Iron-rich sedimentary rocks

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Iron ore from Kryvyi Rih, Ukraine. Banded iron formation.jpg
Iron ore from Kryvyi Rih, Ukraine.

Iron-rich sedimentary rocks are sedimentary rocks which contain 15 wt.% or more iron. However, most sedimentary rocks contain iron in varying degrees. The majority of these rocks were deposited during specific geologic time periods: The Precambrian (3800 to 539 million years ago), the early Paleozoic (539 to 419 million years ago), and the middle to late Mesozoic (205 to 66 million years ago). Overall, they make up a very small portion of the total sedimentary record.

Contents

Iron-rich sedimentary rocks have economic uses as iron ores. Iron deposits have been located on all major continents with the exception of Antarctica. They are a major source of iron and are mined for commercial use. [1] The main iron ores are from the oxide group consisting of hematite, goethite, and magnetite. The carbonate siderite is also typically mined. A productive belt of iron formations is known as an iron range. [2]

Classification

The accepted classification scheme for iron-rich sedimentary rocks is to divide them into two sections: ironstones and iron formations [1]

Ironstones

Ironstones consist of 15% iron or more in composition. This is necessary for the rock to even be considered an iron-rich sedimentary rock. Generally, they are from the Phanerozoic which means that they range in age from the present to 540 million years ago. [1] They can contain iron minerals from the following groups: oxides, carbonates, and silicates. Some examples of minerals in iron-rich rocks containing oxides are limonite, hematite, and magnetite. An example of a mineral in iron-rich rock containing carbonates is siderite and an example of minerals in an iron-rich rock containing silicate is chamosite. [2] They are often interbedded with limestones, shales, and fine-grained sandstones. They are typically nonbanded, however they can be very coarsely banded on occasion. [1] They are hard and non-cherty. [2] The components of the rock range in size from sand to mud, but do not contain a lot of silica. They are also more aluminous. They are not laminated and sometimes contain ooids . Ooids can be a distinct characteristic though they are not normally a main component of ironstones. Within ironstones, ooids are made up of iron silicates and/or iron oxides and sometimes occur in alternating laminae. They normally contain fossil debris and sometimes the fossils are partly or entirely replaced by iron minerals. A good example of this is pyritization. They are smaller in size and less likely to be deformed or metamorphosed than iron formations. [3] The term iron ball is occasionally used to describe an ironstone nodule. [2]

Iron formations

Bog ore. Limonite bog iron cm02.jpg
Bog ore.

Iron formations must be at least 15% iron in composition, just like ironstones and all iron-rich sedimentary rocks. However, iron formations are mainly Precambrian in age which means that they are 4600 to 590 million years old. They are much older than ironstones. They tend to be cherty, though chert can not be used as a way to classify iron formations because it is a common component in many types of rocks. They are well banded and the banding can be anywhere from a few millimeters to tens of meters thick. The layers have very distinct banded successions that are made up of iron rich layers that alternate with layers of chert. Iron formations are often associates with dolomite, quartz-rich sandstone, and black shale. They sometimes grade locally into chert or dolomite. They can have many different textures that resemble limestone. Some of these textures are micritic, pelleted, intraclastic, peloidal, oolitic, pisolitic, and stromatolitic. [1] In low-grade iron formations, there are different dominant minerals dependent on the different types of facies. The dominant minerals in the oxide facies are magnetite and hematite. The dominant minerals in the silicate facies are greenalite, minnesotaite, and glauconite. The dominant mineral in the carbonate facies is siderite. The dominant mineral in the sulfide facies is pyrite. Most iron formations are deformed or metamorphosed simply due to their incredibly old age, but they still retain their unique distinctive chemical composition; even at high metamorphic grades. The higher the grade, the more metamorphosed it is. Low grade rocks may only be compacted while high grade rocks often can not be identified. They often contain a mixture of banded iron formations and granular iron formations. Iron formations can be divided into subdivisions known as: banded iron formations (BIFs) and granular iron formations (GIFs). [3]

The above classification scheme is the most commonly used and accepted, though sometimes an older system is used which divides iron-rich sedimentary rocks into three categories: bog iron deposits, ironstones , and iron formations. A bog-iron deposit is iron that formed in a bog or swamp through the process of oxidation.

Banded iron formations vs. granular iron formations

Banded iron formation close up, Upper Michigan. MichiganBIF.jpg
Banded iron formation close up, Upper Michigan.

