ALBERT

Description: Since 1979, polymetallic massive sulphide deposits have been found at water depths up to 3,700 m in a variety of tectonic settings at the modern seafloor including mid-ocean ridges, back-arc rifts, and seamounts. Many of the sulphide deposits consist of a black smoker complex on top of a sulphide mound which commonly is underlain by a stockwork zone. It has been widely established that circulating seawater which is modified in a reaction zone close to a subaxial magma chamber is the principal carrier of metals and sulphur which are leached out of the oceanic basement. Precipitation of massive and stockwork sulphides at and beneath the seafloor takes place in response to mixing of the hightemperature (up to 400°C) metal-rich hydrothermal seawater fluid with ambient seawater. Polymetallic seafloor sulphide deposits can reach a considerable size (up to 100 million tonnes) and often carry high concentrations of copper (chalcopyrite), zinc (sphalerite), and lead (galena) in addition to gold and silver. Extremely high concentrations of gold have recently been found in a new type of seafloor mineral deposit previously only known as epithermal (magmatic) gold deposits on the continents. Due to the high concentration of base and precious metals, seafloor polymetallic sulphide deposits have recently attracted the interest of the international mining industry. The recovery of those deposits appears to be both economically and environmentally feasible due to certain advantages over land-based deposits and will likely become reality within this decade. For logistical and technical reasons, future mining operations will largely focus on deposits in national rather than international waters.