| Abstract | Columbite-tantalite series and pyrochlore supergroup (betafite, microlite, and pyrochlore groups) minerals are the main constituents of Ta and Nb ores. They are present in carbonatites, pegmatites,
peralkaline intrusions, Li, REE, Be, Zr, W, Sn, and Cs-bearing deposits associated with granitoids, in saprolites overlying these deposits, in associated stream and lake sediments, and till. Based on Ta/ Nb+Ta) and Mn/(Mn+Fe) atomic ratios, members
of the columbite-tantalite series [(Fe,Mn)Nb O and (Fe,Mn)Ta O respectively; can be divided into tantalite-(Fe), columbite-(Fe), columbite- (Mn), and tantalite-(Mn). Li-Nb-Ta-REE-Sn enriched (specialty metal) granites are known to contain
predominantly columbite-(Fe) with Mn/(Mn+Fe) atomic ratio > 0.1, columbite- (Mn), and tantalite-(Mn). Pegmatite-related deposits are known to contain tapiolite [tetragonal equivalent of tantalite-(Fe)], all major varieties of tantalite, and Ta-rich
end members of columbite. There is a paucity of columbite analyses from carbonatite-hosted deposits; however, preliminary compilation suggests that they contain Nb-rich columbite-(Fe) with Mn/(Mn+Fe) atomic ratio <= 0.15. Pyrochlore supergroup
minerals are present in the same deposits as columbitetantalite minerals. Betafite [(Ca,U) (Ti,Nb,Ta) O (OH)] is common in pegmatites and present in some carbonatites. Microlite [(Na,Ca) Ta O (O,OH,F)] is found predominantly in rare element
pegmatites and in miarolitic cavities in granites. Pyrochlore [(Na,Ca) Nb O (OH,F)] is characteristic of carbonatites, but is also present in other alkaline rocks. Discrimination diagrams based on composition of columbitetantalite and/or pyrochlore
minerals may allow exploration geologists to relate indicator mineral chemistry to established specialty metal deposit models. For example, Nbrich pyrochlore and columbite-(Fe) with Mn/Mn+Fe atomic ratio <= 0.15 are expected in carbonatite-related
specialty metal deposits, while tapiolite, microlite, and Ta-rich betafite are characteristic of pegmatite-related mineralisation. |