GEOLOGY OF THE DOMUYO GEOTHERMAL AREA, PATAGONIA, ARGENTINA

Cerro Domuyo (4,709 m of elevation) is the highest mountain in Patagonia and has been considered an extinct stratovolcano of Pliocene age. The existence of a geothermal reservoir on its western slope has been suggested since the 80’s on the basis of the occurrence of thermal springs with boiling fluids as well as silicic domes and pyroclastic deposits dated between 0.55 and 0.11 My. These early studies proposed that the thermal springs were fault-controlled and the reservoir was located in a graben bounded by E-W normal faults. A recent geochemical study estimated a temperature of 220ºC for the fluid reservoir and a thermal energy release of ~ 1.1 GW, one of the world largest advective heat flux from a continental volcanic center.

We have studied the geology of the Domuyo volcanic complex (DVC) to obtain an updated conceptual model of the geothermal system. Our survey indicate that the DVC is not a stratovolcano, but a dome complex formed by several silicic domes. They overlie an older, Middle Miocene to Pliocene volcanic sequence widely exposed to the southwest and to the north, which in turn cover a Paleozoic metamorphic terrain with Permian intrusions, and the Mesozoic Neuquen marine sedimentary succession. These pre-Cenozoic successions are involved in dominantly N-S trending folds and thrust faults later displaced by E-W striking normal faults with a right lateral component of motion that underlie the DVC. The volcanic cycle forming the DVC is distinctly bimodal with the emplacement of massive silicic domes but also less voluminous olivine basalts on its southern slope. The central dome underwent a major collapse that produced a voluminous ash and block flow and associated pyroclastic flows that filled the valley to the southwest up to ~30 km from the source. This was followed by voluminous effusive activity that formed the silicic domes of Cerro Domo, Cerro Covunco and Cerro de la Pampa, which are inferred to overlain a silicic magma chamber, less than 100 ka old. The geothermal reservoir is inferred at a depth of less than 1 km in the highly fractured pre-Cenozoic rocks bounded by ~E-W faults and capped by the pyroclastic deposits and rhyolitic lavas of the DVC. The location of most thermal springs is not controlled by faults. Rather, they are lateral flows emerging at the contact between the fractured basement and the caprock.