banded iron formations Banded iron formations, or BIFs for short, are an unique type of sedimentary rock in the Earth's geological record, which are as important economically as they are scientifically. They comprise the majority of the world's iron reserves, occurring in vast deposits on every continent, with bulk iron contents locally exceeding 50 per cent by weight. For example, the banded iron formations of the Lake Superior region of North America have been the source of most of the iron ore produced in the United States over the past 120 years and this readily available supply contributed significantly to the rapid industrialization of North America that occurred during this period. The iron formations of north-western Australia and the Transvaal in South Africa occur in layers hundreds of metres thick and are exposed over thousands of square kilometres, providing an important source of income for these countries, as well as an assured supply of iron ore for the foreseeable future.

The oldest known rocks on Earth, from the Isua area of West Greenland, are sedimentary deposits that include a banded iron formation and are about 3.8 Ga (billion years) old. In fact, the vast majority of banded iron formations are very old, having formed prior to 1.8 Ga before the present day. As no modern equivalent is known to exist, the process by which banded iron formations form has been debated by geologists since the late 1800s. However, it is not only their mechanism of formation that makes banded iron formations scientifically intriguing. These very old rocks, with their distinct chemical composition, provide important clues about the early development of the Earth's atmosphere and oceans. As such, banded iron formations record important information that contributes towards unravelling the puzzle of when and how early life developed on Earth. It has even been proposed that biological activity was directly responsible for the formation of banded iron formations.

A banded iron formation is defined as an anomalously iron-rich, chemically precipitated sediment which is usually thinly bedded or laminated and often contains layers of interbedded chert, or microcrystalline silica. These deposits are virtually free of detrital, or terrigenous, sediment. The world's occurrences of iron formations can be subdivided into two groups, based upon their geometric form and their association with other types of sediments: the Algoma-type are lenticular-shaped deposits that are intimately associated with volcanic rocks; and Superior-type iron formations, which are the most common in the rock record, have a large aerial extent on the scale of up to tens of thousands of square kilometres and are associated with sequences of marine sediments such as carbonates, black shales, and quartzites. Banded iron formations can also be characterized by their mineralogy, which reflects their bulk chemistry and the environment in which they were deposited. There are four different mineralogical types:(1) oxide iron formation, containing haematite, magnetite, and chert;(2) carbonate iron formation containing siderite, ferro-dolomite, and calcite;(3) silicate iron formation, containing the hydrous iron-silicates greenalite, minnesotaite, stilpnomelane, chlorite, and amphibole; and(4) sulphide iron formation, containing pyrite.

Over time, the banded iron formations have experienced variable degrees of metamorphism, which has modified the original mineralogy to differing degrees. This makes it difficult to determine unequivocally the exact mineralogy of the original chemical sediment that was deposited.

The chemical composition of banded iron formations is unique and provides useful constraints on the prevailing conditions during their deposition. In addition to iron, they are always rich in silica, indicating that the waters from which the iron formations precipitated were saturated in silica as well as iron. Iron formations have extremely low alumina and titanium contents; elements which are generally associated with extensive land erosion. This characteristic, along with the lack of interlayered detrital sediment, suggests that the banded iron formations were deposited well away from any source of land-derived material, such as a river delta. Banded iron formations themselves also contain very little organic carbon, arguing against a direct link between biological activity and the precipitation of the iron and silica. Their remarkably low manganese and trace metal contents stand in contrast to the manganese-rich nodules currently forming on the deep sea floor. On the other hand, the trace element and rare earth element contents have a ‘signature’ compatible with formation from waters that were mixtures of normal sea water and hydrothermal fluids, like those forming the ore-rich ‘black smokers’ located at the mid-ocean ridges. These latter fluids are rich in dissolved metals and other elements through the alteration of the ocean floor by hot percolating fluids.

A further feature of banded iron formations is the chemical nature of iron itself, which can occur in different forms, according to the oxidizing nature of the environment. Under oxidizing conditions, ferric iron is favoured and the most stable iron oxide is haematite (Fe₂O₃). Under more reduced conditions, some or all of the iron can be present as ferrous iron. For example, magnetite (Fe₃O₄) contains a mixture of both ferrous and ferric iron, and many silicate and carbonate minerals contain predominantly ferrous iron (Fe²⁺). It is important to note that while ferrous iron is relatively soluble in water, ferric iron (Fe³⁺) has an extremely low solubility; hence the very low iron contents of today's oxidized oceans. In most banded iron formations, the ratio of ferric to ferrous iron is low, reflecting the association of magnetite and ferrous iron-bearing silicate and carbonate minerals along with haematite. As a result, banded iron formations are not as oxidized as one might initially think, simply based upon the occurrence of haematite. Notable exceptions to this are several relatively younger banded iron formations. such as the Rapitan banded iron formation of north-western Canada, which are highly oxidized and have a simple mineralogy of haematite and quartz.

