Abstract

The Precambrian banded iron-formations and the manganese ores of the Singhbhum region of eastern India occur within a sequence of predominantly volcanogenic metasediments (Iron Ore Supergroup) which occur around a central granitic platform. A review of the stratigraphy and tectonic history of these rocks indicates that they are made up of four separate groups, representing successive phases of sedimentation and deformation, spanning a time range between 2,700 and 950 m.y. ago. Two banded iron-formations occur within these rocks, of which the older one, occurring in the northern, eastern, and southern parts of the platform, is of the Algoma type whereas the younger one, occurring in the western part, is of the Lake Superior type. An offshore zone of deep-seated fracture and volcanism has been envisaged to have supplied the major part of the materials of the Iron Ore Supergroup and associated banded iron-formations and manganese ores which were deposited in the inner shallow sea (miogeosyncline), marginal to the platform. The banded iron-formations are considered to represent exhalative phases of volcanism along the fracture zone, when iron and silica were discharged into the inner sea and were precipitated out in alternating bands. Manganese, in significant amounts, was discharged along with iron only during the deposition of the younger banded iron-formation. Its separation from iron may have been controlled by slowly rising pH of sea waters, through progressive contamination with volcanic exhalations, as a result of which iron oxide precipitated out first, followed by manganese oxide under favorable Eh. The upper parts of the banded iron-formations and the manganese orebodies were subsequently lateritized to give rise to patches of secondarily enriched massive hematite and manganese ores.

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