{{good article}}{{short description|Distinctive layered units of iron-rich sedimentary rock that are almost always of Precambrian age}}{{Use American English|date=June 2020}}{{Use dmy dates|date=March 2017}}

*factoids*

File:Black-band ironstone (aka).jpg|thumb|2.1 billion year old rock from North America showing banded iron formation, displayed in Dresden, Saxony, Germany GermanyBanded iron formations (BIFs; also called banded ironstone formations) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Almost all of these formations are of Precambrian age and are thought to record the oxygenation of the Earth's oceans. Some of the Earth's oldest rock formations, which formed about {{Ma|3700}} (Ma), are associated with banded iron formations.Banded iron formations are thought to have formed in sea water as the result of oxygen production by photosynthetic cyanobacteria. The oxygen combined with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the ocean floor. Each band is similar to a varve, resulting from cyclic variations in oxygen production.Banded iron formations were first discovered in northern Michigan in 1844. Banded iron formations account for more than 60% of global iron reserves and provide most of the iron ore presently mined. Most formations can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States.

Description

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- Banded Iron Formation Barberton.jpg -
Banded iron formation from the Barberton Greenstone Belt, South Africa

A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor chert, often red in color, of similar thickness.JOURNAL, James, Harold Lloyd, Sedimentary facies of iron-formation, Economic Geology, 1 May 1954, 49, 3, 235â€“293, 10.2113/gsecongeo.49.3.235, 1954EcGeo..49..235J, ENCYCLOPEDIA, Trendall, A.F., 2002, Precambrian Sedimentary Environments: A Modern Approach to Ancient Depositional Systems, Altermann, Wladyslaw, Corcoran, Patricia L., 0-632-06415-3, Blackwell Science Ltd, 33â€“36, The significance of iron-formation in the Precambrian stratigraphic record, JOURNAL, Katsuta N, Shimizu I, Helmstaedt H, Takano M, Kawakami S, Kumazawa M, Major element distribution in Archean banded iron formation (BIF): influence of metamorphic differentiation, Journal of Metamorphic Geology, June 2012, 30, 5, 457â€“472, 10.1111/j.1525-1314.2012.00975.x, 2012JMetG..30..457K, 129322335, BOOK, Condie, Kent C., Kent Condie, Earth as an evolving planetary system, 2015, Academic Press, 9780128036891, 3, A single banded iron formation can be up to several hundred meters in thickness and extend laterally for several hundred kilometers.Banded iron formation is more precisely defined as chemically precipitated sedimentary rock containing greater than 15% iron. However, most BIFs have a higher content of iron, typically around 30% by mass, so that roughly half the rock is iron oxides and the other half is silica.ENCYCLOPEDIA, Precambrian iron-formation, A.F., Trendall, J.G., Blockley, Evolution of the Hydrosphere and Atmosphere, P.G., Eriksson, W., Altermann, D.R., Nelson, W.U., Mueller, O., Catuneanu, Developments in Precambrian Geology

, Developments in Precambrian Geology, 12, 2004, 359â€“511

isbn=9780444515063, TRENDALL >FIRST1=A.

