Ch. 2— Resource Assessments and Expectations • 71 



Figure 2-11.— Cobalt-Rich Ferromanganese Crusts 

 on the Flanl(s of Seamounts and Volcanic Islands 



Cobalt-enriched 

 manganese crust 



Iron-manganese crusts enriched in cobalt occur on the flanks 

 of volcanic islands and seamounts in geochemically favorable 

 areas of the Pacific. Samples have been recovered for 

 scientific purposes, but equipment for potential commercial 

 evaluation and recovery has not been developed. 



SOURCES: Office of Technology Assessment, 1987; Bonnie A. McGregor and 

 Terry W. Offield, "The Exclusive Economic Zone: An Exciting New 

 Frontier," U.S. Department of the Interior, Geological Survey. 



3. can be found within the U.S. EEZ which 

 could provide a more stable investment cli- 

 mate, and 



4. may provide alternative sources of strategic 

 metals. 



Photo credit: National Oceanic and Atmospheric Administration 



Pillow basalt on the crest of the Juan de Fuca Ridge. 

 Lava that flows out onto the seafloor from submarine 

 volcanoes and spreading centers commonly forms 

 pillow-like features. Ferromanganese crusts form on 

 features such as these, which makes the microrelief 

 of any future potential mine site an important 

 constraint in developing a crust mining system. 



Geologic Considerations 



Ferromanganese crusts range from thin coatings 

 to thick pavements (up to 4 inches) on rock sur- 

 faces that have remained free of sediment for mil- 

 lions of years. The deposits are believed to form 

 by precipitation of hydrated metal oxides from near- 

 bottom seawater. The crusts form on submarine 

 volcanic and phosphorite rock surfaces or as nod- 

 ules around nucleii of rock or crust fragments. They 

 differ from deep ocean nodules, which form on the 

 sediment surface and derive much of their metals 

 from the interstitial water of the underlying sedi- 

 ment. Several factors appear to influence the com- 

 position, distribution, thickness, and growth rate 

 of the crusts. These factors include metal concen- 

 tration in the seawater, age and type of the sub- 

 strate, bottom currents, depth of formation, lati- 

 tude, presence of coral atolls, development of an 

 oxygen-minimum zone, proximity to continents, 

 and geologic setting. 



The cobalt content varies with depth, with max- 

 imum concentrations occurring between 3,300 and 

 8,200 feet in the Pacific Ocean. Cobalt concentra- 

 tions greater than 1 percent are generally restricted 

 to these depths. Platinum (up to 1 .3 parts per mil- 



lion) and nickel (to 1 percent) are also found asso- 

 ciated with cobalt in significant concentrations in 

 many ferromanganese crust areas. Other metals 

 found in lesser but significant amounts include lead, 

 cerium, molybdenum, titanium, rhodium, zinc, 

 and vanadium (table 2-6). 



At least two periods of crust formation occur in 

 some crusts. Radiometric dating and other analy- 

 ses indicate that crusts have been forming for the 

 last 20 million years, with one major interruption 

 in ferromanganese oxide accretion during the late 

 Miocene, from 8 to 9 million years ago, as detected 

 in some samples. During this period of interrup- 

 tion, phosphorite was deposited, separating the 

 older and younger crust materials. In some areas, 

 there is evidence of even older periods of crust for- 

 mation. Crust thickness is related to age; conse- 

 quently, within limits, the age of the seafloor is an 

 important consideration in assessing the resource 

 potential of an area. However, crust thickness does 

 not ensure high cobalt and nickel concentrations. 



The U.S. Geological Survey found thick crusts 

 with moderate cobalt, manganese, and nickel con- 

 centrations on Necker Ridge, which links the Mid- 



