Geological Aspects of the Banded Iron-Formation of Bihar and Orissa

The metamorphic age (2.55-3.07 Ga) of BIF of Tomka-Daiteri belt has been established from Sm/Nd data which seemingly indicates that the sedimentation age of BIF is older than Singhbhum granite (3.12±0.OI Ga). Intrusive evidences of Singhbhum granite into BIF have been documented. An older age of the GorumahisaniBadampahar- Tomka·Daiteri-Malayagiri BIF with respect to that of Jamda-Koira Valley is supported.

The BIF, consisting predominantly of magnetite-chert, magnetite-jasper (BMJ) hematite-jasper (BHJ) or martite-quartz (BMtQ}form part of a sedimentary succession, underlain by quartz-arenite or lithic wacke and interlayered with altered volcanic tuff (shale ?). Magnetite, hematite and chert were primary precipitates in the BIF. No primary carbonate, silicate or sulphide have been noticed to occur in these rocks excepting sporadic occurrence of carbonate pseudomorphs in the underlying quartz-arenite.

Primary sedimentary structures like current ripples, syneresis cracks, penecontemporaneous deformation structures (e.g. folds, faults, micro-brecciation, pods, etc.) and diagenetic recrystallization features are described.

Similar to other world deposits of Precambrian and Archaean age, the BIF of Bihar and Orissa are also deficient in major, trace (TE) and rare earth elements (REE). Mineralogical composition and geochemistry suggest that they belong to the oxide facies iron-formation, Low concentration of Al₂0₃, CaO, MgO. MnO and of Zr, Cr, Ti, V, Ni and Cu in the BIF with respect to their abundance in. the interlayered pyroclastics presumably suggests a non-volcanogenic source of iron and silica. Eu/Sm ratio (0.541.01) indicates an Archaean age for the BJF. A shallow water marine environment is predicted for the deposition of BIF.