Minerals: Polymetallic Sulphides
Evidence of hydrothermal mineral deposits were first noticed in 1948 during the Swedish Albatross oceanographic expedition in the Red Sea. This was followed by discoveries of metalliferous muds and brines between 1963 and 1966 in the Red Sea Rift system. These metalliferous muds contained large amounts of copper, zinc, lead, iron, silver and gold. These discoveries led to similar searches for the same resources in other parts of the world, resulting in the discovery of several hydrothermal sites along the East Pacific Rise System, Galapagos Ridge, the Juan de Fuca and Gorda Ridges in the 1970s.
The Galapagos Rift was found to contain inactive mounds containing ore-grade metals, some of them up to 35 m high chimneys. Sophisticated instrumentation, like submersibles, has led to several other discoveries since the Galapagos Rift discovery in 1981.
More than 350 sites of high-temperature hydrothermal vents and about two hundred sites of significant massive sulphide accumulation have been identified since the discovery of black smokers in 1978-79. Most of these deposits are associated with the mid-ocean ridges, backarc environments, submarine volcanic arcs and in basins near volcanic arcs. A study based on new deposit occurrence data from about 10,000 km of ridge, arc, and backarc spreading centers of the global oceans shows the total accumulation to be on the order of 6 × 108 tonnes, containing ∼3 × 107 tonnes of copper and zinc.
Biological Communities and Environmental Concerns: Unusually high temperature and thriving biological communities are most conspicuous of the vents phenomenon. As the seawater seeps into the volcanic rocks through fissures, it becomes hot and the metals of surrounding rocks get dissolved, it then rises up due to convection and carries high temperature fluids (350-400 °C) and ‘black smoke’, containing minerals of economic importance- copper, lead, zinc and silver and gold as traces. Hydrothermal vent fields are hosts of thriving biological communities such as giant tube worms, clam shells, crabs and micro fauna. These animals use chemosynthesis – they derive energy from bacterial oxidation of chemicals in the vent fluids for survival. Massive sulphide deposits are 3D deposits, hence the mining technology for these resources is likely to be different from the polymetallic nodule resource technology. Moreover, due to the presence of benthic community with these deposits, appropriate mitigation strategies to minimize the impact of mining activities are being developed with input from scientific communities.