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65-606: It consists of three parts, Transvaal sedimentation began with predominantly clastic sedimentary rocks (Black Reef-Vryburg Formations) followed by carbonate rocks and banded iron formations (Chuniespoort-Ghaap-Taupone Groups). After an erosional hiatus, the clastic sedimentary rocks and volcanics of the Pretoria-Postmasburg-Segwagwa Groups were deposited within the three basins, largely under closed-basin conditions. A final stage of predominantly volcanic succession (Rooiberg Group-Loskop Formation)

130-538: A continental shelf . 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. Banded iron formations are almost exclusively Precambrian in age, with most deposits dating to

195-445: A photic zone inhabited by cyanobacteria that had evolved the capacity to carry out oxygen-producing photosynthesis, but which had not yet evolved enzymes (such as superoxide dismutase ) for living in an oxygenated environment. Such organisms would have been protected from their own oxygen waste through its rapid removal via the reservoir of reduced ferrous iron, Fe(II), in the early ocean. The oxygen released by photosynthesis oxidized

260-422: A specific gravity of 3.96, a white streak and a vitreous lustre or pearly luster . Siderite is antiferromagnetic below its Néel temperature of 37 K (−236 °C) which can assist in its identification. It crystallizes in the trigonal crystal system , and are rhombohedral in shape, typically with curved and striated faces. It also occurs in masses. Color ranges from yellow to dark brown or black,

325-434: A tsunami at least 1,000 m (3,300 ft) high at the point of impact, and 100 m (330 ft) high about 3,000 km (1,900 mi) 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. Although Cloud argued that microbial activity

390-514: 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

455-509: A factor of 50 under conditions of low oxygen. 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

520-657: 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

585-447: A higher energy depositional environment , in shallower water disturbed by wave motions. However, they otherwise resemble other banded iron formations. The great majority of banded iron formations are Archean or Paleoproterozoic in age. However, a small number of BIFs are Neoproterozoic in age, and are frequently, if not universally, associated with glacial deposits, often containing glacial dropstones . They also tend to show

650-734: 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 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 ironstones . Ironstones are relatively rare and are thought to have been deposited in marine anoxic events , in which

715-541: A hydrous silica gel. The conversion of iron hydroxide and silica gels to banded iron formation is an example of 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

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780-430: 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 750  million years ago may be associated with the hypothetical Snowball Earth. The microbands within chert layers are most likely varves produced by annual variations in oxygen production. Diurnal microbanding would require

845-625: A mode of formation does not require a global anoxic ocean, but is consistent with either a Snowball Earth or Slushball Earth model. Banded iron formations provide most of the iron ore 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"

910-472: A peculiar kind of Precambrian evaporite . Other proposed abiogenic processes include radiolysis by the radioactive isotope of potassium , K, or annual turnover of basin water combined with upwelling of iron-rich water in a stratified ocean. 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. However, if

975-408: 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 via

1040-827: A thickness of 60 meters (200 feet). Other examples of early Archean BIFs are found in the Abitibi greenstone belts , the greenstone belts of the Yilgarn and Pilbara cratons , the Baltic shield , and the cratons of the Amazon , north China , and south and west Africa. The most extensive banded iron formations belong to what A.F. Trendall calls the Great Gondwana BIFs. These are late Archean in age and are not associated with greenstone belts. They are relatively undeformed and form extensive topographic plateaus, such as

1105-497: A thin layer on the ocean floor. Each band is similar to a varve , resulting from cyclic variations in oxygen production. Banded iron formations were first discovered in northern Michigan in 1844. Banded iron formations account for more than 60% of global iron reserves and provide most of the iron ore presently mined. Most formations can be found in Australia , Brazil , Canada , India , Russia , South Africa , Ukraine , and

1170-600: A three-chambered concentric roasting furnace, before passing the ore to a separate reducing furnace for smelting. Details of this mill were the invention of Charles Sanderson, a steel maker of Sheffield, who held the patent for it. These differences between spathic ore and haematite have led to the failure of a number of mining concerns, notably the Brendon Hills Iron Ore Company . Spathic iron ores are rich in manganese and have negligible phosphorus. This led to their one major benefit, connected with

1235-485: A twofold division of BIFs into an Algoma type and a Lake Superior type, based on the character of the depositional basin. Algoma BIFs are found in relatively small basins in association with greywackes and other volcanic rocks and are assumed to be associated with volcanic centers. Lake Superior BIFs are found in larger basins in association with black shales, quartzites , and dolomites , with relatively minor tuffs or other volcanic rocks, and are assumed to have formed on

