Analyses of high resolution aeromagnetic data for structural and porphyry mineral deposit mapping of the nigerian younger granite ring complexes, North - Central Nigeria

Journal Pre-proof Analyses of high resolution aeromagnetic data for structural and porphyry mineral deposit mapping of the nigerian younger granite ring complexes, North - Central Nigeria Olawale Olakunle Osinowo, Kingsley Alumona, Abel Idowu Olayinka PII:

S1464-343X(19)30360-7

DOI:

https://doi.org/10.1016/j.jafrearsci.2019.103705

Reference:

AES 103705

To appear in:

Journal of African Earth Sciences

Received Date: 29 March 2019 Revised Date:

2 November 2019

Accepted Date: 5 November 2019

Please cite this article as: Osinowo, O.O., Alumona, K., Olayinka, A.I., Analyses of high resolution aeromagnetic data for structural and porphyry mineral deposit mapping of the nigerian younger granite ring complexes, North - Central Nigeria, Journal of African Earth Sciences (2019), doi: https:// doi.org/10.1016/j.jafrearsci.2019.103705. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Analyses of High Resolution Aeromagnetic Data for Structural and Porphyry Mineral Deposit Mapping of the Nigerian Younger Granite Ring Complexes, North - central Nigeria

Olawale Olakunle Osinowo, Kingsley Alumona and Abel Idowu Olayinka Department of Geology, University of Ibadan, Ibadan, Nigeria

Corresponding Author [email protected], [email protected] Mobile +234 (0) 812 410 9193.

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Abstract This study report the results of the different geophysical analyses carried out on the aeromagnetic data of north-central Nigeria with the object of delineating the occurrence and evaluate the structural influence on the emplacement of the porphyry mineral deposits of the Ring Complexes. The residual magnetic intensity extracted using Low Cut Guassian filter, from the high resolution aeromagnetic data of north-central Nigeria were filtered to remove high frequency near surface cultural noise and subsequently Reduced to Pole at Low Latitude (RTPLL) to focus anomaly peaks on the corresponding geological sources in order to resolve anomaly complexity associated with mid-latitude potential data. The data were further enhanced using upward continuation filter to extract desired magnetic responses at various continuation distances. Analytic Signal (AS), Centre for Exploration Techniques (CET) Grid and Porphyry Analyses were then carried out on the filtered, transformed and enhanced magnetic data to generate diagnostic magnetic attributes employed to map the boundaries of geologic magnetic sources, extract linear features and delineate the occurrence of porphyry mineral deposits across the study area. The AS responses defined twenty-four (24) arcuate / circular shaped AS anomalies which correspond to Younger Granites Ring Complexes in the northeast, northwest, southeast and central part of the study area from the low AS response Basement Complex rocks that host them. The CET grid analysis delineates several linear structures which generally trend NE – SW, NNE – SSW and ENE – WSW directions and their average length (500 m – 17500 m) and population (212 – 32) directly and inversely, respectively, correlate with the upward continuation distances. The CET porphyry analysis also delineates fifty (50) porphyry mineral deposits which vary in radial size from 300 m – 4300 m. The occurrence of the porphyry mineral deposits within or in close proximity with the Younger Granite Ring Complexes which are either crosscut or flanked by the linear structures, suggests hydrothermal non-orogenic origin of the structurally controlled ring complexes and the associated porphyry mineralisation that result from phase mineralisation that accompanied Jurassic hydrothermal activities. Keywords: Younger Granites; aeromagnetic anomalies; analytical signal; CET, lineament; porphyry deposits

