The research area sits within the Igarra Schist Belt, a segment of the West African Precambrian terrain dominated by migmatites, gneisses, quartzite mica schists, a psammite-pelite sequence and hornblende granite gneiss. These rocks were forged and deformed during two major orogenic episodes: the Eburnean event roughly two billion years ago and the Pan-African orogeny around 600 million years ago. That long tectonic history left the region riddled with faults, folds and shear zones, the classic plumbing through which hot, metal-bearing fluids migrate and deposit gold in quartz veins. The terrain also hosts limestone, granite, kaolin, feldspar, quartz, lateritic iron ore and pegmatites carrying gemstones such as tourmaline, garnet and zircon, yet the structural controls on its mineral wealth have remained only partially understood.

The dataset underpinning the study came from a nationwide airborne geophysical survey flown between 2005 and 2009 by Fugro Airborne Surveys, funded by the Federal Government of Nigeria and the World Bank and archived by the Nigerian Geological Survey Agency. Flight lines ran north-south at 500 metre spacing with east-west tie lines every 2,000 metres, at an average terrain clearance of just 80 metres. The researchers processed the total magnetic intensity data in Oasis Montaj, removing the International Geomagnetic Reference Field to isolate residual anomalies, applying tie-line and micro-leveling to suppress survey noise, and gridding the measurements at 125 metre resolution using the minimum curvature method.

Because the study area lies at low magnetic latitude, with an inclination of minus 13.91 degrees and a declination of minus 2.15 degrees, raw magnetic anomalies appear asymmetric and shifted away from the rocks that cause them. To correct this distortion, the team applied a Reduction to the Equator transformation, which mathematically repositions anomalies as if measured at the magnetic equator, centering them directly over their sources. The resulting map, with residual intensity values ranging from minus 79.1 to 60.3 nanotesla, showed noticeably realigned anomaly peaks and troughs, particularly in the southeast and along mapped lithological contacts, sharpening the definition of geological boundaries that the uncorrected data blurred.

The raw total magnetic intensity map itself, spanning 32,931.4 to 33,079.9 nanotesla, already told a geological story. Magnetic highs cluster in the northwest, southwest and northeast, while prominent lows occupy the central and southeastern portions around Aiyetoro, Iretutu, Igarra, Somorika, Ojirami Oke and Akuku. When overlaid on the geological map, the lows correspond to the psammite-pelite sequence and weathered hornblende granite gneiss, both depleted in magnetite through composition and leaching. Pockets of high magnetization within the quartzite mica schist terrain suggest either unaltered basement blocks or intrusive magmatic bodies emplaced during later tectonic events, precisely the kind of magnetization contrasts that flag lithological boundaries and potential mineralization zones.

To separate deep crustal architecture from shallow detail, the team decomposed the field into regional and residual components and extended the reduced grid upward to 10 kilometres above the survey plane. The regional map revealed broad northeast-southwest trending anomalies tied to crustal thickness and deep tectonic fabric, while the residual field, spanning minus 72.7 to 68.3 nanotesla, isolated shallow lithological changes and structural discontinuities superimposed on those regional trends. First and second vertical derivative filters then acted as edge detectors: the first derivative exposed north-south trending shallow faults and fractures in the western and eastern parts of the area, and the second derivative refined those boundaries with high precision, delineating shallow fault corridors and fracture networks that define prospective mineralization zones.

The most diagnostic tool proved to be the Analytic Signal Amplitude, which is independent of both the Earth’s field direction and the magnetization of the source, making it especially robust at low latitudes. Its peaks fall directly over the edges of magnetized bodies, and in this survey its values ranged from 0.01 to 0.294 nanotesla per metre, sorting the terrain into three magnetic domains. Low amplitudes of 0.01 to 0.045 nanotesla per metre mark quartz-rich metasediments and weathered granites; moderate values of 0.045 to 0.101 reflect the granite-gneiss complex; and high amplitudes above 0.294 concentrate along the western margin and parts of the southeast, coinciding with migmatite and the psammite-pelite sequence. Critically, the high-amplitude zones align with lithological boundaries and structural intersections, the very settings where hydrothermal fluids are channeled and gold is deposited.

Lineaments digitized from the derivative and analytic signal maps and plotted in rose diagrams revealed a dominant east-northeast to west-southwest structural grain, followed by west-northwest to east-southeast trends, with minor northeast-southwest and northwest-southeast orientations. This fabric matches the Pan-African tectonic trends documented across the Nigerian Basement Complex and the Benue Trough in earlier studies, indicating that regional deformation, not local accident, dictated where mineralizing fluids could travel. The synthesis yields a hierarchical framework: crustal-scale northeast-southwest basement grain overlain by east-northeast and west-northwest fault corridors where edge-detection anomalies, derivative lineaments and mapped geology converge.