1300-466: A typical European high-phosphorus ore in Bessemer's converter gave a poor quality steel. To produce high quality steel from a high-phosphorus ore, Mushet realised that he could operate the Bessemer converter for longer, burning off all the steel's impurities including the unwanted phosphorus but also the carbon (which is an essential ingredient in steel), and then re-adding carbon, along with manganese, in

1365-429: A very high rate of deposition of 2 meters per year or 5 km/Ma. Estimates of deposition rate based on various models of deposition and sensitive high-resolution ion microprobe (SHRIMP) estimates of the age of associated tuff beds suggest a deposition rate in typical BIFs of 19 to 270 m/Ma, which are consistent either with annual varves or rhythmites produced by tidal cycles. Preston Cloud proposed that mesobanding

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1430-533: Is a mineral composed of iron(II) carbonate (FeCO 3 ). Its name comes from the Ancient Greek word σίδηρος ( sídēros ), meaning "iron". A valuable iron ore , it consists of 48% iron and lacks sulfur and phosphorus . Zinc , magnesium , and manganese commonly substitute for the iron, resulting in the siderite- smithsonite , siderite- magnesite , and siderite- rhodochrosite solid solution series. Siderite has Mohs hardness of 3.75 to 4.25,

1495-742: Is limited to the Transvaal Basin. The Campbellrand-Malmani carbonate platform is part of the Chuniespoort Group and originally covered all of the Kaapvaal Craton. It has a thickness of over 1 km in both the Malmani and Campbellrand Subgroups. The Malmani Subgroup is situated northwest of Johannesburg. It consists of dolomite and chert with only minor clastic sediments. Several workers such as an environmentalist, Isaac Chuene, Raymond Ngobeni and Khutso Masemola noted

1560-493: Is more difficult to smelt than a haematite or other oxide ore. Driving off the carbonate as carbon dioxide requires more energy and so the ore 'kills' the blast furnace if added directly. Instead the ore must be given a preliminary roasting step. Developments of specific techniques to deal with these ores began in the early 19th century, largely with the work of Sir Thomas Lethbridge in Somerset . His 'Iron Mill' of 1838 used

1625-411: Is more precisely defined as chemically precipitated sedimentary rock containing greater than 15% iron . However, most BIFs have a higher content of iron, typically around 30% by mass, so that roughly half the rock is iron oxides and the other half is silica. The iron in BIFs is divided roughly equally between the more oxidized ferric form, Fe(III), and the more reduced ferrous form, Fe(II), so that

1690-645: Is often related to the depositional environment of the enclosing sediments. In addition, a number of recent studies have used the oxygen isotopic composition of sphaerosiderite (a type associated with soils ) as a proxy for the isotopic composition of meteoric water shortly after deposition. Although carbonate iron ores, such as siderite, have been economically important for steel production, they are far from ideal as an ore. Their hydrothermal mineralisation tends to form them as small ore lenses , often following steeply dipping bedding planes . This makes them not amenable to opencast working , and increases

1755-425: Is oxidation by anaerobic denitrifying bacteria . This requires that nitrogen fixation by microorganisms is also active. The lack of organic carbon in banded iron formation argues against microbial control of BIF deposition. On the other hand, there is fossil evidence for abundant photosynthesizing cyanobacteria at the start of BIF deposition and of hydrocarbon markers in shales within banded iron formation of

1820-511: The Bessemer steel-making process . Although the first demonstrations by Bessemer in 1856 were successful, others' initial attempts to replicate his method infamously failed to produce good steel. Work by the metallurgist Robert Forester Mushet showed that the reason for the discrepancy was the nature of the Swedish ores that Bessemer had innocently used; they were very low in phosphorus. Using

1885-834: The Great Lakes region and the Frere Formation of western Australia are somewhat different in character and are sometimes described as granular iron formations or GIFs . Their iron sediments are granular to oolitic in character, forming discrete grains about a millimeter in diameter, and they lack microbanding in their chert mesobands. They also show more irregular mesobanding, with indications of ripples and other sedimentary structures , and their mesobands cannot be traced out any great distance. Though they form well-defined, discrete units, these are commonly interbedded with coarse to medium-grained epiclastic sediments (sediments formed by weathering of rock). These features suggest

1950-797: The Hamersley Range . 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 900 meters (3,000 feet). 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

2015-984: The Mulaingiri Formation of India . Paleoproterozoic banded iron formations are found in the Iron Range and other parts of the Canadian Shield . The Iron Range is a group of four major deposits: the Mesabi Range , the Vermilion Range , the Gunflint Range , and the Cuyuna Range . All are part of the Animikie Group and were deposited between 2500 and 1800 Ma. These BIFs are predominantly granular iron formations. Neoproterozoic banded iron formations include