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Introduction The occurrence of Ring complexes which often present circular or cylindrical shaped granitic body with characteristic ring-shaped outcrop often associated with continental volcanic provinces (Kearey, 2001) have been reported in different parts of the world, notable among which include the Wadi Dib and the Katherina ring complexes in Egypt, Loch Ba in Scotland, ring complexes of the Nuanetsi province in Zimbabwe, the Pilanesberg alkaline ring complex in South Africa, Adrar Bous complex in northern Niger and Nigerian Younger Granite Ring complexes of north-central Nigeria to mention just a few. Their mode of occurrence, distribution, unique structural geometry, mineralogical composition, association with the host rocks and most especially their economic mineral resource potential have been copiously discussed by several authors (e.g. Billings, 1943; Bowden et al., 1976; Gudmundsson, et al., 1997; Johnson, et al., 1999 ). Several studies have also been carried out on the Nigeria Younger Granite Ring Complexes located in the north central part of Nigeria which were reported to mostly stand out as relatively high relief ring dykes and cone sheets bodies of non-orogenic intrusive rocks of Jurrassic age (Bowden, 1970; Obaje, 2009) that discordantly intrude the Basement Complex of North central Nigeria to form high hills. Twenty-four (24) Younger Granite Complexes which include Amo, Buji, Forum, Ganawuri, Jarawa, Jere-Sanga and Jos - Bukuru, Junguru, Kagoro, Kigom, Kofai, Kudaru, Kunkur, Kwandonkaya, Miango, Ririwai, Ropp, Rukuba, Sha - Kalen, Saiya - Shokobo, Sara - Fier, Shere, Sutumi, and Tongolo Complexes, located around Jos, Vom, Sango, Toro, Rahama, Mangu, Pankshin, Kafancha have been identified to occur between Latitude 9ᵒ 001 N – 10o 301 N and Longitude 8ᵒ 001 E - 9ᵒ 301 E (Figure 1) in the north-central part of Nigeria. The north-central Nigeria younger granite ring complexes like many other have been documented to have huge economic mineral resource potential which have only been partly tapped in the form of tin and columbite mining in north central Nigeria. Several other resource potentials having capacity to generate employment and wealth for the nation remain largely untapped. Examples include associated hydrothermal products such as Zinc (Zn) and Niobium (Nb), with Copper (Cu), Iron (Fe), Bithmus (Bi), alluvial deposits of cassiterite and columbite, radioactive mineral such as Thorite, Monazite and Uraninite and rare-earth elements (Obaje, 2009). Although several geological mapping, geochemical, petrological, dating studies among others have been done on the younger granite ring complexes of north-central Nigeria, very few 3

regional geophysical studies have been carried out, especially to establish their structural architecture which has been reported to control the mineralisation of porphyry mineral deposits. This study through the analyses and interpretation of processed, enhanced and transformed High Resolution Aeromagnetic (HRAM) data of north-central Nigeria, acquired by Fugro Geophysical Survey Limited for the Nigerian Geological Survey Agency (NGSA) identified and mapped the geometry of the ring complexes, defined the linear structures and trend across the study area, delineated associated porphyry-style mineralization as well as established the existing relationship between the younger granite rock complexes, structures and porphyry mineral deposits, especially as they play controlling effect upon one and other.

Figure 1 Location map of the study area. Geology of the Nigerian Younger Granites The geology of Nigeria Younger Granite has been described by several workers. Falconer (1911) used the term “Older Granite” to distinguish the orogenic granites of the Basement Complex of Nigeria, which are mainly Pan-African in age from non-orogenic granites, described as Younger Granites, which are mostly Mesozoic in age. Several explanations have been proposed for the mode of emplacement and the structural setting of the anorogenic ring complexes, as well as for their relationship with the dynamic environment. Clough et al., (1909) explained the emplacement of ring structures by a cauldron subsidence process 4

resulting from magma intrusion. Anderson (1936) further explained the mechanics of the successive intrusions (cone sheets and ring dykes) on the basis of strain distribution related to the pressure of an ellipsoidal magma chamber. Those concepts have been used to explain the emplacement of the ring complexes in Nigerian province (Jacobson et al., 1958). The main phase of acid magmatism in Nigerian Younger Granite province commenced during the Triassic times and continued to migrate in a generally southerly direction until the close of the Jurassic (Bowden et al., 1976). Three developmental stages were proposed for the Nigerian ring complexes by Bowden et al., (1976) to include (1) early stage of a large rhyolite volcano, which prior to the end of its stage, large amount of magma accumulated in the syn-volcanic reservoir about 5 km beneath the surface, (2) caldera and ring dike stage when the centre of the volcanic structure within a ring fault collapsed, magma arose along the fault and crystallized as granite porphyry which was extruded into the caldera rocks (3) intrusive stages when smaller granite intrusions were emplaced at increasing deeper levels. Twenty four (24) ring complexes (Figure. 2), whose individual massifs ranges in size from less than 1.68 km2 to as much as 640 km2 have been identified and mostly named after their localities (Adubok, 2008), Their ages were determined to generally increase southerly and with their emplacements controlled by basement fracture systems so that the ring complexes preferentially concentrate along ENE and WSW trends (Rahaman et al., 1984)

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Figure 2 Geologic map of the north-central Nigeria showing Nigerian Younger Granites Ring Complexes (Kinnaird 1985).

