1987 Volume 37 Issue 204 Pages 227-236
High grade gold ores were intersected in 1981 by a scout drill carried out by the Metal Mining Agency of Japan during a reconnaissance drilling programme at Hishikari, Kagoshima Prefecture. The original aim of the hole was to explore a deep target where geophysical anomalies overlapped the abandoned old drifts (gravity-high, resistivitylow by airborne EM, and resistibity-low near the surface and high at the depth by the schlumberger deep-sounding).
Subsequently, a follow-up drilling programme was commenced by the Sumitomo Metal Mining Co., Ltd, the property owner, and all the 18 holes intersected high tirade ores.
The underground development was started in January 1983, and the first crosscut intersected the ore body at 100 mL (a.s.l.) in July 1985.
The deposit is of an epithermal gold-siver bearing quartz-adularia vein with unusually high gold content. The overall Au: Ag ratio is about 1: 0.6. The deposit has a "semi-blind" nature, as the top of the present high grade portions occur some 120 m underneath the abandoned old drifts.
Several major veins and numerous veinlets have so far been encountered in an area some 800 m by 100 m.
Veins strike in N45°to 70°E and dip 70°N to 90°, and their widths vary from a few cm up to 8 m. They occur in both andesitic volcanics of Pleistocene and sedimentary rocks of late Cretaceous to early Palaeogene. The K-Ar ages of mineralization are 0.78±0.07 and 1.04±0.07 m.y. (vein adularia).
The major constituent ore minerals are electrum, naumannite, and chalcopyrite, with minor amounts of pyrite, sphalerite, galena, stibnite, and marcasite. Quartz, adularia, montmorillonite, and a minor amount of calcite, chlorite, and truscottite are so far identified as gangue minerals.
Large volumes of hotwater with a temperature of 59-65°C occur within the vein system. The original static water level was as high as 200 m a.s.l.
