Integrated geophysical methods for delineating crustal structures and hydrothermal alteration zones for mineral exploration projects in parts of west-central, Nigeria

The knowledge that quite many primary deposits of economic minerals in Nigeria are structurally controlled, the principal of which are faults, folds, shear zones, mylonites and hydrothermal alteration zones among others, has informed the choice of this study. Although significant success has been recorded in mapping many of the recognized structures, there are yet many gaps to fill especially in the area of hydrothermal alteration mappings. To provide new insights for mapping alteration zones, the present study integrated two airborne geophysical methods namely magnetic and radiometric methods with ground geophysical surveys comprising the electrical resistivity tomography (ERT) and very-low-frequency electromagnetic (VLF-EM) methods. The airborne data were interpreted using standard geophysical techniques such as horizontal gradient magnitude, radiometric ratio and radiogenic heat production. The results obtained in each case showed that the hydrothermal zone H3, out of the six suspected hydrothermal alteration zones namely H1, H2, H3, H4 H5 and H6, has positive prospects for hydrothermal alteration zone. The length and the average width of 30 km and 5 km respectively of the alteration zone H3 enabled its detection from airborne surveys. The zone H3 was, thereafter, investigated with ERT and VLF-EM methods. The results obtained from the ERT showed a fairly deformed homogeneous layer of high electrical resistivity below fairly weathered heterogeneous formations. The result of the VLF-EM method, on the other hand, showed several weak zones suspected to be fracture zones that may have aided the circulation of the hydrothermal fluids for subsequent mineral precipitation in wall rocks.