Evaluation of the St. Lucia geothermal resource: geologic, geophysical, and hydrogeochemical investigations Page: 7 of 98
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EVALUATION OF THE ST. LUCIA GEOTHERMAL RESOURCE:
GEOLOGIC, GEOPHYSICAL, AND HYDROGEOCHEMICAL INVESTIGATIONS
Mark Ander, Fraser Goff, Bob Hanold, Grant Heiken,
Frangois Vuataz, and Kenneth Wohletz
St. Lucia is a volcanic-island of the Lesser Antilles arc. Much
of the southern portion of the island is, dominated by mountainous
landscape that, along with steam fumaroles and boiling pools near the
town of Soufriere, attests to its volcanic origin. The major event
in this volcanic field was eruption of the Choiseul Pumice and forma-
tion of the Qualibou caldera (32,000 to 39,000 years ago). The
latest magmatic activity within the caldera was the eruption of large
dacitic domes -and associated tephra at Belfond (20,000 to 32,000
Two major NE-SW-trending faults straddle the caldera; they cross
St. Lucia and are-parallel to small graben-forming faults. Caldera
faults are best developed in northern sections of the caldera; they
define an arcuate western caldera margin, whereas those of the
southern margin are crescent shaped and form a scalloped caldera
margin, affected by regional fault trends. Faulting and hydrology
have controlled the location of thermal springs. Regional linear
faults and caldera faults are the most _important in providing
pathways to the surface for thermal waters originating at depth. A
complex, multiple magma body probably underlies the caldera; the size
of the magma chamber is estimated to be in the range of 100 km 3.
- A 5.2-km-long dipole-dipole DC resistivity survey was conducted
along a north-south trending line through the Qualibou caldera. The
survey was centered over Sulphur Springs and the profile location was
selected on the basis of the previous British resistivity investiga-
tion and- the detailed geologic evaluation performed by Los Alamos.
There is an apparent resistivity high, greater than 1000 ohm-m,
located below the Belfond area. Beneath this apparent resistivity
high, there is deeper low-resistivity material that is measured at
less than 10 -ohm-m. There is a zone of very low apparent resis-
tivity, less than 1-ohm-m, underlying the Etangs area. The zone
beneath Etangs is related to thermal upweli.ng along-a fault,
probably the caldera-bounding fault.
Beneath Sulphur Springs and starting at a depth of approximately
600 m, there is higher apparent resistivity material ranging from 40
ohm-m up to 150 ohm-m in the center of a 1-km-diameter high-
resistivity closure. Interpretation of the, data strongly suggests
the presence of a very hot dry steam field beneath Sulphur Springs.
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Ander, M.; Goff, F.; Hanold, B.; Heiken, G.; Vuataz, F. & Wohletz, K. Evaluation of the St. Lucia geothermal resource: geologic, geophysical, and hydrogeochemical investigations, report, August 1, 1984; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1111322/m1/7/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.