2025 Volume 79 Issue 2 Pages 81-101
Siliceous mudstones of the lower part of the Middle to Upper Miocene Yagen Formation are locally exposed and have undergone hydrothermal alteration at the northeastern part of Mt. Hiuchi-dake, a Quaternary volcano of the Shimokita Peninsula, Aomori Prefecture. This study aims to clarify the effects of the hydrothermal system of Mt. Hiuchi-dake on the concentration and composition of organic matter and the lithological composition of the mudstone. Mudstone samples were collected from three outcrop locations, all of which are situated at nearly the same stratigraphic level: (1) National road outcrop, (2) Oakagawa outcrop and (3) Altered continuous outcrop, and analyzed for organic / inorganic geochemical, and mineral investigations. The results showed a clear trend of decreasing total organic carbon (TOC) and hydrocarbon contents closer to Mt. Hiuchi-dake. The C/N and C/S ratios varied more widely closer to Mt. Hiuchi-dake, which indicate that hydrothermal alteration caused pyrolysis of the organic matter in mudstones, decomposition of the amino groups, generating asphaltenes and the dissolution and reprecipitation of indigenous pyrite. It is found that hydrothermal flow promoted hydrocarbon discharge, but the initial relative composition of n-alkanes was almost preserved. The estimated heating temperatures of the hydrothermal fluid were 100-110 °C at the National road outcrop, 200-240 °C at the Oakagawa outcrop, and 150-250 °C at the altered continuous outcrop (Ueda and Sampei 2024). Mudstones of the Oakagawa outcrop correspond to the transition zone between Opal CT and quartz, and preserve high porosity. In the altered continuous outcrop, there is a correlation between SiO2 concentration and porosity, and rock density increases with silicification. According to mineral composition, the altered continuous outcrop was altered by high sulfidation hydrothermal system with a high contribution from volcanic volatiles. On the other hand, the Oakagawa outcrop experienced weakly acidic to neutral hydrothermal alteration originated from heated groundwater. The age of alteration may have been after 0.1 Ma in both outcrops.
