Interpreting the magnetic signatures and radiometric indicators within Kogi State, Nigeria for economic resources

Abstract

The mineral and geothermal potentials of part of Kogi State was investigated through the interpretation of aeromagnetic and radiometric data of the study area. The analysis targeted bridging the gap of insufficient geophysical information of sub-crustal resources that could be of economic value within the area of study. Vertical derivatives, analytical signal and spectral depth analysis were used for the interpretation of the aeromagnetic data while the concentration and ternary images of the three radiogenic elements were used for the interpretation of the radiometric data. The result of first vertical derivative was helpful in delineating mineral potent lineaments labelled F1 to F8. The lineaments were seen trending E-W and NE-SW direction. A principal fault line F6 tends to separate the regions of sediments to the south-east and basement geologic formations to the south-western regions. Result of the analytical signal amplitude revealed regions with shallow intrusive magnetic rocks having high amplitudes ranging from 0.152 to 0.557 nT/m, while regions with magnetic rock intruding into sedimentary formations at greater depths, have medium to low amplitudes ranging from 0.014 to 0.136 nT/m. Regions delineated to be altered through hydrothermal process coincided with areas of major magnetic lineaments. The lineaments which could be fractures, faults or shear zones usually serve as conduits for mineral deposits during hydrothermal process. Result of potassium_thorium ratio map showed evidence of hydrothermal alteration in the NW and SW regions of study area. These regions of alterations also corresponds to regions where major lineaments were mapped and thus represent regions with significant potency for mineralisation. Result of spectral depth analysis on the aeromagnetic data showed that peak values of geothermal gradient and Heat flow were 27°C/km and 68 mW/m2 respectively, the values were recorded at the North-eastern part of study area, at the lower end of Koton-Karfe and part of Lokoja where the shallowest Curie point depth of 24 km also occurred. The estimated values of heat flow falls below the range of 80 to 100 mW/m2 recommended as threshold for a good source of geothermal energy, hence the region will not be prospectively good for a cost effective geothermal energy exploration. In order to assess the heat production within the study area due to activities of radioelements, a comparative analysis of the concentration of the three radiogenic elements was carried out. A relatively high radiogenic heat production (RHP) value of 3.4 μW/m3 was recorded at the North-western region indicating high occurrence of radioactivity within the granitic rocks. The (RHP) value of 3.4 μW/m3 is slightly below 4.0 μW/m3 which is a recommended value for a good source of geothermal energy.