Chemical characteristics of thermal waters in the Central Range of Taiwan, R.O.C.

Abstract

There are 70 hot-spring areas in the Central Range metamorphic terrane of Taiwan, where argillite, slate, phyllite and black schist predominate. The chief mineral constituents of these metapelites are quartz (Qtz), K-mica (Km), chlorite and albite (Ab), with a minor amount of calcite (Cc) but without K-feldspar (Kf). P CO 2 in subsurface water is high, ranging from a few atmospheres at ∼ 150°C to nearly 100 atm. at ∼ 220°C in the Chingshui and Tuchang geothermal fields. The carbonic acid in water reacts with Ab to produce NaHCO 3-type water in the Central Range. The Na, K, Mg and Ca concentrations of these waters are controlled by solubilities of Ab, Km, chlorite and Cc, respectively. These waters have comparatively higher Na/K ratios than waters in equilibrium with Ab and Kf, and the estimated subsurface water temperatures by the NaKCa geothermometer of R.O. Fournier and A.H. Truesdell are generally 15–40°C lower than t SiO 2(Qtz) . Analytical data of thermal waters in the Chingshui, Tuchang and Chihpen geothermal fields are sufficient to define a relationship between Na/K ratio and the reciprocal of absolute temperature, which can be expressed empirically as log( Na K ) = 1670 T − 1.73 , and from which the maximum water temperature can be estimated by the following formula with an uncertainty of less than 15°C: t Na/K(Ab−Musc)( °C) = 1670/[log(Na/K) + 1.73] −273.15 The formula is valid for 350°C > t > 130°C. Since the dissolved SiO 2 in thermal water is more easily re-equilibrated with Qtz than Na + and K + re-equilibrated with Ab and Km and/or Kf in a flowing hydrothermal system, ( t Na K - t SiO 2 ) is small (< 15°C) in deep water but large (up to 50°C or more) in shallow water, and t Na K ( deep water) - t Na K ( shallow water) seldom exceeds 10°C. If thermal water is in equilibrium with Ab, muscovite (Musc) and Qtz according to the reaction 3Ab + 2H + + K + ag 3Na + + Musc + 6Qtz, then: K Ab−Musc = α 3 Na/α Kα 2 H and pH= 1 2 (log K Ab−Musc + log α K−3 log α Na) Calculated pH-values for waters in the Central Range range from 6.4 at ∼ 220°C to 8.4 at ∼ 100°C, which probably have an uncertainty of 0.2–0.5 pH unit. The equation: log(α Na/α K = log K Ab−Musc − 2 log(α Na/α H) indicates decreasing Na/K ratio with increasing pH-value, which probably accounts for the abnormally low Na/K ratios of some low-temperature waters. The reaction CaCO 3 + 2H + ag Ca 2+ + H 2O + CO 2 (g) gives m Ca = a 2 H K Cc − H γ Caf CO 2 . A plot of calculated log(√Ca/NA) vs. 1000 T for the Chingshui and Tuchang thermal waters exhibits a trend similar to that published by Fournier and Truesdell, implying that the √ Ca Na ratio of natural water may be controlled by solubilities of calcite and Ab, and that the NaKCa geothermometer is probably based on solubilities of Cc, Ab, Kf and Km. If only Ab, Kf and Km are considered and at same P- T conditions, waters in equilibrium with Ab and Kf should have lower Na/K ratios than waters in equilibrium with Ab and Km, whereas waters reacting with Ab, Kf and Km may have intermediate Na/K ratios.