Banded iron formations

Banded iron formations (BIFs) were originally chemical muds and contain well developed thin lamination. They are able to have this lamination due to the lack of burrowers in the Precambrian. BIFs show regular alternating layers that are rich in iron and chert that range in thickness from a few millimeters to a few centimeters. The formation can continue uninterrupted for tens to hundreds of meters stratigraphically. These formations can contain sedimentary structures like cross-bedding, graded bedding, load casts, ripple marks, mud cracks, and erosion channels. In comparison to GIFs, BIFs contain a much larger spectrum of iron minerals, have more reduced facies, and are more abundant. [1]

BIFs are divided into type categories based on the characteristics related to the nature of their formation and unique physical and chemical properties. Some categories of banded iron formations are the Rapitan type, the Algoma type, and the Superior type.

Superior type banded iron formation, North America. Black-band ironstone (aka).jpg
Superior type banded iron formation, North America.

Rapitan type

Rapitan types are associated with the glaciogenic sequences of the Archean and Early Proterozoic. The type is distinctive as the hydrothermal-input has notably less influence on this formation's Rare Earth Element (REE) chemistry than other formations during this time period. [5]

Algoma type

Algoma types are small lenticular iron deposits that are associated with volcanic rocks and turbidites. [6] Iron content in this class type rarely exceeds 1010 tons. They range in thickness from 10–100 meters. Deposition occurs in island arc/back arc basins and intracratonic rift zones. [7]

Superior type

Superior types are large, thick, extensive iron deposits across stable shelves and in broad basins. [6] Total iron content in this class type exceeds 1013 tons. They can extend to over 105 kilometers2. Deposition occurs in relatively shallow marine conditions under transgressing seas. [7]

Granular iron formations

Granular iron formations (GIFs) were originally well-sorted chemical sands. They lack even, continuous bedding that takes the form of discontinuous layers. Discontinuous layers likely represent bedforms that were generated by storm waves and currents. Any layers that are thicker than a few meters and are uninterrupted, are rare for GIFs. They contain sand-sized clasts and a finer grained matrix, and generally belong to the oxide or silicate mineral facies. [1]

Depositional environment

Profile illustrating the shelf, slope and rise. Continental shelf.png
Profile illustrating the shelf, slope and rise.

There are four facies types associated with iron-rich sedimentary rocks: oxide-, silicate-, carbonate-, and sulfide-facies. These facies correspond to water depth in a marine environment. Oxide-facies are precipitated under the most oxidizing conditions. Silicate- and carbonate-facies are precipitated under intermediate redox conditions. Sulfide-facies are precipitated under the most reducing conditions. There is a lack of iron-rich sedimentary rocks in shallow waters which leads to the conclusion that the depositional environment ranges from the continental shelf and upper continental slope to the abyssal plain. (The diagram does not have the abyssal plain labeled, but this would be located to the far right of the diagram at the bottom of the ocean). [7]

Water colored by oxidized iron, Rio Tinto, Spain. IronInRocksMakeRiverRed.jpg
Water colored by oxidized iron, Rio Tinto, Spain.
Iron bacteria growing on iron-rich water seeping from a tall bluff, Sipsey Wilderness Area, Bankhead National Forest, Alabama. Iron Bacteria in Bankhead National Forest.JPG
Iron bacteria growing on iron-rich water seeping from a tall bluff, Sipsey Wilderness Area, Bankhead National Forest, Alabama.

Chemical reactions

Ferrous and ferric iron are components in many minerals, especially within sandstones. Fe2+ is in clay, carbonates, sulfides, and is even within feldspars in small amounts. Fe3+ is in oxides, hydrous, anhydrous, and in glauconites. [8] Commonly, the presence of iron is determined to be within a rock due to certain colorations from oxidation. Oxidation is the loss of electrons from an element. Oxidation can occur from bacteria or by chemical oxidation. This often happens when ferrous ions come into contact with water (due to dissolved oxygen within surface waters) and a water-mineral reaction occurs. The formula for the oxidation/reduction of iron is:

Fe2+ ↔ Fe3+ + e−

The formula works for oxidation to the right or reduction to the left.