ENCYCLOPEDIA=ENCYCLOPEDIA OF GEOLOGY

PAGES=37â€“42, Elsevier, The iron in BIFs is divided roughly equally between the more oxidized ferric form, Fe(III), and the more reduced ferrous form, Fe(II), so that the ratio Fe(III)/Fe(II+III) typically varies from 0.3 to 0.6. This indicates a predominance of magnetite, in which the ratio is 0.67, over hematite, for which the ratio is 1. In addition to the iron oxides (hematite and magnetite), the iron sediment may contain the iron-rich carbonates siderite and ankerite, or the iron-rich silicates minnesotaite and greenalite. Most BIFs are chemically simple, containing little but iron oxides, silica, and minor carbonate, though some contain significant calcium and magnesium, up to 9% and 6.7% as oxides respectively.When used in the singular, the term banded iron formation refers to the sedimentary lithology just described. The plural form, banded iron formations, is used informally to refer to stratigraphic units that consist primarily of banded iron formation.Examples of this usage are found in Gole and Klein 1981; Klein 2005; Trendall 2005; and Zhu et al. 2014.A well-preserved banded iron formation typically consists of macrobands several meters thick that are separated by thin shale beds. The macrobands in turn are composed of characteristic alternating layers of chert and iron oxides, called mesobands, that are several millimeters to a few centimeters thick. Many of the chert mesobands contain microbands of iron oxides that are less than a millimeter thick, while the iron mesobands are relatively featureless. BIFs tend to be extremely hard, tough, and dense, making them highly resistant to erosion, and they show fine details of stratification over great distances, suggesting they were deposited in a very low-energy environment; that is, in relatively deep water, undisturbed by wave motion or currents.ENCYCLOPEDIA, Trendall, A.F., 2002, Precambrian Sedimentary Environments: A Modern Approach to Ancient Depositional Systems, Altermann, Wladyslaw, Corcoran, Patricia L., 0-632-06415-3, Blackwell Science Ltd, 33â€“36, The significance of iron-formation in the Precambrian stratigraphic record, BIFs only rarely interfinger with other rock types, tending to form sharply bounded discrete units that never grade laterally into other rock types.

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- MichiganBIF.jpg -
Close-up of banded iron formation specimen from Upper Michigan

