GEOSCAN, résultats de la recherche


TitreColumbite-tantalite series and pyrochlore supergroup minerals-potential indicators in exploration for specialty metal deposits
AuteurMackay, D A; Simandl, G J
SourceWhistler 2013: Geoscience for Discovery, abstracts; 2013.
Séries alt.Secteur des sciences de la Terre, Contribution externe 20120511
RéunionWhistler 2013: Geoscience for Discovery; Whistler; CA; Septembre 24-27, 2013
Documentpublication en série
Mediapapier; en ligne; numérique
ProgrammeÉtude des gîtes des métaux rares, Initiative géoscientifique ciblée (IGC-4)
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.