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Tourmaline in petalite-subtype granitic pegmatites from the Pakeagama Lake and Separation Lake areas of northwestern Ontario provides evidence for the compositional evolution of pegmatite-forming melts through magmatic crystallization and pegmatite melt -host-rock interactions. At the former locality, tourmaline from the pegmatite units is essentially magmatic, whereas at the latter locality, tourmaline in many of the Fe-suite pegmatites displays characteristics (such as elevated Mg, Ti and Ca contents) that indicate significant contamination through the incorporation and digestion of components of amphibolite (mafic metavolcanic units) and banded iron-formation (BIF) host-rocks. The presence of tourmaline in mafic host-rocks indicates that cation migration also involved the introduction of B, Li and Al from pegmatite-generated fluids to form metasomatic haloes. The Li + Mn versus Mg + Ti + Ca diagram is introduced to discriminate tourmaline with a dominantly magmatic signature from tourmaline with a signature influenced by pegmatite -host-rock interaction. The behavior of Ca in tourmaline is also shown to be potentially useful in recognizing late-stage Ca-enrichment in pegmatite-forming melts as a result of Ca-F complexing, but can give similar trends to tourmaline that has suffered Ca-metasomatism as a result of interaction with granitic host-rocks. The tourmaline data suggest that at Separation Rapids, pegmatites mainly crystallized in an open system (i.e., pegmatite -host-rock interaction was common), whereas at Pakeagama Lake, the pegmatite units were predominantly unaffected by interaction with host rocks (i.e., they crystallized in a closed system). This has implications for concentrating rare-elements such as Rb and Cs, which otherwise would be dispersed throughout pegmatite and host rocks (as at Separation Lake). The potential for a buried pollucite deposit associated with the SE pegmatite at Pakeagama Lake has increased as a result of these observations, particularly as Cs is elevated in associated K-feldspar and mica, and is also present in a late pollucite-bearing vein.

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