Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 174

170
NESJAVELLIR
Occurrence of secondary minerols
Depth
0
too
?00
.XX)
400
500
600
700
800-
900-
>000
1100
1200-
1300
1400
I5Ö0 -
1600-
1700
1000
0
100
200
300
- 400
- 500
- 600
- 700
- 800
- 900
1000
-1100
1200
-1300
1400
ÖOO
>600
1700
1800
Fig. 3: The distribution of secondary minerals in drillhole no. 5 in the Nesjavellir
high-temperature geothermal area. A simplified geological section, the amount of
clay minerals and altered glassy matrix and estimated rock temperature are also
shown in the figure. Legend as in figure 2 for hasalt and hayloclastites. Dolerite
is marked by (evenly spaced) small crosses.
minerals olivine is most easily altered, first to iddingsitic aggre-
gates and later it is completely replaced by clay minerals. Pyroxene
is also altered to clay minerals, but this alteration is only com-
pleted in the most intensively altered rocks. The plagioclase is usu-
ally only slightly affected by alteration. The opaque minerals have
not been investigated in detail yet, but some of the original mag-
netite can survive in the basalts to the greatest depths reached by
drilling so far (5).
The temperature is assumed to be the most important factor in
the hydrothermal alteration. The permeability and age of the area
also highly influence the degree and distribution of the alteration.
This is clearly reflected by comparison of the Reykjanes and Nesja-
vellir areas (fig. 2 and 3).