Nature and origin of an LCT-suite pegmatite with late-stage sodium enrichment, Brazil Lake, Yarmouth County, Nova Scotia; I, Geological setting and petrology

The Brazil Lake pegmatite, in Yarmouth County, Nova Scotia, enriched in Li, Ta and Rb, conforms to the LCT-suite classification. The pegmatite group, dominated by two subvertical sheets, is hosted by Silurian sedimentary and mafic volcanic rocks that were deformed and metamorphosed (600 degrees C, 3-4 kbar) during the ca. 410 Ma Acadian Orogeny. A U-Pb tantalite minimum age of 395 Ma for the pegmatite equates to an amagmatic event in this part of southern Nova Scotia, but given its location within a high-grade shear-zone environment, it is likely that a progenitor, crust-derived granite resides at depth. Sequential extraction of progressively more evolved melt-fractions, facilitated by compression in a shear-zone environment, resulted in generation of a melt with several wt.% Li2O. Pegmatite orientation and internal textures are consistent with intrusion into en echelon structures within a regionally extensive dextral shear-zone. The southern pegmatite, the focus of study, is dominated by spodumene and K-feldspar megacrysts (1-2 m) with intergranular spodumene-muscovite-albite-quartz and rare K-feldspar. The megacrysts are generally oriented perpendicular to the contact, and there is a notable absence of an aplite wall-zone, quartz-rich core, and miarolitic cavities. Instead, extensive areas of saccharoidal albite occur after pre-existing K-feldspar megacrysts. The albite-rich zones host most of the Ta-Nb oxide mineralization, as indicated from extensive sampling and geochemical analyses. Most of the muscovite is also of secondary origin, its paragenesis being tied to albite formation. This muscovite sequestered the K and Rb liberated from destruction of K-feldspar and also is enriched in columbite-group phases. The Brazil Lake pegmatite represents an excellent example of an albite-spodumene-type pegmatite overprinted by late-stage albite-rich replacement units. Replacement of K-feldspar by the albite resulted from alkali exchange between a Na-rich melt and an already crystallized, crystal-rich body (K-feldspar+spodumene) that was driven by chemical and thermal gradients. In addition, the Li-rich nature of the melt delayed the onset of Ta-Nb-Sn oxide mineral crystallization, thus leading to enrichment of these elements in the latter stages of melt evolution. The observations noted here are not unique, as Ta-Nb enrichment in albite-rich zones of like pegmatites is observed globally; hence, this interpretation has universal application.