DELINEATION OF STRUCTURES FOR SOLID MINERALS WITHIN KUBIL (SHEET 128) AND WAWA (SHEET 159) NORTH CENTRAL NIGERIA FROM AEROMAGNETIC DATA

ABSTRACT

To resolve the problem of artisanal miners using trials and error method to locate solid minerals, a geophysical technique was employed to delineate structures that host mineral in the study area. The study focused on both (Qualitative and Quantitative) analyses of high resolution Aeromagnetic data to delineate the geological structures that could serve as host to mineral within Wawa (Sheet 128) and Kubil (Sheet 159) Niger State, Nigeria. The area is bounded by Latitude 4º00′ and4º30E’ to Longitude 9º30′ and 10º30’N. The aeromagnetic data was subjected to various filtering method such as generating of Total Magnetic Intensity Map, Analytical Signal, First Vertical Derivatives and Center for Exploration Targeting (CET). The total magnetic intensity map comprises of both positive and negative anomalies with magnetic values within the study area ranges from -66.589 nT to 129.237 nT. Result of analytical signal depicts high amplitude response of magnetic anomalies ranges from 0.232 to 0.355 cycles in regions of shallow magnetic intrusive rocks and low amplitude response of magnetic anomalies ranges from 0.010 to 0.218 cycles in regions of relatively thick sedimentation. The first vertical derivative helped to place both low and high magnetic lineaments which are of interest that could serve as gold veins. These were located around latitude 9o50′ to 10º10’N within Yangari, Lasun Sarabe, Wawa Malete town down to Doro across rivers Yakumosin. The major lineament are mapped as F1 to F7 on the first vertical derivatives. Similar structures were observed from the Centre for exploration targeting (CET) grid analysis only that they are more detailed. The mapped regions on the first vertical derivatives (FVD) hosting major lineaments are the potential minerised zone. Ground trothing should be conducted by relevant geological agency at zones of minerisation to confirm the mineral type and their relative abundance. Resistivity survey is recommended for the delineated regions of minerisation for a more detailed capturing of delineated structures since the current research is of a regional scale.

CHAPTER ONE

1.0       INTRODUCTION

1.1       Background to the Study

Geophysics is a very potent and vital tool of exploration and consistently used in detail surveys. There are a lot of geophysical survey methods which include gravity, magnetic, radiometric, seismic, electrical resistivity etc. Each  of the above survey method has a unique operative physical property like density, magnetic susceptibility, radioactivity, propagation or velocity of seismic waves, electrical conductivity etc. of the Earth (Kearey et al., 2002). These methods had been used to investigate the subsurface geology of an area of interest. Some of these methods can still be applied by flying the geophysical equipment namely magnetic, electromagnetic, radiometric and gravity. Airborne geophysics is an effective way for surveying a very large area quickly for regional exploration. (Kearey et al., 2002).

Aeromagnetic  survey  is  the  frequent  type  of  airborne  geophysical  survey  and  has  been recognised as a principal mapping tool for materials that are strongly magnetised (Murthy, 2007). Magnetic method seeks to probe the geology of the particular area due to the differences in the susceptibility of the field. These differences are as a result of the magnetic features of the rocks subsurface (Kearey et al., 2002).The most vital magnetic minerals in soils are the iron oxides, such as magnetite. In soils the main source of magnetic minerals is the parent material through the soil formation processes. A general practice to identify the existence and concentration of magnetic minerals is the amount of the magnetic susceptibility in soils. Soil magnetic susceptibility can be related to different terrain topographic attributes such as the slope, elevation and concavity-convexity of the surface terrain to explain the distribution of magnetic minerals within soils (Rowland & Ahmed, 2018), Based on geomagnetism, the earth may be considered as made up of three parts: core, mantle and crust. Convection processes in the liquid part of the iron core give rise to a dipolar geomagnetic field that resembles that of a large bar- magnet aligned approximately along the earth’s axis of rotation. The mantle has little effect on earth’s magnetism, while interaction of the geomagnetic field with the rocks of the Earth’s crust produces the magnetic anomalies recorded in detailed surveys carried out close to the earth’s surface (Reeves, 2005). Thus an anomaly is created when the earth’s magnetic field is disturbed by an object that can be magnetized. Survey data is interpreted based on the assumption that magnetic  sources  must  lie  below  the  base  of  the  sedimentary sequence.  This  allows  rapid identification of hidden sedimentary basins in mineral exploration. The thickness of the sedimentary sequence may be mapped by systematically determining the depths of the magnetic sources over the survey area. Depths to the magnetic basement are very useful in basin modeling such as determination of source rock volume and source rock burial depth. The identification and mapping of geometry, scale and nature of basement structures is critical in understanding the influence of basement during rift development, basin evolution and subsequent basin inversion. From regional aeromagnetic data sets, information such as tectonic frame of the upper crust can be obtained. The patterns and amplitude of anomalies reflect the depth and magnetic character of crystalline basement, the distribution and volume of intrusive and extrusive volcanic rocks and the nature of boundaries between magnetic terrains.

