The 20 or more Sulitjelma orebodies are recognized as stratiform, strata-bound pyritic Cu-(Zn) sulfide ores, the products of volcanic-associated hydrothermal sedimentary exhalative formation. The ores are interpreted as having been formed at a single stratigraphic interval on the Ordovician sea floor. Each has a well-developed subjacent zone of alteration, in which the core of each alteration zone appears to be abnormally enriched in potassium relative to the surrounding strata. The core is enveloped by a more typical chloritic alteration zone characterized by an increasing Fe/(Fe + Mg) ratio away from the deposit. Distinct ore facies are recognized as products of primary hydrothermal zonation and of regional amphibolite--grade metamorphism and accompanying penetrative tectonic deformation. The metals in the ores are interpreted to result from leaching of the enclosing basalts. These show widespread physical and chemical effects of hydrothermal leaching. The widespread brecciation associated with the orebodies can partly be explained by tectonism; it may also be evidence of a cyclic catastrophic stage during the evolution of the sea-floor hydrothermal system. The Sulitjelma Gabbro Complex shows textural features in accordance with rapid cooling by seawater and it is believed that the cooling gabbroic magma represents the heat source which drove the hydrothermal convection cell.