Abstract
The Yamada deposit of the Hishikari gold mine, in southern Kyushu, Japan, develops 1.2 km southwest of the Hishikari-Honko deposit. It is composed of the Yusen and Seisen veins. As a result of exploration of the Yusen veins that commenced in 1991 and of the Seisen veins in 1995, the Seisen veins may become a bonanza of the Hishikari mine.
The veins of the Yamada deposit can also be divided into three types. Type-I veins contain white montmorillonite and are characterized by occurrence of the rare mineral truscottite. They develop in the eastern Yusen veins (typically in the Yusen No.1 vein). Type II veins contain green montmorillonite and/or saponite. Examples are the Seisen veins, which occur in the northern part of the Yamada deposit. Type III veins develop in the southwestern Yusen veins. They are compact and contain smaller amounts of clay mineral than Type-I and Type-II veins.
The Seisen No.2 vein, a champion vein among the Type-II veins, is characterized by the dominant presence of saponite. Further, ores and electrum of this vein exhibit exceptionally high Au/Ag ratios (>4 for ores and 0.64 to 0.83 for electrum); these ratios are higher than those of other veins in the Hishikari mine. It is therefore likely that certain depositional environments were responsible for the formation of the Seisen No. 2 vein, which has many other signatures of low sulfidation Au deposits.
The Yamada vein system has three orientations with N30°E, N50°E and N70°E in descending time based on cross-cut relations. This is consistent with K-Ar ages, which range from 0.66±0.06 Ma to 1.25±0.17 Ma.
