Indian Journal of Science

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24(93), July - September, 2017

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Indian Journal of Science

ISSN 2319–7730 EISSN 2319–7749

Ground magnetic prospecting of Precambrian basement rocks of Ayegunle-Oka, Supare-Akoko and Akungba-Akoko, Southwestern Nigeria Okpoli Cyril C Department Of Earth Sciences, Adekunle Ajasin University, Akungba-Akoko, Ondo State, Nigeria; E-mail: [email protected] Article History Received: 10 August 2017 Accepted: 13 September 2017 Published: July - September 2017 Citation Okpoli Cyril C. Ground magnetic prospecting of Precambrian basement rocks of Ayegunle-Oka, Supare-Akoko and Akungba-Akoko, Southwestern Nigeria. Indian Journal of Science, 2017, 24(93), 469-483 Publication License This work is licensed under a Creative Commons Attribution 4.0 International License. General Note Article is recommended to print as color digital version in recycled paper.

ABSTRACT Advances in technique development and data interpretation have greatly improved our ability to visualize the subsurface. A Magnetic survey was carried out at Ayegunle Akoko, Supare Akoko and Akungba Akoko in Ondo state Southwestern Nigeria, using a proton precession magnetometer (GSM-19T). A total of ten traverses were established in an SE-NW, E-W, SW/NE direction in the study areas. The acquired magnetic field data were corrected for drift. Qualitative and quantitative interpretations were adopted to obtain negative peak value and the maximum positive peak value. The contour maps, 3-D surface map and the 1-grid vector map

total length of 1700 m at a line spacing of 10 m. The whole area was characterized by complete varying negative amplitudes from a very low peak value of about -0.997-0.084 nT and a maximum positive peak value of about 48.137-97.047 nT respectively. Ayegunle Akoko, Akungba Akoko and Supare Akoko TMI and Apparent susceptibility range: 0.4 nT-1.7 nT, 48.137 nT-97.074 nT, 1.125 nT-1.42 nT and 0.4211 nT-0.999 nT, 0.089 nT-0.145 nT,-1.392 nT-0.561 nT respectively. The closely spaced, linear sub-parallel orientations of © 2017 Discovery Publication. All Rights Reserved. www.discoveryjournals.com

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breadth of 45 m at 5 m spacing; Supare Akoko has a total length of 1500 m at a line spacing of 10 m while Akungba Akoko has a

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present the subsurface image. The survey perimeters of Ayegunle Akoko has a total length of 1800 m at a line spacing of 10m and a

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contours from the South western part of the map suggest the possibility of faults or local fractured zones. . The comparison of rocks and magnetic susceptibility results shows that the rock in the study area has been subjected to tectonic activities, while the photomicrographs revealed various minerals present in rock types (containing quartz, feldspar, and biotite with some orthopyroxene, typically hypersthenes) which accounted for the formation of magnetic minerals. Key Words: Magnetic Anomaly, Magnetic survey, Proton magnetometer, Fractured zone

