Carbonatite ( /kɑːrˈbɒnətaɪt/) is a type of intrusive or extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geochemical verification.
Carbonatites usually occur as small plugs within zoned alkalic intrusive complexes, or as dikes, sills, breccias, and veins. They are almost exclusively associated with continental rift-related tectonic settings. It seems that there has been a steady increase in the carbonatitic igneous activity through the Earth's history, from the Archean eon to the present.
Nearly all carbonatite occurrences are intrusives or subvolcanic intrusives. This is because carbonatite lava flows, being composed largely of soluble carbonates, are easily weathered and are therefore unlikely to be preserved in the geologic record. Carbonatite eruptions as lava may therefore not be as uncommon as thought, but they have been poorly preserved throughout the Earth's history.
Carbonatite liquid compositions are significantly more alkaline than what is preserved in the fossil carbonatite rock record as composition of the melt inclusions shows.
Only one carbonatite volcano is known to have erupted in historical time, the active Ol Doinyo Lengai volcano in Tanzania. It erupts with the lowest-temperature lava in the world, at 500–600 °C. The lava is natrocarbonatite dominated by nyerereite and gregoryite.
|Primary||carbonate minerals, (> 50%)|
The magmatic origin of carbonatite was argued in detail by Swedish geologist Harry von Eckermann in 1948 basen on his study of Alnö Complex. It was however the 1960 eruption of Ol Doinyo Lengai in Tanzania that led to geological investigations that finally confirmed the view that carbonatite is derived from magma.
Carbonatites are rare, peculiar igneous rocks formed by unusual processes and from unusual source rocks. Three models of their formation exist:
Evidence for each process exists, but the key is that these are unusual phenomena. Historically, carbonatites were thought to form by melting of limestone or marble by intrusion of magma, but geochemical and mineralogical data discount this. For example, the carbon isotopic composition of carbonatites is mantle-like and not like sedimentary limestone.
Primary mineralogy is highly variable, but may include natrolite, sodalite, apatite, magnetite, barite, fluorite, ancylite group minerals, and other rare minerals not found in more common igneous rocks. Recognition of carbonatites may be difficult, especially as their mineralogy and texture may not differ much from marble except the presence of igneous minerals. They may also be sources of mica or vermiculite.
Carbonatites are classed as calcitic sovite (coarse textured) and alvikite (finer textured) varieties or facies. The two are also distinguished by minor and trace element composition. The terms rauhaugite and beforsite refer to dolomite- and ankerite-rich occurrences respectively. The alkali-carbonatites are termed lengaite. Examples with 50–70% carbonate minerals are termed silico-carbonatites. Additionally, carbonatites may be either enriched in magnetite and apatite or rare-earth elements, fluorine and barium.
Natrocarbonatite is made up largely of two minerals, nyerereite (named after Julius Nyerere, the first president of independent Tanzania) and gregoryite (named after John Walter Gregory, one of the first geologists to study the East African Rift and author of the book The Great Rift Valley). These minerals are both carbonates in which sodium and potassium are present in significant quantities. Both are anhydrous, and when they come into contact with the moisture in the atmosphere, they begin to react extremely quickly. The black or dark brown lava and ash erupted begins to turn white within a few hours.
Carbonatite is composed predominantly of carbonate minerals and extremely unusual in its major element composition as compared to silicate igneous rocks, obviously because it is composed primarily of Na2O and CaO plus CO2.
Most carbonatites tend to include some silicate mineral fraction; by definition an igneous rock containing >50% carbonate minerals is classified as a carbonatite. Silicate minerals associated with such compositions are pyroxene, olivine, and silica-undersaturated minerals such as nepheline and other feldspathoids.
Geochemically, carbonatites are dominated by incompatible elements (Ba, Cs, Rb) and depletions in compatible elements (Hf, Zr, Ti). This together with their silica-undersaturated composition supports inferences that carbonatites are formed by low degrees of partial melting.
A specific type of hydrothermal alteration termed fenitization is typically associated with carbonatite intrusions. This alteration assemblage produces a unique rock mineralogy termed a fenite after its type locality, the Fen Complex in Norway. The alteration consists of metasomatic halos consisting of sodium rich silicates arfvedsonite, barkevikite and glaucophane along with phosphates, hematite and other iron and titanium oxides.
Associated igneous rocks typically include ijolite, melteigite, teschenite, lamprophyres, phonolite, foyaite, shonkinite, silica undersaturated foid-bearing pyroxenite (essexite), and nepheline syenite.
