Timing of volcanism and hydrothermal alteration in the Onta-Yamakuni gold area, northern Kyuksu, Japan
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The Onta-Yamakuni gold area is located in the northwestern part of the Neogene Hohi volcanic zone, northern Kyushu. Hydrothermally altered volcanic rocks in the area host gold-bearing quartz veins and clay deposits. The Asahi mine as a well known gold mine has produced at least 0.3 tonnes of gold during 12 years from 1935 to 1953.<BR>On the basis of alteration mineral assemblages, the gold area was divided into four zones, the chlorite zone which widely distributes in the central part, the mixed layer mineral zone which surrounds the chlorite zone, the smectite zone and unaltered zone. The smectite zone is subdivided into the inner quartz-smectite subzone and the outer cristobalite-smectite subzone. Gold-bearing quartz veins and sericitized rocks occur in the chlorite zone. Fluid inclusion studies on quartz and calcite indicate that the mineralization temperatures are 140-260°C in the Asahi gold deposit, and that the alteration temperatures are 220-270°C in the chlorite zone, and ca. 170°C in the mixed layer mineral zone.<BR>Three unaltered volcanic rocks and four sericite separates from argillized andesites were dated with K-Ar method. The Dainichigatake Volcanic Rocks gave 4.49±0.20 Ma, consisting with the reported fission track ages (4.5-4.3 Ma). The Jogusan Andesite and the Bukunoyama Dacite gave 3.37±0.10 Ma'and 3.29±0.08 Ma, respectively, suggesting the later stage of volcanic activity in the southern part of the area. Sericite gave 3.67±0.08 Ma and 3.71±0.08 Main the Onta area and 3.77±0.09 Ma and 4.10±0.09 Ma in the Yamakuni area. The last one indicates the age of gold mineralization at the Asahi deposit. These newly obtained K-Ar ages and field observations strongly suggest that the hydrothermal alteration and gold mineralization occurred in the final stage of the volcanic activity in the Onta-Yamakuni area.
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