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Structural study of Wamba and Environs, north-central Nigeria using aeromagnetic data and NigeriaSat-X image
Journal of African Earth Sciences 111 (2015) 307e321
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Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci
Structural study of Wamba and Environs, north-central Nigeria using aeromagnetic data and NigeriaSat-X image J.K. Ogunmola a, *, E.N. Gajere a, E.A. Ayolabi b, S.B. Olobaniyi b, D.N. Jeb a, I.J. Agene a a b
National Centre For Remote Sensing, P.M.B 2136, Jos, Plateau State, Nigeria Department of Geosciences, University of Lagos, Nigeria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2014 Received in revised form 16 April 2015 Accepted 7 July 2015 Available online 20 August 2015
Wamba 1:100,000 sheet 210 covers Wamba and Nassarawa Eggon area of North-Central Nigeria and consists of basement rocks, biotite granites and Older Granites in most parts of the northern part and by sedimentary rocks of the Cretaceous Middle Benue Trough in the southern part. High resolution aeromagnetic data was interpreted and the results integrated in a GIS environment with data from NigeriaSat-X image to map out the major structural trends within the area. Reduction-to-the-equator (RTE) operation was carried out on the aeromagnetic data after which several data transforms/derivatives such as horizontal derivative, analytical signal, and tilt derivative were calculated to highlight subsurface boundaries and the major structures within the area. Several digital image enhancement techniques such as general contrast stretching and edge enhancement were applied to the NigeraSat-X image in ERDAS IMAGINE 9.2 after which structures from the interpreted magnetic data and the image were mapped out on-screen using ArcMap 10. The results show that the RTE produced a reasonable geological picture of the area. Also the basement configuration consists of several NEeSW and NWeSE structures that range from 1 km to about 17 km in length with the NEeSW structures being the major trend within the area. The lineaments are mainly within the basement and the Older granites and may be related to the Pan-African Orogeny. This study was also able to map out more accurately the contact between the basement and the sediments hence a modified geological map of the area was produced. © 2015 Elsevier Ltd. All rights reserved.
Keywords: NigeriaSat-X GIS Aeromagnetic data Structures/lineaments Geology
1. Introduction The area of study lies within the pegmatite zone of Nigeria. It covers Wamba 1:100,000 sheet 210 with an area of about 3080 km2 0 0 0 between latitude 8 30 N and 9 00 N and longitude 8 30 E and 9 0 00 E. Exploration of mineral resources has been described as a fourstep operation, involving regional reconnaissance, surface and subsurface mapping, ground geophysical and other surveys and actual drilling (Ananaba and Ajakaiye, 1987). This work involves the use of satellite imagery and aeromagnetic data to better understand the geology and structural set-up of the Wamba area which is made up of basement and sedimentary rocks which can be an aid for mineral exploration since the area is well known for mineralized pegmatites (Adekeye and Akintola, 2007). Wamba Falls within the pegmatite zone of North Central
* Corresponding author. E-mail address: [email protected] (J.K. Ogunmola). http://dx.doi.org/10.1016/j.jafrearsci.2015.07.028 1464-343X/© 2015 Elsevier Ltd. All rights reserved.
Nigeria. Kuster (1990) showed that the mineralized pegmatites show similar geochemical characteristics with the late Pan African granites and are fracture controlled. They are also mylonitized along a conjugate set of NEeSW and NWeSE to NNW-SSE striking faults. These pegmatites are mainly quartz-feldspar-muscovite pegmatites rich in NbeTaeSneBeLieW. During the tantalite and wolframite boom between 1999 and 2002 in Nigeria, the area was known for a lot of mining activities during which tantalite and wolframite were mined from mineralized pegmatites. However, there was no systematic and scientific approach to the exploration and mining of these metals. The pegmatites mined were only those discovered on the surface or accidentally stumbled upon which were exhausted in little time. It is in the light of this that the authors took interest in the area in order to explore for hidden or buried pegmatites that maybe hosts to these rare metals. Exploring buried rare-element pegmatites which are either hidden by thick overburden or situated at depth is a difficult task to the geologist. Truemond and Cerny (1982) described them as geophysical nonresponders because they are non-magnetic, contain insufficient
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metallic minerals that make them conductive and may not have sufficient mass/density to allow for contrast from the host rock using gravity methods. However, detailed magnetic mapping can be used to delineate faults in the Pan-African and Older granites which may contain mineralized pegmatites. Magnetic lows can also mark non-magnetic, possibly mineralized shear zones or alteration zones. Some indicators found to be useful in mineral exploration include faults, fractures, lineaments, arched or domed strata, oxidized and hydrothermal altered areas. Ananaba and Ajakaiye (1987) have, in particular, demonstrated positive correlation between lineament densities and primary mineral occurrences in Nigeria. The area of study forms part of the Upper Proterozoic mobile belt extending from Algeria across the Sahara into Nigeria, Benin and the Cameroun. One of the objectives of this work is to differentiate and map the basement rocks and the sediments and tracing out the boundaries between them. It also involves mapping out lineaments within the area using NigeriaSat-X and aeromagnetic data. This work shows how the Geographic Information System (GIS) in combination with geological data sets can be used to solve specific geological problems. Digital image processing focuses on the usage of field and laboratory data to gain a better understanding of the environment. 1.1. Geological setting The area under study lies within the Basement Complex of central Nigeria, which is part of the Upper Proterozoic (Pan-African) mobile belt extending from Algeria across the south Sahara to Nigeria, Benin and Cameroon Republics (Fig. 1). Part of the area also lies within the Benue Trough which is part of an early Cretaceous rift complex known as the West and Central African Rift System. The Nigeria Basement lies to the south of the Tuareg shield. Evidence from the eastern and northern margins of the West African Craton indicates that the Pan-African belt evolved by plate tectonic processes which involved the collision between the passive continental margin of the West African Craton and the active continental margin (Pharusian Belt) of the Tuareg shield about 600 Ma (Burke and Dewey, 1972; Leblanc, 1976, 1981; Black et al., 1979; Caby et al., 1981). The collision at the plate margin is believed to have led to the reactivation of the internal region of the belt. The Nigerian Basement Complex lies in the reactivated part of the belt. Radiometric ages indicate that the Nigerian basement is polycyclic and includes rocks of Liberian (2700 ± 200 Ma), Eburnian (200 ± 200 Ma), Pan-African (600 ± 150 Ma) and questionably, the Kibaran (1100 ± 200 Ma), (Ogezi, 1977; Ajibade et al., 1988). The most obvious effects of the Pan eAfrican orogeny on Nigeria is the emplacement of large volumes of granitoids and the resetting of mineral ages in virtually all rock types in the basement (Ogezi, 1977). A 400 km wide zone of low-grade schist belts occurs in the western part of Nigeria. These schist belts have been variously considered to be Archean in age (Russ, 1957), Kibaran (Ogezi, 1977; 1988) middle Proterozoic (Oyawoye, 1964, 1972) and Pan-Africa (McCurry, 1973). Grant (1978) and Turner (1983) suggested that there are two generations of schist belts, one Kibaran, the other Pan-African. The schist belts are considered critical to the understanding of the evolution of the Nigerian basement. The first reliable radiometric data for the schist belts was reported by Ogezi (1977) who obtained a RbeSr isochron age of 1064 ± 64 Ma from phyllites from the Maru belt. A simplified geological map of Nigeria is shown in Fig. 2. The Nigerian Basement complex is made up of different types of rocks but they have been broadly classified as follows (e.g Ogezi 2002)-
Fig. 1. Generalised Geological map of the Pan-African belt east of west African Craton (after Ajibade et al, 1985).
1. Syntectonic to late tectonic granitic rocks 2. Low grade metasediment dominated by schists, and 3. Polymetamorphic migmatiteegneiss Complex The Polymetamorphic migmatiteegneiss complex is the oldest rock group of the Basement Complex. It covers about 60% of the Nigerian Basement Complex and consists of biotite-gneiss and granulite gneiss, banded gneiss and granite gneiss, but the main constituents are the migmatite and gneiss that are of various compositions. These include amphibolites and relict metasedimentary rocks represented by medium to high grade calcareous, pelitic and quartzitic rocks which are within the migmatites and gneisses. Oyawoye in 1972 described them as “Ancient Metasediments”. The low grade metasediment dominated schists form a narrow belt in the western half of the country and it is believed to be relicts of a supracrustal cover which was infolded into the migmatiteegneiss complex (Russ, 1957; McCurry, 1973). It has been described as “ New Metasediments” (McCurry, 1976) and unmigmatised and slightly migmatised schists (Rahaman, 1976). The Jurassic (145-210Ma) Younger granites in the study area are highlevel, anorogenic granites; they mainly consist of microgranites and biotite granites. The southern part of the area is part of the middle Benue Trough (Fig. 3) and is underlain by Cretaceous sedimentary rocks, namely the Awgu shale and Lafia formation. The Awgu shale is composed mainly of bluish-gray to black shales, whereas the Lafia formation consists mainly of sandstones and claystones. According to Obaje (1994), Paleogene basalt flows and dolerite sills have been encountered within the Lafia formation.
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Fig. 2. Generalized geological map of Nigeria showing the sub-divisions of the Benue Trough and the study area in black box (after Ajibade et al., 1985).
Fig. 3. Geological map of Wamba and its adjoining areas (modified from, Macleod et al., 1971).
