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Integrating Core and Aeromagnetic Datasets to Evaluate Mineral Potential of Southern Nupe Basin, Nigeria


 Aeromagnetic and core drilled data covering parts of southern Nupe Basin was acquired and interpreted with the view to evaluating the mineral potentials of the area through interpretation of the structural features in the area; determination of the curie isotherm depth; and correlation of aeromagnetic outcomes with the core sample data from the area. Two major regional fault trends were interpreted, trending, Northeast–Southwest (NE–SW) and NNE–SSW with minor northwest–southeast (NW–SE) directions. Two depth sources in the area are delineated namely; zone of shallow seated basement which ranges from 0.42km to 1.5km and zone of deeply seated basement which ranges from 1.91 to 3.50km.Results of qualitative interpretation of the Total magnetic intensity map (TMI) and Residual intensity map reveal that the magnetic intensities ranges from 7500 to 8460 nano-Telsa (nT) and -220 to 240 nT respectively. The depth to the centroid and top of the magnetic caustic bodies ranges from 9.00 to 17.10km and 0.4 to 3.10km respectively. Juxtaposing the topographical and core drilling data reveals that the oolitic iron ore level follows the topographical level which implies that the topography of the area controls the configuration of the iron ore deposit level. All these deduction are made considering the geology of the area.


Introduction
The study area is situated within the Nupe Basin also called the Mid Niger Basin. It is an intracratonic basin extending from Kontagora in Niger State to areas slightly beyond Lokoja in the south ( Aeromagnetic survey routinely target caustic magnetic bodies that poses unusual characteristic which mostly reveals themselves in form of anomalies. Magnetic properties are predominating in basement rocks and minimum in sedimentary rocks. According to Onwuemesi, (1995 and1997) and also Anakwuba and Chinwuko (2015), delineation of the subsurface requires a more advanced technique which the magnetic survey provides. These techniques can comfortably help in mapping the subsurface structures, calculate the depth parameter and model them because it is a fast, target oriented and cost effective geophysical method.
The works of some notable authors such as Singh and Biswas (2016), Saibi et al, (2015) Bhattacharyya and Leu, (1975), (Ross et al, 2006;Mandal, et al, 2013;Abraham et al., 2014) suggested that Curie isotherm and geothermal gradients can be used to complement geothermal data information in an areas where borehole information are not available. And this study is not an exception, as it attempt to integrate aeromagnetic and core data to evaluate the mineral potential of the basin. This research work focuses on the Evaluation of the mineral potential of the Nupe basin, through the use of centroid and forward modelling of the spectral peak methods, over some parts of southern Nupe Basin, North-Central Nigeria. This will delineate the geometry of the basin by evaluating the basal depth of the caustic magnetic rock, the thickness of the magnetic basement and crustal temperature information in the area of study. It will also reveal the geologic structures and possibly provide a 3D model of the basin.

Geological Setting
The stratigraphic sequence of the Nupe Basin, are referred to as the Bida Group (Adeleye, 1973), it is sub divided into Northern Nupe Basin (Sub-Basin) and Southern Nupe Sub-Basin or Lokoja Sub-Basin (Fig. 2).
The Nupe Basin is presumed to be a north-western extension of the Anambra Basin (Akande et al., 2005).
The basin ll contains a cretaceous sediment that were believed to be deposited as a result of, subsidence, rifting, block faulting, basement fragmentation and drifting resultant to the Cretaceous inaugural of the South Atlantic Ocean. The main parallel movements laterally to the north-eastern to south-western axis of the adjacent Benue Trough seems to have been interpreted to the north-south and north-western trending shear region to form the Mid-Niger Basin abrupt to the Benue Trough (Ojonugwa et al., 2018).
Nupe basin is an extension of the Anambra basin but it was subdivided in the Santonian. There is an accumulation of Pre-Santonian sediment in the lower Benue Trough and the adjacent southern Anambra Lithologically the Mamu Formation comprises of sandstone, siltstone with shale intercalation and coal seal deposit of deltaic to estuarine environments. Its lateral equivalent is the Bida Formation in the Nupe Basin (Obaje et al, 2013). The Ajali Sandstone Formation underlies the Mamu Formation and it lateral equivalent is the Patti, Sakpe and Enagi Formations of the Nupe Basin (Fig. 3). The predominant lithic component of this formation is well sorted sandstone. This sandstone is commonly interbeded with claystone and siltstone. This is eminent in Patti Formation. Agbaja iron Formation and Batati Formation are iron rich, they overlies the Patti and Batati Formation (Fig. 3).