Banded iron formations are so named in part because of their distinctive banded or layered structure, which occurs at various scales from microscopic to macroscopic. It is notable that this layering can have extreme lateral continuity. For example, in parts of the Hamersley iron formation in north-western Australia, individual millimetre-thick chert laminations have been traced for over 300 km, and sequences of layers can be correlated over an area in excess of 50 000 km². Such fine-scale layering and extensive continuity point to a quiescent environment of deposition for the iron formation. There are other occurrences, however, where layering is locally disrupted and sedimentary structures like ripple marks, scours, and channels are recognizable, together with oolitic and peloidal textures. These features are indicative of deposition in a high-energy environment where currents vigorously rework the sea-bed sediment.

The distribution of banded iron formations in the geological record is limited to an early period in the Earth's history. Radiometric dating reveals that banded iron formations were primarily deposited during the Archaean (>2.5 Ga old) through the Early Proterozoic (between 2.5 and 1.6 Ga) eras, their greatest development occurring between 2.6 and 1.8 Ga ago. After about 1.8 Ga ago, there was essentially no deposition of banded iron formations, except for a slight resurgence of deposition that occurred between 800 and 600 Ma ago. These younger deposits, including the Rapitan iron formation in north-western Canada, have a distinctly different character in comparison with the older banded iron formations, suggesting that they formed under different environmental conditions. Since 600 Ma ago, no true banded iron formations have been deposited.

Origins and formations of BIFS

Hypotheses to explain the origins of banded iron formations have been many and varied; some early geologists even proposed that they had crystallized from a magma. Although their sedimentary origin through precipitation has long been agreed, the mechanism of iron deposition and the type of environment in which they formed are still the subject of debate. Any hypothesis must adequately account for a number of features, namely: the layering, which exhibits continuity over large distances; the variation of mineralogical type; and the limitation of their formation prior to 1.8 Ga ago. In addition, there must be a plausible source and mechanism for the transport and precipitation of large quantities of iron and silica.

It is generally agreed that the iron and silica must have been in solution in order to account for the widespread deposition of banded iron formations. This implies that the iron was in the ferrous state, and it then follows that the amount of free oxygen was much lower than it is in today's oceans and atmosphere, if it was present at all. There are two conceivable sources for the iron and silica. One possibility is that they were the product of weathering of rocks exposed on land. A lack of atmospheric oxygen would facilitate the transport of iron to the oceans by rivers. Another likely source is from volcanic or hydrothermal activity. Indeed, the Algoma-type banded iron formations, which are most common in the Archaean, have volcanic rocks associated with them. The trace-element contents of banded iron formations also point to a hydrothermal source. However, although hydrothermal activity appears to have supplied much of the iron (and silica), other geochemical considerations suggest that river run-off also made a contribution.

The mechanism of precipitation of the iron has exercised the imagination of many geologists. There is general agreement that oxidation and changes in acidity (pH) are necessary to cause iron precipitation and to form the different mineralogical types of iron formations. It is also considered that precipitation of silica may have been more or less continuous, since without the presence of micro-organisms that take silica out of the water to make shells and skeletons the ocean would have been always close to silica saturation. One proposal calls upon evaporation to cause iron precipitation. Such a model entails deposition in a basin with restricted circulation or even a setting of playa-lake type in which the lake periodically dries up. In this case, the microscopic laminations and macroscopic layering would be related to daily and seasonal fluctuations in temperature. A difficulty with this model is that there is often no geological evidence for such a restricted basin, and rocks associated with banded iron formation tend to show marine affinities rather than the characteristics of lake deposits. A second model envisages a direct role of biological activity in the precipitation of iron. Here, primitive algae provide the necessary oxygen (through photosynthesis) to oxidize the iron locally. Support for this hypothesis comes from the presence of iron in biologically produced organic pigments and the observation of iron oxides precipitating directly on some modern bacteria. The layering in the iron formations would then be due to daily and seasonal variations in photosynthetic production of oxygen. Major drawbacks to a direct link between biological activity and the precipitation of iron come from the absence of fossils and the general lack of organic carbon in the iron formations themselves, particularly those of the oxide type. A further implication of both the models so far described is that iron formations were deposited in quite shallow water, where evaporation was effective and where photosynthesis could occur. However, carbonate sediments associated with banded iron formations contain fossils that are indicative of shallow-water conditions and biological activity; and these sediments are remarkably poor in iron. This suggests that although the amount of atmospheric oxygen may have been minimal, the surface waters were oxidizing enough to preclude the presence of significant amounts of dissolved iron.