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2. Materials and methods
2.2. Reduction to the equator
2.1. Data available
The direction (inclination and declination) of the magnetization and the earth's magnetic field affects the shape of a total magnetic intensity anomaly. When the inclinations of the vectors are either 90 or 0 , the magnetic anomaly is not centred over the source. A reduction to the pole is normally used to remove this effect of inclination and declination and hence centre the anomaly over the source. It has however been observed that the RTP introduces errors and false trends at low magnetic latitudes. The reduction to the equator (RTE) reduces the data to the magnetic equator (Leu, 1982) during which magnetic intensity anomalies are recomputed as if the bodies causing the anomaly are magnetized in a horizontal direction which results in anomalous lows to be centred over the body. Jain (1988) showed that even though the magnetic field is more complex at the equator than the actual magnetic field at the pole, an RTE map is less complex. The reduction to the equator was carried out with the aid of Geosoft's magmap using a geomagnetic inclination of 11.595, declination of 2.097 (Derived from IGRF 11). North-South features usually blow up after an RTE operation and to reduce this, a higher latitude was chosen for the amplitude correction. Several values were used but a value of 10 proved to be optimum and gave reasonable view of the geology of the area especially as seen at the contact between the sediments and the basement rocks in the southern part of the area.
2.1.1. Magnetic data The aeromagnetic data for Wamba sheet 210 (Fig. 4) was acquired by Fugro Airborne surveys in 2005 at a flight line spacing of 200 m and a terrain clearance of 80 m. The data available for this study is in grid format only, without the flight line data. This shortcoming made it impossible to remove artefacts that are due to flight lines from the data through processes like cross-over analysis and micro-levelling. 2.1.2. SRTM data SRTM (Fig. 5) stands for Shuttle Radar Topography Mission which was flown by NASA that obtained digital elevation models of the earth's surface. It is useful in surface mapping especially in areas where a detailed geological map is not available. 2.1.3. NigeriaSat-X data In magnetic data interpretation, it is often useful to compare structure or geologic bodies delineated from the derivatives with surface geology. Satellite imagery can give us a picture of the surface where outcrops and features such as dykes can be observed. NigeriaSat-X (Fig. 6) data with a resolution of 22 m was used for this study.
Fig. 4. Total magnetic intensity (TMI) grid of Wamba and environs.
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Fig. 5. Digital Elevation Model (DEM) of Wamba and environs.
2.3. Total horizontal derivative of the RTE The total horizontal derivative (THDR) was used for this study because this method was designed to image faults and contact features which makes it well suited for this study as we intend to delineate faults in the Pan-African and Older granites which may contain mineralized pegmatites as well as mineralized shear zones or alteration zones.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 vT vT Full ðor TotalÞ Horizontal derivative THDR ¼ þ vx vy The horizontal gradient method measures the rate of change in magnetic susceptibility in the x and y directions and produces a resultant grid. There is no change in the frequency content of the TMI and the total gradient but the spectral phase of the gradient changes (Cordell and Grauch, 1985). 2.4. Analytical signal (AS) When working in equatorial regions, techniques to be applied should be chosen carefully because of the complications associated with interpretations of magnetic anomalies at low latitudes
which include e Horizontal ambient field Weak ambient field (about 35,000 nT compared to about 70,000 nT at higher latitudes. Difficulty in identifying structure striking N e S. When flux density cuts the boundary of a structure, magnetic anomalies are generated if the structure strikes parallel with the field, no anomaly is generated in equatorial areas because the flux stays within the structure. However, since structures such as faults have irregular shapes, some parts of the structures will generate dipole shape anomalies where the flux cuts the interface of the structure. Therefore N e S striking structures could be seen as a string of pearls, and it has been shown that the Analytical Signal is the best derivative to recover N e S contacts in equatorial regions (Beard, 2000). It generates a maxima directly over discrete bodies as well as the edges. The analytical signal transform uses a Fast Fourier Transform to compute the derivatives and hence the analytical signal. Analytical signal is the complex output from the differentiation of a potential field data that is also complex (Nabighian, 1972). The magnetic derivatives are calculated in the x,y,z direction, and the square root of the sum of the square of the derivatives gives the analytical signal (Roest et al., 1992). It is also called the total gradient method
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Fig. 6. NigeriaSAT-X image of Wamba and environs.
because it involves the derivatives in all directions.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 vT vT vT A ¼ AðX; YÞ ¼ þ þ vx vy vz The Analytical signal performs well at all magnetic latitudes because the direction of the ambient field does not affect it and the maxima defines the edges of magnetic bodies. For locating contacts and sheet like structures, the Analytical method has been found to be an effective method irrespective of their angle of dip or the magnetic latitude (Phillips, 2000). 2.5. Tilt derivative of RTE The tilt-derivative (TDR) was chosen because of its peculiar characteristics. It tends to equalize the amplitude output of TM anomalies across a grid. All conventional derivatives have amplitude response that is closely linked to the amplitude of the TMI anomaly while the TDR is independent of amplitude of the TMI anomaly and are instead controlled by the reciprocal of the depths of the sources (Verduzco et al., 2004) which in our case are less than 3 km. The TDR
has also been shown to act as an effective signal discriminator in the presence of noise. The TDR also has its zero values close to the edges of the magnetic bodies for RTE and RTP fields. Another advantage of the TDR is the fact that its phase is controlled by the vertical derivative and because of its “AGC” it can image smaller amplitude features. The ratio of the vertical gradient to the total horizontal derivative which is always positive has been defined as the tilt angle (Miller and Singh, 1994). The Tilt derivative (TDR) is defined as-
Tilt derivative TDR ¼ tan1
VDR THDR
The tan1 component of the expression which is between a range of þ1.57 and 1.57 provides an automatic gain control (AGC) that amplifies the amplitude of signals that are low which makes the Tilt derivative a powerful method for RTE and RTP data (Verduzco et al., 2004). The tilt method was applied to the RTE data of Wamba sheet 210 using Geosoft's magmap. 2.6. Image processing Several digital image enhancement techniques such as general
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Fig. 7. Curvature map of Wamba and Environs.
contrast stretching and edge enhancement were applied to the NigeriaSat-X images in ERDAS IMAGINE 9.2. 2.7. Topographic analysis The curvature of the study area was derived using ArcMap spatial analyst. This process calculates the curvature of a raster surface by deriving the second derivative of the raster data set. Curvature is the amount by which a geometric object deviates from being flat. A positive curvature indicates the surface is upwardly convex at that cell. A negative curvature indicates the surface is upwardly concave at that cell. A value of 0 indicates the surface is flat. 2.8. Structural mapping The analysis and interpretation of remote sensing imagery are determined by the objective of the interpretation. The term lineaments was originally used by Hobbs (1904) in his paper titled “Lineaments of the Atlantic Border region.” He defined lineaments as significant lines of landscape that reveal the hidden architecture of rock basement. However O'Leary et al. (1976) define lineament as a mappable, simple or composite linear feature of a surface whose parts are aligned in a rectilinear or slightly curvilinear relationship and which differ from the pattern of adjacent features and presumably reflect some sub-surface phenomenon. The regional analysis of the lineaments/structures presented in this work has
been based on the spatial and directional attributes of their assemblages. ArcMap is powerful GIS tool that can be used to integrate different data sets that have the same spatial reference to extract information that may be common or different among the various data sets. The SRTM data is a digital elevation model which is used to study the terrain of the study area. The relief of the area can give us an impression of the surface geology. The NigeriaSAT-X imagery also shows the geomorphology of the surface. Integrating all the data sets including the derivatives from the magnetic data makes mapping the structures easier than looking at the data sets separately. The first stage of the mapping involved identifying lineaments from the image and digitizing them on-screen and saving them as a feature class in a geodatabase. The second stage of structural mapping involved mapping out magnetic lineaments that could be due to any of the following- the contacts between two rock types of contrasting magnetic susceptibility - edges of structures that could be faults or intrusives within the sediments To achieve this, all the various data sets were displayed in ArcMap and by studying one layer at a time and comparing with other layers in the GIS environment. The geological map was useful because it showed the location where the basement occurs as surface exposure. The SRTM data was able to show the outline
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Fig. 8. Analytical Signal of RTE field of the magnetic data showing the two prominent structures (in black box).
