New conceptual model for the Tocomar geothermal system (Central Puna, NW Argentina): insights from geological, structural and geochemical data.

FILIPOVICH, RUBEN; CHIODI, AGOSTINA; BIGI, SABINA; DE ASTIS, GIANFILIPPO; NORINI, GIANLUCA; VIRAMONTE, JOSE GERMAN; INVERNIZZI, CHIARA; BÁEZ, WALTER; CORRADO, SVEVA; DE BENEDETTI, ARNALDO; TAVIANI, SARA; GIORDANO, GUIDO; AHUMADA, MARIA FLORENCIA; ALDEGA, LUCA; CARICCHI, CHIARA; GROPELLI, GIANLUCA; TASSI, FRANCO

Catania

Congreso; Geosciences for the environment, natural hazard and cultural heritage; 2018

Società Geologica Italiana

The outlining of conceptual models, even in the arliest stages, represents a paramount aspect for geothermal exploration since these schemes are by far the most effective cost-benefit tool. While analogue models of geothermal fields provide an overall picture of reservoirs, they do not help planning exploration programs as each area of geothermal interest is a function of site-specific variables. Thus, reconstructing stratigraphic and structural framework is fundamental for i)understanding the relationship among cap rocks, reservoirs and fluid circulation, ii) geothermal potential, and iii) planning resource explotation. Limited geothermal resource assessment in Argentina is based on weak exploration approaches as anomaly hunting or anomaly stacking, more than on realistic conceptual models generated from detailed geological information. In this sense, a new conceptual model for the Tocomar Geothermal System (TGS) based on geological, structural and geochemical data along with the available geophysical data is presented. The TGS is located in the Central Puna, related to the 0.57 Ma Tocomar volcanic center (TVC) emplaced in a small extensional basin "Tocomar basin (TB)" linked to the active NW-SE trans-Adean tectonic lineament known as Calama?Olacapato?Toro (COT) fault system. The pre-basin succession is made of low-grade metamorphic and sedimentary lower Paleozoic rocks, Cretaceous continental rift-related deposits and ignimbrites from the Aguas Calientes Caldera (10-17 Ma). The basin infill consists of a thick alluvial deposit formed by polymictic conglomerates and sandstones that evolve upwards to several pyroclastic deposits. The updated conceptual model for the TGS consists of a heat source related to the 0.57 Ma rhyolitic magmatic activity of the TVC. The geothermal reservoir has a NW-SE geometry elongated along the COT-like Chorrillos fault, with a minimum area of ca. 6 km2. The average depth of the top of the reservoir is 1,000-1,400 mbgs, probably hosted in Cretaceous sedimentary rocks. The primary permeability is enhanced by intense deformation along the COT. Both the interbedded fine-grained Cretaceous facies and a hydrothermal clay cap (argillic alteration) act as seal rocks. Hot springs occur in association with deep N30-N60 fractures related to the COT. They show a Na-Cl composition, with pH values of 5.84 to 6.87, and outlet temperatures ranging from 30.3°C to 70.2°C. Temperature estimation using the NA-K geothermometer indicates reservoir temperatures of 184-230°C. Estimated local geothermal gradient for the TB is ~150 °C/km and the minimum stored heat in the geothermal reservoir is of about 3.21x1015 kJ.