Hot spring-type gold deposits

Hot spring-type gold deposits (Nansatsu-type by Urashima et al. (1981,1987), high sulfidation-type by Hedenquist (1987), epithermal Au disseminated-type) are distributed in the Nansatsu district of southern Kyushu (Fig. 1.193). The deposits (Kasuga, Akeshi, Iwato) were formed at Pliocene age (5.5-3.7 m.y.) in the calc-alkaline volcanic rocks of nearly same age (Togashi and Shibata, 1984). The deposits, which are similar to Nansatsu-type deposits, occur in Southwest Hokkaido (Date, Hakurhu). 

The deposits are characterized by conspicuous alteration zoning from the centre (orebody) to margin (Tokunaga, 1955 Doi, 1972 Urashima et al., 1981, 1987). They are siliceous zone, alunite zone, kaolinite zone, sericite zone and montmorillonite zone. 

In the siliceous body, electrum and Cu minerals (enargite, luzonite, covelline), and native sulfur occur. The Ag content of electrum is lower (0.0-5.3 wt%) than that from epithermal Au-Ag vein-type deposits (Fig. 1.194) (Shikazono and Shimizu, 1987). Low Ag content of electrum and sulfide mineral assemblage (enargite, native sulfur, covellite, pyrite) indicate high fs2 condition (Fig. 1.194). 

Hedenquist et al. (1994) indicated that homogenization temperature of postmineralization of the Akeshi deposit and salinity of ca. 1 wt% NaCl eq. and the presence at times of a two-phase fluid in the centre of ore zones indicating boiling of ore fluids. 

The /oj-pH conditions for this type of deposits and other types of epithermal deposits and hydrothermal alteration are shown in Fig. 1.196 (Shikazono and Aoki, 1981). It is worth noting that the /oj-pH ranges for the epithermal Au deposits (hot spring-type, Te-type, and Se-typ e) lie between the points A (high fo, and low pH) and B (low /oj 

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