According to reports (2018) by the Nigeria Geological Survey Agency (NGSA), Nigeria as a nation has some 34 known major minerals resources distributed in 450 different locations across the country and given more invaluable attraction for investors. Minerals exploration for several solid minerals, e.g. tin, niobium, lead, zinc and gold, goes back for more than four decades but only tin and niobium production have rated on a world-wide scale (Rowland & Ahmed, 2018).

While the major international exploration groups have paid more than interest, there has been general exploration carried out by the tin mining groups and since the mid-1970s by several organisations and in particular the Nigerian Mining Corporation. Throughout its long history the Nigeria Geological Survey Agency had played an invaluable role in the exploration for mineral deposits many of which have been first presented by its officers. Presently, there is a great interest in the development of solid mineral resources whose production in the last three decades has been depreciating in every case. The privatisation, commercialisation and general reform exercises currently being undertaken by the Federal Republic of Nigeria are expected to lead to a positive change in the exploration, growth and development of Nigeria’s solid mineral resources (Rowland & Ahmed, 2018).

1.1.1    Magnetism of the earth

The Earth may be divided into three parts i.e crust, mantle and core. The core of the Earth may also divide into two parts that is the molten outer core and the solid inner core. The core of the Earth is the main provider of heat energy in the Earth. Inglis (1955) pointed out that, it is impossible at present for determine the types of convectional motion in the molten core. Reeve (2010) indicated that the movement of the charged electric particles within the molten core produces a magnetic field around the Earth after several theoretical and experimental studies. This magnetic field enveloping the Earth give rise to the magnetic features of the various rocks found within or on the surface of the Earth. The flow of these electrical charges successfully creates a huge electromagnet (Clark and Emerson, 1991).

1.1.2    Nature of the geomagnetic field

The Earth’s magnetic field within or at the surface of the Earth is produced from the molten outer core (Rivas, 2009). The Earth’s magnetic field is made up of three parts (Telford et al.,1990) namely;

a.   The major field, which differs comparatively gradually and originates within the Earth.

b.   The minor field  (compared  to  the major  field),  which  differs  rather quickly and  of external origin.

c.   The spatial variation of the major field which are usually lesser than the major field, are almost the same in time and place, and are brought by local magnetic anomalies within the Earth’s crust. These are the areas of interest in magnetic surveying.

The compass needle aligns itself in the direction of magnetic field of the Earth when hanged freely at any position on the surface of the Earth. This alignment creates an angle between the magnetic and geographic north (Kearey et al., 2002).

Almost 90% of the geomagnetic field can be characterized by the field of a theoretical magnetic dipole at the Earth’s centre subtended at an angle of about 11.5° to the rotation axis (Kearey et al., 2002)

1.1.3    The earth’s magnetic field

If an unmagnetized steel needle could be hung at its centre of gravity, so that it is free to orient itself in any direction, and if other magnetic fields are absent, it would assume the course of the total geomagnetic field, a direction which is usually neither horizontal nor in line with the geographic meridian (Telford et al., 1990).

