Carbonatite

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. Trace elements are extremely enriched in carbonatites, and they have the highest concentration of lanthanides of any known rock type. The largest REE-carbonatite deposits are Bayan Obo, Mountain Pass, Maoniuping, and Mount Weld. Vein deposits of thorium, fluorite, or rare-earth elements may be associated with carbonatites and may be hosted internal to or within the metasomatized aureole of a carbonatite. As an example, the Palabora complex of South Africa has produced significant copper (as chalcopyrite, bornite and chalcocite), apatite, vermiculate along with lesser magnetite, linnaeite (cobalt), baddeleyite (zirconium–hafnium), and by-product gold, silver, nickel and platinum.

Dozens of carbonatites are known including:

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.

Carbonatite is considered a major source of valuable rare earth elements, including lanthanum, cerium, praseodymium, and neodymium. However, this rock is only rarely found on the earth's surface, and as such has no uses in and of itself, though the elements harvested from carbonatite, including metals such as copper and titanium, are important for industrial uses.

Overall, 527 carbonatite localities are known on Earth, and they are found on all continents and also on oceanic islands. Most of the carbonatites are shallow intrusive bodies of calcite-rich igneous rocks in form of volcanic necks, dykes, and cone-sheets. These generally occur in association with larger intrusions of alkali-rich silicate igneous rocks. The extrusive carbonatites are particularly rare, only 49 are known, and they appear to be restricted to a few continental rift zones, such as the Rhine valley and the East African rift system. 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 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.