The decisive test came from the ground. Nine known artisanal gold mining sites were overlaid on the analytic signal map, and most sit directly on or adjacent to high-amplitude anomalies and major structural intersections. The northeastern workings in particular follow the contact between the schist and porphyritic granite, close to a major mapped structure. This spatial correlation validates the geophysical interpretation and confirms that gold emplacement here reflects the combined influence of favourable lithologies, including biotite granite, granite gneiss, quartzite-mica schist and migmatites, and tectonic deformation rather than lithology alone. Areas around Ojirami, Akuku, Somorika, Ogbe and the northeastern sector of the study area emerge as priority exploration targets where structural complexity, magnetic signatures and known occurrences converge.

Beyond its immediate findings, the study makes a case for aeromagnetic interpretation as a cost-efficient, non-invasive first step in exploring crystalline basement terrains where surface exposure is poor and conventional mapping struggles. The authors position their structural framework as the essential foundation for future three-dimensional magnetic inversion and magnetization vector inversion modelling, and for integration with aeroradiometric, gravity, remote sensing and geochemical datasets within a GIS-based prospectivity model. For a nation seeking to grow its solid mineral reserves, the message is that the gold-bearing architecture of the Igarra Schist Belt was already recorded in the airborne data; it simply needed the right filters to be seen.

Subject of Research: Aeromagnetic delineation of structurally controlled gold mineralization zones in the Igarra Schist Belt, southwestern Nigerian Basement Complex

Article Title: Preliminary assessment for delineation of mineralized structures and potential gold alteration zones in Igarra Schist Belt southwestern Nigerian Basement Complex

Article References: Ogbamikhumi, A., Salami, S. A., Ehinlaiye, A. O., Worenwu, W. E., Aigbadon, G. O., Ojeaga, K., Igbinigie, N. S., & Tawey, M. D. (2026). Preliminary assessment for delineation of mineralized structures and potential gold alteration zones in Igarra Schist Belt southwestern Nigerian Basement Complex. Discover Geoscience, 4(1), Article 321. https://doi.org/10.1007/s44288-026-00695-z

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00695-z

Keywords: aeromagnetic data, gold mineralization, Igarra Schist Belt, Nigerian Basement Complex, analytic signal, vertical derivatives, Reduction to the Equator, structural geology, Pan-African orogeny, mineral exploration, shear zones, hydrothermal fluids

The Nigerian Younger Granite province is one of the world’s classic examples of anorogenic magmatism. It comprises 53 distinct silica-oversaturated syenite-granite ring complexes spread across roughly 7,500 square kilometres within a north-trending belt about 400 kilometres long and 160 kilometres wide. These complexes were emplaced into the Precambrian Nigerian Basement Complex during the Jurassic period, with radiometric ages ranging from approximately 147 to 173 million years. Geologists interpret the province as the product of within-plate magmatism linked to the early opening of the South Atlantic Ocean, meaning the alkaline igneous activity was decoupled from the compressional forces of subduction zones. Individual complexes typically display multiple intrusive phases, from earlier basic and intermediate rocks to later, highly evolved granites and rhyolites, reflecting progressive fractional crystallisation of mantle-derived or hybrid melts.

The Tongolo area, located in Toro Local Government Area about 80 kilometres southeast of Bauchi city, sits within the broader Jos Plateau-Bauchi ring complex province. It has historically been recognised for tin-niobium mineralisation hosted in biotite granites. However, recent artisanal mining activity and reports of new base-metal occurrences prompted field investigations that confirmed the presence of lead, zinc and copper minerals. The area is underlain by three principal lithological units: the Tongolo Younger Granite Complex, composed of biotite granites, porphyritic granites and rhyolites; the Dagga-Allah Porphyry, a polygonal system of dykes cutting across the basement south of the granite complex; and the Palaeoproterozoic migmatite-gneiss basement reworked during the Pan-African Orogeny around 600 million years ago.

To characterise these rocks, the researchers collected seventeen representative samples from fresh outcrops across the roughly 282-square-kilometre study area during two years of fieldwork. After petrographic analysis at the University of Jos, the samples were crushed, pulverised and digested using a multi-acid mixture of hydrofluoric, nitric and perchloric acids to ensure complete dissolution of resistant minerals such as zircon and monazite. The resulting solutions were analysed for major and trace elements on an Agilent 4200 Microwave Plasma-Atomic Emission Spectrometer. Analytical accuracy was verified against certified reference materials, with recoveries for arsenic in the BGS 119 standard reaching 92.96 percent and relative standard deviations for key elements remaining below 3 percent across replicates.

The major-element results paint a clear picture of highly evolved, ferroan magmatism. The granitic rocks contain high silica dioxide, ranging from 68.50 to 75.25 weight percent with a mean of 72.66 weight percent, excluding a single dolerite sample. They also show elevated iron numbers, averaging 0.74, and low magnesium numbers averaging 36.56, a combination that is diagnostic of A-type granitoid affinity. On the Total Alkali-Silica classification diagram, the granites and rhyolites plot predominantly in the granite field, while the dolerite falls in the gabbroic diorite field. Harker variation diagrams reveal systematic negative correlations between silica and aluminium, iron, magnesium, calcium, manganese and phosphorus oxides, consistent with fractional crystallisation as the dominant differentiation process. Notably, the declining phosphorus trend points to early apatite fractionation, a mechanism widely recognised for generating metal-enriched residual melts in ore-forming granitic systems.

