Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
consisting of alternating layers of
iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...
s and iron-poor
chert
Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...
. 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
The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
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), are associated with banded iron formations.
Banded iron formations are thought to have formed in
sea water
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approx ...
as the result of
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
production by
photosynthetic
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
. The oxygen combined with dissolved
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
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
A varve is an annual layer of sediment or sedimentary rock.
The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (var ...
, resulting from cyclic variations in oxygen production.
Banded iron formations were first discovered in northern
Michigan
Michigan () is a state in the Great Lakes region of the upper Midwestern United States. With a population of nearly 10.12 million and an area of nearly , Michigan is the 10th-largest state by population, the 11th-largest by area, and the ...
in 1844. Banded iron formations account for more than 60% of global iron reserves and provide most of the
iron ore
Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...
presently mined. Most formations can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States.
Description
A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black
iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...
s, either
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
(Fe
3O
4) or
hematite
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
(Fe
2O
3), alternating with bands of iron-poor
chert
Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...
, often red in color, of similar thickness.
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
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
containing greater than 15%
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
. 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.
The iron in BIFs is divided roughly equally between the more oxidized
ferric
In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+.
The adjective ferric or the prefix ferri- is often used to spe ...
form, Fe(III), and the more reduced
ferrous
In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to "ferric" or iron(III), meaning iron in its +3 oxidation state, such a ...
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
Siderite is a mineral composed of iron(II) carbonate (FeCO3). It takes its name from the Greek word σίδηρος ''sideros,'' "iron". It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Zinc, magnesium and ...
and
ankerite
Ankerite is a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral carbonates with the chemical formula . In composition it is closely related to dolomite, but differs from this in having magnesium replaced by varyin ...
, or the iron-rich silicates
minnesotaite
Minnesotaite is an iron silicate mineral with formula: (Fe2+,Mg)3Si4O10(OH)2. It crystallizes in the triclinic crystal system and occurs as fine needles and platelets with other silicates. It is isostructural with the pyrophyllite-talc mineral g ...
and
greenalite
Greenalite is a mineral in the kaolinite- serpentine group with the chemical composition (Fe2+,Fe3+)2-3Si2O5(OH)4.Sleep N.H., Bird D.K. (2007): Niches of the pre-photosynthetic biosphere and geologic preservation of Earth’s earliest ecology. Geo ...
. 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
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
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.
BIFs only rarely interfinger with other rock types, tending to form sharply bounded discrete units that never grade laterally into other rock types.
Banded iron formations of the
Great Lakes region
The Great Lakes region of North America is a binational Canadian–American region that includes portions of the eight U.S. states of Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania and Wisconsin along with the Canadian p ...
and the Frere Formation of western
Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
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
Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.
Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different partic ...
, 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 geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be ...
, in shallower water disturbed by wave motions. However, they otherwise resemble other banded iron formations.
![Rapitan BIF south australia](https://upload.wikimedia.org/wikipedia/commons/c/c5/Rapitan_BIF_south_australia.jpg)
The great majority of banded iron formations are
Archean
The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic.
The Earth
Earth ...
or
Paleoproterozoic
The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
in age. However, a small number of BIFs are
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago.
It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...
in age, and are frequently,
if not universally,
associated with glacial deposits, often containing glacial
dropstones
Dropstones are isolated fragments of rock found within finer-grained water-deposited sedimentary rocks or pyroclastic beds. They range in size from small pebbles to boulders. The critical distinguishing feature is that there is evidence that the ...
.
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 nonexistent
and
soft-sediment deformation structures {{Short description, Geologic formation
Soft-sediment deformation structures develop at deposition or shortly after, during the first stages of the sediment's consolidation. This is because the sediments need to be "liquid-like" or unsolidified ...
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.
Banded iron formations are distinct from most
Phanerozoic
The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...
ironstone
Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement, that contains a substantial proportion of an iron ore compound from which iron (Fe) can be smelted commercially. Not to be con ...
s. Ironstones are relatively rare and are thought to have been deposited in marine
anoxic event
Oceanic anoxic events or anoxic events ( anoxia conditions) describe periods wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulfidic) waters. Although anoxic events have not ...
s, in which the depositional basin became depleted in free
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
. They are composed of iron silicates and oxides without appreciable chert but with significant
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
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
greywacke
Greywacke or graywacke (German ''grauwacke'', signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lit ...
