Origin of amagmatic hydrothermal solutions inferred from TL, FT and K-Ar dating, and fluid inclusion data; A Case study in the southern part of Kii Peninsula, SW Japan

Hanamuro, Takahiro  ; Umeda, Koji; Maeda, Katsuhiko*

Although no evidence of volcanism during the Pliocene and Quaternary is known in the Kii Peninsula, it has long been recognized to be several hot springs with discharge temperatures higher than 60 C. On the other hand, several tens of small-scale vein-type metal deposits are distributed around the southern part of Kii Peninsula, whose heat source are thought to be Middle Miocene acidic magmatism associated with Kumano Acidic Rocks. The results of the dating of altered rocks from these hot spring area, ore deposits and their surroundings altered rocks show that high temperature hydrothermal alteration related to the acidic magmatism occurred in the Middle Miocene, and relatively low temperature alteration has been caused since the Pliocene in Hongu and Totsukawa area. Chemical and isotope data were obtained for fluid inclusions trapped in hydrothermal minerals in the peripheral parts of the high-temperature hot springs, and other vein-type ore deposits. The former inclusions have very low temperatures and low salinities. On the other hand, the latter inclusions are characterized by high temperature fluid with high salinity. The He/He ratios of the former inclusions indicate relatively high values, generally in agreement with those of the present-day hydrothermal fluids, resulting in a significant contribution of deep source gases.