Abstract The Okpella region, located in the eastern Igarra Schist Belt of southwestern Nigeria, is a Neoproterozoic metasedimentary–granitoid terrain known for its gold (Au) mineralization potential. Despite this, the complex interactions between litho-nuclide dynamics, hydrothermal alteration, and associated radiogenic and potentially toxic element (PTE) hazards have not been fully explored in the region. This study, the first to apply magnetic-assisted radiometric analysis combined with geochemical and PTE assessments in Nigerian geology, aims to address this gap. The results reveal significant geochemical heterogeneity, with elevated concentrations of K (up to 5.04%), eTh (up to 51.12 ppm), and eU (up to 13.42 ppm) predominantly found in garnet-biotite schist, calc-silicate gneiss, granite, and their contacts. These concentrations are attributed to hydrothermal fluid activity and fluid/melt-rock interactions. Conversely, charnockite in the southern region shows depleted radionuclide levels, reflecting limited alteration and the presence of K-poor minerals. Advanced mapping techniques have identified hydrothermal alteration zones and Au mineralization belts aligned with NE-SW and NW-SE structural trends. Magnetic depth-structure models suggest basement depths of ~ 0.3 to 3.5 km, with fractures acting as conduits for hydrothermal fluids, facilitating radionuclide redistribution near the surface. The study also highlights elevated radiogenic hazard indices, particularly in the central and northern regions, surpassing crustal limits. The PTE (<InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="41748_2025_675_Article_IEq1.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="53" /> </InlineMediaObject> <EquationSource Format="TEX">\(\:mg/kg\)</EquationSource> </InlineEquation>) trend in sediments (Hg > Cu > As > Pb > Zn > Co > Cr > Ni) indicates environmental and health risks, particularly from Hg contamination linked to artisanal Au mining. These findings underscore the need for controlled gold extraction, Hg-free processing, and continuous environmental monitoring. This study provides a scalable framework for balancing resource extraction with environmental sustainability, offering insights into best practices for mineralized terrains globally. Graphical Abstract This visual summary serves as a pivotal entry point into the research, offering a concise yet engaging overview of the study’s core findings and methodologies. Set within the orogenic Au terrain of Okpella in the Igarra Schist Belt, southwestern Nigeria, the study begins (Panel a) by presenting the geological framework—metasedimentary and granitoid rocks—highlighting the location of artisanal mining activities in the northern region. Building on this, Panel (b) illustrates the conceptual model of Au formation, where hydrothermal fluids migrate through fractured rocks, depositing Au and trace elements along structural conduits. Panel (c) introduces the core methodology: an integrated magnetic-assisted radiometric and environmental speciation framework used to assess radionuclide mobility, RHP, PTE contamination, and radiogenic–PTE health risks. These analyses unfold across Panels (d–f). Radiometric maps (Panel d) show the distributions of K, eTh, and eU, while the fusion map (Panel g) integrates these with total count (TC) data to delineate geochemically enriched zones and major structural trends (NE–SW and NW–SE) that control Au mineralization, especially in the northern part. The TMI–RTE map (with depth-structure profiles) and lineament map (Panels e, h) reveal magnetic contrasts and lineaments/fractures aligned with basement structures to depths of ~ 0.3–3.5 km, where fractures act as conduits for hydrothermal fluids and surface-level radionuclide redistribution. Panel (i) identifies elevated radiogenic hazards (eRa, DoR, RHP, AEDEout, AGDE, and ELCR) aligned with PTE hotspots (Hg, Cu, As), while Pb, Co, Cr, Ni, and Zn remain within safe limits. Panel (j) recommends Hg-free extraction, phytoremediation, nanotech interventions, and continuous monitoring as sustainable remediation strategies. Altogether, this holistic approach offers a detailed understanding of, and pathways for mitigating, mining-induced environmental hazards in structurally complex terrains.

