Textural and Mineralogical Evolution of the Little Nahanni Pegmatite Group (NWT, Canada) with Implications for Metasomatism, Rare-Metal Mineralization, and Pegmatite–Wall Rock Interaction

Abstract

Abstract Situated in the Northwest Territories (Canada), the Cretaceous (ca. 85 Ma) Little Nahanni Pegmatite Group is an LCT-type pegmatite swarm enriched in Li, Sn, and Ta. Displaying intensive albitization associated with rare-metal mineralization (i.e., Nb, Ta, Sn), having a notably high surface-area-to-volume ratio (i.e., hundreds of thin dikes), and being particularly well exposed in a suite of thirteen glacial cirques, the Little Nahanni Pegmatite Group provides an ideal opportunity to investigate some unresolved aspects of pegmatite formation and evolution, such as metasomatism, pegmatite–wall rock interaction, and the nature and origin of rare-metal mineralization (i.e., magmatic versus metasomatic). Using an integrated approach combining field observations with detailed textural and mineralogical studies, two different stages of pegmatite evolution are documented: primary magmatic followed by intense metasomatism. Preservation of primary magmatic features, such as coarse oriented high aspect-ratio crystals (spodumene, K-feldspar), banded aplites, anisotropic fabrics, and skeletal textures, highlight the importance of undercooling (ΔT) of the melt with consequent disequilibrium crystallization and boundary layer effects. The metasomatic stage is dominated by the formation of domains of secondary albite and is attributed to interaction with a residual highly fluxed sodic melt and is responsible for most of the rare-metal mineralization; less abundant micaceous units and cafemic domains reflect incursion of acidic magmatic-sourced and alkaline wall rock derived fluids. Our findings are significant and have widespread application to all pegmatite types. Particularly significant is the finding that the rare-metal mineralization is mostly localized to zones of metasomatism.