of surface geological features that are also evident on the NigeriaSat-X data. One of the advantages of working in a GIS environment using several data sets is the opportunity to examine features that are spatially referenced. A feature that is less pronounced in one data set can be more pronounced in another data set and this can be better studied in a GIS environment. To start digitizing the magnetic lineaments, a feature class was created in ArcCatalog and it was set to the same coordinate and spatial reference as the other data sets. The digitizing tool was then used to map out the magnetic lineaments observed from the various derivatives. 2.9. Depth to basement inversion from magnetic data 2.9.1. Euler deconvolution The Euler deconvolution method used for this research is that developed by Geosoft which uses Euler's homogeneity equation to map out the depth, location and nature of the causative bodies present. Thompson (1982) expressed the Euler's homogeneity as-
ðX Xo Þ
vT vT vT þ ðY Yo Þ þ ðZ Zo Þ ¼ NðB TÞ vx vy vz
where: Xo, Yo, Zo-is the position of the magnetic body
T-Total field measured at (X,Y,Z) N- the degree of homogeneity which can also be interpreted as the structural index (SI) is the variation of the field with distance. B- Background value of the TMI However, Reid et al. (1990) showed that the structural index of 0.5 leads to underestimates of depths and the value of a sloping contact is in fact zero as long as an offset A is introduced and hence Euler's equation can be written as-
ðX Xo Þ
vT vT vT þ ðY Yo Þ þ ðZ Zo Þ ¼A vx vy vz
The southern part of the study area is mainly made of the Lafia formation and Awgu shales while the northern part is mainly of older granites and migmatites/gneisses. Therefore to derive the depths to the basement within the sedimentary part of the study area, the southern part of the TMI data was subset from the entire data for the Euler deconvolution. The data is divided into square windows within the grid and Euler's assumes a structural index and a regional value B to derive least-squares estimates for an optimum source geometry within a data window. Care was taken in choosing a structural index because values that are too low will give estimates that are too shallow and high values can give abnormally deep estimates. The Euler deconvolution requires derivatives in the x,y,z directions so these were derived in Geosoft. A folder was created for
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the solutions that are stored as a.gdb file. Most of the anomalies in the grid are between 1 km and 3 km in width and as the grid cell size of the data is 200 m, a window size of 20 was selected which corresponds to a search window size of 4 km. The maximum distance from the centre of the window to accept was set as 0 which stands for infinity. The survey of the Wamba area was flown at 80 m so the survey elevation was set as 80 m. Since this research is aimed at mapping out magnetic lineaments which could be faults and contacts between rock types, a structural index of 0 was selected (after Reid et al., 1990). The solutions generated were analysed and values greater than 7 km were discarded since the thickest sediments in the Benue Trough have been found to be about 7 km (Carter et al., 1963) and the middle Benue has been observed to be the shallow part of the Trough. The data was exported as a shape file and gridded in ArcMap. 3. Results 3.1. Curvature From the output of the curvature map (Fig. 7), it was observed that areas with large curvature values (patches of red and blue colours) correlate with areas where the basement rocks outcropped on the surface which was useful in mapping areas with thick vegetation on the satellite image. Areas with low curvature (green
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colour) values correlate flat lands and stream channels. 3.1.1. Reduction to the equator The reduced-to-the Equator (RTE) transform gave an honest view of the geology of the area for the Wamba-Nasarawa Egon area because structures are better preserved in the RTE as can be seen where there is a near perfect fit between structures seen on the satellite image and the magnetic data as exemplified in structures observed south of Dange and around Bwina. Hence, the RTE grid was used for this study. 3.2. Analytical signal The transformed Analytical signal grids (Fig. 8) show the maxima over the magnetic sources especially in areas covered by the basement. Two prominent structures that are about 10 km in length between Ambakar and Jimiya trending NEeSW and WNWeESE were highlighted. These prominent structures are however subsurface and are not visible on the satellite images and during the field work. 3.3. Tilt derivative The Tilt derivative grid (Fig. 9) clearly shows that this method is obviously very good at enhancing anomalies as several of the
Fig. 9. Tilt derivative of the RTE field of the magnetic data.
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magnetic lineaments that were not clearly visible in analytical signal grid were greatly enhanced.
at the north-eastern part of the area and some were seen to crisscross each other.
3.4. Total horizontal derivative
3.6. Depth to basement
THDR is a powerful method because it brings out the edges of the structures and they appear like rail lines along narrow features. The output shows the NEeSW,NeS and NWeSE structures in the data clearly defined (Fig. 10).
The grid of the solutions derived from the Euler method shows that the depth to the magnetic basement ranges from about 1 km to 3 km. The IDW interpolation method used to grid the data interpolates a raster surface from points using an inverse distance weighted (IDW) technique. The distributions of depths are shown in Fig. 15.
3.5. Structural analysis The lineaments extracted from the NigeriaSat-X image range in length from 600 m to 16 km and are mostly from the Basement part of the area (Fig. 11). The rose diagram (Fig. 12) suggests predominantly northeast-southwest tectonic trends which is the predominant trend of the Benue Trough. The magnetic lineaments derived from all the magnetic derivatives show remarkable similarity with those derived from the NigeriaSat-X image (Fig. 13). They also range in length from about 600 m to about 17 km and the predominant direction is the NEeSW with a few trending NWeSE (Fig. 14). Some of the lineaments derived from the magnetic data have surface expressions even though not clearly defined as can be seen
4. Discussion The TDR was able to image structures that were non-continuous in the HDR and the AS. Example can be seen in the structure that stretches for about 22 km from Okambu to Monkwar in the Northern part of the area. This structure appears as discreet anomalies in the HDR and the AS but was imaged as a continuous anomaly in the TDR. Similar examples are seen in structures directly below Bwina in the NW part of the area where the TDR imaged the structures as continuous bodies. The roles of the HDR and AS were not overlooked in this study. Both methods
Fig. 10. Total Horizontal derivative of RTE field of the magnetic data.
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Fig. 11. Lineaments derived from the NigeriaSat-X image.
contributed to the study, for example, a fault-like structure North of Azaba was better imaged in the HDR. The total horizontal derivative of the TDR which is theoretically independent of geomagnetic inclination also generated useful magnetic responses in the study area. Overall the TDR generated better defined maxima centered over the body edges compared to the HDR and the AS. The major structural trends in the Pan-African are NEeSW, ENEeWSW, SEeNW and SSEeNNW to SeN (Tairou et al., 2012). The lineaments extracted from the satellite image were derived from the Younger Granites at the north-western part of the area, gneisses and migmatites of the Basement Complex exposed in the central part and the Older Granites of the Konakanoae, Monkwar, Ngolo and Shishem hills at the north-eastern part of the study area which is probably the reason why there are more NNEeSSW trending structures that are have been found to occur within the Younger Granites (Ogunmola et al., 2008). The lineaments from the magnetic data were mostly from the crystalline basement and shows a predominant NEeSW trend and a few NeS and EeW trends which are the common structural trends in the Nigerian Basement. A few NWeSW are also observed in both rose diagrams. (Ekwueme, 1985) attributed this to an earlier tectonic event
regarded as Pre- Pan African. The lineaments extracted from the satellite image around the basement and those from the magnetic data both have a predominant NEeSW trend which suggests that tectonic events that affected the basement may also have been responsible for fracturing within the sediments. This lends credence to the suggestion by Benkhelil et al. (1998) that sedimentary structural features of the Cote d’IvoireeGhana Marginal Ridge (CIGMR) and those of the Benue Trough developed as a result of basement-seated transcurrent faults. These lineaments may also be related to fracture zones that cut the mid-oceanic ridges at or close to right angles. One of these feature zones, the Romanche fracture zone that trends NEeSW has been linked with structural trends of the basement west of the Benue Trough (Wright, 1976). The Chain and Charcot fracture zones are also thought to have continental extensions and are likely to control the major NEeSW fracture system along the Benue Trough. The lineaments extracted from the NigeriaSat-X image maybe fractures that appear on the surface and range in length from 600 m to about 17 km. Some of the lineaments derived from the magnetic data may be surface extensions of the basement seated structures because they coincide with the lineaments
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Fig. 12. Rose diagram from lineaments derived from the NigeriaSat-X image showing distribution of lineaments by direction.
observed on the surface. It is interesting to note that some of these sub-surface structures trend several kilometers and don't appear on the surface. For example, between Jumiya and Ampakar in the northern part of the area, there are two prominent structures that cross each other and are about 10 km in length. These magnetic lineaments could be fractures/faults or pegmatites that are common within the area. The Wamba e Nassarawa Egon area has been known for mineralized pegmatites. The late Pan-African granites that are host to rare-metal pegmatites and gold bearing veins have been associated with the fractures in the Pan-African mobile belts (Kuster, 1990; Ekwueme and Matheis, 1995; Garba, 2002; Okunlola, 2005). The end of the Pan-African tectonic event is marked by a conjugate fracture system of strike-slip faults (Ball, 1980). The pattern of these fracture systems was probably established during the Pan-African orogeny (McCurry, 1971). The pegmatites are associated with the Older Granites and they were initially thought to occur only in an NE-NS zone from Ago-Iwoye in the south-west through Wamba-Jama'a to Bauchi in the north-east. However other occurrences have been found around Zuru-Gusau in the north-west (Garba, 2002; Okunlola, 2005) and Obudu area (Ekwueme and Matheis, 1995). The magnetic data was very useful in determining
the contact between the sediments and the basement. It guided the modification of the geological map of the area. 5. Conclusions ➢ Remote sensing and GIS have become increasingly powerful due to improvements in both hardware and software. One of the geological fields that have benefited from this technology is structural geology and structural studies are very important in geological mapping. ➢ The reduced-to-the Equator (RTE) transform gave an honest view of the geology of Wamba-Nasarawa Egon area because structures are preserved in the RTE as can be seen where there is a near perfect fit between structures seen on the satellite image and the magnetic data as exemplified in structures observed south of Dange and around Bwina. ➢ Overall the TDR performed better and generated better defined maxima centered over the body edges compared to the HDR and the AS. ➢ The predominant fracture direction within the Pan African granites and the basement rocks is NEeSW. Fractures within the
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Fig. 13. Magnetic lineaments derived from the magnetic derivatives.
