Epithermal deposits

The area of the potassic alteration is not wide, compared with the propylitically altered area. The width of potassic alteration zone away from the vein is generally within several tens of meters (ca. 50 m) (Shikazono and Aoki, 1981 Imai, 1986). The potassic alteration is usually found in the intermediate vicinity of the vein in the epithermal deposits in Japan. Thus it is evident that this type of alteration occurs genetically related to the ore deposition. 

Salinities of inclusion fluids from epithermal vein-type deposits clearly indicate that the salinities of inclusion fluids from these types of deposits are distinctly different, that is, 20-2 NaCl equivalent wt% (base-metal vein-type deposits) and 0-3 wt% (Au-Ag vein-type deposits) (Shikazono, 1985b) (Table 1.13). Salinities of inclusion fluids from Kuroko deposits (0.5-5 wt% NaCl equivalent concentration) are between these two types of deposits. This kind of difference is observed in epithermal deposits in other countries (Hedenquist and Henley, 1985). 

SD and 8 0 values for epithermal deposits from other countries are summarized in Fig. 1.105 (Field and Fifarek, 1985). The oxygen shift away from the meteoric water line is always observed, but 8D is similar to meteoric water value, suggesting meteoric water source of epithermal ore fluids. Magmatic contribution to ore fluids has not been found except in some ore fluids responsible for the deposits in the other countries Tui... 

Figure 1.108. Distribution.s of S C in epithermal deposits (Field and Fifarek, 1985).

Lead isotopic data on the epithermal deposits together with Kuroko deposits are plotted in Fig. 1.116 (Sato and Sasaki, 1973 Sato et al., 1973, 1981 Sato, 1975 Sasaki et al., 1982 Sasaki, 1987 Fehn et al., 1983). It is evident that lead isotopic compositions of epithermal vein ores are more scattered than Kuroko ores, although averaged values are similar to the Kuroko ores. This variation seems to be due to the difference in crustal materials underlying the ore deposits Lead isotopic compositions of different ore deposits which formed at different ages in the same district show the same values (Sasaki, 1974). 

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... 

Heald, R, Foley, N.K. and Hayba, D.O. (1987) Comparative anatomy of volcanic-hosted epithermal deposits acid-sulfate and adularia—sericite types. Econ. Geol, 82, 1-26. 

Henley, R.W. (1985) The geothermal framework of epithermal deposits. Reviews in Econ. Geol, 2, 1-24. 

Yamaoka, K. and Nedachi, M. (1978b) Gold and silver minerals from Chitose and Takatama epithermal deposits. In Mining and Metallurgical Institute of Japan (ed.). Gold and silver ore in Japan, pp. 75-100 (in Japanese). 

Fig. 2.24. Relationship between the Ag/Au total production ratios and metal elements from epithermal deposits (Shikazono, 1986).

Chang, Z., Hedenquist, J., White, N et al. Exploration tools for linked porphyry and epithermal deposits Example from the Mankayan intrusion-centered Cu-Au district, Luzon, Philippines. Economic Geology, in review. 

Compared to porphyry copper deposits 5 " S-valnes in epithermal deposits are more variable due to lower temperatures of formation and significant amounts of both sulfide and sulfate in the hydrothermal flnid. 

Epithermal deposits The hydrothermal deposits which formed at relatively shallow environment from surface and at lower temperatures (less than 300 °C) compared with the other hydrothermal ore deposits are classed as epithermal deposits. 

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