Fission track geochronology of Itoshima Granodiorite and Sawara Granite in the Gokayama area, northern Kyushu: timing of Miocene to Pliocene hydrothermal activity in Cretaceous granitoids

摘要

Ruins of gold prospects occur within the Sawara Granite in the Gokayama district, located in the upper reaches of the Naka River. The granitoids around the gold prospects have been altered by hydrothermal activity. This hydrothermal alteration is characterized by the appearance of illite and pyrite. The maximum temperature of the alteration is estimated to be 230-280°C, based on previous fluid inclusion analysis. Fission track (FT) geochronological analysis was carried out to evaluate the cooling history of the granitic basement and the timing of the hydrothermal alteration. Zircon and apatite FT ages range from 83.1-62.0 Ma and 18.2-3.8 Ma, respectively. These FT ages and previous Rb-Sr data define a typical cooling history for the granitic basement in this area. A group of younger apatite FT ages (6.3-3.8 Ma) from the alteration zone successfully indicates the hydrothermal activity in the Miocene to Pliocene period.