ALBERT

Description: The Tuamotu Archipelago in French Polynesia is a Co-rich ferromanganese crust province. The NODCO I survey (1986) provided detailed data on Co-rich crusts in this environment through the exploration of a restricted zone in the vicinity of Niau Island on the southern flank of the archipelago. This flat zone is a fossil atoll which, under the action of subsidence and tectonic movements, has collapsed to a water depth of 1000 m. The plateau is partially filled with coralline sediments. Outcrops of ferromanganese crusts, associated with rare nodules and slabs, are located on the inner side of the coral reef which bounds the ancient lagoon. The successive episodes of plateau history have been recorded in the different growth periods of the ferromanganese crusts. The crusts, nodules and slabs belong to the same morphological, mineralogical and geochemical family. Cobalt contents vary from 0.7 to 1.3%. The highest values belong to the thinnest ferromanganese crusts which are located on the flanks of the plateau. Average Ni contents are about 0.5% and Cu contents about 0.1%; Pt contents vary from 0.2 to 1.3 ppm. Platinum and Co are enriched in the outermost oxide zone of the crusts. Poorly crystallized -MnO2 is the dominant mineralogical phase. Cobalt enrichment seems to be related to -MnO2 particle size. The greatest contents are located in the finest material where the particle size is less than 0.1 m. Cobalt-rich crusts of the Niau Zone have the same characteristics as the Co-rich crusts from the Equatorial North Pacific. They differ in original setting: the reefal environment in the Niau Zone is superficial, overlying a volcanic substrate.

Description: Chemical and mineralogical compositions of ferromanganese oxide coatings on rocks dredged from the New England Seamounts, the Sierra Leone Rise and the Mid-Atlantic Ridge near the Equator have been determined in an investigation of regional differences in Atlantic ferromanganese deposits. Most encrustations are clearly of hydrogenous origin, consisting mainly of todorokite and delta MnO2, but several recovered from the equatorial fracture zones may be hydrothermal accumulations. Differences in the chemistry of the water column and in growth rates of the ferromanganese coatings may be important in producing this regional contrast in composition. Fine-scale changes in element abundances within the encrustations indicate that the nature of the substrate has little influence on compositional variations.