Banded iron formations of the Great Lakes region and the Frere Formation of western Australia are somewhat different in character and are sometimes described as granular iron formations or GIFs. Their iron sediments are granular to oolitic in character, forming discrete grains about a millimeter in diameter, and they lack microbanding in their chert mesobands. They also show more irregular mesobanding, with indications of ripples and other sedimentary structures, and their mesobands cannot be traced out any great distance. Though they form well-defined, discrete units, these are commonly interbedded with coarse to medium-grained epiclastic sediments (sediments formed by weathering of rock). These features suggest a higher energy depositional environment, in shallower water disturbed by wave motions. However, they otherwise resemble other banded iron formations.JOURNAL, Gole, Martin J., Klein, Cornelis, Banded Iron-Formations through Much of Precambrian Time, The Journal of Geology, March 1981, 89, 2, 169â€“183, 10.1086/628578, 1981JG.....89..169G, 140701897, File:Rapitan BIF south australia.jpg|thumb|Thin section Thin sectionThe great majority of banded iron formations are Archean or Paleoproterozoic in age. However, a small number of BIFs are Neoproterozoic in age, and are frequently,JOURNAL, Bekker, A, Slack, J.F., Planavsky, N., Krapez, B., Hofmann, A., Konhauser, K.O., Rouxel, O.J., Iron formation: the sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes., Economic Geology, May 2010, 105, 3, 467â€“508,weblink 10.2113/gsecongeo.105.3.467, 2010EcGeo.105..467B, 10.1.1.717.4846, if not universally,JOURNAL, Abd El-Rahman, Yasser, Gutzmer, Jens, Li, Xian-Hua, Seifert, Thomas, Li, Chao-Feng, Ling, Xiao-Xiao, Li, Jiao, Not all Neoproterozoic iron formations are glaciogenic: Sturtian-aged non-Rapitan exhalative iron formations from the Arabianâ€“Nubian Shield, Mineralium Deposita, 6 June 2019, 55, 3, 577â€“596, 10.1007/s00126-019-00898-0, 2019MinDe..55..577A, 189829154, associated with glacial deposits, often containing glacial dropstones. They also tend to show a higher level of oxidation, with hematite prevailing over magnetite, and they typically contain a small amount of phosphate, about 1% by mass. Mesobanding is often poor to nonexistentJOURNAL, Cox, Grant M., Halverson, Galen P., Minarik, William G., Le Heron, Daniel P., Macdonald, Francis A., Bellefroid, Eric J., Straus, Justin V., Neoproterozoic iron formation: An evaluation of its temporal, environmental and tectonic significance, Chemical Geology, 2013, 362, 232â€“249, 10.1016/j.chemgeo.2013.08.002, 2013ChGeo.362..232C, 56300363,weblink 23 June 2020, and soft-sediment deformation structures are common. This suggests very rapid deposition. However, like the granular iron formations of the Great Lakes, the Neoproterozoic occurrences are widely described as banded iron formations.JOURNAL, Klein, C., Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins, American Mineralogist, 1 October 2005, 90, 10, 1473â€“1499, 10.2138/am.2005.1871, 2005AmMin..90.1473K, 201124189, JOURNAL, Ilyin, A. V., Neoproterozoic banded iron formations, Lithology and Mineral Resources, 9 January 2009, 44, 1, 78â€“86, 10.1134/S0024490209010064, 129978001, JOURNAL, Stern, Robert J., Mukherjee, Sumit K., Miller, Nathan R., Ali, Kamal, Johnson, Peter R., ~750Ma banded iron formation from the Arabian-Nubian Shieldâ€”Implications for understanding neoproterozoic tectonics, volcanism, and climate change, Precambrian Research, December 2013, 239, 79â€“94, 10.1016/j.precamres.2013.07.015, 2013PreR..239...79S, ENCYCLOPEDIA, Gaucher, Cladio, Sial, Alcides N., Frei, Robert, Chapter 17: Chemostratigraphy of Neoproterozoic Banded Iron Formation (BIF): Types, Age and Origin, Chemostratigraphy: Concepts, Techniques, and Applications, 2015, 433â€“449, 10.1016/B978-0-12-419968-2.00017-0, 9780124199682,weblink 22 June 2020, JOURNAL, Li, Zhi-Quan, Zhang, Lian-Chang, Xue, Chun-Ji, Zheng, Meng-Tian, Zhu, Ming-Tian, Robbins, Leslie J., Slack, John F., Planavsky, Noah J., Konhauser, Kurt O., Earth's youngest banded iron formation implies ferruginous conditions in the Early Cambrian ocean, Scientific Reports, 2 July 2018, 8, 1, 9970, 10.1038/s41598-018-28187-2, 29967405, 6028650, 2018NatSR...8.9970L, free, Banded iron formations are distinct from most Phanerozoic ironstones. Ironstones are relatively rare and are thought to have been deposited in marine anoxic events, in which the depositional basin became depleted in free oxygen. They are composed of iron silicates and oxides without appreciable chert but with significant phosphorus content, which is lacking in BIFs.No classification scheme for banded iron formations has gained complete acceptance. In 1954, Harold Lloyd James advocated a classification based on four lithological facies (oxide, carbonate, silicate, and sulfide) assumed to represent different depths of deposition, but this speculative model did not hold up. In 1980, Gordon A. Gross advocated a twofold division of BIFs into an Algoma type and a Lake Superior type, based on the character of the depositional basin. Algoma BIFs are found in relatively small basins in association with greywackes and other volcanic rocks and are assumed to be associated with volcanic centers. Lake Superior BIFs are found in larger basins in association with black shales, quartzites, and dolomites, with relatively minor tuffs or other volcanic rocks, and are assumed to have formed on a continental shelf.JOURNAL, Gross, G.A., 1980, A classification of iron formations based on depositional environments, The Canadian Mineralogist, 18, 215â€“222, This classification has been more widely accepted, but the failure to appreciate that it is strictly based on the characteristics of the depositional basin and not the lithology of the BIF itself has led to confusion, and some geologists have advocated for its abandonment.ENCYCLOPEDIA, The Archean atmosphere, hydrosphere, and biosphere, H., Ohmoto, Evolution of the Hydrosphere and Atmosphere, P.G., Eriksson, W., Altermann, D.R., Nelson, W.U., Mueller, O., Catuneanu, Developments in Precambrian Geology

, Developments in Precambrian Geology, 12, 2004, 5.2

isbn=9780444515063, However, the classification into Algoma versus Lake Superior types continues to be used.TANER >FIRST1=MEHMET F.

