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Hydrogenetic Fe-Mn crusts generally have iron/manganese ratios between 0.4
and 1.2, most commonly 0.7 ± 0.2, whereas mixed hydrogenetic and hydrothermal crusts
and continental margin hydrogenetic crusts have ratios between 1 and 3, mostly 1.3-1.8
(from data used to compile Table 6). Cobalt is the metal with the greatest economic
potential in crusts and ranges from about 0.05-1.7% (500-17,000 parts per million, ppm) in
individual bulk crusts and averages between 0.19% and 0.74% (1900-7400 ppm) for
various parts of the global ocean (Table 6). Cobalt is also considered the element most
characteristic of hydrogenetic precipitation in crusts 64 and is considered to maintain a
constant flux from seawater to Fe-Mn crusts 65, regardless of water depth. Nickel and
platinum are also considered of economic importance and range up to 1.1% and 1.3 ppm
respectively for individual bulk crusts. Platinum ranges up to 3 ppm for individual crust
layers 66. Elements most strongly enriched over abyssal Fe-Mn nodules include iron,
cobalt, platinum, lead, arsenic, bismuth, bromine, vanadium, phosphorus, calcium,
titanium, strontium, tellurium, and REEs, whereas nodules are more enriched in copper,
nickel, zinc, lithium, aluminium, potassium (only Pacific crusts), and cadmium. Fe-Mn
crusts are enriched over seawater in all elements except bromine, chlorine, and sodium;
enrichments over seawater between 10 8 and 10 10 times include bismuth, cobalt,
manganese, titanium, iron, tellurium, lead, and thorium, and between 10 6 and 10 8 times
include tin, hafnium, zirconium, aluminium, yttrium, scandium, thallium, nickel,
calcium, niobium, indium, copper, germanium, zinc, tungsten, and tantalum. Crusts are
enriched over lithospheric concentrations about five thousand times for tellurium and a
hundred to five hundred times for molybdenum, thallium, antimony, cobalt, manganese,
bismuth, arsenic, selenium, and lead. Crusts may have an economic potential not only
for cobalt, nickel, manganese, and platinum, but also for titanium, cerium, tellurium,
thallium, zirconium, and phosphorus.
Elements in crusts have different origins and are associated with different crust
mineral phases 67. Generally elements are associated with five phases in crusts, d-MnO2,
iron oxyhydroxide, detrital (aluminosilicate), CFA, and residual biogenic phases.
Manganese, cobalt, nickel, cadmium, and molybdenum are invariably associated with
the ?-MnO2 phase. In addition, in more than 40% of the regions studied, lead, vanadium,
zinc, sodium, calcium, strontium, magnesium, and titanium are also associated with that
phase. Iron and arsenic are most commonly the only elements associated with the iron
oxyhydroxide phase, although less commonly vanadium, copper, lead, yttrium,
phosphorus, chromium, beryllium, strontium, titanium, and cerium have also been
reported to be associated with that phase. The detrital phase always includes silicon,
aluminium, and potassium, and commonly also titanium, chromium, magnesium, iron,
sodium, and copper. The CFA phase invariably includes calcium, phosphorus, and
carbon dioxide, and also commonly strontium and yttrium; molybdenum, barium,
cerium, and zinc may also be associated with the CFA phase in some regions. The
residual biogenic phase includes barium, strontium, cerium, copper, vanadium, calcium,
and magnesium, and in some regions also iron, arsenic, sodium, molybdenum, yttrium,
56 International Seabed Authority