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 ironformation occurrences envisioned as carbonate-containing sediments, with high concentrations of Fe and Si, and lower contents of Ca, Mg, A1 (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 ironformations 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.

The Nigerian manganese-rich iron-formations and their host rocks—from sedimentation to metamorphism

“…Using isotopic data or structural and lithological criteria together with isotopic arguments, Ogezi (1977), Grant (1978), Turner (1983) and Fitches et al (1985) distinguished the earlier Kibaran (c. 1100 Ma) schist belts from those of Pan-African age (700-600 Ma). However, these ages are now interpreted as a reflection of the youngest tectono-metamorphic event (about 550 Ma = Pan-African) which overprinted the older rocks, considered to be probably the equivalents of the 2.3 Ga Birimian sequence of the neighbouring West African Craton (Mü cke and Annor, 1993;Mü cke et al, 1996;Annor et al, 1997). The PanAfrican reactivation of Eburnian (about 2.0 Ga) and Archaean provinces in Nigeria was recently confirmed by structural and isotopic data (Annor, 1995 andFerré et al, 1996).…”

“…which contains Na instead of K and has the calculated formula (Na 0.361 Ca 0.012 ) 0.383 (Al 1.473 -Fe 0.935 Mg 0.475 Ti 0.103 Mn 0.014 ) 3.000 [(OH) 2 /(Si 2.77 Al 1.230 ) 4.000 -O 10 ] AE 2 H 2 O (microprobe analysis, Mü cke andAnnor, 1993) and quartz, but no garnet and amphibole. Garnet forms idiomorphic porphyroblasts and may contain corroded inclusions of carbonate, amphibole and quartz.…”

Two Contrasting Iron Deposits in the Precambrian Mineral Belt of Cameroon, West Africa

“…Investigations of the economic geology of iron deposits often seek to answer a number of questions, two of the most important being: (1) the age relationships of magnetite and hematite (e.g., Mücke and Annor, 1993); and (2) the nature of the ore-forming process (i.e., whether hypogene and/or supergene). These two questions can be addressed by ore microscopy and REE geochemistry, respectively.…”

Cobalt‐Bearing Grunerite in the Metamorphosed Banded Iron Formation in Orissa, India

“…Dykes intruding into the BIF of Sulaipat were responsible to bring about thermal metamorphism. The present authors considered this as being of local (Mücke and Annor, 1993). 7.…”

“…Mücke and Annor (1993) reported that the increasing grade of metamorphism promotes only recrystallization of the iron oxides, involving grain size enlargement.…”