➢
➢
➢
➢
sedimentary parts of the study area also show dominant NEeSW trend which may be related to basement seated structures The structures observed within the study area which is part of the middle Benue may be related to fracture zones such as the Romanche fracture zones and the Chain and Charcot fracture zones which are thought to have continental extensions and are likely to control the major NEeSW fracture system along the Benue Trough as suggested by earlier authors Geological mapping showed that majority of the pegmatites cutting across the gneisses and migmatites trend in a predominant NEeSW trend particularly between Wamba and Dange which is similar to the major trend in the area and therefore suggests that the emplacements of the pegmatites are fracture controlled The microgranites and cross-cutting pegmatites have parallel foliation to those observed in the Pan-African granites which suggests that the deformation occurred after the emplacement of the pegmatites The depth to the magnetic basement in the area ranges from about 1 km to about 3 km in most of the southern part of the area
➢ Some interesting features that are not observed during field work were delineated from the magnetic derivatives. For example, between Jumiya and Ampakar in the northern part of the area, there are two prominent structures that cross each other and are about 10 km in length. ➢ Proper delineation of the sediment-basement contact with the aid of the magnetic data contributed in producing a modified geological map of Wamba sheet 210 Exploring buried rare-element pegmatites which are either hidden by thick overburden or situated at depth is a difficult task to the geologist. Truemond and Cerny (1982) described them as geophysical non-responders because they are non-magnetic, contain insufficient metallic minerals that make them conductive and may not have sufficient mass/density to allow for contrast from the host rock using gravity methods. However, detailed magnetic mapping can be used to delineate faults in the Pan-African and Older granites which may contain mineralized pegmatites. Magnetic lows can also mark non-magnetic, possibly mineralized shear zones or alteration zones. It is therefore suggested a lithogeochemical survey be carried out around some of the prominent
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Fig. 14. Rose diagram from magnetic lineaments derived from magnetic derivatives showing distribution of magnetic lineaments by direction.
Fig. 15. Grid of depth solutions derived from Euler deconvolution.
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structures that were delineated from this research since emplacement of rare-element pegmatites is accompanied by both the alteration of the adjacent host-rock and the development of an alkali-enriched, exomorphic aureole within the surrounding host rock. Using primary exomorphic aureoles as a pegmatite exploration tool requires that the elements making up the aureole originate from the source pegmatitic melt, are mobile and form thick aureoles. The best elements to test the aureole thickness and intensity are Li, Cs, B, Sn, Be and Rb. Acknowledgements This study has benefited from the facilities provided by the National Centre for Remote Sensing, Jos, Nigeria. References Adekeye, J.I.D., Akintola, O.F., 2007. Geochemical features of rare metal pegmatites in Nassarawa, central Nigeria. J. Min. Geol. 43 (1), 15e21. Ajibade, A.C., Woakes, M., Rahaman, M.A., 1985. Proterozoic crustal development in the Pan-African Regime of Nigeria. In: Kogbe, C.A. (Ed.), Geology of Nigeria, second ed. Elizabethan Publ. Co, Lagos, pp. 227e244. Ajibade, A.C., Rahaman, M.A., Ogezi, A.E.O., 1988. The precambrian of Nigeria, a geochronological summary. In: Oluyide, P.O., Mbonu, W.C., Ogezi, A.E., Egbuniwe, I.G., Ajibade, A.C., Umeji, A.C. (Eds.), Precambrian Geology of Nigeria. Geological Survey of Nigeria, Kaduna, pp. 313e324. Ananaba, S.E., Ajakaiye, D.E., 1987. Evidence of tectonic control of mineralization in Nigeria from lineament density analysis. Int. J. Remote Sens. 8 (19), 1445e1453. Ball, E., 1980. An example of very consistent brittle deformation over a wide intracontinental area: the late Pan-African fracture system of the Tuareg and Nigerian Shield. Tectonophysics 61, 363e379. Beard, L.P., 2000. Detection and identification of north-south trending magnetic structures near the magnetic equator. Geophys. Prospect. 2000 (48), 745±761. Benkhelil, J., Mascle, J., Guiraud, M., 1998. Sedimentary and structural characteristics ^ te d’Ivoire-Ghana transform margin and in the of the cretaceous along the Co Benue trough: a comparison. Proc. Ocean Drill. Program, Sci. Results 159. Black, R., Ball, H., Betrand, J.M.L., Boullier, A.M., Caby, R., Davison, I., Fabre, J., Leblanc, M., Wright, L.I., 1979. Outline of the Pan African geology of Adrar des Iforas (Republic of Mali). Geol. Rundsch 68 (2), 543e564. Burke, K.C., Dewey, J.F., 1972. Orogeny in Africa. In: Dessauvagie, T.F.J., Whiteman, A.J. (Eds.), Proceedings of Conference on African Geology. University of Ibadan. Caby, R., Bertrand, J.M.L., Black, R., 1981. Pan African Ocean closure and continental collision in the Hoggar-Iforas segment, Central Sahara. In: Kroner, A. (Ed.), Precambrian PlateTectonics. Elsevier, Armsterdam, pp. 407e434. Carter, J.D., Barber, W., Tait, E.A., 1963. The Geology of Parts of Adamawa, Bauchi and Bornu Provinces in North-eastern Nigeria. Geol. Surv. Nigeria, Rep. No. 30. Cordell, I., Grauch, V.J.S., 1985. Mapping basement magnetisation zones from aeromagnetic data in the San Juan basin, New Mexico. In: Hinze, W.J. (Ed.), The Utility of Regional Gravity and Magnetic Anomaly Maps: Soc. Expl. Geophys, pp. 181e197. Ekwueme, B.N., 1985. Petrology, Geochemistry and Rb-Sr Geochronology of Metamorphic Rocks of Uwet Area, Southeastern Nigeria. Ph.D. Thesis. Univ. of Nigeria, Nsukka, p. 176 (Unpublished). Ekwueme, B.N., Matheis, G., 1995. Geochemistry and economic value of pegmatites in the Precambrian basement of southeast Nigeria. In: Srivastava, R.K., mChandra, R. (Eds.), Magmatism in Relation to Diverse Tectonic Settings. Oxford & IBH Publishing Co, New Delhi, pp. 375e392. Garba, I., 2002. Late Pan-African tectonics and origin of gold mineralisation and rare-metal pegmatites in the Kushaka chist Belt, North-Western Nigeria. J. Min. Geol. 38 (1), 1e12. Grant, N.K., 1978. Structural distinction between a meta-sedimentary cover and an underlying basement in the 600 Ma old Pan-African domain of North-Western Nigeria. Bull. Geol. Soc. Am. 89, 50e58. Hobbs, W.H., 1904. Lineaments of the Atlantic border regions. Geol. Soc. Am. Bull. 15, 483e506.
