Alkaline sulphate fluids produced in a magmatic hydrothermal system

Though isothermal dilution is a simplification of the actual process, these simulations
demonstrate that simple dilution, when “buffered” by a mineral assemblage, is a plausible
explanation for reducing the chloride content of a hydrothermal fluid whilst maintaining or
even enhancing concentrations of sulphate and silica.
The chemistry of the alkaline sulphate springs suggests considerable fluid mixing and
dilution, probably with a cooler water on the basis of the high Mg 2+ concentrations. The
abundance of cooler waters to dilute the hydrothermal system is most likely a result of the
tropical climate of the Solomon Islands and high annual rainfall (3–5 m; Solomon Islands
Meteorological Service).
5.2 Oxygen and Hydrogen Isotopes of Water
The alkaline sulphate springs discharge rapidly, and the residence time of any mass of water
in the spring is short. Thus, the isotopic composition of the fluids is unlikely to be affected by
post-discharge evaporation. Two exceptions to this are the outliers shown on Fig. 4 (SV207
and 208), that appear to have undergone evaporative enrichment similar to the acid springs
(see below).
The position of the alkaline sulphate springs on Fig. 4 can be produced by a number of
different (although not mutually exclusive) processes. On δ 18 O–δD plots, the condensation of
magmatic vapour into meteoric-derived groundwater produces a straight mixing line between
the two end-member compositions (Hedenquist and Aoki, 1991). However, the chemistry of
the springs (high pH, low chloride) is inconsistent with such an origin, thus meteoric–
magmatic mixing is unlikely to be the sole process determining the isotopic composition of
the alkaline sulphate springs. Water–rock reaction produces horizontal shifts in δ 18 O–δD
space, whilst boiling causes minor increase of δD (Fig. 4; Giggenbach and Stewart, 1982).
The latter two processes can be demonstrated to occur at Savo (sections 5.1 and 5.1.1), whilst
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