View source for Earth:Banded iron formation

{{short description|Distinctive layered units of iron-rich sedimentary rock that are almost always of Precambrian age}}
 <!-- Slightly silly when almost all dates are Ga, Ma, or at worst a year, but seems to be a GA requirement. -->
{{Infobox rock
|name=Banded iron formation
|alternative_name=
|type=Sedimentary
|image=Banded iron formation Dales Gorge.jpg
|image_size=300
|alt=
|caption=Banded iron formation, Karijini National Park, Western Australia
|coordinates=
|composition= [[Chemistry:Iron oxide|iron oxide]]s, [[Earth:Chert|chert]]s
|composition_secondary=Other
}}
[[File:Black-band ironstone (aka).jpg|thumb|2.1 billion year old rock from [[Earth:North America|North America]] showing banded iron formation, displayed in Dresden, Saxony, {{wipe|Germany}}]]

'''Banded iron formations''' ('''BIFs'''; also called '''banded ironstone formations''') are distinctive units of [[Earth:Sedimentary rock|sedimentary rock]] consisting of alternating layers of [[Chemistry:Iron oxide|iron oxide]]s and iron-poor [[Earth:Chert|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 [[Earth:Precambrian|Precambrian]] age and are thought to record the [[Earth:Great Oxygenation Event|oxygenation of the Earth's oceans]]. Some of the Earth's oldest rock formations, which formed about {{Ma|3700}} ([[Year#SI prefix multipliers|Ma]]), are associated with banded iron formations.

Banded iron formations are thought to have formed in sea water as the result of [[Chemistry:Oxygen|oxygen]] production by [[Earth:Photosynthesis|photosynthetic]] [[Biology:Cyanobacteria|cyanobacteria]]. The oxygen combined with dissolved [[Chemistry:Iron|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 [[Earth:Varve|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 [[Earth:Iron ore|iron ore]] presently mined. Most formations can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States.

==Description==
[[Image:Banded Iron Formation Barberton.jpg|thumb|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 [[Chemistry:Iron oxide|iron oxide]]s, either [[Chemistry:Magnetite|magnetite]] (Fe<sub>3</sub>O<sub>4</sub>) or [[Chemistry:Hematite|hematite]] (Fe<sub>2</sub>O<sub>3</sub>), alternating with bands of iron-poor [[Earth:Chert|chert]], often red in color, of similar thickness.<ref name="james-1954">{{cite journal |last1=James |first1=Harold Lloyd |title=Sedimentary facies of iron-formation |journal=Economic Geology |date=1 May 1954 |volume=49 |issue=3 |pages=235–293 |doi=10.2113/gsecongeo.49.3.235|bibcode=1954EcGeo..49..235J }}</ref><ref name="trendall-2002">{{cite encyclopedia |last1=Trendall |first1=A.F. |year=2002 |encyclopedia=Precambrian Sedimentary Environments: A Modern Approach to Ancient Depositional Systems |editor1-last=Altermann |editor1-first=Wladyslaw |editor2-last=Corcoran |editor2-first=Patricia L. |isbn=0-632-06415-3 |publisher=Blackwell Science Ltd |pages=33–36 |title=The significance of iron-formation in the Precambrian stratigraphic record}}</ref><ref name=Katsuta2012>{{cite journal | vauthors = Katsuta N, Shimizu I, Helmstaedt H, Takano M, Kawakami S, Kumazawa M |title=Major element distribution in Archean banded iron formation (BIF): influence of metamorphic differentiation |journal=Journal of Metamorphic Geology |date= June 2012 |volume=30 |issue=5 |pages=457–472 |doi=10.1111/j.1525-1314.2012.00975.x |bibcode=2012JMetG..30..457K |s2cid=129322335 }}</ref><ref name="condie-2015">{{cite book |last=Condie |first=Kent C. |title=Earth as an evolving planetary system |date=2015 |publisher=Academic Press |isbn=9780128036891 |edition=3}}</ref> A single banded iron formation can be up to several hundred meters in thickness and extend laterally for several hundred kilometers.<ref name="trendall-blockley-2004"/>

Banded iron formation is more precisely defined as chemically precipitated [[Earth:Sedimentary rock|sedimentary rock]] containing greater than 15% [[Chemistry:Iron|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.<ref name="trendall-blockley-2004">{{cite encyclopedia |chapter=Precambrian iron-formation |first1=A.F. |last1=Trendall |first2=J.G. |last2=Blockley |title=Evolution of the Hydrosphere and Atmosphere |editor1-first=P.G. |editor1-last=Eriksson |editor2-first=W. |editor2-last=Altermann |editor3-first=D.R. |editor3-last=Nelson |editor4-first=W.U. |editor4-last=Mueller |editor5-first=O. |editor5-last=Catuneanu |encyclopedia=Developments in Precambrian Geology
 |series=Developments in Precambrian Geology |volume=12 |year=2004 |pages=359–511
|doi=10.1016/S0166-2635(04)80007-0|isbn=9780444515063 }}</ref><ref name="trendall-2005">{{cite encyclopedia|last1=Trendall |first1=A. |chapter=Banded iron formations |encyclopedia=Encyclopedia of Geology |year=2005 |pages=37–42 |publisher=Elsevier }}</ref> The iron in BIFs is divided roughly equally between the more oxidized [[Chemistry:Ferric|ferric]] form, Fe(III), and the more reduced [[Chemistry:Ferrous|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.<ref name="condie-2015"/> In addition to the iron oxides (hematite and magnetite), the iron sediment may contain the iron-rich carbonates [[Chemistry:Siderite|siderite]] and [[Chemistry:Ankerite|ankerite]], or the iron-rich silicates [[Chemistry:Minnesotaite|minnesotaite]] and [[Chemistry:Greenalite|greenalite]]. Most BIFs are chemically simple, containing little but iron oxides, silica, and minor carbonate,<ref name="trendall-blockley-2004"/> though some contain significant calcium and magnesium, up to 9% and 6.7% as oxides respectively.<ref name="gole-klein-1981"/><ref name="klein-2005"/>

When used in the singular, the term banded iron formation refers to the sedimentary lithology just described.<ref name="james-1954"/> The plural form, banded iron formations, is used informally to refer to stratigraphic units that consist primarily of banded iron formation.<ref name="explanation">Examples of this usage are found in Gole and Klein 1981; Klein 2005; Trendall 2005; and  Zhu ''et al.'' 2014.</ref>

A well-preserved banded iron formation typically consists of ''macrobands'' several meters thick that are separated by thin [[Chemistry:Shale|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.<ref name="trendall-2002">{{cite encyclopedia |last1=Trendall |first1=A.F. |year=2002 |encyclopedia=Precambrian Sedimentary Environments: A Modern Approach to Ancient Depositional Systems |editor1-last=Altermann |editor1-first=Wladyslaw |editor2-last=Corcoran |editor2-first=Patricia L. |isbn=0-632-06415-3 |publisher=Blackwell Science Ltd |pages=33–36 |title=The significance of iron-formation in the Precambrian stratigraphic record}}</ref> BIFs only rarely interfinger with other rock types, tending to form sharply bounded discrete units that never grade laterally into other rock types.<ref name="trendall-blockley-2004"/>

