Geothermal water

Another potential problem, particularly if geothermal water is not returned to its source, is land subsidence. For example, there has been significant subsidence at the Wanaker field in New Zealand. Finally, an annoying difficulty with geothermal heat has been the noise produced by escaping steam and water. The shriek of the high-pressure fluids is intolerable, and is usually dissipated in towers in which the fluids are... 

ES in ore fluids is generally in a range of 10 -10 mol/kg H2O based on ES in present-day geothermal waters and fluid inclusion analytical data (Shikazono, 1972a). /ci- is represented as a function of ionic strength and temperature. Ionic strength is related to salinity which can be approximated as Cl concentration. Cl concentration can be estimated from fluid inclusion study. 

In Fig. 1.59 the relationship between temperature and concentration of elements (Zn, Ba) at constant Cl concentration which is equal to that of seawater obtained by the experimental studies and analytical data on natural hydrothermal solution (geothermal water) are shown. It is seen that the concentrations of base-metal elements (Zn, Fe, Mn, Cu, Pb) and Ba increase with increasing of temperature. Concentrations of these... 

Figure 1.56. Relationship between the zinc and Cl concentration in geothermal waters and hydrothermal solution experimentally interacted with rocks (Shikazono, 1988c).

Gangue minerals and salinity give constraints on the pH range. The thermochemical stability field of adularia, sericite and kaolinite depends on temperature, ionic strength, pH and potassium ion concentration of the aqueous phase. The potassium ion concentration is estimated from the empirical relation of Na+/K+ obtained from analyses of geothermal waters (White, 1965 Ellis, 1969 Fournier and Truesdell, 1973), experimental data on rock-water interactions (e.g., Mottl and Holland, 1978) and assuming that salinity of inclusion fluids is equal to ffZNa+ -h m + in which m is molal concentration. From these data potassium ion concentration was assumed to be 0.1 and 0.2 mol/kg H2O for 200°C and 250°C. 

Arnorsson, S. and Gunnlaugsson, E. (1983) The chemistry of geothermal waters in Iceland, III. Chemical geothermometry in geothermal investigations. Geochim. Cosmochim. Acta, 47, 567-577. 

Shikazono, N. (1978a) Possible cation buffering in chloride rich geothermal waters. Chem. Geol, 23, 234—259. Shikazono, N. (1978b) Selenium content of acanthite and chemical environments of Japanese vein-type deposits. Econ. Geol, 73, 524—533. 

Takeno, N. (1989) The effect of temperature gradient on the interaction between geothermal water and rock — an approach by numerical simulation. Mining Geology, 39, 295-304. 

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. 

Hence, it is assumed that the minerals in equilibrium with geothermal waters are albite, K-feldspar, muscovite, quartz, calcite, anhydrite, chlorite and wairakite. 

In geothermal waters, the condition of electroneutrality must be fulfilled ... 

It is found that Na" " and Cl are the predominant cation and anion in geothermal waters of high temperatures (200-300°C). If the concentrations of the other species are negligible, compared with tho.se of Na+ and Cl, equation (2-1) is approximated by,... 

This relationship is shown in Fig. 2.2 in which data on Na+ and Cl concentrations of geothermal waters in Japan and in other countries are plotted. 

Fig. 2.2. Relation between the Na+ and CI concentrations of geothermal waters and of inclusion fluids. The solid line indicates the condition of electroneutrality approximated by the equation = mc -. Solid and open circles mean the chemical analytical data on inclusion fluids and geothermal waters, respectively. S = Salton Sea R = Reykjanes W = Wairakei B = Broadlands O = Otake H = Hveragerdi C = Climax D = Darwin P = Providencia (Shikazono, 1978a).

Fig. 2.3. Relation between the K+ and CI concentration of geothermal waters and inclusion fluids. The solid line defines the equilibrium condition between the solution and the assemblage albite-K-feldspar at 250°C. For symbols used, see caption to Fig. 2.2. (Shikazono, 1978a).

The concentrations of Rb and Cs in feldspars in geothermal areas have not been studied. However, if the concentration ranges are similar to those observed for feldspars in common igneous rocks, we can construct curves to show the relationship of the concentration of an alkali element X+ and the Cl concentration of geothermal waters (Fig. 2.4 and Fig. 2.5). It is assumed that the concentrations of Cs and Rb in feldspars range from lO -" to 10 wt% and I0 to 10 wt%, respectively. 

If muscovite, K-feldspar and quartz are saturated with geothermal waters, the following reaction can be written ... 

Using the equilibrium constant for (2-5) and (2-9), the relationship between pH and Cl concentration can be derived as shown in Fig. 2.7. The line shown in Fig. 2.7 has a slope of approximately —1, indicating that the pH of the geothermal waters decreases with increasing Cl concentration. 

Fig. 2.6. Relation between the Li" " and CP concentrations of geothermal waters (Shikazono, 1978a).

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). 

The Na/Li ratio of geothermal waters decreases with increasing temperature and has been used as a geothermometer (Fig. 2.10) (Fouillac and Michard, 1981), suggesting that this ratio is controlled by feldspar-solution equilibrium (Shikazono, 1978a). 

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