GRC Transactions, Vol. 38, 2014

Toward a Continuum Geothermal Model to Explain Coexistence of Medium to High (100 to 250°C) Temperature Geothermal Systems in Martinique and , French West Indies

V. Bouchot1, A. Gadalia1, H. Traineau2, and S. Caritg1 1BRGM, Orléans, 2CFG Services, Orléans, France [email protected]

Keywords (Bouchot et al., 2010). So while is a classical system Martinique, Guadeloupe, , Bouillante, Montagne of high temperature (250°C), according to geothermometers it Pelée, high to medium temperature geothermal systems, appears that the geothermal systems in Martinique show lower continuum geothermal model, evolution in space and time, temperature reservoirs between 100°C and 220°C. These different magmatic heat source, exploration guidelines, fluid equilib- reservoir temperatures coupled with their variations in time pose rium several questions. Do these temperature differences indicate dif- ferent stages of the same type of geothermal development (with successive prograde, peak or retrograde stages)? What impacts Abstract can the age, position, volume and duration of the magmatic ac- tivity as source of heat, have on the evolution of the geothermal The recent surface exploration data acquired in Martinique led system (duration, extension) and the temperature of its reservoir? to a revisit of the conceptual models of three geothermal systems The comparison of different systems, based on selected pa- in active volcanic arc context: Petite Anse in the Southwest of rameters such as reservoir temperature, duration of the geothermal the island, in the center, Montagne Pelée in the North. system using fossils events, age and duration of magmatism, de- These three explored systems are compared with each other and gree of equilibrium of geothermal fluid, should provide relevant with the developed Bouillante geothermal field in Guadeloupe (15 answers to these questions. This comparative synthesis should thus MWe). From parameters, such as reservoir temperature, duration allow a better understanding of the temporal and spatial evolution of the geothermal system using fossils events, age and duration of these systems and contribute to better exploration of them in of magmatism, degree of equilibrium of geothermal fluid, we the Lesser Antilles island volcanic arc (Table 1). propose a continuum model of geothermal system including three A geothermal system associated with an active volcanic successive stages: prograde, peak and retrograde. arc is, a priori, controlled by a magmatic source of heat. It is

Introduction Recent surface exploration data ac- quired in Martinique, French West Indies, between 2012 and 2014, led to revisiting the conceptual models of three geothermal systems in this active volcanic arc setting: Petite Anse in the southwest of the island, Lamentin (close to Pitons du Carbet) in the centre, and Montagne Pelée in the north (Gadalia et al., 2014) (Figure 1). The three systems being explored in Martinique are compared with each other and with the Bouillante geothermal field (Guadeloupe, France) (Figure 1), the only exploited geothermal field in the Carib- Figure 1. Location of the four studied geothermal fields (red star), in Guadeloupe and in Martinique bean, with an installed capacity of 15 MWe (French Lesser Antilles).

363 Bouchot, et al. characterized by i) a geothermal reservoir resulting from hydrothermal convective circulation, ii) leaks from the reservoir to the surface and associated surface expres- sions (thermo-mineral springs, fumarole) and iii) recharge of the system involving, as on islands in the Lesser Antilles, meteoric and marine waters.

