Marine Geosciences

 

II.- POLYMETALLIC NODULES

(Posters 1 et 2)

• History

• Distribution

• Morphology

• Composition

• Origin

• Conclusion

 

II.1.- The history of the discovery of polymetallic nodules

 

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The first sub-marine ferromanganese concretions were discovered in 1868 on the Kara Sea (Russia). 

From 1873 to 1876, during the expedition around the world of the H.M.S. "Challenger", many small dark-brown balls, rich in manganese and iron, were collected. They were given the name of manganese nodules.

Around 1900, the scientist Agassiz found nodules in most sampling made in the Eastern Pacific ocean.

Nodules were also collected in cores and dredges by the scientific expeditions that surveyed the global ocean.

It was only after 1957 that the American John Mero succeeded in convincing some industrials of the economic interest of the nodules and led them to the exploration of the Central Pacific Ocean.

At the beginning of the sixties, manganese nodules started to be considered as potential resources for nickel, copper, cobalt and even manganese. 

The first involved companies were Kennecott and Newport Shipbuilding Company (1962) which started to carry on sampling cruises, while American scientists (Fuerstenau, Arrhenius) were studying the nodule geochemistry  and their metallurgic processing. 

From 1965, new companies undertook important researches in nodule exploration, mining and processing, and among them AFERNOD. 

In 1972, the "National Science Foundation" launched a research program that implied at least fifteen scientific laboratories in the study of nodule genesis. In the Soviet Union, Bezrukov published a voluminous book on Pacific nodules. However in this country, industrial exploration expanded only after 1982. 

 

II.2.- Nodule Distribution in the World Ocean

Scientific and industrial studies give a general idea on the worldwide distribution of the nodules and allow the localization of areas that contain potential ore deposits. 

Ferro-manganese concretions were found in many places of the seabed. The map, published in 1969 by Mc Kelvey, showed already the occurrence of nodules in all the oceans, some have been found also on the bottom of lakes (USA, Canada, etc...). However not all are of economic value because their low abundance and/or their small metal content (Mn, Ni, Cu, Co). 

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In 1973, Horn demonstrated the predominance of the “East-West belt of the Southern part of the Northern Pacific”, between the Clarion and Clipperton fracture zones, known afterward as the "Horn zone". It was in this area that most mining companies were already working. 

 

II.3.- Morphology

Polymetallic nodules are small balls, dark-brown colored and lightly flattened, 5 to 10 centimeters in diameter, which lay on the seabed at 4.000 to 6.000 meters deep. Their wet density is around 2 g/cm³; their water-content is 40 % of their dry-weight and their porosity is 50 %. 

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Size and shape are highly variable (roughly spherical, more or less ovoid), nodules are classified in: 

• mononodule: simple nodule, spherical or ellipsoidal, 

• polynodule: a nodule with several cores, 

• composite nodule: several joined nodules. 

In section, most nodules show concentric layers called "cortex" that correspond to the successive step of growth around a "core", often microscopic. The core can be a fragment of an old nodule, a shark tooth or a rock fragment (basalt, limestone, etc...). 

The layers are formed of hydroxides of manganese and iron more or less crystallized. The more crystallized they are (todorokite, birnessite), the richer in Mn, Ni and Cu, while cryptocristallized structures (vernadite) are richer in Fe and Co. 

Several studies showed that Ni and Cu are, either adsorbed in the ferro-manganese hydroxides, or incorporated in their lattice, filling the Mn⁺⁺ sites of todorokite or birnessite.

Grades are function of the crystallization state. The more crystallized are the nodules, the highest are their metal grades. However, valuable-element recovery (Ni, Cu) seems to be easier when crystallization is incomplete. 

The growth rate of the nodules is one of the slowest phenomenon (in the order of a centimeter by several millions years). The age of Pacific Ocean nodules is 2 to 3 millions years. 

 

II.4.- Composition

Chemical analyses of the nodules show that they contain many components, of which the majors are: 

• manganese (29.40 %); 

• iron (6 %); 

• nickel (1.34 %); 

• copper (1.25 %); 

• cobalt (0.25 %); 

• titanium (0.6 %); 

• aluminum (2.9 %) 

• and sodium, magnesium, silicium, zinc, oxygen and hydrogen (32.16 %).

 

II.5.- Origin of nodules

The problem of the genesis of the nodules is far to have been resolved. Four processes have been proposed by Bonatti (1986):

• "hydrogenous": a slow precipitation of metallic component from the seawater, that forms concretions with similar content in iron and manganese and relatively high grade in Ni+Cu+Co,

• "hydrothermal", producing concretions generally rich in iron, poor in and (Ni+Cu+Co),

• "diagenetic", giving by manganese remobilization in the column and precipitation at the sediment-water interface, nodules rich in manganese and poor in iron and (Ni+Cu+Co),

• "halmyrolitic", where the source of metallic components is the weathering of basaltic debris by the seawater.

These theories, qualified of "mineral", are opposed to the "biologic" ones of which the main defenders are Graham and Cooper (1959). For them organic debris are responsible of Cu and Ni enrichments.

It is rather difficult to opt for one of these theories. However, several processes seem necessary for nodule genesis: 

1. a low rate of sedimentation leaves concretions the possibility to grow before to be buried away from the favorable conditions of their development,

2. metallic hydroxides issued from living mater contain copper and nickel that can be incorporated to ferromanganese concretions,

3. manganese in the seawater seems to come from the leaching of the basalts and essentially from hydrothermal activity along medio-oceanic ridges,

4. concretioning is favored by biologic activity of some micro-organisms.

However fast formations of ferromanganese incrustation have been also observed near ships wrecked during the First World War (Goldberg, 1958) or around motor plugs (Andrews, 1972). 

Such fast growth imposes some restrictions concerning the component origin. If nodules are formed slowly, "hydrogenous" or "diagenetic" processes can be considered, but if they are quickly formed, metal source other than seawater is necessary. In this case, "hydrothermal" or even "halmyrolitic" processes can be envisaged.

II.6.- Conclusion

The present knowledge on polymetallic nodules allows them to be considered as resources of economic interest that merit a particular attention from mining companies.