Náttúrufræðingurinn - 1981, Page 63

SUMMARY
Dissolved Constituents in
Geothermal Waters and
Mineralogical Alteration
by
Stefán A mórsson and Einar Gunnlaugsson
Science Institute
University of Iceland, Reykjavík
and
Hördur Svavarsson
National Energy A uthority
Grensásvegur 9, Reykjavík
The present contribution sumrnarizes
results obtained through investigations of
water and gas chemistry and hydrothermal
alteration in Icelandic geothermal systems.
An overall chemical equilibrium seems to
be attained — or closely approached —
between the dissolved major constituents in
the geothermal waters and the alteration
minerals. The variables that effectively
determine the major element fluid
chemistry are temperature and the supply of
chloride to the water. This supply may be of
marine origin, occasionally derived from
magmas, but mostly it has been leached
from the rock undergoing alteration.
The hydrothermal alteration at depth is
characterized by zonal distribution of
alteration minerals which is directly related
to underground temperature conditions.
Alteration at depth is to be contrasted with
surface alteration in the high-temperature
geothermal fields. This surface alteration is
caused by acid waters formed by
condensation of steam in surface waters and
subsequent oxidation of hydrogen sulphide
in the steam to sulphuric acid. Drastic
mineralogical and chemical changes
accompany alteration of the rock by the acid
surface waters.
Although mineralogical transformations
of the rock at depth in geothermal systems
may be complete, chemical changes
accompanying this alteration appear to be
subtle. The main changes, which are mostly
confined to high-temperature systems,
include addition of water, sulphur and
carbon to the rock. The two latter elements
are thought to be mostly derived from
underlying magma intrusions acting as the
heat sources for the geothermal system.
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