FIRST2=MADJID

JOURNAL=ORE GEOLOGY REVIEWS

VOLUME=70

DOI=10.1016/J.OREGEOREV.2015.03.016

DOI-ACCESS=FREE, DEPOSITIONAL SETTING OF ALGOMA-TYPE BANDED IRON FORMATION > JOURNAL=PRECAMBRIAN RESEARCH

DATE=2016-08-01

DOI=10.1016/J.PRECAMRES.2016.04.019

LAST1=GOURCEROL

LAST2=THURSTON

LAST3=KONTAK

LAST4=CÃ´TÃ©-MANTHA

LAST5=BICZOK

BIBCODE=2016PRER..281...47G,weblink

Occurrence

File:Bif in time.jpg|thumb|Abundance of banded iron formation in the geologic record. Color indicates dominant type. Red = older Archean formations; green = Greater Gondwana formations; blue = granular iron formations; black = Snowball Earth Snowball Earth{{Location map many| Earth| width = | float = | border = | caption = Location of occurrences. Color indicates dominant type. Light yellow = older Archean formations; dark yellow = Greater Gondwana formations; brown = granular iron formations; red = Snowball Earth formations.| alt = | relief = | AlternativeMap = | mark1 = Yellow pog.svg| mark1size = | link1 = Mount Richard-Molard

