An exploration venture that led to the discovery of a small ore-grade Cu deposit in the very active Mattagami Lake exploration area of northwestern Quebec is described. The Garon Lake property was staked to cover a small, isolated E.M. anomaly that was recorded during an airborne survey of a folded post-Keewatin basic intrusion. A ground geophysical survey confirmed the presence of a narrow but good conductor and of a coincident, positive magnetic anomaly. Diamond drilling has indicated the presence of a body of massive sulfides containing a little over 2% Cu. The sulfide deposit is in a group of metamorphosed Keewatin lavas, tuffs, and sediments intruded by older gabbro and by systems of late unmetamorphosed dikes, of which lamprophyre, olivine diabase, and albite granite, are examples. The strike and dip of the mineralization are essentially parallel to the strike and dip of the country rock. The shoot of massive sulfides plunges at about 60 degrees . Pyrrhotite, pyrite, chalcopyrite, and sphalerite occur in varying proportions and show textural evidence of having replaced the pre-existing rock. The mineralization is near the contact between amphibolite (derived from lavas) and quartz-rich schist (derived from sediments). More particularly, the sulfide body is closely associated with an alteration aureole of cordierite-anthophyllite hornfels. Examples of base metal sulfide deposits that are structurally and genetically related to the formation of Mg-rich cordierite-bearing metasomatic zones have been described from other parts of the world. Geographically the closest described example is the Amulet ore body in the Noranda district. The discovery of this type of deposit on the Garon Lake property sheds new light on exploration in the Mattagami Lake area.