The Opawica River Complex is a layered mafic igneous intrusion which cuts Archean volcanic rocks of the Matagami-Chibougamau Greenstone Belt. The volcanic sequence was subsequently intruded by the Opawica river Pluton and then folded into an east-west trending anticline. The exposed portion of the complex is about 15,000 feet thick and over 15 miles long. The complex is divided into two zones: the anorthosite Zone and the Gabbro-Ferropyroxenite Zone. The volcanics and intrusives were syntectonically affected by low grade (greenschist facies) regional metamorphism belieed to be of Kenoran age (~2.7 b.y). Mineralogical studies using a petrographic microscope and electron probe microanalyzer indicate that the original cumulus phases were plagioclase and clinopyroxene. Intercumulus phases were clinopyroxene, plagioclase, magnetite, and ilmenite. Major element chemistry and perserved textural relationships indicate that the regional metamorphism was essentially isochemical. Gabbroic rocks from the Opawica River complex show an increase in FeO with a reciprocal decrease in Mgo and suggest that the Opawica River Complex formed as a result of fractional crystallization of a basaltic magma having tholeiitic affinities.