20|W}}| mark2 = Yellow pog.svg| mark2size = | link2 = Penge, Limpopo

24.383

30.317

}}| mark3 = Yellow pog.svg| mark3size = | link3 = Kaapvaal Craton

26.5

28.9

}}| mark4 = Yellow pog.svg| mark4size = | link4 = Griqualand West

28.75

24.7667

}}| mark5 = Yellow pog.svg| mark5size = | link5 = Kryvyi Rih

36|E}}| mark6 = Yellow pog.svg| mark6size = | link6 = Kursk

11|E}}| mark7 = Yellow pog.svg| mark7size = | link7 = Kostomuksha mine

30|E}}| mark8 = Yellow pog.svg| mark8size = | link8 = Baba Budan giri

13.421

75.763|E}}| mark9 = Yellow pog.svg| mark9size = | link9 = Odisha

20.27|85.82}}| mark10 = Yellow pog.svg| mark10size = | link10 = Pilbara

119|E}}| mark11 = Yellow pog.svg| mark11size = | link11 = Hamersley Range

35|E}}| mark12 = Yellow pog.svg| mark12size = | link12 = Yilgarn Craton

121

57|E}}| mark13 = Brown pog.svg| mark13size = | link13 = Frere Formation

53|E}}| mark14 = Yellow pog.svg| mark14size = | link14 = Middleback Range

33.0

137.14|E}}| mark15 = Yellow pog.svg| mark15size = | link15 = Anshan

41.108

122.994|E}}| mark16 = Red pog.svg| mark16size = | link16 =

62.5

134.6|W}}| mark17 = Yellow pog.svg| mark17size = | link17 = Isua Iron Mine

20|W}}| mark18 = Yellow pog.svg| mark18size = | link18 = Abitibi gold belt

48.30

80.80|W}}| mark19 = Brown pog.svg| mark19size = | link19 = Labrador

63|W}}| mark20 = Brown pog.svg| mark20size = | link20 = Iron Range

56|W}}| mark21 = Yellow pog.svg| mark21size = | link21 = Venezuela

65|W}}| mark22 = Yellow pog.svg| mark22size = | link22 = CarajÃ¡s Mine

37|W}}| mark23 = Red pog.svg| mark23size = | link23 = Corumba

10|W}}| mark24 = Yellow pog.svg| mark24size = | link24 = Minas Gerais

57|W}}}}Banded iron formations are almost exclusively Precambrian in age, with most deposits dating to the late Archean (2800â€“2500 Ma) with a secondary peak of deposition in the Orosirian period of the Paleoproterozoic (1850 Ma). Minor amounts were deposited in the early Archean and in the Neoproterozoic (750 Ma). The youngest known banded iron formation is an Early Cambrian formation in western China. Because the processes by which BIFs are formed appear to be restricted to early geologic time, and may reflect unique conditions of the Precambrian world, they have been intensively studied by geologists.Banded iron formations are found worldwide, in every continental shield of every continent. The oldest BIFs are associated with greenstone belts and include the BIFs of the Isua Greenstone Belt, the oldest known, which have an estimated age of 3700 to 3800 Ma.JOURNAL, Czaja, Andrew D., Johnson, Clark M., Beard, Brian L., Roden, Eric E., Li, Weiqiang, Moorbath, Stephen, Biological Fe oxidation controlled deposition of banded iron formation in the ca. 3770Ma Isua Supracrustal Belt (West Greenland), Earth and Planetary Science Letters, February 2013, 363, 192â€“203, 10.1016/j.epsl.2012.12.025, 2013E&PSL.363..192C, The TemagamiJOURNAL, AFRI 31M04SW0091, Geological and electromagnetic (VLP) surveys on part of Strathy-Cassels Group, Alexander, D.R., Hollinger Mines Limited, Timmins, Ontario, 3, 4, 9, 1977-11-21, banded iron deposits formed over a 50-million-year period, from 2736 to 2687 Ma, and reached a thickness of {{convert|60|meters|feet|abbr=off}}.WEB, Ontario banded iron formation,weblink American Museum of Natural History, 17 June 2020, Other examples of early Archean BIFs are found in the Abitibi greenstone belts, the greenstone belts of the Yilgarn and Pilbara cratons, the Baltic shield, and the cratons of the Amazon, north China, and south and west Africa.The most extensive banded iron formations belong to what A.F. Trendall calls the Great Gondwana BIFs. These are late Archean in age and are not associated with greenstone belts. They are relatively undeformed and form extensive topographic plateaus, such as the Hamersley Range.MacLeod, W. N. (1966) The geology and iron deposits of the Hamersley Range area. Bulletin {{Webarchive|url=https://web.archive.org/web/20160304040152weblink |date=4 March 2016 }} (Geological Survey of Western Australia), No. 117WEB,weblink Geology, Rio Tinto Iron Ore, 2012-08-07, dead,weblink" title="web.archive.org/web/20121023034150weblink">weblink 2012-10-23, The banded iron formations here were deposited from 2470 to 2450 Ma and are the thickest and most extensive in the world,WEB, Banded Iron Formation,weblink Western Australian Museum, 17 June 2020, with a maximum thickness in excess of {{convert|900|meters|feet|abbr=off}}. Similar BIFs are found in the CarajÃ¡s Formation of the Amazon craton, the CauÃª Itabirite of the SÃ£o Francisco craton, the Kuruman Iron Formation and Penge Iron Formation of South Africa, and the Mulaingiri Formation of India.Paleoproterozoic banded iron formations are found in the Iron Range and other parts of the Canadian Shield. The Iron Range is a group of four major deposits: the Mesabi Range, the Vermilion Range, the Gunflint Range, and the Cuyuna Range. All are part of the Animikie Group and were deposited between 2500 and 1800 Ma.JOURNAL, Three Great Basins of Precambrian Banded Iron Formation Deposition: A Systematic Comparison, Trendall, A. F, Geological Society of America Bulletin, 10.1130/0016-7606(1968)79[1527:TGBOPB]2.0.CO;2, 1968, 79, 11, 1527, 1968GSAB...79.1527T, These BIFs are predominantly granular iron formations.Neoproterozoic banded iron formations include the Urucum in Brazil, Rapitan in the Yukon, and the Damara Belt in southern Africa. They are relatively limited in size, with horizontal extents not more than a few tens of kilometers and thicknesses not more than about {{convert|10|meters|feet|abbr=off}}. These are widely thought to have been deposited under unusual anoxic oceanic conditions associated with the "Snowball Earth."

Origins

File:Iron banding 01.jpg|thumb|right|An ashtray carved out of a soft form of banded ironstone from the Barbeton Supergroup in South Africa. The red layers were laid down when Archaean photosynthesizing cyanobacteria cyanobacteria

