[S-VC28] Hydrothermal systems of volcanoes

[SVC28-06] Origin and cycling process of elements in hydrothermal alteration zones: Cause of the strong acidic water in Dokuzawa mineral spring, Shimosuwa Nagano

*Ryosuke Yanagisawa¹, Koichi Sakakibara¹, Hirochika Sumino², Nanae Fukushima³, Kou Takahashi¹, Junichi Yamamoto⁴, Terumi Ejima¹ (1.Faculty of Science, Shinshu University, 2.Research Center for Advanced Science and Technology, The University of Tokyo, 3.Graduate School of Arts and Sciences, The University of Tokyo, 4.Suwa Seiryo high school, Suwa, Nagano)

Keywords:water-rock interaction, highly acid water, leaching test, Hydrothermal Alteration Zones

Understanding the causes of acidic water formation in environment is important for controlling water pollution (Odri et al., 2022) Acidification is caused by the dissolution of volcanic fluids and minerals. Particularly, mineral reactions vary depending on their origin and species, leading to numerous unresolved factors (Welch et al., 2008).The objective of this study is to clarify the effects of volcanic fluids and mineral dissolution on Dokuzawa mineral spring, which is highly acidic, and to discuss the mineral species and origin of the minerals that cause the spring to become highly acidic, in order to clarify the elementary process of strong acidic water discharge. Water samples were collected from Dokuzawa mineral spring, stream water, and surrounding hot springs to evaluate the involvement of volcanic fluids throughout the region. These samples were analyzed for major dissolved ions and various isotope ratios(²H/¹H, ¹⁸O/¹⁶O, ⁴He/²⁰Ne, ³He /⁴He). Water-rock reaction experiments were also conducted using altered rocks collected from Dokuzawa mineral spring source, and dissolved elements were analyzed in the solutions after the reaction experiments. In addition, EPMA was used to observe the microstructure and analyze the chemical composition of the minerals. Analysis of the collected water samples showed that Dokuzawa mineral spring and the surrounding hot springs were of meteoric water origin. However, the dissolved gases from the surrounding hot springs confirmed the involvement of volcanic fluids and other substances. The results of the water-rock reaction experiment were similar to those of Dokuzawa mineral spring, suggesting that the fast reaction rate cannot be explained by pyrite, but by sulfate minerals. The rock samples are composed of mainly quartz, muscovite, pyrite and sulfate minerals, and rarely silver oxide. The rock is thought to have been affected by hydrothermal alteration because of the presence of pseudomorph throughout the rock, veined pyrite, sulfate minerals and silver oxide. These results suggest that the dissolution of sulfate minerals from hydrothermal alteration is more responsible for the strong acidification of Dokuzawa mineral spring than the involvement of volcanic fluids.