Fe2+ is the ferrous form of iron. This form of iron gives up electrons easily and is a mild reducing agent. These compounds are more soluble because they are more mobile. Fe3+ is the ferric form of iron. This form of iron is very stable structurally because its valence electron shell is half filled. [9]

Laterization

Laterization is a soil forming process that occurs in warm and moist climates under broadleaf evergreen forests. Soils formed by laterization tend to be highly weathered with high iron and aluminium oxide content. Goethite is often made from this process and is a major source of iron in sediments. However, once it is deposited it must be dehydrated in order to come to an equilibrium with hematite. The dehydration reaction is: [9]

2 FeO(OH) → Fe2O3 + H2O
Pyritized Lytoceras. Pyritized lytoceras.png
Pyritized Lytoceras.

Pyritization

Pyritization is discriminatory. It rarely happens to soft tissue organisms and aragonitic fossils are more susceptible to it than calcite fossils. It commonly takes place in marine depositional environments where there is organic material. The process is caused by sulfate reduction which replaces carbonate skeletons (or shells) with pyrite (FeS2). It generally does not preserve detail and the pyrite forms within the structure as many microcrystals. In freshwater environments, siderite will replace carbonate shells instead of pyrite due to the low amounts of sulfate. [10] The amount of pyritization that has taken place within a fossil may sometimes be referred to as degree of pyritization (DOP).

Oolitic Hematite, Clinton, Oneida County, NY. Hematite - oolitic with shale Iron Oxide Clinton, Oneida County, New York.jpg
Oolitic Hematite, Clinton, Oneida County, NY.

Iron minerals

Limonite, USGS. LimoniteUSGOV.jpg
Limonite, USGS.

Iron–rich rocks in thin section

Thin section of rhyolite volcanic rock showing an oxidized iron matrix (orange/brown color). Rhyolite pmg ss 2006.jpg
Thin section of rhyolite volcanic rock showing an oxidized iron matrix (orange/brown color).

Magnetite and hematite are opaque under the microscope under transmitted light. Under reflected light, magnetite shows up as metallic and a silver or black color. Hematite will be a more reddish-yellow color. Pyrite is seen as opaque, a yellow-gold color, and metallic. [12] Chamosite is an olive-green color in thin section that readily oxidizes to limonite. When it is partially or fully oxidized to limonite, the green color becomes a yellowish-brown. Limonite is opaque under the microscope as well. Chamosite is an iron silicate and it has a birefringence of almost zero. Siderite is an iron carbonate and it has a very high birefringence. The thin sections often reveal marine fauna within oolitic ironstones. In older samples, the ooids may be squished and have hooked tails on either end due to compaction. [13]

Related Research Articles

<span class="mw-page-title-main">Shale</span> Fine-grained, clastic sedimentary rock

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, Al2Si2O5(OH)4) and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility. Shale is the most common sedimentary rock.

<span class="mw-page-title-main">Banded iron formation</span> Distinctive layered units of iron-rich sedimentary rock that are almost always of Precambrian age

Banded iron formations are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Almost all of these formations are of Precambrian age and are thought to record the oxygenation of the Earth's oceans. Some of the Earth's oldest rock formations, which formed about 3,700 million years ago (Ma), are associated with banded iron formations.

<span class="mw-page-title-main">Limonite</span> Hydrated iron oxide mineral

Limonite is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·nH2O, although this is not entirely accurate as the ratio of oxide to hydroxide can vary quite widely. Limonite is one of the three principal iron ores, the others being hematite and magnetite, and has been mined for the production of iron since at least 2500 BP.

<span class="mw-page-title-main">Iron ore</span> Ore rich in iron or the element Fe

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 form of magnetite (Fe
3O
4, 72.4% Fe), hematite (Fe
2O
3, 69.9% Fe), goethite (FeO(OH), 62.9% Fe), limonite (FeO(OH)·n(H2O), 55% Fe) or siderite (FeCO3, 48.2% Fe).

<span class="mw-page-title-main">Ironstone</span> Sedimentary rock that contains a substantial proportion of iron ore

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.

<span class="mw-page-title-main">Wüstite</span> Iron(II) oxide mineral formed under reducing conditions

Wüstite is a mineral form of mostly iron(II) oxide found with meteorites and native iron. It has a grey colour with a greenish tint in reflected light. Wüstite crystallizes in the isometric-hexoctahedral crystal system in opaque to translucent metallic grains. It has a Mohs hardness of 5 to 5.5 and a specific gravity of 5.88. Wüstite is a typical example of a non-stoichiometric compound.

