Clay-bearing Paragenetic Associations in Riotinto (sw Spain): Disentangling Multiple Pathways of Acidic Bedrock Alterations on Mars

Introduction: Numerous studies at the Riotinto mining district have focused on astrobiology, sulfates and extremophilic microbial communities in the highly acidic aqueous environments [1-3]. In contrast, this project investigates past acidic alteration of nearby volcanic rocks. The present study covers a variety of alteration pathways that may influence volcanic protoliths, with the specific types of clay minerals present acting as the key to decode the degree of acidity. Different acidity-dependent alteration pathways are explored that lead to the formation of a variety of secondary clay minerals and sulfates. Examining the alteration pathways at this site is expected to further our understanding of potential alteration on Mars. Characterization of the spectral properties and XRD patterns of these materials will contribute toward interpreting similar data of Mars. Materials and Methods: Three alteration sequences were sampled at the Riotinto mining district (Fig. 1). Geologically, an Upper Palaeozoic (Late Famennian-Tournaisian) complex including siliciclastic sediments and mafic and felsic volcanics underwent hydrothermal alteration. The latter enriched the bedrock with quartz, chlorite and illite (2M1 muscovite). Oxidation of an extensive pyrite-rich orebody occurred due to fluctuation of the water table (Miocene). As a consequence, moderate to extreme acidic fluxes leached the surrounding rocks for over 20 million years [4]. Rock samples were collected at three selected sites for lab analyses. Powder preparations were analysed using a PanAlytical X’PERT-PRO X-ray diffractometer (45 kV and 40 mA, 2-80 °2Θ range) for full mineralogy (not shown). An aliquot was used to extract the clay fraction (<2 μm) by sonication and centrifugation. Oriented aggregates were prepared on glass slides and measured in both air-dried and ethylene-glycolsolvated conditions [5] over the range 2-40 °2Θ. The XRD patterns were then modeled using ClaySim (from MDI) for the quantitative investigation of clay mineralogy as described in [6]. The spectral properties of these samples were measured for comparison with the lab XRD data and spectra of Martian outcrops. Mid-IR transmittance measurements were carried out at the Natural History Museum of London. VNIR and midIR reflectance spectra were acquired at ReLAB (Brown University). Results and Discussion: The three alteration se-