| dc.description.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. | |