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Materials and Methods High resolution aeromagnetic data (0.001 nT) obtained at 80 m sensor mean terrain clearance by Fugro Airborne Survey Limited for the Nigerian Geological Survey Agency (NGSA) in 2009 were used for this study. The data were acquired with the aid of digital acquisition system using 3 x Scintrex CS3 Cesium Vapour magnetometer which sampled at 0.1 seconds along equally spaced flight line spaced at 500 m and tie line spacing of 5000 m along 135o and 45o respectively. Matching each geophysical data with the corresponding Global Positioning System (GPS), micro-levelling and deculturing were some of the data preprocessing operations that the acquired data were subjected to immediately after acquisition before handling the data to NGSA. The pre-processing of the data tied each of the data points to corresponding sampled location. It also minimizes mis-tie and eliminate cultural (man-made) magnetic effects resulting from artificial features such as wirelines, railways, steel tower, pipelines among others (Mauring, et al., 2002).

The pre-processed aeromagnetic data obtained from NGSA were subsequently subjected to several data processing / reduction operations to further reduce remaining noise to the barest minimum, extract data corresponding to the domain of interest, properly focus the data on the corresponding anomaly sources as well as enhance desired geological and structural signatures with the desire to ease interpretation. The Total Magnetic Intensity (TMI) data which comprises of both regional and residual magnetic intensity data were subjected to Gaussian low cut filtering in order to extract the relatively high frequency but short wavelength residual anomalies and cut off the low frequency but longer wavelength regional anomalies. This is essential since the desired targets (Ring Complexes will generate relatively local anomalies. The residual magnetic intensity data were further filtered using low pass Butterworth filter to eliminate near surface high frequency and other spiky/spurious data, mostly corresponding to noisy / damaged data, artefacts and cultural noise, that were not removed during the pre-processing stage (Lahti, 2010).

Reduction to Pole at Low Latitude The resultant filtered data were transformed using Reduction to the Pole at Low Latitude (RTPLL) transformation filter to simplify the magnetic anomalies and focus the anomaly peaks positively symmetrical over the corresponding geologic sources. This operation was necessary because the data were acquired at low latitude (around 9o) where due to the midlatitude effect, the anomaly peaks were asymmetrical skewed away either to the west or east 7

over the causative bodies. This is because at mid latitude, the magnetic inclination I is neither 90o as at the pole, nor 0o as at the equator, where the anomaly peaks are usually situated directly over the causative bodies. A modification of the Reduction to Pole (RTP) transformation filter, which is able to overcome the stability challenges associated with low latitude potential data due to low magnetic inclination value was applied. This is because at low latitude the conventional RTP breaks down, such that noise becomes amplified so much that the resultant data is dominated by linear features aligned along the direction of declination (Baranov and Naudy, 1964; Keating and Zerbo, 1996). RTPLL was preferred to Reduction to Equator (RTE) filter which was sometimes used for data acquired close to the equator in order to overcome the challenges associated RTP at low latitude. However, RTE generates negatively symmetrically focus data solutions, which is the inverted form of the magnetic anomalies.

Upward Continuation Upward continuation geophysical technique which is apt at suppressing shorter wavelengths corresponding to shallow magnetic effects and thereby enhancing longer wavelength signals that correspond to increasingly deeper depths was applied. This geophysical technique enhances progressively deeper signatures by focusing on magnetic signals wavelength corresponding to the upward continuation distance and attenuates all others by migrating the measuring magnetic sensor upward. The aeromagnetic data of north-central Nigeria were upwardly continued to 500 m, 1000 m, 2000 m, 3000 m, 4000 m, 5000 m, 6000 m up to 7000 m to obtain smoothened magnetic signatures at those continuation distances, thus enabling the examination of the subsurface magnetic structures and associated porphyry mineral deposits at different depths.

