Volcanic activity related to the formation of the Kuroko-type deposits in the Kosaka district, Japan

Abstract

In the Miocene Kosaka formation of NE-Japan, submarine volcanic sedimentary deposits of the Kuroko-type occur. This formation consists mainly of volcanics which erupted in a submarine environment. The “Kosaka Volcano” was built up by nine or more volcanic events of a single eruptive cycle. The mode of eruption during a representative single eruptive cycle changed as follows: The Uwamuki tuff breccia is a contact product between dacitic magma and the sea water. Dacitic magma pushed the Motoyama dacite dome upward, as a result of the decrease in vesicularity and perhaps also in temperature. Next, a steam explosion occurred at a flank of this lava dome. The hydrothermal activity which began in this steam explosion center is responsible for the formation of the Kuroko-type Cu-Zn-Pb-mineral deposits. Similar examples of a single eruptive cycle as at Kosaka are also found in Quaternary terrestrial volcanoes of Japan.