GEOCHEMICAL CONTROLS ON ARSENIC MOBILITY IN ICELANDIC GEOTHERMAL SYSTEMS

Arsenic concentrations in geothermal fluids (20-240 ppb) from basalt-hosted geothermal systems in Iceland are elevated above background surface- (<0.03-1.3 ppb) and groundwater (0.08-10.2 ppb) levels and above the World Health Organization’s safe drinking water level (10 ppb). Mineral-fluid reactions involving titanomagnetite and pyrite are possible controls on arsenic concentration based upon geothermal fluid chemistry and mineral stability. However, the amount of arsenic in these minerals and arsenic’s distribution within Icelandic geothermal systems is poorly known. We studied arsenic’s behavior in water-rock interactions by conducting petrographic and bulk and mineral separate element analyses of variably altered basaltic rocks, including drill cuttings from 350 to 3050 m depth in the active Reykjanes geothermal system, basalt erupted in 1984 from the Krafla caldera, and samples from fossil geothermal systems exposed at Vesturhorn and Geitafell. Analyses were completed using ICP-MS, ICP-OES, electron microprobe microanalysis, and optical microscopy. Basalts and gabbros that had not experienced hydrothermal alteration contained 0.8 to 1.2 ppm As. Altered rocks contained 0.5 to 16 ppm As. These results indicate that arsenic may be slightly depleted by hydrothermal alteration of mafic rocks or locally concentrated by up to a factor of 10. This enrichment is of similar magnitude to the arsenic enrichment that is observed in felsic glasses worldwide and is attributed to arsenic’s incompatible behavior during magmatic differentiation. Bulk rock arsenic and sulfur content were closely correlated, suggesting that hydrothermal sulfides are important arsenic sinks and that pyrite, rather than titanomagnetite, is a dominant mineral control on arsenic concentration in Icelandic geothermal fluids. The apparent relationship of sulfide stability and arsenic mobility can be used to relate arsenic movement to geothermal system structure and geochemistry.