[[Image:MichiganBIF.jpg|thumb|Close-up of banded iron formation specimen from Upper Michigan]]
Banded iron formations of the Great Lakes region and the Frere Formation of western {{wipe|Australia}} are somewhat different in character and are sometimes described as ''granular iron formations'' or ''GIFs''.<ref name="gole-klein-1981"/><ref name="trendall-blockley-2004"/> Their iron sediments are granular to [[Earth:Oolite|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 [[Earth:Sedimentary structures|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 [[Earth:Depositional environment|depositional environment]], in shallower water disturbed by wave motions. However, they otherwise resemble other banded iron formations.<ref name="gole-klein-1981">{{cite journal |last1=Gole |first1=Martin J. |last2=Klein |first2=Cornelis |title=Banded Iron-Formations through Much of Precambrian Time |journal=The Journal of Geology |date=March 1981 |volume=89 |issue=2 |pages=169–183 |doi=10.1086/628578|bibcode=1981JG.....89..169G |s2cid=140701897 }}</ref>

[[File:Rapitan BIF south australia.jpg|thumb|[[Earth:Thin section|Thin section]] of Neoproterozoic banded iron formation from Australia]]
The great majority of banded iron formations are [[Earth:Archean|Archean]] or [[Earth:Paleoproterozoic|Paleoproterozoic]] in age.  However, a small number of BIFs are [[Earth:Neoproterozoic|Neoproterozoic]] in age, and are frequently,<ref name="klein-2005"/><ref name="ilyin-2009"/><ref name="bekker-etal-2010">{{cite journal |last1 = Bekker |first1=A |last2=Slack |first2=J.F. |last3=Planavsky |first3=N. |last4=Krapez |first4=B. |last5=Hofmann |first5=A. |last6=Konhauser |first6=K.O. |last7=Rouxel |first7=O.J. | title = Iron formation: the sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes. | journal = Economic Geology | date = May 2010 | volume = 105 | issue = 3 | pages = 467–508 | url = http://faculty.eas.ualberta.ca/konhauser/Reprints/EconomicGeology-AB%282010%29.pdf | doi = 10.2113/gsecongeo.105.3.467 |bibcode=2010EcGeo.105..467B | citeseerx = 10.1.1.717.4846 }}</ref> if not universally,<ref name="abd-etal-2019">{{cite journal |last1=Abd El-Rahman |first1=Yasser |last2=Gutzmer |first2=Jens |last3=Li |first3=Xian-Hua |last4=Seifert |first4=Thomas |last5=Li |first5=Chao-Feng |last6=Ling |first6=Xiao-Xiao |last7=Li |first7=Jiao |title=Not all Neoproterozoic iron formations are glaciogenic: Sturtian-aged non-Rapitan exhalative iron formations from the Arabian–Nubian Shield |journal=Mineralium Deposita |date=6 June 2019 |volume=55 |issue=3 |pages=577–596 |doi=10.1007/s00126-019-00898-0|bibcode=2019MinDe..55..577A |s2cid=189829154 }}</ref> associated with glacial deposits, often containing glacial dropstones.<ref name="klein-2005"/> They also tend to show a higher level of oxidation, with hematite prevailing over magnetite,<ref name="ilyin-2009"/> and they typically contain a small amount of phosphate, about 1% by mass.<ref name="ilyin-2009"/> Mesobanding is often poor to nonexistent<ref name="cox-etal-2013">{{cite journal |last1=Cox |first1=Grant M. |last2=Halverson |first2=Galen P. |last3=Minarik |first3=William G. |last4=Le Heron |first4=Daniel P. |last5=Macdonald |first5=Francis A. |last6=Bellefroid |first6=Eric J. |last7=Straus |first7=Justin V. |title=Neoproterozoic iron formation: An evaluation of its temporal, environmental and tectonic significance |journal=Chemical Geology |date=2013 |volume=362 |pages=232–249 |doi=10.1016/j.chemgeo.2013.08.002 |bibcode=2013ChGeo.362..232C |s2cid=56300363 |url=https://francismacdonald.fas.harvard.edu/files/fmacdonald/files/cox_2013_chemgeo_bifs.pdf |access-date=23 June 2020}}</ref> and soft-sediment deformation structures are common. This suggests very rapid deposition.<ref name="stern-etal-2013"/> However, like the granular iron formations of the Great Lakes, the Neoproterozoic occurrences are widely described as banded iron formations.<ref name="klein-2005">{{cite journal |last1=Klein |first1=C. |title=Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins |journal=American Mineralogist |date=1 October 2005 |volume=90 |issue=10 |pages=1473–1499 |doi=10.2138/am.2005.1871|bibcode=2005AmMin..90.1473K |s2cid=201124189 }}</ref><ref name="ilyin-2009">{{cite journal |last1=Ilyin |first1=A. V. |title=Neoproterozoic banded iron formations |journal=Lithology and Mineral Resources |date=9 January 2009 |volume=44 |issue=1 |pages=78–86 |doi=10.1134/S0024490209010064|s2cid=129978001 }}</ref><ref name="stern-etal-2013">{{cite journal |last1=Stern |first1=Robert J. |last2=Mukherjee |first2=Sumit K. |last3=Miller |first3=Nathan R. |last4=Ali |first4=Kamal |last5=Johnson |first5=Peter R. |title=~750Ma banded iron formation from the Arabian-Nubian Shield—Implications for understanding neoproterozoic tectonics, volcanism, and climate change |journal=Precambrian Research |date=December 2013 |volume=239 |pages=79–94 |doi=10.1016/j.precamres.2013.07.015|bibcode=2013PreR..239...79S }}</ref><ref name="condie-2015"/><ref name="gaucher-etal-2015">{{cite encyclopedia |last1=Gaucher |first1=Cladio |last2=Sial |first2=Alcides N. |last3=Frei |first3=Robert |title=Chapter 17: Chemostratigraphy of Neoproterozoic Banded Iron Formation (BIF): Types, Age and Origin |encyclopedia=Chemostratigraphy: Concepts, Techniques, and Applications |date=2015 |pages=433–449 |doi=10.1016/B978-0-12-419968-2.00017-0 |isbn=9780124199682 |url=https://www.sciencedirect.com/science/article/pii/B9780124199682000170 |access-date=22 June 2020}}</ref><ref name="li-etal-2018">{{cite journal |last1=Li |first1=Zhi-Quan |last2=Zhang |first2=Lian-Chang |last3=Xue |first3=Chun-Ji |last4=Zheng |first4=Meng-Tian |last5=Zhu |first5=Ming-Tian |last6=Robbins |first6=Leslie J. |last7=Slack |first7=John F. |last8=Planavsky |first8=Noah J. |last9=Konhauser |first9=Kurt O. |title=Earth's youngest banded iron formation implies ferruginous conditions in the Early Cambrian ocean |journal=Scientific Reports |date=2 July 2018 |volume=8 |issue=1 |page=9970 |doi=10.1038/s41598-018-28187-2|pmid=29967405 |pmc=6028650 |bibcode=2018NatSR...8.9970L |doi-access=free }}</ref>

Banded iron formations are distinct from most [[Earth:Phanerozoic|Phanerozoic]] [[Earth:Ironstone|ironstone]]s. Ironstones are relatively rare and are thought to have been deposited in marine [[Earth:Anoxic event|anoxic event]]s, in which the depositional basin became depleted in free [[Chemistry:Oxygen|oxygen]]. They are composed of iron silicates and oxides without appreciable chert but with significant [[Chemistry:Phosphorus|phosphorus]] content, which is lacking in BIFs.<ref name="bekker-etal-2010"/>

