Here is an essay on the iron ore deposits in India which have been broadly divided into the following six groups on the basis of their modes of occurrence and origin.
Essay on Iron Ore Deposits in India
Essay # 1. Banded Iron Formations of Precambrian Age (BIF):
The BIF deposits are very well developed in India. Most of the Indian deposits are similar to those of the Lake Superior region of U.S.A., Brazil and Venezuela. Extensive outcrops of BIF are found in the states of Jharkhand, Orissa, Chhattisgarh, Maharashtra, Karnataka, Goa and Tamil Nadu.
The common names used in India to designate BIF are Banded Hematite Quartzite (BHQ), and Banded Magnetite Quartzite (BMQ). In other parts of the world, names used are Taconite (Lake Superior), Itabirite (Brazil), Jaspilite (Western Australia) and Calico Rock (South Africa) has been used to designate BIF. In the recent years BIF has come to be generally acceptable both in field term as well as stratigraphic term to designate iron rich sedimentary rocks.
i. Classification of BIF:
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Gross (1965) distinguished two main types of iron formations from the Precambrian era viz. Algoma type and Lake Superior. The Algoma types is dominating the Archean age and are characterized by their banding and absence of ooliatic and granular texture. These are generally limited to lateral extent and closely associated with volcanic rocks and grey sediments. Carbon and pyrite rich black slate are common in them.
The Lake Superior type on the other hand, has the characteristic of the Protozoic era and is characteristically very extensive and closely associated with clastic sediments like quartzite and pelitic rocks without showing any direct relationship will volcanic associates. The BIF of India generally possesses the characteristics of Algoma and Lake Superior types especially in the Archean schist belt.
There is yet another distinct type of iron formation which is seen mostly confined to the granulite terrain. This is generally a weakly-banded magnetite-quartzite forming part of a supra-crustal sequence of quartzite, mica schist marbles, meta-volcanic, and amphibolites, completely engulfed in a voluminous mass of tonalitic gneiss.
The formation is highly folded and metamorphosed under granulite facies condition. Typical example are iron-ores, formation of granulite terrain of southern India like, Tamil Nadu, Andhra Pradesh, Karnataka, and Kerala and these are different from those of the Archean Schist Belt.
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Therefore, the iron formation in the Indian shield can be divided into two main types:
(a) Such iron are deposits lying within the high grade region and
(b) Such iron ore deposit which are confined to Archean Schist belt.
Among these, the first type, namely those occurring within the high grade regions of Archean granulites and gneisses and represent formation of an older age group (> 3,000 m.y.) formed in distinct tectonic environment and were later incorporated within high grade mobile belt.
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The next type, namely those occurring in the Archean schist belt and the more extensive types, are the ones confined to the schist belts formed during the period between 2,900-2,600 m.y.. These types of deposits are confined to the states of Jharkhand, Orissa, Karnataka, Maharashtra, Chhattisgarh and Goa. They represent an important source of rich iron ore deposits in India.
ii. Origin of BIF:
The origin of the BIF is still not settled, for they have always been debated and as such a final option is not viable at this point of time. There are inputs from several attempts and investigations in India as well as in other parts of the world where similar BIF deposits occur. Yet broad generalizations exist, which suggests that the large and most widespread proportions of deposits are of sedimentary origin.
The volcanic nature of the period during which the Archean iron ore formation were deposited have been also recognized (e.g. Isua Formation). A controlling factor was possibly the nature of the ocean water during the Archean times. The Eh and pH of ocean water appears to have been significantly different from those of later years.
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One view is that iron formation was deposited in shallow inland lakes, fresh water being considered as a more likely way of transport of silica, and iron from earth’s crust quite similar to the present day laterite crust. Periods of intense deposition was proceeded by a long period of accumulation of dissolved iron and silica in such sedimentary forms.
A different view is that the iron formations are essentially a produce of diageretic replacement of primary carbonates. Although there are evidences of replacement but the process cannot fully satisfy for vast amounts of iron in such BIF formations.
The character of such late Archean – early Protozoic atmosphere is also taken into account to understand deposition of BIFs. The atmosphere at that time, as deciphered, was rich in carbon dioxide, nitrogen and was deficient in oxygen. Enormous quantities of iron thus got accumulated in the oceans and lakes. Subsequently, it is postulated that when life first appeared, photosynthesis took place, which resulted in the release of bulk oxygen.
Thus oxygen combined with dissolved iron, precipitated it, which resulted in formation of iron deposits. However, iron formations as old as 3,000 m.y. show that the build of oxygen in hydrosphere took place much earlier.
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Thus, the distinctive chemical composition as suggested by iron ores formation (BIFs) restricted mainly to iron and silica, to the exclusion of other metallic compounds and origin of uniform banding cannot be explained through inorganic chemical precipitation process alone.
There exists, thus, differing opinions and concepts of various works in explaining the origin of BIFs. The existing knowledge about the BIFs appears not only inadequate to build a good theory of origin, suggesting perhaps that a single mode of origin for all BIFs does not seem to be feasible.
iii. Characterization of BIF Type of Iron Ores:
Let us briefly examine what makes the BIF formation so special? We know that the BIF’s have given rise to huge accumulations of iron to produce a commercial interest of both quantity and quality in having a high percentage of Fe in them. In India nearly 90 per cent of the iron ore mined comes from the BIF’s. In these the main ore minerals are hematite, and magnetite. Important deposits are located, the southern regions of Jharkhand; Keorjhar; Sundergarh and Bonai districts of Orissa; Bellary district in Karnataka; Baster district in Chhattisgarh; and in Goa.
