Arsenic is a trace element in Icelandic basalts that can be concentrated in hydrothermal fluids (~0.15 ppm As) due to water-rock reactions in geothermal systems. Arsenic concentrations above 0.01 ppm As in domestic water are a health concern and, as a mobile element in hydrothermal systems, arsenic has the potential to contaminate surface waters as a result of geothermal development. Therefore, understanding the behavior of arsenic in hydrothermal fluids is important to minimizing the environmental impacts of geothermal energy development in Iceland and elsewhere.

Reactions between host rock and geothermal fluids ultimately control arsenic concentration and mobility within the active volcanic zone of Iceland. Iron-sulfides and titano-magnetite are believed to control arsenic mobility and distribution in Icelandic geothermal systems, but the amount of arsenic in Icelandic rocks, its distribution in different minerals, and the effect of hydrothermal alteration on arsenic distribution are poorly constrained.

Fossil geothermal systems preserve spatial relationships and so allow comparison of arsenic behavior in rocks with different chemistry and/or permeability that have experienced the same geothermal conditions. Fossil geothermal systems provide an excellent opportunity for comparative analysis with active geothermal systems accessed through geothermal drilling projects, such as the Iceland Deep Drilling Project (IDDP). Considered together, active and fossil systems will help us more completely evaluate arsenic mobility in geothermal environments.

We analyzed arsenic in 50 drill hole cutting samples from the seawater-dominated geothermal system at Reykjanes, 30 representative samples from fossil geothermal systems exposed at Geitafell and Vesturhorn in eastern Iceland, and one representative sample of recent volcanic eruption products from Krafla. Samples were analyzed for bulk rock trace element concentrations by ICP-MS and ICP-OES. Results indicate that sulfide mineralization accompanying hydrothermal alteration locally concentrates arsenic by a factor of 2-10 to produce basaltic rocks with 2 to possibly 19 ppm As. However, bulk rock arsenic and sulfur contents are not linearly related, suggesting that other factors in addition to sulfide abundance control arsenic concentration. This study also shows that hydrothermal arsenic enrichment is of the same magnitude as but geochemically distinct from arsenic enrichment in silicic rocks that occurs during magmatic differentiation.

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