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Jain, S., 1988. Total magnetic field reduction-the pole or equator? A model study. Can. J. Explor. Geophys. 24 (2), 185e192. Kuster, D., 1990. Rare-metal pegmatites of Wamba, Central Nigeria-their formation in relationship to late Pan-African granites. Miner. Deposita 25, 25e33. Leblanc, M., 1976. Proteorozoic oceanic crust at Bou Asser. Nature 261, 34e35. Leblanc, M., 1976. Proterozoic oceanic crust at Bou Asser. Nature 261, 34e35. Leu, L., 1982. Use of reduction-to-the-equator process for magnetic data interpretation. Geophysics 47, 445. Macleod, W.N., Turner, D.C., Wright, E.P., 1971. The geology of jos plateau. Geol. Surv. Niger. Bull. 2 (32). McCurry, P., 1971. Pan-African orogeny in northern Nigeria. Geol. Soc. Amer. Bull. 82, 3251e3262. McCurry, P., 1973. Geology of Degree Sheet 21 (Zaria), vol. 45. Overseas Geological Mineral Research, London. McCurry, P., 1976. The geology of the Precambrian to lower paleozoic rocks of Northern Nigeria- a review. In: Kogbe, C.A. (Ed.), Geology of Nigeria. Elizabethan Publishing Co, Ibadan, pp. 15e40. Miller, H.G., Singh, V., 1994. Potential field tilt a new concept for location of potential field sources. J. Appl. Geophys. 32, 213e217. Nabighian, M.N., 1972. The analytical signal of two-dimensional magnetic bodies with polygonal cross-section: Its properties and use for automated anomaly interpretation. Geophysics 37 (3), 507e517. Obaje, N.G., 1994. Coal Petrography, Microfossils and Paleoenvironments of Cretaceous Coal Measures in the Middle Benue Trough of Nigeria. Tuebinger Mikropalaeontologische Mitteilungen No. 11, pp. 1e165. Ogezi, A.E.O., 1977. Geochemistry and Geochronology of Basement Rocks from Northwestern Nigeria. Ph.D. thesis. Leeds University. Ogezi, A.E.O., 1988. Origin and evolution of the Basement complex of northwestern Nigeria in the light of new geochemical and geochronological data. In: Oluyide, P.O., Mbonu, W.C., Ogezi, A.E., Egbuniwe, I.G., Ajibade, A.C., Umeji, A.C. (Eds.), Precambrian Geology of Nigeria. A Publication of the Geological Survey of Nigeria, pp. 301e312. Ogezi, E.A., 2002. Geology Atlas of Nigeria. Africa Atlases: Nigeria. Les Editions, J.A. Paris, France, pp. 60e61. Ogunmola, J.K., Ugodulunwa, F.X.O., Eguaroje, O.E., Alaga, A.T., Ogbole, J.O., 2008. A structural study of some parts of the younger granite province of Nigeria using Nigeriasat-1 imagery. J. Sci. Technol. Res. 7 (3). Okunlola, O.A., 2005. Metallogeny of Tantalum-Niobium mineralization of Precambrian pegmatites of Nigeria. Mineral Wealth 137, 38e50. O'leary, D.W., Friedmon, J.D., Pohn, H.A., 1976. Lineament, Lenear, Lineations: some proposed new standards for old terms. Geol. Soc. Am. Bull. 87, 1463e1469. Oyawoye, M.O., 1964. The geology of the Nigerian basement complex. J. Niger. Min. Geol. Metall. Soc. 1. Oyawoye, M.O., 1972. The basement complex of Nigeria. In: Dessauvagie, Whiteman (Eds.), African Geology Ibadan 1970. Geology Department, University of Ibadan, Nigeria, pp. 67e99. Phillips, J.D., 2000. Locating Magnetic Contacts: a Comparison of the Horizontal Gradient, Analytic Signal, and Local Wavenumber Methods. SEG 2000 Expanded Abstracts. Rahaman, M.A., 1976. Review of the basement geology of south-western Nigeria. In: Kogbe, C.A. (Ed.), Geology of Nigeria. Elizabethan Publication. Company, Lagos, pp. 41e58. Reid, A.B., Allsop, J.M., Granser, H., Millets, A.J., Somerton, I.W., 1990. Magnetic interpretation in three dimensions using Euler deconvolution. Geophysics 55 (1), 80e91. Roest, W.R., Verhoef, J., Pilkington, M., 1992. Magnetic interpretation using the 3-D analytic signal. Geophysics 57 (1), 116e125. Russ, W., 1957. The geology of part of Niger, Zaria and Sokoto provinces with special reference to the occurrence of gold. Niger. Geol. Surv. Bull. 27, 42. Tairou, M.S., Affaton, P., Anum, S., Fleury, T.J., 2012. Pan-African paleostresses and reactivation of the Eburnean basement complex in southeast Ghana (West Africa). J. Geol. Res. 2012. Thompson, D.T., 1982. EULDPH-A new technique for making computer-assisted depth estimates from magnetic data. Geophysics 47, 31e37. Turner, D.C., 1983. Upper proterozoic Schist belts in the Nigerian sector of the PanAfrican province of West Africa. Precambrian Res. 21, 55e79. Verduzco, B., Fairhead, J.D., Green, C.M., MacKenzie, C., 2004. New insights into magnetic derivatives for structural mapping. Lead. Edge 23, 116e119. Wright, J.B., 1976. Fracture systems in Nigeria and initiation of fracture zones in the south Atlantic. Tectonophysics 34, T43eT47.
Contents lists available at ScienceDirect
Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci
Structural study of Wamba and Environs, north-central Nigeria using aeromagnetic data and NigeriaSat-X image J.K. Ogunmola a, *, E.N. Gajere a, E.A. Ayolabi b, S.B. Olobaniyi b, D.N. Jeb a, I.J. Agene a a b
National Centre For Remote Sensing, P.M.B 2136, Jos, Plateau State, Nigeria Department of Geosciences, University of Lagos, Nigeria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2014 Received in revised form 16 April 2015 Accepted 7 July 2015 Available online 20 August 2015
Wamba 1:100,000 sheet 210 covers Wamba and Nassarawa Eggon area of North-Central Nigeria and consists of basement rocks, biotite granites and Older Granites in most parts of the northern part and by sedimentary rocks of the Cretaceous Middle Benue Trough in the southern part. High resolution aeromagnetic data was interpreted and the results integrated in a GIS environment with data from NigeriaSat-X image to map out the major structural trends within the area. Reduction-to-the-equator (RTE) operation was carried out on the aeromagnetic data after which several data transforms/derivatives such as horizontal derivative, analytical signal, and tilt derivative were calculated to highlight subsurface boundaries and the major structures within the area. Several digital image enhancement techniques such as general contrast stretching and edge enhancement were applied to the NigeraSat-X image in ERDAS IMAGINE 9.2 after which structures from the interpreted magnetic data and the image were mapped out on-screen using ArcMap 10. The results show that the RTE produced a reasonable geological picture of the area. Also the basement configuration consists of several NEeSW and NWeSE structures that range from 1 km to about 17 km in length with the NEeSW structures being the major trend within the area. The lineaments are mainly within the basement and the Older granites and may be related to the Pan-African Orogeny. This study was also able to map out more accurately the contact between the basement and the sediments hence a modified geological map of the area was produced. © 2015 Elsevier Ltd. All rights reserved.
Keywords: NigeriaSat-X GIS Aeromagnetic data Structures/lineaments Geology
1. Introduction The area of study lies within the pegmatite zone of Nigeria. It covers Wamba 1:100,000 sheet 210 with an area of about 3080 km2 0 0 0 between latitude 8 30 N and 9 00 N and longitude 8 30 E and 9 0 00 E. Exploration of mineral resources has been described as a fourstep operation, involving regional reconnaissance, surface and subsurface mapping, ground geophysical and other surveys and actual drilling (Ananaba and Ajakaiye, 1987). This work involves the use of satellite imagery and aeromagnetic data to better understand the geology and structural set-up of the Wamba area which is made up of basement and sedimentary rocks which can be an aid for mineral exploration since the area is well known for mineralized pegmatites (Adekeye and Akintola, 2007). Wamba Falls within the pegmatite zone of North Central
* Corresponding author. E-mail address: [email protected] (J.K. Ogunmola). http://dx.doi.org/10.1016/j.jafrearsci.2015.07.028 1464-343X/© 2015 Elsevier Ltd. All rights reserved.
Nigeria. Kuster (1990) showed that the mineralized pegmatites show similar geochemical characteristics with the late Pan African granites and are fracture controlled. They are also mylonitized along a conjugate set of NEeSW and NWeSE to NNW-SSE striking faults. These pegmatites are mainly quartz-feldspar-muscovite pegmatites rich in NbeTaeSneBeLieW. During the tantalite and wolframite boom between 1999 and 2002 in Nigeria, the area was known for a lot of mining activities during which tantalite and wolframite were mined from mineralized pegmatites. However, there was no systematic and scientific approach to the exploration and mining of these metals. The pegmatites mined were only those discovered on the surface or accidentally stumbled upon which were exhausted in little time. It is in the light of this that the authors took interest in the area in order to explore for hidden or buried pegmatites that maybe hosts to these rare metals. Exploring buried rare-element pegmatites which are either hidden by thick overburden or situated at depth is a difficult task to the geologist. Truemond and Cerny (1982) described them as geophysical nonresponders because they are non-magnetic, contain insufficient
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metallic minerals that make them conductive and may not have sufficient mass/density to allow for contrast from the host rock using gravity methods. However, detailed magnetic mapping can be used to delineate faults in the Pan-African and Older granites which may contain mineralized pegmatites. Magnetic lows can also mark non-magnetic, possibly mineralized shear zones or alteration zones. Some indicators found to be useful in mineral exploration include faults, fractures, lineaments, arched or domed strata, oxidized and hydrothermal altered areas. Ananaba and Ajakaiye (1987) have, in particular, demonstrated positive correlation between lineament densities and primary mineral occurrences in Nigeria. The area of study forms part of the Upper Proterozoic mobile belt extending from Algeria across the Sahara into Nigeria, Benin and the Cameroun. One of the objectives of this work is to differentiate and map the basement rocks and the sediments and tracing out the boundaries between them. It also involves mapping out lineaments within the area using NigeriaSat-X and aeromagnetic data. This work shows how the Geographic Information System (GIS) in combination with geological data sets can be used to solve specific geological problems. Digital image processing focuses on the usage of field and laboratory data to gain a better understanding of the environment. 1.1. Geological setting The area under study lies within the Basement Complex of central Nigeria, which is part of the Upper Proterozoic (Pan-African) mobile belt extending from Algeria across the south Sahara to Nigeria, Benin and Cameroon Republics (Fig. 1). Part of the area also lies within the Benue Trough which is part of an early Cretaceous rift complex known as the West and Central African Rift System. The Nigeria Basement lies to the south of the Tuareg shield. Evidence from the eastern and northern margins of the West African Craton indicates that the Pan-African belt evolved by plate tectonic processes which involved the collision between the passive continental margin of the West African Craton and the active continental margin (Pharusian Belt) of the Tuareg shield about 600 Ma (Burke and Dewey, 1972; Leblanc, 1976, 1981; Black et al., 1979; Caby et al., 1981). The collision at the plate margin is believed to have led to the reactivation of the internal region of the belt. The Nigerian Basement Complex lies in the reactivated part of the belt. Radiometric ages indicate that the Nigerian basement is polycyclic and includes rocks of Liberian (2700 ± 200 Ma), Eburnian (200 ± 200 Ma), Pan-African (600 ± 150 Ma) and questionably, the Kibaran (1100 ± 200 Ma), (Ogezi, 1977; Ajibade et al., 1988). The most obvious effects of the Pan eAfrican orogeny on Nigeria is the emplacement of large volumes of granitoids and the resetting of mineral ages in virtually all rock types in the basement (Ogezi, 1977). A 400 km wide zone of low-grade schist belts occurs in the western part of Nigeria. These schist belts have been variously considered to be Archean in age (Russ, 1957), Kibaran (Ogezi, 1977; 1988) middle Proterozoic (Oyawoye, 1964, 1972) and Pan-Africa (McCurry, 1973). Grant (1978) and Turner (1983) suggested that there are two generations of schist belts, one Kibaran, the other Pan-African. The schist belts are considered critical to the understanding of the evolution of the Nigerian basement. The first reliable radiometric data for the schist belts was reported by Ogezi (1977) who obtained a RbeSr isochron age of 1064 ± 64 Ma from phyllites from the Maru belt. A simplified geological map of Nigeria is shown in Fig. 2. The Nigerian Basement complex is made up of different types of rocks but they have been broadly classified as follows (e.g Ogezi 2002)-
Fig. 1. Generalised Geological map of the Pan-African belt east of west African Craton (after Ajibade et al, 1985).
