Examples and genetic significance of the formation of iron oxides in the Nigerian banded iron-formations

Abstract

Ore microscopic studies reveal two main parageneses in the banded iron-formations of Nigeria. In the low-grade metamorphic schist belts of northern Nigeria, a magnetitic paragenesis comprising magnetite, silicates (grunerite and garnet), and quartz is developed. Magnetite which sometimes contains carbonate inclusions is markedly martitized. In contrast, the higher-grade metamorphic terrains of central Nigeria exhibit a different paragenesis consisting of hematite (including specularite) and quartz. Here, minerals of the magnetitic paragenesis only occur as relics. The protolith of these banded iron-formation occurrences envisioned as carbonate-containing sediments, with high concentrations of Fe and Si, and lower contents of Ca, Mg, Al (and also Mn where they are associated with gondite) underwent both submarine weathering and metamorphic changes in their evolution. During submarine weathering, sheet silicates and porphyroblasts of Fe-Mn-(Mg-Ca)-carbonate solid solutions, were formed. At the outset of a regional metamorphic episode, grunerite, garnet and porphyroblastic magnetite were developed. Magnetite formed at the expense of carbonate and sheetsilicates but was later martitized under post-metamorphic conditions. In the course of a later heterogeneous tectono-metamorphic event, martitized magnetite was transformed as follows: under low-grade metamorphism, as observed in the northern Nigerian schist belts, recrystallization into coarse-grained martite occurred, while at the higher grades of metamorphism in central Nigeria, recrystallization into hematite and, ultimately, specularite, took place. This relationship between magnetite and hematite has also been observed in many other banded iron-formations from different parts of the world, thus underscoring its widespread significance. Magnetite crystallizes first at the expense of carbonate and silicate minerals and hematite is subsequently derived from it directly or generally through martitization. This metamorphic phenomenon contradicts the common assumption that magnetite and hematite in banded iron-formations are invariably the products of direct precipitation from solution, in response to changes in environmental Eh/pH or different (reducing/oxidizing) diagenetic alterations of precipitated ferric hydroxide.