B.V.S.Murthy, B.Madhusudan Rao, A.K. Dubey and Srinivasulu - IGU

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B.V.S.Murthy et al.From the table of physical properties it is clear thatthe manganese ores pyrolusite and psylomelane havesignificantly high density values compared to laterite,phyllite, shale or BHJ/BHQ. Iron ores namelyhematite, limonite or goethite show higher densityvalues comparable to manganese. In the case ofmagnetic properties manganese (pyrolusite &psylomelane) show paramagnetic (antiferro magnetic)responses comparable to that of hematite. Howeveramidst the environment of phyllite, shale,conglomerate or quartzites the manganese ores can beexpected to show detectable magnetic responses. Theintergranular pore spaces in the manganese ores andtheir shallow occurrence in the region make themrelatively conductive compared to the BHJ/BHQs orlaterite. The difficulty is however, in distinguishingmanganese ores from the phyllites and shales. In viewof these physical properties information and theexperience of the earlier scientists in other areas wehave chosen magnetic method as the primary toolwhich is given support by electrical resistivity andgravity. Cost effectiveness and ease of conducting fieldsurveys were also given due weight in the selectionof geophysical methods and their sequence.Present Geophysical InvestigationsThe scientific interest of the present authors and theeconomic interest of private lease holders in differentblocks of the Jamda-Koira belt have led the authorschoosing three blocks (marked in fig.1) for the presentgeophysical investigations. In view of the limited sizesof occurrence of manganese deposits in these areas analmost uniform network of 10mx20m size formagnetics was chosen. Electrical resistivity profilingpoints with two spacings of AB/2 = 10m and 30m,soundings with AB/2 up to 90m and gravityobservations were located along related traverses ineach block. The instruments used are protonprecession magnetometer of IGIS & Terra Sciencemake, Resistivity meters Aqua Meter–Pune and DDR–III of IGIS and Lacoste–Romberg Gravimeter (ModelG1106). Elevation and position location data wereobtained using Total Station.Figure 2. Block-1 Layout of Geophysical Survey Net.152
Geophysical exploration for manganese-some first hand examples from Keonjhar district, OrissaFigure 3. Magnetic Total Field Map of Block-1 : (Contour Interval: 25n T).BLCOK-1This block is nearly in a square shape spreading forabout 25 hectares. The topography of the area is in afashion that is elevated towards SE with a relief ofabout 60meters. The surface is covered by thickforest. From east to west the area is marked by lateritecapping; BHJ, BHQs in the central elevated portionsand gradually thickening soil cover towards west andsouthwest.Though number of boreholes were drilled earlierin the area by a private mining company, no geologicallog about the occurrence or otherwise of manganeseat depth was available to the present authors.However, a rough idea of the surface distribution ofsome pockets of manganese lumps/lensoids with orwithout the association of iron and/or laterite isavailable from the lease companies.Geophysical traverses (mainly magnetic) were laidin the West to East direction at 25m traverse intervalalong which stations were located at 10m interval. Tohave more control, additional traverses were laid inthe N-S direction with a view to map the response ofore bodies and also to detect and demarcate theirboundaries. Additional stations were laid at 5minterval on majority of the traverses with a view totrace the lateral spread of ore lenses. In total about650 magnetic observations, 193 resistivity profilingpoints, 38 schulmberger soundings and 140 gravityobservations were made in the area. The layout ofgeophysical observations is shown in Fig 2.Correction for the diurnal variation of magneticfield was effected through maintaining a secondinstrument at local base.The magnetic map (Fig.3) interestingly is distinctand characteristic in revealing changes in themagnitudes of anomalies, trends and alignmentsattributable to known and unknown surface andsubsurface geological situation. The magneticvariations shown at 25 nT contour interval correspondwell with the trends of manganese ore distributed inlumps/lensoid form. These anomalous zones areshowing alignments in NW–SE, N–S and E–Wdirections. In the SW part of the Block, the highintensity anomaly closures are related to theoccurrence of iron ores or iron rich BHJ/BHQs. Atplaces manganese associated with iron is also showinglocalized magnetic anomaly closures like in the southand also in the north. Magnetic anomalies in lateriticzones are also of low order variation. Low order153
Page 1 and 2: J. Ind. Geophys. Union ( July 2009
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B.V.S.Murthy, B.Madhusudan Rao, A.K. Dubey and Srinivasulu - IGU

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