and MICHELS

POTENTIAL FOR GEOTHERMAL DEVELOPMENT

IN GRENADA, WEST

Gerald W. Huttrer'
Donald E.

'Geothermal Management C o . , P.O. Box 2425, Frisco, Colorado 80443

'Don Michels Associates, P.O. Box 8652, Missoula, Montana 59807
words: surface manifestations, geothermal exploration, prospective areas, Grenada, West lndies

ABSTRACT In 1991, Grenada Electricity Services, Ltd. (GRENLEC), the

Pre-feasibility studies were conducted between 1992 and national electric utility, had a nameplate generating capacity 18.5
March 1993. Resource assessment included literature reviews. air photo all provided by medium-sized diesel generators that were
evaluations, on-site geology and acquisition of new data for hydro- 23 yearn old. Although several of these had been refurbished and
geochemistry. Additionally, interviews with key of the rebuilt since 1985, GRENLEC to decommission several plants
Grenadan public and private yielded insight how geothermal before 1995 EO that about o f new capacity will be needed.
development could proceed in this island environment. Geothermally generated power Could economically provide this
Geological and hydro-geochemical studies identified sites with electricity.
surface manifestations in central and northern Grenada and apparently 1991 fuel were reported to be $US 0.088 per
commercial temperatures, Additional study of the three while their generation costs are raid to be SUS 0.0688 per
most prospective sites recommended on the basis of accessibility and GRENLEC charger a commercial rate $US per With
apparent resource quality. reasonable allowances far the of distribution, O&M
The national power Utility, GRENLEC, the and administrative there s t i l l room far
decommissioning of several small diesel powered electric generators by geothermal power into the fuel at an estimated 0.09
1995, with need replace about of capacity. Additional new without adversely affecting the of power the public.
demand can be expected. For several reasons, privatization island
infrastructure will become practical during the next few years. Power favoring the privatization
sales contracts could be obtained at prices exceeding today's fuel price, components. For example. GRENLEC has historically been Owned and
$US 0.088 per controlled by the government. But, recently it, like the national
Economic modelling based on rite-specific conditions and telephone service bought by a private entity. These
plausibilities suggests a 7.5 facility could yield about 18% per year sales are the first in what appears be a trend towards privatization of
internal rare o f return. infrastructure in Grenada. Their consummation has resulted in the
Creation of laws, rules, and regulations that the other
SCOPE O F WORK private projects, such as geothermal power generation.
Pre-feasibility studies were conducted between 1992 and Interviews with government officials indicated that the few existing
March 1993 determine the potential for commercial development of that might be perceived barriers energy projects are being
geothermally-fueled electric power generation in Grenada, These favorably modified.

.
included
review 51 publications identified via the 'GEOREF" Tax and other investment incentives are now provided Grenadan
law that should significantly enhance the economics of a geothermal

.
files (USGS and Colorado School of Mines)
.acquisition and review of 67 framer of color air project. They reflect the growing government effort entice all kinds
new businesses Grenada. Grenada's financial and economic
review a previous reconnaissance of geothermal resources in
Grenada: (Geatermica Italiana, 1981) conditions are not robust, but they are improving and currently
field geologic and hydra-geochemical studies in 1992. a neutral factor with regard to geothermal development.

Field work. October 11-27, 1992. included geologic are that they would constitute
and sampling of 17 thermal springs. of there were indicated in impediments geothermal development. Grenadans are quite
Geotermica (1981) and others were located with the help of their rtill-pristine environment, but their laws are practical and
renowned Grenadan hiker and guide, Bedeau. reasonable.
Though the most important aspect of any geothermal development
the availability a commercial-quality resource, it critical in Grenada Strong and willing. The
that several conditions unrelated the resource be favorable. facilities, roads, telephones, range from adequate
related information summarized in subsequent sections was to
gleaned via with 28 Grenadan representatives of the
public, private, academic and industrial sectors. I n summary, all related conditions are favorable for
geothermal Economic conditions in Grenada are
NON-RESOURCE FACTORS improving and are comparable to, better than those in other
Important information was obtained in regard developing in which geothermal projects have been successfully
demand for and present price of electric power built.
institutional and environmental factors that promote hinder
geothermal development PLANT ECONOMIC ANALYSIS
factors and incentives regarding taxes. economics, and finances Proforma economic analyses hypothetical, reasonably structured
available labor and logistical facilities. projects were made. One example is provided here; basic assumptions
are provided in Huttrer and Michels (1993). highlights are given were:
below. 7500 KW capacity 20-year project life
90% capacity factor 15 year loan period
Electric demand in Grenada in 1991 peaked at 11.6 with an 70% debt, 30% equity interest rate
average bare load of Peak was expected rise to 14.8 3% inflation rate power plant
by reflecting the expected continuation of a 10% annual 4 prod. wells, 2 inj. wells 5000 f t prod. well depth
increase in power sales. well 3000 f t well depth
year one insurance and
20-yr tax holiday million for exploration
Ltd, Vancouver. B.C Power sales begin in year 3
 and MICHELS

