research article indian journal of...

© 2017 Discovery Publication. All Rights Reserved. www.discoveryjournals.com OPEN ACCESS

ARTICLE

Page

469

RESEARCH

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 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 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 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

RESEARCH 24(93), July - September, 2017

Indian Journal of Science ISSN 2319–7730

EISSN 2319–7749


ARTICLE

Page

470

RESEARCH

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 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.

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.


ARTICLE

Page

471

RESEARCH

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 latitudes 7o43l and 7o45l, and longitudes 5o81l and 5o83l 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 Source: AFOLABI 2012


ARTICLE

Page

472

RESEARCH

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 metasediments” (McCurry, 1976). Rocks recognized in the area are described (Fig 3.) Grey gneiss widely distributed in the area. Its colour varies from light to dark grey and it is usually of medium to coarse grained with phaneritic texture. This rock contains 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-


ARTICLE

Page

473

RESEARCH

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.

Figure 3 Geological Map of Ondo State Showing the Study Area Source: Adapted from the Nigeria Geological Survey Agency, 2006

STUDY AREA STUDY AREA


ARTICLE

Page

474

RESEARCH

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 Trimming of the chips Grinding of rock chips to smoothing surface using a grinding machine Lapping using different grades of carborundum on each plate until a smooth surface is achieved. 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 7024’3490’’N - 70241’2124’’N and longitude 5041’0675’’E – 5040’8815’’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 arealies within latitude 7028.’8523’’N-7028.’7122’’N and longitude5044.’5432’’E -5044’6238’’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 3, measurements were carried Over and across Granite Gneissalong three traverses lies within latitude 7037’40.20’’N - 7038’26.33’’N and longitude 5056’11.98’’E - 5056’20.45’’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 expected magnetic field of the area or to any positive value or zero. 2. Filtering processing using reduction to poles, vertical derivatives (2nd order), upward and downward continuations, susceptibilities, power spectrum.


ARTICLE

Page

475

RESEARCH

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:

퐹(휇,푉) = ∬푓(푥, 푦). 푒 ( ) 푑푥푑푦

And its reciprocal relation is given as:

푓(푥, 푦) = ∬퐹(푢, 푣). 푒 ( ) 푑휇푑푣

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” 푓x and 푓y (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 of infinite depth and strike extent (MontajTM 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 (휃) = (1/푠푖푛퐼 + 푖푐표푠퐼. 푐표푠(퐷 − 휃))

Where; I Geomagnetic inclination

퐼 Inclination for amplitude correction (never less than I) D Geomagnetic declination Parameter:

퐼 −Inclination to use for the amplitude correction. Default is ± 20. (퐼 = 20), if I > 0;

퐼 = (-20), if I <0). If |퐼 | is specified to be less then |I |, it is set to I.


ARTICLE

Page

476

RESEARCH

Reduction to the pole has an amplitude component (the 푠푖푛퐼 term) and a phase component (the 푖푐표푠퐼. 푐표푠(퐷 − 휃)

term). When reducing to the pole from equatorial latitudes, North- South features can blow-up due to the strong amplitude

correction (the sin(I) term) that is applied when (퐷 − 휃) is 휋/2 (i.e. a magnetic east-west wave number). By specifying higher

latitude for the amplitude correction alone, this problem can be reduced or eliminated at the expense of under-correcting the amplitudes of North- South features. An amplitude inclination of 90 causes only the phase component to be applied to the data (no amplitude correction), and a value of zero causes phase and amplitude corrections to be applied over the entire range.

4. RESULTS DISCUSSION Figure 4 and 5 showed the results of photomicrograph of the various rock types studied under crossed and plane polarized light. The minerals are plagioclase (Na (AlSi3O8- Ca(Al2Si) O8) which is a colourless mineral in thin section. It occurs as plate or lath-shaped section. It is characterized by an imperfect cleavage. Relief is low and birefringence is weak with interference colours which are pale yellow of the first order. Maximum extinction varies and polysynthetic twinning is common in the mineral according to albeit law. Quartz, Q (SiO2) mineral is colourless in thin section and occurred as euhedral prismatic crystals as observed in figure 4 and 5. It is characterized by the absence of cleavage; however cleavage substance showed on the edge of the slide. Relief of the mineral is low and it has a weak birefringence, thus showing as a maximum first order white interference colour. Extinction is parallel in euhedral crystals and symmetrical to cleavage traces. Twining is absent in thin section of the mineral. It is ubiquitous in all the sampled analyzed. Biotite K(Mg,Fe)2(AlSi3)O10(OH)2) ranges from brown, yellowish brown to reddish brown as observed under thin section. The mineral is pleochroic, occurring as a plate and laths showing mineral stretching lineation on a microscopic scale. Cleavage is perfect in one direction. But section cut parallel do not show cleavage. Polysynthetic twining also occurs in some crystals, the mineral occur in minor amount in all the samples. Extinction is usually parallel to the cleavage traces which intersect at 90 degrees. More so, Hornblende (NaCa2(Mg,Fe)4AlSi6Al2O22(OH,F)2) mineral occurs in the rocks. It is pleochroic in various shades of green and brown in plain polarized light, hornblende ranges from yellow to green to dark brown while the opaque minerals (zircon) appear black under plain polarized light and cross nicol as studied under thin section.