1. Syntectonic to late tectonic granitic rocks 2. Low grade metasediment dominated by schists, and 3. Polymetamorphic migmatiteegneiss Complex The Polymetamorphic migmatiteegneiss complex is the oldest rock group of the Basement Complex. It covers about 60% of the Nigerian Basement Complex and consists of biotite-gneiss and granulite gneiss, banded gneiss and granite gneiss, but the main constituents are the migmatite and gneiss that are of various compositions. These include amphibolites and relict metasedimentary rocks represented by medium to high grade calcareous, pelitic and quartzitic rocks which are within the migmatites and gneisses. Oyawoye in 1972 described them as “Ancient Metasediments”. The low grade metasediment dominated schists form a narrow belt in the western half of the country and it is believed to be relicts of a supracrustal cover which was infolded into the migmatiteegneiss complex (Russ, 1957; McCurry, 1973). It has been described as “ New Metasediments” (McCurry, 1976) and unmigmatised and slightly migmatised schists (Rahaman, 1976). The Jurassic (145-210Ma) Younger granites in the study area are highlevel, anorogenic granites; they mainly consist of microgranites and biotite granites. The southern part of the area is part of the middle Benue Trough (Fig. 3) and is underlain by Cretaceous sedimentary rocks, namely the Awgu shale and Lafia formation. The Awgu shale is composed mainly of bluish-gray to black shales, whereas the Lafia formation consists mainly of sandstones and claystones. According to Obaje (1994), Paleogene basalt flows and dolerite sills have been encountered within the Lafia formation.
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Fig. 2. Generalized geological map of Nigeria showing the sub-divisions of the Benue Trough and the study area in black box (after Ajibade et al., 1985).
Fig. 3. Geological map of Wamba and its adjoining areas (modified from, Macleod et al., 1971).
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2. Materials and methods
2.2. Reduction to the equator
2.1. Data available
The direction (inclination and declination) of the magnetization and the earth's magnetic field affects the shape of a total magnetic intensity anomaly. When the inclinations of the vectors are either 90 or 0 , the magnetic anomaly is not centred over the source. A reduction to the pole is normally used to remove this effect of inclination and declination and hence centre the anomaly over the source. It has however been observed that the RTP introduces errors and false trends at low magnetic latitudes. The reduction to the equator (RTE) reduces the data to the magnetic equator (Leu, 1982) during which magnetic intensity anomalies are recomputed as if the bodies causing the anomaly are magnetized in a horizontal direction which results in anomalous lows to be centred over the body. Jain (1988) showed that even though the magnetic field is more complex at the equator than the actual magnetic field at the pole, an RTE map is less complex. The reduction to the equator was carried out with the aid of Geosoft's magmap using a geomagnetic inclination of 11.595, declination of 2.097 (Derived from IGRF 11). North-South features usually blow up after an RTE operation and to reduce this, a higher latitude was chosen for the amplitude correction. Several values were used but a value of 10 proved to be optimum and gave reasonable view of the geology of the area especially as seen at the contact between the sediments and the basement rocks in the southern part of the area.
2.1.1. Magnetic data The aeromagnetic data for Wamba sheet 210 (Fig. 4) was acquired by Fugro Airborne surveys in 2005 at a flight line spacing of 200 m and a terrain clearance of 80 m. The data available for this study is in grid format only, without the flight line data. This shortcoming made it impossible to remove artefacts that are due to flight lines from the data through processes like cross-over analysis and micro-levelling. 2.1.2. SRTM data SRTM (Fig. 5) stands for Shuttle Radar Topography Mission which was flown by NASA that obtained digital elevation models of the earth's surface. It is useful in surface mapping especially in areas where a detailed geological map is not available. 2.1.3. NigeriaSat-X data In magnetic data interpretation, it is often useful to compare structure or geologic bodies delineated from the derivatives with surface geology. Satellite imagery can give us a picture of the surface where outcrops and features such as dykes can be observed. NigeriaSat-X (Fig. 6) data with a resolution of 22 m was used for this study.
Fig. 4. Total magnetic intensity (TMI) grid of Wamba and environs.
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Fig. 5. Digital Elevation Model (DEM) of Wamba and environs.
2.3. Total horizontal derivative of the RTE The total horizontal derivative (THDR) was used for this study because this method was designed to image faults and contact features which makes it well suited for this study as we intend to delineate faults in the Pan-African and Older granites which may contain mineralized pegmatites as well as mineralized shear zones or alteration zones.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 vT vT Full ðor TotalÞ Horizontal derivative THDR ¼ þ vx vy The horizontal gradient method measures the rate of change in magnetic susceptibility in the x and y directions and produces a resultant grid. There is no change in the frequency content of the TMI and the total gradient but the spectral phase of the gradient changes (Cordell and Grauch, 1985). 2.4. Analytical signal (AS) When working in equatorial regions, techniques to be applied should be chosen carefully because of the complications associated with interpretations of magnetic anomalies at low latitudes
which include e Horizontal ambient field Weak ambient field (about 35,000 nT compared to about 70,000 nT at higher latitudes. Difficulty in identifying structure striking N e S. When flux density cuts the boundary of a structure, magnetic anomalies are generated if the structure strikes parallel with the field, no anomaly is generated in equatorial areas because the flux stays within the structure. However, since structures such as faults have irregular shapes, some parts of the structures will generate dipole shape anomalies where the flux cuts the interface of the structure. Therefore N e S striking structures could be seen as a string of pearls, and it has been shown that the Analytical Signal is the best derivative to recover N e S contacts in equatorial regions (Beard, 2000). It generates a maxima directly over discrete bodies as well as the edges. The analytical signal transform uses a Fast Fourier Transform to compute the derivatives and hence the analytical signal. Analytical signal is the complex output from the differentiation of a potential field data that is also complex (Nabighian, 1972). The magnetic derivatives are calculated in the x,y,z direction, and the square root of the sum of the square of the derivatives gives the analytical signal (Roest et al., 1992). It is also called the total gradient method
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Fig. 6. NigeriaSAT-X image of Wamba and environs.
because it involves the derivatives in all directions.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 vT vT vT A ¼ AðX; YÞ ¼ þ þ vx vy vz The Analytical signal performs well at all magnetic latitudes because the direction of the ambient field does not affect it and the maxima defines the edges of magnetic bodies. For locating contacts and sheet like structures, the Analytical method has been found to be an effective method irrespective of their angle of dip or the magnetic latitude (Phillips, 2000). 2.5. Tilt derivative of RTE The tilt-derivative (TDR) was chosen because of its peculiar characteristics. It tends to equalize the amplitude output of TM anomalies across a grid. All conventional derivatives have amplitude response that is closely linked to the amplitude of the TMI anomaly while the TDR is independent of amplitude of the TMI anomaly and are instead controlled by the reciprocal of the depths of the sources (Verduzco et al., 2004) which in our case are less than 3 km. The TDR
has also been shown to act as an effective signal discriminator in the presence of noise. The TDR also has its zero values close to the edges of the magnetic bodies for RTE and RTP fields. Another advantage of the TDR is the fact that its phase is controlled by the vertical derivative and because of its “AGC” it can image smaller amplitude features. The ratio of the vertical gradient to the total horizontal derivative which is always positive has been defined as the tilt angle (Miller and Singh, 1994). The Tilt derivative (TDR) is defined as-
Tilt derivative TDR ¼ tan1
VDR THDR
The tan1 component of the expression which is between a range of þ1.57 and 1.57 provides an automatic gain control (AGC) that amplifies the amplitude of signals that are low which makes the Tilt derivative a powerful method for RTE and RTP data (Verduzco et al., 2004). The tilt method was applied to the RTE data of Wamba sheet 210 using Geosoft's magmap. 2.6. Image processing Several digital image enhancement techniques such as general
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Fig. 7. Curvature map of Wamba and Environs.