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,
quartzite
Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...
s, and
dolomite Dolomite may refer to:
*Dolomite (mineral), a carbonate mineral
*Dolomite (rock), also known as dolostone, a sedimentary carbonate rock
*Dolomite, Alabama, United States, an unincorporated community
*Dolomite, California, United States, an unincor ...
s, with relatively minor
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...
s or other volcanic rocks, and are assumed to have formed on a
continental shelf
A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...
.
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.
However, the classification into Algoma versus Lake Superior types continues to be used.
Occurrence
![Bif in time](https://upload.wikimedia.org/wikipedia/commons/3/3d/Bif_in_time.jpg)
Banded iron formations are almost exclusively
Precambrian
The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
in age, with most deposits dating to the late Archean (2800-2500 Ma) with a secondary peak of deposition in the
Orosirian
The Orosirian Period (; grc, ὀροσειρά, translit=oroseirá, meaning "mountain range") is the third geologic period in the Paleoproterozoic era (geology), Era and lasted from annum, Mya to Mya (million years ago). Instead of being based ...
period
Period may refer to:
Common uses
* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertainment, and media
* Period (music), a concept in musical composition
* Periodic sentence (or rhetorical period), a concept ...
of the
Paleoproterozoic
The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
(1850 Ma). Minor amounts were deposited in the early Archean and in the
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago.
It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...
(750 Ma).
The youngest known banded iron formation is an
Early Cambrian
The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...
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
A shield is a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. These rocks are older than 570 million years and sometimes date back to around 2 to 3.5 billion years. T ...
of every continent. The oldest BIFs are associated with
greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies.
The name comes from the green ...
s and include the BIFs of the
Isua Greenstone Belt, the oldest known, which have an estimated age of 3700 to 3800 Ma.
The
Temagami
Temagami, formerly spelled as Timagami, is a municipality in northeastern Ontario, Canada, in the Nipissing District with Lake Temagami at its heart.
The Temagami region is known as ''n'Daki Menan'', the homeland of the area's First Nations commu ...
banded iron deposits formed over a 50-million-year period, from 2736 to 2687 Ma, and reached a thickness of .
Other examples of early Archean BIFs are found in the
Abitibi greenstone belt
The Abitibi greenstone belt is a 2,800-to-2,600-million-year-old greenstone belt that spans across the Ontario–Quebec border in Canada. It is mostly made of volcanic rocks, but also includes ultramafic rocks, mafic intrusions, granitoid rocks, ...
s, the greenstone belts of the
Yilgarn and
Pilbara craton
The Pilbara Craton is an old and stable part of the continental lithosphere located in the Pilbara region of Western Australia.
The Pilbara Craton is one of only two pristine Archaean 3.6–2.7 Ga (billion years ago) crusts identified on the ...
s, the
Baltic shield
The Baltic Shield (or Fennoscandian Shield) is a segment of the Earth's crust belonging to the East European Craton, representing a large part of Fennoscandia, northwestern Russia and the northern Baltic Sea. It is composed mostly of Archean and ...
, and the cratons of the
Amazon
Amazon most often refers to:
* Amazons, a tribe of female warriors in Greek mythology
* Amazon rainforest, a rainforest covering most of the Amazon basin
* Amazon River, in South America
* Amazon (company), an American multinational technology c ...
,
north China
North China, or Huabei () is a List of regions of China, geographical region of China, consisting of the provinces of Beijing, Tianjin, Hebei, Shanxi and Inner Mongolia. Part of the larger region of Northern China (''Beifang''), it lies north ...
, and
south
South is one of the cardinal directions or Points of the compass, compass points. The direction is the opposite of north and is perpendicular to both east and west.
Etymology
The word ''south'' comes from Old English ''sūþ'', from earlier Pro ...
and
west
West or Occident is one of the four cardinal directions or points of the compass. It is the opposite direction from east and is the direction in which the Sunset, Sun sets on the Earth.
