archaean age. Flat to slightly depleted Ce pattern can be observed in the iron formations of the present study.
Some workers suggest that a Eu anomaly can be used as an indicator for oxidising versus reducing environments in the Precambrian atmosphere. Fryer (1983) showed that the behaviour of Eu appears to be time dependent, as the Archaean iron formation was consistently enriched in Eu whereas early Proterozoic iron formations exhibit slightly positive to slightly negative Eu a˜amolies. However, the REE patterns of the iron formations of the present study show enriched to slightly depleted Eu anamoly.
Based on the behaviour of Eu, it seems rather difficult to say anything about the oxidisation state of the environment during the Archaean period in southern Karnataka. The behaviour of Eu anamoly in the iron formations of southern Karnataka is similar to the observation made by Appel (1983) on the Isua iron formation of Greenland.
Therefore, it can be summarised that the iron formations of the high grade region are associated with pelitic and carbonate sediments and are found as enclaves within the gneisses. Based on petrography, two distinct types are recognised, namely, oxide and silicate facies.
A peculiar feature of these iron formations is the occurrence of a variety of silicate minerals, such as orthopyroxenes, clinopyroxenes, amphiboles, garnet, pyroxferroite, lamellar pyroxenes and biotite. Chemical investigations of these minerals indicate that the pyroxenes are represented by ferrohypersthene and ferroaugitc whereas the garnet belongs to pyralspite series.
The lamellar pyroxenes are Ca-poor and Ca-rich pyroxenes containing Ca-rich and Ca-poor lamellae respectively. Major element chemistry has revealed that these iron formations are in general comparable with the international iron formation standard with one major difference, that is, that the iron formations of the present study have a very high MnO content (3.97 per cent).
Regarding trace elements, the Ba and Ni content of the iron formations of the present study is relatively lower compared with that of iron formations elsewhere. Rare earth elements’ concentration is consistent with the REE concentration of Archaean iron formations elsewhere.
It is possible to this postulate that the iron formations of this region mainly consist of silicate and oxide facies. The presence of a variety of silicate minerals in these iron formations is controlled by a combination of parameters, namely, bulk composition, oxygen fugacity and pressure/temperature conditions of metamorphism.
The pressure/temperature conditions of metamorphism making use of well-known geothermobarometric models have yielded an average PIT estimate of 7.5-8 kb and 650°C. These values suggest that the iron formations have undergone upper amphibolite to granulite facies conditions of metamorphism.
The major, trace and REE geochemistry indicate that these are chemical sediments and silica and iron might have come from volcanic exhalations. Although large amounts of basic volcanic rocks are absent, there are, however, considerable amounts of basic granulites and amphibolites associated with these iron formations which have a tholeiitic precursor.
Further, though it is difficult to apply any new model for the origin of the iron formations, with the aid of geochemistry, it does seem probable that these might have evolved under shelf conditions, since the associated sediments namely, pelites, and carbonates have characteristics typical of shallow water marginal facies depositional environments.
The high MnO content in these iron formations suggests that there was little separation of Fe and Mn during deposition. This may be accounted for by assuming a narrow range of Eh. Regarding the nature of the original material, nothing can be deduced definitely since precursor mineralogy is completely obliterated because of high grade metamorphism.