Iron Ore Deposits in North Orissa (With Map) | India | Metallurgy

Banded Iron Formation (BIF) and iron ore constitute a key horizon in the Badampahar-Gorumahisani-Suleipat (BGS) belt lying to the northeastern periphery of the North Orissa Iron Ore Craton (NOIOC) (Figure 6.1).

The era ton comprises Singhbhum granite complex having enclaves of Older Metamorphic Group (OMG) and Older Metamorphic Tonalite Gneiss (OMTG). The BGS belt extends in an arcuate pattern from Badampahar in the south upto Gorumahisani in the north through Suleipat. The details are from Beura and Singh (2008).

The litho assemblages consist of Banded Magnetite Quartzite (BMQ), Banded Hematite Quartzite (BHQ) Banded Magnetite Grunerite Quartzite (BMGQ) and Banded Cherty Quartzite (BCQ) invaded by ultrabasics and numerous dolerite dykes. The litho-unit of BGS belt is assigned as the oldest Iron Ore Group (BIF-I) of Iron Ore Super-group.

The older rocks exposed in the area belong to OMG, which are amphibolites, tremolite-actinolite schist and granite (≡ Singhbhum granite). The litho units have been subjected to multiple phases of folding and faulting. They have suffered low-grade metamorphism. The episode of tectonism has been correlated to the Iron Ore Orogeny.

The iron formation comprises magnetite, hematite, martite, goethite, specularite and grunerite, and silica minerals. The iron mineral in the basin is predominately oxide facies. The mineral assemblages reveal sedimentary signature and subsequently modified by diagenesis and metamorphism.

The iron-formation and ore deposit are developed in a fault bounded basinal structure .The spatial disposition of iron deposits in this basin is influenced by the tectono-structural events. The iron formation reveals characteristic features of Archean schist belt and has given rise to commercially exploitable deposit.

Geological Framework of BGS Belt:

BGS belt is located in the northeastern flank of the North Orissa Iron Ore Craton (NOIOC). It is underlain by metamorphosed mafic volcanics and clastic sedimentary rocks. The supracrustal assemblages of the belt comprises Banded Magnetite Quartzite.

Banded Magnetite Grunerite Quartzite, Banded Cherty Quartzite is assigned as Archaean schist belt belonging to the oldest Iron Ore Group under Iron Ore Super-group of north Orissa. The iron-formation of the belt constitutes Banded Magnetite Quartzite as the predominant rock type characterized by alternate bands of iron oxide mineral and silica.

The iron-formation consists of iron oxide minerals such as magnetite, martite, specularite, hematite and silica in form of chert. It is characterised by well-developed bands, small-scale folds, faults and certain non-diastrophic structures such as pinch and swell and scour and fill indicate primary sedimentary nature.

The general trend of the area is NE-SW, however towards the northern part it deviates truncating towards N-NNW. The rocks are complexly folded and faulted and have been intruded by ultrabasic and mafic dyke. In fact the proportion of grunerite increases towards the contact of basic intrusive.

The depositional basin of BGS belt has probably been originated due to progressive tectonic activities. During the initial stage the cratonic massif witnessed spreading and extension along its margin. In subsequent period, thinning of the cratonic margin was resulted, which led to rifting on further extension. The post rifting grabenisation could develop the BGS basin. This intracratonic basin has been considered as depository of iron formation and iron ore.

In many instances supracrustal rocks including BIF were laid down in Early Proterozoic rift basins. Stretching of the sialic crust gave rise to thinning and fracturing near the margins of the ancient continental block; this led to subsidence troughs and on further spreading led to volcanism accompanied by deposition of chemical precipitates. Archaean continental crust experienced tectonic- magmatic reactivation in disruption and fragmentation in early Proterozoic time.

The Iron ore basins of Karnataka have been developed as “intra-cratonic” fringed by volcanics that erupt along fracture zones. Ultimately it led to block-rifting causing zone of depression and subsequent deposition of sediments.

The sedimentary setting of Proterozoic iron formation of Canadian Shield, Hamersley Group and Transvaal Super Group has also undergone tectonic system of basin development. Sawkin (1990) while discussing the spectrum of rift-related metal deposits stated that the iron deposits along with volcaniclastic were formed in the rift-related environments.

The basin so formed in the process of tectonic activities might have become the receptacle to receive chemical precipitates and volcano-clastic materials. These materials have been derived from diversified sources through different processes, which in turn operated by the basin architect and external dynamic events.

The basin structure of present study area is an intra- cratonic one that received sediments from terrestrial source through continental denudation, sea water through transgression and regression, deep circulation of marine or meteoric water and from volcanic exhalation product inside the basin.

The ancient cratonic massif, probably of late Archaean started, providing denuded material to the basin of low relief in form of solution, instead of clastic inputs because of prolonged period of crustal stability. In the course of continued spreading of crustal block, the basin initiated volcanism accompanied by the deposition of chemical precipitates. Low relief basin in the cratonic margin was able to arrest the mineralizing water during the transgressive and regressive stages.

