The characterisation of an exhumed high-temperature paleo-geothermal system on Terre-de-Haut Island (the Les Saintes archipelago, Guadeloupe) in terms of clay minerals and petrophysics

International audienceThe volcanic island of Terre-de-Haut (in the Les Saintes archipelago, Lesser Antilles arc) is described in the literature as an exhumed paleo-geothermal system. Outcropping conditions provide a good, nearly horizontal 2D view of the system. This present study focuses on clay minerals, as they are known to be good markers of hydrothermal alterations worldwide, and on flow pathways. At Terre-de-Haut, the distribution of the clay minerals has a ‘concentric’ zonation with smectite, illite and chlorite comprising the outer part to the inner part of the altered zone, respectively. In the active Bouillante geothermal system, located on Basse-Terre island, 35 km NNW of Terre-de-Haut, drillings have revealed a vertical succession of the same clay minerals: from bottom to top, chlorite above 240 °C, illite above 180 °C and smectite at lower temperatures. Both sites have the same clay minerals and can, therefore, be compared. Within those geothermal systems, pathways for the fluids are encountered at all scales, from the micrometre-scale (µm-scale; through microcracks within phenocrysts and at grain joints) to the metre- and kilometre-scale (m- and km-scale) due to fractures and faults. Petrophysical measurements indicate that fresh rock samples are porous and permeable, allowing hydrothermal alteration on a km-scale. The combination of 1D vertical data at Bouillante and this new 2D horizontal distribution of clay minerals and fractures on Terre-de-Haut allows for a 3D schematic representation of the geothermal systems of the Guadeloupe archipelago and suggests that they are of hectometre-size (hm-size)