Banded iron formation provided some of the first evidence for the timing of the Great Oxidation Event, 2,400 Ma.JOURNAL, Cloud, P., Preston Cloud, 10.2113/gsecongeo.68.7.1135, 1135â€“1143, 68, Economic Geology, Paleoecological Significance of the Banded Iron-Formation, 1973, 7, 1973EcGeo..68.1135C, JOURNAL, Holland, Heinrich D, The oxygenation of the atmosphere and oceans, Philosophical Transactions of the Royal Society B: Biological Sciences, 19 May 2006, 361, 1470, 903â€“915, 10.1098/rstb.2006.1838, 16754606, 1578726, With his 1968 paper on the early atmosphere and oceans of the Earth,JOURNAL, Cloud, Preston E., Atmospheric and Hydrospheric Evolution on the Primitive Earth., Science, 160, 3829, 1968, 729â€“736, 10.1126/science.160.3829.729, 1724303, 5646415, 1968Sci...160..729C, Preston Cloud established the general framework that has been widely, if not universally,JOURNAL, Ohmoto, H., Watanabe, Y., Yamaguchi, K.E., Naraoka, H., Haruna, M., Kakegawa, T., Hayashi, K., Kato, Y., Chemical and biological evolution of early Earth: Constraints from banded iron formations, Geological Society of America Memoir, 2006, 198, 291â€“331, 10.1130/2006.1198(17), 9780813711980,weblink 19 June 2020, BOOK, Banded iron formations, to iron ore : an integrated genesis model, 978-1536109719, Desmond Fitzgerald, Lascelles, 2017, Nova Science Publishers, accepted for understanding the deposition of BIFs.Cloud postulated that banded iron formations were a consequence of anoxic, iron-rich waters from the deep ocean welling up into a photic zone inhabited by cyanobacteria that had evolved the capacity to carry out oxygen-producing photosynthesis, but which had not yet evolved enzymes (such as superoxide dismutase) for living in an oxygenated environment. Such organisms would have been protected from their own oxygen waste through its rapid removal via the reservoir of reduced ferrous iron, Fe(II), in the early ocean. The oxygen released by photosynthesis oxidized the Fe(II) to ferric iron, Fe(III), which precipitated out of the sea water as insoluble iron oxides that settled to the ocean floor.Cloud suggested that banding resulted from fluctuations in the population of cyanobacteria due to free radical damage by oxygen. This also explained the relatively limited extent of early Archean deposits. The great peak in BIF deposition at the end of the Archean was thought to be the result of the evolution of mechanisms for living with oxygen. This ended self-poisoning and produced a population explosion in the cyanobacteria that rapidly depleted the remaining supply of reduced iron and ended most BIF deposition. Oxygen then began to accumulate in the atmosphere.Some details of Cloud's original model were abandoned. For example, improved dating of Precambrian strata has shown that the late Archean peak of BIF deposition was spread out over tens of millions of years, rather than taking place in a very short interval of time following the evolution of oxygen-coping mechanisms. However, his general concepts continue to shape thinking about the origins of banded iron formations. In particular, the concept of the upwelling of deep ocean water, rich in reduced iron, into an oxygenated surface layer poor in iron remains a key element of most theories of deposition.JOURNAL, Simonson, Bruce M., Bruce Simonson, Hassler, Scott W., Was the Deposition of Large Precambrian Iron Formations Linked to Major Marine Transgressions?, The Journal of Geology, November 1996, 104, 6, 665â€“676, 10.1086/629861, 1996JG....104..665S, 128886898, The few formations deposited after 1,800 MaJOURNAL, Slack, J.F., Cannon, W.F., 10.1130/G30259A.1, Extraterrestrial demise of banded iron formations 1.85 billion years ago, Geology, 37, 11, 1011â€“1014, 2009, 2009Geo....37.1011S, may point to intermittent low levels of free atmospheric oxygen,JOURNAL, Lyons, T.W., Reinhard, C.T., Early Earth: Oxygen for heavy-metal fans, Nature, 461, 7261, 179â€“81, September 2009, 19741692, 10.1038/461179a, 2009Natur.461..179L, 205049360, free, while the small peak at {{Ma|750}} may be associated with the hypothetical Snowball Earth.JOURNAL, Hoffman, P.F., Kaufman, A.J., Halverson, G.P., Schrag, D.P., A neoproterozoic snowball earth, Science, 281, 5381, 1342â€“6, August 1998, 9721097, 10.1126/science.281.5381.1342,weblink 1998Sci...281.1342H, 13046760,

