Epithermal system

Although a wide range of alteration minerals has been recognized in epithermal systems considered here, few of their chemical compositions have been determined. 

Gold precipitation due to mixing offluids in epithermal system... 

Figure 1.197, Subduction mode of the Pacific plate beneath the Northeast Japan arc, and style of the epithermal systems in the Kameda peninsula, Hokkaido. Normal, oblique and highly oblique subduction are tentatively defined by the angle (a) between the assumed subduction direction of the Pacific plate and the trend of the Northeast Japan arc. Normal subduction (60° < a = 90°), oblique subduction (30° < a = 60°) and highly oblique subduction (0° < a = 30°). Highly oblique subduction has not occurred along the Northeast Japan arc since 11 Ma. (Watanabe et al., 1996).

Reed, M.H. and Spycher, N.F. (1985) Boiling, cooling and oxidation to epithermal systems. A numerical modeling approach. In Berger, B.R. and Bethke, P.M. (eds.). Geology and Geochemistry of Epithermal System. Reviews in Economic Geology, 2, 249-272. 

Such evolution of a hydrothermal system from acidic sulfate hydrothermal solution to neutral is common in the epithermal system associated with precious metal mineralization. For example, advanced argillic alteration and intense silicification occurred at earlier stage of hydrothermal system in the Seigoshi Au-Ag mine area. 

As discussed in previous chapters, gold deposition in epithermal systems and back-arc basins (Kuroko deposits) occurs in relatively higher /oj conditions than base metal... 

Cu, Pb, Zn, Fe) deposition in epithermal systems. Thus, it is interesting to summarize the gold distribution in Besshi-type deposits, considering the geochemical environment of gold deposition. 

Trace element contents in rocks intersected by drill holes WB-08-03 to WB-08-06 are associated with altered rhyolite Spearman rank correlation coefficients for Ag, Mo and S with Au are moderate to high. Correlation coefficients for Au with As, Cu, Sb and Mo in drill hole WB08-11 (vuggy quartz) are moderate to high. These element correlations suggest that mineralization may be related to an epithermal system (Panteleyev 1995). 

Fournier, R. O. 1985. In Geology and geochemistry of epithermal systems, ed B. R. Berger and P. M Bethke. Reviews in Econ. Geol. 2. Chelsea, MI Soc. Econ. Geologists. 

The DESMOS cauldron vent field has up to 120°C vent fluids that are sulfate-rich and have very low pH (<2.1). Gamo et al. (1997) have suggested that these fluids may be acid-sulfate fluids similar to those in some continental epithermal systems due to disproportionation of magmatic SO2. Disproportionation of SO2 is a plausible mechanism to explain a measured 6 " Sh2S value of -5.6%o and a measured 5D value of -8.1%o, but these are very unusual fluids and one would like to see more than single determinations of these critical isotope values. Furthermore, the DESMOS vent fluids seem to have nearly normal seawater Mg contents and do not seem to track to zero Mg. Gamo et al. 

Field C.W. and Fifarek R.H., 1986, Light stable isotope systematics in the epithermal environment. In Berger B.R. and Bethke P.M. (eds,), Geology and geochemistry of epithermal systems. Society of Economic Geologists, Pev. Bcon. GeoL, 2, 99-128. 

For the same reason, this system will be self-scavenging. It thus seems unlikely that much can be learned about the hot or epithermal reactions in Ni(CO)4. 

It is widely accepted that boiling of ore fluids took place in the epithermal Au— Ag mineralization system from the fluid inclusion data (Nakayama and Enjoji, 1985 ... 

Figure 1.123. Schematic model for the formations of the Te-type and Se-type epithermal gold depositions in the fossil geothermal system. Reference Henley and Ellis (1983) (Shikazono et al., 1990).

Evolution of tectonics and hydrothermal system associated with epithermal and Kuroko mineralizations... 

However, in contrast to these geologic and tectonic studies, very few studies on the relationship between tectonics and hydrothermal system in Neogene age have been carried out. Therefore, these studies are briefly summarized and then the relationship between geologic and tectonic evolution and evolution of hydrothermal system associated with the mineralizations (Kuroko deposits, epithermal veins) are considered below. 

Etoh, J., Taguchi, S. and Izawa, E. (2001) Gas compositions in fluid inclusions from the Hishikari epithermal gold deposit, southern Kyushu, Japan. Proceedings of International Symposium on Gold and Hydrothermal Systems, pp. 99-104. 

Hedenquist, J.W. and Henley, R.W. (1985) The importance of CO2 on freezing point mea.surements of fluid inclusions evidence from active geothermal systems and implications for epithermal ore deposition. Econ. Geol, 50, 1379-1406. 

Shikazono, N., Yonekawa, N. and Karakizawa, T. (2002) Mass transfer, oxygen isotope, and gold precipitation in epithermal gold system A case study of the Hishikari deposit, southern Kyushu Japan. Resource Geology, 52, 211-222. 

Another interesting characteristic of the Osorezan hydrothermal system is that it is located at a volcanic front. This is different from low sulfidation epithermal Au-Ag veins... 

Comparison of active geothermal systems with epithermal vein-type deposits... 

Close similarities between epithermal vein-type deposits and active geothermal systems have been cited by various authors (e.g.. White, 1955, 1981 Henley and Ellis, 1983 Shikazono, 1985a,b Izawa and Aoki, 1991). 

In this section (2.2), geochemical, mineralogical and geological characteristics of epithermal vein-type deposits summarized in section 1.4 will be compared with subaerial active geothermal systems associated with base metal and Au-Ag mineralizations mentioned in sections 2.1.1 and 2.1.2. 

Fig. 2.23 shows the distributions of major geothermal systems and epithermal gold deposits of Japanese Islands. It is interesting to note that their distributions are similar and they are distributed close to the volcanic front. 

Fig. 2.23. The major geothermal system and epithermal gold deposits of Japan. Epithermal gold deposits are represented by gold mines or mining areas that have produced more than 10 metric tons of gold (Izawa and Aoki, 1991).

It has been pointed out by Giggenbach (1981) on the basis of thermochemical calculations that epidote occurs at higher temperatures of at least more than 240°C, and K-feldspar occurs at restricted temperatures, i.e. below ca. 250°C, in active geothermal systems. These theoretical results seem to be consistent with those observed in epithermal vein-type deposits in Japan. 

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