Light colored patches, ranging to 0.5 m across, are common in some gabbro dykes and volcanic rocks at Matagami. Skeletal ilmenite and outlines of plagioclase grains, traced from patches to fresh gabbro, indicate an origin by alteration and metasomatism. Gabbro near patches is composed of actinolite, hornblende, labradorite, and composite ilmenite–magnetite grains, with minor epidote, albite, quartz, and chlorite. Patches are mainly epidote, quartz, hornblende, with minor albite, chlorite, apatite, and sphene. Compared to gabbro, patches are enriched in CaO, SiO2, and P2O5, and depleted in FeO*, MgO, and alkalis.Present features of the gabbro developed in three stages: (1) magmatic, (2) autometamorphic (or deuteric), and (3) regional metamorphic. The patches formed during stage 2 by channeling of fluids through permeable zones in cooling dykes and overlying volcanic rocks.