a b c

d e f Figure 4 Photomicrograph of Rock Samples Thin Section under crossed nicols Grey gneiss (a-b), Granite gneiss (c-d) and Granite (e-f) respectively Source: AFOLABI 2012

a b c

a b


ARTICLE

Page

477

RESEARCH

d e f

Figure 5 Photomicrograph of Rock Samples Thin Section under Plain Polarized Light (PPL) Grey gneiss (a-b), Granite gneiss (c-d) and Granite (e-f) respectively Source: AFOLABI 2012 Location 1 (Ayegunle) Four traverses were taken at the survey location and they were all inputted into Oasis Montaj datasheet to produce respective maps for the location in order to determine the rock magnetic intensities and susceptibilities.

The shaded colour total magnetic intensity map (TMI) (Figure 6a) over location 1 of Ayegunle area, showed magnetic signature regions (A, and B) of this map is characterized by relative high magnetic intensity values (0.4 nT – 1.7 nT) dominantly seen towards some part of Northwestern, Northeastern and small portion of the Southwestern parts of the study area which could be as a result of presence of Granite-gneiss which is the most widespread rock formation in the area under consideration. These compared favorably well with geologic map on Figure 4 and 5. While the Southwestern, Northwestern and small portion of the northeastern part of the area (C, D and E) with relative low magnetic intensity values (-2.4 nT - 0.1 nT) coincides with the presence of weathered/fractured rocks and (E) the saturated zone/low magnetized geologic material. The NE – SW low amplitude lineament anomaly marked (F – F’) in Figure 4.1a may represent fault or fracture zone.

The study area is characterized by moderate to fairly high magnetic intensities. The fairness is due to the tectonic activities in which the rocks had been subjected to because from the TMI map has proven to show large portion on the maps with intense weathered/fractured/folding relative to portions that are marked by high magnetic intensities (undisturbed/undistorted/non-deformed Gneissic rocks) which correlate with geologic structures seen during the survey exercise.

Figure 6a shows combination signals of regional, residual and noise but with the help of matched filtering, separation of signals was made possible as can be seen from Figure 4.1b, 4.1c, and 4.1d respectively.

a b

f


ARTICLE

Page

478

RESEARCH

c d

Figure 6 Magnetic maps of location 1(a) total magnetic intensity map; (b) downward continuation map (c) second vertical derivative and (d) apparent susceptibility map Source: AFOLABI 2012

Figure 7 3D surface topographic relief map of location 1 Source: AFOLABI 2012

The downward continuation map (Figure 6b) enables the easy observation of short wavelength anomalies not seen in the original total magnetic map, marked A, B, C, D, E, and G. Figure 6a and owing to small size mineralized bodies in the study area.


ARTICLE

Page

479

RESEARCH

Three of the short wavelength high amplitude anomalies A, and B have no similar trend (approximately N-S) with prominent anomalies C, E and G (-1.371 nT – 0.305 nT) identified as Granite- gneiss because of their correlation with the geological map. In Figure 6a, areas with sharp change in magnetic intensity from high amplitude anomaly to low amplitude anomaly 309 exhibited as narrow boundaries are inferred fault zones. The downward continuation map 310 has shown that both low and high magnetic intensity anomaly patterns characterize the study 311 area and that the tectonic framework of the area is contributed mainly by faults.

In Figure 6c, the second vertical derivative map helps to highlight details and breaks in anomaly texture of near surface rocks i.e. subtle, local and short wavelength anomalies are emphasized. The short wavelength anomalies marked A, C, and G on this map is caused by near surface small sized mineralized bodies and/or geological features. The reason why these anomalies are not seen in the original total magnetic ground map is because they have been obscured by stronger effect of broader regional features in the area.

