NIOBIUM (COLUMBIUM) AND TANTALUM 



447 



minerals of the complex in addition to loparite and 

 are also present in minor amounts in many more. 



Another important area of niobium mineraliza- 

 tion is the Ilimaussaq complex in southwest Green- 

 land where agpaitic nepheline syenites are overlain 

 by lavas, gabbros, anorthosites, and augite syenite. 

 All the rocks contain niobium minerals, chiefly pyro- 

 chlore and members of the epistolite-murmanite 

 group, in shear zones and hydrothermal veins. The 

 agpaitic rocks contain 500-1,500 ppm NbaOs and 

 15-60 ppm TasOs. The niobium content of the rocks 

 of the complex increases from the preagpaitic rocks 

 to the agpaitic rocks and further to the late hydro- 

 thermal veins (Hansen, 1968). 



Alkalic complexes dominated by agpaitic rocks are 

 found at various other places in the world, including 

 the United States, but so far as is known, only those 

 in the Kola Peninsula, U.S.S.R., have been economic- 

 ally exploited for niobium. Economic deposits are 

 chiefly in the more numerous complexes of domi- 

 nantly miaskitic rocks which are more favorable for 

 the development of carbonatite bodies and veins. 

 Although there are some exceptions (Sorensen, 

 1960), the miaskitic types are characterized by far 

 less of the numerous zirconium-titanium minerals 

 in which much of the niobium and tantalum in the 

 agpaites is dispersed and are more apt to contain 

 the niobium and tantalum in richer and more eco- 

 nomically interesting species, notably pyrochlore, in 

 minable concentrations in their related carbonatites. 



Nepheline syenite bodies containing about 200 

 ppm niobium are the source rocks of the bauxite 

 deposits of Pulaski and Saline Counties, Ark. Inves- 

 tigation of the trace-element content of the bauxite 

 showed that a significant enrichment in niobium had 

 taken place, the bauxite containing about 2.5 times 

 the amount of niobium present in the parent rock 

 (Gordon and Murata, 1952; Fleischer and others, 

 1952). Niobium in the syenite appears to be largely 

 contained in accessory sphene which during bauxiti- 

 zation was altered to various other titanium min- 

 erals, chiefly ilmenite, which then became the prin- 

 cipal carriers of niobium in the bauxite. The niobium 

 enrichment parallels that of alumina, suggesting 

 that the concentration was entirely residual. 



CARBONATITES 



Carbonatite bodies contain the most extensive 

 known world resources of niobium. They commonly 

 are found in the interior parts of alkalic intrusive 

 complexes which contain a wide variety of rock types 

 ranging from nepheline syenite to alkalic gabbro 

 and pyroxenite. Many of these complexes are circu- 



lar in plan (though some are not) and the consti- 

 tuting rocks including the carbonatites commonly 

 are arranged in arcuate, radial, or irregular dis- 

 cordant masses within the complexes. Some car- 

 bonatites are found intruding unrelated country 

 rocks, but most of the occurrences can be associated 

 with nearby alkalic rocks. 



Carbonatites and associated alkalic rocks range 

 widely in age from Precambrian to Holocene and 

 are npt confined to any particular geologic period. 

 Some complexes are linked to volcanic activity at the 

 earth's surface, and actual carbonatite lavas are 

 recorded at Oldoinyo Lengai in Tanzania (Tangan- 

 yika). Others not obviously related to any known 

 extrusive rocks represent complexes exposed at 

 deeper levels in the crust by a combination of ero- 

 sion and geologic structure. 



Carbonatites and related alkalic rock complexes 

 are for the most part restricted to the stable parts 

 of the continent. This distribution is true for Africa, 

 South America, North America, and Eurasia. Some 

 complexes tend to cluster along major crustal breaks 

 such as the African rifts, whereas some show little 

 relation to identifiable structures. 



Carbonatites have been intensively explored over 

 the world primarily in the search for niobium, and 

 considerable information is now available on their 

 nature and composition. Many carbonatites have 

 been shown to contain deposits of one or more of 

 the following metals or mineral commodities: Nio- 

 bium, rare earths, copper, nickel, titanium, thorium, 

 uranium, vermiculite, fluorite, zirconium, barite, 

 apatite, and strontium. The carbonatite itself ranges 

 from iron- and magnesium-bearing carbonates to 

 nearly pure calcite, and some of this material is 

 marketed for agricultural and other purposes. Some 

 carbonatites conceivably could be exploited profit- 

 ably for the collective recovery of more than one 

 commodity, whereas on an individual basis the com- 

 modities might be marginal or submarginal eco- 

 nomically. 



Explored niobium deposits in carbonatites con- 

 tain immense reserves of niobium. The grades range 

 from below 0.2 percent NbgOs to more than several 

 percent. Some of the largest deposits exceed 8 mil- 

 lion tons of NbaO^ which occurs in the carbonatites 

 chiefly as the mineral pyrochlore. 



Some representative niobium-bearing carbonatites 

 in various parts of the world are listed below : 



Iron Hill, Colo. Temple and Grogan (1965). 



Gem Park, Colo. Parker and Sharp (1970). 



Oka, Quebec Gold (1966). 



Axara, Brazil de Souza and Castro (1968). 



Fen, Norway Saether (1957). 



