The Davis nepheline pegmatite and associated nepheline gneisses near Bancroft, Ontario

Abstract

The Davis pegmatite consists largely of albite with some nepheline and biotite, and minor amounts of microline and several other minerals. It is discordant to a series of gneissic nepheline-amphibolites and plagioclase–nepheline–biotite–hastingsite–calcite gneisses. These rocks have been sampled by about 150 m of diamond drill core. Analyses of this material are interpreted to mean that the present composition of the pegmatite cannot be the direct result of magmatic crystallization. It is suggested that post-magmatic metasomatism has exchanged Na for K and Fe for Mg, among other changes, and this is consistent with the mineral textures observed.