No classification scheme for banded iron formations has gained complete acceptance.<ref name="trendall-blockley-2004"/> 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,<ref name="james-1954"/> but this speculative model did not hold up.<ref name="trendall-blockley-2004"/> 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 [[Earth:Greywacke|greywacke]]s 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, [[Earth:Quartzite|quartzite]]s, and [[Earth:Dolomite (rock)|dolomite]]s, with relatively minor [[Earth:Tuff|tuff]]s or other volcanic rocks, and are assumed to have formed on a [[Earth:Continental shelf|continental shelf]].<ref name="gross-1980">{{cite journal|last1=Gross |first1=G.A. |year=1980 |title=A classification of iron formations based on depositional environments |journal=The Canadian Mineralogist  |volume=18 |pages=215–222}}</ref> 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.<ref name="trendall-2002"/><ref name="ohmoto-2004">{{cite encyclopedia |chapter=The Archean atmosphere, hydrosphere, and biosphere |first1=H. |last1=Ohmoto  |title=Evolution of the Hydrosphere and Atmosphere |editor1-first=P.G. |editor1-last=Eriksson |editor2-first=W. |editor2-last=Altermann |editor3-first=D.R. |editor3-last=Nelson |editor4-first=W.U. |editor4-last=Mueller |editor5-first=O. |editor5-last=Catuneanu |encyclopedia=Developments in Precambrian Geology
 |series=Developments in Precambrian Geology |volume=12 |year=2004 |at=5.2
|doi=10.1016/S0166-2635(04)80007-0|isbn=9780444515063 }}</ref> However, the classification into Algoma versus Lake Superior types continues to be used.<ref name="taner-2015">{{cite journal |last1=Taner |first1=Mehmet F. |last2=Chemam |first2=Madjid |title=Algoma-type banded iron formation (BIF), Abitibi Greenstone belt, Quebec, Canada |journal=Ore Geology Reviews |date=October 2015 |volume=70 |pages=31–46 |doi=10.1016/j.oregeorev.2015.03.016|bibcode=2015OGRv...70...31T |doi-access=free }}</ref><ref name="Precambrian Research 2016 pp. 47–79">{{cite journal | title=Depositional setting of Algoma-type banded iron formation | journal=Precambrian Research | volume=281 | date=2016-08-01 | issn=0301-9268 | doi=10.1016/j.precamres.2016.04.019 | pages=47–79 | last1=Gourcerol | first1=B. | last2=Thurston | first2=P.C. | last3=Kontak | first3=D.J. | last4=Côté-Mantha | first4=O. | last5=Biczok | first5=J. | bibcode=2016PreR..281...47G | url=https://hal-brgm.archives-ouvertes.fr/hal-02283951/file/proof.pdf }}</ref>

==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 formations. Adapted from Trendall 2002.]]
{{Location map many
| Earth
| width       = 
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}}
Banded iron formations are almost exclusively [[Earth:Precambrian|Precambrian]] in age, with most deposits dating to the late Archean (2800–2500 Ma) with a secondary peak of deposition in the [[Earth:Orosirian|Orosirian]] period of the [[Earth:Paleoproterozoic|Paleoproterozoic]] (1850 Ma). Minor amounts were deposited in the early Archean and in the [[Earth:Neoproterozoic|Neoproterozoic]] (750 Ma).<ref name="trendall-blockley-2004"/><ref name="condie-2015"/> The youngest known banded iron formation is an [[Earth:Early Cambrian|Early Cambrian]] formation in western China.<ref name="li-etal-2018"/> 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.<ref name="trendall-blockley-2004"/><ref name="condie-2015"/>

Banded iron formations are found worldwide, in every continental shield of every continent. The oldest BIFs are associated with [[Earth:Greenstone belt|greenstone belt]]s and include the BIFs of the Isua Greenstone Belt, the oldest known, which have an estimated age of 3700 to 3800 Ma.<ref name="trendall-blockley-2004"/><ref name="czaja-etal-2013">{{cite journal |last1=Czaja |first1=Andrew D. |last2=Johnson |first2=Clark M. |last3=Beard |first3=Brian L. |last4=Roden |first4=Eric E. |last5=Li |first5=Weiqiang |last6=Moorbath |first6=Stephen |title=Biological Fe oxidation controlled deposition of banded iron formation in the ca. 3770Ma Isua Supracrustal Belt (West Greenland) |journal=Earth and Planetary Science Letters |date=February 2013 |volume=363 |pages=192–203 |doi=10.1016/j.epsl.2012.12.025|bibcode=2013E&PSL.363..192C }}</ref> The [[Earth:Temagami Greenstone Belt|Temagami]]<ref name="AHI">{{cite journal|id=AFRI 31M04SW0091|title=Geological and electromagnetic (VLP) surveys on part of Strathy-Cassels Group|last=Alexander|first=D.R.|publisher=Hollinger Mines Limited|location=Timmins, Ontario|pages=3, 4, 9|date=1977-11-21}}</ref> 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}}.<ref name="amnh-2020">{{cite web |title=Ontario banded iron formation |url=https://www.amnh.org/exhibitions/permanent/planet-earth/how-has-the-earth-evolved/a-special-planet/how-do-we-know-about-the-early-atmosphere/ontario-banded-iron-formation |website=American Museum of Natural History |access-date=17 June 2020}}</ref> 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, [[Earth:North China Craton|north China]], and [[Earth:Kalahari Craton|south]] and [[Earth:West African Craton|west]] Africa.<ref name="trendall-blockley-2004"/>

The most extensive banded iron formations belong to what A.F. Trendall calls the Great [[Earth:Gondwana|Gondwana]] BIFs. These are late Archean in age and are not associated with greenstone belts. They are relatively undeformed and form extensive topographic plateaus,<ref name="trendall-2002"/> such as the Hamersley Range.<ref name="henrietta.liswa.wa.gov.au">MacLeod, W. N. (1966) [http://henrietta.liswa.wa.gov.au/record=b2251313~S2 The geology and iron deposits of the Hamersley Range area. Bulletin] {{Webarchive|url=https://web.archive.org/web/20160304040152/http://henrietta.liswa.wa.gov.au/record=b2251313~S2 |date=4 March 2016 }} (Geological Survey of Western Australia), No. 117</ref><ref name="Geology">{{cite web |url=http://www.riotintoironore.com/ENG/operations/497_geology.asp |title=Geology |publisher=Rio Tinto Iron Ore |access-date=2012-08-07 |url-status=dead |archive-url=https://web.archive.org/web/20121023034150/http://www.riotintoironore.com/ENG/operations/497_geology.asp |archive-date=2012-10-23 }}</ref><ref name="Iron2002" /> The banded iron formations here were deposited from 2470 to 2450 Ma and are the thickest and most extensive in the world,<ref name="condie-2015"/><ref name="wam-2014">{{cite web |title=Banded Iron Formation |url=http://museum.wa.gov.au/research/collections/earth-and-planetary-sciences/rock-collection/banded-iron-formation |publisher=Western Australian Museum |access-date=17 June 2020}}</ref> with a maximum thickness in excess of {{convert|900|meters|feet|abbr=off}}.<ref name="gole-klein-1981"/> Similar BIFs are found in the Carajás Formation of the Amazon craton, the Cauê Itabirite of the São Francisco craton, the Kuruman Iron Formation and Penge Iron Formation of South Africa, and the Mulaingiri Formation of India.<ref name="trendall-blockley-2004"/>

Paleoproterozoic banded iron formations are found in the [[Chemistry:Iron Range|Iron Range]] and other parts of the [[Place:Canadian Shield|Canadian Shield]].<ref name="trendall-blockley-2004"/>  The Iron Range is a group of four major deposits: the [[Earth:Mesabi Range|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.<ref name=af>{{cite journal|title=Three Great Basins of Precambrian Banded Iron Formation Deposition: A Systematic Comparison|author=Trendall, A. F|journal=Geological Society of America Bulletin|
doi=10.1130/0016-7606(1968)79[1527:TGBOPB]2.0.CO;2|year=1968|volume=79|issue=11|page=1527|bibcode = 1968GSAB...79.1527T }}</ref> These BIFs are predominantly granular iron formations.<ref name="trendall-blockley-2004"/>

