Banded iron-formations origin

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

Although under chemolithoautotrophic growth conditions, cell densities of only 3-5 x 10 cells per milliliter were observed, the specific rate of Fe(III) reduced per cell unit was about 10 times faster than what had been published for any other Fe(lll) reducer. This strengthens the hypothesis that microbially mediated Fe(III) reduction by obligately anaerobic thermophiles could have been an important process on early Earth, when elevated temperatures were predominant (Baross 1998 Kashefi and Lovley 2000), which includes the involvement in the formation of specific Banded Iron Formations. In light of the properties of the above Fe(lll) reducers, the theories on the origin and biogeochemistry of Banded Iron Formations should be revisited. 

Rapid embedding in silica is particularly favourable for the good preservation of the organic matter, and this was surely the case with the well-preserved microstructures. But there is much indication from the thin section petrography that the Precambrian banded iron-formations were primarily deposited as carbonates (Lougheed, 1983)The origin of the embedding chert has been a major problem,... 

Markov, M. S., Shchipansky, A. A., Shukolyukov, Yu. A., Verkhovsky, A. B. (1990) Distribution and origin of noble gases in magnetites from Precambrian banded iron ores. In Ancient banded iron formations (regional presentation), pp. 281—4. Athens Theophrastus Publications, S.A. 

In the precambrian (or on present day Mars), the absence, of an ozone layer allowed solar UV radiation with energies as high as 40,000 cm"1 (0.25 microns). Photoreduction transitions via the Fe(3d) to Fe(4s) transition may have been very significant. The photochemical oxidation of Fe2+, and the precipitation of FeOOH, may be the origin of the extensive precambrian banded iron formations (38-40). Moreover, the photooxidation of Fe2 may have reduced C02 to organic molecules (37.41) ... 

Present ideas on the origin of the banded iron-formations... 

Drever, J.I., 1974, Geochemical model for the origin of Precambrian banded iron formations. Bull. Geol. Soc. Am., 85 1099-1106. 

Govett, G.J.S., 1966. Origin of banded iron formations. Geol. Soc. Am. Bull., 77 II9I-I2II. 

Anbar A. D. and Holland H. D. (1992) The photochemistry of manganese and the origin of banded iron formations. Geochim. Cosmochim. Acta 56, 2595 - 2603. 

Dymek R. F. and Klein C. (1988) Chemistry, petrology and origin of banded iron-formation lithologies from the 3,800 Ma Isua supracrustal belt. West Greenland. Precamb. Res. 39, 247-302. 

Most of the discussions relating organic activity to the origin of banded iron formations have centered on three areas (1) the presence of fossil microorganisms in cherts from banded iron formations (2) the similarities between laminations in banded iron formations and modem algal and bacterial stromatolites and (3) role of photosynthesis in the evolution of oxygen in the ocean and atmosphere, and consequently in the origin of banded iron formations. 

Similarly, banded iron formation (BIF), a sedimentary rock produced by chemical precipitation, is extremely rare in the Phanerozoic but common in the Archaean and Proterozoic record. Explaining its origin in terms of the atmospheric or ocean chemistry of the early Earth is an important part of recovering the history of early Earth. This is discussed in Chapter 5 (Section 5.4.3.2). 

Figure 12-4 will now be discussed in more detail. The record of reduced carbon in sedimentary rocks and the associated, 3C/I2C isotope shift date back to the oldest deposits of 3.5 Gyr ago, and even further if one makes allowance for the metamorphic alterations of the still older Isua formation (see Fig. 11-2). The biological origin of the isotope shift has been convincingly demonstrated, as reviewed by Schidlowski et al (1983). The implication that life had been in full swing 3.5 billion yr ago is supported by several lines of fossil evidence stromatolites, microfossils, and banded iron formations.

The oldest existing rocks on Earth, from the Isua supracrustal belt of West Greenland, are dated at 3.8 X 10 years (Moorbath et al., 1973) and consist of metamorphosed sediments. It has been suggested that the carbon in the Isua banded iron formation may be photosynthetic in origin (Schidlowski et al., 1979). Whether this remarkable conclusion is substantiated by further work remains to be seen. It is in any case abundantly clear from the available evidence that the conditions of deposition of the oldest known sediments were remarkably like those prevailing during the rest of the Archaean (4.6 — 2.6 X 10 years). Furthermore, with the exception of the appearance... 

In the seas, banded iron formations once again painted the bottom with iron and oxygen, like in the original Great Oxidation Event, creating the second BIF peak in Figure 8.1 in Chapter 8. At least three or four cold snaps passed over the planet like an inverse fever. The chemistry of the oceans changed dramatically, and soon after, so did life. 

Inthelab, a mixture oftypical microbes from this era... E. ChiFruetaL Fossilized iron bacteria reveal a pathway to the biological origin of banded iron formation. 2013. Nat Commun. 4,p. 2050. DOI 10.1038/ncomms3050. 

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 ... 

Khodyush, L.Ya., 1969. The banded structure of ferruginous quartzites and its origin. In Problemy obrazovaniya zhelezistykh porod dokembriya (Problems of Formation of the Precambrian Iron Formations). Izd. Naukova Dumka, Kiev, pp. 242-258 (in Russian). 

The variations in distribution of oxygen isotopes in the oxides and carbonates of iron-formations have been examined in detail and the complex origin of the main ore mineral—magnetite—has been demonstrated. Isotopic investigations were supplemented by a special study of the chemistry of the rocks of the cherty-iron formations as a whole and of the individual layers, bands, and minerals constituting them. 

No attempt is made here to summarize the enormous literature on Precambrian iron formations A recent comprehensive statement on their occurrence and origin is for example that of Mel nik [26] The one property of iron formations I am discussing here is their banding, which often occurs on two or even three size scales microbands of submillimeter thickness, meso-bands about 1 cm thick (these are the ones after which these formations are called "banded"), and macrobands meters thick. 

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