The compass needle direction when hanged freely at any position of the Earth gives the direction of the geomagnetic field. The direction can be specified in terms of declination, the  angle between the true north and the horizontal and the direction of the total field. The field magnitude is proportional to the maximum torque exerted on the compass needle by the field. The geomagnetic field to which the compass needle is reacting seems to be caused by complex interactions between the Earths hot, liquid, metal outer core as it rotates and convection with it, generating circular current at the core-mantle boundary. The Earth’s magnetic field sources vary in nature and place. The dynamo action of the molten core produces the most extreme field recorded at the equator is in the range 30,000 nT whiles the poles record a range of 60,000 nT (Kono and Schubert, 2007).

Whitham, (1960) indicated that the magnetic elements are illustrated in (Figure 1.1.) below The declination ‘D’ is taken positive or negative depending on the deviation east or west of the geographic north. So declination can be defined as the angle the geographic north makes with the magnetic north of the Earth. The Fig. below in which the magnetic field is vertical plane, it is passing through the total magnetic force “T”. Hence, the magnetic field of the Earth at every position on the surface of the Earth „V‟ and “H” are vertical and horizontal components of “F”.

The universe of classical electrodynamics begins with a vacuum containing matter solely in the form of electric charges, possibly in motion, and electric and magnetic fields. We can detect the presence  of  these  fields  by the  forces  they exert  on  a  moving  point  charge  q.  Maxwell‟s equations provide the curl and divergence of the electric fields and magnetic fields in terms of other things. The universe we are operating in comprises an infinite vacuum containing electrical charges, represented by a local density ρ, which may be moving, and hence generating electric current, represented by a local current density J. The equations in vector form are: where is measured in (coulomb/m3), J is also measured in (ampere/m3), is permeability of vaccum (4π x 10-7 henries/m), is capacitivity of vaccum (107/ 4πc2 farads/m), B is in teslas and E is in volt/m

1.1.5    Magnetism of rocks and minerals

Most  rock-forming minerals  are non-magnetic.  Only a few magnetic  minerals,  that  include magnetite (Fe3O4), ilmenite (FeTiO3) and pyrrhotite (FeS), significantly affect the magnetization field of the particular area. Magnetic rocks contain these minerals, usually in small quantities. Because subsurface temperatures increase with depth, substantial magnetization can occur only above certain depths. In areas with relatively high geothermal gradients, the maximum depth of magnetization is shallower than it is in areas with lower geothermal gradients. Most sedimentary rocks contain negligible quantities of magnetic minerals, and are therefore non-magnetic. Most basic igneous rocks, on the other hand, have high magnetic susceptibilities, while acid igneous rocks and metamorphic rocks can have susceptibilities ranging from negligible to extremely high (Reeves, 1989; Petersen, 1990; Reynolds, 1997).

Below the Curie temperature is when the magnetic features of rocks can exist. This temperature varies for different rock types but ranges from 5500C to 600oC. Present day geothermal studies have indicated that Curie point can be reached at depths 30 to 40 Km beneath the Earth. Based on these assumptions, it is estimated that all crustal rocks are very potent to carry magnetic features. Reeves (1989) suggested that the upper part of mantle has no magnetic properties hence the bottom of the Earth’s crust may be effective depth where magnetic sources can be found. Magnetic materials can be grouped on the basis of their behavior when placed in an external field (Telford et al., 1990). Many materials have equal numbers of electrons spinning and orbiting in opposite direction so that, in absence of some external magnetic field, their effects cancel out. If a magnetic field is applied, the electron orbits are very slightly disturbed  electromagnetism induction. This very slightly weakens the field inside the material giving a minute magnetic effect  called  diamagnetism.  Diamagnetism  is  independent  of  the  temperature.  Diamagnetic materials include quartz, feldspar, calcite, graphite, salt. The diamagnetic substances have negative magnetic susceptibilities (Reynolds, 1997).

