IDEAN   23403
INSTITUTO DE ESTUDIOS ANDINOS "DON PABLO GROEBER"
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
THE GEOTHERMAL FIELD IN THE SW SECTOR OF THE DOMUYO VOLCANIC SYSTEM (PATAGONIA, ARGENTINA): A GEOCHEMICAL APPROACH FOR ASSESSING A LIKELY ENORMOUS GEOTHERMAL POTENTIAL
Autor/es:
LICCIOLI C.; CHIODINI G.; VASELLI, O.; TASSI, F.; AGUSTO M. R.; CASELLI A.T.
Lugar:
Córdoba
Reunión:
Congreso; XIX Congreso Geologico Argentino; 2014
Institución organizadora:
Asociación Geologica Argentina
Resumen:
A geochemical survey of the main thermal waters discharging in the SW sector of the Domuyo volcanic complex (36°34?S, 70°25?W, Fig. 1) was carried out to estimate the thermal release and geothermal potential of this remote site of Patagonia. The chemical compositions of the boiling waters were used to: i) infer the temperatures of the deep reservoir, ii) restore the original composition of the stored un-boiled liquid and iii) compute the enthalpy of the geothermal system based on the equilibrium temperatures and conservative elements (e.g. Cl, Na, Li). By using original geochemical data and rivers flow rates into which these hot waters flow (Fig. 1), the advective heat flux was calculated as in tectonically young and active areas of the Earth where meteoric waters infiltrate and deeply circulate, it may represent the dominant form of heat transfer. The magnitude and spatial distribution of heat flux are relevant for both evaluating potential geothermal resources and inferring the deep structure underlying volcanic areas, with reference to the nature and size of heat sources (such as magma or hot intrusive rocks) at depth (Manga, 1998). The Domuyo volcano (Middle Miocene to Pleistocene) is a broad (estimated volume of ~ 200 km3 with a base area of 138 km2; Völker et al., 2011) structural dome located in the northern edge of the Cordillera del Viento chain. The volcanic rocks belong to two distinct magmatic series originated from different volcanic activities by two feeding magmas: a calc-alkaline series that produced andesitic rocks and a shoshonitic-alkaline series that originated dacitic to rhyolitic rocks whose main expression is the Cerro Domo (0.72±10 Ma), located in the southern slope of Domuyo (Brousse and Pesce 1982). The youngest volcanic products were dated between 0.55±0.10 and 0.11±0.02 Ma (JICA, 1983) by fission-tracks method, suggesting that Domuyo volcanism had been active at least until the Late Pleistocene. In the SW slope of the Domuyo volcano several thermal springs at boiling point discharge into roughly ENEWSW oriented creeks, which at their turn flow into the main drainage system of the zone: the NS-oriented Rio Varvarco (Fig. 1). This area shows strikingly impressive signs of high release of thermal energy: consistent flow rates (hundreds L s-1) of boiling fluids enter the hydrographic network, where the temperature of the creeks are maintained up to 50 °C after 5 to 10 km from the fluid source. In February 2003 one of the investigated thermal spots suffered two violent hydrothermal explosions likely triggered by local sealing processes of the near-surface channels from where the hydrothermal fluids are discharged (Mas et al., 2009). The sampling strategy (Fig. 1) was consisting in the sampling of: i) four thermal springs (red circles), ii) Rio Varvarco waters, upstream and downstream of the entrance of the hot waters (dor6 and dor12) and in correspondence of the main creeks before flowing into the main drainage system (dor7,8,9,5,10,11) and iii) two cold springs (green circles) not far from some of the thermal manifestations. All the thermal waters have a Na- Cl composition similar to that observed for the streams into which the thermal waters discharge, although the receiving creeks show a lower salinity due to dilution processes. The two cold springs and two stream waters have a Ca(Na)-HCO3 composition and low salinity, whilst only the upstream sample (dor6) has a Ca-SO4 geochemical facies, likely derived by the dissolution of gypsum-bearing formation occurring in the hydrographic basin of the Rio Varvarco upstream of the surveyed area (Palacio and Llambias, 1978). Equilibrium temperatures of the geothermal reservoirs were calculated by using the Na-K-Mg geothermometer, assuming that the liquid phase was in contact with a typical hydrothermal mineral assemblage to achieve a rocksolution iso-chemical equilibrium. Furthermore, their very low Mg contents were compatible with a ?mature? geothermal liquid (Giggenbach, 1988). Three out of the four thermal waters resulted to be fully equilibrated with the assumed solid phase. Only one of them was partially equilibrated, likely due to a secondary process, i.e. mixing with cold ground waters. The resulting equilibrium temperatures were between 200 and 230 °C. The three thermal springs out-flowing at boiling temperature are likely affected by shallow steam separation, a process that is enhanced by the formation of steam vents nearby the thermal emissions. During steam separation, CO2, originally dissolved in the water, is lost favoring the increase of the pH values (up to 8.3). As a consequence, the precipitation of CaCO3, as testified by the presence of travertine deposits, occurs. According to Mas et al. (2009), the emitted gas phase is mainly made up of steam (99% by vol.) and other gases among which XIX Congreso Geológico Argentino, Junio 2014, Córdoba VOLCANES ACTIVOS S23-23 CO2 is the most abundant. The steam loss also induces the increase of solute concentrations in the residual phase. Assuming that the original geothermal liquid was affected, during its rise to the surface, by an iso-enthalpic boiling process, the calculated fraction of separated steam (from 0.17 to 0.26) allowed to restore the original composition of the un-boiled liquid referring to the mobile species concentrations. Sodium, Cl and Li were chosen as tracers of the thermal component flux in the main river (Rio Varvarco) and its tributaries because of their conservative behavior. The Na, Cl and Li fluxes showed a sharp increase from Rio Varvarco upstream to Rio Varvarco downstream as the hot waters by the tributaries enter the main drainage system of the SW part of Domuyo, although part of the feeding hot waters may be lost as they infiltrate into the permeable alluvial deposits. Being Cl the most conservative element and considering that thermal waters are usually enriched with respect to superficial and shallow ground waters, the Cl flux computed at the downstream section of Rio Varvarco River corresponded to a thermal component of 1,100-1,300 L sec-1. Referring to a liquid originally at 220 °C, the thermal energy release (QH) can be computed by the following simple relation: QH = (QCl/Clt)×Ht where QCl is the flux of Cl in the river (g s-1), Clt is the Cl concentration in the original thermal liquid (g g-1, the unity was converted from mg L-1 considering a water density of 1,000 g L-1) and Ht is the enthalpy (J g-1) of the liquid at the estimated reservoir temperature (220 °C). The computed value was of ~ 1.1 GW, which to the best of our knowledge is the second ever measured advective heat flux from any hydrothermal system of the Earth after Yellowstone Caldera (USA). Setting aside this latter case, where the total discharge of the thermal waters was estimated to be 3,200 L s-1 (Fournier, 1989), thermal discharges in other geothermal systems are rarely exceeding 100 L s-1, this value being computed during the pre-exploitation discharge at Wairakei (New Zealand, Ellis and Wilson, 1955). This observation should pose the attention on the significant, still unexploited, geothermal potential of Domuyo volcano. Further studies at the Domuyo volcanic system are presently in progress, although in our opinion the occurrence of such relevant estimated heat flux may refer to either a not documented younger volcanic activity or a magma intrusion at shallow depth, possibly related to a recent reactivation of the Domuyo volcanic system.