The Moose II rare-metal granitic pegmatite is located approximately 115 km east of Yellowknife, NWT, on the north shore of Great Slave Lake. The irregularly zoned dike is -430 m long by 61 m wide, and is discordantly hosted within polydeformed metaturbidites of the Neoarchean Yellowknife Supergroup. This deposit was mined for both lithium and tantalum (1946-1954). The pegmatite formed ca. 2652 Ma (U-Pb columbite ), which corresponds to a late magmatic period following a phase of extensive plutonism in the Slave Province. The size and orientation of the pegmatite suggest that it was emplaced into a dilatant zone along a north-trending shear zone. The dike displays extreme fractionation, manifested by the irregular spatial zonation of mineral assemblages, rare-metal enrichment, and the very high degree of chemical evolution. Mineralogical zones include: a narrow border zone, a fine-grained wall zone, several megacrystic intermediate zones, massive quartz and amblygonitemontebrasite core zones, saccharoidal ( aplitic) al bite zones, and muscovite-rich replacement zones. The degree of chemical evolution of the dike suggests that the pegmatite melt was injected a considerable distance from the progenitor pluton. Detailed internal fractionation trends show progressive evolution from the margins of the pegmatite inwards, and from the south section of the pegmatite towards the north. The economically important minerals present include: amblygonite-montebrasite (Li), spodumene (Li), and columbite-group minerals (Nb-Ta). Processes of niobiumtantalum mineralization are primarily magmatic, with enrichment during magmaticmetasomatism, and minor remobilization during hydrothermal metasomatism.
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