The economic geology of columbium (niobium) and of tantalum

With the development of carbonatite deposits, the resources of niobium have been increased to rank in importance with the major metals. Tantalum, the valuable twin element, still however, remains scarce. The two metals have a remarkable range of occurrence; carbonatites, placers, pegmatite, granites, basic alkaline rocks. In most of these occurrences the twin metals are part of a paragenesis that includes RE, Zr, Th, Sn (though only as a trace in carbonatites); W, Li, Be (only in pegmatites); Ti, Fe, P (not in granites), Ba, Sr, Mn, Fe, Mg. Niobium and tantalum are already disseminated in the earliest magmatic stages. The main concentrations of the earth acids are related to late processes of differentiation and replacement. However, the most valuable concentrations occur in products of alteration. The limit of tropical weathering often divides economic deposits from mere occurrences. The structural and zoneographic control of the mineralization is distinct. The majority of niobium and tantalum deposits are restricted to specific belts in Africa and South Amemrica. (auth)