The progress in the study and survey of oceanic cobalt-rich crust resources

Cobalt-rich crusts constitute another deep-sea sedimentary solid mineral resource discovered after oceanic polymetallic nodule. Cobalt-rich crusts are distributed on the seafloor of the Pacific Ocean, the Atlantic Ocean and the Indian Ocean. It is estimated that the amount of dry crust resources in the three oceans is (1081.1661~2162.3322)×10⁸ t. Survey of the cobalt-rich crust by various countries in the world began in the early 1980s. So far, 4 countries, i.e., Japan, China, Russia and Brazil, have signed a cobalt-rich crust exploration contract with the International Seabed Authority. The application of cobalt-rich crusts mining area submitted by South Korea was also approved by International Seabed Authority in 2016. Cobalt-rich crusts can be divided into plate curst, gravel-like crust and nodule-like crust according to their shapes. Cobalt-rich crusts are usually three-layer structure in the macro. Upper layer is called bright coal seam layer, intermediate layer is called loose layer, and bottom layer is called relatively loose layer. Microscopically, cobalt-rich crusts are mainly characterized by such structures as columnar structure, callenia structure, plaque structure, and laminar structure. The minerals of cobalt-rich crusts are mainly authigenic iron and manganese minerals, which include vemadite, todorokite, amakinite, akaganeite, feroxyhyte, goethite, and so on. Cobalt-rich crusts are rich in Mn, Fe, Co, Ni, Cu, Pb, Zn as well as REEs and PGEs. The content of Co of cobalt-rich crusts is particularly significant. The average content of Co in cobalt-rich crusts from the Pacific Ocean is the highest in the three large oceans in the world. Formation process of cobalt-rich crusts is extremely slow. Only several millimeters of crust can be formed in one million years. Studies show that cobaltrich crusts from the West Pacific Ocean were formed as early as Eocene to early Miocene. It's generally accepted that the cobaltrich crust is hydatogenic. Co, Fe, Mn and other metal elements in crusts originate from sea water. In addition, studies have shown that microorganisms play a very important role in the formation of cobalt-rich crusts. The distribution and characteristics of cobaltrich crust are affected by such factors as topography, water depth, substrate rocks type, hydrochemical characteristics of seawater, latitude and longitude. Cobalt-rich crusts are formed on the slope of seamount and island and submarine highland above the depth of carbonate compensation and below oxygen minimum zone, with the water depth of 800~2500m. West and Central Pacific seamounts are considered to be the main production area of cobalt-rich crusts in the world.