- Copyright © 2000 Society of Exploration Geophysicists
The Hishikari epithermal gold-silver deposit, one of the world's richest gold mines, is in northeastern Kagoshima on Kyushu Island, southwest of Japan (Figure 1). The area is characterized by volcanic activity, ranging from the active Sakurajima and Kirishima volcanoes to local hot springs.
Although gold prospecting in the area dates from 1750, it formerly was intermittent and small scale. The major deposits currently being developed were not discovered until geophysical surveys were applied to the area. They played an important role in the 1981 discovery and subsequent development of the mine.
Regional geologic mapping and reconnaissance geophysical surveys were conducted by the Metal Mining Agency of Japan (MMAJ) in 1975–78. A gravity survey in 1975–76 revealed a northeast-trending gravity high (amplitude = 4 mGal). In 1976, electrical sounding using a Schlumberger array and a heliborne EM survey were carried out. Both detected a low-resistivity zone. Exploration drilling was carried out where the high-gravity anomaly overlapped the low-resistivity anomaly. All three holes intersected high-grade gold veins.
Subsequently, 18 more holes were drilled and carried out by Sumitomo Metal Mining (SMM), the property owner; all intersected high-grade ores. Underground development commenced in 1983. Between 1983 and 1987, SMM also tested various geophysical methods in the area to examine their potential application for exploration for this type of gold deposit. These included several methods not applied routinely to mineral exploration in Japan—MT, CSAMT, timedomain IP (TDIP), TEM, etc.
Geology and ore deposit
The area is underlain by the Shimanto Supergroup of Cretaceous age and volcanic rocks/alluvial deposits of Quaternary age. Figure 2 shows geologic sections of the Honko and Yamada areas.
The Shimanto Supergroup, the basement rocks of the southern half of Kyushu Island, are interbedded shales and sandstones. No …