Hydrogeochemistry

Geochemical studies of this area have been performed by Bath (1976; 1977), Aquater (1982), and Goff and Vuataz (1984). Consideration of new data and previous studies led

Fig. 5.39
Plot of seismic frequency, hot-spring temperature, and bicarbonate content for the
period 1975 through 1977 at Usu volcano. These three indicators rose
sharply in months preceding the 1977 eruptions.
(Adapted from Katsui et al ., 1981.)

Goff and Vuataz to conclude that a geothermal reservoir underling the Sulphur Springs area consists of an upper-steam condensate zone, an intermediate depth, two-phase zone, and a lower brine zone. Sulphur Springs display chemical compositions (Table 5.5) very characteristic of acid-sulfate systems in which mixtures of steam and other gases condense in the near-surface environment and oxidation of H₂ S and H₂ SO₄ leads to acidic conditions. The springs have relatively low pH, high SO₄ , and a low Cl content; divalent and trivalent cations (Ca, Mg, Al, and Fe) dominate Na + K, and most trace elements other than B are relatively scarce. Bath's (1977) brine analyses of well 4 (see Fig. 5.41) show considerable variability because of wet and dry cycling of the flow from the well, which indicates variable steam loss from the brines sampled. Goff and Vuataz (1984) noted that this brine is extremely unusual because the Ca content is twice that of Na by weight and it is very rich in B.

Although drilling encountered temperatures >220°C at depths of 700 m (Williamson, 1979), oxygen isotope composition, gas geothermometry, steam enthalpy, and B abundances indicate brine reservoir temperatures are near 280°C. Figure 5.44 (Goff and Vuataz, 1984) shows a model of the geothermal system beneath the Sulphur Springs area in which geothermal upflow could occur beneath Sulphur Springs and possibly below Belfond—the areas of most recent silicic dome eruptions. Lateral outflow occurs near the surface in the condensation zone, but there is also a strong possibility that it occurs below the vapor zone, where deep brines flow northward and pool against the north caldera-wall

Fig. 5.40
Generalized stratigraphy of Qualibou caldera near
the Terre Blanche dacite dome on St. Lucia. The
stratigraphic section represents a thickness of
~2 km and reflects a general trend from
precaldera mafic andesites and basalts through
intermediate products of caldera-related
eruptions to postcaldera eruptions associated
with silicic dome rock. A vapor-dominated
hydrothermal system is thought to exist in
pumice sections just below the intracaldera
dome lavas, and there is probably a brine
reservoir in the fractured caldera-
fill and precaldera rocks below.

faults. The brine composition indicates it originated from sea water that reacted with basaltic rocks in the subsurface and was subsequently replaced by meteoric water.