Mineralogy and Textures of Metal-rich Scales from the Reykjanes Seawater-dominated System, Iceland: Comparison with Seafloor Hydrothermal Systems

Abstract

The Reykjanes high-temperature geothermal system is the onshore extension of the Mid-Atlantic Ridge in Iceland. The high-temperature liquid has salinity similar to seawater, but is depleted in SO4 -2 and Mg by two and three orders of magnitude, respectively, due to precipitation of anhydrite and magnesium minerals at depth. The liquid is enriched in SiO2, K, and Ca relative to seawater due to reaction with the surrounding basalts. The range of inflow temperature to wells is between 275°C and 315°C. Hydrothermal manifestations at the surface cover 1 km2, but the geothermal system is believed to be as large as 10 km2 in the subsurface. It has been utilized discontinuously since 1983. Only two wells were in operation during this time and both were closed due to scaling, but 14 new wells have been drilled the last three years for power generation.

Scales that formed at the wellhead of one of the closed wells (no. 9, 35 bar-g well-head pressure) consist of fine black, sooty sulfides as well as bands 4 mm thick of very fine layers of anhedral sphalerite intergrown with chalcopyrite, galena, and clay minerals. In pipes on surface, upstream (on the high pressure side) of the orifice plate, the scales (