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The Nature and Distribution of Tantalum-bearing Minerals in Newly Discovered, Rare-Element Pegmatites at the Musselwhite Mine, Northwestern Ontario

Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada, K1S 5B6

ROD HENHAM

Placer Dome (CLA) Limited, Musselwhite Mine, Ontario, Canada

Newly discovered pegmatites distributed throughout the sub-surface environs of the Musselwhite Archean gold deposit occur as narrow dikes that intrude most of the major lithological units. The mineralogy and textures of the pegmatite dikes are characteristic of granitic pegmatites of the "Rare-Element Class"; more specifically, they closely resemble varieties of the "Complex-type" granitic pegmatites that are commonly associated with zones of tantalum mineralization. The essential minerals in the dikes comprise albite, quartz, and K-feldspar, together with lesser amounts of mica (including lepidolite), spessartine, tourmaline, and locally, spodumene. Fine-grained (30 µm to 300 µm), tantalum-bearing minerals occur in the quartzo-feldspathic matrix of the dikes as disseminations, in interstices, and in small vugs. The nature of the tantalum-bearing mineral assemblage present in individual pegmatite dikes varies from a simple mineral assemblage composed of ferrocolumbite (with around 39 wt% Ta₂O₅) in the most northerly dike, to more complex assemblages comprised of manganotantalite (with up to 75 wt% Ta₂O₅), ferrotapiolite (with up to 79 wt% Ta₂O₅), and microlite (with up to 76 wt% Ta₂O₅) that are present in dikes located several kilometers to the south and southwest. The compositional characteristics and textural relationships of the tantalum-bearing minerals in the dikes provide evidence of igneous fractionation trends that are typical of rare-element granitic pegmatites. Such trends suggest that the newly discovered pegmatites represent part of a more extensive group of cogenetic, tantalum-mineralized dikes.