Geothermal Fluids and Reser- voir Temperatures In Martinique and Guadeloupe, geo- thermal reservoirs show little variations in terms of chemical composition of geother- mal waters. They are usually close to the pole chlorinated sodium mainly because of the strong influence of seawater in the reservoir recharge (Figure 2). In fact, the Figure 2. Position of the different geothermal waters (from Martinique and Guadeloupe geothermal involvement of seawater with a salinity fields) in the Na-K-Mg diagram (left) and Cl-HCO3-S04 diagram (right). around 35 g/l, complicates the interpreta- tion of geothermal fluids in terms of water maturity. To do this, it diagram [Cl-HCO3-SO4] (Giggenbach, 1988) whose concept of favors the use of isotopic analysis of fluids (O, C), binary diagrams “mature waters” is inappropriate because of sea water influence (e.g. Cl/Br), and the ternary diagram [Na-K-Mg] rather than the (Figure 2). In Lamentin, current geothermal water from hot springs and wells are of sodium- Table 1. Comparison between the geothermal systems of Martinique and Guadeloupe French islands. chloride type, at pH <5.6 to 6.2 with a Geothermal Field Montagne Pelée Bouillante Petite Anse Lamentin-Carbet salinity of 10-11 g/l, resulting from a mix- Stage of the geo- Peak stage Retrograde stage Advanced ture of 30-35% sea water and 65-70% fresh Prograde stage thermal evolution (with plateau) (trend) retrograde stage water. According to the ternary diagram Duration of the 125 ky 1 Ma ~1.1 Ma 700 ky [Na-K-Mg] (Figure 2), theses sodium- magmatic activity (550 ky if Mont- (between 1.2 My (between 1,5 My (between 1 Ma chloride waters are in the field of “partial in the region Conil included) and 230 ky ago) and 350 ky ago) and 337 ky ago) equilibrium” close to “immature waters”. Latest volcanic Active volcanism In Montagne Pelée, two reservoirs are activity recorded ~500 ky ago ~350 ky ago > 600 ky ago (1929, last event) distinguished: a first reservoir, located on in the field the SW flank of Pelée stratovolcano, is of 3He/4He signature 7.2 Ra 4 - 4.7 Ra 7.75 -7.93 Ra 2.2 Ra sodium-chloride bicarbonate-type with an (from hot spring) outflow feeding the springs of Rivière Pi- Temperature of 100-140°C 180-200°C (R2) codo (0.4 g/l, pH 7.7 - 8.4), while a second the reservoir (geo- 250°C 190-210°C (estimated) 200-220°C (R1) thermometer or (measured) (estimated) 90°C reservoir, located in the apex area is com- (estimated) measured) (measured in well) posed of bicarbonate fluids with an outlet at Palaeo-caprock the springs of Rivière Chaude (1.4 g/l, pH Adularia and Illitic alteration in and palaeo-acidic Fossil geothermal quartz breccia depth and palaeo- 6.4-6.8), and which appeared after the 1902 No identify alteration (fumarole) stage (250 ± 50 ky silica sinters (300 to eruption. The proportion of seawater in the with evidence of by 40Ar-39Ar) 250 ky by U/Pb) boiling (undated) Picodo reservoir is estimated around 2% (Gadalia et al. 2014). Unlike conventional Duration of the ~ 100 ky geothermal (from 125 ky ago > 350 ky ~200 ky >> 500 ky magmatic-hydrothermal model associated system to today) with an active stratovolcano, the summit reservoir (Rivière chaude), whose waters Current fluid Na-HCO3 to Na-Cl Na-Cl Na-Cl Na-Cl composition with 5.6