<span class="mw-page-title-main">Skarn</span> Hard, coarse-grained, hydrothermally altered metamorphic rocks

Skarns or tactites are coarse-grained metamorphic rocks that form by replacement of carbonate-bearing rocks during regional or contact metamorphism and metasomatism. Skarns may form by metamorphic recrystallization of impure carbonate protoliths, bimetasomatic reaction of different lithologies, and infiltration metasomatism by magmatic-hydrothermal fluids. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate minerals. These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a granitic pluton found in and around faults or shear zones that commonly intrude into a carbonate layer composed of either dolomite or limestone. Skarns can form by regional or contact metamorphism and therefore form in relatively high temperature environments. The hydrothermal fluids associated with the metasomatic processes can originate from a variety of sources; magmatic, metamorphic, meteoric, marine, or even a mix of these. The resulting skarn may consist of a variety of different minerals which are highly dependent on both the original composition of the hydrothermal fluid and the original composition of the protolith.

<span class="mw-page-title-main">Petrifaction</span> Process of fossilisation

In geology, petrifaction or petrification is the process by which organic material becomes a fossil through the replacement of the original material and the filling of the original pore spaces with minerals. Petrified wood typifies this process, but all organisms, from bacteria to vertebrates, can become petrified. Petrifaction takes place through a combination of two similar processes: permineralization and replacement. These processes create replicas of the original specimen that are similar down to the microscopic level.

<span class="mw-page-title-main">Volcanogenic massive sulfide ore deposit</span> Metal sulfide ore deposit

Volcanogenic massive sulfide ore deposits, also known as VMS ore deposits, are a type of metal sulfide ore deposit, mainly copper-zinc which are associated with and created by volcanic-associated hydrothermal events in submarine environments.

<span class="mw-page-title-main">Gunflint chert</span> Geologic formation in Minnesota and Ontario

The Gunflint chert is a sequence of banded iron formation rocks that are exposed in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. The Gunflint Chert is of paleontological significance, as it contains evidence of microbial life from the Paleoproterozoic. The Gunflint Chert is composed of biogenic stromatolites. At the time of its discovery in the 1950s, it was the earliest form of life discovered and described in scientific literature, as well as the earliest evidence for photosynthesis. The black layers in the sequence contain microfossils that are 1.9 to 2.3 billion years in age. Stromatolite colonies of cyanobacteria that have converted to jasper are found in Ontario. The banded ironstone formation consists of alternating strata of iron oxide-rich layers interbedded with silica-rich zones. The iron oxides are typically hematite or magnetite with ilmenite, while the silicates are predominantly cryptocrystalline quartz as chert or jasper, along with some minor silicate minerals.

<span class="mw-page-title-main">Ore genesis</span> How the various types of mineral deposits form within the Earths crust

Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.

<span class="mw-page-title-main">Cumulate rock</span> Igneous rocks formed by the accumulation of crystals from a magma either by settling or floating.

Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks. Cumulates can be deposited on top of other older cumulates of different composition and colour, typically giving the cumulate rock a layered or banded appearance.

<span class="mw-page-title-main">Red beds</span> Sedimentary rocks with ferric oxides

Red beds are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain thin beds of conglomerate, marl, limestone, or some combination of these sedimentary rocks. The ferric oxides, which are responsible for the red color of red beds, typically occur as a coating on the grains of sediments comprising red beds. Classic examples of red beds are the Permian and Triassic strata of the western United States and the Devonian Old Red Sandstone facies of Europe.

<span class="mw-page-title-main">Mineral redox buffer</span>

In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions (or equivalently, oxygen fugacities) at which a rock forms and evolves can be important for interpreting the rock history. Iron, sulfur, and manganese are three of the relatively abundant elements in the Earth's crust that occur in more than one oxidation state. For instance, iron, the fourth most abundant element in the crust, exists as native iron, ferrous iron (Fe2+), and ferric iron (Fe3+). The redox state of a rock affects the relative proportions of the oxidation states of these elements and hence may determine both the minerals present and their compositions. If a rock contains pure minerals that constitute a redox buffer, then the oxygen fugacity of equilibration is defined by one of the curves in the accompanying fugacity-temperature diagram.

Channel iron deposits (CID) are iron-rich fluvial sedimentary deposits of possible Miocene age occupying meandering palaeochannels in the Early to Mid-Cenozoic Hamerlsey palaeosurface of Western Australia. Examples are also known from Kazakhstan.

