GENESIS OF HIGH-GRADE IRON ORES OF THE ARCHEAN IRON ORE GROUP AROUND NOAMUNDI, INDIA

High-grade hematite ores of the Iron Ore Group in the Noamundi area, Jharkhand State, India, are hosted by a laterally extensive, 220m thick banded iron formation in a folded greenstone belt succession of Paleoarchean age. Single ore bodies, which are up to 3 km long along strike and several hundred meters wide, depending on dip of the beds, are stratabound and composed of two major ore types, namely hard hematite ore that is of ancient geological origin and ores related to recent weathering along a lateritized Cretaceous-Cenozoic land surface. The ancient hard hematite ore bodies comprise laminated hematite ores in which microplaty hematite is dominant, and massive ores composed almost entirely of martite. Supergene ores, in contrast, are comprised of goethite-rich duricrust and soft saprolitic hematite ores representing the leached zone of the Cretaceous-Cenozoic laterite profile.

Hard hematite-martite ores formed by hydrothermal replacement of banded iron-formation protolith, not only through leaching of silica, but also introduction of iron by hydrothermal fluids of meteoric origin. The hydrothermal fluids evolved from one that was warm, acidic and reducing, to allow for transport of ferrous iron to one that was cooler and more alkaline to allow for dissolution of silica and precipitation of ferric iron, i.e. hematite and martite. Hematite iron ore pebbles in an overlying Mesoproterozoic conglomerate attest to the antiquity of the hard ores.

In Cretaceous-Cenozoic times leaching of silica and/or carbonate from partly mineralized banded iron-formation gave rise to formation of soft saprolitic ores. Weathering took place under the influence of highly reducing and acidic meteoric water, which is typical for lateritic soil profiles developed under humid tropical climatic conditions with lush plant cover..