364 Bouchot, et al. by a salinity of 20 g/l at pH 7 to 6.3. The reservoir waters could Th, in Chovelon et al, 1984) and chlorite-epidote and illite-I/S as- result from a mixture of sea water (54%) and freshwater (46%), semblages identified in wells (> 200°C) in total imbalance with the as in Bouillante. According to the Br/C diagram, a large excess temperatures (80-90°C) of the current fluid, testify to a high tem- of chloride (34%) could have a magmatic origin (Gadalia et al., perature palaeo-system active 300 ky ago in the plain of Lamentin 2014). According to the ternary diagram Na-K-Mg, the geothermal (Mas et al. 2003). In addition, in the Morne Rouge area, a large water is partially equilibrated (Figure 2) but close to “immature siliceous hydrothermal breccia (explosion breccia type), associated waters” field. In addition, high concentrations of certain trace ele- with silica sinter, suggests that the HT palaeo-system came into ments (As, B, Cs, Li, Rb, ...) and the isotopic ratios of 18O water sharp boiling 300 ky ago. Later, temperatures inferior to 200°C, indicates a water-rock interaction at high temperature. expressing the beginning collapse of the palaeo-system, has been In Bouillante, the geothermal fluid, taken from the reservoir or suspected due to the presence of laumontite and corrensite in the reconstituted from hot springs, is homogeneous in its composition. wells (Mas et al. 2003). Today, kaolinite and smectite assemblage It corresponds to sodium chloride brine with a salinity approaching is stable with geothermal water (80-90°C). From a spatial point of 20 g/l and a pH of 5.3 ± 0.3 at 250°C. Its chemical and isotopic view, the witnesses of the high temperature palaeo-system (silica compositions indicate that the reservoir is supplied both by sea sinters, explosion breccia, hydrothermal alteration and related ar- water (58%) and fresh surface water (42%), the mixture of which senic anomaly) are distributed over a wide area of approximately is at chemical equilibrium with the host rock at a temperature of 10 km² (at the surface), while the current water flow at 90°C and 250 °C (Sanjuan et al., 2001). According to the ternary diagram related kaolinite-smectite alteration (in wells) and conductive [Na-K-Mg] (Figure 2), theses sodium-chloride waters reach the signature (CSEM method, Gadalia et al., 2014) are very limited “full equilibrium” field. in space (~ 1 km² at the surface). Therefore, assuming that both Geothermometers calculated from hot spring and wells waters geothermal stages (fossil and current) are connected to the same of the four studied fields show strong variations in temperature of geothermal system and comparing the current low temperature the geothermal reservoirs, ranging from 100 to 250°C. reservoir to the fossil high temperature reservoir, it can be seen In Lamentin, the current reservoir is a low temperature type that, between 250 ky and today, the Lamentin geothermal system (100-140°C) according to geothermometers from Lamentin hot gradually cooled (from ~ 250°C to 100°C in the reservoir) while water (90°C) (Figure 3), while fossil reservoir known in Lamentin contracting in space (from 10 km² to 1 km ² according to surficial 250 ky is of high temperature type (>200°C). expressions). In Montagne Pelée, where at least two high temperature In Petite Anse (Figure 3), the warm springs of Eaux Ferrées, reservoir have been characterized (Gadalia et al., 2014), the located on the coast, are hosted in an intense argillic alteration chloride-bicarbonate reservoir (related to Rivière Picodo) would with pyrite-montmorillonite smectite dominant (indicating ~ have an estimated temperature of 155-180°C while the bicarbon- 100°C), which forms the caprock of a fossil geothermal system ate reservoir (in relationship with Rivière Chaude spring) would (Traineau et al., 2013). The scarce presence of interstratified be a little warmer around 180-200°C. illite-smectite and illite (> 200°C) in this palaeo-caprock is In Petite Anse, the reservoir is a high temperature type, interpreted as the result of warmer fluid upflow coming from between 190-210 °C (Sanjuan et al., 2003), according to the the underlying geothermal fossil reservoir. Above this caprock geothermometers obtained from the Eaux Ferrées springs (close (Figure 3), a typical kaolinite-alunite palaeo-alteration was to Petite Anse village). identified, on the relief, in several sites distributed along a 2 km In Bouillante, the measured temperature in the developed long NNW-SSE corridor between Petite Anse and Anse d’Arlet. reservoir, up to 250°C, is the highest of the four study geothermal This acid alteration is symptomatic of a palaeo-fumarolic activ- fields. ity, likely related to an early boiling event of a larger and / or In terms of fluid equilibrium, it appears that the dominantly shallower reservoir than the current one. These undated palaeo- chloride-sodium type reservoir waters, characterized by a strong alterations (neutral caprock on the sea level and acidic alteration recharge of sea water (except for Montagne Pelée) show: i) a full on the relief) are the witnesses of an intense high temperature equilibrium for Bouillante reservoir fluid (250°C), ii) only partial geothermal palaeo-system, whose the caprock is today at the equilibrium for Petite Anse and Lamentin hot spring (close to surface, because of the flank collapse of the Morne Jacqueline immature field), and, ii) clearly immature waters for Montagne volcano (Traineau et al., 2013) (Figure 3). Therefore in Petite Pelée (Rivière Picodo and Rivière Chaude). Anse, we suggest that the geothermal system could begin sev- eral hundreds of thousands of years ago (300-500 ky?) with an Early Hydrothermal Stages as Fossil Witnesses intense geothermal activity recognized on several kilometers of the Geothermal Systems (under boiling conditions), whereas the current activity seems to have decreased in intensity (and perhaps slightly in temperature Identification of early fossil stages of the geothermal systems 190-210°C) and is contracted around Morne Jacqueline volcano. can make a relevant contribution to explain their evolution in time We interpreted this evolution as a retrograde trend of the system. and space (Figure 3): i) determining how long they are active, However, strong magmatic signature of the 3He/4He ratio (7.3-7.9 ii) estimating their thermal temporal evolution (cooling, heating Ra according to Pedroni et al., 1999 and Gadalia et al., 2014) or thermal peak?) and iii) assessing if their spatial extent varies suggests a continuation (or renewal) of magmatic activity in the with time. region despite the absence of volcano less than 350 ky, which In Lamentin (Figure 3), prominent fossil silica sinter as high could eventually maintain the current geothermal system at high temperature occurrences, emplaced 250-300 ky ago (dated by U/ temperature (~ 200°C).

