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The geochemistry and mineralization of H 2 S in the geothermal system hosted by basaltic rock formation at Hellisheidi, SW Iceland, was studied. Injection of mixtures of H 2 S with geothermal waste water and condensed steam into the N 230°C geothermal aquifer is planned, where H 2 S will hopefully be removed in the form of sulphides. The natural H 2 S concentrations in the aquifer average 130 ppm. They are considered to be controlled by close approach to equilibrium with pyrite, pyrrhotite, prehnite and epidote. Injection of H 2 S will increase significantly the reservoir H 2 S equilibrium concentrations, resulting in mineralization of pyrite and possibly other sulphides as well as affecting the formation of prehnite and epidote. Based on reaction path modelling, the main factors affecting the H 2 S mineralization capacity are related to the mobility and oxidation state of iron. At temperatures above 250°C the pyrite mineralization is greatly reduced upon epidote formation leading to the much greater basalt dissolution needed to sequestrate the H 2 S. Based on these findings, the optimum conditions for H 2 S injection are aquifers with temperatures below~250°C where epidote formation is insignificant. Moreover, the results suggest that sequestration of H 2 S into the geothermal system is feasible. The total flux of H 2 S from the Hellisheidi power plant is 12,950 tonnes yr − 1. Injection into 250°C aquifers would result in dissolution of~1000 tonnes yr − 1 of basalt for mineralization of H 2 S as pyrite, corresponding to~320 m 3 yr − 1 .

Geothermal waters often are enriched in trace metal­(loid)­s, such as arsenic, antimony, molybdenum, and tungsten. The presence of sulfide can lead to the formation of thiolated anions; however, their contributions to total element concentrations typically remain unknown because nonsuitable sample stabilization and chromatographic separation methods convert them to oxyanions. Here, the concurrent widespread occurrence of thioarsenates, thiomolybdates, thiotungstates, and thioantimonates, in sulfide-rich hot springs from Yellowstone National Park and Iceland is shown. More thiolation was generally observed at higher molar sulfide to metal­(loid) excess (Iceland > Yellowstone). Thioarsenates were the most prominent and ubiquitous thiolated species, with trithioarsenate typically dominating arsenic speciation. In some Icelandic hot springs, arsenic was nearly quantitatively thiolated. Also, for molybdenum, thioanions dominated over oxyanions in many Icelandic hot springs. For tungsten and antimony, oxyanions typically dominated and thioanions were observed less frequently, but still contributed up to a few tens of percent in some springs. This order of relative abundance (thioarsenates > thiomolybdates > thiotungstates ≈ thioantimonates) was also observed when looking at processes triggering transformation of thioanions such as mixing with non-geothermal waters or H2S degassing and oxidation with increasing distance from a discharge. Even though to different extents, thiolation contributed substantially to speciation of all four elements studied, indicating that their analysis is required when studying geothermal systems.

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