Petrography, lithostratigraphy and major element geochemistry of Mesoarchean metamorphosed banded iron formation-hosted Nkout iron ore deposit, north western Congo craton, Central West Africa

Abstract

The Nkout banded iron formation (BIF) or iron deposit is located in the northwestern margin of the Congo Craton (CC) and is hosted within high-grade metamorphic Ntem complex. It is mainly made up of metamorphosed banded iron formations with resource estimation of 1.8 Bt @ 32.6% Fe indicated, and 0.9 Bt @ 30.3% Fe inferred. Nkout iron deposit is subdivided into three prospects namely East, Centre and West deposits. Results from lithostratigraphic studies allow detailed correlations between units in the Nkout succession. The litho-package from the top to the base stratigraphy comprises iron-rich member and the gneissic member. The first member forms the upper and middle units while the second member forms the lower unit and represents the footwall of the deposit. The upper unit is ∼200 m deep and consists of oxidized (hematite) and un-oxidized (fresh metamorphosed BIF) ore types. The middle unit is approximately 150 m thick and consists of fresh metamorphosed BIF with minor intercalation of biotite gneiss. The lower unit is made up of garnet-magnetite micaschist, biotite gneiss and pyroxene-biotite gneiss.The Nkout banded iron formation belongs to oxide facies magnetite-rich type which has suffered greenschist to amphibolite facies metamorphism, leading to recrystallization of chlorite, amphibole and garnet minerals. Its overall major oxide bulk chemistry is very similar to that of most Archaean iron-formations worldwide, with high SiO2 and Fe2O3 contents. However, the garnet-rich BIF has higher contents of Al2O3 (8.73 wt%), TiO2 (0.27 wt%), K2O (2.37 wt%) relative to magnetite-rich BIF, suggesting clastic contamination, thus a depositional environment closer to the continent. Furthermore, the absence of carbonate facies BIF in the entire Nkout deposit suggests deep water distal depositional setting, in an area of little or no organic carbon supplies.The lithostratigraphic correlation of metamorphosed BIF unit reveals both thicknesses and depth variations from West to the East, inferring a folded structure for the Nkout deposit. However the occurrence of BIF units in the entire deposit supports the fact that BIF deposition took place on a basin-wide scale. Considering the extent of the Ntem basin, it is suggested that a huge amount of Fe2+ in sea water was required. Thus, the deposition of Nkout BIF probably involved an iron rich environment of reduced Fe2+ rich hydrothermal fluid in chemically stratified sea water, similar to the depositional model proposed for the Minas BIF Supergroup of Quadrilátero Ferrífero in Brazil.