Radiogeochemistry, uranium migration, and radiogenic heat of the granite gneisses in parts of the southwestern Basement Complex of Nigeria

Abstract

The radiogeochemistry of granite gneisses in Akunu-Akoko (AK) and Ayere (AY) areas of the southwestern Basement Complex of Nigeria was studied using gamma-ray spectrometry, and geostatistical analysis and modeling. This study was aimed at determining the concentrations of radionuclides (K, U, and Th) in these rocks, as well as alteration scenarios, nature and rates of uranium migration (UM), levels of radiological hazards, and rate of radiogenic heat production (RHP). The interquartile (lowest Q1 < highest Q2 < highest Q3) elemental concentrations for 40K, 238U, and 232Th were 0.23 < 3.75<4.06%; 1.00 < 4.20<5.40 ppm; and 4.10 < 16.10<30.20 ppm, respectively, within the minimum and maximum limits of BDL (below detection limit) to 5.23%, BDL–9.70 ppm, and 0.40–60.20 ppm. The average activity concentrations of 40K, 238U, and 232Th with values of 629.28, 38.58, and 49.96 Bqkg−1, respectively, were above their crustal averages in the order of AK3<AK2<AK3<AY3<AY1<AY2. Variability in K-eU-eTh content and alteration in the rocks were affirmed by radioelement ratios and F-parameter. The high radionuclide concentrations may be attributed to high levels of radioactive materials, as well as trace and rare earth elements. Low radionuclide concentration in rocks is due to weathering, metamorphism, and 12–86% outward UM. The absorbed dose rate (ADR) average of 75.09 nGyh−1 was within the allowable range, with regression R-squared (R2) prediction values of 25–92% from the radionuclides. The average radium activity was below the permissible limit of 370 Bqkg−1, and the annual effective doses, hazard indices, gamma activity, and activity utilization index were all far below the safe limit of 1 mSvy−1. However, gonad equivalent and excess lifetime cancer risk were above their permissible limits. RHP average of 1.89 μWm−3 above the crustal range of 0.8–1.2 μWm−3 will considerably contribute to the heat flux in the areas. Based on the prediction accuracy of the R2 results, 36–86% of RHP rates in the rocks are achievable from the radionuclides.