Etymology
The word "west" is a Germanic languages, German ...
Africa.
The most extensive banded iron formations belong to what A.F. Trendall calls the Great
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
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
The Hamersley Range is a mountainous region of the Pilbara region of Western Australia. The range was named on 12 June 1861 by explorer Francis Thomas Gregory after Edward Hamersley (senior), Edward Hamersley, a prominent promoter of his explo ...
.
[MacLeod, W. N. (1966]
The geology and iron deposits of the Hamersley Range area. Bulletin
(Geological Survey of Western Australia), No. 117 The banded iron formations here were deposited from 2470 to 2450 Ma and are the thickest and most extensive in the world,
with a maximum thickness in excess of .
Similar BIFs are found in the
Carajás Formation Carajás may refer to:
* Carajás Mine
* Carajás Airport
* Canaã dos Carajás
* Eldorado dos Carajás
* Carajás (proposed Brazilian state)
* Carajas (spider), ''Carajas'' (spider)
* Karajá or Carajás, an indigenous tribe of Brazil
* Gurgel Cara ...
of the Amazon craton, the
Cauê Itabirite of the
São Francisco craton
The São Francisco Craton is an ancient craton in the eastern part of South America. The craton crops out in the Brazilian states of Minas Gerais and Bahia.
It includes a number of blocks of Archean basement, separated by orogenic belts. The b ...
, the
Kuruman Iron Formation and
Penge Iron Formation of South Africa, and the
Mulaingiri Formation of
India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
.
Paleoproterozoic banded iron formations are found in the
Iron Range
The term Iron Range refers collectively or individually to a number of elongated iron-ore mining districts around Lake Superior in the United States and Canada. Much of the ore-bearing region lies alongside the range of granite hills formed by ...
and other parts of the
Canadian Shield
The Canadian Shield (french: Bouclier canadien ), also called the Laurentian Plateau, is a geologic shield, a large area of exposed Precambrian igneous and high-grade metamorphic rocks. It forms the North American Craton (or Laurentia), the anc ...
.
The Iron Range is a group of four major deposits: the
Mesabi Range
The Mesabi Iron Range is a mining district in northeastern Minnesota following an elongate trend containing large deposits of iron ore. It is the largest of four major iron ranges in the region collectively known as the Iron Range of Minnesota. ...
, the
Vermilion Range, the
Gunflint Range
The Gunflint Range is an iron ore deposit in northern Minnesota in the United States and Northwestern Ontario, Canada. The range extends from the extreme northern portion of Cook County, Minnesota into the Thunder Bay District, Ontario.
The Gun ...
, and the
Cuyuna Range
The Cuyuna Range is an inactive iron range to the southwest of the Mesabi Range, largely within Crow Wing County, Minnesota. It lies along a line between Brainerd, Minnesota, and Aitkin, Minnesota. The width ranges from 1 to 10 miles (1.6 to 16 ...
. All are part of the
Animikie Group
The Animikie Group is a geologic group composed of sedimentary and metasedimentary rock, having been originally deposited between 2,500 and 1,800 million years ago during the Paleoproterozoic era, within the Animikie Basin. This group of formati ...
and were deposited between 2500 and 1800 Ma.
These BIFs are predominantly granular iron formations.
Neoproterozoic banded iron formations include the Urucum in Brazil, Rapitan in the
Yukon
Yukon (; ; formerly called Yukon Territory and also referred to as the Yukon) is the smallest and westernmost of Canada's three territories. It also is the second-least populated province or territory in Canada, with a population of 43,964 as ...
, 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 .
These are widely thought to have been deposited under unusual anoxic oceanic conditions associated with the "
Snowball Earth
The Snowball Earth hypothesis proposes that, during one or more of Earth's Greenhouse and icehouse Earth, icehouse Climate, climates, the Earth's surface, planet's surface became entirely or nearly entirely Freezing, frozen. It is believed that ...
."
