Geophysical exploration for manganese-some first hand examples from Keonjhar district, Orissa

Manganese ores in India are being exploited since the past hundred years. Continuous exploitation of shallower and massive deposits led to searching for further occurrences and also necessitated application of geophysical methods. Geological survey of India since 1940s, has been exploring for manganese deposits in various parts of the country employing different geophysical methods (Ganokar, Das & Srirama 2001). The authors of this paper had opportunity to conduct geophysical surveys comprising gravity, magnetic and electrical resistivity methods in some selected blocks in the JAMDA-KOIRA belt of Keonjhar District, Orissa. Manganese ore, mainly psilomelane and pyrolusite occur in this belt as small lenses/lumps discontinuously and, in general under cover of laterite. These ore occurrences appear to be confined to near NW-SE or N-S belts and associated with shales/phyllitic shales below which are occurring occasionally brecciated conglomerates with chert and cherty quartz. The basement is Banded Iron Formations, mainly BHJ and BHQs. In the first block, which is about 300mx300m size, magnetic, gravity and electrical resistivity profiling and soundings were conducted. The geophysical signatures, though feeble, showed the trends and alignments of ore bodies and the intervening faults/ fractures associated with iron concentrations. Based on these results two more blocks (Block – II about 89hectares and Block – III about 24 hectares) were covered by magnetic mapping, electrical resistivity profiling and sounding and gravity survey on selected traverses. The geophysical anomalies in these two blocks also are characteristic in delineating the probable alignments of manganese bodies. Critical analysis of, essentially, the magnetic contour maps and resistivity and magnetic profile data helped visualizing pockets of likely occurrence of manganese ore.