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.

CHAPTER ONE

1.0       INTRODUCTION

1.1       Background to the study

Potential field methods of geophysical survey such as the magnetic and the radiometric methods have been useful in investigating subsurface geological structures. They have also been useful in giving information about the basement rocks and geologic structures like fractures, faults and contacts which may have influenced the occurrence of mineralized rocks. Magnetic survey has been extensively used to delineate subsurface structures  and  provide  an  estimate  of  the  thickness  of  non-  magnetic  sediments overlying magnetic rocks. However, the interpretation of magnetic data can be ambiguous due to the dipolar nature of the magnetic field and the direction of magnetization in rocks. Despite these complications, magnetic surveys and their interpretations can give very useful geological information when applied to the right types of problems (Ahmed et al., 2012). Magnetic variation or susceptibility may be analysed using either total intensity or residual maps. Detailed geologic features such as the  geometry  and  configuration  of  individual  basement  blocks  can  be  shown  by magnetic residual maps. They bring out the subtle magnetic anomalies that result from the changes in rock type across the basement rocks.  On the other hand, total magnetic intensity maps show large scale geologic features such as basin shape or anomalous rock types within the basement (Nettleton, 1962). Generally aeromagnetic maps reflect the variations in the magnetic field of the earth. These variations are related to structural and magnetic susceptibility change (Ozebo et al., 2014).

Aeromagnetic survey is one of the geophysical survey techniques carried out using a magnetometer attached to an aircraft and flown within the survey area. This enables quick coverage of much larger areas of the earth’s surface . The resulting magnetic maps shows the spatial distribution and relative abundance of magnetic minerals (Most commonly magnetite) in the upper levels of the crust. The magnetic map shows the geology and the geological structures of the upper crust of the earth. This is very helpful where bedrock is hidden by thick sediments. The aeromagnetic method is very useful in mineral exploration as it helps to delineate structures like faults, folds, contacts, shear zones and intrusions which are favourable areas for mineral deposits. These structures play important roles in the localization of minerals. Areas that are affected by deep weathering and thick overburden usually complicate the problem of rock delineation. One of the ways to achieve greater accuracy in facies delineation is by the use of radiometric method (Amadi et al., 2012)

The radiometric method measures the naturally occurring radioactive materials that emits the ionization radiation (the alpha (α), Beta (β) and gamma (γ)) from rocks. Out of these ionization radiations mentioned, the gamma rays have the highest penetration power and are used in radiometric survey. In the field, the gamma rays are detected by a spectrometer coupled with the scintillation detector. Theoretically, the energy of the gamma rays emitted from the natural radionuclides range from 0 to 3 MeV, but in the geological survey, the interest lies between 0.2 and 3 MeV (Kearey et al., 2002). Peaks in the spectrum are attributed to potassium (%K), thorium (eTh) and uranium (eU). The count rate of the whole spectrum is referred to as the total count (TC). The rocks near the earth surface are often weathered. During weathering thorium is often freed by the breakdown of minerals and may be remained in Iron (Fe) or Titanium (Ti) oxides/hydroxides and with clays. Uranium is a reactive metal and easily removed from the origin places. It may be present in rocks as oxide and silicate minerals, uraninite and uranothorite; as trace amounts in other minerals or along grain boundaries possibly as uranium oxides or silicates (Kearey et al., 2002; Milsom, 2003).

The radiometric method is one of the most cost-effective and rapid techniques for geochemical mapping based on the distribution of the radioactive elements: potassium, uranium  and  thorium.     The  method  is  very  useful  for  geological  mapping  and exploration of other types of economic minerals. It is also applied in geochemical and environmental monitoring such as the detection of radioactive contamination as a result of  nuclear  accidents  and  environmental  pollution.    

It  allows  the  interpretation  of regional features over large areas, and applicable in several fields of science (IAEA, 1991; 2003). They may be used to estimate and assess the terrestrial radiation dose to the  human  population  and  to  identify  areas  of  potential  natural  radiation  hazard. Regional   surveys   also   provide   a   baseline   data   set   against   which   man-made contamination can be estimated. The airborne radiometric datasets can also provide detailed information about the characteristics of the soil and its parent rocks, including surface texture, weathering, leaching, soil depth, moisture, and clay mineralogy (Bierwirth, 1997).The airborne radiometric data may be less reliable in urban areas because a significant proportion of the ground area is covered with buildings and/or asphalt paving, and the flight altitude is approximately 240 m. As a result of that, the number of gamma ray from the Earth   that reach to the detector resulting from the measured concentrations of eU, eTh and K for the urban area are very low (Appleton et al., 2008).

In this study, aeromagnetic and radiometric datasets of the study area were acquired, processed and interpreted and areas with high potentials of minerals were delineated.

The Total Magnetic Intensity (TMI) map of the study area was produced and interpreted to indicate regions of high and low magnetic susceptibility. The aeromagnetic were passed through various filtering methods and the results were used to delineate geology structures like faults, folds and veins which are possible hosts of solid minerals.

On the other  hand,  the  radiometric data were  used  to  produce the Potassium  (K), Uranium (U) and Thorium (Th) concentration map to delineate the regions of highs and low concentration of these elements. Also radiometric ratio maps (K/U, Th/U, Th/K and U/K) were produced and these were compared and related to indicate areas where there are veins. Finally a Ternary map was produced by merging the data of potassium (red), Thorium (green) and Uranium (blue) and correlating it with geology map of the area. The Ternary map gives the picture of the concentration of the elements combined together.

1.2       Statement of the Research Problem

The dwindling and fluctuating price of crude oil is biting hard on the Nigeria economy. Therefore, there is a great need to diversify and explore the solid mineral sector which can help boost the Gross Domestic Product (GDP) of the country.

The problem this work tries to solve is to use the aeromagnetic and aeroradiometric data to interprete possible geology structures (fractures, faults, folds and shear zones) that could be possible host of gold deposits and evaluate the exact position where these minerals are available.

1.3       Aim and Objectives of the Study

The aim of this study is to identify potential regional structures with possible solid minerals (gold) deposits in part of Nasarawa state through the use of aeromagnetic and aero-radiometric data interpretations.

Objectives

The objectives of the study are to:

1.   Compare the residual magnetic intensity map with the geologic map to reveal major geological features, magnetic trend and susceptibility variance.

2.   Delineate  geological  structures  and  lineaments  within  the  study  area  by computing the analytical signal, horizontal and vertical derivative within the field.

3.    Delineate regions of hydrothermal alteration from ternary map and ratio maps and to correlate the structures obtained from first vertical derivative map with that obtained from the ternary map and geology maps.

1.4   Justification of the Study

There is an agitation for economic restructuring of Nigeria which suggests that every state should manage a large chunk of their resources. Nigeria is blessed with abundant mineral resources that if well harnessed, every part of the country will develop its resources  and  be  self-sustaining,  thereby  reducing  these  agitations  to  the  barest minimum hence the need for this study.

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