Banded iron-formations chemical composition

Ewers WE (1983) Chemical factors in the deposition and diagenesis of banded iron-formation. In Iron Formations Facts and Problems. Trendall AF, Morris RC (eds) Elsevier, Amsterdam, p 491-512 Fantle MS, DePaolo DJ (2002) The isotopic composition of continental iron and implications for the global Fe cycle. EOS Trans Am Geophys Union 83 V22B-1234... 

The find is associated with other specimens of the same kind and together with non-structured carbonaceous debris of similar chemical composition. The assemblages are arranged along bedding planes or other primary patterns of the sediment which is a chert, stromatolite, banded iron formation, shale or related rock. 

The next stages in the evolution of atmospheric oxygen and consequently in the evolution of chemical composition of the Ocean, lasted from approximately 2.4 billion years to 1.7 billion years. During this time, the atmosphere appears to have been oxidizing (based, again, on red beds and detrital mineral deposition, see 3.2), but the deep ocean remained reduced. The evidence for an anoxic deep ocean comes from the occurrence of massive banded iron formations (BIFs), which are considered to require a deep ocean rich in dissolved ferrous iron (Figure 14). 

The Precambrian banded iron formations are huge, flat accumulations, ( V 500 km across and V 500 m thick) mainly of silica and iron minerals arranged in bands, present in the Precambrian cores of all continents They are the world s main ore of iron. The consensus is that they formed by precipitation out of shallow water bodies at earth s surface temperatures, and that somehow their origin appears to be tied to the composition and evolution of the earth s early atmosphere. Beyond this, everything else is controversial How did such extreme chemical winnowing take place over whole basins up to hundreds of kilometers across ... 

On an AFM diagram (Fig. 16) all the minerals from iron-formations proper fall into a narrow and clearly defined field, while the rock-forming minerals of the associated iron-rich shales from BIF of Superior type (chamosite, ripidolite and other chlorites, garnets and some biotites) form another broader and less clear-cut field. These two fields do not overlap, which indicates the isochemical character of metamorphism and the limited mobility of the main rock-forming minerals, at least within individual layers and bands of the iron-formations and shales. The chemical composition of stilpnomelane occupies an intermediate position both in iron content and in alumina content. However, a high content of potassium (up to 2.2% K2O), an element not typical of most BIF, is necessary for the formation of this mica. 

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