Carbonatites are typically associated with undersaturated (low silica) igneous rocks that are either alkali (Na2O and K2O), ferric iron (Fe2O3) and zirconium-rich agpaitic rocks or alkali-poor, FeO-CaO-MgO-rich and zirconium-poor miaskitic rocks.
The Mount Weld carbonatite is unassociated with a belt or suite of alkaline igneous rocks, although calc-alkaline magmas are known in the region. The genesis of this Archaean carbonatite remains contentious as it is the sole example of an Archaean carbonatite in Australia.
Carbonatite is known to form in association with concentrically zoned complexes of alkaline-igneous rocks, the typical example of this being Phalaborwa, South Africa.
The Mud Tank and Mount Weld carbonatites take the form of multi-stage cylindrical intrusive bodies with several distinct phases of carbonatite intrusion. Smaller carbonatite sills and dikes are present in other Proterozoic mobile belts in Australia, typically as dikes and discontinuous pods.
Dozens of carbonatites are known including:
Carbonatites may contain economic or anomalous concentrations of rare-earth elements, phosphorus, niobium–tantalum, uranium, thorium, copper, iron, titanium, vanadium, barium, fluorine, zirconium, and other rare or incompatible elements. Apatite, barite and vermiculite are among the industrially important minerals associated with some carbonatites.
As an example, the Palabora complex of South Africa has produced significant copper (as chalcopyrite, bornite and chalcocite), apatite, vermiculte along with lesser magnetite, linnaeite (cobalt), baddeleyite (zirconium–hafnium), and by-product gold, silver, nickel and platinum.
The Alnö Complex or Alnö Alkaline Complex is a group of carbonatite and alkaline igneous rocks in Alnö in the eastern coast of central Sweden that intruded the basement in Late Ediacaran times. The Alnö Complex is made up by a series of concentric dykes within a radius of 25 km of a main "central complex" of intrusions. In addition the Alnö Complex proper is surrounded by a 500 to 600 m broad zone of metasomatic rock that was formed by metasomatic alteration of the existing Precambrian migmatite gneiss basement. The specific type of metasomatic rock is referred by some authors as "fenite". The dykes of the complex consist of carbonatite and alkaline rocks such melilite and sövite.It has been proposed that both the Fen Complex in Southern Norway and the Alnö Complex formed as consequence to mild extensional tectonics in the ancient continent of Baltica following the opening of the Iapetus Ocean.Harry von Eckermann published a landmark study on the Alnö Complex in 1948 correctly claiming a magmatic origin of carbonatite, albeit his finds were only widely accepted after the Ol Doinyo Lengai eruption of carbonatite lava in the 1960s showed contemporary evidence on the existence of such magmas.Cerite
Cerite is a complex silicate mineral group containing cerium, formula (Ce,La,Ca)9(Mg,Fe+3)(SiO4)6(SiO3OH)(OH)3. The cerium and lanthanum content varies with the Ce rich species (cerite-(Ce)) and the La rich species (cerite-(La)). Analysis of a sample from the Mountain Pass carbonatite gave 35.05% Ce2O3 and 30.04% La2O3.Cerite was first described in 1803 for an occurrence in Bastnäs in Västmanland, Sweden. The lanthanum rich species, cerite-(La) was first described for an occurrence in the Khibina massif, Kola Peninsula, Russia in 2002.Eluvium
In geology, eluvium or eluvial deposits are those geological deposits and soils that are derived by in situ weathering or weathering plus gravitational movement or accumulation.
The process of removal of materials from geological or soil horizons is called eluviation or leaching. There is a difference in the usage of this term in geology and soil science. In soil science, eluviation is the transport of soil material from upper layers of soil to lower levels by downward precipitation of water across soil horizons, and accumulation of this material (illuvial deposit) in lower levels is called illuviation. In geology, the removed material is irrelevant, and the deposit (eluvial deposit) is the remaining material. Eluviation occurs when precipitation exceeds evaporation.
A soil horizon formed due to eluviation is an eluvial zone or eluvial horizon. In a typical soil profile, the eluvial horizon refers to a light-colored zone located (depending on context and literature) either at the lower part of the A horizon (symbol: Ae) or within a distinct horizon (E horizon) below the A, where the process is most intense and rapid. Yet some sources consider the eluvial zone to be the A horizon plus the (distinct) E horizon, as eluviation technically occurs in both.
The strict eluvial horizon (E horizon) is typically light gray, clay-depleted, contains little organic matter and has a high concentration of silt and sand particles composed of quartz and other resistant minerals.