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2080-575: The Sturtian glaciation . An alternative mechanism for banded iron formations in the Snowball Earth era suggests the iron was deposited from metal-rich brines in the vicinity of hydrothermally active rift zones 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

2145-553: The United States . A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides , either magnetite (Fe 3 O 4 ) or hematite (Fe 2 O 3 ), alternating with bands of iron-poor chert , 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

2210-452: The oxygenation of the Earth's oceans . Some of the Earth's oldest rock formations, which formed about 3,700  million years ago ( Ma ), are associated with banded iron formations. Banded iron formations are thought to have formed in sea water as the result of oxygen production by photosynthetic cyanobacteria . The oxygen combined with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming

2275-592: The Archean. These older BIFs tend to show a positive europium anomaly consistent with a hydrothermal source of iron. By contrast, Lake Superior-type banded iron formations primarily formed during the Paleoproterozoic era, and lack the europium anomalies of the older Algoma-type BIFs, suggesting a much greater input of iron weathered from continents. The absence of hydrogen sulfide in anoxic ocean water can be explained either by reduced sulfur flux into

2340-458: The Fe(II) to ferric iron, Fe(III), which precipitated out of the sea water as insoluble iron oxides that settled to the ocean floor. Cloud suggested that banding resulted from fluctuations in the population of cyanobacteria due to free radical damage by oxygen. This also explained the relatively limited extent of early Archean deposits. The great peak in BIF deposition at the end of the Archean

2405-528: The Great Oxygenation Event. Prior to 2.45 billion years ago, the high degree of 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

2470-486: The Pilbara craton. The carbon that is present in banded iron formations is enriched in the light isotope, 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: Trendall and J.G. Blockley proposed, but later rejected, the hypothesis that banded iron formation might be

2535-585: The Precambrian world, they have been intensively studied by geologists. Banded iron formations are found worldwide, in every continental shield of every continent. The oldest BIFs are associated with greenstone belts and include the BIFs of the Isua Greenstone Belt , the oldest known, which have an estimated age of 3700 to 3800 Ma. The Temagami banded iron deposits formed over a 50-million-year period, from 2736 to 2687 Ma, and reached

2600-631: The Urucum in Brazil, Rapitan in the Yukon , and the Damara Belt in southern Africa. They are relatively limited in size, with horizontal extents not more than a few tens of kilometers and thicknesses not more than about 10 meters (33 feet). These are widely thought to have been deposited under unusual anoxic oceanic conditions associated with the " Snowball Earth ." Banded iron formation provided some of

2665-424: The availability of reduced iron on time scales of decades. In the case of granular iron formations, the mesobands are attributed to winnowing of sediments in shallow water, in which wave action tended to segregate particles of different size and composition. For banded iron formations to be deposited, several preconditions must be met. There must be an ample source of reduced iron that can circulate freely into

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2730-424: The cost of working them by mining with horizontal stopes . As the individual ore bodies are small, it may also be necessary to duplicate or relocate the pit head machinery, winding engine and pumping engine, between these bodies as each is worked out. This makes mining the ore an expensive proposition compared to typical ironstone or haematite opencasts. The recovered ore also has drawbacks. The carbonate ore

2795-542: 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

2860-417: The deep ocean became sufficiently oxygenated at that time to end transport of reduced iron. Heinrich Holland argues that the absence of manganese deposits during the pause between Paleoproterozoic and Neoproterozoic BIFs is evidence that the deep ocean had become at least slightly oxygenated. The "Canfield ocean" model proposes that, to the contrary, the deep ocean became euxinic and transport of reduced iron

2925-472: The deep ocean or a lack of dissimilatory sulfate reduction (DSR), the process by which microorganisms use sulfate in place of oxygen for respiration. The product of DSR is hydrogen sulfide, which readily precipitates iron out of solution as pyrite. The requirement of an anoxic, but not euxinic, deep ocean for deposition of banded iron formation suggests two models to explain the end of BIF deposition 1.8 billion years ago. The "Holland ocean" model proposes that

2990-412: The deposition basin. Plausible sources of iron include hydrothermal vents along mid-ocean ridges, windblown dust, rivers, glacial ice, and seepage from continental margins. The importance of various sources of reduced iron has likely changed dramatically across geologic time. This is reflected in the division of BIFs into Algoma and Lake Superior-type deposits. Algoma-type BIFs formed primarily in