The apparent susceptibility map (Figure 6d) was used to define the magnetization domains within the study area and rock types were inferred based on the range of susceptibility values within these domains i.e. relative high magnetic susceptible zones (0.4211 nT – 0.999 nT) could be inferred as area that encloses the saturated zones/low magnetized geologic material and low

The surface 3D map shows the topographic relief of the location 1, with places that are of high and low topography. The 3D surface relief map (figure 7) has established correlation between the total ground magnetic maps and the geological map of the study area. Locaton 2 (Akungba-akoko) Three traverses were taken at the survey location and they were all inputted into Oasis Montaj datasheet to produce respective maps for the location in order to determine the rock magnetic intensities and susceptibilities.

The shaded colour total magnetic intensity map (TMI) (Figure 8a) over location 2 of Akungba-Akoko area, showed magnetic signature regions (A, and B) of this map is characterized by relative high magnetic intensity values (48.137 nT – 97.074 nT) dominantly seen towards some part of Northern, Eastern and small portion of the Southwestern parts of the study area which could be as a result of presence of Gneissic rocks (Granite-gneiss and Grey-gneiss (Migmatite)) which is the most widespread rock formation in the area under consideration. These compared favorably well with geologic map on Figure 4 and 5. While the Southwestern, Northwestern and small portion of the northeastern part of the area (C, D and E) with relative low magnetic intensity values (-87.008 nT 39.808 nT) coincides with the presence of weathered/fractured rocks and (E) the saturated zone/low magnetized geologic material. The portion marked E is characterized by low amplitude lineament anomaly in Figure 8a which represent fracture zone which can be detected when filter is applied to this map.

a b


ARTICLE

Page

480

RESEARCH

c d

Figure 8 Magnetic maps of location 2 (a) total magnetic intensity map; (b) second vertical derivative (c) downward continuation map and (d) apparent susceptibility map. Source: AFOLABI 2012

The study area is characterized by moderate to fairly high magnetic intensities. The fairness is due to the tectonic activities in which the rocks had been subjected to because from the TMI map has proven to show large portion on the maps with intense weathered/fractured/folding which correlate with geologic structures seen during the survey exercise.

Figure 8a shows combination signals of regional, residual and noise but with the help of matched filtering, separation of signals was made possible as can be seen from Figure 8b, 8c, and 8d respectively.

The downward continuation map (Figure 8b) enables the easy observation of short wavelength anomalies not seen in the original total magnetic map, marked A, B, C, D, E, and G in Figure 8b owing to small size mineralized bodies in the study area. Three of the short wavelength high amplitude anomalies A, and B have similar trend (approximately East–West) with prominent anomalies (32.507 nT – 77.342 nT) identified as Gneissic rocks because of their correlation with the geological map. In Figure 4.2a, areas with sharp change in magnetic intensity from high amplitude anomaly to low amplitude anomaly exhibited as narrow boundaries are inferred fault zones. Five faults were delineated and grouped into two based on their trend i.e. NE – SW group (H – H’, J – J’, K - K’ and N – N’) and NW - SE group (M - M’) While Q – Q’ which is likely to be a deep seated fault that was not seen clearly on the TMI map. The NE – SW faults are observed to correlate with the surface mapped faults shown on the geological map of the area. The downward continuation map has shown that both low and high magnetic intensity anomaly patterns characterize the study area and that the tectonic framework of the area is contributed mainly by faults.

In Figure 8c the second vertical derivative map helps to highlight details and breaks in anomaly texture of near surface rocks i.e. subtle, local and short wavelength anomalies are emphasized. The short wavelength anomalies marked A, B, C, E and G on this map is caused by near surface small sized mineralized bodies and/or geological features. The reason why these anomalies are not seen in the original total magnetic ground map is because they have been obscured by stronger effect of broader regional features in the area. The East –West trending elongated low magnetic intensity anomaly marked (G) at the Southwestern of the map is identified as a fault zone.

The apparent susceptibility map (Figure 8d) was used to define the magnetization domains within the study area and rock types were inferred based on the range of susceptibility value within these domains i.e. relative high magnetic susceptible zones (0.089 nT – 0.145 nT) could be inferred as areas that enclose the saturated zones/low magnetized geologic material and low susceptible zones (-0.005 nT to 0.069 nT) as area with Gneissic rocks. The rocks have appreciable magnetic susceptibilities.


ARTICLE

Page

481

RESEARCH

The surface 3D map shows the topographic relief of the location 2, with places that are of high and low topography. The 3D surface relief map (figure 9) has established correlation between the total ground magnetic maps and the geological map of the study area.

