The Geochemistry of As, Mo, Sb, and W in Natural Geothermal Waters, Iceland

Abstract

Arsenic, Mo Sb and W concentrations in Icelandic geothermal waters lie in the range 0.11-223 ppb, 0.15-48 ppb, 0.01-36 ppb and 0.14-78 ppb, respectively. The concentrations of As, Sb and W increases with increasing temperature, whereas Mo has irregular behaviour with temperature. Arsenic, Sb and W are mobile in the geothermal water. They are, however, less mobile than B and this is considered to be due to non-stoichiometric dissolution of the bulk rock with As, Sb and W being concentrated in the relatively insoluble titano-magnetite but B on a very soluble form. Molybdenum concentration in high-temperature waters is controlled by molybdenite solubility. Boiling of sulphide rich water leads to molybdenite supersaturation and precipitation whereas boiling of low sulphide water leads to insignificant molybdenite formation. This variable behaviour upon boiling is considered to be the main cause of the variable Mo concentration between geothermal areas in Iceland.