Geology and mineralogy of niobium deposits

Niobium is element no. 33 in the list of abundance in the crust of the Earth; it is less abundant than zinc, nickel, copper or tungsten but more abundant than cobalt, molybdenum or tantalum. Niobium makes 24 ppm of the crust but only 5 ppm of the whole Earth; it is enriched in the mantle and the crust but depleted in the core. Pyrochlore is the most abundant mineral phase containing niobium: it corresponds to […] where A is principally Na+, Ca2+ Ba2+ and B is principally Nb5+. The most important commercial pyrochlores are bariopyrochlore from residually enriched carbonatite at Araxá Brazil (Nb2O5 66, BaO 14 and H2O 8% and pyrochlore from primary carbonatite at St-Honoré, Quebec, Canada (Nb205 68, CaO 14 and Na2O 5.8%). Columbite corresponds to Fe Nb2O6; it is derived by weathering of granites and concentration in placer deposits (Nigeria); Nigerian columbites contain 60 percent Nb2O5, 6.5 percent Ta2O5 and 15 percent FeO. Two different mineral species with a perovskite structure are known to contain niobium as a principal constitutent: lueshite NaNbO3 and latrappite (Ca,Na) (Nb,Ti,Fe)O2. Several other multiple oxide minerals contain Nb as principal element: fergusonite (RE3+) NbO4, euxinite Y(Nb,Ti) 2O6, aeschynite La(Nb,Ti) 206 and stibiocolumbite Sb NbO4 occur in granites principally. Three silicate minerals are known to contain principal quantities of niobium: niocalite Ca7Nb Si4O16(O,Oh,F), nenadkevichite Na2NbSi2O6 (OH).2H2O and niobophyllite K2Fe6Nb2Si8 (.O,OH,F) 38’. In all its minerals, niobium occurs in octahedral coordination with O2- and Nb-O distances vary from 1.83 to 2.15 A; thus rNb5+/-0.58 A. Carbonatites (magmatic rocks containing more than 50% carbonate minerals) are the principal rocks in which niobium deposits are formed. Three types are common: sovite (calcite rich), rauhaugite or dolomitite (dolimite rich) and ferro carbonatite (siderite, ankerite or ferran dolomite rich). Soda carbonatire containing nyererite Na2Ca(CO3)2 and thermonatrite Na2CO3H2O has been observed to issue forth as lava from modern volcanoes (e.g. Oldoinyo Lengai, Tanzania). Silicate rocks are frequently associated with carbonatites; they are in general alkaline and contain 11 to 14 percent (Na2O+K2O). Principal intrusive ones are nepheline syenite, ijolite, urtite, while their extrusive equivalents are trachyte, phonolite and nephelinite. Some ultramafic varieties are also common: dunite, peridotite and pyroxenite. Carbonatites favor three assemblages: carbonatite-nephelinite, basalt-nephelinite and ultramafite-carbonatite. Carbonatites occur along rift zones in the crust; these rift zones are located on tectonic lineaments in the Precambrian cratons reactivated at various times since late Precambrian. Mesozoic and Cenozoic carbonatites are the best documented geologically. Two types of niobium ore deposits are known: Primary: the host is carbonatite, the ore mineral is principally pyrochlore and the grade approximately 0.5 to 0.7 percent Nb2O5. The Niobec deposit (St-Honoré Quebec, Canada) and the Oka deposits (near Montreal, Quebec, Canada) are of this type. Enriched by leaching: This type is probably derived from the former by the leaching out of carbonate minerals. The ore, thus, contains iron oxides, phosphate minerals, some barite, some quartz and bariopyrochlore: ore grade may be 2 to 10 times the grade of the primary carbonatite. The Araxá and Catalao deposits of Brazil are of this type. Whereas columbite concentrates from placer deposits in Nigeria were important between 1950 and 1970, they now account for only 4 percent of the total niobium production. Ore reserves at the various producing mines are considerable, e.g. 460 MT at 2.5 percent Nb2O5 at Araxá, Brazil and 16 MT at 0.68 percent Nb2O5 at St-Honoré, Quebec, Canada. Niobium resources in various manners of definition are also reported from many areas: Brazil (Tapira), Africa (Lueshe, Panda Hill, Chilwa Island, Kalubwe), Canada (James Bay, Nemegosenda, Lackner Lake, Manitou Islands, Oka.). Proven reserves and indicated resources are sufficient to support columbium utilization at a level increased by one order of magnitude for the foreseeable future. Niobium analyses in ores, concentrates and metallurgical processes are best done by X-ray fluorescence techniques. Precision attained is readily […] at the 1 percent N2O5. However, recent interlaboratory comparisons show much broader variations and indicate that accuracy in N2O5 measurements is still difficult to attain. (AU)