The extensional tectonic regimes along the cratonic margin during time of basin formation generated continuous permeable zones where pressure of mineralizing solution was nearly equal to the hydrostatic pressure that facilitated the deep circulation of marine and meteoric mineralisers.

The BIF was precipitated as chemical sediment in the studied basin from the hydrous Fe (OH)₃ solution collected through above different processes. Tectonically frame of basin assisted on availing iron and silica minerals, principally, from diversified sources, in interacting in stratified ocean water in association with the Precambrian environment.

The stratified ocean water had maintained two major zones; one below chemocline as anoxic water zone and the other above the chemocline as oxic zone. By the process of upwelling the dissolved iron and silica in anoxic water come up to the oxic zone probably in form of hydroxide and precipitated as oxides in shallow water.

The basin environment maintained a restricted environment with a chemical equilibrium condition in scanty oxygen level, alkaline medium, low pressure and temperature, and positive Eh that started precipitating magnetite .In certain conditions, with scanty supply of iron minerals, silica minerals deposited cyclic nature of deposition. Increase in grain size and blurring of bands is the result of recrystallization during diagenesis and low-grade metamorphism.

In the event the basin underwent slow rate of supply of oxygen hydrous Fe₃O₄ rather than hydrous Fe₂O₃ could be obtained by oxidation of ferrous hydroxide in an alkaline (low Eh). However, with a large supply of oxygen (high Eh) the rate of oxidation was so high that little or no Fe₃O₄ is formed, where in hematite was the stable mineral of iron. Hematite precipitated as a thermodynamically unstable phase in a gradually increasing oxidizing condition in preference to magnetite, where magnetite was the stable phase.

As the chemical system approaches the equilibrium, magnetite would form at the expense of hematite, when equilibrium not attained completion, some hematite would remain though unstable. This would permit that at the apparent range of condition both hematite and magnetite occur together. Magnetite and hematite pairs of iron minerals are stable together under specific pH and Eh. as shown in Figure 6.2.

The BGS belt, due to the above cited tectono-sedimentary system; carry a domain of iron ore deposits dominated by magnetite with patchy occurrences of hematite. The sheets/beds of syn-sedimentary origin rich in iron minerals in the area refer a syngenetic mode of origin.

Regional faults have been traced along the periphery of the basin as evidenced by differential altitude and displacement between the adjoining litho-units exhibiting escarpment and vein quartz. The major faults extend in NE-SW direction lying parallel to the regional structural disposition of the BGS belt. Besides, a number of E-W faults have also been noted across the regional trend.

There are number of faults along the periphery as well as inside the belt. The basin structure is affected mainly by continuous transverse faults and thrusts followed by numerous concordant to sub-concordant boundary faults.

Poly-phase deformation as well as folding episodes of vectorial contrasting nature generated intense cleavages in beds, which led to an increase in permeability of the rocks. This had enhanced the percolation of water into BIF that results in iron ore deposits of supergene type confined to cleavage cracks.

Structural setting in the basin has relationship with ore bodies having being controlled over by the solution channels. The downward movement of water responsible for leaching and subsequent oxidation of ore bodies has been controlled by structure.

The iron deposits are discontinuous and appear lenticular in outcrop pattern; this can be attributed to the folding and faulting rather than to an irregular pattern of iron deposition. Achaean BIF and chert are significant source of iron ore suggesting syngenetic character. Mineralisation is controlled by structural style; cross faults, closely spaced fractures and folds influence the localisation of the ore deposits.

The diastrophisms under iron ore orogeny also have witnessed faults of various dimensions and directions. The iron formation has been cut by a group of major faults striking along the general trend of the area i.e. NE and a good number of minor faults in the peripheral zones of the basin.

The faulting framework has become the pathways for dykes and ultra-basics. In due course the contact zones have undergone thermal metamorphism to produce grunerite. Elsewhere the pressure zones give rise to specularite.

The BGS depository basin was intra-cratonic formed by the rifting and grabenization of the craton along the boundary. The present starched out basin shape is due to the effect of deformations. The fault bounded closed basin activated the formation of BIF and ore in thermodynamic regime through chemical equilibrium system.

The BGS belt is experienced with poly-phase deformational episodes as well as folding episodes of vectorial contrasting nature generated intense cleavages in beds, which leads to an increase in percolation feasibility of the rocks. This resulted in cleavage cracks filled up with ores. Fault zones act as avenues for migrating large scale meteoric water to form ore bodies by supergene- enrichment process.

Oxidation is facilitated along fault zone, the result of which is the concentration of martitised magnetite. The shears zones are marked by specularite indicate the control of pressure on mineralisation.

Presence of grunerite demands its origin in specific P- T environment during deposits metamorphism, which might have been derived from ultrabasic intrusions controlled by tectonostructural events. Iron ore deposits, though discontinuous in nature, have their trend parallel to the regional strike of the belt indicate the structural influence on ore.

Presence of volcaniclastic rocks and tuffs in the BGS belt indicate the basin to have undergone active tectonic process. Younger ultrabasics and dolerite dykes that invaded the meta-sediments may be responsible for isochemical change in BIF, endothermic reaction in magnetite, rise in P-T condition and thermal metamorphism.