The Reykjanes Geothermal System: Alteration Mineralogy and Fluid-Rock Interaction in an Icelandic High Temperature System

Abstract

The Reykjanes geothermal system is a seawater-recharged hydrothermal system that appears to be analogous to seafloor hydrothermal systems, and for this reason is a useful proxy for seafloor vents. During the Pleistocene, the system was dominated by meteoric water recharge, and fluid composition at Reykjanes has evolved through time as a result of changing proportions of meteoric water influx as well as differing pressure and temperature conditions. The purpose of this study is to characterize secondary mineralization, degree of metasomatic alteration, and bulk composition of cuttings from well RN-17 from the Reykjanes geothermal system. The progressive hydrothermal alteration sequence observed with increasing depth results from reaction of geothermal fluids with the basaltic host rock. Alteration phase textures of drill cuttings reveal alteration style to be strongly dependant on protolith. Hyaloclastites are intensely altered with calc-silicate alteration assemblages comprising calcic hydrothermal plagioclase, grandite garnet, prehnite, epidote, hydrothermal clinopyroxene, and titanite. In contrast, crystalline basalts and intrusives display a range in alteration intensity from essentially unaltered to pervasive and nearly complete albitization of igneous feldspar and uralitization of clinopyroxene. 87Sr/86Sr isotopic values of alteration phases including epidote range from 0.7037 to 0.7076 and indicate isotopic exchange with seawater Sr.