Neoproterozoic banded iron formations include the Urucum in Brazil, Rapitan in the [[Earth:Yukon|Yukon]], and the Damara Belt in southern Africa.<ref name="trendall-blockley-2004"/> 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}}.<ref name="ilyin-2009"/> These are widely thought to have been deposited under unusual anoxic oceanic conditions associated with the "[[Earth:Snowball Earth|Snowball Earth]]."<ref name="trendall-2002"/>

==Origins==
 [[File:Iron banding 01.jpg|thumb|right|An ashtray carved out of a soft form of banded ironstone from the [[Biology:Archean life in the Barberton Greenstone Belt|Barbeton Supergroup]] in South Africa. The red layers were laid down when Archaean photosynthesizing cyanobacteria produced oxygen that reacted with dissolved iron compounds in the water, to form insoluble iron oxide (rust). The white layers are sediments that settled when there was no oxygen in the water, or when dissolved Fe<sup>2+</sup> was temporarily depleted.<ref name=Margulis>{{cite book | last1=Margulis |first1=L |last2=Sagan |first2=D |author-link2=Dorion Sagan | date = August 2000 | title = What is Life? | pages = 81–83 | publisher = University of California Press | isbn = 978-0-520-22021-8 }}</ref>]]

Banded iron formation provided some of the first evidence for the timing of the [[Earth:Great Oxidation Event|Great Oxidation Event]], 2,400&nbsp;Ma.<ref name= "Cloud_1973">{{cite journal |last1 = Cloud |first1=P. | doi = 10.2113/gsecongeo.68.7.1135| pages = 1135–1143| volume = 68| journal = Economic Geology| title = Paleoecological Significance of the Banded Iron-Formation| year = 1973 | issue = 7 |bibcode=1973EcGeo..68.1135C }}</ref><ref name="holland-2006">{{cite journal |last1=Holland |first1=Heinrich D |title=The oxygenation of the atmosphere and oceans |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=19 May 2006 |volume=361 |issue=1470 |pages=903–915 |doi=10.1098/rstb.2006.1838|pmid=16754606 |pmc=1578726 }}</ref> With his 1968 paper on the early atmosphere and oceans of the earth,<ref name="cloud-1968">{{cite journal |last1=Cloud |first1=Preston E. |title=Atmospheric and Hydrospheric Evolution on the Primitive Earth. |journal=Science |volume=160 |number=3829 |year=1968 |pages=729–736 |doi=10.1126/science.160.3829.729 |jstor=1724303 |pmid=5646415 |bibcode=1968Sci...160..729C }}</ref> Preston Cloud established the general framework that has been widely, if not universally,<ref name="ohmoto-etal-2006">{{cite journal |last1=Ohmoto |first1=H. |last2=Watanabe |first2=Y. |last3=Yamaguchi |first3=K.E. |last4=Naraoka |first4=H. |last5=Haruna |first5=M. |last6=Kakegawa |first6=T. |last7=Hayashi |first7=K. |last8=Kato |first8=Y. |title=Chemical and biological evolution of early Earth: Constraints from banded iron formations |journal=Geological Society of America Memoir |date=2006 |volume=198 |pages=291–331 |doi=10.1130/2006.1198(17) |isbn=9780813711980 |url=https://books.google.com/books?id=U_QqAd1QlIgC&pg=PA291 |access-date=19 June 2020}}</ref><ref>{{cite book |title=Banded iron formations, to iron ore : an integrated genesis model |isbn=978-1536109719 |first1=Desmond Fitzgerald |last1=Lascelles |date=2017 |publisher=Nova Science Publishers}}</ref> accepted for understanding the deposition of BIFs.<ref name="trendall-blockley-2004"/><ref name="condie-2015"/>

Cloud postulated that banded iron formations were a consequence of anoxic, iron-rich waters from the deep ocean welling up into a [[Earth:Photic zone|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 [[Biology:Superoxide dismutase|superoxide dismutase]]) for living in an oxygenated environment. Such organisms would have been protected from their own [[Biology:Reactive oxygen species|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.<ref name="cloud-1968"/><ref name= "Cloud_1973"/>

Cloud suggested that banding resulted from fluctuations in the population of cyanobacteria due to [[Biology:Free radical damage to DNA|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.<ref name="cloud-1968"/><ref name= "Cloud_1973"/>

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.<ref name="trendall-2002"/> 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.<ref name="trendall-blockley-2004"/><ref name="simonson-hassler-1996">{{cite journal |last1=Simonson |first1=Bruce M. |last2=Hassler |first2=Scott W. |title=Was the Deposition of Large Precambrian Iron Formations Linked to Major Marine Transgressions? |journal=The Journal of Geology |date=November 1996 |volume=104 |issue=6 |pages=665–676 |doi=10.1086/629861|bibcode=1996JG....104..665S |s2cid=128886898 }}</ref>

The few formations deposited after 1,800&nbsp;[[Year#SI prefix multipliers|Ma]]<ref name="Slack_2009">{{Cite journal |last1 = Slack |first1=J.F. |last2=Cannon |first2=W.F. | doi = 10.1130/G30259A.1 | title = Extraterrestrial demise of banded iron formations 1.85 billion years ago | journal = Geology | volume = 37 | issue = 11 | pages = 1011–1014 | year = 2009 |bibcode = 2009Geo....37.1011S }}</ref> may point to intermittent low levels of free atmospheric oxygen,<ref name="Lyons_2009">{{cite journal |last1 = Lyons |first1=T.W. |last2=Reinhard |first2=C.T. | title = Early Earth: Oxygen for heavy-metal fans | journal = Nature | volume = 461 | issue = 7261 | pages = 179–81 | date = September 2009 | pmid = 19741692 | doi = 10.1038/461179a | bibcode = 2009Natur.461..179L | s2cid = 205049360 | doi-access = free }}</ref> while the small peak at {{Ma|750}} may be associated with the hypothetical Snowball Earth.<ref name="Hoffman_1999">{{cite journal |last1 = Hoffman |first1=P.F. |last2=Kaufman |first2=A.J. |last3=Halverson |first3=G.P. |last4=Schrag |first4=D.P. | title = A neoproterozoic snowball earth | journal = Science | volume = 281 | issue = 5381 | pages = 1342–6 | date = August 1998 | pmid = 9721097 | doi = 10.1126/science.281.5381.1342 | url = http://marine.rutgers.edu/ebme/HistoryEarthSystems/HistEarthSystems_Fall2008/Week6a/Hoffman_et_al_Science_1998.pdf | bibcode = 1998Sci...281.1342H |s2cid=13046760 }}</ref>

===Formation processes===

The microbands within chert layers are most likely [[Earth:Varve|varve]]s produced by annual variations in oxygen production. [[Earth:Diurnal cycle|Diurnal]] microbanding would require a very high rate of deposition of 2 meters per year or 5&nbsp;km/Ma. Estimates of deposition rate based on various models of deposition and [[Physics:Sensitive high-resolution ion microprobe|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 [[Earth:Rhythmite|rhythmite]]s produced by tidal cycles.<ref name="trendall-blockley-2004"/>

Preston Cloud proposed that mesobanding was a result of self-poisoning by early cyanobacteria as the supply of reduced iron was periodically depleted.<ref name= "Cloud_1973"/> 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.<ref name="trendall-blockley-2004"/> Another theory is that mesobands are primary structures resulting from pulses of activity along [[Earth:Mid-ocean ridge|mid-ocean ridge]]s that change the availability of reduced iron on time scales of decades.<ref name="morris-horwitz-1983">{{cite journal |last1=Morris |first1=R.C. |last2=Horwitz |first2=R.C. |title=The origin of the iron-formation-rich Hamersley Group of Western Australia — deposition on a platform |journal=Precambrian Research |date=August 1983 |volume=21 |issue=3–4 |pages=273–297 |doi=10.1016/0301-9268(83)90044-X|bibcode=1983PreR...21..273M }}</ref> In the case of granular iron formations, the mesobands are attributed to [[Earth:Winnowing (sedimentology)|winnowing]] of sediments in shallow water, in which wave action tended to segregate particles of different size and composition.<ref name="trendall-blockley-2004"/>

