Summary:
The Khandbandh brecciated iron deposit is located in the eastern limb of Bonai-Keonjhar belt. The Bonai-Keonjhar belt, designated as BIF-III occupies a distinct ‘U’-shaped pattern in the western flank of the NOIOC that rests over the Dhanjori Quartzite (Acharya, 1993; and Beura, 2008). The Precambrian IOG largely contains BIF in addition to the other volcano-sedimentary rocks forming a significant portion of the Singhbhum–North Orissa Craton of eastern Indian shield (Saha et.al, 1988).
The litho-associations of this area form the youngest Iron Ore Group in the Iron Ore Supergroup of Odisha and named as BIF-III (Acharya,1984, 2000, 2005). Various names have been given to this belt such as Barbil Group (Iyengar and Murty, 1982), Koira Group (Murty and Acharya,1975) and Khondadhar group (Sarangi and Acharya, 1975).The IOG in addition to BIF also contains other rock units like two generations of shales, tuffs that underlie or are interlayered with BIF. The litho-associations of this area form the youngest Iron Ore Group comprising of banded hematite jasper, banded hematite quartz/chert, banded shale, banded manganese formation and ferruginous shale. The brecciated iron deposit occurs as small horizons in discontinuous manner along with hard, massive iron formation. The banded iron formation consists of predominantly iron oxide minerals such as hematite, martite, specularite and, magnetite and the silica as chert, jasper and quartz. The litho-assemblages of this youngest Iron ore belt are un-metamorphosed and lack of intrusives.
The general structural disposition of the rocks of the area shows a synclinorium trending NNE-SSE direction having low plunge towards NNE. The rocks of the area are experienced with three phases of folding. Successive phases of folding lead to plunge reversal that results in open cross folds.
The brecciated iron deposits (BID) are found to be genetically different from the detrital ones. Both the components, the breccia and the matrix are hematite in BID. A number of polish sections of brecciated iron deposit of the area were studied for micro-morphological characters under the microscope (Leica labour lux 12 pol. and Leica DM 2500 P). In a network of supply-sedimentation-lithification process of iron material, the layers/bands were broken down to breccias in a disturbed milieu and got cemented by the same iron material while remained floating in the hydroplastic stage. In some instances bands of hematite and BHJ as well are broken due to load pressure and sagging. The fragmentation process continues, while the sedimentation process runs. In the later stage they have been undergone recrystallisation, martitisation and goethitisation processes to form other minerals. The degree of disturbance and viscosity of hydroplastic base determine the position, orientation and concentration of clasts in BID.
Broken down hematite fragments of different shapes, size and angularity attains granular status with intergranuar spaces filled with already supplied material like hematite, silica or their recrystallised/altered products. The arrangements and sorting of the clasts in BID are irregular and ill respectively. Mostly the fragments occupy the nearby area where they are degenerated from the bedded or layered masses. Some clasts of folded geometry remain in juxtaposed with non-folded bands attest penecontemponeous nondiastrophic features produced simultaneously with sedimentation. Large fragments of banded hematite jasper embedded within the small clasts of hematite indicate differential load/stress bearing capacity between them.
The brecciated iron deposit of Khandbandh occurs in the same stratigraphic horizon with banded iron formation and iron ores in the eastern limb of Horseshoe synclinorium. Similar type of deposits detrital iron deposits are also seen, but they are genetically different from BID. In BID fragmentation is not erosion driven; rather clasts are produced due to the in situ environmental disturbances in the depositional site. Simultaneous production of clasts with sedimentation process endorses the accumulation and cementation of those at the same place and with the same material. Therefore the matrix is mostly hematite, which is compositionally same as the fragments. Some rounded pebbles of iron and shale are found to be appear along with the hematite clasts in BID clearly indicated they have been transported from nearby areas. It is obvious from these evidences that DID and BID were occurred in same time without any clear cut discrimination in connection of depositional environment.