Recent discovery of the high-grade gold deposit at Hishikari not in Cenozoic volcanics but in the underlying basement sediments leads us to examine role of the basement for the gold mineralization. Type of the basements for late Cenozoic gold deposits was briefly reviewed, and the basement rocks, mostly of sedimentary, were analyzed for trace amount of gold in selected areas of southwestern Japan. The results indicate that Ag/Au ratio and the size of gold deposits are independent of the type of the basement but dependent upon the position in the magmatic arcs, and the gold contents are as low as 1 ppb even in the basement of rich gold fields including Hishikari, Kushikino and Taio deposits in Kyushu Island.
Sulfur isotopic ratio of pyrites from altered zones and ore veins of the Hishikari deposit is averaged as δ³⁴SCDT 0‰, while the sulfur in unaltered rocks of the Shimanto Supergroup has an average of -12‰, which is unlikely to be the source for the 0‰-ore sulfur. Thus, a magmatic source is proposed for sulfur, and possibly gold, of the Hishikari deposit. In referring to the recent geological and geophysical works at the Hishikari mine, the ore deposit is considered being related to a hidden magnetite-series felsic porphyry plug which intruded at western margin of the kakuto caldera. Rising magmatic ore solutions through a normal fault zone in the uplifted basement block met the meteoric water and cooled down near the level of unconformity plane about 500 meters below the surface existing ca. 1 Ma ago, and precipitated native gold. Thus, the Hishikari deposit is of a unconformity-controlled type. The unusually high-grade ores of the ore deposit can be explained by strong concentration of gold in the primary magmatic ore solution, proximity to the hidden intrusion and the suitable condition for the gold precipitation.