Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 171

HYDROTHERMAL ALTERATION IN ICELANDIC
GEOTHERMAL FIELDS
HREFNA KRISTMANNSDÓTTIR
and
JENS TÓMASSON
National Energy Authority, Department of Natural Heat,
Laugavegur 116, Reykjavík.
ABSTRACT
In all investigated Icelandic geothermal areas the following alteration zones
are found as characterized by dominant alteration minerals:
1) Smectite-zeolite zone.
2) Mixed-layer clay minerals (prehnite) zone
3) Chlorite-epidote zone
By progressive alteration of the rocks they go from zone 1 to 3, which repre-
sents the most strongly altered rocks so far reached hy drilling. The alteration
scheme from the Icelandic geothermal fields and the effect of low temperature
metamorphism on Mid-Atlantic Ridge rocks are compared and are found to he
very similar.
INTRODU CTION
Investigation of the 17 active high-temperature geothermal areas
in Iceland is in progress. Deep drilling (to 1000-1850 m) has been
carried out in seven of them. Detailed mineralogical investigation
of the drill cuttings has been performed in some of these and is
proceeding for the others. This summary is based mainly on our re-
sults from the Nesjavellir (Hengill) and Reykjanes areas (fig. 1)
(1,2 and unpublished results) and work by Sigvaldason (3) and