448 



UNITED STATES MINERAL RESOURCES 



Lueshe, Congo Wambeke (1965). 



Sukulu, Uganda Davies (1956). 



Kaiserstiihl, Germany Wambeke and others (1964). 



Although some disagreement exists on the origin 

 of carbonatites, prevaihng opinion is that they are 

 intimately related to alkalic rocks including kimber- 

 lites and that they probably originated from material 

 derived at great depth, possibly within the mantle 

 (Heinrich, 1966). 



GRANITIC ROCKS 



Niobium and tantalum are present in most granitic 

 rocks in trace amounts in iron- and iron-titanium- 

 bearing minerals and to some extent in zirconium 

 minerals. A few rare granites contain columbite, 

 euxenite, fergusonite, or pyrochlore as accessory 

 minerals to the extent that economic concentrations 

 of these minerals may develop through the weather- 

 ing of the rocks either in residual deposits or in 

 placers. 



The Jos Plateau in central Nigeria is an important 

 producer of columbite which is obtained in part from 

 alluvial and eluvial concentrations from Precambrian 

 pegmatites, but mainly from alluvial deposits and 

 saprolite derived from biotite granites. These gran- 

 ites are part of an igneous complex of possible 

 Jurassic age (Black and Girod, 1970) known as the 

 Jos-Bokuru Younger Granite complex which occu- 

 pies an area of more than 100 square miles. The 

 complex includes gabbros, porphyries, rhyolites, bio- 

 tite granite bodies, and riebeckite granite (Mackay 

 and others, 1949). Several phases of the biotite 

 granite, all of which contain accessory columibite, 

 have been recognized. The Rayfield-Gona and Forum 

 phases are the richest and contain an average of 

 0.4 pound columbite per ton, and as much as several 

 pounds per ton are found in local concentrations 

 (Williams and others, 1956). Columbite is also an 

 accessory mineral in the Lunyo albite granite in 

 Uganda (Knorring, 1960) . Other accessory minerals 

 in this rock include magnetite, cassiterite, zircon, 

 thorogummite, fluorite, and pyrite. Columbite gran- 

 ites may not result entirely from magmatic processes 

 but may involve autometasomatic processes resem- 

 bling greisenization in which calcium is removed 

 from plagioclase and iron, titanium, niobium, and 

 tantalum are removed from biotite. These elements 

 are recombined to form fluorite, magnetite, and 

 columbite (Parker and Fleischer, 1968). 



Euxenite-bearing granitic rocks (quartz mon- 

 zonite or quartz diorite) in the Idaho batholith con- 

 tain from a trace to 0.05 pound of euxenite per cubic 

 yard and with their related pegmatites are the 

 apparent source of the euxenite placer deposits in 

 Bear Valley, Idaho (Mackin and Schmidt, 1956). A 



fergusonite granodiorite in the Nang-ling Range in 

 southern mainland China has been described by 

 Wang (1964) . From the English summary of Wang's 

 report (Lee, 1970, p. 22), the fergusonite is most 

 abundant in the marginal zone of the granodiorite. 

 Fergusonite granite has also been found in the basin 

 of the Gava Say River in the U.S.S.R. (Parker and 

 Fleischer, 1968). 



Pyrochlore granites are highly alkalic rocks com- 

 posed principally of microperthite, sodic plagioclase, 

 quartz, sodic amphibole, and sodic pyroxene. Pyro- 

 chlore is present in the Mount Rosa Granite, a 

 riebeckite granite in the Pikes Peak area, Colorado 

 (Gross and Heinrich, 1965), and in a riebeckite 

 granite phase of the Jos-Bokuru complex in Nigeria. 

 The pyrochlore content of the Nigerian granite from 

 one occurrence was estimated to be about 0.7 per- 

 cent (Mackay and others, 1949, p. 79). 



PEGMATITES 



Pegmatites are a type of igneous rock generally 

 considered to represent the crystalline product of 

 residual magmatic fluids and as such may contain 

 concentrations of a number of rare elements whose 

 properties inhibited their entry into minerals of 

 earlier formed rocks. Pegmatites are commonly of 

 granitic composition, but pegmatitic phases of other 

 rocks, such as syenites, are found in some districts. 

 A characteristic of pegmatites is the large but 

 extremely variable grain size of the constituent 

 minerals. Some pegmatites contain roughly con- 

 centric layers or zones of contrasting mineral assem- 

 blages, while others are unzoned wdth an essentially 

 random distribution of minerals. 



Pegmatites were for many years the major source 

 of niobium and tantalum, and whereas niobium is 

 now obtained largely from carbonatites, pegmatites 

 still remain the dominant primary source of tanta- 

 lum. These elements may be present in pegmatites, 

 either in one or more of a large number of inde- 

 pendent minerals of which columbite-tantalite is the 

 most common or in the structure of such carrier 

 minerals as micas, garnets, tourmaline, ilmenite, or 

 zircon. Certain types of pegmatites seem to be more 

 intensely mineralized with niobium and tantalum 

 than others. These are commonly the complexly 

 zoned pegmatites characterized by abundant albite 

 and a strong development of lithium and beryllium 

 minerals. 



The tendency for pegmatite minerals to occur in 

 large crystals and to be localized in distinct zones 

 has favored the recovery of niobium-tantalum min- 

 erals from deposits that would otherwise be unmin- 

 able. Pegmatites in tropical areas may also be deeply 



