|Résumé||(disponible en anglais seulement)|
Minerals belonging to the columbite [(Fe, Mn)Nb2O6]-tantalite [(Fe, Mn)Ta2O6] solid solution series (with densities of 5.2 and 8.0 g/cm3, respectively) are an
important source of Ta and Nb, and ferroniobium. They are also commonly present in other specialty metal-bearing deposits containing Li, REE, Be, Zr, W, Sn, and Cs that are associated with granitoids, pegmatites, carbonatites, and peralkaline
intrusions. By extension, they are also present in saprolites overlying these deposits, in associated stream and lake sediments, and till. In combination with pyrochlore supergroup minerals (pyrochlore, microlite, and betafite groups), they have the
potential to be used as indicator minerals. Compositionally, following classification of Cerný and Ercit (1989), based on Ta/(Nb+Ta) and Mn/(Mn+Fe) atomic ratios, members of the columbite-tantalite series can be divided into ferrotantalite
[tantalite-(Fe)], ferrocolumbite [columbite-(Fe)], manganocolumbite [columbite-(Mn)] and manganotantalite [tantalite-(Mn)]. Structurally, most of these mineral species are orthorhombic; however, the Ta-rich tantalite-(Fe) end member of the
quadrilateral is tetragonal and commonly referred to as tapiolite. Li-F-Rb-Cs-Be-Nb-Ta-REE-Sn-U-Th-Zr enriched granites are known to contain predominantly ferrocolumbite with Mn/(Mn + Fe) atomic ratio > 0.1, manganocolumbite and manganotantalite.
Pegmatites and related deposits are known to contain tapiolite, 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 predominantly Nb-rich ferrocolumbite with Mn/(Mn + Fe) atomic ratio =0.1. Mineral groups belonging to the pyrochlore supergroup (betafite, microlite and pyrochlore) as defined by Atencio et al. (2010) are commonly
associated with the same deposits as columbite-tantalite series minerals and have densities that are expected to be in the range of 3.7-6.4 g/cm3. Betafite group minerals [(Ca,U)2(Ti,Nb,Ta)2O6(OH)] are common in granite pegmatites and present in some
carbonatites; microlite group minerals [(Na,Ca)2Ta2O6(O,OH,F)] are found predominantly in rare metal pegmatites and in miarolitic cavities in granites; and pyrochlore group minerals [(Na,Ca)2Nb2O6(OH,F)] are characteristic of carbonatites, but are
also present in other alkaline rocks.
Development of discrimination diagrams based on composition of minerals belonging to the columbite-tantalite series and/or pyrochlore supergroup may allow exploration geologists to better relate mineral
chemistry of these minerals to established ore deposit models. For example, Nb-rich pyrochlore and ferrocolumbite with Mn/Mn+Fe atomic ratio < 0.1 are expected in carbonatite-related specialty metal deposits, while tapiolite, microlite and Ta-rich
betafite are characteristic of pegmatite-related mineralization. Weathering is known to affect chemical composition of minerals belonging to the columbite-tantalite series and pyrochlore supergroup. It should be taken into consideration during the
development and interpretation of discrimination diagrams.