Eluvial ore deposits are those such as tungsten and gold placer deposits formed by settling and enriched by the winnowing or removal of lower density materials. Diamonds within yellow ground (weathered portions of kimberlites) may be considered to be eluvial deposits. Cassiterite and columbite-tantalite deposits also occur as residual or eluvial concentrations. The Pitinga tin deposit in Brazil, an eluvial deposit, is one of the largest tin mines in the world. Weathering supergene enrichment of an apatite rich carbonatite in Ontario has produced a significant eluvial phosphate ore deposit.Fen Complex
The Fen Complex (Norwegian: Fensfeltet) in Nome, Telemark, Norway is a region noted for an unusual suite of igneous rocks. Several varieties of carbonatite are present in the area as well as lamprophyre, ijolite and other highly alkalic rocks. It is the type locality for fenite, a metasomatic rock commonly found around carbonatite and alkali intrusives.The Fen Complex is a roughly circular area about three kilometres in diameter. It is located just west of the Oslo graben. Radiometric age dating on the carbonatites gave an age of 539 +/- 14 Myr. The host rocks for the intrusions are middle Proterozoic granites and gneiss and the complex was associated with the Cambrian rifting of the cratonic rocks.The complex is a protected location because of the rare minerals and rock types found there. The rocks were first described by Waldemar Christofer Brøgger in 1921.Harry von Eckermann
Harry von Eckermann (1886–1969) was a Swedish industrialist, mineralogist and geologist. His studies were centered around anorogenic alkaline igneous rocks occurring in the Baltic Shield. Following this line he studied the Alnö Complex, Norra Kärr Alkaline Complex and various Rapakivi granites.
In a 1948 publication on Alnö, von Eckermann correctly claimed a magmatic origin of carbonatite, albeit his finds were only widely accepted after the Ol Doinyo Lengai eruption of carbonatite lava in the 1960s showed contemporary evidence on the existence of such magmas. In relation to the mid-20th century granitization controversy von Eckermann rejected the notion that rapakivi granites were Jotnian sediments turned into granite.Hogenakkal Falls
Hogenakkal is a waterfall in South India on the Kaveri river in the Dharmapuri district of the Indian state of Tamil Nadu. It is located 180 km (110 mi) from Bangalore and 46 km (29 mi) from Dharmapuri. Sometimes referred to as the "Niagara Falls of India," it is known for bathing areas and hide boat rides, projecting itself as a major tourist attraction. Carbonatite rocks in this site are considered to be the oldest of its kind in South Asia and one of the oldest in the world. The Government of Tamil Nadu made a proposal to convert the falls into providing drinking water for the state.Khanashin
Khanashin, or Khan Neshin, (other names: Khān Neshīn, Khannesin, Khan Nashin, Khān Nashīn, Khan Nashim, Khānnešīn) is a village located in the Reg District of Helmand Province, Afghanistan at 30.5494°N 63.7897°E / 30.5494; 63.7897 at 642 altitude. It is close to the Helmand River and 168 km southwest of Lashkargah. It has been identified by the USGS as the site of a deposit, called the Khanneshin carbonatite, of at least 1 million tons of rare earth element ore.Kola Alkaline Province
The Kola Alkaline Province or Kola Alkaline Carbonatite Province is a discontiguous group of unusual igneous rocks centered in the Kola Peninsula of Russia and with ouliers in nearby areas of Finland and in Arkhangelsk Oblast across the White Sea. The province is made up of alkaline-ultramafic rock complexes often associated to carbonatites and stand-alone dykes and pipes made up of carbonatites, kimberlites and similar rocks. To this it adds the large nepheline syenite bodies of the Lovozero Massif and the Khibiny Mountains. An estimate puts the total volume of the rocks of the Kola Alkaline Province at 15,000 ±2,700 km3.The more mafic silicate rocks of the province originated from small degrees of partial melting in a source region in Earth's mantle made up of garnet-bearing peridotite. The lithosphere had thicknesses similar to present-day (200 km) conditions when magmas originated in the Devonian. Prior to Devonian magmatism the Kola and Karelia region had expierenced a long history of low-frenquency alkaline and carbonatite volcanism.The Permian rocks of the Kola Alkaline Province is commonly presumed to represent an igneous hotspot created by a mantle plume. The relation of Kola Alkaline Province to other igneous or tectonic features is not clear. Some have suggested a link to the Permian Dnieper-Donets Rift while others have considered it as part of a much larger a "North Atlantic Alkaline Province".Magnet Cove igneous complex
The Magnet Cove igneous complex is a small alkalic ring complex lying to the west of the town of Magnet Cove in Hot Spring County, Arkansas. It and the adjacent town are so named due to the existence of magnetite and the terrain being a cove, a basin-shaped valley.The complex is of Mesozoic age, intruded into Paleozoic sediments. Mapping was conducted by the geologists Erickson and Blade in 1963. Units within the complex include carbonatite, nepheline syenite, phonolite, and ijolite.
