Massive sulfide deposits result from the steady state or transient discharge to the sea floor of submarine geothermal systems. The elements of such systems are discussed with respect to their terrestrial counterparts. Sustained low power discharges result in metalliferous sediments or banded massive sulfide ores. Kuroko and Archean massive sulfides are generally brecciated and may deposit from short-lived high power discharges through stockwork zones formed by seismic- or gas pressure-induced hydrothermal explosion in the sealed surface zone of the geothermal system. It is shown that during deposition these fine-grained ore deposits may be maintained as hot fluidized beds. By analogy with porphyry copper-type deposits it may be speculated that, whereas some ore metals may be derived by rock leaching, magmatic vapour deep in the geothermal system may also be a source of base metals, sulfur, and carbon dioxide. With reference to laboratory analogues it is suggested that high power discharges may focus magmatic fluid input through the entire system to the surface until stockwork sealing reverts the system to low power discharge.