Carbonatite

The element is found in niobite (or columbite), niobite-tantalite, parochlore, and euxenite. Large deposits of niobium have been found associated with carbonatites (carbon-silicate rocks), as a constituent of parochlore. Extensive ore reserves are found in Canada, Brazil, Nigeria, Zaire, and in Russia. 

Quartz sand IAEA-NBS127 Barium sulphate Carbonatite... 

Occurrence. Niobium and tantalum usually occur together. Niobium never occurs as the metal, ie, ia the free state. Sometimes it occurs as a hydroxide, siUcate, or borate most often it is combiaed with oxygen and another metal, forming a niobate or tantalate ia which the niobium and tantalum isomorphously replace one another with Htde change ia physical properties except density. Ore concentrations of niobium usually occur as carbonatites and are associated with tantalum ia pegmatites and alluvial deposits. Principal niobium-beariag minerals can be divided iato two groups, the titano- and tantalo-niobates. 

Intrusive Deposits. Deposits included in the intmsive deposit type are those associated with intmsive or anatectic rocks of different chemical composition, eg, alaskite, granite, monzonite, peralkaline syenite, carbonatite, and pegmatite. Examples include the uranium occurrences in the porphyry copper deposits such as Bingham Canyon and Twin Butte in the United States, the Rossing Deposit in Namibia, and Ilimaussaq deposit in Greenland, Palabora in South Africa, and the deposits in the Bancroft area, Canada (15). 

The Phalaborwa complex ia the northeastern Transvaal is a complex volcanic orebody. Different sections are mined to recover magnetite, apatite, a copper concentrate, vermicuhte, and baddeleyite, Hsted in order of aimual quantities mined. The baddeleyite is contained in the foskorite ore zone at a zirconium oxide concentration of 0.2%, and at a lesser concentration in the carbonatite orebody. Although baddeleyite is recovered from the process tailings to meet market demand, the maximum output could be limited by the requirements for the magnetite and apatite. The baddeleyite concentrate contains ca 96% zirconium oxide with a hafnium content of 2% Hf/Zr + Hf. A comminuted, chemically beneficiated concentrate containing ca 99% zirconium oxide is produced also. 

The comprehensive studies of rare-metal - rare-earth minerals of carbonatit associations were carried out. Some rarest mineral species were investigated. Two minerals were attested by the Commission on new minerals of International Mineralogical Association as new. 

Figure 6. Schematic representation of the model used by Williams et al. (1986) to calculate the age of the Oldoinyo Lengai (Tanzania) caibonatite magma. The model assumes an instantaneous Ra-Th fractionation produced by the exsolution of a carbonatite melt from a nephelinite parental magma in radioactive equilibrium for both Ra-Th pairs. The existence of Ra- Th disequihbria indicates that the fractionation occurred shortly before eruption, and thus the ( Tla/ °Th) ratios have not significantly changed since the exsolution. By assuming the same Ra-Th fractionation for both pairs, the ( Ra/ °Th) in the carbonatite gives the ( Ra/ h) ratio just after the exsolution, and its age can then be calculated from the equation ...

Alley RB, Mayewski PA, Sowers T, Stuiver M, Taylor KC, Clark PU (1997) Holocene climatic instability A prominent, widespread event 8200 yr ago. Geology 25 483-486 Amelin Y, Zaitsev AN (2002) Precise geochronology of phoscorites and carbonatites The critical role of U-series disequilibrium in age interpretations. Geochim Cosmochim Acta 66 2399-2419 Amundson RG, Chadwick OA, Sowers JM, Doner HE (1988) Relationship between climate and vegetation and the stable carbon isotope chemistry of soils in the eastern Mojave Desert, Nevada. Ouat Res 29 245-254... 

B. Residual liquid injection Same as above, followed by injection Most pegmatite and carbonatite veins... 

Intrusive-type (carbonatite, pegmatite, quartz-monazite, etc.)... 

A. J. G. Notholt, D. E. Higley and T. Deans, Economic Minerals in Carbonatites and Associated Alkaline Igneous Ores, Trans. Inst. Min. Metall. Sect. B, vol. 99, p. B59-B80,1990. 

Carbonatites have been defined as rare-metal-bearing rocks composed mainly of carbonate of mantle origin, formed in close association with alkaline rocks (Woolley Kempe 1989). However, there... 