1. INTRODUCTION It is quite understandable that sufficient knowledge of true magnetization direction of a causative body is vital in order to accurately interpret magnetic data by quantitative methods such as inversion. Most recently available algorithms require the knowledge of magnetization direction, since it is an essential piece of information for carrying out the forward modeling (Li and Oldenburg, 1996; Pilkington, 1997). Such a prerequisite has been the driving force for development of many well recognized approaches for estimating total magnetization when strong remanence or self-demagnetization are present. Example problems to which these methods are routinely applied include the interpretation of magnetic data over ferrous unexploded military ordnance, banded iron formations, nickel deposits, kimberlite pipes, and depth to basement problems. Rocks in the basement complex are Precambrian meta-sediments, Migmatite, gneiss, granites, schists etc. (Sawkins and Chase, 1989). In their natural form where no disturbance has taken place they are non-porous and impermeable. Hence, they cannot store or transmit water. However, alternation can result as the rocks are susceptive to geological processes such as weathering, fracturing and faulting. Ground magnetic prospecting is the oldest method of exploration, it gives information from one can determine depth to basement rocks and locate and define the extent of sedimentary basins, such information is of particular value in previously unexplored areas such as continental shelves, and can be used to map topographic features on the basement surface that might influence the structure of overlying sediments (Telford, 1990). The magnetic method is a geophysical technique that measures variations in the earth’s magnetic field to determine the location of subsurface features. This non destructive technique has numerous applications in engineering and environmental studies, including the location of voids, near-surface faults, igneous dikes, and buried ferromagnetic objects like storage drums, pipes etc. (Weymouth, 1985). In recent years, geophysical methods have been employed in various applications. These include engineering applications such as assessment of road failure, pipe leakages, ground-water contamination and assessment of construction sites for dams and bridges (Musset and Khan, 2000). Because of the nature of the random test-pitting method generally employed by archaeologists which is both tedious and inaccurate, the need has arisen for the introduction of better techniques which could help in the location of these materials (Audah and Okpokob 1994; Gibson, 1986). Magnetic method measures variation in the Earth’s magnetic field caused by changes in the subsurface geological structure or the differences in near-surface rocks’ magnetic properties (Telford, 1990). Magnetic surveys can be performed on land, at sea and in the air. Consequently, the technique is widely employed, and the speed of operation of airborne surveys makes the method very attractive in the search for types of ore deposit that contain magnetic minerals (Kearey et al., 2005). In further search for magnetic minerals in Southwest Nigeria, a ground magnetic survey carried out in Orile-Ilugun, shows that the depth to center and top of magnetic bodies are shallow and exhibit positive and negative magnetic susceptibilities indicating that paramagnetic and diamagnetic minerals are present. Previous studies have also shown that this region is underlain by Precambrian rocks typical of the basement complex of Nigeria (Rahaman, 1976, Kogbe, 1976). The main rock types found in the region are amphibolites complex, schist, quartzite and quartz schist (Kayode, 2006; Ajayi, 2003,; Folami, 1992; Folami, 1991; Ajayi, 1981a; Elueze, 1986; Kayode, 2009; Kayode, 2010). In further search for magnetic minerals in Southwest Nigeria, a ground magnetic survey carried out in OrileIlugun, shows that

In this paper, we present results ground magnetic surveys data that has been inverted quantitatively in estimating the subsurface geology on the basis of the anomalies in the earth's magnetic field resulting from the magnetic properties of the underlying rocks. We likewise demonstrate photomicrograph of various rock types in the surveyed area. © 2017 Discovery Publication. All Rights Reserved. www.discoveryjournals.com

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that paramagnetic and diamagnetic minerals are present.

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the depth to center and top of magnetic bodies are shallow and exhibit positive and negative magnetic susceptibilities indicating

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2. DESCRIPTION OF THE STUDY AREAS The study area is located in Ayegunle- Akoko (Stone World Mining Limited), Supare-Akoko (Abandon Mine) and Akungba- Akoko (Adekunle Ajasin University Campus) all in Akoko South West Local Government Area of Ondo State Nigeria. The area lies between o

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o

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o

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latitudes 7 43 and 7 45 , and longitudes 5 81 and 5 83 in the Southwestern part of Nigeria. Outcrops of rocks observed in the study area are basically granite gneisses which are of the Pan African time. Topographically, the area is characterized by a relatively rugged, undulating, topography with outcrops of charnockites, migmatite gneiss with other gneissic rocks as highlands which range between 600 and 1500feet above sea level. It is situated within the Precambrian Basement Complex with the outcrops which are predominantly gneiss and migmatite. Study areas are easily accessible with massive and well exposed Precambrian basement rocks, but the road linking the mining industries (Stone World Mining Limited) with the main road were not tarred, residential settlements are also at a distant of about 300m away from the mining industries. A typical feature (i.e. landmark) found in the area of study is a farm house by the road side at approximately 560m away from the mining industries(Stone World Mining Limited), also at the abandon mine in Supare-Akoko, there is a palm oil factory at approximately 200m from the mining industries. The third location is the main campus of Adekule Ajasin University Akungba-Akoko (AAUA). A typical feature found around the area includes building approximately 400m from point of survey.

