Seafloor massive sulfides (SMS), also known as black smoker deposits, are occurrences of metal-bearing minerals that form on and below the seabed as a consequence of the interaction of seawater with a heat source (magma) in the sub-seafloor region of volcanically active oceanic spreading centers and along volcanic arcs (see Figure below). These occurrences are commonly associated with “oasis of life” harboring chemosynthetic faunal communities. By far the majority of SMS occurrences presently known are small, 3-dimensional bodies that can contain metals such as copper, zinc, gold, and silver. Other trace elements that are important for a variety of industry uses can be enriched at certain sites and are commonly considered as possible important by-products. Known SMS deposits at the seafloor rarely exceed a few million tonnes of metal with the exception of the metalliferous muds in the Atlantis II Deep of the Red Sea. The amount of sulfide close to the current spreading centres has been estimated to be 600 million tons globally, but more sulfides might be present in inactive deposits away from the spreading centres. New technologies to explore for such inactive sites are currently being developed in order to investigate the full ocean potential for sulfides. As a result, there is considerable interest in exploration both within the EEZ of coastal states and international waters. As of January 2022, the International Seabed Authority (ISA), responsible for seabed activities in areas beyond national jurisdiction has approved 7 contracts for exploration, 3 in the Atlantic and 4 in the Indian Ocean.

For a complete list of exploration contracts from ISA see here.

The technology for mining these deposits is currently being built, and first deep-sea tests were performed by the Japanese in their coastal waters. The Canadian company Nautilus Minerals that built deep-sea SMS collectors for its "Solwara 1" deposit in the territorial waters of Papua New Guinea is no longer existing. Solwara 1, long thought to become the first deepsea mine site for SMS, had a mining license issued by the PNG government in 2011. The future of this operation is, however, currently unclear.

The connection between volcanism and life on Earth is nowhere more evident than at the undersea spreading zones, the longest structure on our planet. By far the majority of Earth's volcanic activity occurs along these 55,000 km long ridges, where magma rises from the Earth's interior and forms new oceanic crust.

The discovery of hot springs, massive sulfides, and associated communities at 21°N on the East Pacific Ridge in 1979 clearly demonstrated that the formation of new seafloor is linked to the formation of metallic deposits and the emergence of spectacular communities. The formation of these hot springs is due to the convection of seawater through the oceanic crust. Seawater heated at depth and altered by chemical reactions emerges from the seafloor at high temperatures (up to about 400°C). The metals contained in these solutions, precipitation on contact with the cold, oxygen-rich seawater, forming the vent-like structures known as "black smokers" in reference to factory chimneys. 

Since discovery in 1979, ~430 sulfide deposits (see also InterRidge database) have been found in the world's oceans, of which about 300 are currently hydrothermally active. Early investigations were limited to fast-spreading mid-ocean ridges, eg, in the eastern Pacific, where magma chambers could be detected 1 to 3 km below the seafloor. In recent years, however, a number of deposits have been found in tectonic environments where such hydrothermal systems had not been expected. In particular, these include slow-spreading mid-ocean ridges in the Atlantic, Arctic, and Indian Oceans, where rocks of the Earth's mantle are exposed on the seafloor, and shallow-marine island arc volcanoes in the western Pacific.

Some of the sulfide deposits contain considerable amounts of copper and zinc and, depending on the type of deposit, also high concentrations of the precious metals gold and silver, making them potentially interesting for the raw material supply of the future. However, despite sometimes quite high tonnages in individual deposits, the available sample material from about 100 deposits so far does not represent more than a few hundred tons of material.

Based on the existing data and the lack of information about the depth of the mineralization, it seems premature at the moment to assume an economic importance of these deposits. However, published geochemical analyses indicate that the metal grades are equivalent to or significantly higher than those of onshore deposits. The latter, however, is partly due to unrepresentative sampling by submersibles and robots. Quite a number of these deposits, mostly at shallow water depths, are highly geogenically contaminated by high concentrations of heavy metals such as lead, arsenic, antimony and mercury. The impact of the released metals on the environment (e.g., fishing grounds) has been poorly understood.