Active geothermal system

Figure 1.142 shows the dependence of solubility of Si02 minerals (quartz, cristobalite) on temperature. As described already, cristobalite occurs in peripheral and shallower part of hydrothermal alteration zone. Quartz is present in zones occurring in deeper and closer to the gold-quartz veins. Such zoning from quartz to cristobalite is also common in main active geothermal systems (Hayashi, 1973 Takeno et al., 2000). 

Temperature of each reservoir was estimated from the assemblage of hydrothermal alteration minerals and temperature of alteration zone in active geothermal system (e.g., Hayashi, 1973 Takeno et al., 2000). 

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. 

White, D.E, (1981) Active geothermal systems and hydrothermal ore deposits. Econ. Geol, 75th Anniv. Vol., 392-423. 

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. 

Pb, Zn) sulfides occur in deeper part as in Broadlands (New Zealand) and in Okuaizu (Japan). Pb and Zn sulfides occur dominantly at 400 m from the surface in Broadlands. In the boreholes of Broadlands, concentrations of precious and base metal elements in the boreholes change with depth as studied by Ewers and Keayse (1977), Au, As, Sb and T1 decrease with depth, but Ag increases with depth. In active geothermal systems in Japan, Au, Te, Se, T1 and Hg are enriched on the surface in the Osorezan hydrothermal system. 

The generalized sequence of alteration minerals from shallower to deeper portions and/or from lower to higher temperatures in active geothermal systems, which is constructed mainly based on the work by Henley and Ellis (1983), is given in Fig. 2.25. It is shown in Fig. 2.25 that the change in alteration and gangue minerals largely depends on temperature as well as on the other physicochemical parameters such as /s2, /o2> /c02> and pH. 

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. 

Fig. 2.26. Range of carbon dioxide fugacity (fco ) and temperature for the propylitic alteration (epidote zone) in the Seigoshi area and same active geothermal systems. Seigoshi = propylitic alteration of the Seigoshi district. The curves A-B and A -B are equilibria for epidote (Xpis = 0.30) - K-mica (oK-mica = 0-9) -K-feldspar (aK-feidspar = 0.95) - calcite assemblages at saturated water vapor pressure condition (Shikazono, 1985a).

During the last three decades, many hydrothermal deposits have been discovered at midoceanic ridges, back-arc basins and subaerial active geothermal systems. Characteristic features of back-arc deposits at the western Pacific region (e.g., Okinawa Trough, Izu Ogasawara, North Fiji and Mariana deposits) are very similar to those of Kuroko deposits. 

Several active geothermal systems in Japanese Islands are associated with precious- and base-metal mineralization. Base metal mineralization takes place from hot springs containing high chloride concentration probably due to the contribution of seawater. Precious-metal mineralization occurs in the Osorezan hot springs which are characterized by neutral pH, high H2S concentration, and low salinity. These chemical features are similar to those of epithermal precious metal vein-type deposits in Japan. 

In and near the Japanese Islands many Neogene hydrothermal ore deposits have been formed from the middle Miocene to the present time, and many subaerial active geothermal systems occur. Some of them are associated with base-metal (Cu, Zn, Pb, Fe, Mn) and precious-metal (Au, Ag) mineralizations. 

During the last three decades, subaerial geothermal areas in the Japanese Islands have been explored considerably and geothermal energy plants were developed. It was recognized that some active geothermal systems are accompanied by present-day base-metal and precious-metal mineralizations. 

One of the main applications of hydrogen and oxygen isotope thermometry in geochemistry is the estimation of the reservoir temperatures of active geothermal systems or the evaluation of the ruling T conditions during deposition or alter-... 

Figure 18. Plot of oxygen isotope fractionation factor between magnetite and water at low temperatures. Data sources Becker and Clayton (1976) and Rowe et al. (1994) - empirical-theoretical calculations Zheng (1995) -empirical increment method O Neil and Clayton (1964) - extrapolation of high-temperature experimental results to an analysis of magnetite teeth from marine chiton Blattner et al. (1983) -natural samples from active geothermal system in New Zealand Zhang et al. (1997) - extracellular magnetite...

Lambert SJ, Epstein S (1980) Stable isotope investigations of an active geothermal system in Valles Caldera, Jemez Monntains, New Mexico. J Volcan Geotherm Res 8 111-129. 

Alteration mineral assemblages characterize the many varied hydrothermal-chemical reactions that occur in active geothermal systems. The effect of chemical and temperature conditions on the occurrence of secondary minerals is well known, based on field observation and mineral synthesis laboratory experiments. It is less clear, however, how specific chemical species become enriched in one part of an active hydrothermal system. 

Hara, J., Tsuchiya, N. and Inoue, A. 2001. Mineralogical alteration zoning in active geothermal systems of the Hachimantai volcanic region, northeast Japan. 10 WRI proceedings, pp. 847 - 850. 

Low-temperature isotopic thermometry is also applicable to ascertaining the temperatures of diagenesis and low-grade metamorphism, and estimating the temperatures of active geothermal systems, both in the condnental crust and on the ocean floor. 

Propylitic and advanced argUlic alterations are commonly found in active geothermal systems and mining areas. 

Gold (Appendix, Plate 18) is frequently enriched in active geothermal system and epithermal gold deposits (Appendix, Plates 19-23). 

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