For banded iron formations to be deposited, several preconditions must be met.<ref name="cox-etal-2013"/>

# The deposition basin must contain waters that are ferruginous (rich in [[Chemistry:Iron|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 [[Earth:Euxinia|euxinic]] (rich in [[Chemistry:Hydrogen sulfide|hydrogen sulfide]]), since this would cause the ferrous iron to precipitate out as [[Chemistry:Pyrite|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.<ref name="trendall-blockley-2004"/> Plausible sources of iron include hydrothermal vents along mid-ocean ridges, windblown dust, rivers, glacial ice, and seepage from continental margins.<ref name="cox-etal-2013"/>

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.<ref name="Nadoll_2014">{{cite journal|last1 = Nadoll |first1=P. |last2=Angerer |first2=T. |last3=Mauk |first3=J.L. |last4=French |first4=D. |last5=Walshe |first5=J |title=The chemistry of hydrothermal magnetite: A review|journal=Ore Geology Reviews|volume=61|pages=1–32|doi=10.1016/j.oregeorev.2013.12.013|year=2014|bibcode=2014OGRv...61....1N }}</ref><ref name="Zhu_2014">{{Cite journal|last1 = Zhu |first1=X.Q. |last2=Tang |first2=H.S. |last3=Sun |first3=X.H. |title=Genesis of banded iron formations: A series of experimental simulations|journal=Ore Geology Reviews|volume=63|pages=465–469|doi=10.1016/j.oregeorev.2014.03.009|year=2014|bibcode=2014OGRv...63..465Z }}</ref><ref name="Li_2015">{{cite journal |last1 = Li |first1=L.X. |last2=Li |first2=H.M. |last3=Xu |first3=Y.X. |last4=Chen |first4=J. |last5=Yao |first5=T. |last6=Zhang |first6=L.F. |last7=Yang |first7=X.Q. |last8=Liu |first8=M.J. |title=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=Journal of Asian Earth Sciences|volume=113|pages=1017–1034|doi=10.1016/j.jseaes.2015.02.007|bibcode=2015JAESc.113.1017L|year=2015}}</ref> Algoma-type BIFs formed primarily in the Archean. These older BIFs tend to show a positive [[Chemistry:Europium anomaly|europium anomaly]] consistent with a [[Earth:Hydrothermal vent|hydrothermal]] source of iron.<ref name="condie-2015"/>  By contrast, Lake Superior-type banded iron formations primarily formed during the [[Earth:Paleoproterozoic|Paleoproterozoic]] era, and lack the europium anomalies of the older Algoma-type BIFs, suggesting a much greater input of iron weathered from continents.<ref name="klein-2005"/><ref name="Li-etal-2015">{{cite journal |last1=Li |first1=Weiqiang |last2=Beard |first2=Brian L. |last3=Johnson |first3=Clark M. |title=Biologically recycled continental iron is a major component in banded iron formations |journal=Proceedings of the National Academy of Sciences |date=7 July 2015 |volume=112 |issue=27 |pages=8193–8198 |doi=10.1073/pnas.1505515112|pmid=26109570 |pmc=4500253 |bibcode=2015PNAS..112.8193L |doi-access=free }}</ref><ref name="condie-2015"/>

====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 [[Biology:Dissimilatory sulfate reduction|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.<ref name="holland-2006"/>

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 [[Chemistry:Manganese|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.<ref name="holland-2006"/>

Banded iron formations in northern Minnesota are overlain by a thick layer of ejecta from the [[Earth:Sudbury Basin|Sudbury Basin]] impact. An [[Astronomy:Asteroid|asteroid]] (estimated at {{convert|10|km|abbr=on}} across) [[Astronomy:Impact event|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|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.<ref name="Slack_2009"/>

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

=====Oxygenic photosynthesis=====
[[Image:CSIRO ScienceImage 4203 A bluegreen algae species Cylindrospermum sp under magnification.jpg|thumb|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:<ref name= "Cloud_1973"/><ref name="cox-etal-2013"/>

:{{chem2|4 Fe(2+) + O2 + 10 H2O -> 4 Fe(OH)3 + 8 H+}}

The oxygen comes from the photosynthetic activities of cyanobacteria.<ref name="cox-etal-2013"/> 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.<ref name="cox-etal-2013"/>

=====Anoxygenic photosynthesis=====
[[Image:Iron bacteria burn.JPG|right|thumb|A [[Earth:Burn (landform)|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 [[Biology:Anoxygenic photosynthesis|anoxygenic phototrophs]].<ref>{{cite journal |last1 = Kappler |first1=A. |last2=Pasquero |first2=C. |last3=Konhauser |first3=K.O. |last4=Newman |first4=D.K. | title = Deposition of banded iron formations by anoxygenic phototrophic Fe (II)-oxidizing bacteria. | journal = Geology | date = November 2005 | volume = 33 | issue = 11 | pages = 865–8 | url = http://www.ess.uci.edu/~cpasquer/papers/kappleretal_GEO2005.pdf | archive-url = https://web.archive.org/web/20081216220557/http://www.ess.uci.edu/~cpasquer/papers/kappleretal_GEO2005.pdf | archive-date=16 December 2008 | doi = 10.1130/G21658.1 | bibcode = 2005Geo....33..865K }}</ref> The concentrations of phosphorus and trace metals in BIFs are consistent with precipitation through the activities of iron-oxidizing bacteria.<ref name="konhauser-etal-2002">{{cite journal |last1=Konhauser |first1=Kurt O. |last2=Hamade |first2=Tristan |last3=Raiswell |first3=Rob |last4=Morris |first4=Richard C. |last5=Grant Ferris |first5=F. |last6=Southam |first6=Gordon |last7=Canfield |first7=Donald E. |title=Could bacteria have formed the Precambrian banded iron formations? |journal=Geology |date=2002 |volume=30 |issue=12 |pages=1079 |doi=10.1130/0091-7613(2002)030<1079:CBHFTP>2.0.CO;2|bibcode=2002Geo....30.1079K }}</ref>

Iron isotope ratios in the oldest banded iron formations (3700-3800 Ma), at Isua, Greenland, are best explained by assuming extremely low oxygen levels (<0.001% of modern O<sub>2</sub> levels in the photic zone) and anoxygenic photosynthetic oxidation of Fe(II):<ref name="czaja-etal-2013"/><ref name="cox-etal-2013"/>

:{{chem2|4 Fe(2+) + 11 H2O + CO2 + hv → CH2O + 4 Fe(OH)3 + 8 H+}}

This requires that dissimilatory iron reduction, the biological process in which microorganisms substitute Fe(III) for oxygen in respiration, was not yet widespread.<ref name="czaja-etal-2013"/> By contrast, Lake Superior-type banded iron formations show iron isotope ratios that suggest that dissimilatory iron reduction expanded greatly during this period.<ref name="johnson-etal-2008">{{cite journal |last1=Johnson |first1=Clark M. |last2=Beard |first2=Brian L. |last3=Klein |first3=Cornelis |last4=Beukes |first4=Nic J. |last5=Roden |first5=Eric E. |title=Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis |journal=Geochimica et Cosmochimica Acta |date=January 2008 |volume=72 |issue=1 |pages=151–169 |doi=10.1016/j.gca.2007.10.013|bibcode=2008GeCoA..72..151J }}</ref>

