Structural analysis and fluid geochemistry as tools to assess the potential of the Tocomar geothermal system, Central Puna (Argentina).

The Argentinean Andean region hosts a vast geothermal resource clustered by active magmatic and tectonic activity. One of the most studied geothermal areas is the Tuzgle-Tocomar geothermal system in Central Puna (NW Argentina). However, despite the existence of several studies since the 1970′ s highlighting the geothermal potential of the area, only highly schematic and dissimilar conceptual models have been proposed for the Tocomar Geothermal System. This study presents new detailed geological-structural and hydrogeochemical data, together with in-situ permeability analysis of fault zones and Raman spectroscopic characterization of hydrothermal-alteration minerals. The electrical generating capacity has also been evaluated using the volumetric method and a stochastic approach. A new comprehensive conceptual model of the studied area was constructed highlighting the role of the Calama-Olacapato-El Toro (COT) fault-system in the circulation of hydrothermal fluids. The reservoir of the Tocomar geothermal system has a Na+-Clˉ(HCO3)ˉ composition and an estimated temperature of ~235 ◦C. Such a reservoir is hosted in fractured Ordovician rocks and controlled by the COT-like Chorrillos transpressive fault at 1000–1500 m depth b.g.l. The water isotopic data and hydrological features indicate a regional recharge beyond the Tocomar sub-basin boundaries (>5000 m a.s.l.). Additionally, the main hydrothermal reservoir receives inputs of magmatic fluids from the degassing of the intra-crustal rhyolitic magma chamber of the Tocomar volcanic center. The Monte Carlo simulations suggest that the Tocomar geothermal system has a probable power production capacity above 1.23 MWe (P90), 6.18 MWe (P50) and 11.67 MWe (P10) at different confidence levels. All calculations were biased towards minimum values, thus a tighter definition of the resource size and fracture porosity would significantly impact on the predictions. Notwithstanding, the strategic position of the Tocomar geothermal field encourages move forward towards more in-depth exploration phases.