Forms on mid-ocean ridges, in back-arc basins and along submarine volcanic arcs. SMS deposits form through hydrothermal activity; cold sea water percolates down through the seafloor, is heated through geothermal energy, becomes buoyant and rises, dissolving metals and sulphides from the surrounding rocks.
Rapid precipitation of metal sulphides from their host hydrothermal fluid in tight systems leads to chimney formation, with chimney collapse and coalescence forming sulphide mounds.
They usually form in water depths between 1000 and 4000 metres and are active for several tens of thousand years. It is only the extinct SMS that are targeted for mining. The key minerals are copper and zinc plus potential high concentrations of gold and silver.