In comparison to other commodities, e.g. gold, niobium-ores are related to a small number of igneous rocks, in particular carbonatites and genetically-related silicate-carbonate rocks.
Within these rocks niobium is segregated principally in oxide minerals such as the pyrochlore-microlite solid solution series (Ca,Na)2(Nb,Ta)2O6(O,OH,F) and niobian perovskite (Ca,Na)(Ti,Nb)O3. At least four other Nb minerals can occur in sufficient quantities to dominate or affect the niobium content of a carbonatite. They include ferrocolumbite FeNb2O6, fersmite (Ca,REE,Na)(Nb,Ta,Ti)2(O,OH,F)6, niocalite Ca4NbSi2O10(O,F), and wöhlerite NaCa2(Zr,Nb)Si2O8(O,OH,F). Pyrochlore is the dominant niobium host and the only mineral that is mined for Nb in carbonatite rocks.
The econonomically important niobium deposits are related to carbonatite complexes and their lateritic horizons. Araxá (Minas Gerais State, Brazil) is the world largest deposit of pyrochlore. The second largest deposit is Catalão (Goiás State, Brazil). In both deposits, pyrochlore is mined in open pits. The third largest deposit is St. Honoré (Canada), which contains both pyrochlore and columbite.
Although more than 400 carbonatite occurrences are known worldwide, not all of the complexes carry pyrochlore or are rich in niobium content. There are still outstanding problems in understanding niobium mineralization in carbonatites including the determination of factors controlling the crystallization of pyrochlore or Nb-bearing perovskite-like minerals e.g. lueshite (NaNbO3). Fluorine seems to be present to stabilize pyrochlore as a primary phase. It is also concluded that pyrochlore crystallization is associated with the early stages of evolution of F-bearing carbonatite magmas. In most carbonatites with niobium mineralization pyrochlore is the major component rather than a perovskite structured phase. Early precipitaion of pyrochlore indicates that in natural systems pyrochlore can be concentrated by differentiation processes. Thus, economic deposits of pyrochlore might result from the rheological or gravitational concentration of this mineral.
In this study, pyrochlore rich, as well as niobium poor carbonatite samples from different carbonatite complexes are petrographically described and geochemically analysed to gain information about the above mentioned possible factors controlling the formation of pyrochlore minerals. Further, it will be discussed if associated elemental distributions or elemental ratios can be used as tracer for the indication of pyrochlore enrichment.
Electron microprobe analyses of pyrochlore are performed to show similarities and differences between the different carbonatite samples and to give information about the elemental distribution, especially thorium and uranium. The content of those elements has implications for the import/ export and processing of ore.
Microthermometric fluid inclusion analyses on apatite in a pyrochlore containing carbonatite and one carbonatite without pyrochlore are done to give information on the fluids participating in pyrochlore formation as well as to give first ideas of minimum crystallisation temperatures.
This study was performed on behalf of the German Federal Institute for Geosciences and Natural Resources (BGR).
Warnsloh, J.M. and Meyer, F.M. (2009) Minerals of the pyrochlore group as niobium ore : final report.
Berichte zur Lagerstätten- und Rohstoffforschung, Vol. 60, pp131, BGR Hannover.