1997 Volume 47 Issue 2 Pages 45-58
The Seta low sulfidation epithermal gold deposit is the most recently discovered gold deposit in Japan. The deposit is characterized by well preserved near surface hydrothermal activity features such as the presence of in-situ silica sinter, kaolin deposit, mercury deposit and hydrothermal breccias. These features were the clues for discovery of the deposit. The main mineralization zone is located at approximately 250 m below the present land surface, and a very shallow depth of gold mineralization is proposed.
Mineralization occurs in two stages, (i) white crustiform quartz veins and (ii) black sulfide-rich breccia. Both types are hosted predominantly by tuff breccia and andesite lavas. The former at maximum reaches 19 m in core length and locally they may grade up to 50g/t. They formed from an ascending near neutral hydrothermal fluid. The veins in the north of the area are steep and has a NW-SE succession but at the south to southwest area, the veins become narrower and irregular and at the southernmost of the area, the veins become to be horizontal. This suggests that the hydrothermal fluids were restricted by the hydrostatic gradient when flowing from north to south. The latter exists in four parts and are oriented in a NW-SE trend forming an en-echelon pattern. Width reaches 25 m in core length but gold grade is generally low. It is proposed that it formed by mixing of following upwelling neutral hydrothermal fluid and low pH water percolating downwards from the aerated ground which were developed by condensation and oxidation of H2S rich steam accompanied by reduction of the watertable.
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