GEOCHEMISTRY AND GEOCHRONOLOGY OF A HYDROTHERMAL FERROMANGANESE DEPOSIT FROM THE NORTH FIJI BASIN

A study of North Fiji Basin surface sediment geochemistry has indicated one core to be of predominately hydrothermal origin. This core contains a 38 cm thick ferromanganese oxide deposit that conformably overlies foraminiferal ooze. Uranium series, and major, rare earth, and trace element chemical analyses were performed in order to understand the origin and age of this unusual deposit. The onset of hydrothermal deposition at 128 +/- 18 ka B.P. has been determined by excess Th-230 dating of the oxide/ooze contact. Both Ra-226 and Rn-222 are supplied to the overlying water column by diffusion from the Th-230-rich ferromanganese deposit. Foraminifera tests within the deposit are replaced by and coated with the ferromanganese-rich material. Calculated metal accumulation rates are comparable to, or exceed, rates from other known hydrothermal areas such as the East Pacific Rise, Bauer Basin, FAMOUS and TAG areas. Rare earth element results indicate a partial hydrogenetic contribution to a predominantly hydrothermal signature in the ferromanganese part of the deposit. Vertical element distributions reveal variations that are interpreted as changes in the nature of hydrothermal activity at this site. R-mode factor analysis of the elemental data indicates three major factors: low-temperature hydrothermal, volcanic ash-detrital, and high-temperature hydrothermal that together account for > 95% of the sample variability. The combined geochemical data reveal a sequence of deposition at this site of initial volcanism (ash fall), followed by high-temperature and then low-temperature hydrothermal mineralization lasting for up to 128 ka.