Methodology
The input data's used in the study are nine aeromagnetic data namely; 100,000 and the total surveyed are is 27,225 square kilometers. The aeromagnetic data was enhanced using mathematical lters such as downward continuation, upward continuation, rst vertical derivative, analytical signal and reduction to the pole. For we to interpret the local body, the enhanced data is separated into religion eld and the residual eld using the multiple regression techniques. The input data used for the spectral analysis is the residual data and it helps in evaluation of depth to the magnetic sources; delineate Curie isotherm temperature and heat ow. Hence, the residual data were subjected to both graphical and mathematical modelling for quantitative interpretation, this help us to deduce the thermomagnetic properties in the area.
The borehole data obtained is the Enegbaki -Akpogu Oolitic iron ore deposit is situated at about 9km North East of Akpogu village which lies along Lokoja-Abuja Road. The deposit being investigated has an area of 1.0 sq.km and is capped by lateritic top layer, clay and some silica. The iron ore bearing plateau trends in a NE -SW direction with an average height of 263m above sea level. Twenty-ve (25) core holes placed at 250m grid interval were drilled using rotary type of drilling (Massenza Multipurpose drill).
Qualitatively interpretations were done using visual inspection of the total magnetic eld data, residue eld data and analytical signal map of the area. More so, core data were analysed and interpreted in order to produce the oolitic iron model in the area. Lastly, areas with geothermal energy potential were delineated using integrated geological and geophysical data. The work ow is shown in Fig. 4.

Results And Discussion
The total magnetic intensity (TMI) and residual anomaly maps ( Fig. 5 and Fig. 6) were contoured from the digitized data. Visual assessment of the TMI and residual anomalous maps reveals intricate form of magnetic signatures of both short and long wavelengths ( Fig. 5 and Fig. 6), this is an indication of variable magnetic intensities from diverse geologic sources as supported by the work of Okonkwo et al.
(2012) and these variable magnetic intensities is well pronounced in the study area. The mangetic intensity values of the TMI and residual anomalous maps ranges from 7800 to 8200 nT and -220 to 240 nT respectively ( Fig. 5 and 6). There are strong evidences of igneous intrusion when juxtaposed with the geologic map of the area. These intrusions account for the high level of faulting and folding around the study area.
More so, these areas mentioned above possess mostly close-spaced contour lines and this implies thinner sedimentary in llings (that is the depth to the top of the basement is shallow); Again, oval contours also encounter in this area may also be an indication of intrusive igneous bodies or lineaments containing groups of mineral deposit of great economic values as evident in the study area like Koton-Kar , Abaji, Bida and Paiko.
At the central parts (Pategi, Baro, Gulu, La agi) of the study area,contains widely spaced contour, signifying thick sedimentary in lling (that is deeper magnetic sources) within these areas (Fig. 5). It is therefore logical to conclude that the widely spaced contours in conjunction with the high magnetic intensity values detected in the area may have been related with a vein bearing mineral this assertion is supported by the geologic map of the study ( Fig. 7 and 8).
The structural alignment of the area was evaluated using the anomalous residual shaded maps ( Fig. 9) produced from the residual acoustic eld intensity map. The major trend of the lineament were NE-SW and minor ones trending E-W and NW-SE. This structural direction is in conformity with previous work by (Chinwuko,  The residual magnetic anomalies data were subjected to both Peter's half-slope method and spectral analysis for depth calculation (sedimentary thicknesses) and depth modelling within the study area ( Fig. 10). The result of the analysis is shown in Table 1. The interpreted result reveals two-layer depth model; the shallower magnetic depth bodies which varies from 0.54 to 1.87km (for Peter's slope method), and 1.27 to 1.96km; for spectral analysis interpretation, deeper magnetic bodies of depth range of 2.01 to 3.27km (for Peter's slope method) and 2.01 to 4.27km for spectral analysis. Again, the depth to the centroid obtained through the spectral analysis reveals depth range of 9.79 to 15.75km across the area (Table 1). From the depth calculations using spectral analysis, two anomalous depth sources were interpreted. The deeply seated magnetic anomalous sources varying from 1.91 to 3.78 km, and shallower seated magnetic anomalous sources which ranges from 0.98 to 1.98 km (Fig. 11). Deeply seated magnetic anomalous sources may possibly denote depth to the basal crystalline rocks, while the shallower seated magnetic anomalous sources maybe associated with intrusive igneous and/or magnetized bodies coved by sediment. The basal depth is abysmal in the entire south and central part of the study area, whereas, at other parts of the area such as Kutawenji, La aji, Koton-Kar and Lapai areas have shallower sources.
More so, the 3-D surface plot of depth to the top of the anomalous magnetic body shows presence of peaks (uplifts) and depressions (troughs). Around Agaje, Egbako, Olle, Mopa and Abaji areas, there visible linear depressions and these areas reveal higher sediments than the other parts such as Kutaeregi, Paiko, Lapai, La agi, Aiyegunle, and Koton-Kar areas which have prevalent uplifts (peaks) in conjunction with lower sedimentary thicknesses (Fig. 12). The presence of these peaks (uplifts) suggests that there are numerous intrusive bodies around these areas; as a result, there are more tectonic activities in the areas associated with depressional feature. According to Biswas et al, (2016 and 2017) and also Biswas (2015), these identi ed igneous intrusives generally occur as silly and dykes.

