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Carbonatites are intrusive igneous rocks defined by a mineralogy which is greater than 50% carbonate minerals. Carbonatites may be confused with marble, and may require geochemical verification.
Carbonatites are usually intrusions with central plugs within zoned alkalic intrusive complexes, or as dikes, sills, breccias, and veins. The majority of carbonatites are Proterozoic in age, and hosted in Proterozoic fold belts adjacent to Archaean cratons.
Nearly all carbonatite occurrences are intrusives or subvolcanic intrusives. This is because carbonatite lava flows are unstable and react quickly in the atmosphere. Carbonatite lavas may not be as uncommon as thought, but have been poorly preserved throughout Earth's history.
Only one carbonatite volcano is known to have erupted in historical time, Ol Doinyo Lengai in Tanzania. It erupted the lowest temperature lava in the world, at 500-600 °C (930-1,100 °F). The lava is dominated by natrolite and trona, sodic calcite.
Genesis
Carbonatites are rare, peculiar igneous rocks formed by unusual processes and from unusual source rocks. Three models of their formation exist:
- direct generation by very low degree partial melts in the mantle and melt differentiation
- liquid immiscibility between a carbonate melt and a silicate melt
- peculiar, extreme crystal fractionation
Mineralogy
Primary mineralogy is highly variable, but may include natrolite, sodalite, sovite, apatite, magnetite, barite, fluorite, ancylite group minerals, and other rare, peculiar minerals not found in more normal igneous rocks. Recognition of carbonatites may be difficult, especially as their mineralogy and texture may not differ much from marble save for the presence of igneous minerals. They may also be sources of mica or vermiculite.
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 Great Rift Valley 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 of 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.
Geochemistry
Carbonatites may contain economic or anomalous concentrations of rare earth elements, phosphorus, niobium, uranium, thorium, copper, iron, titanium, barium, fluorine, zirconium, and other rare or incompatible elements. Geochemically, carbonatites are dominated by incompatible elements (Ba, Cs, Rb) and depletions in compatible elements (Hf, Zr, Ti).
Examples
Deposits exist in Oka, Quebec; Iron Hill and Gem Park, Colorado; Magnet Cove, Arkansas; St. Honore, Quebec; Mountain Pass, California; Phalaborwa, South Africa; Jacupiranga, Brazil; Kovdor, Russia, India; Mount Weld, Mud Tank Australia; the Fen Complex, Norway.
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Spatially and (or) genetically related deposit types
Vein deposits of thorium, fluorite, or rare earth elements may be associated with carbonatites.
Carbonatite is typically associated with concentrically zoned complexes of alkaline-igneous rocks, though some deposits may consist of dikes or thick sheets. Associated igneous rocks typically include ijolite, melteigite, teschenite, lamprophyres, phonolite, foyaite, shonkinite, pyroxenite (essexite), and nepheline syenite. Carbonatites are typically associated with undersaturated igneous rocks that are miaskitic (nearly peralkaline) rather than agpaitic (peralkaline).
References
"Carbonatite"