3055-551: The depositional basin became depleted in free oxygen . They are composed of iron silicates and oxides without appreciable chert but with significant phosphorus content, which is lacking in BIFs. No classification scheme for banded iron formations has gained complete acceptance. In 1954, Harold Lloyd James advocated a classification based on four lithological facies (oxide, carbonate, silicate, and sulfide) assumed to represent different depths of deposition, but this speculative model did not hold up. In 1980, Gordon A. Gross advocated

3120-501: The first evidence for the timing of the Great Oxidation Event , 2,400 Ma. With his 1968 paper on the early atmosphere and oceans of the Earth, Preston Cloud 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

3185-480: The form of a previously obscure ferromanganese ore with no phosphorus, spiegeleisen . This created a sudden demand for spiegeleisen. Although it was not available in sufficient quantity as a mineral, steelworks such as that at Ebbw Vale in South Wales soon learned to make it from the spathic siderite ores. For a few decades, spathic ores were therefore in demand and this encouraged their mining. In time though,

3250-442: 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. 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

3315-566: The late Archean (2800–2500 Ma) with a secondary peak of deposition in the Orosirian period of the Paleoproterozoic (1850 Ma). Minor amounts were deposited in the early Archean and in the Neoproterozoic (750 Ma). The youngest known banded iron formation is an Early Cambrian formation in western China. Because the processes by which BIFs are formed appear to be restricted to early geologic time, and may reflect unique conditions of

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3380-441: 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

3445-446: The latter being due to the presence of manganese. Siderite is commonly found in hydrothermal veins , and is associated with barite , fluorite , galena , and others. It is also a common diagenetic mineral in shales and sandstones , where it sometimes forms concretions , which can encase three-dimensionally preserved fossils . In sedimentary rocks , siderite commonly forms at shallow burial depths and its elemental composition

3510-592: 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

3575-399: The original 'acidic' liner of the Bessemer converter, made from siliceous sandstone or ganister , was replaced by a 'basic' liner in the newer Gilchrist Thomas process . This removed the phosphorus impurities as slag produced by chemical reaction with the liner, and no longer required spiegeleisen. From the 1880s demand for the ores fell once again and many of their mines, including those of

3640-819: The presence of fossils in the Chunniespoort mountains of the Chunniespoort Group. This article about a specific stratigraphic formation in Africa is a stub . You can help Misplaced Pages by expanding it . Banded iron formation Banded iron formations ( BIFs ; also called banded ironstone formations ) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert . They can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Almost all of these formations are of Precambrian age and are thought to record

3705-587: The ratio Fe(III)/Fe(II+III) typically varies from 0.3 to 0.6. This indicates a predominance of magnetite, in which the ratio is 0.67, over hematite, for which the ratio is 1. In addition to the iron oxides (hematite and magnetite), the iron sediment may contain the iron-rich carbonates siderite and ankerite , or the iron-rich silicates minnesotaite and greenalite . Most BIFs are chemically simple, containing little but iron oxides, silica, and minor carbonate, though some contain significant calcium and magnesium, up to 9% and 6.7% as oxides respectively. When used in

3770-505: The reaction The iron may have originally precipitated as 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. 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

3835-528: 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. A well-preserved banded iron formation typically consists of macrobands several meters thick that are separated by thin shale beds. The macrobands in turn are composed of characteristic alternating layers of chert and iron oxides, called mesobands , that are several millimeters to

3900-529: Was a key process in the deposition of banded iron formation, the role of oxygenic versus anoxygenic photosynthesis continues to be debated, and nonbiogenic processes have also been proposed. 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

3965-406: Was a result of self-poisoning by early cyanobacteria as the supply of reduced iron was periodically depleted. Mesobanding has also been interpreted as a secondary structure, not present in the sediments as originally laid down, but produced during compaction of the sediments. Another theory is that mesobands are primary structures resulting from pulses of activity along mid-ocean ridges that change

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4030-565: 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. 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

4095-479: Was blocked by precipitation as pyrite. Banded iron formations in northern Minnesota are overlain by a thick layer of ejecta from the Sudbury Basin impact. An asteroid (estimated at 10 km (6.2 mi) across) impacted into waters about 1,000 m (3,300 ft) deep 1.849 billion years ago, coincident with the pause in BIF deposition. Computer models suggest that the impact would have generated

4160-589: Was coined in the iron districts of Lake Superior , where the ore deposits of the Mesabi, Marquette , Cuyuna, Gogebic , and Menominee iron ranges were also variously known as "jasper", "jaspilite", "iron-bearing formation", or taconite . Banded iron formations were described as "itabarite" in Brazil, as "ironstone" in South Africa, and as "BHQ" (banded hematite quartzite) in India. Siderite Siderite

4225-432: 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

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