STRUCTURAL AND TEXTURAL EVOLUTION OF THE STRANGE LAKE PERALKALINE RARE-ELEMENT (NYF) GRANITIC PEGMATITE, QUEBEC-LABRADOR

RANDY R. MILLER
Newfoundland Department of Mines and Energy, Geological Survey Branch, P.O. Box 8700, St. John's, Newfoundland A1B 4J6

Abstract

The Strange Lake peralkaline complex, on the Quebec-Labrador border north of Schefferville, hosts magmatic rare- element (NYF) pegmatite-aplite lenses. The largest pegmatite-aplite lens, the Main Lens, hosts significant rare-metal (Zr, Y, Nb, Be and REE) mineralization. It forms a partly eroded dome-shaped body that is commonly 6 to 10 m thick. Layering within the lens consists of an aplite footwall zone, a pegmatite-aplite central contact zone, and a pegmatite hanging-wall zone. The footwall zone contains two chilled units, representing influxes of parental magma, and flow- lineated aplite units, representing differentiates of parental magma. The hanging-wall zone contains a lineated aplite unit on the upper contact and a massive pegmatite unit, representing a volatile-rich, less dense differentiate of the parental magma. Extremely enriched residual magma collected in the pegmatite-aplite contact zone to form red aplite-pegmatite units. Mineral variation within the lens defines systematic trends in mineral abundances and textures. Albite abundances increase and K-feldspar abundances decrease upward in the footwall zone, whereas the uppermost unit contains mostly albite. Conversely, feldspar in the pegmatite-aplite contact zone and the hanging-wall pegmatite unit is mostly K-feldspar. The central unit of the lens has low abundances of both feldspar and quartz. Zr-silicate mineral morphology varies, changing from squat grains in the chill units to elongate grains in the pegmatite-aplite contact zone. Crystal-liquid differentiation produced incompatible element-enriched residual melts in a boundary layer at the footwall crystallization interface. These less dense melts moved both vertically and laterally to the upper part of the lens to form rare-metal- enriched magmatic mineralization in the pegmatite-aplite contact zone. Models of formation and classification of granitic pegmatite, recently developed from observations on mainly subaluminous to peraluminous compositions, also apply to peralkaline pegmatites in the Strange Lake peralkaline complex.