Characteristics of Co-rich ferromanganese nodules and crusts sampled in French Polynesia

Co-rich ferromanganese nodules and crusts were sampled at water depths of about 1000 m on the southern flanks of the Tuamotu Archipelago in French Polynesia. Crusts developed on a hard sedimentary substrate form a continuous ferromanganese encrustation, about 2.5 cm in average thickness, on the plateaus and slopes of seamounts. Nodule clusters occur within the crusts on the tops of the plateaus. A detailed study of the structure, mineralogy and geochemistry of the nodules and crusts established the following: (1) the nodule and crust cores are made up of phosphatized breccia with brecciated biogenous limestone elements, which reflects the gradual subsidence of the archipelago since the middle Eocene; (2) two discontinuous growth episodes of ferromanganese oxide zones were separated by a deposition of carbonate-apatite; (3) Co increases according to the morphological oxide layers, from the inner layer (0.5% Co) to the peripheral layer where Co concentrations reach up to 2%; and (4) Co enrichment appears to be related to the Oxygen Minimum Layer. The characteristics of samples from French Polynesia are similar to those of Co-rich crusts from other regions of the North Pacific. Co-rich nodules and crusts from the Tuamotu Archipelago should be considered as a potential metal resource, similarly to those from the Hawaiian-Emperor Archipelago and the Line Islands-Mid-Pacific Mountains system.