Banded iron formation (BIF) consists of finely interstratified chemical sediments rich in iron oxides, carbonates and/or silicates that are precursor deposits to the world`s largest iron ore deposits (see Chapter 13.13). The second class of deposits, granular iron formations (GIFs), includes iron oxide and silicate-rich granular materials that are deposited in high-energy environments over large continental shelves. The enrichment of OFI and GIF to create higher-grade ore deposits is variously attributed to supergenic and hydrothermal processes. Iron formation deposition was most prevalent between 2.65 and 1.85 Ga and was closely related to atmospheric oxygen development, but the causes and effects of ocean oxidation are still widely discussed (Bekker et al., 2010; Clout and Simonson, 2005). The few formations deposited after 1,800 Ma[36] may indicate intermittent low concentrations of oxygen in the open air[37], while the small peak of 750 million years ago could be associated with the hypothetical snowball earth. [38] Most of the world`s largest iron deposits are found in rocks called banded iron formations (or BIF for short), which are finely stratified sedimentary rocks made up of alternating bands of chert (a form of quartz) and iron oxide. In an aqueous environment, UV oxidation of Fe2+ forms Fe3+ and is followed by co-presence of iron and iron oxides (Fe2O3 and FeO, respectively). These oxides are formed from iron ions and water molecules that release protons as a by-product, as well as mineral magnetite: banded iron formations, although widespread, remain mysterious in several ways. The vast majority of banded iron formations are archaic or Paleoproterozoic in old age.
However, a small number of BIFs are neoproterozoic in old age and are often associated,[8][10][11], if not universal,[12] with glacial deposits, which often contain glacial drops. [8] They also tend to show higher oxidation levels, with hematite predominant over magnetite,[10] and they usually contain a small amount of phosphate, about 1% by mass. [10] Mesobandization is often poor or non-existent[13] and soft sediment deformation structures are common. This indicates a very fast deposit. [14] However, like the granular iron formations of the Great Lakes, neoproterozoic deposits are widely described as banded iron formations. [8] [10] [14] [4] [15] [16] The absence of hydrogen sulphide in the anoxic water of the oceans can be explained either by a reduction in sulphur flow in the deep seabed or by a lack of reduction in dissimilatory sulphate (DSR), in which microorganisms use sulphate instead of oxygen for respiration. The product of the DSR is hydrogen sulfide, which easily precipitates iron from the solution in the form of pyrite. [31] The peak of the deposition of banded iron formations at the end of the Archaic and the end of the deposit in Orosirium have been interpreted as markers of the Great Oxygenation Event.
2.45 billion years ago, the high degree of independent fractionation of sulfur mass (MIF-S) indicates an extremely oxygen-poor atmosphere. The peak of banded iron formation deposition coincides with the disappearance of the MIF-S signal, which is interpreted as a permanent occurrence of oxygen in the atmosphere 2.41 to 2.35 billion years ago. This was accompanied by the development of a stratified ocean with a deep anoxic layer and a shallow oxidized layer. The end of the BIF deposit 1.85 billion years ago is attributed to the oxidation of the deep seabed. [31] The isotopic ratios of iron in the oldest banded iron formations (3700-3800 Ma) in Isua, Greenland, are best explained by the hypothesis of extremely low oxygen levels (<0.001% of modern O2 content in the photic zone) and anoxygenic photosynthetic oxidation of Fe(II)[21][13] Isley AE (1995) Hydrothermal plumes and iron delivery to band iron formation. J Geol 103:169-185 Various mining districts have coined their own names for IFFs. The term "band iron formation" was coined in the iron districts of Lake Superior, where ore deposits in the Mesabi, Marquette, Cuyuna, Gogebic, and Menominee iron regions were also known as "Jasper," "Jaspilit," "Iron Formation," or Taconite. Banded iron formations have been described as "itabarite" in Brazil, "iron stone" in South Africa, and "BHQ" (banded hematite quartzite) in India. [6] The dark layers of this rock are mostly made up of magnetite (Fe3O4), while the red layers are chalcedony, a form of silica (SiO2) colored red by tiny particles of iron oxide.
Some geologists suspect that the layers form every year with the changing seasons. Fig. 5.2-8. Age distributions of Rapitan/Clinton iron formations, Algoma and Superior band iron formations, volcanic massive sulphide deposits, venous Au-U deposits and quartz-pebble conglomerate Au-U deposits. Data from Meyer (1985), Kimberley (1989), Barley and Groves (1992) and Ohmoto (1996a). The Algoma type is the oldest (of the Archaeum) and seems to be associated with volcanic arcs. They are mainly found in ancient greenstone belts. Iron-rich minerals are usually magnetite. Algoma-type iron ore deposits are relatively small, usually less than 100 metres thick and several kilometres wide laterally. Algoma-type accumulations are exploited in Bjørnevatn (Norway), in the Abitibi greenstone belt (Ontario, Canada), in Kostomuksha (Russian Karelia), etc. Konhauser K, Newman DK, Kappler A (2005) The potential significance of microbial Fe(III) reduction during deposition of Precambrian banded iron formations. Geobiology 3:167–177 Nealson KH, Myers CR (1990) Reduction of iron by bacteria: a possible role in the formation of banded iron formations.
On J Sci 290:35-45 When used in the singular, the term banded iron formation refers to the sedimentary lithology that has just been described. [1] The plural form, banded iron formations, is used informally to refer to stratigraphic units consisting mainly of banded iron formation. [9] Paleoproterozoic band iron formations are found in the iron range and other parts of the Canadian Shield. [5] The Iron Firing Range is a group of four major deposits: the Mesabi Firing Range, the Vermilion Firing Range, the Gunflint Firing Range and the Cuyuna Firing Range. All are part of the Animikie group and were deposited between 2500 and 1800 Ma. [28] These BIFs are primarily granular iron formations. [5] The world ocean was almost entirely on ice and therefore separated from the atmosphere. As a result, decreasing conditions were reintroduced into the water column, which corresponded to those before the oxygen supply to the atmosphere. This near-global anoxia in seawater is generally considered to be the reason why BIFs again deposited in water in the form of iron and then accumulated as the ice age calmed down and the ocean was reoxygenated.