An alternate route is oxidation by anaerobic [[Biology:Denitrifying bacteria|denitrifying bacteria]]. This requires that [[Biology:Nitrogen fixation|nitrogen fixation]] by microorganisms is also active.<ref name="cox-etal-2013"/>

:{{chem2|10 Fe(2+) + 2 NO3- + 24 H2O → 10 Fe(OH)3 + N2 + 18 H+}}

=====Abiogenic mechanisms=====
The lack of organic carbon in banded iron formation argues against microbial control of BIF deposition.<ref name="klein-beukes-1989"/> On the other hand, there is [[Biology:Fossil|fossil]] evidence for abundant photosynthesizing cyanobacteria at the start of BIF deposition<ref name="trendall-blockley-2004"/> and of [[Astronomy:Biosignature|hydrocarbon markers]] in shales within banded iron formation of the Pilbara craton.<ref name="brocks-etal-1999">{{cite journal |last1=Brocks |first1=J. J. |first2=Graham A. |last2=Logan |first3=Roger |last3=Buick |first4=Roger E. |last4=Summons |title=Archean Molecular Fossils and the Early Rise of Eukaryotes |journal=Science |date=13 August 1999 |volume=285 |issue=5430 |pages=1033–1036 |doi=10.1126/science.285.5430.1033|pmid=10446042 |bibcode=1999Sci...285.1033B }}</ref> The carbon that is present in banded iron formations is enriched in the light isotope, <sup>12</sup>C, an indicator of a biological origin. If a substantial part of the original iron oxides was in the form of hematite, then any carbon in the sediments might have been oxidized by the decarbonization reaction:<ref name="trendall-2002"/>

:{{chem2|6 Fe2O3 + C <-> 4 Fe3O4 + CO2}}

Trendall and J.G. Blockley proposed, but later rejected, the hypothesis that banded iron formation might be a peculiar kind of Precambrian [[Earth:Evaporite|evaporite]].<ref name="trendall-blockley-2004"/>  Other proposed abiogenic processes include [[Physics:Radiolysis|radiolysis]] by the radioactive isotope of [[Chemistry:Potassium|potassium]], <sup>40</sup>K,<ref name="draganic-etal-1991">{{cite journal |last1=Draganić |first1=I.G. |last2=Bjergbakke |first2=E. |last3=Draganić |first3=Z.D. |last4=Sehested |first4=K. |title=Decomposition of ocean waters by potassium-40 radiation 3800 Ma ago as a source of oxygen and oxidizing species |journal=Precambrian Research |date=August 1991 |volume=52 |issue=3–4 |pages=337–345 |doi=10.1016/0301-9268(91)90087-Q|bibcode=1991PreR...52..337D }}</ref> or annual turnover of basin water combined with upwelling of iron-rich water in a stratified ocean.<ref name="klein-beukes-1989">{{cite journal |last1=Klein |first1=Cornelis |last2=Beukes |first2=Nicolas J. |title=Geochemistry and sedimentology of a facies transition from limestone to iron-formation deposition in the early Proterozoic Transvaal Supergroup, South Africa |journal=Economic Geology |date=1 November 1989 |volume=84 |issue=7 |pages=1733–1774 |doi=10.2113/gsecongeo.84.7.1733|bibcode=1989EcGeo..84.1733K }}</ref>

Another abiogenic mechanism is photooxidation of iron by sunlight. Laboratory experiments suggest that this could produce a sufficiently high deposition rate under likely conditions of pH and sunlight.<ref name="braterman-etal-1983">{{cite journal |last1=Braterman |first1=Paul S. |last2=Cairns-Smith |first2=A. Graham |author-link2=Graham Cairns-Smith |last3=Sloper |first3=Robert W. |title=Photo-oxidation of hydrated Fe2+—significance for banded iron formations |journal=Nature |date=May 1983 |volume=303 |issue=5913 |pages=163–164 |doi=10.1038/303163a0|bibcode=1983Natur.303..163B |s2cid=4357551 }}</ref><ref name="braterman-cairns-smith-1987">{{cite journal |last1=Braterman |first1=Paul S. |last2=Cairns-Smith |first2=A. Graham |title=Photoprecipitation and the banded iron-formations — Some quantitative aspects |journal=Origins of Life and Evolution of the Biosphere |date=September 1987 |volume=17 |issue=3–4 |pages=221–228 |doi=10.1007/BF02386463|bibcode=1987OrLi...17..221B |s2cid=33140490 }}</ref> However, if the iron came from a shallow hydrothermal source, other laboratory experiments suggest that precipitation of ferrous iron as carbonates or silicates could seriously compete with photooxidation.<ref name="konhauser-etal-2007">{{cite journal |last1=Konhauser |first1=Kurt O. |last2=Amskold |first2=Larry |last3=Lalonde |first3=Stefan V. |last4=Posth |first4=Nicole R. |last5=Kappler |first5=Andreas |last6=Anbar |first6=Ariel |title=Decoupling photochemical Fe(II) oxidation from shallow-water BIF deposition |journal=Earth and Planetary Science Letters |date=15 June 2007 |volume=258 |issue=1–2 |pages=87–100 |doi=10.1016/j.epsl.2007.03.026 |bibcode=2007E&PSL.258...87K |url=https://www.sciencedirect.com/science/article/abs/pii/S0012821X07001823 |access-date=23 June 2020}}</ref>

====Diagenesis====
Regardless of the precise mechanism of oxidation, the oxidation of ferrous to ferric iron likely caused the iron to precipitate out as a ferric hydroxide gel. Similarly, the silica component of the banded iron formations likely precipitated as a hydrous silica gel.<ref name="trendall-blockley-2004"/> The conversion of iron hydroxide and silica gels to banded iron formation is an example of [[Earth:Diagenesis|diagenesis]], the conversion of sediments into solid rock.

There is evidence that banded iron formations formed from sediments with nearly the same chemical composition as is found in the BIFs today. The BIFs of the Hamersley Range show great chemical homogeneity and lateral uniformity, with no indication of any precursor rock that might have been altered to the current composition. This suggests that, other than dehydration and decarbonization of the original ferric hydroxide and silica gels, diagenesis likely left the composition unaltered and consisted of crystallization of the original gels.<ref name="trendall-blockley-2004"/> Decarbonization may account for the lack of carbon and preponderance of magnetite in older banded iron formations.<ref name="trendall-2002"/> The relatively high content of hematite in Neoproterozoic BIFs suggests they were deposited very quickly and via a process that did not produce great quantities of biomass, so that little carbon was present to reduce hematite to magnetite.<ref name="cox-etal-2013"/>

However, it is possible that BIF was altered from carbonate rock<ref name="kimberley-1974">{{cite journal |last1=Kimberley |first1=M. M. |title=Origin of iron ore by diagenetic replacement of calcareous oolite |journal=Nature |date=July 1974 |volume=250 |issue=5464 |pages=319–320 |doi=10.1038/250319a0|bibcode=1974Natur.250..319K |s2cid=4211912 }}</ref> or from hydrothermal mud<ref name="krapez-etal-201">{{cite journal |last1=Krapez |first1=B. |last2=Barley |first2=M.E. |last3=Pickard |first3=A.L. |title=Banded iron formations: ambient pelagites, hydrothermal muds or metamorphic rocks? |journal=Extended Abstracts 4th International Archaean Symposium |date=2001 |pages=247–248}}</ref> during late stages of diagenesis. A 2018 study found no evidence that magnetite in BIF formed by decarbonization, and suggests that it formed from thermal decomposition of [[Chemistry:Siderite|siderite]] via the reaction
::{{chem2|3 FeCO3 + H2O → Fe3O4 + 3 CO2 + H2}}
The iron may have originally precipitated as [[Chemistry:Greenalite|greenalite]] and other iron silicates. Macrobanding is then interpreted as a product of compaction of the original iron silicate mud. This produced siderite-rich bands that served as pathways for fluid flow and formation of magnetite.<ref>{{cite journal |last1=Rasmussen |first1=Birger |last2=Muhling |first2=Janet R. |title=Making magnetite late again: Evidence for widespread magnetite growth by thermal decomposition of siderite in Hamersley banded iron formations |journal=Precambrian Research |date=March 2018 |volume=306 |pages=64–93 |doi=10.1016/j.precamres.2017.12.017|bibcode=2018PreR..306...64R }}</ref>

