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abyssal hills where the rocks have been swept clean of sediments at least intermittently
for millions of years. Crusts form pavements up to 250 mm thick on rock outcrops, or
coat talus debris. Fe-Mn crusts form by precipitation from cold ambient bottom waters
(hydrogenetic), or by a combination of hydrogenetic and hydrothermal precipitation in
regions where hydrothermal venting occurs, such as near oceanic spreading axes,
volcanic arcs, and hotspot volcanoes. Fe-Mn crusts contain sub-equal amounts of iron
and manganese and are especially enriched in cobalt, manganese, lead, tellurium,
bismuth, and platinum relative to their lithospheric and seawater concentrations (Table
1). There are two practical interests in Fe-Mn crusts, the first being their economic
potential for cobalt, but also for manganese, nickel, and platinum, and possibly also
titanium, rare earth elements (REEs), tellurium, thallium, phosphorus, and others. The
second interest is the use of crusts as recorders of the past 60 million years (Ma) of
oceanic and climatic history. Besides the high cobalt contents compared to abyssal Fe-
Mn nodules, exploitation of crusts was viewed as advantageous because most high
quality crusts occur within the Exclusive Economic Zone (EEZ; Fig. 1) of island nations
and, therefore, are not subject to some of the perceived problems of recovery of mineral
resources occurring in international waters.
1.1. Classification
Up until the late 1970s Fe-Mn crusts were usually not distinguished from abyssal
Fe-Mn nodules. If a distinction was made, crusts were called seamount nodules.
However, there are distinct differences between Fe-Mn nodules and crusts, other than
just morphology (Table 2) 2. Nodules nucleate on small bits of rock, bone, or old nodule
fragments on the surface of sediments. Nodules commonly form by both diagenetic
(components derived from sediment pore waters) and hydrogenetic precipitation and
thus their composition reflects input from both seawater and sediments. However, some
nodules form predominantly by either diagenetic or hydrogenetic processes. Nodules
have sometimes been referred to as hydrogenous, regardless of their origin;
consequently, we use the term hydrogenetic to avoid any confusion about a substrate
contribution to crusts--a substrate contribution is not found in Fe-Mn crusts 3. Generally,
crusts and nodules have different mineralogical (vernadite versus todorokite and
vernadite) and chemical compositions (for example high cobalt versus high copper)
because of their genetic differences as well as differences in water depths of formation,
although there is much overlap.
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