Iron oxide copper gold ore deposits (IOCG) are important and highly valuable concentrations of copper, gold and uranium ores hosted within iron oxide dominant gangue assemblages which share a common genetic origin.

<span class="mw-page-title-main">Permineralization</span> Type of fossilization

Permineralization is a process of fossilization of bones and tissues in which mineral deposits form internal casts of organisms. Carried by water, these minerals fill the spaces within organic tissue. Because of the nature of the casts, permineralization is particularly useful in studies of the internal structures of organisms, usually of plants.

Microspherulites are microscopic spherical particles with diameter less than two mm, usually in the 100 micrometre range, mainly consisting of mineral material. Only bodies created by natural physico-chemical processes, with no contribution of either biological or human activity, are considered to be microspherulites. Generally speaking, the common feature (sphericity) indicates that each sphere represents an internal equilibrium of forces within a fluid medium.

<span class="mw-page-title-main">Primary mineral</span>

A primary mineral is any mineral formed during the original crystallization of the host igneous primary rock and includes the essential mineral(s) used to classify the rock along with any accessory minerals. In ore deposit geology, hypogene processes occur deep below the Earth's surface, and tend to form deposits of primary minerals, as opposed to supergene processes that occur at or near the surface, and tend to form secondary minerals.

The geology of Mauritania is built on more than two billion year old Archean crystalline basement rock in the Reguibat Shield of the West African Craton, a section of ancient and stable continental crust. Mobile belts and the large Taoudeni Basin formed and filled with sediments in the connection with the Pan-African orogeny mountain building event 600 million years ago and a subsequent orogeny created the Mauritanide Belt. In the last 251 million years, Mauritania has accumulated additional sedimentary rocks during periods of marine transgression and sea level retreat. The arid country is 50% covered in sand dunes and has extensive mineral resources, although iron plays the most important role in the economy.

References

  1. 1 2 3 4 5 6 7 Boggs Jr., Sam, 2006, Principles of Sedimentology and Stratigraphy (4th ed.), Pearson Education Inc., Upper Saddle River, NJ, pp. 217–223
  2. 1 2 3 4 Jackson, Julia A., 1997, Glossary of Geology, American Geologic Institute, Ventura Publisher, Alexandria, VA, pp. 335–336
  3. 1 2 Middleton, Gerard V. (and others), 2003, Encyclopedia of Sediments and Sedimentary Rocks, Kluwer Academic Publishers, Dordrecht, Netherlands, pp. 124–125, 130–133, 159–160, 367–368, 376–384, 486–489, 555–557, 701–702
  4. ↑ "Banded iron formation". www.sandatlas.org. Retrieved 2020-03-29.
  5. ↑ Klein, Cornelis; Beukes, Nicolas J. (1993-05-01). "Sedimentology and geochemistry of the glaciogenic late Proterozoic Rapitan Iron-Formation in Canada". Economic Geology. 88 (3): 542–565. doi:10.2113/gsecongeo.88.3.542. ISSN   1554-0774.
  6. 1 2 Stow, Dorrik Av, 2005, Sedimentary Rocks in the Field, Academic press - Manson Publishing, London, UK, p. 218
  7. 1 2 3 Harnmeijer, Jelte P., 2003, Banded Iron-Formations: A Continuing Enigma of Geology, University of Washington, WA, USA
  8. ↑ Pettijohn, Potter, and Siever, 1987, Sand and Sandstone, Springer-Verlag Publishing Inc., New York, NY, pg. 50-51
  9. 1 2 Leeder, Mike, 2006, Sedimentology and Sedimentary Basins, Blackwell Publishing, Malden, MA, pp. 20–21, 70–73
  10. ↑ Parrish, J. Michael, 1991, The Process of Fossilization, Belhaeven Press, Oxford, UK, pp. 95–97
  11. ↑ Collison, J.D., 1989, Sedimentary Structures, The University Printing House, Oxford, Great Britain, pp. 159–164
  12. ↑ Scholle, Peter, 1979, Constituents, Textures, Cements, and Porosities of Sandstones and Associated Rocks, The American Association of Petroleum Geologists, Tulsa, OK, pp. 43–45
  13. ↑ Adams, A.E., MacKenzie, W.S., and Guilford, C., 1984, Atlas of Sedimentary Rocks Under the Microscope, William Clowes Ltd., Essex, Great Britain, pp. 78–81
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