Occurrence of K-feldspar-bearing hydrothermal breccias in the Bouillante geothermal field (Basse Terre - Guadeloupe)
Résumé
Surficial indicators of recent to present geothermal activity are present around Bouillante (Guadeloupe, French West Indies). Until lately, most mineralogical parageneses identified in this area were typical of low temperature alteration (< 100°C) with the occurrence of dioctahedral aluminous smectites accompanied by zeolites (heulandite-clinoptilolite) ± calcite ± silica ± kaolinite, as an evidence of the cap rock of the geothermal system. Recently, numerous boulders of highly silicified breccias containing high temperature minerals (> 200°C) have been identified in the Bouillante bay (Anse Marsolle). Their petrographic study revealed several hydrothermal parageneses typical of a multistage alteration process. Stage 1) An early potassic alteration facies typical of a high-temperature geothermal system characterized by K-feldspar/adularia + quartz + pyrite. K-feldspars have been shown to be present both in the clasts (replacement) and cement of these breccias. They are associated with quartz whose textural properties have revealed fracturing associated with boiling. Stage 2) An acid-sulphate advanced argillic alteration facies with the occurrence of jarosite, gypsum ± silica (≥ 150°C) as usually observed in the upper part of epithermal systems worldwide as marker of deeper boiling. Stage 3) An argillic alteration facies (illite/smectite mixed layers ± smectite ± calcite). With temperatures typically ranging from 100 to 200°C, this alteration facies is associated with near neutral fluids of mainly meteoric origin as known in the present geothermal reservoir (pH = 5.4). This alteration is the later one as evidenced by petrographic observations. These results highlight mineral assemblages and mineral textures characteristic of high temperature hydrothermal alteration in epithermal settings. The occurrence of these breccias involved the existence of eruptive events (magmatic/hydrothermal explosion) which ejected this material. Isotopic (oxygen, argon) and geochemical (trace elements) studies are now necessary to clarify the timing of these breccias and the nature and the connection of the original fluids with current geothermal fluids