| Abstract |
Hydrothermal alteration of basaltic rocks in the geothermal area on Þeistareykir was studied in a drillcore from well ÞR-7 on the edges of the Theistareykir geothermal field, NE Iceland. Emphasis was laid on the study of clay minerals and zeolites formed by hydrothermal alteration. The reservoir rocks in the area are mainly tholeiitic basalt lavas and hyaloclastites, with occasional occurrence of acidic volcanic rocks that are moderately to highly altered. The rock forming minerals have been transformed to clay minerals or sheet silicates and several secondary minerals have been precipitated in vugs and factures. Several different clay minerals were identified in the altered rocks: smectite, chlorite, mixed layer chlorite/smectite, mostly irregular types and irregular chloritic mixed layer sheet silicates. Smectite/illite mixed-layer minerals were also encountered. The clay minerals are mostly poorly crystalline and a regular clay zonation from smectite through mixed layer smectite-chlorite to chlorite, that is common in high-temperature geothermal fields in Iceland, is not observed in the core. The zeolites identified in the core are laumontite, mordenite, wairakite, and the quite rare yugawaralite, previously only encountered in three localities in Iceland. The higher temperature zeolite wairakite is found in the middle of a zone dominated by the lower temperature zeolite laumontite. The dispersion of the secondary minerals does thus not show a very clear zonation of the alteration minerals and correlation with rock temperature is not easily obtained. Some of the clay minerals/ sheet silicates encountered suggest a retrograde alteration of previously formed clay minerals at lower temperatures than the original hydrothermal alteration. The occurrence of zeolites in the core implies a similar development. The rock temperature in well ÞR-7 appears to have been higher at earlier times than at present showing an overprint of lower temperature secondary minerals over a previous high-temperature alteration. |