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Epithermal Gold 2012
Epithermal Gold Deposits of the Western Pacific
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[Geothermal system]
Kyushu Field Workshop - Kuju Volcano:   The tour commenced when the group assembled in Fukuoka, northern Kyushu, in western Japan, on the evening of Friday 11 May for an introductory meeting and dinner.   On the following morning, the group drove south to the Hatchobaru geothermal power station (right) where in the central pipe of the double-flash steam separator unit, (upright cylindrical unit to the left in the right image, where steam is separated from fluid), banded quartz-alunite-adularia 'scale' carrying ~1 g/t Au (lower-left) is accumulated - this pipe is ~60 cm in diameter.   The Hatchobaru power station taps hydrothermal vapour and fluid at 200 to 240°C from the NW-trending Yokoo fault zone at depths of up to 1000 m below surface in zones of quartz, alunite kaolinite-pyrophyllite and clay minerals within host pyroxene andesites.   These fluids and vapours are sourced from the Otake Geothermal Field on the flanks of the <0.3 Ma Kuju Volcano.   The hydrothermal fluids and vapour are drawn from wells and brought to the surface under pressure which is partially released, when the fluid and vapour are separated, and CO2 is released in the steam separator, accompanied by the precipitation of gold and are silica.   The group also visited the adjacent steaming ground (top-left) along the trace of the Yokoo fault, where steam and boiling mud is seeping to the surface and silica (cristobalite) is precipitated in a concentrically zoned alteration system, grading outwards from a 50 to 60 m wide, elongate, lenticular alunite core through kaolinite to a smectite halo.   The steaming ground is ~300 to 500 m above the geothermal reservoirs, which are driven by a deep igneous heat and partial fluid source.   Professor Sachihiro Taguchi of Fukuoko University was the technical leader of the field workshop, which continued for 4 days, interspersed with visits to the Kasuga (Nansatsu-type) high sulphidation, and Hishikari low sulphidation epithermal gold deposits (see following images).   Photographs by Mike Porter.

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