Analytic Signal Analytical Signal (AS) which is known to generate a form over the causative geologic body thus tracing out the boundary that defines the edges and shapes of the target geologic sources (Blakely and Simpson 1986; Fedi and Florio 2006) was carried out to map out the geometry and the distribution of the Nigerian Ring Complexes as they occur around the north-central part of Nigeria. The fact that the AS solution also referred to as the energy envelop of the magnetic anomalies (Nabighian, 1972, 1974) generated through the combination of horizontal and the vertical gradient (Equation 1) combines all vector components of the field 8

and transform the shape of magnetic anomaly from any magnetic inclination to positive symmetrical signature over the causative body (Nabighian, 1972; Pilkington et al., 2004) to generate a magnetization map (Roest et al., 1992) that is independent of the dip and magnetization direction (Nabighian, 1974; Roestet al., 1992) indicate the credibility of AS solution to correctly delineate the true location and geometry of the causative geologic bodies.

=

+

+

1

CET Grid and Porphyry Analyses The Centre for Exploration Targeting (CET) grid Analysis and CET Porphyry Analysis algorithm developed by the Geophysics and Image Analysis Group of the University of Western Australia and incorporated as extension into the Geosoft Oasis Montaj software (Geosoft, 2014) were used in this study for lineament. Structural and porphyry mineral deposit mapping.CET Grid Analysis was used for lineament and structural complexity mapping using the CET grid algorithm through five (5) different processing stages which include Texture Analysis, Phase Symmetry, Amplitude Thresholding, Skeletonisation and Skeleton to Vector conversion (Kovesi, 1991; Kovesi, 1997; Holden et al., 2008; Holdenet al, 2010; Lam et al., 1992). The Texture Analysis which determines the Standard Deviation of the magnetic anomaly distribution provides an estimate of the local variation in the magnetic anomalies. Phase Symmetry isolates laterally continuous line-like features of the complex texture while noise and background signals were suppressed by Amplitude Thresholding. Thinning of the extracted linear features until one pixel wide was achieved by Skeletonisation. Finally, the generated skeleton containing pixel-by-pixel coordinates of each of the extracted linear features were converted to Vectors to delineate lineaments as shown by the magnetic anomaly distribution of the processed aeromagnetic data of north-central Nigeria. The CET Porphyry analysis mapping extension was employed to map porphyry mineral deposits in four (4) different analytical steps. The first, known as Circular Feature Transform highlights the locations of circular features within the magnetic anomaly signatures. The Central Peak Detection finds the probable centres of the circular features highlighted by the Circular Feature Transform output, the Amplitude Contrast Transform algorithm emphasises 9

the boundaries of circular features while the Boundary Tracing stage outlines detected porphyry – like features. For detail information on the operation of the CET Porphyry algorithm, refer to Holden et al., (2011), Loy and Zelinsky, (2003), Williams and Shah, (1990) and Geosoft, (2014).

A simplified flow diagram which presents the different data processing steps adopted to refine, transform, enhance and extract relevant information for the evaluation of the aeromagnetic data of the Nigeria Ring Complexes, north-central Nigeria is presented in Figure 3.

Figure 3 Methodology flow chart of the study

Results This section presents the outcome of some of the data refining and enhancing processing operations performed on the aeromagnetic data of north-central Nigeria. The section also presents the various results in the form of profiles, maps, sections and plots generated to map the location and geometry of Nigeria Ring Complexes, define the structural architecture of

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the study area as well as delineate the occurrences of porphyry mineral deposits within the Nigerian Younger Granites Ring Complexes. Residual magnetic intensity map generated after the removal of the regional core field effect is presented in (Figure 4) with magnetic intensity distribution ranging from 141.7 nT to 122.2 nT and showing regions of high and low magnetic intensity. Relatively high positive residual magnetic intensity dominates the northeast, southwest, southeast and north-central parts of the study area, while low negative magnetic intensity distribution occur in the eastern and north-western parts of the study area.

