Granitoids and rare-element pegmatites of the Georgia Lake area, northwestern Ontario

Abstract

The Georgia Lake pegmatite field is located in the Quetico Gneiss Belt of the Superior Province. Spodumene-bearing and subordinate beryl-bearing pegmatites of the Georgia Lake area are flanked to the south and east by an extensive granitoid terrain, which previously has not been subdivided. Granitoids of the immediate Georgia Lake area were investigated in conjunction with rare-element pegmatites to determine the character of the granitoids as parental intrusions to rare-element pegmatites. The granitoids include two-mica leucogranites occurring as a large plutonic mass south of the pegmatite field and as smaller satellitic intrusions, the Kilgour Lake Group granitoids centered on a small gabbroic-metagabbroic unit near Kilgour Lake and tonalitic sills dispersed throughout the pegmatite field. The distinction of the three types of granitoids was made on the basis of field observations, petrography and analytical geochemistry. Two-mica leucogranites and tonalitic sills were derived as partial melts of politic metasediments and metagreywacke, respectively. The Kilgour Lake Group granitoids were presumed to be the products of fractional crystallization of a mafic melt generated in the upper mantle or lower crust. Mineralogical studies were carried out on perthitic microcline, tantalite-columbite and Sn oxide minerals from rare-element pegmatites. Results indicate that perthitic microcline in all pegmatites is of the maximum microcline structural state, tantalite-columbite minerals occur in a partly to completely disordered structural state and the dominant Sn oxide mineral is staringite. Division of spodumene-bearing rare-element pegmatites into Southern, Central and Northern Groups was made on the basis of internal textural variations, mineralogy and differences in geochemistry of perthitic microcline and muscovite. The Southern Group consists of one pegmatite which is unique to the Georgia Lake pegmatite field with respect to development of mineralogical zones and strong internal fractionation of Rb and Cs. Central Group pegmatites are linked by a fractionation trend, with respect to Rb and Cs, across the group. A similar fractionation trend is not observed across the Northern Group pegmatites. The pegmatite groupings reflect different modes of source fluid derivation, although all pegmatites of the Georgia Lake area originated as the result of a common anatectic event responsible for the intrusion of two-mica leucogranites. Central and Southern Group pegmatites were derived from low viscosity fluids differentiated from granitic melts, while Northern Group pegmatites are presumed to be the products of fluids generated by direct anatexis of metasediments.