The Hishikari gold deposit: high-grade epithermal veins in Quaternary volcanics of southern Kyushu, Japan

Abstract

The Hishikari epithermal gold-silver deposit is located in northeastern Kagoshima Prefecture, Kyushu, Japan. Geological and geophysical surveys played important roles in the discovery, made in 1981. Subsequent mine development has proved Hishikari to be one of the major gold deposits in the western Pacific. The production from July, 1985, to December, 1988, totaled 21.7 metric tons of gold and 14.3 metric tons of silver. Ore reserves are estimated to be 1.4 million metric tons at an average grade of 70 g/metric ton gold (98 metric tons of contained gold) in the Honko ore zone and approximately 2 million metric tons at 20 to 25 g/metric ton gold in the newly discovered Yamada ore zone. The geology of the mine area is composed of basement sediments of the Cretaceous Shimanto Supergroup, and volcanic rocks of Quaternary age. The deposit is of the quartz-adularia vein type, with associated electrum, naumannite-aguilarite, pyrargyrite and smectite. K-Ar age dating of adularia bearing ore indicates a Pleistocene age of 0.84 ± 0.07 to 1.01 ± 0.08 Ma for gold mineralization. Fluid-inclusion studies indicate that the representative temperature of gold deposition was 210°C in the basement and less than 200°C in the overlying volcanic rocks. δ 18O values of quartz veins range from +8.8 to +6.8%. Chlorite and sericite alteration is directly associated with high-grade gold mineralization. Alteration zones of interstratified clay minerals and quartz-smectite envelope the mineralized center and form a near-horizontal layer of intense argillization, located 50–100 m above the Hishikari vein system. These alteration zones are surrounded by a zone of cristobalite-smectite and essentially unaltered rocks. Subsurface structures were recognized by a detailed gravity survey, with uplifted basement blocks represented by gravity highs. Schlumberger vertical soundings are useful in estimating the depth to the basement and to identify the approximate resistivity structure, while resistivity and IP surveys define the areal distribution of the hydrothermal alteration related to mineralization. Geochemical exploration using Hg, CO 2 and radon in soil gas has been effective in tracing fracture zones. The extremely high-grade gold mineralization is focused near the unconformity between basement sediments and overlying volcanic rocks of the Honko area. The high grades of gold may be explained as a combination of two processes. As the higher-temperature fluids ascended, boiling resulted in gold deposition; further mineralization was favored by subsequent mixing of the deep fluids with steam-heated groundwaters near the unconformity, causing rapid cooling and oxidation.