Figure 4 Residual magnetic intensity map of the study area.Reduction to Pole at Low Latitude (RTPLL) The resultant magnetic intensity distribution map after the application of the RTPLL filter on the residual magnetic data is presented in figure 5. The map which displayed focused 11

magnetic anomaly peaks directly over the causative bodies presents relatively high positive magnetic intensity distributions (173.6 - 83.4 nT) in the north-west, north-east and central parts of the study area. The southwest, southeast and some parts of the central parts on the other hand present intermediate to low magnetic intensity values which range from 55.7 nT to-160.1nT.

Figure5 Reduction to pole at low latitude map

Upward Continuation The upward continuation filter applied to the reduced to pole at low latitude residual aeromagnetic data of north-central Nigeria smoothens and filters out high frequency noisy data which often correspond to shallow / near surface effects. The smoothened and enhanced

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deeper source effects which are most likely contributions from subsurface geology presents regions having relatively high positive magnetic intensity distribution which ranges in value from 132.7 nT to 45.4 nT (Figure 6), predominantly in the north-east and north-central parts of the study area. Intermediate to low magnetic intensity distribution (36.6 nT to -73.1 nT) are also visible in different parts of the study area, especially part of the north and some parts south-west. The upward continuation solutions also indicate that the distribution of the low magnetic intensity signatures decreases in value and extent with increasing continuation distances as shown in figures 6 (a & b) which present the magnetic intensity distribution at 3000 m and 7000 m continuation distances respectively. This implies that most geologically relevant magnetic signatures are

Figure 6: Upward continuation maps at (a) 3000 m and at (b) 7000 m.

Analytical Signal Map The AS map generated from the transformed and enhanced residual magnetic intensity data of the north-central Nigeria is presented in Figure 7. The AS map which presents magnetic intensity distribution enhances and clearly defined the boundaries of the Younger Granite Ring Complexes (0.03 – 0.01 nT), thus delineating them from the low AS surrounding Basement Complex (≤ 0.01 nT) that hosts the ring complexes (Figure 7).. The AS map 13

correlate positively with the existing geologic map of the study area which shows the location and geometry of some mapped Younger Granite Ring Complexes. Twenty-four (24) relatively high amplitude AS anomalies which vary in size and shape were delineated in the study area and they correspond to the Younger Granites intrusions. Sixteen (16) of the ring complexes are concentrated in the north and eight situated in the south of the study area. The most prominent ones among the delineated Younger Granites (Figure 7) are the Jos - Bukuru and Shere in the central part, Sara - Fier, Ropp and Jarawa in the south-east, Kagoro in the south-west, Sha - Kalan in the south, Geshere and Rishiwa in the north-west and Kwandonkaya and Kofia in the north-eastern part (Kinnaird, 1985; and Nwankwoala et al, 2017).

Imori Rishiwa Geshere

SaiyaShokobo

Dagga Allah Kwandonkaya

Jere-Sanga Amo Rukubo

Buji

Kofia

Shere

Shona

Miango Jos Bukuru

Kigom

Jarawa

Ganawuri

Forum Ropp

Kagoro Nok

Sara-Fiar Sha-kaleri Shawai

Analytical signal magnetization (nT)

Figure 7 Positive correlation between the AS map and the Geological map of the study area.

CET Grid Analysis for Lineament Mapping The lineament maps generated through CET grid analysis on the upwardly continued aeromagnetic data at different continuation distance (0 m – 7000 m: 1000m) indicate an average of 212 local and regional lineaments which range in length from 500 m to 17500 m, were extracted. The lineament map of extracted linear magnetic structures which reflect the 14

structural framework of the study area generated through CET grid analysis of upwardly continued aeromagnetic data at 3000 m is presented in figure 8a. The rose diagram generated from the plot of the extracted linear features to define the structural trend (Figure 8b) indicates that the extracted structures generally trend NE – SW, NNE – SSW and ENE – WSW.

` Figure 8(a) Extracted lineaments map and (b) Rose diagram at3000 m upward continution distance.

