Enrichment of Mo in hydrothermal Mn precipitates: possible Mo sources, formation process and phase associations

Hydrothermal Mn crusts were sampled at a water depth of 1970 m from the SO-99 field, which lies within a hydrothermally active area just south of the Triple Junction of the Central Fiji Ridge in the North Fiji Basin (NFB). The crusts are up to 30 mm thick and consist of dense massive Mn oxide layers with submetallic lustre. Individual layers are only a few micron thick, occur as rhythmic sequences and form spheroidal or botryoidal structures when infilling cavities. The crusts consist of pure todorokite ((Mn2+,Ca,Mg)Mn34+O7·H2O) and birnessite (Na4Mn14O27·9H2O) with no other crystalline mineral phases detected. lybdenum concentrations are between 826 and 1090 μg/g. The Mn/Fe ratio varies between about 100 and >1000. The crusts also contain high Ba concentrations between 1400 and 1930 μg/g and slightly higher Cu and Zn contents than MOR and other back-arc crusts. Leaching experiments demonstrated that Mo is mainly (>90%) associated with the Mn oxide phase. X-ray absorption spectroscopy (XAS) shows that Mo is present predominantly as octahedrally coordinated polymolybdate and that any adsorbed tetrahedral Mo constitutes only a small fraction of the total adsorbed species. Thermodynamic calculations demonstrate that heptamolybdate is the dominant Mo species which is only stable in slightly acid fluids. It is therefore suggested that Mo was retained by the Mn crusts through adsorption from Mo-rich hydrothermal fluids.