Title: 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-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 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.