The sedimentary deposits of the tiki basin (south-east pacific) passage from carbonate oozes to "metalliferous sediments"

A sedimentological, mineralogial and geochemical study was made on surface sediment samples associated with nodules in the Tiki Basin (Central South Pacific). The aim of this study was to find variations in the sediment composition with respect to bathymetry and to thus show a relationship between nodule formation and sedimentation. It is now well known that nodules rich, from an economical point of view, are located in this area at a depth more than 4100 meters. The progressive transformation of a carbonate ooze rich in Foraminifera and poor in siliceous biogenic material into a metalifferous sediment has been shown. The CaCO(3), dissolution involves Fe and Mn enrichment of the sediment. The content of these elements becomes stable and independant of the CaCO(3), dissolution when the sediment in which nodules are present contains less than 70 % CaCO(3). The carbonate dissolution induces successive neoformations : Fe-Mn hydroxides precipitation, then, Fe-rich smectite formation related to the dissolution of siliceous organisms, and finally micronodule and zeolite formation. The metalliferous sediments of the present study are similar to those described in the Bauer depression. Their great geographical extend leads us to reconsider the hypothesis of hydrothermal enrichment in Fe and Mn. Conditions of metalliferous sediments formation are proposed : - no terrigenous removal, - reduced planktonic activity, - strong dissolution of siliceous biogenic material until complete disappearance, - strong dissolution of carbonate biogenic material, - little volcanic material. These observations have allowed us to explain why metalliferous surface sediments have been described in the South Pacific where all of the above conditions have been met. This must also explain the parlicularities of the nodules in this area. [NOT CONTROLLED OCR]