Origins
![Iron banding 01](https://upload.wikimedia.org/wikipedia/commons/c/ca/Iron_banding_01.jpg)
Banded iron formation provided some of the first evidence for the timing of the
Great Oxygenation Event
The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust, was a time interval during the Paleoproterozoic era when the Earth's atmosphere ...
, 2,400 Ma.
With his 1968 paper on the early atmosphere and oceans of the earth,
Preston Cloud
Preston Ercelle Cloud, Jr. (September 26, 1912 – January 16, 1991) was an American earth scientist, biogeologist, cosmologist, and paleontologist. He served in the United States Navy (in which he was a bantamweight boxing champion), and led s ...
established the general framework that has been widely, if not universally,
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
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
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
Superoxide dismutase (SOD, ) is an enzyme that alternately catalyzes the dismutation (or partitioning) of the superoxide () radical into ordinary molecular oxygen (O2) and hydrogen peroxide (). Superoxide is produced as a by-product of oxygen me ...
) 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
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approx ...
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.
The few formations deposited after 1,800
Ma may point to intermittent low levels of free atmospheric oxygen,
while the small peak at may be associated with the hypothetical Snowball Earth.
Formation processes
The microbands within chert layers are most likely
varve
A varve is an annual layer of sediment or sedimentary rock.
The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (var ...
s 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
The sensitive high-resolution ion microprobe (also sensitive high mass-resolution ion microprobe or SHRIMP) is a large-diameter, double-focusing secondary ion mass spectrometer (SIMS) sector instrument produced by Australian Scientific Instrume ...
(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
rhythmite
A rhythmite consists of layers of sediment or sedimentary rock which are laid down with an obvious periodicity and regularity. They may be created by annual processes such as seasonally varying deposits reflecting variations in the runoff cycle, b ...
s produced by tidal cycles.
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 ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
s that change the availability of reduced iron on time scales of decades.
In the case of granular iron formations, the mesobands are attributed to
winnowing
Winnowing is a process by which chaff is separated from grain. It can also be used to remove pests from stored grain. Winnowing usually follows threshing in grain preparation. In its simplest form, it involves throwing the mixture into the ...
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
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
).
# 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
Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified, have an oxic, highly ...
(rich in
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
), since this would cause the ferrous iron to precipitate out as
pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic Luster (mineralogy), lust ...
.
# 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
![BlackSmoker](https://upload.wikimedia.org/wikipedia/commons/b/b0/BlackSmoker.jpg)
There must be an ample source of reduced iron that can circulate freely into the deposition basin.
Plausible sources of iron include
hydrothermal vents
A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...
along mid-ocean ridges, windblown dust, rivers, glacial ice, and
seepage
Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and wat ...
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.
Algoma-type BIFs formed primarily in the Archean. These older BIFs tend to show a positive
europium anomaly
The europium anomaly is the phenomenon whereby the europium (Eu) concentration in a mineral is either enriched or depleted relative to some standard, commonly a chondrite or mid-ocean ridge basalt (MORB). In geochemistry a europium anomaly is sa ...
consistent with a
hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
source of iron.
By contrast, Lake Superior-type banded iron formations primarily formed during the
Paleoproterozoic
The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
era, and lack the europium anomalies of the older Algoma-type BIFs, suggesting a much greater input of iron weathered from continents.
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
Dissimilatory sulfate reduction is a form of anaerobic respiration that uses sulfate as the terminal electron acceptor to produce hydrogen sulfide. This metabolism is found in some types of bacteria and archaea which are often termed sulfate-reduc ...
(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 Heinrich Dieter 'Dick' Holland (May 27, 1927 – May 21, 2012) was an emeritus professor in the Earth and Planetary Sciences department of Harvard University. He made major contributions to the understanding of the Earth's geochemistry, especia ...
argues that the absence of
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
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
Minnesota () is a state in the upper midwestern region of the United States. It is the 12th largest U.S. state in area and the 22nd most populous, with over 5.75 million residents. Minnesota is home to western prairies, now given over to ...
are overlain by a thick layer of ejecta from the
Sudbury Basin
The Sudbury Basin (), also known as Sudbury Structure or the Sudbury Nickel Irruptive, is a major geological structure in Ontario, Canada. It is the third-largest known impact crater or astrobleme on Earth, as well as one of the oldest. The cra ...