Figure 1 Location Map of the Study Areas

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Source: AFOLABI 2012

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Figure 2 Map of Nigeria showing the Study Areas and Location Source: AFOLABI 2012 The area under study is situated in the deciduous rain forest area within south-western Nigeria. It has evergreen vegetation and urban settlement. The vegetation of this area reflects the rainforest and Guinea Savannah’s vegetation, which is characterized by different plants and trees which may reach a height of 5 m and even more. They consist of light forests, shrubs and scattered cultivation. There are areas where rocks are covered by vegetation which is also an indication of the porosity of the rocks and function of the grain size. Trees and plants like timber, oil palm, kolanut, rubber, cocoa and citrus are very prominent in these areas. A high forest zone is found in the north while the southern part is mostly Sub-Savannah due to the farming activities in the area, which had actually reduced the thickly vegetated area. The climate can be said to be subequatorial with two peaks of rainfall. The first peak comes up between April and July while the second peak comes up between late August and late October. These two peaks are marked by heavy rainfall and the mean annual rainfall is 1500 – 2000 mm with a relative humidity of about 75 - 95%. Since the climate is sub – equatorial, temperature could sometimes be severe. The mean annual temperature is 23 - 26°C. The topography of the study area is high with undulating relief of about 200m to more than 1500m high. There are knolls, ridges and flat lying exposures observed in the area. The western part is characterized by a conical hill relief – outcrops of batholiths is scattered in the southern part of the area towards Etioro and Ayegunle. Eastward, the area is featured by lowland outcrops and sparsely characterized with gneissic ridges. The major road runs from southwest to northwest. The study areas are underlain by Migmatite gneiss, according to Rahaman (1976, 1988). The Migmatite-Gneiss-Quartzite complex is a heterogeneous rock group compromising Quartzofeldspathic gneisses and Migmatites with high-grade supracustal relies of basic and calcareous schist, marbles and quartzites termed “Ancient metasediments” (Oyawoye, 1964: 1973) or “Older

mineralogical, banding of quarts, feldspar, biotite and sometimes garnets. Granite gneiss is foliated mineralogically, it contains quarts, feldspar, and biotite with intrusions of pegmatite and quarts feldspathic veins.Granite Gneiss in AkungbaAkoko is part of the older granite suites in Nigeria. The granites have phenocryst and one matrix 130 supported and the phenocryst made up of k© 2017 Discovery Publication. All Rights Reserved. www.discoveryjournals.com

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colour varies from light to dark grey and it is usually of medium to coarse grained with phaneritic texture. This rock contains

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metasediments” (McCurry, 1976). Rocks recognized in the area are described (Fig 3.) Grey gneiss widely distributed in the area. Its

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feldspar while the matrix is dominated by quartz. charnokite is pyroxene bearing granite which occur as boulders with a weak foliation and usually coarsely grained. It consists of k-feldspar, plagioclase, hornblende and biotite. It is an intrusive Migmatite rock, which shows granuloblastic texture. Charnockite has a dark greenish in colour. Diorite is an igneous rock, which is not widely distributed like the major rock type. It is very dark grey in colour and it is medium grained. The nature of the exposure is mostly boulders and consists of quartz hornblende biotite, feldspar and amphibole. The minerals are randomly oriented. The boundary nature is as a result of fracturing. This rock contains mineralogical, Biotite, Pyroxene, Plagioclase, and Orthoclase feldspar, also Quartz. Quartzo-felspathic and pegmatitic intrusions are widely distributed in all the rocks. It belongs to the younger generation and it is very coarse grained consisting Alkali feldspar and quartz.

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STUDY STUDY AREA AREA

Figure 3 Geological Map of Ondo State Showing the Study Area


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Source: Adapted from the Nigeria Geological Survey Agency, 2006

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3. MATERIALS AND METHODS 3.1. Thin Sectioning and Microscopic Study We adopted the following steps in preparation for the petrographic thin section: 

Cutting rock samples into chips using rock cutter

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Trimming of the chips

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Grinding of rock chips to smoothing surface using a grinding machine

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Lapping using different grades of carborundum on each plate until a smooth surface is achieved.

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Mounting, which involves the placing of then smoothened surface on glass slides after lapping araldite which has been thoroughly mixed, hence, acting as the mounting media.

We went ahead to mount the thin section under plane and crossed (Nicol) polarized light, which involved: modal counts and naming respective minerals seen under the microscope using two different viewing potentials under a polarizing microscope in the laboratory. The minerals were identified using their optical properties such as colour, form, relief, cleavage, birefringence, pleochroism, extinction, and twinning. Modal analysis was done using the visual estimation of the proportions of the minerals in rocks through a chart produced by Terry and Chilinger (1955). 3.2. Magnetic Data Acquisition and Processing The magnetic measurements in the survey area were made with a GSM-19T Proton Precession magnetometer, the equipment measures three components which are vertical, horizontal and total magnetic intensities in gamma (nanotesla). Magnetic profiling was carried out at three locations over and across buried granite-gneiss (location 1), over and cross granite-gneiss/chanockite gneiss (location 2), and granite gneissic (location 3). The areas were being gridded both horizontally and vertically with a spacing of 10m for both forward, backward, upward and downward readings respectively. Observations were made along series of traverses at three locations at equal spacing with a base station carefully selected, where the magnetic intensities are being measured at a stationary point. The observed magnetic field data was corrected for diurnal variations and offset by subtracting base station regional magnetic field from magnetic field measurements taken along the traverse at synchronized times. The corrected magnetic data were used for preparing the residual maps. At location 1, measurements were carried out across and over Granite along four traverses, the area lies within latitude 0

