Structural, mineralogical and petrophysical characterization of the hydrothermal system in Saint-Kitts Island (Lesser Antilles)

In Lesser Antilles arc several volcanic islands present high potential for the high enthalpy geothermal production. Saint-Kitts Island is one of them, where numerous surface evidences of active hydrothermal system such as fumaroles, boiling water are present. All of these activities are structurally controlled. Field analysis highlights four main families of faults or fractures with NE-SW, NW-SE, N-S, and E-W trends respectively.

The interaction of fluids (rain and sea waters) with host rock leads to meteoric and hydrothermal alterations of rocks according to depth of infiltration, usually consisting in their argillization. The characterization of these alterations allows a better understanding of the hydrothermal system of St Kitts and to estimate the geothermal potential of the resource in this island.

In the Frigate Bay area, in the southern part of the island, the rocks present an intensive structural and petrological transformations related to hydrothermal fluid flows. From study of mineralogical transformations with microscopic, SEM, EPMA, Raman and XRD analysis, we identify clays minerals (kaolinite, smectite), sulphates (jarosite, alunite, gypsum), quartz, opal, calcite and chlorite. These paragenesis are consistent with fluids above 100°C and allow to constrain the spatio-temporal activity of the hydrothermal system and the geothermal reservoir. Petrophysical properties, on a selected set of representative petro-structural facies, show large ranges of variation, porosity from 2 to 40%, permeability from 10^-3 to 3 D; grain density between 2.84 and 2.31 g.cm^-3, thermal conductivity is relatively low, 0.5 to 2 W.m^-1.K^-1. Samples alteration results in increasing of porosity and decrease in density. In turn the porosity increasing causes a decrease in the thermal conductivity.

These investigations allows us to interpret this site as part of a hydrothermal paleosystem and consider as an analogue of the deep northern part of the island under a current hydrothermal activity.