A volcanic seamount. Kick-'em-Jenny, i s building about four

Positive cash flow in year northeast of the Grenadan shore. At eight eruptions have occurred
Cumulative cash flow after 20 years million . since 1939 involving alkali basalts. and companion
rate of return is 17.8%. island. de Caille, are the youngest and most active volcanoes in the
Caribbean. Since the seamount and the island are so the north
GEOLOGY 0 F GRENADA A BRIEF DESCRIPTION shore of Grenada, they may be considered as part of the main edifice.
Since Cretaceous time, regional compressional forces oriented In the near geologic future. they likely grow and rise above the
ENE-WSW have persistently acted Grenadan rocks. These forces are waters now separating them.
due the westward and southward subduction of the North Atlantic
Plate beneath the relatively immobile Caribbean plate. (Figure I ) . The TABLE VOLCANIC ROCKS OF GRENADA

.
results of the subduction and the compression hare been:
development of NNW-SSE folds within the sediments, basalt
dikes intruding the sediments, and in many volcanic units
eak cr-

21
faulting with predominantly orthogonal NE-SW and NW-SE Basalt
trends with N-S trends Hill 7.1
generation of magmas that have above level. forming 1.7-1.L tuff.
Grenada and ten other islands to the in this Caribbean
archipelago.
.
Grenada can he divided into five groups: the
sedimentary basement, the volcanic pile, rocks related Kick-'em-
de Caille, intrusive rocks, and hot spring deposits. The
sedimentary basement includes and sandstones of the Craters various ages and are apparent in
Oligocene age Hall formation. The Tempe Pamassus-Hope Vale photos, away from the Steep interior. One of the largest
may have been deposited slightly later than appears to form the harbor at (largest town) which
contemporaneous with the Hall. relatively old and eroded. Younger examples with complete perimeters
and enclosing circular ponds occur in the northern part of the island.
The volcanic pile Grenada comprises the products eight
eruptive stages that began in the early Miocene (21 MYBP) and ended
than 1 MYBP. From oldest youngest, these volcanic units are A travertine occurs a t Chambord Estate, just north of
listed in Table I (an explosion crater of Recent ape) the northeast end
the island. Travertine covers area about 250 feet long by feet
There is no prominent pattern the geographic distribution of wide formed by coalescence of thermal outflows. Uranium
these eruptive rocks with respect time, but, the most recent activity thorium dating of the travertine yielded a young age. Ka, with large
seems be toward the north central and northeastern parts the uncertainty Warm springs (95 flaw weakly
island. from in the travertine, evidencing the continued
existence of a nearby heat source.

NORTHATLANTIC
PLATE

PLATE

FIGURE SELECTED PLATE BOUNDARIES

IN THE CARIBBEAN REGION
ON UPPER

130
 and MICHELS

Intrusive be scarce Grenada. Only two bodies TABLE 2: CH EMICAL DATA G RENADAN GEOTHERMAL
have been mapped to date, quartz diorite and dacite porphyry located
in the Hill area of the north-central part of the island. These
rocks have both intruded the sedimentary basement rocks and they are
both overlain by the Mt. Catherine Thus fheir age is
between 21 and MYBP. Both these host anomalous W 5.76 16307 915 120
zinc. lead and copper. Their presence suggests the existence 700 6.92 457 137
1000 6.53 2726 105 125
of other. yet unmapped, shallow that could be heat 600 7.47 216 77 16 53
f a r geothermal cells. 6.M 31 30 12.3 Q4
e4 6.42 35 36 4.3
Thermal mostly located in valleys eroded along the traces
of prominent faults. This is evidence that the network 97 6.13 962 168 352 30 120
7929 725 167 33
needed sustain viable geothermal open, permeable and
28 61 121
conducting thermal waters to the surface in Grenada. Significant 92 6.W 725
rainfall in Grenada. I 4 meters per year, depending on elevation. il 7.87 368 IT e4
imparts significant dilution and cooling the emergent thermal waters. 40 7.31 8.5 14 2.34 78
e4 7.74 117 3e4 44 15 1.01 78

HYDROGEOCHEMISTRY FOR THERMAL WATERS OFGRENADA 1000 102 6.W 13113 736 281
82 8.31 85 30 5.8 0.92 53
Seventeen water samples collected in 1992 represent six prospective 81 28 671 57.1 33
geothermal sites in the northern part of Grenada. of these appear 6.80 27 417 1.0 30.6 14 37
to have Commercial temperatures and at least three of those deserve
additional work to outline the associated resource. 134 527 30.7 163

Same of the areas were also sampled in 1981 (Geotermica Italiana).

Because the chemical data from their sampling and ours are generally 24 13 7 3.1 20 27
consistent bath of results were combined (Table 2) for this overall
assessment. The significant chemico-thermal areas are shown in 102 57 256 16 13.4 302
Figure 2. 1300 76 3.12 27 6.3 82
23 6.6 81
Groups of sampled spring waters from single areas show 13 62 262 1.2 12.1 PO
77 12 23 34 3.9
a wide range of dilution. In fact, it is the dilution of otherwise 77 3.63 V 3.9 14.6 365 0.02 78
common compositions that define these potential resources. The 73 6.00 7 13 22 2.3 4.9 0.02 51
geographic Clustering of sample sites With chemical
characteristics permits assigning the chemical features proscribed 700 91 7.06 85 114 812 62.6 3.7
areas. three groups especially, Castle Hill South, Chambord, and 5.92 39 81 130 63.2 3.8 0.25
Hermitage-Peggy's Whim, dilution by more than orders of 96 e4 6.88 68 4.8 5.0 0.08
magnitude that 1001 dilution) still yields mired 77 7.85 9 11 2.50.03 26

that can be recognized belonging to one group and not some other. in
in
The effects of dilution must be recognized in order that the
temperatures may be cautiously interpreted. Figure 3 examples show how related sample data yield remarkably light fits to
and Figure 4 YE I) show how distinctly the groups are
straight In these log-log plots, all liner that are parallel to the
defined by what we may call their dilution profiles. These figures are reference lines represent dilution effects'. Both linearity and
parallelism in the tend to become lost when chloride
Concentrations are diluted to than about 50
.
3

Same plots have Slightly shallower than the reference lines.

This due to the dilution fluid (local groundwater) being pure
water, but containing elements weathered from rock. (Weathering
components exceed the concentrations due to salt entrained with the
original rain.] Background chloride concentration to be about
to 13 and may be presumed due to aerosols of sea
incorporated into local rain. Higher concentrations of chloride indicate
a thermal water Component

FIGURE PROSPECT LOCATIONS

 and MICHELS

Same are notably but no gas sampler were taken.

Thermal area6 Seem especially indicated by elevated concentrations of
chloride 16,000 Spring surface temperatures are only
slightly elevated, a further indication of the substantial dilution effects
in the near-surface zones. Conductive coaling also appears important
because there negligible Correlation between spring temperature and
chloride concentration.

High chloride concentrations were viewed earlier (Geatermica

Italiana, 1981) d u e to connection. but consideration of the
observed chloride with ratios for conserved and
elements us that seawater is not involved.
Some values are shown in Table 3. Graphical
of seawater dilutions are given in Figures 4. and 6
K, SO,). Figure 6 shows that high chloride concentration
sulfate to be relatively less than in seawater whereas f a r dilute
chloride sulfate tends be relatively greater than in
Correlations of SO, and of
shown) are poor at all concentrations, indicating separate origins.
TABLE 3: REPRESENTATIVE R ATIOS
FIGJKF 5
289 26 19100

"ill 225 17
235 830 0
230
218

One objective of chemical studies to estimate the deep thermal

temperatures that these resources may have. The approach is than
straightforward because of substantial dilution of thermal waters by
shallow near-surface waters.

Silica geothermometers were judged be not reliable, based

poor to negligible correlation with chloride (Figure relatively
Concentrations. Although would limit about I I
it is generally absent from the surface volcanic racks from
which weathering can deliver to about 50 from other
minerals. Higher silica Concentrations are viewed residual from a
thermal water component. One could that silica deposition
occurred prior lo mixing and possibly afterward in some Circumstances.

Ratios of and were often highly preserved over large

dilution factors: Figure shows the Na K correlations for the
prospect sites. Accordingly. were selected
most reliable. TO help verify this, three-component of
were made in accordance with Giggenbach (1988) and (1990).
These indicate that the diluted ratios are valid estimates of
the pre-dilution temperatures.

From among the several versions of that

due et (1983) preferred here because it was
calibrated against basalis, which most closely match the predominant
lithology of Grenada. Selected results, with some
associated data, shown in Table 4.

Results of the NaKCa geothermometer higher and

lower than the results. For all groups, the mismatch increaser
with increasing dilution, evidence of bias due to the dilution waters
being relatively enriched in calcium. Far Castle Hill South and
Chambord, for the least diluted samples. In there cases
the temperature value is suspected be numerically more
accurate because its calibration was against a more relevant rock type.

132
 and MICHELS

CASTLE (Figure the highest concentrations

o f chloride. were mainly collected from the walls of the
valley, where bedrock are exposed near the river.
Chloride-rich water trickles from several zones nearly
traces of fractures through the volcanic rocks above the Stream level.
chloride Concentrations were generally found at higher
elevations of this valley. Measured concentration of chloride in stream
water at lower elevation was 400 indicating a Significant
collective contribution of thermal water the stream.
chloride in surface waters expose a
pattern may be a useful exploration for this resource. The
and NaKCa vary with chloride but
their trends are of opposite sense (Table 4). Both indicate
more than far the least diluted samples. From the limited data
available, this area be the most promising in terms
elevation. extent, accessibility, and thermal characteristics.
THE AREA, in nearly level terrain with elevation 25 50
feet. includes a travertine terrace about 250 feet long by feet wide
formed by of several thermal outflows. Currently, the
have negligible discharge rates. Elevated chloride in the adjacent
stream, to 15 feet below the pool elevations, indicates
hidden contribution from the thermal source. Uranium-thorium dating
of the travertine yielded a young age. Ka, with large uncertainty
This terrace may be the largest geothermal deposit an
the island. However, its relatively low temperatures for surface flow
and geothermometer suggest it may not be rhe best prospect. However.
a careful survey focussing water temperatures, chloride
and flow of surface waters be made
here as an effort define the amounts of heat discharged and better
resolve the deep
are chloride
HOPE involves steep terrain at higher elevation
probably not r e l i a b l e due severe d i l u t i o n feet and four widely separated sample points. One sample,
apparently much diluted, but with chloride at more than times
Two areas not Table 4 are River and background, yields an encouraging temperature. The other three
Fluids from both of there were dilute yield have chloride about three times background,
reasonable geothermometer components. At Plaisance-Red River. but their associated geothermometer temperatures are likely not reliable.
abundant alteration rocks near fractures. but with near-background involves less terrain at lower
chloride levels. is interpreted an effect of steam. The small ratio than Hope. One Contains chloride at
is consistent with this view. No fumaroles elevated temperatures were more than 100 times background with a significant chemical
temperature. T h e mismatch between and NaKCa geothermometer
found there. T h e area, near the MI. Catherine. the
values indicates that it substantially diluted. These two latter areas
highest point in Grenada 2757 included a fumarole with the
highest found. Chloride concentrations of associated deserve more study define better their prospective natures.
liquids were 13 not distinguishable from background. but A DELPHI -S T . yielded sampler with chloride concentrations u p
possibly related four times background. indicating a area. However, the
dilute temperatures.
SUMMARY A N D CONCLUSIONS T h e location, in mild terrain (01000 feet east of Gran (2nd
highest peak in Grenada, 2509 suggests favorable recharge in
the vicinity of significant volcanism. Further study there seems
warranted.
Grenada has all the attributes necessary for the existence of
commercial quality geothermal systems. Reservoirs may be within
fractured in the central part of the island where the volcanic Plant Economics
is thick enough reach depths of several thousand feet. Plausibly, the A project reasonably around production wells feet
underlying carbonate formations may be involved in the northern part with average capability of 3 and a power sales price
the island, particularly beneath Cantle Hill and Chambord. an internal rate of return near
Some heat sources may be a Continuation of magma intrusion that
surfaced the Catherine (0.94 Ma), expressed now as ACKNOWLEDGEMENTS
the buildup o f Kick-'em-Jenny. Intermediate age magma activity has We indebted for his enthusiastic and accurate
built de Caille and one may presume that unexposed location of warm in the field. We are grateful Neil
underlie some northern parts of Grenada in addition the Chambord National Laboratory, far the U- Th age determination
travertine.

Conduits for thermal and fluids are certainly

associated with the regional orthogonal systems. These
features are likely by interconnected networks of
and fractures within the reservoir rocks. Probably. these conduits are
kept open and permeable by Contemporary seismicity that
the plate subduction which generates the volcanic Caribbean islands,
including Grenada.

Water for recharge the hydrothermal circulation systems widely

available. The higher mountains of Grenada are drenched by over 4000
mm in.) of rain annually. while coastal areas get about I meter
annually. The sub-surface water circulation system is quite complex
within Inter-flaw boundaries. fractures, and buried
weathered surfaces provide a three-dimensional of and
barriers which that mort, or all, beneath the shallow
water table will be saturated.

133