In addition to the magnetite which forms both massive lodestone and crystals, the complex is strewn with odd and rare minerals, and is the type locality for five mineral species. Over 100 different minerals have been identified from the area.
There are many titanium minerals such as rutile, anatase, brookite, and perovskite, as well as some vanadium mineralization. Some rare-earth-bearing minerals have also been identified.
Many companies have mined the area over the last century, including the Diamond Jo Quarry.The complex has developed a unique soil series, Magnet, which has several inches of dark reddish brown loam over reddish brown clay or clay loam. The Magnet is present only over 5000 acres at the complex and a small outlier at Potash Sulphur Springs.Arkansas Highway 51 runs generally east–west through Magnet Cove.Mount Homa
Mount Homa is a mountain located in western Kenya. It forms a broad peninsula on the southern shore of Winam Gulf, an extension of Lake Victoria. This peninsula defines Homa Bay and the mountaintop is about 20 kilometres north of the town of that name.
In the Luo language Got Uma or God Marahuma means "famous mountain".
The mountain is formed of carbonatite lava and dates from Miocene to Pleistocene. Along with the active Ol Doinyo Lengai, it is one of the very few carbonatite volcanoes in the world.Mount Weld
Mount Weld is a mountain and a mine site in Western Australia, located about 30 km (20 mi) south of Laverton and 120 km (75 mi) east of Leonora. It ranks as one of the richest major rare-earth deposits in the world.Rare earths are contained in secondary phosphates and aluminophosphates, presumably derived from weathering of the Proterozoic Mount Weld carbonatite. The primary commercial interest at the site is targeted towards oxides as well as further niobium and tantalum deposits within the intrusive pipe of the Mount Weld carbonatite, which is approximately three kilometers (1.9 mi) in diameter.The main deposits are hosted within the soil and regolith horizon that blankets the entire carbonatite and form shallow lenses within 60 m (200 ft) of the surface. The most important rare-earth oxide deposit, the Central Lanthanide Deposit, CLD, is located at the center of the carbonatite with the niobium/tantalum and other deposits generally located towards outer fringes. Discovered in 1988, the CLD represents a spectacular enrichment of rare-earth deposit sediments. The deposit is believed to be the largest and highest grade of its type in the world.The Mount Weld deposit is owned by ASX-listed Lynas Corporation, which raised A$450 million equity from J. P. Morgan in 2009 to fund the development of a mine and also a processing plant in Kuantan, Malaysia. Once operational, the Mount Weld mine is expected to be the largest source of rare-earth elements outside of China.Mining began at the Mount Weld site in 2011.Mountain Pass rare earth mine
The Mountain Pass Rare Earth Mine is an open-pit mine of rare-earth elements on the south flank of the Clark Mountain Range, just north of the unincorporated community of Mountain Pass, California, United States. The mine once supplied most of the world's rare-earth elements. Today, it is owned by MP Materials, and is the country's only rare earth mining and processing facility.Nephelinite
Nephelinite is a fine-grained or aphanitic igneous rock made up almost entirely of nepheline and clinopyroxene (variety augite). If olivine is present, the rock may be classified as an olivine nephelinite. Nephelinite is dark in color and may resemble basalt in hand specimen. However, basalt consists mostly of clinopyroxene (augite) and calcic plagioclase.
Basalt, alkali basalt, basanite, tephritic nephelinite, and nephelinite differ partly in the relative proportions of plagioclase and nepheline. Alkali basalt may contain minor nepheline and does contain nepheline in its CIPW normative mineralogy. A critical ratio in the classification of these rocks is the ratio nepheline/(nepheline plus plagioclase). Basanite has a value of this ratio between 0.1 and 0.6 and also contains more than 10% olivine. Tephritic nephelinite has a value between 0.6 and 0.9. Nephelinite has a value greater than 0.9. Le Maitre (2002) defines and discusses these and other criteria in the classification of igneous rocks.
