Alkaline sulphate fluids produced in a magmatic hydrothermal system

average meteoric water composition for the south west Pacific (Madang varies from δ 18 O =
−14 to −2‰ and δD = −92 to −3‰; average −7, −46‰). Thus, for the purposes of the
following discussion, the average isotopic composition of well and cold spring waters is
interpreted as being representative of meteoric-derived groundwater on Savo.
4.4 Strontium Isotopes
87 Sr/ 86 Sr values (Fig. 5) for all hot and cold springs (average = 0.70420 ± 9) overlap with the
values for local rocks (average = 0.70414 ± 11, based on 14 samples ranging from basalt to
trachyte; Smith et al., 2009). A seawater sample collected by the same method from offshore
Savo has a 87 Sr/ 86 Sr value of 0.709164 ± 12 (the accepted value for modern seawater is
0.709211 ± 37; Elderfield 1986). Coastal wells analysed in this study have 87 Sr/ 86 Sr values
slightly higher (0.70442–0.70467) than those of the inland springs.
4.5 Native Sulphur
Native sulphur collected from areas of steaming ground and fumarolic / solfataric activity
shows negative δ 34 S values, ranging from −4.2 to −5.9‰, lower than sulphate recovered from
acid springs, and considerably lower than that of alkaline springs (Table 5; Fig. 3).
5 Discussion
5.1 Water Chemistry
Surface discharge from the hydrothermal system at Savo is dominated by alkaline sulphatetype
springs. The features of these springs (high sulphate, high silica, moderate Na-K-Ca-Mg,
high pH, low chloride) are unusual in that sulphate-rich, chloride-poor springs are typically
acidic, steam-heated springs. This is not the case at Savo.
Near-neutral or alkaline fluids from hydrothermal areas are typically close to equilibrium
with a secondary (hydrothermally altered) mineral assemblage (Giggenbach, 1988). Although
15