The paramagnetic materials have unbalanced electrons so that the individual atoms or molecules act like very tiny magnets. In the absence of an external magnetic field these molecular magnets are arranged at random, giving no resultant magnetic effect to the material as a whole but if a magnetic  field  is  applied  the  molecular  magnetic  becomes  partially  aligned  with  it  thus increasing its strength. This is small effect is called paramagnetism. With paramagnetic materials which have positive values of magnetic susceptibilities. The total magnetic intensity will be bigger than the original magnetic field. Examples of such materials include pyroxene, olivine, pyrite, and biotite (Reynolds, 1997).

Strong paramagnetic materials such as iron, nickel and cobalt are said to be ferromagnetic. With ferromagnetic materials, there is almost a perfect arrangement of their domains. Ferromagnetic materials have all their magnetic dipoles aligned hence there is a magnetization of high effect being produced. With anti-ferromagnetic materials such as hematite its adjacent magnetic dipoles are  opposite  to  the  direction  of  magnetization  hence  produces  zero  magnetization  effect. Materials such as magnetite, ilmenite show very strong magnetization effect and its domains align themselves in the direction of applied external field (Reynolds, 1997).

1.1.6    Magnetic susceptibility

The degree to which a material can be magnetized in an applied external field is a physical parameter known as magnetic susceptibility (Dalan, 2006). In geology, magnetic susceptibility is one characteristic of a mineral type. Its measurement gives us information about the type and quantity of minerals present in the sample. The measure of magnetization is solely characterized by the amount and composition (shape and size) of iron oxide in the rocks (Dearing, 1994;

Wemegah  et  al.,  2009).The  magnetic  susceptibility  effectively  is  the  magnetization  effect divided by the applied magnetic field. If the magnetic field is H (A/m) and the magnetization is M (A/m), the magnetic susceptibility is к =Xv = ��⁄𝐻 (2.3) Where Xv is volume of susceptibility Although susceptibility has no units, to rationalize its numerical value to be compatible with the SI or rationalized system of units, the value in c.g.s. equivalent units should be multiplied by (Clark, 1997).

Reynolds (1997) indicates that most sedimentary rocks contain negligible quantities of magnetic minerals, and are therefore non-magnetic. Most basic igneous rocks, on the other hand, have high magnetic susceptibilities, while acid igneous rocks and metamorphic rocks can have susceptibilities ranging from negligible to extremely high. Magnetic susceptibility is a trace parameter of rocks, because the percentage of magnetic minerals is usually one percent or less, even in basic igneous rocks. Slight differences in iron oxide content of a mineral can cause large magnetic susceptibility variations. Remke et al. (2004) also pointed out that the amount of iron oxides in rocks are influenced by the parent rock, age of rock and weathering processes.

1.2       Statement of the Research Problem

Artisanal miners have engaged themselves by wildcat chase for gold, gemstones and other solid minerals digging and destroying places because they cannot really detect the exact locations where those solid minerals are, which have yielded low productivity. Hence there is a need for application of aeromagnetic data analysis (a geophysical method for revealing the subsurface) to determine accurate location of the potential minerals which would enhance better mining operation in the study area.

1.3       Aim and Objectives of the Study

The aim of this research work is to delineate geological structures that could host solid minerals such as gold and gemstone within the study area using the aeromagnetic data interpretation.

The Objectives of the Study are to:

i.   generate the Total Magnetic Intensity (TMI) Map of the study area from gridded data; ii.  extract derivatives, Centre for exploration targeting (CET) and analytical signal maps; iii. correlate delineated zone favorable to mineral accumulation in the study area.

1.4       Justification of the Study

Aeromagnetic methods are very effective in environmental monitoring and geological mapping. This work targets the structures that host solid minerals such as gold and gemstones that exist within the study area and delineate their precise location in terms  of coordinate. With this database, knowledge of prospective investors in minerals exploration will greatly be enhanced and exploration activities be made easier.

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