The No. 5 Zone of the Horne Mine in the Noranda District consists of massive to disseminated sulfides, mainly pyrite, enclosed conformably within steeply dipping, interbedded tuffs, rhyolite tuff breccias, and rhyolite breccias of Archean age. Large sulfide masses are elongate down dip and have oval to roughly tabular cross sections. They are closely spaced and overlapping, due apparently to successive submarine slumping of soft, unconsolidated sulfide masses and rhyolitic pyroclastics on the flanks of a volcano. Textures of the sulfides suggest that they underwent subsequent dynamic and regional metamorphism probably during the Kenoran orogeny (2.5 billion years).Pyrite in the No. 5 Zone contains 12-60 ppm cobalt and 33-88 ppm selenium, whereas that in the adjacent economic H ore bodies contains about 1,000 ppm cobalt and 390-1,000 ppm selenium. These differences suggest an independent formation of the No. 5 Zone.Large fragments of sulfides similar in composition and texture to the sulfides in the massive bodies of the No. 5 Zone occur in the overlying breccias and were apparently derived from the No. 5 Zone during periods of explosive volcanism. The massive sulfides therefore must have formed during the same period of volcanism.