A study has been made of several dozen small, stock-like ultramafic to mafic igneous bodies that occur in two clusters, 50 and 75 miles west of the head of Lake Superior. One cluster, the Quetico intrusions, is dominantly hornblende peridotite, hornblendite, and feldspathic hornblendite; the other, the Shebandowan intrusions, comprises serpentinized peridotite and subordinate gabbro. Both intrude eugeosynclinal deposits and are followed by extensive granitic, and minor syenitic intrusions, and both have associated Ni–Cu sulphides containing minor Pt and Pd.The Quetico intrusions are shown to have formed by fractional crystallization of tholeiitic olivine basalt liquid, possibly carrying suspended olivine and clinopyroxene when emplaced. Crystallization was at a water partial pressure of at least 2500 bars and, thus, a depth of 5 or more miles. The Shebandowan intrusions resemble alpine-type peridotite bodies structurally but, chemically and petrographically, are more like the differentiates of stratiform mafic intrusions. Their peridotite has formed from tholeiitic magma by early fractionation of olivine; the gabbro probably represents residual magmatic liquid. Serpentinization, which is postmagmatic, apparently has not caused extensive chemical change beyond addition of H2O, O2, and some CO2, and local redistribution of lime.The rocks in the two groups of intrusions differ appreciably in Cu and Ni, and a correlation between the Cu/Ni ratios of the rocks and their respective ores is illustrated. This correlation, and the sulphur isotopic composition, both indicate derivation of the sulphides from the intrusions.