Chemical composition of geothermal waters

Chemical compositions of geothermal waters controlled by hydrothermal alteration mineral assemblage... 

Previous studies clearly indicated that the chemical compositions of geothermal waters are intimately related both to the hydrothermal alteration mineral assemblages of country rocks and to temperature. Shikazono (1976, 1978a) used a logarithmie cation-Cl concentration diagram to interpret the concentrations of alkali and alkaline earth elements and pH of geothermal waters based on thermochemical equilibrium between hydrothermal solution and alteration minerals. 

The chemical compositions of geothermal water plot in Figs. 23-2.9. All data plot near the equilibrium curves. This indicates that the geothermal waters considered... 

The above discussions are based on the assumption of constant temperature. However, temperature varies widely. The chemical compositions of geothermal waters intimately relate to temperature. For example, the correlation between Na/K ratio in geothermal waters and temperature has been interpreted as indicating that this ratio is controlled by albite and K-feldspar (White, 1965 Ellis, 1969, 1970). 

These results indicate that the chemical composition of geothermal water at 250°C is largely controlled by such minerals commonly occurring in geothermal area as albite, K-feldspar, sericite, calcite, wairakite and quartz. 

Range of chemical compositions of geothermal waters discharged from some production wells and Osorezan hot spring (Izawa and Aoki, 1991)... 

The chemical compositions of geothermal waters are characterized by low total dissolved solid (<300 mg/1), near neutral pH, and Na-Cl domination (Sakai et al., 1986 Ueda et al., 1991). 

Chemical composition of geothermal water associated with base-metal deposits (White, 1967)... 

Interpretation of Chemical Compositions of Geothermal Water in Terms of Chemical Equilihrium Model... 

The chemical compositions of geothermal water are interpreted in terms of chemical equihbrium model between aqueous solution and altered rocks (Shikazono 1978a, b). 

Figure 1.14 shows typical chemical composition of geothermal water in equilibrium with dominant hydrothermal alteration minerals (quartz, Na-feldspar, K-feldspar, K-mica). Therefore, the chemical compositions of geothermal water in equilibrium with hydrothermal alteration minerals (quartz, Na-feldspar, K-feldspar, calcite, Ca-feldspar, anhydrite, Mg-chlorite) are derived below. 

Michard, G. 1987. Controls of the chemical composition of geothermal waters. / Helgeson, H. C. (ed.) Chemical Transport in Metasomatic Processes. NATO Advanced Study Institute Series C Mathematical and Physical Sciences, 218, 323-353. 

Table 23.2. Chemical compositions of water and steam discharged from wet-steam geothermal wells in Iceland (Arnorsson et al., 1983)...

Michard, G. and E. Roekens, 1983, Modelling of the chemical composition of alkaline hot waters. Geothermics 12, 161-169. 

Giggenbach WF, Corrales SR (1992) Isotopic and chemical composition of water and steam discharges from volcanic-magmatic-hydrothermal systems of the Guanacaste Geothermal Province, Costa Rica. Appl Geochem 7 309-332... 

For both surface waters and groundwaters, input of or mixing with geothermal waters can have dramatic impacts on both major ion and trace element composition because of the enrichment of geothermal waters in many chemical constituents. In particular, geothermal waters are often enriched in arsenic (11) and contribute significant amounts of arsenic to surface waters, for example, in the eastern Sierra Nevada in California (19,20). 

Fig. 1.3 Activity diagram for Na20-K20-Al203-H20 system 250 °C, 1Kb (Henley 1984). Solid circle represents chemical composition of typical geothermal water... 

Chapter 1 provides an extensive discussion of chemical equilibria as they pertain to minerals. This covers in detail chemical equilibria of solids and aqueous solutions, the solubility of various types of minerals, the weathering of silicate and aluminosilicate minerals, the composition of geothermal and groimd water, hydro-thermal alteration, oxidation and reduction reactions, and the partitioning of elements between aqueous solutions and crystals. 

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