ABSTRACT
Mo-rich hydrothermal Mn crusts have been sampled in 1970 m water depth in the hydrothermal ly active SO-99 field just south of the Triple Junction of the Central Fiji Ridge in the North Fiji Basin. The up to 30 mm thick crusts consist of dense massive Mn oxide layers of pure todorokite and birnessite with submetallic lustre. Mo concentrations are between 826 and 1090 ppm, Mn/Fe fractionation is between about 100 and larger than 1000. Leaching experiments revealed that Mo is mainly (> 90%) associated with the Mn oxide phase. Mineralogical and chemical analyses suggest substitution of Mn by Mo in the octahedral texture of the phyllomanganate lattice. Enrichment of Mo in the hydrothermal fluids is due to leaching of Mo-rich source rocks: it may escape from precipitation from high-temperature fluids due to subsurface boiling of these fluids.