Prevalent Intrusive sources and Geologic model across a pro le in the area
The geologic model ( Fig. 13 and 14) obtained from the quantitative interpretation of the residual magnetic data reveals four geological stages namely: sedimentary stage, ferromagnetic bodies, paramagnetic bodies and lower crust bodies. The sedimentary in lling along this pro le has a variable thickness which ranges from 0.20-3.21 km. The deepest sedimentary cover occurs at the eastern and western ends of the pro le (Egbe) (Fig. 14a) while the shallowest is at the middle. More so, the ferromagnetic bodies underlain the sedimentary layer and it has an average thickness of 6.1 km, the depth to the ferromagnetic bodies range from 12.04 to 13.0 km in this pro le. Also, the paramagnetic bodies which are associated with Curie point depths and upper mantle have depth range of 13.10 to 24.01 km. It is believe that the heat from the mantle must have reworked the magnetic body making them to loss the magnetism. The model suggests that the deepest Curie point depth occurs in both left and right hand side of the model while at the middle, the Curie point depth is shallowest. The model interpretation is also shown in Fig. 14b.

Core Sample Analysis and Its Implication around Koton-Kar
Core sample analysis within Koton-Kar reveals different lithologic units within the sample locations. The depth to the oolitic iron ore obtained across the area ranges from 20.79 to 101.2m with an average of 38.52m (Table 2). The elevation map of Koton-Kar area (Fig. 15) was produced for effective correlation with the ooolitic iron ore level map (Fig. 15). Nevertheless, the ooolitic iron ore level with respect to mean sea level across Koton-Kar area were computed by subtracting depths to oolitic iron ore layers deduced from the core drilling from surface elevations obtained during the data acquisition (Table 2). Thus, maps of the oolitic iron ore levels with respect to elevations were generated in order to depict the possible trend of the oolitic iron ore level across the study area (Fig. 16). Considering elevation and oolitic iron ore maps of Koton-Kar area (Figs. 15 and 16), it is evident that the trend of the oolitic iron ore level is predominantly in NE-SW direction and mineralogically, the mineralization will ow this trend.
More so, Fig. 15 reveals that the anomalous body level (Ooilitic iron ore level) is deep around northeast and southwestern parts of Koton-Kar ; whereas at northwestern part of Koton-Kar , the depth to the anomalous body (Ooilitic iron ore) is shallow with average oolitic iron ore deposit level as 225m.
Furthermore, pro les running from T-T l at Fig. 14 and U-U l at Fig. 15 were superimposed in order to establish the variation phenomenon across the area. Here, it was observed that the oolitic iron ore level follows the topographical level which implies that the topography controls the con guration of the iron ore level (Fig. 17).
Result from qualitative interpretation reveals that the area is extremely fractured, with fractures trending NE-SW direction and minor ones in NW-SE and E-W directions, the trend of these structures is in conformity with the trend of structures within the Nupe basin. These fractures can serve as migrating pathway for both geothermal energy ow and uids like hydrocarbon. The work of Ojonugwa et al, (2018) believes that the strati ed nature of the carbonaceous shale with intercalated sandstone found within the study area may probably favour uid migration into potential reservoir rocks that are made up mostly of uvial deposit, ood plain and shelf sandstones of the Lokoja and the Patti Formations.
