Integrated Aerogeophysical Methods for Litho-Structural Characterisation and Radiological Hazard Assessment of Parts of the Igarra Schist Belt, Southwestern Nigeria

ABSTRACT

Integrated aerogeophysical datasets (aeromagnetic and airborne radiometric) were used for the litho-structural and depth characterisation and to assess the environmental radiation risks in part of Igarra, southwestern Nigeria. The total magnetic intensity (TMI) reduced to the magnetic equator gridded data were enhanced using first vertical derivative, analytic signal, tilt derivative, and spectral and Euler deconvolution analyses to improve and enhance the subsurface rocks’ textural complexities and subtle features. The results were then integrated with radionuclides concentration, radioelements composite and ratio maps for the delineation of litho-structural features and their depths, which are viable for mapping of the hydrothermal and mineralized zones hosting economic minerals, especially gold in the study area. The high anomalous magnetic signature of the study area ranging from 23.49 to 223.60 nT may be attributed to basic rocks and magnetic minerals (olivine, amphibole and iron minerals). The low anomalous zone, on the other hand, with intensities ranging from -402.23 to -0.83 nT characterise areas with sandstone with intercalated shale and felsic rocks. The results of the radionuclides concentration and radioelements composite maps with high and low radionuclides signatures coincide with the litho-structural patterns identified in the aeromagnetic maps. The hydrothermal and mineralized zones in the study area were delineated at boundaries with variability in intensities and radionuclides signatures, and they serve to host the economic minerals in the study area. The radiogenic heat production (RHP) with values ranges from 0.51 to 10.73 µWm-3; the RHP values in some parts of the study area are above the crustal average range of 0.8 – 1.2 , hence, may have contributed to the experienced heat flux. Similarly, the radiological hazard indices; absorbed dose rate(18.35 to 156.52 nGy/h) and annual effective dose equivalent (0.11 to 0.96 mSv/y) were higher than the permissible limits of ≤60 nGy/h and 0.41 mSv/y, respectively, in some sections; hence, the rocks may pose radiological threats to humans, especially rocks with high radionuclides concentration arising from high fraction of K-feldspar, plagioclase, biotite and accessory minerals – zircon and monazites.