Jökull - 01.12.1987, Page 4

field are almost entirely located within the ring structure
and cover an area of about 140 km2 (Fig. 1). They consist
mostly of steaming ground which is, as a rule, intensely
altered by acid surface leaching. Steam heated waters,
both of the acid sulphate and the bicarbonate types, are
relatively common. In the northeastern part of the field,
around Landmannalaugar, sodium-chloride type water
springs are common representing boiled and variably
mixed reservoir water.
Chemical and mineralogical alteration associated
with acid surface leaching was studied in detail in one
locality by Sigvaldason (1959). Arnórsson (1969) carried
out a reconnaissance survey of the major and trace
element chemistry of hot spring discharges. The chem-
istry of the sodium-chloride waters and fumarole gases
in the Landmannalaugar area were investigated by Ar-
nórsson (1985) and Arnórsson and Gunnlaugsson
(1985). A special study of germanium and molybdenum
in Icelandic geothermal waters (Arnórsson 1984, Ar-
nórsson and ívarsson 1985) included data from the
Landmannalaugar area. Using chemical geothermom-
etry and mixing models Arnórsson (1985) concluded that
subsurface temperatures around Landmannalaugar
were some 265°C.
The deuterium and ðlsO content of the sodium-chlo-
ride waters indicate that they are local precipitation
(Árnason 1971, Arnórsson 1985). Oxygen shift of as
much as 2.5 per mil ðlsO units are observed for the
waters highest in chloride.
In the present contribution we summarize geochem-
ical studies which have been carried out on the hot
springs and fumaroles in the Torfajökull geothermal
field including the associated surface hydrothermal al-
teration.
THE HEAT SOURCE
Bödvarsson (1961) estimated the natural heat output
of the Torfajökull geothermal field to be 125-750 • 106
cal/s (525-3150 • 106 J/s). This corresponds to 190-930
kg/s of steam at atmospheric pressure. Pálmason (1980)
has evaluated the stored heat in the uppermost 3 km of
the geothermal reservoir to be 281 • 1018 J and the theo-
retical energy potential to be 964 MW electric for a 50
year production period. According to these authors Tor-
fajökull is the largest geothermal field in Iceland.
The acid volcanics in Iceland are generally considered
to have originated by partial melting of hy drated basaltic
crust due to intrusion of mantle derived olivine tholeiite
magma (Óskarsson et al. 1982, 1985). Accordingly the
abundant acid rocks at Torfajökull indicate emplace-
ment of an unusually large body of basalt magma into
the lowest part of the crust.
The Torfajökull volcanic complex overlies a mantle
plume (Óskarsson et al. 1985, Kurz et al. 1985). Rifting
has not been active in the volcanic zone where this
complex is located until during the latter part of the last
glaciation (ívarsson et al. 1987). The combined effects of
the mantle plume and the lack of rifting may be the cause
of the presumed voluminous basaltic intrusion.
A negative gravity anomaly coincides approximately
with the distribution of the surface outcrops of acid
volcanics (Sœmundsson 1972). A prominent gravity high
occurs within the negative anomaly. Walker (1974) con-
siders that this high reflects basaltic sheet intrusions and,
due to the relatively low density of the acid volcanics,
that the basaltic magma tended to form intrusives at the
base of these rocks rather than rising through them.
Walker (1974) further considers that the presumed bas-
altic sheet intrusions at the base of the acid rocks consti-
tute the heat source to the geothermal field.
ACID SURFACE LEACHING
Sigvaldason (1959) showed by his study of surface
alteration of rhyolite at Hrafntinnusker that all the ma-
jor elements are leached as the rock undergoes complex
mineralogical changes, yet to a different extent. The
ultimate alteration product becomes enriched in those
elements which are leached to the least extent. They
include silica and, in particular, titanium. Montmorillo-
nite appears during the earliest alteration stages. When
all the primary minerals have been decomposed, hema-
tite, anatase and, to a lesser extent, kaolinite are found
with the montmorillonite. At later alteration stages
montmorillonite decomposes conjuncture with progres-
sive leaching of Na, K, Ca and Mg from the rock and
kaolinite grows in abundance, becoming as much as
50% of the rock by volume. Pyrite and amorphous silica
are found in association with the kaolinite and montmo-
rillonite at this alteration stage. In the ultimate alter-
ation product montmorillonite is absent and the rock
consists mostly of amorphous silica, anatase and kaoli-
nite with variable amounts of native sulphur and pyrite.
INTERPRETATION OF WATER
CHEMISTRY
Analyses exemplifying the bicarbonate, acid sulphate
and sodium-chloride waters are given in Table 1. The
bicarbonate waters are characterized by low chloride
concentrations, near neutral pH and, in general, they
2