Petrography, lithostratigraphy, and geochemistry of iron formations from Toko-Nlokeng area (northwestern Congo craton): Implications for origin, depositional tectonic setting, and potential iron ore resource deposit

The genesis of the ore-forming material, the depositional environment, the Fe enrichment process, and the resource estimation of the Toko-Nlokeng iron deposit in the Nyong Complex of southern Cameroon were studied using detailed field investigations, petrography, lithostratigraphy, and geochemical analyses, and deposit modeling methods. This study shows a case of coexistence of granular and banded iron formations (65% GIFs and 35% BIFs) in an Algoma-type deposit, both in the Nyong Complex and worldwide. Representative samples of IFs were collected from drillcores. The Iron Formations (IFs) were associated with metavolcanic rocks consisting of garnet amphibolite, garnet-bearing gneiss, hornblendite, and mafic granulite, which underwent several episodes of deformation and metamorphism of amphibolite to granulite facies. The lithostratigraphic results show sharp correlations between the lithological units of the IFs. The bulk chemistry exhibits SiO₂ and Fe₂O₃ content similar to that of most Archean IFs worldwide, with SiO₂+Fe₂O₃T ranging between 96.31 and 98.10 wt%. Positive Eu anomalies (> 1.8) for BIFs and their association with metavolcanic rocks are consistent with Algoma type IFs. Geochemical data show that the IFs precipitated from a mixture of seawater and high-temperature (< 0.1%) hydrothermal fluids in a submarine environment with volcanic activity under relatively low oxygen conditions. Large amounts of iron and silica were likely produced under these conditions. Thus, the deposition of the Toko-Nlokeng IFs probably involved an iron-rich environment of Fe²⁺-rich hydrothermal fluid in chemically stratified seawater. The block models estimated for TNF08 and TNF11 are constrained by robust geological models of the IFs mineralization with a total measured Mineral Resource estimate of 19.1 Mt grading 36.44% Fe and an Indicated Mineral Resource estimate of 113.6 Mt grading 34.79% Fe. The Toko-Nlokeng can be a reasonable prospect for eventual economic extraction. It is suggested that the transformation of the low-grade IFs protolith into high-grade iron ore was controlled by deformation and metamorphic events.