impact. An
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
(estimated at 10 km across)
impacted into waters about 1,000 m deep 1.849 billion years ago, coincident with the pause in BIF deposition. Computer models suggest that the impact would have generated a
tsunami
A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explo ...
at least 1,000 meters high at the point of impact, and 100 meters high about 3,000 kilometers 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
Bacterial anoxygenic photosynthesis differs from the better known oxygenic photosynthesis in plants by the reductant used (e.g. hydrogen sulfide instead of water) and the byproduct generated (e.g. elemental sulfur instead of molecular oxygen).
Ba ...
continues to be debated, and nonbiogenic processes have also been proposed.
=Oxygenic photosynthesis
=
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
=
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. The concentrations of phosphorus and trace metals in BIFs are consistent with precipitation through the activities of iron-oxidizing bacteria.
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
2 levels in the photic zone) and anoxygenic photosynthetic oxidation of Fe(II):
:
This requires that dissimilatory iron reduction, the biological process in which microorganisms substitute Fe(III) for oxygen in respiration, was not yet widespread.
By contrast, Lake Superior-type banded iron formations show iron isotope ratios that suggest that dissimilatory iron reduction expanded greatly during this period.
An alternate route is oxidation by anaerobic
denitrifying bacteria Denitrifying bacteria are a diverse group of bacteria that encompass many different phyla. This group of bacteria, together with denitrifying fungi and archaea, is capable of performing denitrification as part of the nitrogen cycle. Denitrification ...
. This requires that
nitrogen fixation
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
by microorganisms is also active.
:
=Abiogenic mechanisms
=
The lack of organic carbon in banded iron formation argues against microbial control of BIF deposition.
On the other hand, there is
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
evidence for abundant photosynthesizing cyanobacteria at the start of BIF deposition
and of
hydrocarbon markers in shales within banded iron formation of the Pilbara craton.
The carbon that is present in banded iron formations is enriched in the light isotope,
12C, an
indicator
Indicator may refer to:
Biology
* Environmental indicator of environmental health (pressures, conditions and responses)
* Ecological indicator of ecosystem health (ecological processes)
* Health indicator, which is used to describe the health o ...
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:
:
Trendall and J.G. Blockley proposed, but later rejected, the hypothesis that banded iron formation might be a peculiar kind of Precambrian
evaporite
An evaporite () is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocea ...
.
Other proposed abiogenic processes include
radiolysis
Radiolysis is the dissociation of molecules by ionizing radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux. The radiation in this context is associated with ionizing radiation; radiolysis is ...
by the
radioactive isotope
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...
of
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
,
40K,
or annual turnover of basin water combined with upwelling of iron-rich water in a stratified ocean.
Another abiogenic mechanism is
photooxidation
In polymer chemistry photo-oxidation (sometimes: oxidative photodegradation) is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidatio ...
of iron by sunlight. Laboratory experiments suggest that this could produce a sufficiently high deposition rate under likely conditions of pH and sunlight.
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.
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
Iron(III) oxide-hydroxide or ferric oxyhydroxideA. L. Mackay (1960): "β-Ferric Oxyhydroxide". ''Mineralogical Magazine'' (''Journal of the Mineralogical Society''), volume 32, issue 250, pages 545-557. is the chemical compound of iron, oxygen, ...
gel. Similarly, the silica component of the banded iron formations likely precipitated as a hydrous silica gel.
The conversion of iron hydroxide and silica gels to banded iron formation is an example of
diagenesis
Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...
, 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.
Decarbonization may account for the lack of carbon and preponderance of magnetite in older banded iron formations.
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.
However, it is possible that BIF was altered from carbonate rock
or from hydrothermal mud
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
siderite
Siderite is a mineral composed of iron(II) carbonate (FeCO3). It takes its name from the Greek word σίδηρος ''sideros,'' "iron". It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Zinc, magnesium and ...
via the reaction
::
The iron may have originally precipitated as
greenalite
Greenalite is a mineral in the kaolinite- serpentine group with the chemical composition (Fe2+,Fe3+)2-3Si2O5(OH)4.Sleep N.H., Bird D.K. (2007): Niches of the pre-photosynthetic biosphere and geologic preservation of Earth’s earliest ecology. Geo ...
