Carbonatites


Also found in: Dictionary.

Carbonatites

 

rocks of magmatic or metasomatic origin, composed primarily of carbonates (calcite, dolomite, ankerite) and spatially related to complexes of ultrabasic-alkalic composition. The term “carbonatite” was introduced by the Norwegian petrographer W. Brûgger (1921). He also proposed that calcite carbonatites be called sövites, dolomite carbonatites be called rauhaugites, biotite-dolomite veined carbonatites be called befor-sites, and red-colored carbonatites (in which the carbonate is partially replaced by iron oxides, primarily hematite) be called redbergites.

The complexes of ultrabasic-alkalic rock in which carbonatites are found are generally located along major faults on the platforms. They may be “blind,” that is, not reach the earth’s surface, or “open,” reaching the surface in the form of volcanoes that erupt carbonatite lava (the Oldoinyo Lengai volcano in Tanzania). Geophysical data show that the complexes are dozens of kilometers deep. Carbonatites make up the central parts of the complexes, forming stocks and chimney deposits measuring from 0.1 to 15–20 km and more in area as well as irregularly shaped deposits, branching zones, stockworks, and circular, conical, and radial dikes. In open-type complexes they fill volcanic vents, frequently cementing brecciated volcanic rocks. Where carbonatites develop on ultrabasites and ijolites, in some complexes there occur forsterite-apatite-magnetite rocks with small amounts of calcite (phoscorites, kamaphorites); these rocks are sometimes high-quality magnetite ores (for example, Kovdor on the Kola Peninsula in the USSR) or are rich in apatite (the Palabora Massif in the Republic of South Africa). When carbonatites develop on nepheline syenites, aureoles of albitites with tantalum-niobium mineralization often form.

Carbonatites are multistage formations that form in the temperature interval between 600° and 300°C. Early-stage carbonatites consist of calcite, diopside or forsterite, biotite or phlogopite, apatite, and magnetite. They are enriched with Ti, Zr, Ta, Nb, and U.

Carbonatites of the late stages consist of 80–95 percent dolomite or ankerite and calcite, and, more rarely, siderite and stron-tianite. They contain alkaline amphiboles, serpentine, ferroferriphlogopite, aegirite, chlorite, and epidote. Sulfides typically appear (pyrite, pyrrhotite), as well as fluorite, barite, magnetite, rutile, pyrochlore, lueshite, columbite, fersmite, bur-bankite, bastnaesite, parisite, carbocernaite, and ancylite. Carbonatites are characterized by high concentrations of Sr, Ba, F, Nb, Ce, Th, Pb, Zn, and Mo.

Carbonatites and their associated rocks are an important form of mineral deposit. Related to them are large deposits of phlogopite and vermiculite (Kovdor and Gulinskoe in the USSR), iron (Kovdor in the USSR, Palabora in the Republic of South Africa), phosphorus (Palabora in the Republic of South Africa, Sukulu in Uganda), and rich deposits of niobium ore (Araxá in Brazil, Lueshe in Zaïre, Oka in Canada), well as deposits of tantalum (Nqombwa, Zambia), zirconium (Palabora, Republic of South Africa), rare earths (Mrima, Kenya), copper (Palabora, Republic of South Africa), fluorite (Tagna, USSR), and raw materials for cement and lime (Tororo and Sukulu in Uganda). In addition, it is possible to extract barite and strontianite from some of the deposits. Under conditions of hypergenesis a weathering mantle develops on carbonatites; the content of useful components (apatite, pyrochlore, bastnaesite) in it is three to five times greater than in bedrock.

REFERENCES

Ginzburg, A. I. [et al.] “Redkometal’nye karbonatity.” In Geologiia mestorozhdenii redkikh elementov, fasc. 1. Moscow, 1958.
Ginzburg, A. I., and E. M. Epshtein. “Karbonatitovye mestorozhdeniia.” In Genezis endogennykh rudnykh mestorozhdenii. Moscow, 1968.
Smirnov, V. I. Geologiia poleznykh iskopaemykh, 2nd ed. Moscow, 1969.
Karbonatity. Edited by O. Tuttle and J. Gittins. Moscow, 1969. (Translated from English.)
Heinrich, E. W. The Geology of Carbonatites. Chicago, 1966.

A. I. GINZBURG

References in periodicals archive ?
This project will make a step-change in exploration models for alkaline and carbonatite provinces, using mineralogy, petrology, and geochemistry, and state-of-the-art interpretation of high resolution geophysics and downhole measurement tools, to make robust predictions about mineral prospectivity at depth.
The Tajno massif is composed of alkaline rocks ranging from syenite to pyroxenite, cut by carbonatite veins (Krystkiewicz & Krzeminski 1992; Ryka 1992; Ryka et al.
However, comparisons between Ti-clinohumite from the Cienaga Marbles and that found in JAC carbonatites, xenoliths in kimberlites, RGP Alpine peridotites and Archean ultramafics may carry out as shown in Figure 11, highlighting that the Ti-clinohumite analyzed in this study represents not only an almost pure in Mg but also may be considered as the most Mg-rich and TiO2-rich varieties around the world for skarn environments.
They also provide a good explanation of where REEs tend to be located, from iron and carbonatite deposits to lateritic, placer, peralkaline, vein, and other deposits.
Carbonatites are mantle derived intrusions and show often a spatial relationship with kimberlites, host to diamond deposits.
Another player that has received significant attention since its listing last year is GeoMegA Resources, which has just released its first 43-101 resource calculation at the Montviel carbonatite property, with 184 million mt at 1.
The lavas of Oldoinyo Lengai volcano are comprised of carbonatites, which erupts as a liquid at approximately 540 degrees C.
It is also found in a variety of exotic settings such as kimberlites (Malkov 1974) and carbonatites (Lee et al.
1988) The Blackburn carbonatites near Ottawa, Ontario, dykes with fluidized emplacement.
1979: Classification y nomenclature of volcanic rocks, lamprophyres, carbonatites, and melilitic rocks: recommendations and suggestions of the IUGS Subcommission on the Systematics of Igneous Rocks.
SEQUENTIAL ALTERATION OF LARGE PYRRHOTITE CRYSTALS TO MARCASITE, SKELETAL PYRITE, AND GOETHITE IN PEGMATITIC CARBONATITES AND FLUORITE ORES AT OKORUSU, NORTH CENTRAL NAMIBIA.
Natural carbonatites have melting points as low as 500 [degrees] C, barely warmer than Venus's surface, and viscosities hardly greater than that of water.