Minerals Report - International Seabed Authority
Minerals Report - International Seabed Authority Minerals Report - International Seabed Authority
economic value. Phosphatisation has also been found in slope deposits, probably in relation with up-welling phenomena [21]. The wet specific gravity of crust material is reported to vary from 1.6 to 2.1 g cm -3 [24]. The crust material, as is the case with polymetallic nodules, is very porous (43-74%). Accordingly, in the Tuamotu area, the average dry specific gravity was 1.4 g cm -3 . During the surveys made by CNEXO 3 (1970) then IFREMER 4 in this area [25-26], a submerged old atoll was discovered near Niau Island. The depth of the plateau is 1 000-1 200 m limited by steep flanks of 400 m where the slope is more than 25%. Of the total area of 270 km 2 , at least 80 km 2 are coated with encrustations, with an apparent coverage ratio of 70%. The surface of the crust is bumpy with smooth decimetric microtopogaphy. Sandy sediments with ripple marks occupy enclosed sectors. One can suspect the existence of buried crust beneath this sediment, as found in other deposits [27]. The large blocks of crust, that have been dredged, showed a phosphaticcalcareous core, light brown and well consolidated. Fossil foraminifers give an age of 45 Ma (middle Eocene). The outer part is altered with micro fissures impregnated by ferromanganese hydroxides. The crust, dark black and 2 to 5 cm thick, is more continuous and compact at the top part of the blocks. From the surface of this crust, the cobalt content decreases from 2% in the first 3 mm to 1.7% in the next 15 mm and 0.6% in the following 15 mm. A bulk sample taken from the crust had average grades of 0.33% Co, 0.2% Ni, 0.06% Cu, 9.7% Mn, 7.9% Fe. In 1986, an attempt was made to sample the deposit with a pyrotechnic multicorer with relative success [28]. The representativeness of such sampling versus future mining is questionable, as is the case for many of the surveys conducted elsewhere. An attempt to be more effective was made using a large gravity corer [29]. However, the penetration through the crust and its substrate is limited and consequently does not show a fair picture of the deposit. 3/ Centre national pour l'exploitation des océans, that formed IFREMER in 1984 by merging with the French Institut des pêches maritimes. 4/ Institut Francaise pour la Recherche et l'Exploitation de la Mer (IFREMER). INTERNATIONAL SEABED AUTHORITY 430
At Niau, the tonnage of material with economic merit has been estimated to be 1 million dry tonnes with average cobalt content of 1.2%, based on recovering only the first 2 cm of the crusts. Dilution of this layer by the underlying material will certainly occur, but it seems possible to separate the crust by ore processing techniques. Several similar deposits have been discovered in the vicinity [26] that could increase this speculative inferred resource to five Mt, which could then produce 50 000 t of cobalt. Total resources of cobalt from Co-rich crusts in the central Pacific have been estimated to 500 million tonnes [30] (100 deposits similar to Niau). The exploration techniques must be improved considerably in order to provide the necessary parameters for the design of mining and processing methods. A better knowledge of the micro topography can be obtain by using deep-towed multibeam sonar associated with continuous high resolution TV recordings. Sampling methods must be completely retailored. Rotary diamond drilling machines equipped with a multicorer system must be developed to provide a fast and cheap sampling method with better recovery. 2.2.3. Massive sulphides deposits Unlike polymetallic nodules and crusts, massive sulphides were discovered rather recently. In February 1978, the French manned submersible Cyana went to an area where it found tall conical mounds made of scoriaceous material that was sampled. Several months later, the samples proved to be mostly composed of zinc and copper sulphides [31]. One year later, from the American submersible Alvin, hot springs were observed blowing black smoke at the top of similar "chimneys". Since, many occurrences of black smokers and massive sulphides mounds have been discovered, not only on spreading ridges, but also in back-arc subduction zones and the flanks of intraplate volcanoes, as predicted from fossil massive sulphides deposits [32-34]. In spite of the considerable amount of survey work that has been done, the available information is still inadequate to evaluate the economic potential of these occurrences. INTERNATIONAL SEABED AUTHORITY 431
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economic value. Phosphatisation has also been found in slope deposits,<br />
probably in relation with up-welling phenomena [21].<br />
The wet specific gravity of crust material is reported to vary from 1.6<br />
to 2.1 g cm -3 [24]. The crust material, as is the case with polymetallic nodules,<br />
is very porous (43-74%). Accordingly, in the Tuamotu area, the average dry<br />
specific gravity was 1.4 g cm -3 .<br />
During the surveys made by CNEXO 3 (1970) then IFREMER 4 in this<br />
area [25-26], a submerged old atoll was discovered near Niau Island. The<br />
depth of the plateau is 1 000-1 200 m limited by steep flanks of 400 m where<br />
the slope is more than 25%. Of the total area of 270 km 2 , at least 80 km 2 are<br />
coated with encrustations, with an apparent coverage ratio of 70%. The<br />
surface of the crust is bumpy with smooth decimetric microtopogaphy. Sandy<br />
sediments with ripple marks occupy enclosed sectors. One can suspect the<br />
existence of buried crust beneath this sediment, as found in other deposits<br />
[27]. The large blocks of crust, that have been dredged, showed a phosphaticcalcareous<br />
core, light brown and well consolidated. Fossil foraminifers give an<br />
age of 45 Ma (middle Eocene). The outer part is altered with micro fissures<br />
impregnated by ferromanganese hydroxides. The crust, dark black and 2 to 5<br />
cm thick, is more continuous and compact at the top part of the blocks. From<br />
the surface of this crust, the cobalt content decreases from 2% in the first 3 mm<br />
to 1.7% in the next 15 mm and 0.6% in the following 15 mm. A bulk sample<br />
taken from the crust had average grades of 0.33% Co, 0.2% Ni, 0.06% Cu, 9.7%<br />
Mn, 7.9% Fe. In 1986, an attempt was made to sample the deposit with a<br />
pyrotechnic multicorer with relative success [28].<br />
The representativeness of such sampling versus future mining is<br />
questionable, as is the case for many of the surveys conducted elsewhere. An<br />
attempt to be more effective was made using a large gravity corer [29].<br />
However, the penetration through the crust and its substrate is limited and<br />
consequently does not show a fair picture of the deposit.<br />
3/ Centre national pour l'exploitation des océans, that formed IFREMER in 1984 by<br />
merging with the French Institut des pêches maritimes.<br />
4/ Institut Francaise pour la Recherche et l'Exploitation de la Mer (IFREMER).<br />
INTERNATIONAL SEABED AUTHORITY 430
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