The Rainy Creek Alkaline-Ultramafic Igneous Complex near Libby, Montana. I: Ultramafic Rocks and Fenite

The Rainy Creek complex represents a composite of successive intrusions of igneous rocks emplaced into the Precambrian Belt Series, probably in middle Cretaceous time. The succession began with the emplacement of a body of coarse-grained biotite pyroxenite with a core of coarse-grained biotitite, which is considered to have formed from the accumulation of alkalies and volatiles near the roof of the pyroxenite pluton. Nodules in the biotitite appear to be xenoliths of altered ultramafic rock. Following this igneous phase, magnetite pyroxenite intruded a zone of weakness between the biotite pyroxenite and the Belt Series, forming a ring dike surrounding the inner pyroxenite body. All of the ultramafic rocks are considered to be products of fractional crystallization of a common parent magma. Available mineralogic and experimental data suggest that the biotitite and biotite pyroxenite were emplaced under high P_H₂O and f_O₂-T conditions. Fen-itizing ions, which probably originated in deep-seated ijolite or carbonatite, have altered part of the Belt Series adjacent to the magnetite pyroxenite.