Formation processes

The microbands within chert layers are most likely varves produced by annual variations in oxygen production. Diurnal microbanding would require a very high rate of deposition of 2 meters per year or 5 km/Ma. Estimates of deposition rate based on various models of deposition and sensitive high-resolution ion microprobe (SHRIMP) estimates of the age of associated tuff beds suggest a deposition rate in typical BIFs of 19 to 270 m/Ma, which are consistent either with annual varves or rhythmites produced by tidal cycles.Preston Cloud proposed that mesobanding was a result of self-poisoning by early cyanobacteria as the supply of reduced iron was periodically depleted. Mesobanding has also been interpreted as a secondary structure, not present in the sediments as originally laid down, but produced during compaction of the sediments. Another theory is that mesobands are primary structures resulting from pulses of activity along mid-ocean ridges that change the availability of reduced iron on time scales of decades.JOURNAL, Morris, R.C., Horwitz, R.C., The origin of the iron-formation-rich Hamersley Group of Western Australia â€” deposition on a platform, Precambrian Research, August 1983, 21, 3â€“4, 273â€“297, 10.1016/0301-9268(83)90044-X, 1983PreR...21..273M, In the case of granular iron formations, the mesobands are attributed to winnowing of sediments in shallow water, in which wave action tended to segregate particles of different size and composition.For banded iron formations to be deposited, several preconditions must be met.

The deposition basin must contain waters that are ferruginous (rich in iron).
This implies they are also anoxic, since ferrous iron oxidizes to ferric iron within hours or days in the presence of dissolved oxygen. This would prevent transport of large quantities of iron from its sources to the deposition basin.
The waters must not be euxinic (rich in hydrogen sulfide), since this would cause the ferrous iron to precipitate out as pyrite.
There must be an oxidation mechanism active within the depositional basin that steadily converts the reservoir of ferrous iron to ferric iron.

Source of reduced iron

(File:BlackSmoker.jpg|alt=|thumb|upright=1.5|Hydrothermal vents were one important source for the reduced iron that was later oxidized to form banded iron formations.)There must be an ample source of reduced iron that can circulate freely into the deposition basin. Plausible sources of iron include hydrothermal vents along mid-ocean ridges, windblown dust, rivers, glacial ice, and seepage from continental margins.The importance of various sources of reduced iron has likely changed dramatically across geologic time. This is reflected in the division of BIFs into Algoma and Lake Superior-type deposits.JOURNAL, Nadoll, P., Angerer, T., Mauk, J.L., French, D., Walshe, J, The chemistry of hydrothermal magnetite: A review, Ore Geology Reviews, 61, 1â€“32, 10.1016/j.oregeorev.2013.12.013, 2014, 2014OGRv...61....1N, JOURNAL, Zhu, X.Q., Tang, H.S., Sun, X.H., Genesis of banded iron formations: A series of experimental simulations, Ore Geology Reviews, 63, 465â€“469, 10.1016/j.oregeorev.2014.03.009, 2014, 2014OGRv...63..465Z, JOURNAL, Li, L.X., Li, H.M., Xu, Y.X., Chen, J., Yao, T., Zhang, L.F., Yang, X.Q., Liu, M.J., Zircon growth and ages of migmatites in the Algoma-type BIF-hosted iron deposits in Qianxi Group from eastern Hebei Province, China: Timing of BIF deposition and anatexis, Journal of Asian Earth Sciences, 113, 1017â€“1034, 10.1016/j.jseaes.2015.02.007, 2015JAESc.113.1017L, 2015, Algoma-type BIFs formed primarily in the Archean. These older BIFs tend to show a positive europium anomaly consistent with a hydrothermal source of iron. By contrast, Lake Superior-type banded iron formations primarily formed during the Paleoproterozoic era, and lack the europium anomalies of the older Algoma-type BIFs, suggesting a much greater input of iron weathered from continents.JOURNAL, Li, Weiqiang, Beard, Brian L., Johnson, Clark M., Biologically recycled continental iron is a major component in banded iron formations, Proceedings of the National Academy of Sciences, 7 July 2015, 112, 27, 8193â€“8198, 10.1073/pnas.1505515112, 26109570, 4500253, 2015PNAS..112.8193L, free,