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.
The 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
mass-independent fractionation Mass-independent isotope fractionation or Non-mass-dependent fractionation (NMD), refers to any chemical or physical process that acts to separate isotopes, where the amount of separation does not scale in proportion with the difference in the mass ...
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.
Snowball Earth hypothesis
Until 1992 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).
Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron.
Banded iron formations of this period are predominantly associated with the
Sturtian glaciation
The Sturtian glaciation was a Snowball Earth glaciation, or perhaps multiple glaciations, during the Cryogenian Period when the Earth experienced repeated large-scale glaciations. The duration of the Sturtian glaciation has been variously defined, ...
.
An alternative mechanism for banded iron formations in the Snowball Earth era suggests the iron was deposited from metal-rich
brine
Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for br ...
s in the vicinity of
hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
ly active
rift zone
A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a set of linear cracks (or rifts) develops in a volcanic edifice, typically forming into two or three well-defined regions along the flanks of the vent. Believed t ...
s due to glacially-driven thermal overturn.
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.
Economic geology
![Hull–Rust–Mahoning Open Pit Iron Mine](https://upload.wikimedia.org/wikipedia/commons/1/18/Hull%E2%80%93Rust%E2%80%93Mahoning_Open_Pit_Iron_Mine.jpg)
Banded iron formations provide most of the
iron ore
Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...
presently mined.
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.
Different mining districts coined their own names for BIFs. The term "banded iron formation" was coined in the iron districts of
Lake Superior
Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh wa ...
, where the ore deposits of the Mesabi,
Marquette, Cuyuna,
Gogebic, and
Menominee
The Menominee (; mez, omǣqnomenēwak meaning ''"Menominee People"'', also spelled Menomini, derived from the Ojibwe language word for "Wild Rice People"; known as ''Mamaceqtaw'', "the people", in the Menominee language) are a federally recog ...
iron ranges
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in fro ...
were also variously known as "jasper", "jaspilite", "iron-bearing formation", or
taconite
Taconite () is a variety of iron formation, an iron-bearing (over 15% iron) sedimentary rock, in which the iron minerals are interlayered with quartz, chert, or carbonate. The name "taconyte" was coined by Horace Vaughn Winchell (1865–1923) ...
. Banded iron formations were described as "itabarite" in Brazil, as "ironstone" in South Africa, and as "BHQ" (banded hematite quartzite) in India.
Banded iron formation was first discovered in northern
Michigan
Michigan () is a state in the Great Lakes region of the upper Midwestern United States. With a population of nearly 10.12 million and an area of nearly , Michigan is the 10th-largest state by population, the 11th-largest by area, and the ...
in 1844, and mining of these deposits prompted the earliest studies of BIFs, such as those of
Charles R. Van Hise
Charles Richard Van Hise (May 29, 1857 – November 19, 1918) was an American geologist, academic and Progressivism, progressive. He served as president of the University of Wisconsin–Madison, University of Wisconsin (UW) in Madison, Wisconsin, ...
and
Charles Kenneth Leith
Charles Kenneth Leith (January 20, 1875 – September 13, 1956) was an American geologist. He was head of the University of Wisconsin geology department for 30 years. In 1942, he was awarded the Penrose Medal by the Geological Society of America, ...
.
Iron mining operations on the Mesabi and Cuyuna Ranges evolved into enormous
open pit mines
Open-pit mining, also known as open-cast or open-cut mining and in larger contexts mega-mining, is a surface mining technique of extracting rock (geology), rock or minerals from the earth from an open-air pit, sometimes known as a Borrow pit, b ...
, where
steam shovel
A steam shovel is a large steam-powered excavating machine designed for lifting and moving material such as rock and soil. It is the earliest type of power shovel or excavator. Steam shovels played a major role in public works in the 19th and e ...
s and other industrial machines could remove massive amounts of ore. Initially the mines exploited large beds of hematite and
goethite
Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...
weathered out of the banded iron formations, and some 2.5 billion tons of this "natural ore" had been extracted by 1980.
