Geochemical and mineralogical studies of ca.12 Ma hydrothermal manganese-rich rocks in the Hokuroku district in Japan

Abstract Geological and geochemical studies were performed on ca. 12 Ma Mn-rich rocks in the Hokuroku district in Japan to constrain the origin of Mn-rich hydrothermal fluids and precipitation pathways of Mn oxides. Mn-rich rocks occur in hyaloclastites of rhyolite lavas in the studied areas. Colloform textures of Mn oxides were common in those Mn-rich rocks. Carbonaceous layers uniquely alternate with Mn oxides in the colloform textures. The Mn-oxides layers in the colloform textures were composed of aggregates of nano-scale todorokite, birnessite, and pyrolusite. The carbonaceous layers also host apatite nano-crystals. These findings imply that the carbonaceous layers most likely represent fossilized microbial mats of Mn-oxidizing bacteria. The close association of nano-scale Mn oxides with organic matter, clay and silica minerals suggests that Mn oxides initially precipitated near venting areas of low-temperature hydrothermal fluids. The early Mn oxides were trapped in the hyaloclastites and experienced diagenetic modifications combined with hydrothermal alteration, seawater penetration, and microbial activities in the hyaloclastites. Such secondary processes further enriched Mn in the hyaloclastites, forming various Mn silicates (e.g., pennantite and shirozulite) and veins made of Mn (II and III) silicate and oxide/silicate (e.g., rhodonite and braunite). Later hydrothermal fluids also brecciated the massive Mn-rich rocks. Rhodochrosite and rhodonite occurred in such brecciated Mn-rich rocks or sheared hyaloclastites preferentially. The carbon isotope compositions of rhodochrosite ranged from -26.7‰ to -20.8‰ (VPDB). The mudstone and early colloform textures in the hyaloclastites contained appreciable amounts of organic matter. Such organic matter was degraded by microbial processes under the influence of the later hydrothermal activities, followed by the formation of rhodochrosite. These findings contribute not only to understanding the formation mechanisms of hydrothermal Mn-rich rocks, but also to future exploration of Mn deposits.