Geochemical and mineralogical composition of ferromanganese precipitates from the southern Mariana arc: Evaluation, formation, and implications

Abstract The Mariana intraoceanic volcanic arc system in the western Pacific Ocean hosts abundant ferromanganese (Fe Mn) precipitates. A suite (n = 22) of Fe Mn precipitates were collected from the southern portion of the arc and their mineralogies and chemical compositions were determined. These results were used to decipher their genetic assemblage, assess their potential as a source of trace metals, and place them into context with respect to Fe Mn precipitates sampled at higher latitudes in the Mariana arc and from other locations, globally. Minerals identified include vernadite, birnessite, 10 A manganate, manganite, hematite, goethite, maghemite, calcite, rhodochrosite, quartz, phillipsite, various feldspar, pyroxene, and clay minerals. Element discrimination diagrams indicate that the samples are predominantly of hydrothermal and hydrogenetic-hydrothermal (i.e., mixed) origin, with most reflecting some influence of both. Rare earth element and Y (REY) profiles are distinguished by negative Ce and Y anomalies and positive Eu anomalies. Together, the samples form a continuum from the hydrogenetic to hydrothermal endmembers. Samples with the largest hydrogenetic component are friable with branching oxide/oxyhydroxide growth structures, contain mostly vernadite, and have the greatest concentrations of most metals (including the REY elements). Hydrothermal input produces denser, cemented deposits that contain more 10 A and 7 A manganate minerals and lower minor-metal contents. Calculated growth rates range from 6 mm to >190 m Ma−1. Average metal contents of Fe Mn precipitates from the southern Mariana arc are low relative to hydrogenetic Fe Mn crusts and hydrothermal Fe Mn deposits from the northern Mariana arc and elsewhere, globally, and are therefore unlikely to be viable exploration targets.