The purpose of this work was to study jointly the volcanic-hydrothermal
system of the high-risk volcano La Soufriere, in the southern part of
Basse-Terre, and the geothermal area of Bouillante, on its western
coast, to derive an all-embracing and coherent conceptual geochemical
model that provides the necessary basis for adequate volcanic
surveillance and further geothermal exploration. The active andesitic
dome of La Soufriere has erupted eight times since 1660, most recently
in 1976-1977. All these historic eruptions have been phreatic.
High-salinity, Na-CI geothermal liquids circulate in the Bouillante
geothermal reservoir, at temperatures close to 250 degrees C. These
Na-CI solutions rise toward the surface, undergo boiling and mixing with
groundwater and/or seawater, and feed most Na-CI thermal springs in the
central Bouillante area. The Na-Cl thermal springs are surrounded by
Na-HCO3 thermal springs and by the Na-Cl thermal spring of Anse a la
Barque (a groundwater slightly mixed with seawater), which are all
heated through conductive transfer. The two main fumarolic fields of La
Soufriere area discharge vapors formed through boiling of hydrothermal
aqueous solutions at temperatures of 190-215 degrees C below the
``Ty'' fault area and close to 260 degrees C below the dome summit.
The boiling liquid producing the vapors of the Ty fault area has SD and
delta(18)O values relatively similar to those of the Na-CI liquids of
the Bouillante geothermal reservoir, whereas the liquid originating the
vapors of the summit fumaroles is strongly enriched in O-18, due to
input of magmatic fluids from below. This process is also responsible
for the paucity of CH;I in the fumaroles. The thermal features around La
Soufriere dome include: (a) Ca-SO4 springs, produced through absorption
of hydrothermal vapors in shallow groundwaters; (b) conductively heated,
Ca-Na-HCO3 springs; and (c) two Ca-Na-Cl springs produced through mixing
of shallow Ca-SO4 waters and deep Na-Cl hydrothermal liquids. The
geographical distribution of the different thermal features of La
Soufriere area indicates the presence of: (a) a central zone dominated
by the ascent of steam, which either discharges at the surface in the
fumarolic fields or is absorbed in shallow groundwaters; and (b) an
outer zone, where the shallow groundwaters are heated through conduction
or addition of Na-Cl liquids coming from hydrothermal aquifer(s).