The Hoydal volcanogenic massive sulfide deposit is associated with thick basaltic sheet flows toward the middle of a 1- to 2-km-thick, low-metamorphic, inverted, Lower Ordovician ophiolitic volcanite sequence in the central Norwegian Caledonides, just east of the major (25+ million metric tons) orebody at Lokken.Hoydal is one of the very few Scandinavian Caledonian strata-bound volcanogenic massive sulfide deposits which retains dominantly sea-floor depositional features, due to the minimal effects of later deformation and metamorphism. This study documents these features and compares them with those of recently formed and presently forming sea-floor sulfides.The layered massive sulfides at Hoydal are closely related to a sheetlike feeder zone as well as to extensive jasperoid layers and fault-scarp talus deposits. A palinspastic reconstruction relates the ores to sea-floor deposition along elongated, fissure-controlled, hydrothermal conduits; the talus breccias, involving sulfides, jaspers, and the volcanites, are considered to be due to movements along the feeder-fissure-fault system.Chemically the ore is dominated by Fe, Zn, and Cu, the two latter elements being highly variable and showing no significant correlation with each other. Ag is correlated with Cu, suggesting that most Ag is contained in chalcopyrite. Au shows a strong association with both As and Sb, and tennantite is suggested as a possible Au bearer.Investigations of ore textures have led to definition of several stages of mainly sulfide deposition. An early period of fine-grained colloform pyrite (+ chalcopyrite, sphalerite, and galena) deposition succeeded an earlier phase in which tabular pyrrhotite was probably formed. The colloform pyrite phase was succeeded conformably by coarser grained, clean, crystalline, sub- to euhedral pyrite, deposited in cockadelike structures outside the colloform structures. Contemporaneously, recrystallization of parts of the colloform pyrite produced (internally) euhedral, often poikilitic, pyrite.Both these noncolloform pyrites are attributed to a period of higher temperature conditions resulting from the sealing-off of cold seawater from the rising hydrothermal fluids. These conditions led further to the deposition of chalcopyrite as both open-space fillings and replacement of early-formed sulfides, accompanied by chalcopyrite disease in earlier sphalerite. The last main stage of sulfide deposition introduced the major part of the sphalerite apparently by open-space filling.Comparisons with present-day sea-floor sulfides, as described in the literature, reveal certain special features of the Hoydal sulfides. In common with the major orebody at Lokken, they appear to have been deposited in connection with a phase of vigorous volcanic activity, consequent upon the intrusion of new, hot, mafic magma to a high level in the oceanic crust which gave rise to large-scale hydrothermal convective systems.The main period of high-temperature chalcopyrite deposition was followed by a phase when hot solutions, deficient in Cu and Fe, caused redissolution of parts of the sulfides deposited earlier and the formation of a highly porous, residual pyrite + or - chalcopyrite ore. This was filled in and cemented by late, low Fe sphalerite and quartz introduced by low-temperature solutions during the waning stages of hydrothermal activity.Marked enrichment of elements such as Zn, Pb, Ag, Au, and Sb, relative to Cu and Fe, seems to be due to the purging of the last-mentioned elements from the solutions by large-scale deposition of chalcopyrite and pyrite within the underlying oceanic crust.