Hydrogeological Study of the the Rio Tinto Mars Analog: Implications for Mars Underground Water Fluxes

Introduction: The Iberian Pyritic Belt (IPB) consists of a 250-km long geological unit included into the South-Portuguese geotectonical zone of the Iberian Peninsula. It is comprised by an acid volcanosedimentary complex ranging in age from upper Devonian to Carboniferous where different metallic ores were formed in response to hydrotermalism and tectonic related to the Variscan Orogeny [1, 2]. Inside the IPB, the Rio Tinto area contains some of the largest world’s massive sulfide deposits. In Río Tinto, the interaction between groundwater and sulfide deposits and sulfide-free rocks provides geochemical processes controlling the geochemistry of both surface fluids and groundwater. The resulting waters are characterized by an acidic chemistry and high concentrations of sulfur and iron in solution, which control the formation of analogous mineral associations that have been recognized in Meridiani [3, 4, 5]. As Mars basement has been intensively fractured by meteoritic and geological events, same geochemical reactions ending in the formation of sulfates and oxides that occur in the Río Tinto system may have occurred in the Mars early environments. To understand the processes that drive the mineral formation of sulfur and iron bearing phases, special attention has been paid to the aquifer located on Peña de Hierro, North of Rio Tinto, which are the headwaters that supply the acidic waters of Río Tinto. Given that Peña de Hierro aquifer is mainly controlled by the recharge from rainwater through the present fault system, a hydrogeological study has been conducted in order to accurately characterize underground water fluxes of the aquifer. Geological setting: Peña de Hierro is located at the contact between two different tectonic units included inside the IPB, Concepcion Unit to the North and RioTinto-Nerva Unit to the South (Fig. 1). The first one is composed by shales acidic tuffs and volcanic materials. The second one is composed of dark shales and greywackes. A highly deformed thin unit of purple shales delineates the contact between both units, defined by the San Miguel-Minas del Castillo Shear Zone (SMCSZ). The SMCSZ formed during the second phase of deformation associated to the Variscan Orogeny (Fig. 1) and corresponds to a transpressional ductile deformation with associated penetrative foliation [1, 2]. A Late-Variscan episode of deformation is responsible for the development of a conjugate strike-slip system of brittle fractures trending NNW-SSE (dextral) and NNE-SSW (sinistral) respectively. These fractures are extensively distributed all around the IPB and represents the transition form ductile transpressional deformation to a brittle transtensional event related to the collapse of the Variscan Orogen. The tectonic framework of Peña de Hierro aquifer has been analysed using satellite imaginery analysis and artifical shading of a Digital Elevation Model [6]