The trace-element data are where the story becomes genuinely striking. Compared with average upper continental crust, the Tongolo suite is dramatically enriched in uranium, with concentrations of 96 to 319 parts per million representing enrichment factors of roughly 35 to 118 times crustal values. Lead is enriched 4 to 19 times, zinc 3 to 37 times, and gallium about 4 times crustal abundances, while thorium, barium, vanadium, chromium and nickel are depleted. Primitive mantle-normalised spider diagrams show strong positive anomalies at uranium, lead and gallium alongside pronounced negative anomalies at barium, chromium, vanadium and nickel, a signature consistent with A-type affinity and advanced fractional crystallisation. Europium anomalies calculated for five viable samples range from 0.059 to 0.228, with a mean of 0.111, indicating extensive plagioclase fractionation characteristic of high-temperature, anhydrous A-type magmatic systems.

The base-metal concentrations themselves are remarkable. Lead was detected in every sample, ranging from 72.7 to 324.2 parts per million, indicating a consistent and widespread enrichment across the study area. Zinc spans a much wider range, from 154.7 to 8,537.9 parts per million, while copper varies from 10.6 to a staggering 20,498.7 parts per million. The extreme upper values for both zinc and copper occur in a single sample of pegmatitic granite-gneiss from Dagga-Allah, which records copper at approximately 2.05 weight percent and zinc at 0.85 weight percent, one to two orders of magnitude above the suite average. Elevated cadmium, reaching 72.8 parts per million in another sample, tracks the zinc enrichment and serves as a geochemical pathfinder for sphalerite-hosted mineralisation. Copper and zinc show a strong positive correlation of 0.77, pointing to a shared paragenetic control, whereas lead correlates only moderately with both, suggesting a broader and possibly earlier stage of lead mobilisation.

The spatial architecture of the mineralisation tells an equally compelling story. Kriging-interpolated anomaly maps for lead, zinc and copper, overlaid with lineaments interpreted from satellite imagery and field structural measurements, reveal that the highest concentrations of all three metals cluster in the southern sector of the study area, precisely where lineament density is greatest. Two dominant lineament orientations, northwest-southeast and northeast-southwest, coincide with structural trends identified in rose diagrams of joints and veins. Copper anomalies sit proximal to intrusive contacts, reflecting magmatic-hydrothermal fluid inputs, whereas lead and zinc anomalies are more widespread and structurally controlled, signifying distal dispersion by hydrothermal fluids migrating along the dominant lineament sets. High copper-to-lead ratios above 5, as seen in the Dagga-Allah sample, are generally associated with proximity to a high-temperature mineralising centre above 300 degrees Celsius, while distal expressions of the same system typically fall below 0.1 as lead becomes dominant.

Perhaps the most consequential insight concerns the origin of the controlling structures. The dominant north-northeast to south-southwest structural fabric mapped across all rock types aligns with regional Pan-African structural fabrics in the northeastern Nigerian basement. This suggests the brittle structures now hosting mineralisation were not generated by the Younger Granite emplacement itself but instead reactivated pre-existing basement-scale weaknesses inherited from the Pan-African orogeny. The implication for exploration is profound: similar structural corridors elsewhere in the Pan-African basement of northeastern Nigeria may represent comparable base-metal targets, independent of whether a Younger Granite intrusion is exposed at the surface.

The study’s authors acknowledge limitations, including an incomplete rare-earth-element dataset limited to five elements, the absence of isotopic or fluid-inclusion constraints, and the qualitative nature of the lineament analysis. Future work will integrate aeromagnetic data to map subsurface structural architecture, apply uranium-lead zircon geochronology to constrain the emplacement age of the Tongolo Complex, and conduct fluid-inclusion microthermometry to determine the temperature, pressure and salinity of the mineralising fluids. For now, the southern Tongolo sector stands as a high-priority exploration target, and the study adds fresh geochemical constraints on how A-type magmatism and magmatic-hydrothermal processes conspire to build base-metal deposits in one of Africa’s most distinctive granite provinces.

Subject of Research: Lithogeochemistry, petrogenesis and base-metal mineralisation of A-type granites in the Tongolo area of the Nigerian Younger Granite province

Article Title: Lithogeochemistry and petrogenesis of A-type granites and associated rocks from the Tongolo area of northeastern Nigeria and implications for lead-zinc-copper mineralisation

Article References: Olorunyomi, A. E., Olobaniyi, S. B., & Odukoya, A. M. (2026). Lithogeochemistry and petrogenesis of A-type granites and associated rocks from the Tongolo area of northeastern Nigeria and implications for lead-zinc-copper mineralisation. Discover Geoscience, 4(1), Article 326. https://doi.org/10.1007/s44288-026-00691-3

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00691-3

Keywords: A-type granite, Nigerian Younger Granite province, Tongolo complex, Pb-Zn-Cu mineralisation, lithogeochemistry, petrogenesis, fractional crystallisation, magmatic-hydrothermal fluids, lineament control, MP-AES, Bauchi State, base-metal exploration