Metal Concentrations and Metal Deposition in Deep Geothermal Wells at the Reykjanes High-temperature Area, Iceland

Sulfide scale is precipitating at 2770 m depth and ∼330 °C in well RN-17B from unboiled, seawater-dominated hydrothermal fluids. The scale consists of well-crystallized pyrrhotite with minor sphalerite, and traces of galena, pyrite, and chalcopyrite. Gold concentrations in the bulk sulfides reach 40 ppm, and Ag concentrations reach 20 ppm. In another well, with similar fluid composition, sulfides scale formed due to boiling at 670 to 140 m depth (267 °C – 260 °C), and contains abundant skeletal sphalerite, minor chalcopyrite, trace of bornite, pyrrhotite, pyrite, and gold. Bulk concentrations of Au in this scale reach 100 ppm, and Ag concentrations range from 35 to 450 ppm.