Magnetite ore deposits are mainly restricted to Chickmaglur district of Karnataka and Salem and north Arcot districts of Tamil Nadu, as well as Prakasam district of Andhra Pradesh.
Therefore, different types of iron ores derived from Banded Hematite rock found within the deposits of the group are:
(i) Massive ore,
(ii) Laminated ore,
(iii) Blue dust.
In addition, float ore accumulated on the slopes and foothills appear to be a result of disintegration of in-situ ore bodies. The float ores are of different sizes and of different quality in regard to the iron percentage.
Essay # 2. Sedimentary Iron Ore Deposits of Siderite and Limonitic Composition:
These types of iron ores are also called bog iron deposits. They are generally consisting of siderite and limonitic ores. They occur often in association with the iron stone shells of Lower Gondwana formations in India, within the coalfields of Jharkhand and West Bengal (namely Jharia and Raniganj coalfields).
They occur also with the Tertiary formations of Assam and other small occurrences elsewhere in the Himalaya and appear to be a result of hydration, which converts siderite into limonite stones at the surface. Such deposits are mostly heterogeneous in grade, as well as in their mode of occurrences.
Iron minerals are here accumulated as irregular bodies in stream beds, and typically at the bottom levels of bogs and marshes where Lower Gondwana sedimentary beds have deposited along with the organic debris.
In upper Assam such deposits occur in Lakhimpur and Sibsagar districts and are mainly of two types:
(a) Clay ironstone, and
(b) Impure limonite.
In Raniganj area, the sedimentary iron ores occur in the form of thin beds of iron ores of variable thickness and frequently in Ironstone shale group of the Damuda series in Raniganj coalfield. The ore is mostly grey iron carbonate but near the surface it has been weathered to form into brown hydrated oxide.
Clay ironstones are also known to occur in the Karanpura and Auranga coalfields.
Essay # 3. Laterite Ores Derived from Sub-Aerial Alternations:
Laterite types of iron ores are derived from the sub aerial alteration of rocks, such as gneisses, schists, mafic lava and so on, in humid and hot climatic conditions in the eastern states of India. By examining the composition of these laterites it is possible to attribute that some of these may be of economic value and could be exploited in the time to come.
One limitation of them is a high level of alumina content along with such elements as titanium, nickel, chromium and manganese. Such deposits of laterite ore are mostly concentrated at the top due to alteration process of the iron bearing parent rocks.
They could be of variable colours – red, yellow or brownish hematite and goethite being common. The capping is usually thicker over the mafic rocks for obvious reasons, such occurrence contain high percentage of primary iron, associated with coal, nickel, chromium manganese and titanium.
Large stretches of Deccan Basalts flows and, gnises in the Western Ghats and in the Chota Nagpur, (Ranchi, and Plateau) in Jharkhand state as well as the schistose rocks of many areas like those of Sandur are covered by such laterite tops.
Essay # 4. Iron Ores Formed by Magnetic Activity:
A zone of apatite magnetite rock is found closely associated with the Singhbhum Sheer Zone (the copper belt thrust) occurs in minor bodies. It is understood that these have been formed by magmatic activity associated with Precambrian diastrophic cycle when the rocks of the shear zone were thrusted and subsequently intruded by acid to intermediate igneous rock (soda granites and associated suite).
The magnetite containing rocks is a mixture of apatite and magnetite with some biotite, chlorite and subordinates quartz. This is mostly found to occur on the hanging well side of the copper lode. Here the apatite magnetite ores are associated with granodiorites.
Essay # 5. Titaniferous – and Vanadiferous Magnetite:
The titaniferus vanadiferous magnetite deposits of south eastern Singhbhum, Mayurbhanj, Keonjhar in Jharkhand and Orissa states, and in Hassan district of Karnataka are associated with gabbroid and ultra-mafic rocks.
These types of ores of Jharkhand and Orissa occur as thin veins, lenses and pockets in gabbroid and ultra- mafic rocks which are often altered to serpentine, steatite and even to epidiorite. Magnetite and limonite are present in them, and in several occurrences appreciable quantities of hematite ore is also obtained.
In the state of Karnataka, small lenses like bodies of titaniferious magnetites occur in Tumkur district. These are generally associated with ultramafic rocks and they occasionally contain chromite as well. Such deposits contain up to 55 per cent to 61 per cent of iron content.
In Hassan district, titaniferous magnetities occur as linear bands with prominent outcrops in a narrow belt of Dharwarian rocks, comprising amphibolites and hornblende-schist. These are surrounded by the Peninsular Gneiss having been intruded by an ultramafic complex suite.
Essay # 6. Fault and Fissure Filling Deposits:
Fault and fissure filling deposits of hematite are minor occurrences found in Veldurty and Ranalla, Kota in Kurnool district of Andhra Pradesh. Such ore bodies occur in a fault zone traversing the gneisses and formation of the Cuddapah Group, they extend over a length of several kilometers.
The ore lenses form low hillock and ridges, project out well above the ground. Some of these occur as veins and stringers in the fault zone. The ore is generally hematite and often slightly specular in character, at places even jaspary when it is somewhat siliceoes.