The volcanic island of Savo, Solomon Islands, hosts an active hydrothermal system discharging unusual alkaline (pH 7–8) sulphate-rich, chloride-poor fluid, with variable admixtures of Ca–Mg–HCO3-\ud rich fluid. Hot springs and their outflow streams precipitate a variety of deposits, including travertine, silica sinter and unusual mixed silica–carbonate rocks. Travertine fabrics are dominated by ray-crystal calcite,\ud associated with rapid abiotic precipitation from a supersaturated solution. Sinter is produced by evaporation of thermal waters, and downstream samples contain preserved traces of micro-organisms, which potentially acted as templates for precipitation. Trace element chemistry of sinters and travertines includes anomalously high levels of Te, indicating a magmatic origin for a component fluid in the hydrothermal system. Springs are close to or at saturation with both calcite and amorphous silica. Increased contributions from the Ca–Mg–HCO3 end-member favours calcite formation; this fluid is of low-temperature origin, and as such is favoured by high rainfall. Mixed samples show cyclical changes between silica and carbonate precipitation, potentially as a result of seasonal variation in rainfall
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