A careful examination of the lineament population which ranges from 212 to 30 in number and their respective average lengths which range from 500 m to 17500 m, extracted at progressively increasing (0 - 7000 m) upward continuation distances, respectively, indicate that while the length of individual lineament increases with continuation distances, the population / number of the extracted linear structure decreases with continuation distances (Figure 9). This indicates that increase in continuation distances filters out shorter wavelengths to focus on longer wavelength magnetic signals that corresponds to continuously deeper depths. At shorter continuation distances, numerous relatively short length lineaments were extracted whereas at longer continuation distances fewer longer length regional lineaments were generated. This implies that the depth sensitive upward continuation filters are variedly sensitive to the subsurface linear structures that occur at different depths that are themselves limited in depth distribution, where the subsurface lineaments decreases in population with depth as they become more regional and longer in length. 15

Figure 9 Relationship Lineament population and lengthswith upward continuation distance. CET Porphyry Analysis The CET Porphyry analysis for the delineation of porphyry mineral deposits across the study area carried out at continuation distances (3000 m), which reflects the geology of the area (Figure 7) identified fifty (50) likely porphyry mineral deposit locations (Figure 10a) mostly occurring within the Younger Granite Ring Complexes, which become very visible when the generated porphyry mineral deposit are superimposed on the AS map of the study area (Figure 10b). The Porphyry mineral deposits, carried out at continuation distances (3000 m), ranges in size from 900 m to 4300 m. The lower limit of the estimated porphyry radii for different continuation distances, which generally range from 500 m – 4300 m (Figure 11) could be a better conservative value to be adopted. However, porphyry radii of similar and larger sizes have been reported by several authors, e.g. Lowell and Guilbert (1970), Tosdal and Richards (2001), Seedorff et al., (2005) and Perelló et al., (2008). The delineated porphyry mineral deposit locations mainly concentrate in the northwest of the study areas within the Rishiwa and Geshere Complexes. Some deposits were also delineated in the central part of the study area within the Jos - Bukuru and Shere Complexes. Deposits also occurred within the Kwandonkaya and Kafia Complexes in the north-east as well as around Sara - Fier and Ropp Complexes in the south-eastern part of the study area (Figure 10). 16

Fewer porphyry deposits were also identified around the Sha - Kalan and Kagoro Complexes in the south and northwest of the study area, respectively.

Analytical signal magnetization (nT)

Figure 10(a) Porphyry mineral deposit map, (b) Superimposed Porphyries distribution on AS map of north-central Nigeria. An increasing relationship of the size and decrease in number / population of delineated porphyry mineral deposits were established with increasing upward continuation distance as shown in figure 11.

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Figure 11 Relationship between upward continuation distance and number of porphyries Relationship between Lineaments with Analytical Signal and Porphyry mineral deposits The integration of the extracted lineaments and AS result indicate a positive correlation where most of the delineated zones of high AS signatures are either intercepted by a linear structure or flanked by the extracted linear structure(s) (Figure 12). A positive correlation was also established between the mapped porphyry mineral deposits and the extracted linear structures with most of the delineated porphyry mineral deposit occurring in very close proximity to the extracted linear structures as shown in figure 13. This indicates that the occurrence and the emplacement of the porphyry mineral deposits is greatly controlled by the fault system across the study area.

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Fig. 12: Extracted lineament on AS map of the study area.

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Figure 13 Porphyry mineral deposits superimposed on the extracted lineament map of the study area.

Discussion The processed, transformed and enhanced magnetic intensity data of the north-central part of Nigeria indicate widely varied magnetic intensities distribution which ranges in value from 141.7 nT to -123.2 nT (Figure 3). This suggests occurrence of rocks of different magnetic properties within the study area. Relatively high magnetic intensity distribution (141.7 nT to 58.3 nT) which dominate most of the north-western and central part of the study area mapped out regions having relatively high magnetic susceptibility rocks. Intermediate (58.3 nT to 1nT) and low (0 to -123.2 nT) magnetic intensity distribution which dominate part of the north-east and southern part of the study area have intermediate and low magnetic susceptible rocks, respectively. The AS map (Figure 8) identified two prominent geologic units, the relatively high AS responses having distinct boundaries which traced out circular / ring or nearly actuate boundaries and they correspond to the Younger Granite Ring Complexes of north-central Nigeria. The Basement Complex rocks on the other hand mostly displayed lower AS signatures that formed the background rocks hosting the ring complexes. The lineament map presented in figure 8 shows the population and the lengths of individual extracted linear structures that general trend along ENE - WSW and NE – SW directions (Chukwu-Ike and Norman, 1997; Bala et al., 2000; Raimi et al., 2014; Yenne et al., 2015; Opara et al., 2015; Nwankwoala et al., 2017). An increasing relationship of the delineated lineament’s lengths and decreasing trend of the lineament population with increasing continuation distances indicate the occurrence of numerous local short length linear structures closer to the surface. However, at relatively considerable depth the linear structure become more regional and are few in number (Hronsky, 2013; Opara, et al., 2015).

