The Kushikino gold mine, now a gold park, which is ~50 km SW of Hishikari in Kyushu, Japan, produced 55 tonnes of gold and 497 tonnes of silver since mining commenced in 1660.
Ore occurs in quartz veins hosted by propylitically altered Miocene to Pliocene andesite. These volcanic rocks, which unconformably overlie a basement of Cretaceous sedimentary and older accretionary complexes, are part of the ~60 km diameter Kakuto caldera that embraces many of the significant gold deposits of southern Kyushu, including Hishikari.
The host andesitic volcanic suite has been divided into three units, from the base to top of Hokusatsu Koki Andesites, Hokusatsu Chuki Volcanics and the Hokusatsu Shinki Volcanics. The Kushikino veins are hosted by massive, altered tuff breccia of the Hokusatsu Koki Andesites, which yielded the K-Ar ages of 4.2 to 4.0 Ma. The host rock is composed of poorly sorted matrix including granule to pebble sized breccias. Bedding surfaces were sporadically observed with a westward dip of 15°.
The NE trending main Kushikino No. 1 vein is 2.6 km long and dips at 40° S, with a width of between 0.01 to 5.0 m. It has been exploited from the surface to a depth of 450 m.
Three major stages of mineral deposition are recognised as follows: (i) opening of fractures and deposition of translucent quartz; (ii) repeated fracturing and the main stage of the Au-Ag mineralisation with deposition of milky quartz and calcite, and (iii) deposition of barren calcite.
Quartz and calcite are the predominant vein material, together with adularia, interstratified chlorite/smectite or illite/smectite and smectite, accompanied by electrum, naumannite, polybasite and pyrargyrite. A major zone of silicification (quartz with kaolinite and interstratified illite/smectite) is developed immediately to the east of the mine.
The Kushikino veins were formed at temperatures of 250 to 210°C and at the depths of 850 to 450 m (Izawa et al., 1981). Adularia and illite from a vein of the cluster yielded the K-Ar ages of 3.55±0.11 and 3.67±0.09 Ma, respectively.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2008)