contrast stretching and edge enhancement were applied to the NigeriaSat-X images in ERDAS IMAGINE 9.2. 2.7. Topographic analysis The curvature of the study area was derived using ArcMap spatial analyst. This process calculates the curvature of a raster surface by deriving the second derivative of the raster data set. Curvature is the amount by which a geometric object deviates from being flat. A positive curvature indicates the surface is upwardly convex at that cell. A negative curvature indicates the surface is upwardly concave at that cell. A value of 0 indicates the surface is flat. 2.8. Structural mapping The analysis and interpretation of remote sensing imagery are determined by the objective of the interpretation. The term lineaments was originally used by Hobbs (1904) in his paper titled “Lineaments of the Atlantic Border region.” He defined lineaments as significant lines of landscape that reveal the hidden architecture of rock basement. However O'Leary et al. (1976) define lineament as a mappable, simple or composite linear feature of a surface whose parts are aligned in a rectilinear or slightly curvilinear relationship and which differ from the pattern of adjacent features and presumably reflect some sub-surface phenomenon. The regional analysis of the lineaments/structures presented in this work has
been based on the spatial and directional attributes of their assemblages. ArcMap is powerful GIS tool that can be used to integrate different data sets that have the same spatial reference to extract information that may be common or different among the various data sets. The SRTM data is a digital elevation model which is used to study the terrain of the study area. The relief of the area can give us an impression of the surface geology. The NigeriaSAT-X imagery also shows the geomorphology of the surface. Integrating all the data sets including the derivatives from the magnetic data makes mapping the structures easier than looking at the data sets separately. The first stage of the mapping involved identifying lineaments from the image and digitizing them on-screen and saving them as a feature class in a geodatabase. The second stage of structural mapping involved mapping out magnetic lineaments that could be due to any of the following- the contacts between two rock types of contrasting magnetic susceptibility - edges of structures that could be faults or intrusives within the sediments To achieve this, all the various data sets were displayed in ArcMap and by studying one layer at a time and comparing with other layers in the GIS environment. The geological map was useful because it showed the location where the basement occurs as surface exposure. The SRTM data was able to show the outline
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Fig. 8. Analytical Signal of RTE field of the magnetic data showing the two prominent structures (in black box).
of surface geological features that are also evident on the NigeriaSat-X data. One of the advantages of working in a GIS environment using several data sets is the opportunity to examine features that are spatially referenced. A feature that is less pronounced in one data set can be more pronounced in another data set and this can be better studied in a GIS environment. To start digitizing the magnetic lineaments, a feature class was created in ArcCatalog and it was set to the same coordinate and spatial reference as the other data sets. The digitizing tool was then used to map out the magnetic lineaments observed from the various derivatives. 2.9. Depth to basement inversion from magnetic data 2.9.1. Euler deconvolution The Euler deconvolution method used for this research is that developed by Geosoft which uses Euler's homogeneity equation to map out the depth, location and nature of the causative bodies present. Thompson (1982) expressed the Euler's homogeneity as-
ðX Xo Þ
vT vT vT þ ðY Yo Þ þ ðZ Zo Þ ¼ NðB TÞ vx vy vz
where: Xo, Yo, Zo-is the position of the magnetic body
T-Total field measured at (X,Y,Z) N- the degree of homogeneity which can also be interpreted as the structural index (SI) is the variation of the field with distance. B- Background value of the TMI However, Reid et al. (1990) showed that the structural index of 0.5 leads to underestimates of depths and the value of a sloping contact is in fact zero as long as an offset A is introduced and hence Euler's equation can be written as-
ðX Xo Þ
vT vT vT þ ðY Yo Þ þ ðZ Zo Þ ¼A vx vy vz
The southern part of the study area is mainly made of the Lafia formation and Awgu shales while the northern part is mainly of older granites and migmatites/gneisses. Therefore to derive the depths to the basement within the sedimentary part of the study area, the southern part of the TMI data was subset from the entire data for the Euler deconvolution. The data is divided into square windows within the grid and Euler's assumes a structural index and a regional value B to derive least-squares estimates for an optimum source geometry within a data window. Care was taken in choosing a structural index because values that are too low will give estimates that are too shallow and high values can give abnormally deep estimates. The Euler deconvolution requires derivatives in the x,y,z directions so these were derived in Geosoft. A folder was created for
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the solutions that are stored as a.gdb file. Most of the anomalies in the grid are between 1 km and 3 km in width and as the grid cell size of the data is 200 m, a window size of 20 was selected which corresponds to a search window size of 4 km. The maximum distance from the centre of the window to accept was set as 0 which stands for infinity. The survey of the Wamba area was flown at 80 m so the survey elevation was set as 80 m. Since this research is aimed at mapping out magnetic lineaments which could be faults and contacts between rock types, a structural index of 0 was selected (after Reid et al., 1990). The solutions generated were analysed and values greater than 7 km were discarded since the thickest sediments in the Benue Trough have been found to be about 7 km (Carter et al., 1963) and the middle Benue has been observed to be the shallow part of the Trough. The data was exported as a shape file and gridded in ArcMap. 3. Results 3.1. Curvature From the output of the curvature map (Fig. 7), it was observed that areas with large curvature values (patches of red and blue colours) correlate with areas where the basement rocks outcropped on the surface which was useful in mapping areas with thick vegetation on the satellite image. Areas with low curvature (green
315
colour) values correlate flat lands and stream channels. 3.1.1. Reduction to the equator The reduced-to-the Equator (RTE) transform gave an honest view of the geology of the area for the Wamba-Nasarawa Egon area because structures are better preserved in the RTE as can be seen where there is a near perfect fit between structures seen on the satellite image and the magnetic data as exemplified in structures observed south of Dange and around Bwina. Hence, the RTE grid was used for this study. 3.2. Analytical signal The transformed Analytical signal grids (Fig. 8) show the maxima over the magnetic sources especially in areas covered by the basement. Two prominent structures that are about 10 km in length between Ambakar and Jimiya trending NEeSW and WNWeESE were highlighted. These prominent structures are however subsurface and are not visible on the satellite images and during the field work. 3.3. Tilt derivative The Tilt derivative grid (Fig. 9) clearly shows that this method is obviously very good at enhancing anomalies as several of the
Fig. 9. Tilt derivative of the RTE field of the magnetic data.
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magnetic lineaments that were not clearly visible in analytical signal grid were greatly enhanced.
at the north-eastern part of the area and some were seen to crisscross each other.
3.4. Total horizontal derivative
3.6. Depth to basement
THDR is a powerful method because it brings out the edges of the structures and they appear like rail lines along narrow features. The output shows the NEeSW,NeS and NWeSE structures in the data clearly defined (Fig. 10).
The grid of the solutions derived from the Euler method shows that the depth to the magnetic basement ranges from about 1 km to 3 km. The IDW interpolation method used to grid the data interpolates a raster surface from points using an inverse distance weighted (IDW) technique. The distributions of depths are shown in Fig. 15.
3.5. Structural analysis The lineaments extracted from the NigeriaSat-X image range in length from 600 m to 16 km and are mostly from the Basement part of the area (Fig. 11). The rose diagram (Fig. 12) suggests predominantly northeast-southwest tectonic trends which is the predominant trend of the Benue Trough. The magnetic lineaments derived from all the magnetic derivatives show remarkable similarity with those derived from the NigeriaSat-X image (Fig. 13). They also range in length from about 600 m to about 17 km and the predominant direction is the NEeSW with a few trending NWeSE (Fig. 14). Some of the lineaments derived from the magnetic data have surface expressions even though not clearly defined as can be seen
4. Discussion The TDR was able to image structures that were non-continuous in the HDR and the AS. Example can be seen in the structure that stretches for about 22 km from Okambu to Monkwar in the Northern part of the area. This structure appears as discreet anomalies in the HDR and the AS but was imaged as a continuous anomaly in the TDR. Similar examples are seen in structures directly below Bwina in the NW part of the area where the TDR imaged the structures as continuous bodies. The roles of the HDR and AS were not overlooked in this study. Both methods
Fig. 10. Total Horizontal derivative of RTE field of the magnetic data.
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Fig. 11. Lineaments derived from the NigeriaSat-X image.
contributed to the study, for example, a fault-like structure North of Azaba was better imaged in the HDR. The total horizontal derivative of the TDR which is theoretically independent of geomagnetic inclination also generated useful magnetic responses in the study area. Overall the TDR generated better defined maxima centered over the body edges compared to the HDR and the AS. The major structural trends in the Pan-African are NEeSW, ENEeWSW, SEeNW and SSEeNNW to SeN (Tairou et al., 2012). The lineaments extracted from the satellite image were derived from the Younger Granites at the north-western part of the area, gneisses and migmatites of the Basement Complex exposed in the central part and the Older Granites of the Konakanoae, Monkwar, Ngolo and Shishem hills at the north-eastern part of the study area which is probably the reason why there are more NNEeSSW trending structures that are have been found to occur within the Younger Granites (Ogunmola et al., 2008). The lineaments from the magnetic data were mostly from the crystalline basement and shows a predominant NEeSW trend and a few NeS and EeW trends which are the common structural trends in the Nigerian Basement. A few NWeSW are also observed in both rose diagrams. (Ekwueme, 1985) attributed this to an earlier tectonic event
regarded as Pre- Pan African. The lineaments extracted from the satellite image around the basement and those from the magnetic data both have a predominant NEeSW trend which suggests that tectonic events that affected the basement may also have been responsible for fracturing within the sediments. This lends credence to the suggestion by Benkhelil et al. (1998) that sedimentary structural features of the Cote d’IvoireeGhana Marginal Ridge (CIGMR) and those of the Benue Trough developed as a result of basement-seated transcurrent faults. These lineaments may also be related to fracture zones that cut the mid-oceanic ridges at or close to right angles. One of these feature zones, the Romanche fracture zone that trends NEeSW has been linked with structural trends of the basement west of the Benue Trough (Wright, 1976). The Chain and Charcot fracture zones are also thought to have continental extensions and are likely to control the major NEeSW fracture system along the Benue Trough. The lineaments extracted from the NigeriaSat-X image maybe fractures that appear on the surface and range in length from 600 m to about 17 km. Some of the lineaments derived from the magnetic data may be surface extensions of the basement seated structures because they coincide with the lineaments
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Fig. 12. Rose diagram from lineaments derived from the NigeriaSat-X image showing distribution of lineaments by direction.
