From Slab to Sinter: The Magmatic-Hydrothermal System of Savo Volcano, Solomon Islands

The SW Pacific hosts world-class alkaline-related epithermal gold deposits. Savo Island, a recently active volcano in the Solomon Islands, is dominated by alkaline, sodic (≤7.5 wt % Na2O) lavas and pyroclastic deposits and has an active hydrothermal system, with hot springs and fumaroles. It thus represents a natural laboratory for studying the magmatic and hydrothermal processes that can form epithermal mineralisation. The magmatic suite is divided into mugearites (plagioclase–clinopyroxene–magnetite ± amphibole ± olivine) and trachytes (plagioclase–amphibole–magnetite ± biotite). Mineralogy, geochemistry, and cumulate xenoliths within the lavas indicate that amphibole fractionation drove magmatic differentiation. Hydrous, alkali-rich magmas were likely derived from partial melting of metasomatised mantle, but radiogenic isotope data cannot discriminate the origin of metasomatic agents. Hot springs at Savo include high pH, sulphate-rich discharges (with high Na, Si, Ca, K, low Cl−); atypical for magmatic-hydrothermal systems. These fluids form by the condensation of magmatic volatiles into meteoric-derived groundwater (high Ca, Mg, HCO3−) generating acidity by SO2 disproportionation into H2SO4 and H2S. Water chemistry, δ18O, and δD data indicate that rock reaction, dilution and boiling increase the fluid pH to 7–8. H2S oxidises at the surface, producing H2SO4 and native sulphur in steam-heated springs and fumaroles. The lack of isotopic equilibrium between the various sulphur species indicates that acidity is rapidly neutralised, and that the system is dominated by high pH fluids. Precipitates around hot springs include sinter, travertine and mixed silica-carbonate. These are often enriched in Au and Te, indicating potential for mineralisation at Savo. Varying contributions from meteoric and hydrothermal fluids leads to alternating carbonate and silica precipitation, underlining the importance of high rainfall to the hydrothermal system. Sinter and travertine may be useful tools for the exploration of alkaline-related epithermal deposits, as they provide preservable records of hydrothermal activity and fluid chemistry.