| Abstract |
Due to its geodynamic location on the Lesser Antilles arc, Basse-Terre of Guadeloupe displays an active volcanism which is favorable to the development of geothermal energy associated to power generation. Thus, twenty years ago, the Bouillante geothermal field was brought into production and now produces 15 MWe (Bouchot et al., 2010). An exhumed analog of the Bouillante geothermal reservoir has been identified on Terre-de-Haut island (Les Saintes archipelago, 35km SE of Bouillante; Verati et al., 2016), which represents a key area to study the paleo-hydrothermal alterations, and the interactions between the paleo-hydrothermal fluids and the surrounding rocks. To characterize the paleo-fluids in terms of composition and temperature, and to determine the mineralogical and petrographic features of the paleo-hydrothermal alterations, we propose to couple crystallographic, geochemical and geothermometric approaches, i.e. X-ray diffraction (XRD), microprobe analysis (EMPA), chlorite thermometry and in-quartz fluid inclusion study (FI) on selected samples. The clay minerals display a specific concentric distribution at the scale of the island, with chlorite crystallization in the core, smectite in the surroundings, and illite in an intermediate halo. Considering the data obtained in boreholes in Bouillante, which show that illite and chlorite are in the deeper parts, chlorite being located in the heart of the geothermal reservoir (Bouchot et al., 2010), the clay distribution allows to identify the temperature profile. Hence, thanks to the exhumation, we are looking to a horizontal section through the paleo-system of Terre-de-Haut, where we observe a lateral temperature gradient responsible for the paleo-hydrothermal alteration. Chlorite geothermometry, based on a model specifically developed for low temperature contexts (T less than 350°C) and pressures below 4 kbar (Bourdelle et al., 2013), has been applied on chlorites from both Terre-de-Haut paleo-system and from the active geothermal system of Bouillante. The results show a strong difference between the temperature estimates for chlorite formation on Terre-de-Haut (around 120°C) and for Bouillante (around 230°C, in agreement with the temperature measured in boreholes in Bouillante; Mas et al., 2006). Observations of newly formed euhedral quartz, sampled in a geode located in a N70 oriented fracture, reveal the presence of numerous primary fluid inclusions. Preliminary results show that quartz underwent at least two growth stages recorded in the crystal core and clear overgrowths. Data indicate very low salinity (2% NaCl), and a minimum trapping temperature of around 250-280°C in inclusions located in the core, and around 70°C or less in the outer growth zones. These two events can be interpreted as a record of the fluid cooling during system evolution. Moreover, fluid inclusions from a second quartz sample (in another N70 fracture) are composed of nearly pure CO2 with very low density, indicating a shallow CO2 paleo-circulation episode. This study shows that the clay mineral zonation cropping out in Terre-de-Haut is similar to that found by drilling in the active system of Bouillante. However, the temperatures of formation of some newly formed minerals indicate that some alteration episodes occurs at lower temperature than the fluid circulation occurring in Bouillante’s active geothermal system. Hence, these results show that clay minerals study and geothermometry of newly formed minerals (chlorite and quartz) are key steps to provide new thermal constrains on the paleo-geothermal reservoir of Terre-de-Haut and its evolution. |