===The Great Oxidation Event===
[[Image:Oxygenation-atm-2.svg|thumb|upright=2|[[Great Oxidation Event|Oxygen (O<sub>2</sub>) build-up]] in the Earth's atmosphere. Red and green lines represent the range of the estimates while time is measured in billions of years ago (Ga).<ref name="holland-2006"/>
<br />Banded formation iron deposition peaks at the beginning of Stage 2 and pauses at the beginning of Stage 3.]]
{{Main|Earth:Great Oxidation Event}}
The peak of deposition of banded iron formations in the late Archean, and the end of deposition in the Orosirian, have been interpreted as markers for the Great Oxygenation Event. Prior to 2.45 billion years ago, the high degree of [[Chemistry:Mass-independent fractionation|mass-independent fractionation]] of sulfur (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 the permanent appearance of oxygen in the atmosphere between 2.41 and 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 deposition of BIF at 1.85 billion years ago is attributed to the oxidation of the deep ocean.<ref name="holland-2006"/>

===Snowball Earth hypothesis===
{{Main|Earth:Snowball Earth}}
[[Image:Jaspilite banded iron formation, Soudan Underground State Park.jpg|thumb|Neoarchean banded iron formation from northeastern Minnesota]]
Until 1992<ref>{{cite book | last = Kirschvink | first = Joseph | date = 1992 | chapter = Late Proterozoic low-latitude global glaciation: the Snowball Earth | veditors = Schopf JW, Klein C | title = The Proterozoic Biosphere: A Multidisciplinary Study. | publisher = Cambridge University Press }}</ref> it was assumed that the rare, later (younger) banded iron deposits represented unusual conditions where oxygen was depleted locally. Iron-rich waters would then form in isolation and subsequently come into contact with oxygenated water. The Snowball Earth hypothesis provided an alternative explanation for these younger deposits. In a Snowball Earth state the continents, and possibly seas at low latitudes, were subject to a severe ice age circa 750 to 580 Ma that nearly or totally depleted free oxygen. Dissolved iron then accumulated in the oxygen-poor oceans (possibly from seafloor hydrothermal vents).<ref name="cheilletz-etal-2006">{{cite journal |last1=Cheilletz |first1=Alain |last2=Gasquet |first2=Dominique |last3=Mouttaqi |first3=Abdellah |last4=Annich |first4=Mohammed |last5=El Hakour |first5=Abdelkhalek |title=Discovery of Neoproterozoic banded iron formation (BIF) in Morocco |journal=Geophysical Research Abstracts |date=2006 |volume=8 |url=https://meetings.copernicus.org/www.cosis.net/abstracts/EGU06/04635/EGU06-J-04635.pdf |access-date=23 June 2020}}</ref> Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron.<ref name="trendall-blockley-2004"/><ref name="condie-2015"/> Banded iron formations of this period are predominantly associated with the [[Earth:Sturtian glaciation|Sturtian glaciation]].<ref name="stern-etal-2006">{{cite journal |last1=Stern |first1=R.J. |last2=Avigad |first2=D. |last3=Miller |first3=N.R. |last4=Beyth |first4=M. |title=Evidence for the Snowball Earth hypothesis in the Arabian-Nubian Shield and the East African Orogen |journal=Journal of African Earth Sciences |date=January 2006 |volume=44 |issue=1 |pages=1–20 |doi=10.1016/j.jafrearsci.2005.10.003 |bibcode=2006JAfES..44....1S |url=https://personal.utdallas.edu/~rjstern/pdfs/Snowball.JAES06.pdf |access-date=23 June 2020}}</ref><ref name="cox-etal-2013"/>

An alternative mechanism for banded iron formations in the Snowball Earth era suggests the iron was deposited from metal-rich [[Earth:Brine|brine]]s in the vicinity of hydrothermally active [[Earth:Rift zone|rift zone]]s<ref>{{cite journal | last1 = Eyles |first1=N. |last2=Januszczak |first2=N | title = Zipper-rift': A tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma | journal = Earth-Science Reviews | volume = 65 | issue = 1–2 | pages = 1–73 | url = http://courses.eas.ualberta.ca/eas457/Eyles_2004.pdf | archive-url = https://web.archive.org/web/20071128105306/http://courses.eas.ualberta.ca/eas457/Eyles_2004.pdf | archive-date=28 November 2007 | bibcode = 2004ESRv...65....1E | year = 2004 | doi = 10.1016/S0012-8252(03)00080-1 }}</ref> due to glacially-driven thermal overturn.<ref name="young-2002">{{cite journal |last1=Young |first1=Grant M. |title=Stratigraphic and tectonic settings of Proterozoic glaciogenic rocks and banded iron-formations: relevance to the snowball Earth debate |journal=Journal of African Earth Sciences |date=November 2002 |volume=35 |issue=4 |pages=451–466 |doi=10.1016/S0899-5362(02)00158-6|bibcode=2002JAfES..35..451Y }}</ref><ref name="stern-etal-2006"/> The limited extent of these BIFs compared with the associated glacial deposits, their association with volcanic formations, and variation in thickness and facies favor this hypothesis. Such a mode of formation does not require a global anoxic ocean, but is consistent with either a Snowball Earth or Slushball Earth model.<ref name="young-2002"/><ref name="cox-etal-2013"/>

==Economic geology==
[[File:Hull–Rust–Mahoning Open Pit Iron Mine.jpg|thumb|Hull-Rust-Mahoning Open Pit Iron Mine in the Iron Range]]
Banded iron formations provide most of the [[Earth:Iron ore|iron ore]] presently mined.<ref name="trendall-2005"/> More than 60% of global iron reserves are in the form of banded iron formation, most of which can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States.<ref name="Nadoll_2014" /><ref name="Zhu_2014" />

Different mining districts coined their own names for BIFs. The term "banded iron formation" was coined in the iron districts of Lake Superior, where the ore deposits of the Mesabi, Marquette, Cuyuna, Gogebic, and Menominee [[Chemistry:Iron Range|iron ranges]] were also variously known as "jasper", "jaspilite", "iron-bearing formation", or [[Earth:Taconite|taconite]]. Banded iron formations were described as "itabarite" in Brazil, as "ironstone" in South Africa, and as "BHQ" (banded hematite quartzite) in India.<ref name="trendall-2005"/>

