| Abstract |
Exploration for geothermal resources in the Sabalan mountains of northern Iran, undertaken in the summer of 1998, identified several low resistivity anomalies around the flanks of the volcanic complex that are worth investigating by deep drill-holes. Interpretation of 212 magnetotelluric soundings was assisted by the use of coincident TEM and shallow DC resistivity measurements. Large areas (>50km2) of relatively low resistivity (<5 ohm-m) were found in the vicinity of mixed chloride-sulphate-bicarbonate springs at Gheynarge (to the NW) and at Sareyn (SE). Other significant areas containing low resistivities were found near Dollar Spring, along an inferred southern outflow path towards the Bouchli higher-chloride springs, and near the acid-sulphate spring at Ghotur Suii, which is near the northern rim of an inferred caldera collapse structure. Another significant low resistivity anomaly is located close to the northern caldera margin, between Tous Goali and Houshang Meidani. Here, there are relatively young trachydacite/rhyolite lava domes and flows and some surface evidence of hydrothermal alteration, but no active thermal features. On the northeastern side of the volcanic complex, a thick layer (1-1.5km) of moderately low resistivity (7-15 ohm-m) is associated with surface exposures of relict alteration. This layer overlies a dome-shaped resistive basement in an area of high Bouguer gravity values, which supports an inferred structural model of a dense intrusive mantled by conductive clays. Evidence of structural control on the location of some of the low resistivity anomalies is also noted, particularly where strong 2D effects occur along NW and NE trending corridors, and along the caldera margin. Conceptual hydrological models, based on layered resistivity sounding interpretations, support the geochemical evidence, from springs, of fluid mixing processes in the upper few hundred meters. Rising chloride fluids tend to discharge in valleys where the lowest resistivity layer is closest to the surface, while mixed and steam-heated waters discharge from overlying layers of intermediate resistivity, and cold groundwater springs occur within an uppermost high resistivity layer. |