Hydrothermal iron-rich deposits from the Teahitia-Mehitia and Macdonald hot spot areas, Southwest Pacific

During cruise SO-47 of R.V. Sonne, iron oxyhydroxide crusts of restricted extent were recovered from Teahitia and Moua Pihaa seamounts in the Teahitia-Mehetia hot spot area and from Macdonald Seamount in the Macdonald hot spot area. The crusts show complex layering and incorporate volcanic ash layers. They consist of goethite, feldspar and pyroxene and display low contents of transition elements apart from Fe. It is believed that the crusts were deposited from relatively low temperature hydrothermal fluids. These low temperatures preclude the formation of sulphide deposits. The type of hydrothermal mineral recovered may be a function of the thermal regime of the system and the elevation, which controls the pressure of the venting fluids. The shallower depths of these seamounts compared to the axes of mid-ocean ridges may therefore be responsible for the style of mineralization. Initially, magmatic vapours migrating upwards through the hot spot volcano are thought to be absorbed into seawater at comparatively shallow levels to deposit sulphide minerals within the volcanic pile. The Fe oxyhydroxide deposits are the residual low-temperature deposits formed when the spent hydrothermal fluid reached the crest of the seamount. These deposits therefore differ fundamentally from those found at the crest of mid-ocean ridges where sulphide mounds are deposited directly upon interaction of the boiling hydrothermal fluid with seawater. The discovery of hydrothermal iron oxyhydroxide crusts in these two hot spot areas is only the second such finding after Loihi Seamount, southeast of Hawaii.