The Cedar Bay mine, northern Quebec, is entirely within the Chibougamau gabbro-anorthosite complex. The Cu and Au minerals occur as replacement bodies and possibly cavity-filling along faults and shears. Because the host rock is essentially of one composition, an exceptional opportunity for the study of alteration is available. The differences in modes of occurrence and quantities of alteration minerals make it possible to delimit alteration zones, which are parallel to the ore shoots. The thickness of the alteration zones varies directly with the thickness of the ore shoots. The use of these zones is valuable in estimating the proximity to ore.