MOUNT BEGBIE PEGMATITE GROUP, BRITISH COLUMBIA, CANADA – EVOLUTION FROM BARREN TO COMPLEX RARE-ELEMENT PEGMATITES

The Mount Begbie pegmatite group, near Revelstoke, B.C., Canada, comprises more than 50 relatively small pegmatite bodies ranging from barren to beryl-columbite, beryl-columbite-phosphate, and lepidolite subtype pegmatites. Detailed study of tourmaline (dravite, schorl, fluor-elbaite, Mn-rich elbaite), cordierite-sekaninaite, garnet, rare-element silicates (beryl, chrysoberyl, bertrandite, euclase, trilithionite, Li-muscovite, petalite, pollucite), rare-element oxides (columbite-tantalite, bismutotantalite, Nb-rutile, cassiterite, hübnerite, qitianlingite), phosphates (triplite, lithiophilite, Mn-rich apatite, xenotime, monazite), and zircon provides insight into the mineralogy and geochemistry of the individual dikes. Fractionation within the pegmatite group increases from the southeast to the northwest, as evidenced by whole-rock data, composition of K-feldspar, micas, Nb,Ta,W-oxides and tourmaline.

Columbite-group minerals only occur in the more fractionated rare-element pegmatites; their compositional trends generally reflect the abundance of F in the pegmatite. Columbite shows relatively extensive solid solutions towards wolframite and rutile. Wolframite shows only limited miscibility with columbite. Rutile and cassiterite show a solid solution towards the Fe-analogue of heftetjernite. Qitianlingite is found in aggregates with columbite or hübnerite; its composition is usually close to the ideal end-member formula.

Tourmaline from the lepidolite-subtype pegmatites shows typical compositional trends from foitite, schorl in border and intermediate zones to fluor-schorl, fluor-elbaite and rossmanite in the pegmatite core. Tourmaline from one of the beryl-columbite-phosphate pegmatites exhibits an unusual compositional evolution from Ca-rich dravite, Mg-rich schorl and Mg-rich schorl-foitite in border and intermediate zones to schorl-foitite, (Fe,Mn)-elbaite, and Mn-rich fluor-elbaite in the pegmatite core.

The occurrence of Mn-rich elbaite in a beryl-columbite-phosphate pegmatite, elevated amounts of Be and Li in sekaninaite, and the relatively common presence of hübnerite and qitianlingite in the Mount Begbie pegmatite group, are rather unusual compared to rare-element pegmatite groups worldwide.