Metals, sulphides and nodules
The non-living resources of the deep ocean floor are increasingly attractive for the mineral industry. The major mineral resource potential is held in iron-manganese nodules, cobalt-rich polymetallic crust, and polymetallic sulphides that are being turned to as an alternative to land-based resources when their scarcity will be emerging or offshore exploitation will be more feasible through technological advances.
Four elemental metals are the main components of value in manganese nodules and cobalt crust: manganese, copper, nickel and cobalt. In addition, cobalt crust contains platinum group metals and rare earth elements, and precious metals such as gold and silver are present in sulphides. Although the gross revenue is potentially high, technological, legal and environmental problems are slowing down the rise of a flourishing commercial industry.
Polymetallic manganese nodules
Polymetallic manganese nodules (also known as manganese nodules) are small potato-sized (from millimetres to tens of centimetres in diameter) lumps of material precipitated from seawater and sediment pore water at slow rates over millions of years (Murton et al., 2002; ISA, 2010), and occur mainly on the deep-seafloor (Figure 1).
They contain approximately 24% manganese, compared to 35 to 55% manganese in land ore bodies, so they do not offer solid economics as a manganese source, but they also contain iron (14%), copper (>1%), nickel (>1%), and cobalt (0.25%). The last three metals together, can make up to 3 wt. %. Apart from these metals, nodules include trace amounts of molybdenum, platinum and other base metals (Cronan, 1980; Manheim, 1986).
Table 1 reports the average elemental concentration in different basins, showing why the commercial interests have centred on the region of the eastern Pacific in the Clarion and Clipperton Fracture Zones between Hawaii and Mexico. Here, abundant nodules have up to 30 wt. % manganese, 1.5 wt % copper, 10,000 ppm cobalt and 2 wt % nickel. Nodules in this region are more concentrated than in most other areas, with averages up to 10 kg/m2, but with high variability: in isolated sites it ranges up to 30 kg/m2 (Hayes et al.,1985; Frazer, 1977).
Cobalt-rich ferromanganese crusts
Cobalt-rich ferromanganese crusts (also known as cobalt crusts; Figure 2) form pavements up to 250 mm thick on rock outcrops (ISA, 2004) in water depths of 400 to 4,000 m at the seafloor on the flanks and summits of seamounts, ridges, plateaus, and abyssal hills, where the rocks have been swept clean of sediments at least intermittently for millions of years (Halbach et al., 1989; Hein et al., 2000).
The composition of manganese crusts varies from 15 to 31 wt. % manganese, 7 to 18 wt % iron, and with Mn/Fe ratios ranging from 1.0 to 3.4. Although the cobalt content is generally higher for crusts than for nodules, with crusts containing up to 2 wt % cobalt occurring on the summits of some seamount of less than 1,500 m depth, the average cobalt content is 0.8 wt %. In addition, crusts can also contain significant amounts of nickel, lead, cerium, molybdenum, vanadium and other minor metals including those of the platinum group elements (Manheim, 1986, Halbach et al., 1989; Hein et al., 2000).
Important areas where cobalt-rich manganese crusts have been found in the Pacific include the north-west Hawaiian Ridge, Johnston Island, Huwlarid-Baker Islands, Marianas Island, Guam, Marshall Islands, Central Seamounts, Palmyra-Kingman, Micronesia and Wake Island (Manheim 1986).
Polymetallic sulphides (PMS, Figure 4) are typically composed of iron pyrite, but contain varying proportions of pyrrhotite, pyrite/marcasite, sphalerite/wurtzite, chalcopyrite, bornite, isocubanite and galena. Copper and zinc are the most likely metals to be recovered, but some deposits exhibit significant gold (0–20 ppm) and silver (0–1200 ppm) grades as well.
Submarine massive polymetallic sulphide bodies are principally found along the earth’s major tectonic belts, as indicated in the Figure 5 (Murton, 2001). Up to 40% of the known deposits occur at shallower depths in back-arc basins and on submarine volcanic ridges within 200 nautical miles of the coast and within the jurisdiction of national exclusive economic zones (EEZs).
PMS are formed by precipitation of metals leached by hydrothermal fluid as it interacts with the cooler ambient seawater at or beneath the seafloor at hydrothermal vent sites. These fluids may discharge at temperatures up to 350°C from “black smoker” chimneys (at depths in excess of 2,500 m), but also at lower temperature (200°C) from “white smokers” generating in some cases mineralization of considerable economic potential, with inclusion of precious metals such as gold.