Banded iron formation was first discovered in northern Michigan in 1844, and mining of these deposits prompted the earliest studies of BIFs, such as those of Charles R. Van Hise and Charles Kenneth Leith.<ref name="trendall-blockley-2004"/> Iron mining operations on the Mesabi and Cuyuna Ranges evolved into enormous open pit mines, where [[Engineering:Steam shovel|steam shovel]]s and other industrial machines could remove massive amounts of ore. Initially the mines exploited large beds of hematite and [[Chemistry:Goethite|goethite]] weathered out of the banded iron formations, and some {{convert|2500000000|t|abbr=on}} of this "natural ore" had been extracted by 1980.<ref name="explore-2016">{{cite web |title=Explore Minnesota: Iron Ore |url=http://files.dnr.state.mn.us/lands_minerals/mcc_docs/2016_explore_iron_ore.pdf |publisher=Minnesota Minerals Coordinating Council |access-date=18 June 2020}}</ref> By 1956, large-scale commercial production from the BIF itself began at the Peter Mitchell Mine near Babbitt, Minnesota.<ref name=Ridge>{{Cite book |last=Marsden |first=Ralph |title=Geology of the Iron Ores of the Lake Superior Region in the United States, in Volume 1 of Ore Deposits of the United States, 1933–1967 |publisher=The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. |year=1968 |editor=John D. Ridge |pages=490–492}}</ref> Production in Minnesota was {{convert|40000000|t|abbr=on}} of ore concentrate per year in 2016, which is about 75% of total U.S. production.<ref name="explore-2016"/> Magnetite-rich banded iron formation, known locally as taconite, is ground to a powder, and the magnetite is [[Physics:Magnetic separation|separated with powerful magnets]] and [[Chemistry:Pelletizing|pelletized]] for shipment and smelting.<ref name="DNR1">{{cite web |title=Taconite |url=https://www.dnr.state.mn.us/education/geology/digging/taconite.html |publisher=Minnesota Department of Natural Resources |access-date=10 October 2020}}</ref>

[[File:Mount Tom Price mine, September 2006.jpg|thumb|Tom Price Mine, Hamersley Range, Australia]]
Iron ore became a global commodity after the Second World War, and with the end of the embargo against exporting iron ore from Australia in 1960, the Hamersley Range became a major mining district.<ref name="trendall-blockley-2004"/><ref name="henrietta.liswa.wa.gov.au"/><ref name="Geology"/><ref name="Iron2002">{{cite web|url=http://www.portergeo.com.au/tours/iron2002/iron2002depm1.asp |title=Iron 2002 – Key Iron Deposits of the World – Module 1, Australia |publisher=Porter GeoConsultancy|date=2002-09-18 |access-date=2012-08-07}}</ref> The banded iron formations here are the thickest and most extensive in the world,<ref name="condie-2015"/><ref name="wam-2014"/> originally covering an area of {{convert|150000|km2|sp=us|mi2|abbr=off}} and containing about {{convert|300000000000|t|abbr=on}} of iron.<ref name="wam-2014"/> The range contains 80 percent of all identified iron ore reserves in Australia.<ref>{{cite web |title=Iron Fact Sheet |date=15 May 2014 |url=http://www.australianminesatlas.gov.au/education/fact_sheets/iron.html |publisher=Geoscience Australia |access-date=10 October 2020 |archive-url=https://web.archive.org/web/20170218005217/http://www.australianminesatlas.gov.au/education/fact_sheets/iron.html# |archive-date=18 February 2017}}</ref> Over {{convert|100000000|t|abbr=on}} of iron ore is removed from the range every year.<ref>{{cite web|url=http://www.riotintoironore.com/ENG/operations/497_mining.asp|title=Mining|publisher=Rio Tinto Iron Ore|year=2010|access-date=6 November 2011|url-status=dead|archive-url=https://web.archive.org/web/20100612111105/http://www.riotintoironore.com/ENG/operations/497_mining.asp|archive-date=12 June 2010}}</ref>

The Itabarite banded iron formations of Brazil cover at least {{convert|80000|km2|sp=us|mi2|abbr=off}} and are up to {{convert|600|meters|feet|abbr=off}} thick.<ref name="gole-klein-1981"/> These form the Quadrilatero Ferrifero or Iron Quadrangle, which resembles the Iron Range mines of United States in that the favored ore is hematite weathered out of the BIFs.<ref name="mdo-2020">{{cite web |title=Minas Itabirito Complex |url=https://miningdataonline.com/property/1364/Minas-Itabirito-Complex.aspx |website=Mining Data Solutions |publisher=MDO Data Online Inc. |access-date=22 June 2020}}</ref> Production from the Iron Quadrangle helps make Brazil the second largest producer of iron ore after Australia, with monthly exports averaging {{convert|139299|t|abbr=on}} from December 2007 to May 2018.<ref>{{Cite web|url=https://www.ceicdata.com/en/brazil/iron-ore-exports-by-port|title=Brazil Iron Ore Exports: By Port|publisher=CEIC Data|access-date=2019-02-16}}</ref>

[[File:鞍山市齐大山铁矿.JPG|thumb|461x461px|Qidashan open cast iron ore mine, one of three large pits surrounding Anshan city]]
Mining of ore from banded iron formations at Anshan in north China began in 1918. When Japan occupied Northeast China in 1931, these mills were turned into a Japanese-owned monopoly, and the city became a significant strategic industrial hub during the Second World War. Total production of processed iron in [[Place:Manchuria|Manchuria]] reached {{convert|1000000|t|abbr=on}} in 1931–1932. By 1942, Anshan's Shōwa Steel Works total production capacity reached {{convert|3600000|t|abbr=on}} per annum, making it one of the major iron and steel centers in the world.<ref name="Beasley 1991">{{cite book
 | last = Beasley
 | first = W.G.
 | year = 1991
 | title = Japanese Imperialism 1894–1945
 | publisher = Oxford University Press
 | isbn = 0-19-822168-1
}}</ref> Production was severely disrupted during the Soviet occupation of Manchuria in 1945 and the subsequent Chinese Civil War. However, from 1948 to 2001, the steel works produced {{convert|290000000|t|abbr=on}}290 million tons of steel, {{convert|284000000|t|abbr=on}} of [[Chemistry:Pig iron|pig iron]] and {{convert|192000000|t|abbr=on}} of rolled steel. Annual production capacity {{As of|2006|lc=y}} is {{convert|10000000|t|abbr=on}} of pig iron, {{convert|10000000|t|abbr=on}} of steel and {{convert|9500000|t|abbr=on}} of rolled steel. A quarter of China's total iron ore reserves, about {{convert|10000000000|t|abbr=on}}, are located in Anshan.<ref>{{cite book
  | last = Huang
  | first = Youyi
  |author2=Xiao Siaoming |author3=Li Zhenguo |author4=Zhang Zouku
   | title = Liaoning, Home of the Manchus & Cradle of Qing Empire
  | publisher = Foreign Languages Press, Beijing
  | year = 2006
  | pages = 227
  | isbn = 7-119-04517-2}}</ref>

==See also==
* {{annotated link|Earth:Iron-rich sedimentary rocks}}
* {{annotated link|Earth:Stromatolite}}

== References ==
{{reflist}}

== Further reading ==
{{refbegin}}
* {{cite web |ref=no | last1 = Harnmeijer |first1=J.P. | title = Banded Iron Formation: A Continuing Enigma of Geology. | publisher = University of Washington | date = 2003 | url = http://earthweb.ess.washington.edu/~jelte/essays/BIFs.doc | archive-url = https://web.archive.org/web/20060908034327/http://earthweb.ess.washington.edu/~jelte/essays/BIFs.doc | archive-date = 8 September 2006 }}
* {{cite journal |ref=no | last1 = Klein |first1=C. | title = Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins. | journal = American Mineralogist | date = October 2005 | volume = 90 | issue = 10 | pages = 1473–99 | doi = 10.2138/am.2005.1871 | bibcode = 2005AmMin..90.1473K | s2cid = 201124189 }}
{{refend}}

==External links==
*{{Britannica | id=51509| title=Banded-iron formation}}
* {{Cite Americana|wstitle=Jaspilite |short=x}}

{{Clear}}
{{ores}}


{{DEFAULTSORT:Banded Iron Formation}}
[[Category:Iron ores]]
[[Category:Economic geology]]
[[Category:Sedimentary rocks]]

{{Sourceattribution|Banded iron formation}}