Surficial clay assemblage associated with the hydrothermal activity of Bouillante (Guadeloupe, French West Indies)

Surficial clay minerals as indicators of present-day geothermal activity were studied around Bouillante, Guadeloupe, French West Indies. The study is based on widespread sampling (>100 surface samples) in a zone extending 5 km north and 9 km south from Bouillante. Three types of mineral associations were distinguished: (1) dioctahedral smectites (in which the ‘beidellite–nontronite’ behavior predominates), with zeolites, calcite±quartz±kaolinite; (2) ordered illite–smectite mixed-layered clay with adularia, silica±calcite; and (3) kaolinite–smectite mixed-layered clays±halloysite±kaolinite±smectite±silica. The first association characterizes steaming grounds and areas subjected to hydrothermal fluid infiltrations along fractures or breccia zones in marine and subaerial environments. The second association was only identified in the matrix of epithermal breccias. This assemblage appears as a tracer of high-temperature fluids and is a good indicator of the proximity to the geothermal reservoir. The third assemblage was found both within and beyond the bounds of the known geothermal area, suggesting a pedogenic origin. We interpret the occurrence of the three assemblages in the Bouillante geothermal area as a background of clayey tropical weathering (kaolinite–smectite) with a hydrothermal overprint (dioctahedral smectites and illite–smectite).