New Mineralogical and Geochemical Features of Ferromanganese Ore Deposits in the Izu-Bonin and Mariana Island Arcs

Some features of ferromanganese deposits in the Izu-Bonin and Mariana Island arcs tested in cruises 1 (1977) and 5 (1978) of the R/V Vulkanolog are considered. Two types of genetic (hydrogenic and low-temperature hydrothermal) deposits have been identified. The main ore minerals in hydrogenic ferromanganese deposits are poorly crystallized structures with a low ordering of Fe-vernadite and Mn-feroxyhite, with a smaller amount of goethite and birnessite. Low-temperature hydrothermal deposits consist either primarily of birnessite along with vernadite and goethite or of hematite, goethite, and feroxyhite. Hydrogenic ferromanganese deposits are characterized by a Mn/Fe ratio of 0.84‒1.36 in the Izu-Bonin and Mariana Island arcs; low-temperature hydrothermal deposits of the Izu-Bonin arc, by 6.13‒13.9. It was found that the content of Co, Ni, and Cu is significantly higher in crusts of the Mariana arc compared with the crusts of the Izu-Bonin arc. The contents of the remaining cations of heavy and rare metals (Pb, Cd, Ba, Sr, and others) in the crusts of both arcs are close to each other. The content of most rare earth metals (REM) in hydrogenic deposits from the Izu-Bonin and Mariana arcs is comparable to each other. Low-temperature hydrothermal ferromanganese deposits in the Izu-Bonin arc differ significantly from the hydrogenic deposits by a low (1 or 2 orders of magnitude) content of nonferrous, heavy, and rare metal cations. The contents of REM cations in low-temperature hydrothermal samples from the Izu-Bonin arc are low and range from 0.24 (Tm, Lu) to 32.35 µg/g (Y). Among them, Y, Ce, and Nd are most abundant in these samples.