DELINEATION OF SOLID MINERAL STRUCTURES WITHIN PART OF NASARAWA STATE NIGERIA FROM AEROMAGNETIC AND AERORADIOMETRIC DATA

Abstract

Aeromagnetic data was analysed and used to delineate the structural features of the upper part of Nasarawa State of Nigeria which consists of Kuje, Keffi and Akwanga. These structural features include: lineaments, faults and folds which are channels for the accumulation of solid minerals. In delineating these structural features, some enhancement techniques such as horizontal derivatives, first vertical derivative and analytical signal were employed. Concentration maps were produced for the three radioelements viz Potassium (K), Thorium(Th) and Uranium (U). Furthermore , a ternary map and K/Th, U/K and U/Th ratio maps were produced. The residual magnetic intensity (RMI) map showed regions of different magnetic susceptibility which correspond to different lithology and depth of source rock. The RMI values range from -25.0 to 110.7 nT. The high magnetic signature occupied mostly the north-eastern and south- western part of the map corresponding to Akwanga and Kuje area respectively. The extreme south-eastern part of the map which is the sedimentary region has low magnetic intensity. The horizontal derivatives showed that most part of the area is characterized by a mixture of low and high magnetic closure of short wavelength that are high in frequency of occurrence. Few areas with relative sedimentation are denoted with averagely long wavelength. The analysis of the first vertical derivative map revealed fault zones, lineaments and intrusions. These lineaments trend in the NE-SW direction while the fault lines trend mostly in the NE – SW and E – W directions. The analytical signal showed amplitude range of -119.1255 - 28.4702 nT/m. Most of the lineaments, faults and intrusions observed in the map have high analytical signal. Low analytical signal was observed on the sedimentary region .The K/Th ratio map showed high K/Th ratio in the biotite-granite region having contact with the sedimentary rock in the south-eastern part of the map. Also the long stretch of lineament located towards the south-western area at Longitude 7.24oE-7.25oE and Latitude 8.300N-8.500N has high K/Th ratio while the sedimentary region at the extreme south-eastern part of the map has low K /Th. These areas of high K/Th ratio indicate regions of hydrothermal alteration which are possible areas of gold mineralization. The ternary map confirms the region of hydrothermal alteration indicated in the K/Th ratio map .The ternary map also showed that the biotite granite intrusions at Longitude 7.460E-8.030E and Latitude 8.300N-8.340N and the one located at Latitude 8.440N-8.500N and Longitude 8.200E-8.300E have high concentration of K,Th and U. The magnetic lineaments that represents fractures where hydrothermal alteration took place are potential hosts of gold minerals. Rocks in areas of high magnetic anomaly associated with these fractures are conduits for solid mineral accumulation.