observed on the surface. It is interesting to note that some of these sub-surface structures trend several kilometers and don't appear on the surface. For example, between Jumiya and Ampakar in the northern part of the area, there are two prominent structures that cross each other and are about 10 km in length. These magnetic lineaments could be fractures/faults or pegmatites that are common within the area. The Wamba e Nassarawa Egon area has been known for mineralized pegmatites. The late Pan-African granites that are host to rare-metal pegmatites and gold bearing veins have been associated with the fractures in the Pan-African mobile belts (Kuster, 1990; Ekwueme and Matheis, 1995; Garba, 2002; Okunlola, 2005). The end of the Pan-African tectonic event is marked by a conjugate fracture system of strike-slip faults (Ball, 1980). The pattern of these fracture systems was probably established during the Pan-African orogeny (McCurry, 1971). The pegmatites are associated with the Older Granites and they were initially thought to occur only in an NE-NS zone from Ago-Iwoye in the south-west through Wamba-Jama'a to Bauchi in the north-east. However other occurrences have been found around Zuru-Gusau in the north-west (Garba, 2002; Okunlola, 2005) and Obudu area (Ekwueme and Matheis, 1995). The magnetic data was very useful in determining
the contact between the sediments and the basement. It guided the modification of the geological map of the area. 5. Conclusions ➢ Remote sensing and GIS have become increasingly powerful due to improvements in both hardware and software. One of the geological fields that have benefited from this technology is structural geology and structural studies are very important in geological mapping. ➢ The reduced-to-the Equator (RTE) transform gave an honest view of the geology of Wamba-Nasarawa Egon area because structures are preserved in the RTE as can be seen where there is a near perfect fit between structures seen on the satellite image and the magnetic data as exemplified in structures observed south of Dange and around Bwina. ➢ Overall the TDR performed better and generated better defined maxima centered over the body edges compared to the HDR and the AS. ➢ The predominant fracture direction within the Pan African granites and the basement rocks is NEeSW. Fractures within the
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Fig. 13. Magnetic lineaments derived from the magnetic derivatives.
➢
➢
➢
➢
sedimentary parts of the study area also show dominant NEeSW trend which may be related to basement seated structures The structures observed within the study area which is part of the middle Benue may be related to fracture zones such as the Romanche fracture zones and the Chain and Charcot fracture zones which are thought to have continental extensions and are likely to control the major NEeSW fracture system along the Benue Trough as suggested by earlier authors Geological mapping showed that majority of the pegmatites cutting across the gneisses and migmatites trend in a predominant NEeSW trend particularly between Wamba and Dange which is similar to the major trend in the area and therefore suggests that the emplacements of the pegmatites are fracture controlled The microgranites and cross-cutting pegmatites have parallel foliation to those observed in the Pan-African granites which suggests that the deformation occurred after the emplacement of the pegmatites The depth to the magnetic basement in the area ranges from about 1 km to about 3 km in most of the southern part of the area
➢ Some interesting features that are not observed during field work were delineated from the magnetic derivatives. For example, between Jumiya and Ampakar in the northern part of the area, there are two prominent structures that cross each other and are about 10 km in length. ➢ Proper delineation of the sediment-basement contact with the aid of the magnetic data contributed in producing a modified geological map of Wamba sheet 210 Exploring buried rare-element pegmatites which are either hidden by thick overburden or situated at depth is a difficult task to the geologist. Truemond and Cerny (1982) described them as geophysical non-responders because they are non-magnetic, contain insufficient metallic minerals that make them conductive and may not have sufficient mass/density to allow for contrast from the host rock using gravity methods. However, detailed magnetic mapping can be used to delineate faults in the Pan-African and Older granites which may contain mineralized pegmatites. Magnetic lows can also mark non-magnetic, possibly mineralized shear zones or alteration zones. It is therefore suggested a lithogeochemical survey be carried out around some of the prominent
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Fig. 14. Rose diagram from magnetic lineaments derived from magnetic derivatives showing distribution of magnetic lineaments by direction.
Fig. 15. Grid of depth solutions derived from Euler deconvolution.
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structures that were delineated from this research since emplacement of rare-element pegmatites is accompanied by both the alteration of the adjacent host-rock and the development of an alkali-enriched, exomorphic aureole within the surrounding host rock. Using primary exomorphic aureoles as a pegmatite exploration tool requires that the elements making up the aureole originate from the source pegmatitic melt, are mobile and form thick aureoles. The best elements to test the aureole thickness and intensity are Li, Cs, B, Sn, Be and Rb. Acknowledgements This study has benefited from the facilities provided by the National Centre for Remote Sensing, Jos, Nigeria. References Adekeye, J.I.D., Akintola, O.F., 2007. Geochemical features of rare metal pegmatites in Nassarawa, central Nigeria. J. Min. Geol. 43 (1), 15e21. Ajibade, A.C., Woakes, M., Rahaman, M.A., 1985. Proterozoic crustal development in the Pan-African Regime of Nigeria. In: Kogbe, C.A. (Ed.), Geology of Nigeria, second ed. Elizabethan Publ. Co, Lagos, pp. 227e244. Ajibade, A.C., Rahaman, M.A., Ogezi, A.E.O., 1988. The precambrian of Nigeria, a geochronological summary. In: Oluyide, P.O., Mbonu, W.C., Ogezi, A.E., Egbuniwe, I.G., Ajibade, A.C., Umeji, A.C. (Eds.), Precambrian Geology of Nigeria. Geological Survey of Nigeria, Kaduna, pp. 313e324. Ananaba, S.E., Ajakaiye, D.E., 1987. Evidence of tectonic control of mineralization in Nigeria from lineament density analysis. Int. J. Remote Sens. 8 (19), 1445e1453. Ball, E., 1980. An example of very consistent brittle deformation over a wide intracontinental area: the late Pan-African fracture system of the Tuareg and Nigerian Shield. Tectonophysics 61, 363e379. Beard, L.P., 2000. Detection and identification of north-south trending magnetic structures near the magnetic equator. Geophys. Prospect. 2000 (48), 745±761. Benkhelil, J., Mascle, J., Guiraud, M., 1998. Sedimentary and structural characteristics ^ te d’Ivoire-Ghana transform margin and in the of the cretaceous along the Co Benue trough: a comparison. Proc. Ocean Drill. Program, Sci. Results 159. Black, R., Ball, H., Betrand, J.M.L., Boullier, A.M., Caby, R., Davison, I., Fabre, J., Leblanc, M., Wright, L.I., 1979. Outline of the Pan African geology of Adrar des Iforas (Republic of Mali). Geol. Rundsch 68 (2), 543e564. Burke, K.C., Dewey, J.F., 1972. Orogeny in Africa. In: Dessauvagie, T.F.J., Whiteman, A.J. (Eds.), Proceedings of Conference on African Geology. University of Ibadan. Caby, R., Bertrand, J.M.L., Black, R., 1981. Pan African Ocean closure and continental collision in the Hoggar-Iforas segment, Central Sahara. In: Kroner, A. (Ed.), Precambrian PlateTectonics. Elsevier, Armsterdam, pp. 407e434. Carter, J.D., Barber, W., Tait, E.A., 1963. The Geology of Parts of Adamawa, Bauchi and Bornu Provinces in North-eastern Nigeria. Geol. Surv. Nigeria, Rep. No. 30. Cordell, I., Grauch, V.J.S., 1985. Mapping basement magnetisation zones from aeromagnetic data in the San Juan basin, New Mexico. In: Hinze, W.J. (Ed.), The Utility of Regional Gravity and Magnetic Anomaly Maps: Soc. Expl. Geophys, pp. 181e197. Ekwueme, B.N., 1985. Petrology, Geochemistry and Rb-Sr Geochronology of Metamorphic Rocks of Uwet Area, Southeastern Nigeria. Ph.D. Thesis. Univ. of Nigeria, Nsukka, p. 176 (Unpublished). Ekwueme, B.N., Matheis, G., 1995. Geochemistry and economic value of pegmatites in the Precambrian basement of southeast Nigeria. In: Srivastava, R.K., mChandra, R. (Eds.), Magmatism in Relation to Diverse Tectonic Settings. Oxford & IBH Publishing Co, New Delhi, pp. 375e392. Garba, I., 2002. Late Pan-African tectonics and origin of gold mineralisation and rare-metal pegmatites in the Kushaka chist Belt, North-Western Nigeria. J. Min. Geol. 38 (1), 1e12. Grant, N.K., 1978. Structural distinction between a meta-sedimentary cover and an underlying basement in the 600 Ma old Pan-African domain of North-Western Nigeria. Bull. Geol. Soc. Am. 89, 50e58. Hobbs, W.H., 1904. Lineaments of the Atlantic border regions. Geol. Soc. Am. Bull. 15, 483e506.
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