10:45 AM - 12:15 PM
[BCG06-P05] Geological and mineralogical studies of ca. 12 Ma submarine hydrothermal iron oxides in the Hokuroku area, Akita Prefecture, Japan
Fe-oxide-ores, including Precambrian banded iron formations, occur in various geological ages. On the other hand, sources and transportation mechanisms of Fe were poorly constrained for Fe-oxide ore formations. Besides, the role of microbial activities has not been well understood in the deposition of Fe-oxides. Here we report the geological and geochemical characteristics of ca. 12 Ma Fe-oxide ores at the Hokuroku district to in Akita, Japan to approach above problems.
Geological surveys revealed the different occurrences of Fe-oxide ores. Fe-oxide ores are mostly found in peperite on the surfaces of submarine rhyolite lava domes. It is found that shallow submarine hydrothermal systems developed and migrated in peperite. Rhyolite magma itself was the heat source to drive submarine hydrothermal circulations. Minor ores are found in mudstone pebbles in rhyolitic pyroclastic rocks, or mudstone layers intruded by dolerite. Shallow submarine hydrothermal systems were driven by heat energy from dolerite, and alteration extended in mudstone pebbles or mudstone layers.
Fe-rich green clay minerals, e.g., celadonite or nontronite, were found in the green-colored peperite. Fe-rich green clay minerals were often associated with organic matter. These indicate that organic matter in sediments played as reductant of Fe in host rocks allowing Fe2+ extraction from host rocks and, then, a part of Fe were trapped in clays simultaneously.
Various forms of hematite are found in examined "red" rocks. Hematite ores are mainly hosted in altered peperite and altered mudstones. Hematite-quartz colloforms often appear in hydrothermal veins. Fe and Si formed separated microscopic layers in each colloforms in most cases. Such segregation of Fe and Si may represent redox conditions of injected hydrothermal fluids at each layer-forming stage. Fe and Si alternation is apparently mimicking Precambrian banded iron formations. Organic-rich layers, interlayered with Fe or Si, are found in the colloforms. Raman spectroscopic analyses indicate immature characteristics of interlayered organic matter, rather than typical sedimentary organic matter. This suggests “in situ” microbial activities around the site of hematite formation, leaving organic matter on Fe-oxide layers. All observation in the present study imply the role of organic matter or microbial activities to enhance mobility and transportation of Fe2+, and deposition of Fe-oxides even in the hydrothermal environments.
Geological surveys revealed the different occurrences of Fe-oxide ores. Fe-oxide ores are mostly found in peperite on the surfaces of submarine rhyolite lava domes. It is found that shallow submarine hydrothermal systems developed and migrated in peperite. Rhyolite magma itself was the heat source to drive submarine hydrothermal circulations. Minor ores are found in mudstone pebbles in rhyolitic pyroclastic rocks, or mudstone layers intruded by dolerite. Shallow submarine hydrothermal systems were driven by heat energy from dolerite, and alteration extended in mudstone pebbles or mudstone layers.
Fe-rich green clay minerals, e.g., celadonite or nontronite, were found in the green-colored peperite. Fe-rich green clay minerals were often associated with organic matter. These indicate that organic matter in sediments played as reductant of Fe in host rocks allowing Fe2+ extraction from host rocks and, then, a part of Fe were trapped in clays simultaneously.
Various forms of hematite are found in examined "red" rocks. Hematite ores are mainly hosted in altered peperite and altered mudstones. Hematite-quartz colloforms often appear in hydrothermal veins. Fe and Si formed separated microscopic layers in each colloforms in most cases. Such segregation of Fe and Si may represent redox conditions of injected hydrothermal fluids at each layer-forming stage. Fe and Si alternation is apparently mimicking Precambrian banded iron formations. Organic-rich layers, interlayered with Fe or Si, are found in the colloforms. Raman spectroscopic analyses indicate immature characteristics of interlayered organic matter, rather than typical sedimentary organic matter. This suggests “in situ” microbial activities around the site of hematite formation, leaving organic matter on Fe-oxide layers. All observation in the present study imply the role of organic matter or microbial activities to enhance mobility and transportation of Fe2+, and deposition of Fe-oxides even in the hydrothermal environments.