Geology and geochemical constrains on the origin and depositional setting of the Kpwa–Atog Boga banded iron formations (BIFs), northwestern Congo craton, southern Cameroon

Abstract

The Kpwa–Atog Boga (KAB) area lies within the Paleoproterozoic Nyong unit which defines the northwestern edge of the Archean Congo craton in southern Cameroon. Rock types in the area are mainly comprises of garnet gneiss associated with banded iron formations (BIFs). The BIFs and its host rocks were subjected to amphibolite facies metamorphism. The Rare Earth Elements (REE) patterns and the discrimination ratios (e.g., Al2O3/TiO2 Cr/Th Th/Sc La/Sc) of the garnet gneiss suggest that they are metasedimentary rocks derived from a felsic parent sources that have undergone intensive weathering. The KAB BIFs exhibit significant concentration of TFe2O3 + SiO2 (>93 wt%), and lower Al2O3 and TiO2 contents and high field strength elements (e.g., Hf, Nb, Ta, and Zr). This involves little contamination with continental detritus at their depositional environment. The Post Archean Australian Shale (PAAS)-normalized REE patterns of the KAB BIFs show enrichment in heavy REE and the depletion in light REE with positive La anomalies, which imply that these BIFs precipitated in ambient seawater. Moreover, the contribution of low-temperature hydrothermal fluids during the deposition of the studied BIFs were highlighted through their high superchondritic Y/Ho ratio (mean: 31.48), weak positive Eu/Eu∗(SN) ratio (mean: 1.83) and the low contents of transition metals. This suggests that the geochemical characteristics of the KAB BIFs and those of the Paleoproterozoic superior-type BIFs are almost similar. In summary, low-temperature hydrothermal fluids mixed with seawater contributed to the deposition of the KAB BIFs on the continental margin. This depositional setting similar to its host rocks is consistent with the Paleoproterozoic geological history recorded within the Nyong unit. This preliminary study reveals that the KAB BIFs are associated with low grade gold (<5 ppb), which could constitute further prospective mineralization besides the iron ore in the KAB area.