Figure 9 3D surface topographic relief map of location 2 Source: AFOLABI 2012

Figure 10 (a) Total magnetic intensity map and (b) 3D surface topographic relief map of location 3 Source: AFOLABI 2012


ARTICLE

Page

482

RESEARCH

Location 3 (Supare akoko) Three traverses were taken at the survey location and they were all inputted into Oasis Montaj datasheet to produce respective maps for the location in order to determine the rock magnetic intensities and susceptibilities.

The shaded colour total magnetic intensity map (TMI) (Figure 10) over location 3 of Supare- Akoko area, showed magnetic signature regions (A, and B) of this map is characterized by relatively high magnetic intensity values (1.125 nT – 1.421 nT) dominantly seen towards some part of Northern, Eastern and small portion of the Southwestern parts of the study area which could be as a result of presence of Gneissic rocks (Granite-gneiss and Grey-gneiss). While the Southwestern, Northern and small portion of the Eastern part of the area (C, D and E) with low magnetic intensity values (-1.392nT- 0.561nT) coincides with the presence of weathered/fractured rocks and (E) the saturated zone/low magnetized geologic material. The portion marked E is characterized by low amplitude lineament anomaly in Figure 4.5 may represent fracture zone which can be a deep seated fault.

The study area is characterized by moderate to fairly high magnetic intensities. The fairness is due to the tectonic activities in which the rocks had been subjected t; since the TMI map has proven to show large portion on the maps with intense weathered/fractured/folding which correlate with geologic structures seen during the survey exercise.

5. CONCLUSION Based on the results of this geophysical interpretation, the rock magnetic intensities and susceptibilities in the study areas, range of depth to magnetic sources, lithologic contacts and basement structures were established. The maps showed the magnetic signature region, location 1 and 3 showed relative high magnetic intensities. This is as a result of the presence of Granite-gneiss which is the most widespread rock formation in the areas under consideration. Location 2 showed magnetic signature regions characterized by relative high magnetic intensity values (48.137 nT – 97.074 nT) dominantly seen towards some part of Northern, Eastern and small portion of the Southwestern parts of the study area which could be as a result of presence of Gneissic rocks (Granite-gneiss and Grey-gneiss (Migmatite). The study area is characterized by moderate to fairly high magnetic intensities. The magnetic susceptibility results shows that the rock in the study area has been subjected to tectonic activities as proven by the Total magnetic intensity (TMI) map. More so, large portion on the maps with intense weathered/fractured/folding correlate to portions that are marked with high magnetic intensities while the undisturbed/undistorted/non-deformed Gneissic rocks correlate with geologic structures seen during the survey exercise. In the same vein it the photomicrographs revealed various minerals present in rock types (containing quartz, feldspar, and biotite with some orthopyroxene, typically hypersthene) in the study area and those that could have accounted from the formation of magnetic minerals.

The remarkable correlation between the geologic map (Figure 4 and 5) and total magnetic ground maps of the selected locations produced have shown that the geological structures has relative high magnetic intensity, thus showing detail geological information of the study area. These modifications are evidenced in the lithologic boundaries and mapping of more faults, predominantly those with no surface expression.

The magnetic intensities and susceptibility of rocks in the study areas show that the rock is structurally controlled vis-a-viz trending approximately northeast – southwest in the south and east – west in the central area and north-south in the northern zone of the maps, but structurally controlled by two major groups of fault trends which are the NE – SW and the NW – SE, while other zones (N – S, and E – W) are fairly high magnetic intensity which did not contribute to the structural control of the magnetic minerals in the study areas. The number and varying trend of faults is evident that the region has undergone more than one tectonic event.

RREEFFEERREENNCCEE 1. Afolabi, M. 2012. Ground Magnetic prospecting of

Precambrian Basement rocks of Ayegunle-Oka, Supare-Akoko and Akungba-Akoko Southwestern Nigeria. BSc. Project Mat.No: 090406055. Department of earth Sciences, Adekunle Ajsin University, Akungba-Akoko, Nigeria

2. Ajayi, T. R., 1981. Ground Magnetic Studies of Ilesa east Southwest Nigeria; African Journal of Environmental Science & Technology, vol. 4(3) pp 122-131

3. Ajayi, T. R., 2003. Opportunities for the Exploitation of Precious and Rare Metals in Nigeria: Prospect for Investment in Mineral Resources pp: 15-26

4. Ajibade, 1989. Geological Map of Nigeria Showing the area of study, pg, 112.

5. Barraclough, D.R., and Malin, S.R.C., 1971.Synthesis of International Geomagnetic Reference Field Values. Inst. Geol. Sci. Rep.No. 71/1.