Absence of oxygen or hydrogen sulfide

The absence of hydrogen sulfide in anoxic ocean water can be explained either by reduced sulfur flux into the deep ocean or a lack of dissimilatory sulfate reduction (DSR), the process by which microorganisms use sulfate in place of oxygen for respiration. The product of DSR is hydrogen sulfide, which readily precipitates iron out of solution as pyrite.The requirement of an anoxic, but not euxinic, deep ocean for deposition of banded iron formation suggests two models to explain the end of BIF deposition 1.8 billion years ago. The "Holland ocean" model proposes that the deep ocean became sufficiently oxygenated at that time to end transport of reduced iron. Heinrich Holland argues that the absence of manganese deposits during the pause between Paleoproterozoic and Neoproterozoic BIFs is evidence that the deep ocean had become at least slightly oxygenated. The "Canfield ocean" model proposes that, to the contrary, the deep ocean became euxinic and transport of reduced iron was blocked by precipitation as pyrite.Banded iron formations in northern Minnesota are overlain by a thick layer of ejecta from the Sudbury Basin impact. An asteroid (estimated at {{convert|10|km|abbr=on}} across) impacted into waters about {{convert|1000|m|abbr=on}} deep 1.849 billion years ago, coincident with the pause in BIF deposition. Computer models suggest that the impact would have generated a tsunami at least {{convert|1000|m|abbr=on}} high at the point of impact, and {{convert|100|m|abbr=on}} high about {{convert|3000|km|abbr=on}} away. It has been suggested that the immense waves and large underwater landslides triggered by the impact caused the mixing of a previously stratified ocean, oxygenated the deep ocean, and ended BIF deposition shortly after the impact.

Oxidation

Although Cloud argued that microbial activity was a key process in the deposition of banded iron formation, the role of oxygenic versus anoxygenic photosynthesis continues to be debated, and nonbiogenic processes have also been proposed.

Oxygenic photosynthesis

missing image!
- CSIRO ScienceImage 4203 A bluegreen algae species Cylindrospermum sp under magnification.jpg -
Cyanobacteria species Cylindrospermum sp. under magnification

Cloud's original hypothesis was that ferrous iron was oxidized in a straightforward manner by molecular oxygen present in the water:

The oxygen comes from the photosynthetic activities of cyanobacteria. Oxidation of ferrous iron may have been hastened by aerobic iron-oxidizing bacteria, which can increase rates of oxidation by a factor of 50 under conditions of low oxygen.

Anoxygenic photosynthesis

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- Iron bacteria burn.JPG -
A burn in Scotland with iron-oxidizing bacteria.

Oxygenic photosynthesis is not the only biogenic mechanism for deposition of banded iron formations. Some geochemists have suggested that banded iron formations could form by direct oxidation of iron by microbial anoxygenic phototrophs.JOURNAL, Kappler, A., Pasquero, C., Konhauser, K.O., Newman, D.K., Deposition of banded iron formations by anoxygenic phototrophic Fe (II)-oxidizing bacteria., Geology, November 2005, 33, 11, 865â€“8,weblink weblink" title="web.archive.org/web/20081216220557weblink">weblink 16 December 2008, 10.1130/G21658.1, 2005Geo....33..865K, The concentrations of phosphorus and trace metals in BIFs are consistent with precipitation through the activities of iron-oxidizing bacteria.JOURNAL, Konhauser, Kurt O., Hamade, Tristan, Raiswell, Rob, Morris, Richard C., Grant Ferris, F., Southam, Gordon, Canfield, Donald E., Could bacteria have formed the Precambrian banded iron formations?, Geology, 2002, 30, 12, 1079, 10.1130/0091-7613(2002)0302.0.CO;2, 2002Geo....30.1079K, Iron isotope ratios in the oldest banded iron formations (3700-3800 Ma), at Isua, Greenland, are best explained by assuming extremely low oxygen levels (

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