Alteration of Basaltic Rocks by Hydrothermal-Activity at 100-300°C

Abstract

Six high-temperature geothermal areas in Iceland have been investigated by deep drilling. The underground rocks down to 2 km depth are basaltic hyaloclastites and lavas. Olivine-tholeiites are dominant in some areas, but tholeiites in others. Hydrothermal alteration has taken place at 100-300°C. In all the fields the same groups of alteration minerals are formed at the same rock temperature. However, minor differences are observed due to the varying composition of the basaltic rocks. Smectites (iron-rich saponites), zeolites, calcium silicates, calcite, pyrite and quartz are formed at rock temperatures below 200°C. The smectites have transformed into mixed-layer clay minerals and swelling chlorites at 200-230°C. Most zeolites and the calcium silicates disappear in this temperature interval. Chlorites become the dominant sheet-silicates when the rock temperature exceeds 240°C. Epidote and prehnite are formed at slightly higher temperature. Actinolite appears near to 300°C. The clay minerals are quantitatively the most significant alteration minerals. Further they respond quickly to temperature changes in the geothermal system. An examination of the clay minerals and their relation to other alteration minerals is very useful for interpretation of the thermal history of a geothermal field.