The occurrence of higher number of porphyry mineral deposits at shallow depths (Figure 12) indicate that the porphyries are mostly concentrated closer to the surface, though fewer large size porphyries also occurred relatively deeper into the subsurface. A positive correlation of the delineated porphyry mineral deposits with the AS anomalies which often traced out the geometries and boundaries of Younger Granite Ring Complexes (Figures 12 & 13) (Billingsley and Locke, 1935; Holden et al., 2011; Hronsky, 2013; Sayed and Ahmed, 2016) 20

together with the extracted linear subsurface structures that mostly trend along ENE - WSW and NE - SW indicate that the delineated porphyry mineral deposits mostly occurred within the Younger Granite Ring Complexes (regions of high AS anomalies) and are either intercepted by or in close proximity to the linear structures. This positive relationship as indicated by the coexistence of the ring complexes, regional and local fractures as well as porphyry mineral deposits suggests tectonic origin of the Nigeria Younger Granite Ring Complexes which occurred due to the reactivation that occurred during the Jurassic tectonic activities. The Jurassic tectonic events have been reported to be associated with some hydrothermal activities that resulted in phase mineralisation and the formation of NE – SW and some NW – SE trending faults which guided the origin and the emplacement of economic mineral of alluvial cassiterite and associated minerals like arsenopyrite, chalcopyrite, columbite and tantalite (Kinnaird and Bowden, 1987; Obaje, 2009). This agrees with the account of Kinnaird et al, 2016 and Kogbe, 1989 who suggested that over 95 per cent of the tin (cassiterite) produced in Nigeria was mined in the Younger Granite Province and were won from alluvial deposits (porphyries) derived from the tin-bearing granites and lodes. . Conclusion The invaluable quality of subsurface geological information derived from the analyses of HRAM data over the Nigeria Younger Granite Ring Complexes, north-central Nigeria has been demonstrated in this study. Analytic Signal analysis which generated AS map that clearly discriminate the Younger Granites Ring Complexes from the surrounding rocks of the Basement Complex of north-central Nigeria defined the characteristic arcuate or ring shaped geometry of the ring complexes that visibly discriminate them from the surrounding host rocks. Several lineament extraction analyses including the CET Grid Analysis for lineament mapping have also generated significant linear features which generally trend NE - SW, NNE - SSW and ENE - WSW directions and defined the structural architecture of the terrain. The CET Porphyry Detection for mapping porphyry-style mineralisation also generated credible deposit mapping solution which did not only identify the porphyry mineral deposits within the ring complexes but also establish the controlling influence that structures play on the emplacement of the porphyries which mainly concentrated along the NW, NE, south-central and SE of the study area.

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The positive correlation and the interrelationship among the AS delineated geobodies (younger granite ring complexes), the linear structures and the identified porphyry mineral deposits provide very salient information about the anarogenic origin of the ring complexes as well as provide information that could assist in the design, planning and development of exploration and exploitation campaign of the porphyry mineral deposits, which could play important role in the diversification from the present oil dependent mono economy.

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Research Highlights •

Twenty four (24) arcuate shaped Younger Granites Ring Complexes were defined.

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Linear structures ranging in length from 0.5 – 17.5 km were delineated

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Delineated lineaments generally trend NE – SW, NNE – SSW and ENE – WSW.

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Fifty (50) porphyry mineral deposits which vary in radial size from 300 m – 4300 m.

Declaration of interests

xThe authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: