Evaluation of the St. Lucia geothermal resource: geologic, geophysical, and hydrogeochemical investigations Page: 25 of 98
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central vent locations near Mt. Gimie. Our conclusions are based on the
location of the caldera structure, outflow thickness variations, and well log
data within the caldera. For example, Choiseul Pumice was not distributed
towards the northeast because of the shielding effect of the high composite
cones of Mt. Gimie and Mt. Tabac.
At this time we can only provide a crude estimate of the volume of the
Choiseul Pumice. Much of it must have been deposited in the sea. Pyroclastic
flow and surge deposits studied on land have a minimum volume of 11 km3, about
3
6.5 km DRE. This compares favorably with an estimated volume for caldera
collapse of between 5 and 10 km3.*
Wright et al. (1984) dated one piece of carbonized wood in the Choiseul
Pumice, collected south of Saltibus; it has an age of >32,480 years. They
suspect that the carbon sample collected by Tomblin (1965) was from one of the
Choiseul Pumice deposits; it has an age of 39,050 1500 years.
C. Intracaldera Volcanic Rocks
1. Morne Bonin Dome. Located in the southeast corner of the caldera,
Morne Bonin dome has been called a pale andesite by Tomblin (1964). The
summit is about 330 m above the caldera floor at Belle Plaine. It is believed
to be a postcaldera dome, erupted from a caldera-bounding fault. All samples
are from talus; there are no in situ exposures. A sample from this talus
consists of a quartz-poor dacite; a porphyritic, holocrystalline lava with
phenocrysts of orthopyroxene, plagioclase, quartz, and Fe-Ti oxides.
2. Terre Blanche. Terre Blanche is a 1.5-km-diameter, 450-m-high dome,
located immediately northeast of Sulphur Springs. Associated with the dome
are two craters and one small dome (100 m high) located between it and the
Bel fond dome-crater complex. Terre Blanche dome appears to have erupted
through a tuff ring, with well-bedded medium-ash size surge deposits exposed
on the northeast flank. Drilling at Sulphur Springs indicates a total dome
3
thickness of 600 m, with an approximate volume of 0.6 km . The dome consists
of pink or grey dacite with good flow banding. The west side of the dome has
*Volume was estimated -on the assumption that tuffs -n the plateaus filled
paleovalleys that are now resurrected in part. This observation was confirm-
ed in seacliff exposures. Volume in the caldera was determined by an esti-
mate of caldera area and tuff thickness seen -in drill holes. No estimate was
made of the ash that went into the sea or was carried off in a plume. The
DRE was detenined by multiplying the volume by 0.6.19
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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/m1/25/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.