By 1956 large-scale commercial production from the BIF itself began at the Peter Mitchell Mine near
Babbitt, Minnesota
Babbitt is a city in Saint Louis County, Minnesota, United States. As of the 2020 census, the city had a population of 1,462.
Saint Louis County Highway 21 (CR 21) serves as a main route in the community.
History
The city grew out of the form ...
.
Production in Minnesota was 40 million tons of ore concentrate per year in 2016, which is about 75% of total U.S. production.
Magnetite-rich banded iron formation, known locally as taconite, is ground to a powder, and the magnetite is
separated with powerful magnets and
pelletized for shipment and smelting.
![Mount Tom Price mine, September 2006](https://upload.wikimedia.org/wikipedia/commons/d/d3/Mount_Tom_Price_mine%2C_September_2006.jpg)
Iron ore became a global commodity after the
Second World War
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, and with the end of the embargo against exporting iron ore from Australia in 1960, the Hamersley Range became a major mining district.
The banded iron formations here are the thickest and most extensive in the world,
originally covering an area of and containing about 300 trillion metric tons of iron.
The range contains 80 percent of all identified iron ore reserves in Australia. Over 100 million metric tons of iron ore is removed from the range every year.
The Itabarite banded iron formations of Brazil cover at least and are up to thick.
These form the Quadrilatero Ferrifero or
Iron Quadrangle
The Iron Quadrangle ( pt, Quadrilátero Ferrífero) is a mineral-rich region covering about in the central-southern part of the Brazilian state Minas Gerais. The area is known for its extensive deposits of gold, diamonds, and iron ore, being the s ...
, which resembles the Iron Range mines of United States in that the favored ore is hematite weathered out of the BIFs.
Production from the Iron Quadrangle helps make Brazil the second largest producer of iron ore after Australia, with monthly exports averaging 139,299 metric tons from December 2007 to May 2018.
Mining of ore from banded iron formations at
Anshan
Anshan () is an inland prefecture-level city in central-southeast Liaoning province, People's Republic of China, about south of the provincial capital Shenyang. As of the 2020 census, it was Liaoning's third most populous city with a population ...
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
Manchuria
Manchuria is an exonym (derived from the endo demonym " Manchu") for a historical and geographic region in Northeast Asia encompassing the entirety of present-day Northeast China (Inner Manchuria) and parts of the Russian Far East (Outer Manc ...
reached 1,000,000 metric tons in 1931–32. By 1942, Anshan's Shōwa Steel Works total production capacity reached 3,600,000 metric tons per annum, making it one of the major iron and steel centers in the world.
Production was severely disrupted during the
Soviet occupation of Manchuria
The Soviet occupation of Manchuria took place after the Red Army invaded the Japanese puppet state of Manchukuo in August 1945; the occupation would continue until Soviet forces withdrew in May 1946.
History
On 11 February 1945, the Big Three ( ...
in 1945 and the subsequent
Chinese Civil War
The Chinese Civil War was fought between the Kuomintang-led government of the Republic of China and forces of the Chinese Communist Party, continuing intermittently since 1 August 1927 until 7 December 1949 with a Communist victory on m ...
. However, from 1948 to 2001, the steel works produced 290 million tons of steel, 284 million tons of
pig iron
Pig iron, also known as crude iron, is an intermediate product of the iron industry in the production of steel which is obtained by smelting iron ore in a blast furnace. Pig iron has a high carbon content, typically 3.8–4.7%, along with silic ...
and 192 million tons of
rolled steel
In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is simil ...
. Annual production capacity is 10 million tons of pig iron, 10 million tons of steel and 9.5 million tons of rolled steel. A quarter of China's total iron ore reserves, about 10 billion tons, are located in Anshan.
See also
*
*
References
Further reading
*
*
External links
*
*
*
{{DEFAULTSORT:Banded Iron Formation
Precambrian
Iron ores
Chert
Economic geology
Sedimentary rocks