Another hypothesis is based upon the mixing of reduced, iron-rich waters with oxidized water to cause precipitation of iron. This involves the notion that the early oceans were stratified into two layers by contrasts in density and chemical composition, effectively isolating them from each other. A relatively thin upper layer, containing some oxygen resulting from photosynthetic activity near the surface, would overlie relatively reduced waters that had some input from hydrothermal sources on the ocean floor. Iron precipitation would occur in regions where upwelling currents brought the deep, iron-rich waters up on to the continental shelves to mix with the relatively oxidizing surface waters. Upwelling currents occur on a regional scale in today's oceans, as, for example, along the western coast of South America. Such currents could be more effective during periods of rising sea level. The precipitated iron-bearing minerals would settle to the ocean bottom and accumulate in more or less cyclic layers related to daily, monthly, or seasonal variations in the strength of the upwelling currents, and possibly in the supply of oxygen from photosynthetic activity. Although this mixing hypothesis requires a biological input of oxygen, it is indirect since iron precipitation generally occurs away from the location of biological activity.

The different mineralogical types of banded iron formation formed in response to differing degrees of acidity and oxidation and the supply of organic material, reflecting distinct environments of deposition. Sulphide-type iron formations, which are predominantly black shales or cherts, have relatively high organic carbon contents, indicating that they were deposited close to the site of biological activity and, therefore, in shallow protected basins where upwelling water periodically penetrated. Carbonate-type iron formations have a lower content of organic carbon and contain sedimentary structures that indicate deposition in shallow water, but further from the site of biological activity. Oxide-type iron formations, with their generally undisturbed fine-scale layering and low organic carbon contents, indicate a quiet, deep-sea environment. Some oxide-type BIFs can, however, possess sedimentary structures indicative of a high-energy environment with current action, implying formation in shallow waters. It is likely that the oxide-type iron formations can be deposited in a variety of water depths and that the transition between the oxide and carbonate types may be controlled by chemical factors, namely acidity and the supply of organic material.

The formation of the younger banded iron formations that were deposited between 800 and 600 Ma ago requires a different explanation, since they have a different mineralogy and there is ample geological evidence for an oxidizing atmosphere at this time. The intimate association of these deposits with glacial sediments suggests a cause and effect. It has been proposed that extensive ice sheets covered much of the Earth's surface during this period in its history, essentially isolating the oceans from the atmosphere. Other evidence in the geological record also supports the notion of extensive glaciation. This could have led to stagnant reducing conditions and the progressive build-up of dissolved ferrous iron in the oceans. Subsequent melting of the ice sheets would have restored water circulation patterns and caused oxidation and precipitation of the iron from solution, producing haematite-rich deposits.

Evidence gleaned from the study of banded iron formations has made important contributions towards understanding the early evolution of the Earth. The earliest occurrence of a banded iron formation, deposited 3.8 Ga ago, indicates that oceans had already developed by this time. It seems that the early oceans were stratified, with a thin oxygenated surface layer overlying reduced, deep-ocean water. The amount of oxygen in the ocean surface layer was probably very low relative to that in today's oceans, but it indicates the establishment of oxygen-producing micro-organisms as early as 3.8 Ga ago. Some oxygen could also have been present in the atmosphere, but the amounts must have been very low and possibly transient. It is likely that locking some of the biologically produced oxygen into the precipitating iron-bearing minerals in banded iron formations by oxidation helped to limit the oxygen content of the early atmosphere. Conversely, the banded iron formations owe their existence to photosynthetic micro-organisms that supplied the oxygen necessary for the oxidation of the dissolved ferrous iron in the oceans. By comparison with current conditions, there must have been a point in time in the past when the atmosphere and oceans changed from being dominantly reducing to being dominantly oxidizing. This would represent the time when the rate of supply of biologically produced oxygen overwhelmed the rate at which the oxygen could be consumed through various oxidation reactions. The rather abrupt end to the formation of banded iron formations about 1.8 Ga ago could coincide with this transition. In fact, there is other corroborating evidence in the geological record that points to 1.8 Ga ago as being an important time in the evolution of the Earth's hydrosphere and atmosphere. Except for an anomalous period between 800 and 600 million years ago, the oceans have since remained oxygenated and, as a result, contain only small amounts of dissolved iron.

