| Summary: | Sulphur and carbon analyses of drill cuttings, mineral saturation state calculations and reaction path modelling were used to assess the effects of boiling and phase separation, cooling, magmatic gas input and extent of fluid-rock reaction in order to get insight into the source, transport and precipitation of sulphur, carbon and associated metals in the Reykjanes geothermal system. Reservoir temperatures range from 275 to 310°C and the estimated pH from ~4.5 to ~5.0 with uncertainties up to 0.5 pH units. Geothermal reservoir waters seem to be the formed upon mixing of seawater with very small amount of magmatic gas followed by reaction with basalts. The reservoir water is relatively close to saturation with respect to most minerals observed in the system including quartz, albite, chlorite, epidote, prehnite and pyrite. Carbon dioxide concentrations in the reservoir water are close to an apparent equilibrium with the clinozoisite-calcite-quartz-prehnite mineral assemblage but CO2 may also be source controlled by magmatic input corresponding to about 0.1-1% magmatic gas to seawater ratio. The H2 and H2S concentrations are considered to be controlled by the buffer pyrite-prehnite-magnetite-quartz-clinozoisite-anhydrite or pyrite-wollastonite-magnetite-anhydrite-quartz. Carbon content in drill cuttings notably increases from ~0.01 to ~2.0 wt% as depth decreases in the uppermost 1100 m. Below that depth, concentrations range from <0.5 ppm to a maximum of ~0.03 wt%. At reservoir conditions, carbon precipitation may be precluded, due to the effects of temperature, pH and reduced availability of cations, and leaching from rocks seems to occur. Calcite precipitates and builds up in the altered rocks above background carbon at depths < 1100 m, corresponding to depth of boiling. Sulphides concentrations range from <0.01 to ~1.2 wt% in altered rocks with no markable trend as a function of depth. Reservoir water with metal concentrations based on downhole samples was observed to be supersaturated with respect to ...
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