Jökull


Jökull - 01.12.1987, Síða 4

Jökull - 01.12.1987, Síða 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
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