The Meshkin Shahr Geothermal Prospect, Iran

Abstract

Mt. Sabalan, an immense trachyandestic stratovolcano with a prominent caldera, hosts a geothermal prospect near the town of Meshkin Shahr in northwest Iran. The volcanic rock chemistry indicates that volcanism has been produced in response to crustal thickening by thrusting; there is no current day subduction. An associated shallow magma chamber beneath the mountain is also indicated by the rock chemistry and Ar-Ar and K-Ar dating. The volcanic pile sits on an Early Miocene monzonitic batholith in the southwest and Late Miocene sediments in the northeast. Deformation of the sediments by the weight of the volcanic pile has resulted in a very large slump that has displaced the volcanic pile and opened the Moil valley. This has been followed by the extrusion of a small trachydacite dome in the Moil valley that is likely to be derived from an apophysis of the shallow magma chamber. Such an apophysis may represent the heat source of a geothermal system. Hotspring chemistry indicates that perched groundwater aquifers that exhibit storage behaviour contain neutralised magmatic volatile condensate. Such chemistry is consistent with the presence of the shallow magma chamber inferred from the geology. A significant area of deep low resistivity indicated by an MT survey is found in the Moil valley but not in the vicinity of the trachydacite dome, possibly due to inherently higher resistivities within a vapour core. Therefore, since the area of deep low resistivity is large an exploitable geothermal system is likely to be present as a neutralised zone around a magmatic vapour core.