The Algoma-type iron-formations of the Nigerian metavolcano-sedimentary schist belts

Field relationships as well as petrographical and geochemical considerations form the basis of a model for the origin of the protoliths of the iron-formations and the associated phyllitic host rock of the Palaeoproterozoic schist belts of northern Nigeria. The iron-formations which consist of both the magnetite-subfacies and silicatefacies occur as relatively small, sporadic tabular bodies throughout the belts. They are concordantly interbanded with metasedimentary phyllites with which they share common metamorphic and deformational imprints. The iron-formations have high contents of Mn, Ca, Fe and P₂O₅ and low concentrations of alkalis (Na,K, Rb) Ba and Sr, Ti, Al and Si, whereas the phyllite exhibits exactly the opposite character. These results and other features (e.g. the composition of tourmaline in the phyllite and the occurrence of hydroclastic Cr-Mn-spinel and sulphides in the iron-formation) indicate a supply of materials from two different sources to the marine basin of Nigeria probably during Birimian time: slow but continuous deposition of continentally derived material of pelitic to psammitic composition; and rapid, sometimes intermittent, sporadic pulses of submarine-volcanic exhalations. During regional metamorphism (probably of Eburnian age) at greenschist to lower amphibolite fades conditions, the continental materials were transformed into phyllites and the mudstone-like sediments derived from volcanic exhalations into iron-formations. In the northern Nigerian schist belts two types of metamorphic parageneses in the iron-formations are recognized, both with various subtypes and without transitions between these two facies: (1) silicate-rich parageneses without magnetite (silicatefacies) and (2) magnetite-rich parageneses (magnetite-subfacies). In contrast to these parageneses, the iron-formations in the higher-grade metamorphic terrains of central Nigeria turn out to be hematitic (hematite-subfacies), and are derived from magnetite-bearing iron-formations by a second tectono-metamorphic event of Pan-African age (Mücke and Annor 1993). Whole-rock analyses of the Nigerian iron-formations explain the abundance of garnet (mainly spessartine) and clearly show that the formation of metamorphic minerals depended not only on temperature and pressure but also on the existing redox conditions. These environmental conditions controlled the formation of either magnetite parageneses (low redox conditions) or silicate parageneses without magnetite (high redox conditions). The environmental conditions are also an indication that magnetite (and hematite) could not have been constituents of the original sedimentary protolith of the Nigerian schist belts, but are exclusively of metamorphic origin.