GEOTHERMAL SYSTEMS IN ICELAND - STRUCTURE AND CONCEPTUAL MODELS .1. HIGH-TEMPERATURE AREAS

There are 20 known high-temperature geothermal areas in Iceland and an other eight potential areas. Surface manifestations are meagre in thes e eight areas and not conclusive, and no drilling has been carried out to prove or disprove the existence of high-temperature geothermal sys tems at depth. The high temperature areas are located within the activ e volcanic belts or marginal to them. The heat source is considered to be magmatic,shallow level crustal magma chambers in the case of high- temperature systems associated with central volcanic complexes, but dy ke swarms for the systems on the Reykjanes Peninsula where no central volcanoes have developed. Fossil high-temperature systems are abundant in Quaternary and Tertiary formations as witnessed by alteration of t he basaltic eruptive rocks into lower-greenschist mineral assemblages. The fossil systems are typically associated with central volcanoes wh ere intrusives account for 50% or more of the rock. The fossil systems are considered to have formed within the active volcanic belts but dr ifted out of them in conjunction with crustal accretion within these b elts. In the process they may develop into low-temperature geothermal systems. Permeability is very variable within the drilled high-tempera ture areas, in the range 1-150 millidarcies. The best permeability gen erally appears to be associated with sub-vertical fractures and faults . Permeability is poorest when the reservoir rock consists dominantly of intrusives, such as at Krafla, northeastern Iceland. It appears tha t intrusives are most abundant in reservoirs associated with central c omplexes that have developed a caldera. Temperatures follow the boilin g point curve with depth, at least to the level of the deepest wells, in some areas, but in others they are lower. The highest recorded down hole temperature is >380 degrees C. Hydrological considerations and pe rmeability data favour that convection is density driven and that the source water is shallow groundwater in the vicinity of these systems. This groundwater is in most cases of meteoric origin. However, in thre e areas on the Reykjanes Peninsula it is largely or solely marine. The deuterium content of geothermal waters of meteoric origin is often lo wer than that of local precipitation. This has been taken to indicate that the source of supply is precipitation that has fallen on higher g round inland. This may indeed be the case, but Row from the source are a is considered to be shallow. In some cases the low delta D-values ma y stem from the presence of a component of an old water, which is isot opically lighter than today's precipitation at any particular site bec ause the climate in Iceland was colder in the past. The geothermal sea water at Reykjanes and Svartsengi, southwestern Iceland, is considerab ly lower in deuterium than seawater. The cause of this is not known. H owever, reaction between seawater and basaltic rocks at very low tempe ratures may contribute, as well as rising of H-2 gas from deep levels and its reaction at shallower levels in the geothermal system to form water, but H-2 gas is much more depleted in deuterium than the associa ted water. Degassing of the magma heat source appears to add chemical constituents to the geothermal waters, such as boron, carbon and sulph ur. Sometimes there may also be addition of Cl and H2O during events o f recharge of new magma into the magma chambers in the roots of the ge othermal system such as has been observed in the Krafla area. The high -temperature geothermal waters are close to chemical equilibrium with alteration minerals for all major components, except Cl and B. The alt eration minerals typically display depth zoning because many of them a re stable only over a limited temperature range. At temperatures above about 250 degrees C the alteration mineral assemblage is that of the greenschist metamorphic facies. Precipitation of carbon as calcite and sulphur as sulphides, where boiling occurs in upflow zones of high-te mperature geothermal systems, leads to strong enrichment of carbon and sulphur in the altered rock.