Sulfide Mineralization in the Reykjanes Geothermal System, Iceland ? Potential Applications for Geothermal Exploration

Abstract

The nature and distribution of sulfide mineralization in the Reykjanes geothermal field were determined through the study of cuttings and core from wells that intersect different regions of the subsurface hydrothermal system. Samples were evaluated using optical and scanning electron microscopy, major and trace element analysis by in-situ spectroscopic methods, isotopic analyses, and bulk rock chemical analyses. The data suggest that seawaterderived hydrothermal fluids have interacted with primary igneous sulfides present in the host rocks causing alteration and leaching of their base and precious metals. Similar processes have been proposed as a major control of aqueous sulfide production in midocean ridge environments. Hydrothermal sulfide mineralization in the Reykjanes geothermal field is focused around the present-day upflow region, with the most abundant mineralization occurring in the reservoir at a depth of between ~1300 and 2000 m, and at shallow depths above the inferred caprock to the system (<~500 m depth), where mixing with oxidized near-surface waters can be inferred. The downhole distribution of sulfide phases correlates well with the present-day thermal structure of the system.