Transport and precipitation of carbon and sulphur in the Reykjanes geothermal system, Iceland

Abstract

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-quartzprehnite 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-magnetitequartz- 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 most sulphide minerals and become increasingly more supersaturated upon boiling and cooling, in addition to sulphide being quantitatively removed from solution upon boiling mostly into pyrite but also sphalerite, galena and covellite. Under these conditions, sulphide minerals have the potential to form both in the reservoir and upon fluid ascent resulting in homogeneous sulphide precipitation as a function of depth. Sulphates in rocks range from ~0.02 to 1.8 wt%, with the highest values observed in the uppermost 400 m. Anhydrite precipitation occurs preferentially in the shallowest part of the system mainly due to heated seawater-rock interaction. Total sulphur vertical distribution pattern at Reykjanes may reflect either significant sulphides precipitation at all depths due to high metal concentrations in the reservoir water or enhanced sulphur precipitation due to additional sulphur supply from seawater intruding at all depths, or both. Finally, based on age and extension constrains for the geothermal system, average mass of mineralization of 700, 315 and 1054 tonne/yr were obtained for sulphide, sulphate and carbon, respectively, over the life time of the system taken to be 20,000 years.