0

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7 24’3490’’N - 7 241’2124’’N and longitude 5 41’0675’’E – 5 40’8815’’E covering a distance of 1800m respectively, these were covered for both forward and backward at 10m spacing between them which amount to a total distance of 1.8km. At Location 2, measurement were carried out Over and across Granite Gneiss/Grey Gneiss and Charnockite gneiss along three traverses, the 0

0

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arealies within latitude 7 28.’8523’’N-7 28.’7122’’N and longitude5 44.’5432’’E -5 44’6238’’E. The forward and backward horizontal traverses of three (3) traverses measurement covered a distance of 1500m (1.5km) and with spacing of 10m respectively. At location 0

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3, measurements were carried Over and across Granite Gneissalong three traverses lies within latitude 7 37’40.20’’N - 7 38’26.33’’N 0

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and longitude 5 56’11.98’’E - 5 56’20.45’’E ,covering a 184 distance of 800m respectively, these were covered for both forward and backward at 20m 185 spacing between them which amount to a total distance of 1700m. The magnetic data collected in the study area were processed so as to prepare the dataset for interpretations. We applied the following processing steps to magnetic data: Diurnal correction due to diurnal variations was applied. The diurnal correction removes the diurnal variations from the mobile data so the anomalies can be better appreciated. Diurnal correction data is obtained by combining the readings of a mobile unit with the readings of a base station unit. The diurnal correction is obtained from the following equation: Correction field = mobile field – base field + datum This correction is done on the magnetometer. The datum is just a positive shift of the corrected data, it is set to the average or

nd

2. Filtering processing using reduction to poles, vertical derivatives (2

order), upward and downward continuations, susceptibilities,

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power spectrum.

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expected magnetic field of the area or to any positive value or zero.


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The desired improvements on the quality of the magnetic data were achieved by the application of two dimensional Fast Fourier transform filters i.e. reduction to poles, upward and downward continuation filters, vertical derivative filter and magnetic susceptibility filter. Mathematically according to Dobrin and Savit (1988), the Fourier transform of a space domain function f(x,y) is defined as: (

( , ) = ∬ ( , ).

)

And its reciprocal relation is given as:

( , ) = ∬ ( , ).

(

)

Where μ and v are wave numbers in the x and y directions respectively, measured in radians per meter i.e. when x and y are in units of meter these relates to spatial “frequencies” x and

y (in cycles per meter).

3.2.1. The upward continuation filter Operation allows the transformation of data measured on one surface to some higher surface (Nabighian et al, 2005) and tends to smooth the original data by attenuating short-wavelength anomalies relative to their long – wavelength counterparts. 3.2.2. The downward continuation filter According to Trompat et al, 2003, when applied to potential field data brings the observation surface closer to the source therefore enhancing the responses from sources at depth. 3.2.3. The second vertical derivative filter This is applied to the magnetic data to enhance local anomalies obscured by broader regional trends. It accentuates short wavelength components of the anomaly field, while deemphasizing long-wavelength components. 3.2.4. The susceptibility filter It computes the apparent magnetic susceptibility of magnetic sources with certain assumption applied viz: (i) that the IGRF has been removed from the data (ii) no remanent magnetization and (iii) that all magnetic response is caused by a collection of vertical prisms TM

of infinite depth and strike extent (Montaj

Tutorial, 2004).

3.2.5. Reduction to magnetic pole Reduction to the pole is used in low magnetic latitudes to change an anomaly to its equivalent as would be observed at the north magnetic pole. This transformation simplifies the interpretation and visualization of anomalies from low magnetic latitudes. The reduction to pole is; L(

) = (1/

+

.

( − ))

Where; I

Geomagnetic inclination Inclination for amplitude correction (never less than I)

D

Geomagnetic declination

− Inclination to use for the amplitude correction. Default is ± 20. (

= 20), if I > 0;

| is specified to be less then |I |, it is set to I.


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= (-20), if I