Nephelinite is an example of a silica-undersaturated igneous rock. The degree of silica saturation can be evaluated with normative mineralogy calculated from chemical analyses, or with actual mineralogy for completely crystallized igneous rocks with equilibrated assemblages. Silica-oversaturated rocks contain quartz (or another silica polymorph). Silica-undersaturated mafic igneous rocks contain magnesian olivine but not magnesian orthopyroxene, and/or a feldspathoid. Silica-saturated igneous rocks fall in between these two classes.
Silica-undersaturated, mafic igneous rocks are much less abundant than silica-saturated and oversaturated basalts. Genesis of the less common mafic rocks such as nephelinite is usually ascribed to more than one of the following three causes:
relatively high pressure of melting;
relatively low degree of fractional melting in a mantle source;
relatively high dissolved carbon dioxide in the melt.Nephelinites and similar rocks typically contain relatively high concentrations of elements such as the light rare earths, as consistent with a low degree of melting of mantle peridotite at depths sufficient to stabilize garnet. Nephelinites are also associated with carbonatite in some occurrences, consistent with source rocks relatively rich in carbon dioxide.
Nephelinite is found on ocean islands such as Oahu, although the rock type is very rare in the Hawaiian islands. It is found in a variety of continental settings. An example is the Hamada nephelinite lava flow in southwest Japan which occurred in the late Miocene age. Nephelinite is also associated with the highly alkalic volcanism of the Ol Doinyo Lengai volcanic field in Tanzania. Nyiragongo, another African volcano known for its semipermanent lava lake activity, erupts lava made of melilite nephelinite. The unusual chemical makeup of this igneous rock may be a factor in the unusual fluidity of its lavas.
Olivine nephelinite flows also occur in the Wells Gray-Clearwater volcanic field in east-central British Columbia and at Volcano Mountain in central Yukon Territory. Melilite olivine nephelinite intrusives of Cretaceous age are found in the area around Uvalde, Texas.Ol Doinyo Lengai
Ol Doinyo Lengai, "Mountain of God" in the Maasai language, is an active volcano located in the Gregory Rift, south of Lake Natron within the Arusha Region of Tanzania, Africa. Part of the volcanic system of the East African Rift, it uniquely produces natrocarbonatite lava. The 1960 eruption of Ol Doinyo Lengai led to geological investigations that finally confirmed the view that carbonatite rock is derived from magma.Ore
An ore is an occurrence of rock or sediment that contains sufficient minerals with economically important elements, typically metals, that can be economically extracted from the deposit. The ores are extracted from the earth through mining; they are then refined (often via smelting) to extract the valuable element, or elements.
The ore grade, or concentration of an ore mineral or metal, as well as its form of occurrence, will directly affect the costs associated with mining the ore. The cost of extraction must thus be weighed against the metal value contained in the rock to determine what ore can be processed and what ore is of too low a grade to be worth mining. Metal ores are generally oxides, sulfides, silicates, or native metals (such as native copper) that are not commonly concentrated in the Earth's crust, or noble metals (not usually forming compounds) such as gold. The ores must be processed to extract the elements of interest from the waste rock and from the ore minerals. Ore bodies are formed by a variety of geological processes. The process of ore formation is called ore genesis.Panda Hill Carbonatite
The Panda Hill Carbonatite is an apatite and pyrochlore bearing sovite carbonatite located in the Mbeya District of Tanzania. The deposit was found in the mid-1950s by the then Geological Survey of Tanganyika (now Tanzania). Niobium ore occurs largely in pyrochlore bearing sovite (carbonatite composed largely of calcite) and dolmite-rich carbonatite.The deposit also contains lesser phosphate mineralisation associated with apatite.Sabinaite
Sabinaite (Na4Zr2TiO4(CO3)4) is a rare carbonate mineral. It crystallizes in the monoclinic crystal system as colorless to white prisms within cavities. It is more typically found as powdery coatings and masses. It has a specific gravity of 3.36.
It has been found in vugs in a carbonatite sill on Montreal Island and within sodalite syenite in the alkali intrusion at Mont Saint-Hilaire in Quebec, Canada.
It was first described in 1980 for an occurrence in the Francon quarry, Montreal Island. It is named after Ann Sabina (1930–2015), a mineralogist working for the Geological Survey of Canada.Sovite
Sovite (or sövite) is the coarse-grained variety (or facies) of carbonatite, an intrusive, igneous rock. The finer-grained variety of carbonatite is known as alvikite. The two varieties are distinguished by minor and trace element compositions. Sovite is often a medium-to-coarse-grained calcite rock with variable accessory amphibole, biotite, pyrite, pyrochlore and fluorite.