Compared to typical values from marine limestone, the 5f80 values of Group I are slightly lower, while both 513C and 51sO of Group I are far from those usually associated with carbonatite (Bowman 1998). 

Trace elements and rare-earth elements (REEs) of the same calcite samples used for the stable isotope analysis have significantly lower concentration of REE as well as most trace elements relative to typical carbonatites. The total REE contents of the Ulsan carbonates range from 3 to 17 ppm, which are much lower than any igneous rocks and even lower than those of some sedimentary rocks. REE and trace-element abundances may have changed sufficiently due to alteration, thus, affecting petrogenetic... 

The REE data suggest that these carbonate rocks cannot be carbonatites of mantle origin. However, it is noteworthy that the range of compositional variations of typical carbonatite family rocks overlaps with those of sedimentary, metamorphic, and endogenic carbonate rocks (Samoilov 1991). Samoilov (1991) explained that one reason for the significant compositional variations of the carbonatites were due to different types of the associated alkaline rocks under different geologic-tectonic settings. 

Geochemistry of oceanic carbonatites compared with continental carbonatites mantle recycling of oceanic crustal carbonate. Contributions to Mineralogy and Petrology, 142, 520-542. 

Lentz, D.R. 1998. Late-tectonic U-Th-Mo-REE skarn and carbonatitic vein-dyke systems in the southwestern Grenville Province a Pegmatite-Related Pneumatolytic Model linked to Marble Melting (limestone syntexis). In Lentz D.R. (ed) Mineralized intrusion-related skarn systems, 519-657, Mineralogical Association of Canada Short Course 26. 

Lentz, D.R. 1999. Carbonatite genesis A reexamination of the role of intrusion-related pneumatolytic skarn processes in limestone melting. Geology, 27, 335-38. 

Ray, J., Ramesh, R. Pande K. 2000. Carbon isotopes in Kerguelen plume-derived carbonatites evidence for recycled... 

Samoilov, V.S. 1991. The main geochemical features of carbonatites. Journal of geochemical Exploration, 40, 251-262. 

There are two major types of pyrochlore-containing ores pegmatite ores and carbonatites. This classification is based on the mineral composition of these ore types. The main waste minerals contained in the pegmatite ores include quartz and nepheline. This ore type also includes granites, where pyrochlore is represented in a coarse crystalline form. Granites are composed of cryolite and topaz as the main gangue minerals. 

Carbonatite ores are mainly composed of calcite, dolomite and phosphates as the main gangue minerals. The beneficiation process for pegmatites containing pyrochlore mostly includes gravity preconcentration. Such deposits are common in Africa (Kongo, Madagascar). 

The major minerals contained in pyrochlore-containing ores are pyrochlore, columbite and sometimes ilmenorutile to a lesser extent. Table 22.1 shows pyrochlore minerals present in pegmatite and carbonatite ores. 

The gangue composition of the various carbonatite ores varies considerably. Calcite-dolomite content in some ores ranges from 30% (Niobec, Canada) up to 70% (Panda Hills, Africa). From a mineralogical point of view, pyrochlore usually occurs in crystallized form, as well as octahedron form. Pyrochlore occurs in considerable range of colours, varying from translucent white to opaque black appearance with glassy surfaces. The Nb205... 

Pyrochlore minerals contained in pegmatite and carbonatite ores... 

The treatment process and flotation properties of pyrochlore are very much dependent on the gangue composition of the ore. The selective flotation of pyrochlore from carbonatite ore is not possible since calcite and dolomite have similar flotation properties as pyrochlore. In addition, in the presence of carbonates, the stable pH required for flotation of pyrochlore (i.e. 5.0-5.5) cannot be maintained. 

In the case of carbonatite ores, a beneficiation process involves preflotation followed by reactivation and flotation of pyrochlore. In the case of pegmatitic ores that contain silicates, biotite, albite and limonite, as the gangue minerals, direct flotation of pyrochlore can be achieved with a variety of different collectors. 

The successful flotation of pyrochlore from carbonatite ores depends on a number of factors ... 

Figure 22.1 Effect of pH on calcite-dolomite recovery from carbonatite ores.

Extensive studies have been carried out using orthodihydroxybenzene, known as catechol (commercial name). This reagent has improved the rate of fine pyrochlore flotation and also has a beneficial effect on selectivity. Research work with this reagent was conducted on carbonatite ore from Canada. 

---