(Table 3) Chemical composition of phosphatized rocks from the Kammu Seamount

Phosphatized biogenic limestones and phosphorites with initial Fe-Mn mineralization dredged from the summit surface of the Kammu Seamount (Milwaukee Seamounts, northwestern Pacific) are studied. The rocks are largely composed of nannofossils and planktonic foraminifers with an admixture of benthic foraminifers, bryozoans, and other organic remains, presumably including bacterial ones. The nannofosssil and foraminiferal assemblages indicate Quaternary age of sediments, and their phosphatization is consistent with the phosphatization age determined previously based on nonequilibrium uranium (within the limits of 1 My). The age of phosphatization and the Fe-Mn mineralization in the sediments from Pacific seamounts that young implies dependence of these ore-forming processes on oceanic environments favorable for ore accumulation rather than on their age.

Chemical composition of rocks from the Kammu Seamount and from low atolls of the Western Indian Ocean

Devoted to studies of phosphatized rocks from the Kammu Seamount.

(Table 1) Chemical analyses of rock samples from low atolls of the Western Indian Ocean

Compositions, structures, and microstructures of different types of phosphorites and poorly phosphatized rocks from low atolls in the near-equatorial part of the Western Indian Ocean are described. The rocks were examined under optical and scanning microscopes using microprobe techniques and etching of selected samples with weak solvents as well as with the help of chemical analyses. It is proved that phosphorites have been formed owing to the uneven phosphatization of primary carbonate rocks; degree of their phosphatization ranges from traces to 40% P2O5. In the phosphorites numerous organic remains were encountered; they included fragments of plankton, debris of tortoise shells, and coccoidal and filamentous bacteria-like formations. It is suggested that the phosphorites formed due to high local biological productivity over the outer edges of coral reefs and are not related to guano accumulation or to endoupwelling.