Fractionation trends of the Nb- and Ta-bearing oxide minerals in the Greer Lake pegmatitic granite and its pegmatite aureole, southeastern Manitoba

The Greer Lake pegmatitic granite and related exterior rare-element pegmatites of the beryl-columbite type intrude metabasalt and tonalite gneiss in the Archean English River Subprovince of southeastern Manitoba. Columbite-tantalite is the predominant Nb- and Ta-bearing mineral, associated with subordinate ixiolite, microlite, niobian-tantalian rutile, and rare tantalian cassiterite, wodginite, and ilmenite. In coexisting mineral pairs, Ta/(Ta + Nb) of microlite exceeds that of columbite-tantalite, ixiolite, rutile, and cassiterite; in cassiterite and rutile, Ta/(Ta + Nb) is higher and Mn/(Mn + Fe) lower than in columbite-tantalite and ixiolite.

In Li-, Rb-, Cs-, and F-poor environments, limited Mn enrichment accompanies the fractionation of Ta, which culminates in ixiolite and subordinate microlite. In Li-, Rb-, Cs-, and F-rich parageneses, extensive Mn enrichment precedes the main Ta fractionation, which subsequently generates near-end-member manganotantalite, wodginite, and microlite. The Greer Lake and other fractionation trends indicate that a late-stage F-rich environment promotes extreme Fe/Mn fractionation prior to the main stage of Nb-Ta separation. The abundances of Sn, Ti, and Sc are not related to fluorine or rare-alkali enrichment, but increase from pegmatitic granite to pegmatites. The relative accumulation of Ti and Sc in the pegmatite aureole seems to be due to internal fractionation rather than assimilation.

Columbite-tantalite in the pegmatitic granite shows an intermediate to near-ordered structural state, but is highly disordered in the pegmatite veins. Crystallization in the disordered state is suggested, with subsequent ordering in slowly cooling granite but thermal quenching in the exterior pegmatites. However, the increase in Ti, Sn, Sc, and Fe³⁺ in the Nb- and Ta-bearing minerals of the pegmatite aureole may also have retarded ordering.