Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 175

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In both areas the rocks are dominantly basaltic hyaloclastites in
the uppermost 1000 m. Beneath 1000 m basalts dominate in both
areas. In the Nesjavellir area dolerite intrusions are very common
beneath 1450 m and this is seen to influence the course of altera-
tion beneath that depth.
In the Reykjanes area the alteration is intense and the originally
glassy rocks are commonly composed of 80-90% by volume of
clay minerals. In the former basalts up to 50-60% is converted
to clay minerals. Zeolites and smectites are formed at estimated
rock temperature lower than 200°C and have disappeared com-
pletely at 230°C. Chlorite and epidote are formed at temperatures
above 230°C. The mixed-layer clay minerals are most commonly
found at rock temperatures 200-230° C.
In the Nesjavellir area the amount of alteration is low except
in certain lenses or zones. Zeolites are found at rock temperatures
above 250°C as compared to their disappearance at 200-230°C in
the Reykjanes area. Epidote first occurs at much higher tempera-
ture than in other known areas. The clay minerals also occur at
higher temperatures than found for other areas. This is partly be-
cause we are at the northern margin of the Hengill area and the
temperature has increased fairly recently and partly because of the
different permeability and age of the rocks in the Nesjavellir cross
section.
The water in the Reykjanes geothermal area is seawater in origin,
but Nesjavellir has as most other of the high-temperature geother-
mal areas meteoric water (Table 2). This variation is found to 'have
no great effect on the overall course of alteration. The same main
groups of secondary minerals are formed in all the areas. The
effect of the different water composition is expressed by smaller
differences in the course of alteration.
The types and distribntion of alteration minerals
In fig. 4 are summarized the main alteration zones distinguished
in Icelandic geothermal areas. The boundaries of the zones are
somewhat approximate but the lower limits of the upper two zones
are at approximately 200°C and 230°C. The zones are named after