Abnormal concentrations of magnesia are frequently associated with massive, Cu–Zn-bearing, sulfide (pyrite and/or pyrrhotite) deposits. Research on some reported occurrences of magnesia concentrations associated with volcanogenic, massive sulfide deposits suggests that such concentrations may result from syngenetic, metasomatic alteration of volcanic rocks in the vicinity of submarine volcanic centers by hydrothermal solutions and sea-water. Inasmush as the hydrothermal solutions consist of sea-water heated during convective circulation through the volcanic pile, the eventual source of magnesia is, in all cases, the sea-water. Cordierite–anthophyllite rocks are developed through thermal metamorphism of the altered volcanic rocks. It is suggested that this explanation is also appropriate to the cordierite–anthophyllite and other rocks of the “magnesia metasomatic” suites of Orijärvi in Finland and other similar deposits occurring in south-western Finland and central Sweden.