Morphology and chemical composition of ferromanganese deposits in the Southwestern Pacific Basin

An extensive field of iron-rich, nickel- and copper-poor manganese nodules covers the deep sea-floor of the northeastern quadrant of the Southwestern Pacific Basin, and the eastern side of the Samoan Basin. The nodule field has a relatively well-defined western boundary, the location of which is determined by differences in sedimentation regimes. Immediately east of the Tonga-Kermadec volcanic arc and New Zealand, as well as around Rarotonga (Cook Islands) and the Samoan chain, rapid accumulation of volcanic ash and microfossils has buried potential nodule nuclei before significant ferromanganese oxide encrustation could occur. Further east, where sedimentation rates are lower, the nodule field has developed. The nodules occur almost exclusively on medium to dark brown silty clays which contain a significant proportion (5-15%) of red-brown semi-opaque oxides (RSO's). The brown silty clays consist principally of phyllosilicates (illite, montmorillonite, chlorite and kaolinite) plus subsidiary quartz and feldspar. Microfossils are generally rare, but all of the brown silty clays contain trace quantities of calcareous nannofossils.