Quantitatively, the depth to the top of magnetic basement rock (sedimentary thickness) within the study area have been delineated along seven different pro le lines using Peter's half slope and Spectral analysis of aeromagnetic data and the result shows two depth model across the area, namely; the shallower magnetic sources which vary from 0.56 to 1.96 km and the deeper magnetic sources which Result from the core drill data reveals that the anomalous body level (Ooilitic iron ore level) is deep around northeast and southwestern parts of Koton-Kar ; whereas at northwestern part of Koton-Kar , the depth to the anomalous body (Ooilitic iron ore) is shallow with average oolitic iron ore deposit level as 225m.
Integrating all the results and deductions obtained in this work, it can be deduce that those regions (such as Baro, Kutaeregi, Kutiwenji and Koton-karif) with shallow or low sedimentary thicknesses are possible areas that are associated with magnetic mineral ore deposits like oolitic iron ore identi ed in Koton-Kar area (Fig. 18). Actually, these areas might not support hydrocarbon generation.
However, the sedimentary in llings around Pategi, Agaje, Olle and Baro area is relatively high in conjunction with the high geothermal gradient and heat ow; will possibly pave way for hydrocarbon potential in this area provided that all other conditions for hydrocarbon generation is available. Previous workers such as, Ikumbur  suggest that the study area contains carbonaceous shales intercalated with sandstone and clay, which is a potential source rock.

Conclusions
The qualitative interpretation of the TMI data and residual anomalous data shows a complex con guration of magnetic signatures of the wavelengths. This varying amplitude of the anomalous bodies denotes change in magnetic intensities from diverse acoustic sources and it evident in the study area (such as magnetic intensity range between 7200 nT and 8500 nT ). Around Paiko, Kutaeregi, Kotonkar , Bida, Lapai, Agaje, and Aiyegunle areas, The TMI and residual anomalous maps shows the underlying basement as having magnetic intensities range ranges from 7500 to 8460 nT and -220 to 240 nT respectively. There are strong evidences of igneous intrusion when juxtaposed with the geologic map of the area. However, at the central parts (Pategi, Baro, Gulu, La agi areas) and beyond, the contour lines are extensively spaced indicating that the sedimentary in lling within these areas is thick. We then conclude that the widely spaced contours in conjunction with the high magnetic acoustic eld intensity values interpreted may have been related to the vein bearing mineral and this a rmation is reinforced by the geologic information of the study.
The 3D structural model shows the presence of uplifts and depressions, a characteristic manifestation of secondary structural feature named folds. Lineament alignment in the area was produced from the residual anomalous magnetic data. The main structural trends were NE-SW with minor E-W and NW-SE trends. Juxtaposing these lineaments on the geological of the area, it shows high concentration of structural lineament in area with intense tectonic activities which was believed to be due to intrusive igneous bodies while the less concentrated area fall around zone of high sedimentary in llings suggesting deeply seated basement rock.
The structural con guration of the study area is in conformity with the trend of structures within the basin which trend NE-SW and NNE-SSW as prominent trend, and E-W, NW-SE, and W-E are the minor trends. Core sample analysis within some parts of the study area reveals that depth to top of the oolitic iron ore across the area ranges from 20.79 to 101.2m with an average of 38.52m. Thus, maps of the oolitic iron ore levels with respect to elevations were generated in order to depict the possibly trend of the oolitic iron ore level across the study area to be predominantly in NE-SW direction and mineralogical, the mineralization will ow this trend. Figure 1 Accessibility Map of the Area   Real view distribution of total magnetic eld intensity in the area Figure 8 Analytical Map of the study the area    Juxtaposed topography and oolitic ore deposit level of Koton-Kar area Figure 18 A generalized model for possible mineral deposits and hydrocarbon accumulation