Title: The geochemistry of silica in Icelandic geothermal systems

Abstract

The geochemistry of silica in Icelandic geothermal fluids was studied. The data considered span
over 60 years of a sample collection of both sub-boiling liquid water and two-phase well
discharges, in total ~1650 samples from >30 geothermal areas. The measured concentration of silica
in the samples ranged from >10 to ~1000 ppm. It was observed to increase with increasing
temperature, typically <200 ppm at
<75°C whereas for boiling and close to boiling water (90-100°C) concentrations up to 700 ppm were
observed. Silica concentrations in two-phase well discharges of high-temperature areas were much
greater than for the low-temperature areas, typically ~400-1000 ppm with highest values observed at
Krafla, Hellisheidi and Nesjavellir. Aqueous speciation calculations revealed that the dominant Si
species in all cases were H₄SiO₄(aq) and HSiO ⁻ at acid to neutral and alkaline pH values,
respectively, with NaHSiO₃(aq) also being important at elevated Na concentration. pH was observed
to be the major factor controlling aqueous silica species distribution with temperature and
salinity being less important. Geothermal fluids with neutral to alkaline pH values and
temperatures >20°C were generally observed to be close to saturation with respect to common silica
containing geothermal minerals. In contrast, acid fluids were observed to be undersaturated. At low
temperatures, these are typically clays and zeolites whereas, at temperatures >230°C feldspars,
prehnite and epidote together with quartz are often most important. Geothermal geothermometry
relies on the assumption of such fluid-mineral equilibria, that are in turn primarily dependent on
the temperature at geothermal conditions. It follows, that equilibrium of the fluids with many
silica containing geothermal minerals may be used to predict reservoir temperatures. The simplest
is only involving silica like quartz and chalcedony, but more complex reactions involving two or
more minerals and many components may also be applied. It is further demonstrated that the silica
concentrations in geothermal fluids may be used to predict indirectly spatial reservoir temperature
variations, here demonstrated for emperature systems at the southern lowlands and at the
high-temperature
system at Krafla.