Alan B. Woodland

Banded iron formations

Banded iron formations (BIFs) are chemically precipitated sedimentary rocks . They are composed of alternating thin (millimeter to centimeter scale) red, yellow, or cream colored layers of chert or jasper and black to dark gray iron oxides (predominantly magnetite and hematite), and/or iron carbonate (siderite) layers. Banded iron formations have greater than 15% sedimentary iron content. Banded iron formations are of economic interest as they host the world's largest iron ore deposits and many gold deposits.

Algoma-type banded iron formations were deposited as chemical sediments along with other sedimentary rocks (such as greywacke and shale) and volcanics in and adjacent to volcanic arcs and spreading centers. Iron and silica were derived from hydrothermal sources associated with volcanic centres. Algoma-type iron formations are common in Archean green-stone belts, but may also occur in younger rocks.

Lake Superior-type banded iron formations were chemically precipitated on marine continental shelves and in shallow basins. They are commonly interlayered with other sedimentary or volcanic rocks such as shale and tuff . Most Lake Superior-type banded iron formations formed during the Paleoproterozoic, between 2.5 and 1.8 billion years ago. Prior to this, Earth's primitive atmosphere and oceans had little or no free oxygen to react with iron, resulting in high iron concentrations in seawater. Iron may have been derived from the weathering of iron-rich rocks, transported to the sea as water-soluble Fe⁺². Alternatively, or in addition, both iron and silica may have been derived from submarine magmatic and hydrothermal activity. Under calm, shallow marine conditions, the iron in seawater combined with oxygen released during photosynthesis by Cyanobacteria (primitive blue-green algae, which began to proliferate in near-surface waters in the Paleoproterozoic) to precipitate magnetite (Fe₃O₄), which sank to the sea floor, forming an iron-rich layer.

It has been proposed that during periods when there was too great a concentration of oxygen (in excess of that required to bond with the iron in the seawater) due to an abundance of blue-green algae, the blue-green algae would have been reduced in numbers or destroyed. A temporary decrease in the oxygen content of the seawater then eventuated. When magnetite formation was impeded due to a reduction in the amount of oxygen in seawater, a layer of silica and/or carbonate was deposited. With subsequent reestablishment of Cyanobacteria (and thus renewed production of oxygen), precipitation of iron recommenced. Repetitions of this cycle resulted in deposition of alternating iron-rich and silica- or carbonate-rich layers. Variations in the amount of iron in seawater, such as due to changes in volcanic activity, may have also led to rhythmic layering. The large lateral extent of individual thin layers implies changes in oxygen or iron content of seawater to be regional, and necessitates calm depositional conditions. Iron and silica-rich layers, originally deposited as amorphous gels, subsequently lithified to form banded iron formations. The distribution of Lake Superior-type banded iron formations of the same age range in Precambrian cratons worldwide suggests that they record a period of global change in the oxygen content of the earth's atmosphere and oceans. Also, the worldwide abundance of large, calm, shallow platforms where cyanobacterial mats flourished and banded iron formations were deposited may imply a global rise in sea level.

Primary carbonate in banded iron formations may be replaced by silica during diagenesis or deformation. The pronounced layering in banded iron formations may be further accentuated during deformation by pressure solution; silica and/or carbonate are dissolved and iron oxides such as hematite may crystallize along pressure solution (stylolite) surfaces. Banded iron formations are highly anisotropic rocks. When shortened parallel to their layering, they deform to form angular to rounded folds , kink bands, and box folds. Folds in banded iron formations are typically doubly plunging and conical. Banded iron formations may interact with hot fluids channeled along faults and more permeable, interbedded horizons such as dolomite during deformation. This may remove large volumes of silica, resulting in concentration of iron. Iron, in the form of microplaty hematite can also crystallize in structurally controlled sites such as fold hinges and along detachment faults. If there is sufficient enrichment, an iron ore body is formed. Iron may also be leached, redeposited and concentrated during weathering to form supergene iron ore deposits. Fibrous growth of quartz and minerals such as crocidolite (an amphibole, also known as asbestiform riebeckite) commonly occurs in banded iron formations during deformation due to dilation between layers, especially in fold hinges. Replacement of crocidolite by silica produces shimmering brown, yellow and orange "tiger-eye," which is utilized in jewelry and for ornamental use.

See also Industrial minerals

banded iron formation (BIF) Finely banded, siliceous, hematite deposits found in Precambrian rocks, forming stratiform units often several hundred metres thick and persistent over 150 km or more. They probably formed by chemical—organic processes during sedimentation in stable, shallow basins with little detritus, and so are syngenetic deposits. In their enriched form (40–60% iron) BIFs are mined opencast. They include the world's most important sources of iron ore, e.g. at Hammersley, Western Australia; Lake Superior, USA; Labrador, Canada; Ukraine, and Brazil.