2023 年 129 巻 1 号 p. 17-33
We identified six Kuroko-type deposits in the Izu collision zone, central Japan, which formed as seafloor massive sulfides (SMS) in the Paleo-Izu Arc before its collision with the Honshū Arc during the middle Miocene. These deposits are found in the same stratigraphic horizon (~15 Ma) in the Nishiyatsushiro and Koma groups, between the basaltic volcanic sequence of the Furusekigawa Formation (or its equivalent) and the hanging-wall mudstone of the Tokiwa Formation of the Nishiyatsushiro Group (or the equivalent sedimentary unit in the Koma Group).
The most remarkable difference between the Kuroko-type deposits in this region and typical Kuroko deposits in the back-arc troughs of the Honshū Arc is the close association of the former with basalt, in contrast to the common association of the latter with rhyolite- or dacite-dominant bimodal volcanism. We interpret the deposits in this region as Kuroko-type deposits because they are the products of arc volcanism and related hydrothermal activity. This conclusion is supported by the sulfur isotopic compositions [δ34S vs. Canyon Diablo Troilite (‰)] of gypsum ores from the Takara and Mogura deposits, which fall within a narrow range of values (+21.9‰ to +22.5‰ and +20.1‰ to +22.0‰, respectively) that are consistent with those of middle Miocene seawater sulfate.
Recent exploration in the present-day Izu-Bonin arc indicates that SMS deposits occur exclusively at the summit or within the summit crater or caldera of submarine volcanoes where high-temperature hydrothermal activity could be focused. Therefore, it is highly likely that the footwall basaltic lava and pyroclastic units of these Kuroko deposits are components of the volcanic edifices that hosted the mineralization. The SMS deposits collided as parts of the thin-skinned uppermost crust of the Paleo-Izu Arc and accumulated near major faults, including the Itoigawa-Shizuoka Tectonic Line and Tonoki-Aikawa Tectonic Line.