6. Dobrin M.B., Savit C.H., 1988. Introduction to Geophysical Prospecting. McGraw-Hill Book Co. Inc. New York. P152-190, 498-578, 691-745

7. Elueze, A.A., 1986. Vertical Components of Ground Magnetic Studies of Ilesha are, Southwest Nigeria: Geological Survey of Nigeria


ARTICLE

Page

483

RESEARCH

8. Folami, S.L., 1991. Gravity and magnetic investigation over marble deposits in the Igara area of Bendel State. J. Min. Geol., 27(1), 49-54

9. Folami, S.L., 1992. Interpretation of aeromagnetic data in Iwaraja area, Southwestern Nigeria: J. Min. Geol 28(2); 391-396.

10. Gibson, T. 1986. Magnetic prospection on prehistoric sites in Western Canada. Geophysics 51:553–560

11. Grant, F. S and West, G.F., 1965. An Interpretation Theory in Applied Geophysics: McGraw-Hill Book Co., Ney York, 179-191.

12. Kayode, J.S., 2006. Structural Trends of Ijeda-Ilokoarea As Interpreted from Total Magnetic Gradient and its geological implications. M. Tech. Thesis, Federal University of Tech. Akure, Nigeria

13. Kayode, J.S., 2009. Vertical components of the ground Magnetic study of Ijebu-jesa, Southwestern Nigeria: Global Journal of Engr. &Tech. 2(3) pp 475-484

14. Kayode, J.S., 2010. Vertical components of the ground Magnetic study of Ijebu-jesa, South western: J. Applied Sci. Research. 2(1) pp 109-120

15. Kearey, P. and Brooks, M., 2002. An Introduction to Geophysical Exploration: Blackwell Science Ltd, Oxford 3rd ed. p. 255

16. Kogbe, C.A., 1976. Review of the sedimentation and structure of Niger Delta: Geology and Mineral Resources of Nigeria; Elizabeth Publishing Co., pp. 41-58, pp. 260-267

17. Li, Y., and Oldenburg, D.W., 1996. 3-D inversion of magnetic data: Geophysics, 61, 394.

18. Milsom, J., 2003. Field Geophysics: The geological field guide series, John Milsom University College, London. Published by John Wiley and Sons Ltd. Third edition. Pp. 51

19. MontajTM Tutorial, (2004): Two - Dimensional frequency domain processing of potential Field data.

20. Musset, A.E, and Khan, M.A., (2000): Looking into the Earth: An Introduction to Geological Geophysics, Cambridge Univ.

21. Nabighian, M.N., Grauch, J.S., Hansen, R.O., LaFehr, T.R., Li Y., Peirce, J.W., Phillips, 2005. The historical development of the magnetic method in exploration.Geophysics, 70: 33ND-61ND.

22. Obot, V.E.D., and Wolfe, P.J., 1981. Ground-Level Magnetic study of Greene County, Ohio, Ohio J. Sci. 81: 50-54.

23. Oyawoye, M. O., 1973. The Basement Complex of Nigeria. African Geol. 1, Ibadan Univ. press. Pp.66-102.

24. Peddie, N.W., 1983. International geomagnetic reference field – its evolution and the difference in total field intensity between new and old models for 1965–1980. Geophysics, 48, 1691–6.

25. Pilkington, M., 1997. 3-D magnetic imaging using conjugate gradients, Geophysics, 62, 1132–1142.

26. Rahaman, M., 1976. A review of the Basement Geology of Southwestern Nigeria. Kogbe, C.A. (ed.), Geol. Nig. Elizabeth Publi.Co. pp. 41-58.

27. Rahaman, M.A., 1988. A Review of the Basement Geology of Southwestern Nigeria; Elizabeth Publishing Co. pp. 41-58.

28. Sawkins and Chase.1989. The Evolving Earth: A Text in Physical Geology. Published by Macmillan Publishing Company, Second edition pp.4 2-84.

29. Sharma, P., 1976. Geophysical Methods in Geology. Elsevier, Amsterdam.

30. Telford, WM., Geldart LP, Sheriff RE, Keys DA., 1976. Applied Geophysics. Cambridge Univ. Press p. 770

31. Terry, and Chilinger, J.S.P., 1955. Chart for visual estimation of percentage of minerals in rocks.

32. Trompat, H., Boschetti, F., Hornby, P., 2003. Improved downward continuation of potential field data, Exploration Geophysics 34, 249–256

33. Weymouth, J.W., 1985. Geophysical surveying of archaeological sites surveying: Archaeological Geology Yale University Press. New Haven and London, pp 191-235

research article indian journal of...

Documents