Magnetic-Assisted Radiometric, Speciation, and Environmental Studies of an Orogenic Gold Terrain: Okpella, Igarra Schist Belt, SW Nigeria

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2025年6月4日

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Adedibu Sunny Akingboye,
Andy Anderson Bery,
Abimbola Chris Ogunyele,
Mbuotidem David Dick,
Temitayo Olamide Ale,
Emmanuel Adebayo Titus

摘要

Abstract

The Okpella region, located in the eastern Igarra Schist Belt of southwestern Nigeria, is a Neoproterozoic metasedimentary–granitoid terrain known for its gold (Au) mineralization potential. Despite this, the complex interactions between litho-nuclide dynamics, hydrothermal alteration, and associated radiogenic and potentially toxic element (PTE) hazards have not been fully explored in the region. This study, the first to apply magnetic-assisted radiometric analysis combined with geochemical and PTE assessments in Nigerian geology, aims to address this gap. The results reveal significant geochemical heterogeneity, with elevated concentrations of K (up to 5.04%), eTh (up to 51.12 ppm), and eU (up to 13.42 ppm) predominantly found in garnet-biotite schist, calc-silicate gneiss, granite, and their contacts. These concentrations are attributed to hydrothermal fluid activity and fluid/melt-rock interactions. Conversely, charnockite in the southern region shows depleted radionuclide levels, reflecting limited alteration and the presence of K-poor minerals. Advanced mapping techniques have identified hydrothermal alteration zones and Au mineralization belts aligned with NE-SW and NW-SE structural trends. Magnetic depth-structure models suggest basement depths of ~ 0.3 to 3.5 km, with fractures acting as conduits for hydrothermal fluids, facilitating radionuclide redistribution near the surface. The study also highlights elevated radiogenic hazard indices, particularly in the central and northern regions, surpassing crustal limits. The PTE ( \(\:mg/kg\) ) trend in sediments (Hg > Cu > As > Pb > Zn > Co > Cr > Ni) indicates environmental and health risks, particularly from Hg contamination linked to artisanal Au mining. These findings underscore the need for controlled gold extraction, Hg-free processing, and continuous environmental monitoring. This study provides a scalable framework for balancing resource extraction with environmental sustainability, offering insights into best practices for mineralized terrains globally.

Graphical Abstract

This visual summary serves as a pivotal entry point into the research, offering a concise yet engaging overview of the study’s core findings and methodologies. Set within the orogenic Au terrain of Okpella in the Igarra Schist Belt, southwestern Nigeria, the study begins (Panel a) by presenting the geological framework—metasedimentary and granitoid rocks—highlighting the location of artisanal mining activities in the northern region. Building on this, Panel (b) illustrates the conceptual model of Au formation, where hydrothermal fluids migrate through fractured rocks, depositing Au and trace elements along structural conduits. Panel (c) introduces the core methodology: an integrated magnetic-assisted radiometric and environmental speciation framework used to assess radionuclide mobility, RHP, PTE contamination, and radiogenic–PTE health risks. These analyses unfold across Panels (d–f). Radiometric maps (Panel d) show the distributions of K, eTh, and eU, while the fusion map (Panel g) integrates these with total count (TC) data to delineate geochemically enriched zones and major structural trends (NE–SW and NW–SE) that control Au mineralization, especially in the northern part. The TMI–RTE map (with depth-structure profiles) and lineament map (Panels e, h) reveal magnetic contrasts and lineaments/fractures aligned with basement structures to depths of ~ 0.3–3.5 km, where fractures act as conduits for hydrothermal fluids and surface-level radionuclide redistribution. Panel (i) identifies elevated radiogenic hazards (eRa, DoR, RHP, AEDE_out, AGDE, and ELCR) aligned with PTE hotspots (Hg, Cu, As), while Pb, Co, Cr, Ni, and Zn remain within safe limits. Panel (j) recommends Hg-free extraction, phytoremediation, nanotech interventions, and continuous monitoring as sustainable remediation strategies. Altogether, this holistic approach offers a detailed understanding of, and pathways for mitigating, mining-induced environmental hazards in structurally complex terrains.