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Rare-element-enriched granitic pegmatites in and near the Lamotte and Lacome plutons of the Archean Preissac-Lacorne batholith (Quebec) are zoned from a least fractionated beryl type in the parental pluton through a transitional spodumene + beryl type at the edge of the plutons to a most fractionated spodumene type in the country rocks. Columbite-tantalite is the only primary niobium tantalum oxide mineral; its composition is dominated by Nb, Ta, Fe, and Mn. Ti is present in minor quantities (≤ 0.1 atoms per formula unit). In both plutons, the Ta/(Ta + Nb) and Mn/(Mn + Fe) of columbite-tantalite correlate positively with the degree of pegmatite evolution. These correlations are interpreted to reflect the greater solubility of Mn relative to Fe and of Ta relative to Nb in the melt. In the Lacorne suite, there is a much greater increase in Mn/(Mn + Fe) over Ta/(Ta + Nb) of columbite-tantalite with pegmatite evolution. In pegmatite bodies in the Lacorne suite, where columbite-tantalite crystallization was accompanied by garnet, the Ta/(Ta + Nb) of columbite-tantalite increases at constant Mn/(Mn + Fe) values. We interpret this to indicate that garnet saturation buffered the activities of Fe and Mn in the liquid. The degree of order of columbite-tantalite decreases with the evolution of the pegmatite suites and reflects increased cooling rates of the more evolved, and more distal, bodies of pegmatite.

american mineralogist

Compositional variation and structural state of columbite-tantalite in rare-element granitic pegmatites of the Preissac-Lacorne batholith, Quebec, Canada