Within the footwall volcanic sequence five chemically and mineralogically distinct alteration types have been defined: least altered, sericite, chlorite, chloritoid, and ankerite. An alteration model is proposed in which shallow circulating seawater (1-3 km), heated by a subvolcanic intrusion, encountered rocks of felsic composition within the upper part of the volcanic succession. Seawater-rock reactions evolved an acidic, potassium-rich solution which migrated upward, reacting with overlying felsic rocks, converting plagioclase to sericite and quartz. Porosity of the altered rocks decreased and a widespread sericitic alteration zone formed. At depths greater than 3 km, circulating seawater came into contact with basaltic rocks and ensuing reactions enriched the seawater in Fe, Mg, Mn, Ca, and CO 2 . This second hydrothermal solution moved upward and encountered sericite-rich rocks. Near the sea floor, boiling of the solution and/or mixing with relatively cold seawater rapidly decreased CO 2 solubility causing mass precipitation of iron carbonate. This led to the formation of the ankerite assemblage in the immediate footwall rocks and the siderite iron-formation on the sea floor.--Modified journal abstract.