Distribution and petrogenesis of lithium pegmatites in the Western Superior province of the Canadian Shield

Systematic search for economically viable pegmatite deposits of lithium and associated rare elements requires detailed information about their petrogenetic histories in differing geological frameworks. First results of investigating these aspects of pegmatite mineralization in the Western Superior province of the Canadian Shield are presented in this paper. In a lithologic, structural and metamorphic variety of subprovinces and their segments, mineralized pegmatites are restricted to those characterized by predominant gneissic and granitic composition, extensive deformation, high metamorphic grades, steep metamorphic gradients and widespread anatexis. Subprovinces characterized predominantly by greenstone belt-granitoid diapir association are devoid of noteworthy pegmatites. The study of the Cat Lake-Winnipeg River pegmatite district in the English River subprovince has yielded a working hypothesis which considers sizeable pegmatite deposits as final products of advanced anatexis of gneissic-granatic-greenstone masses and slowly rising quartz-monzonitic magmas. The gradual cooling and multiple pressure quenching of the quartz monzonites promoted advanced fractionation of volatile-saturated residual magmas and repetitively separated supercritical fluids. Formation of pegmatitic granites was an important intermediate step of the process leading to extreme enrichment of the last pegmatite generations in rare elements. Intrusive suites similar to that of the Cat Lake-Winnipeg River quartz monozonite-pegmatitic granite-pegmatite sequence are documented from the Black Hills of South Dakota, the Kitee-Tohmajärvi area in Finland, and from the Georgia Lake area in Ontario.