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Burgos to Diamant and characterized with a 1.1 My-long magmatic activity ending 350 ky ago with the eruption of Morne Larcher volcano (Figure 3). It frequently exhibits evidences of mixing between basaltic magmas and differentiated, quartz-bearing dacitic magmas. The latter is thought to be evolving within shallow magmatic chambers at around 6 km depth ac- cording to Gourgaud and Gerbe (1991). A MT survey evidenced a 5-km long, resistive and high-density body striking north-south which could be viewed as a large dyke-like magmatic intrusion (Gadalia et al., 2014). Its top is at a Figure 3. Surficial expressions as witness of fossil and active geothermal (hot springs) stages of the Lamentin depth of 500 m. A dyke of dioritic com- (left) and Petite Anse (right) fields (Martinique). position outcropping close to the Eaux Ferrées hot springs is probably related. In Bouillante, the high temperature geothermal system (250°C) Such a large and shallow intrusive body could be considered to be is existing for at least 300 ky as indicated by an adularia-silica the heat source of the hydrothermal system. Gases collected with bearing breccia, partially eroded, and visible in outcrop above the thermal fluids (CO2, He) have clear magmatic origin as shown current reservoir (Patrier et al., 2013). Recently dated at 250 ± 50 by the 3He/4He ratio (7.75-7.93 Ra; Pedroni et al., 1999) and are ky by 40Ar/39Ar on adularia (Vérati et al., 2014), this explosion consistent with the occurrence of a magma cooling and degassing. breccia contains mainly adularia emplaced at >200°C, which is Lamentin. The main magmatic activity in this area is related to compatible with the current reservoir temperature. Thus, since the Pitons du Carbet volcano. Germa et al. (2011) evidenced two 250-300 ky, the Bouillante geothermal system has maintained main stages of activity. During a first stage (1 to 0.6 My), dacitic at high temperature (~250°C) without modification of its spatial lava domes were emplaced in the summit area of the volcano. extension, centered on the Bouillante Bay. This system appears Around 337 ± 5 ky, a large sector collapse occurred yielding a particularly stable during a long period. large horse-shoe caldera open to the west (Boudon et al., 2013). It In Montagne Pelée, geothermal activity appears at most as old was immediately followed by the eruption of several large dacitic as the building of Pelée stratovolcano, which began 127 ky ago lava domes which are well observed today. The low 3He/4He ratios (Germa et al., 2011; Boudon et al., 2013). Thus, the stratovolcano (2.2-2.3 Ra; Pedroni et al., 1999) of gases collected with thermal is the main host rock of the geothermal reservoirs, located between fluids of the Lamentin hydrothermal system seem to be indicative 1000 and 2000 m in depth. of a long-lived degassing magma which is rather consistent with Comparing the spatial and temporal evolution of the four stud- the first stage of magmatic activity (1-0,6 My). Moreover, the oc- ied geothermal systems, it appears that the system of Bouillante currence of hydrothermalized formations (acidic alteration) within remains stable in temperature (250°C) and in space during 300 ky, the deposits of the sector collapse dated 337 ± 5 ky is indicative while the geothermal system of Petite Anse is marked by a spatial of the existence of an early hydrothermal system which has been contraction of its reservoir and a reduction of its hydrothermal active before this collapse event. Therefore it is suggested that activity, which could be combined with a slight decrease of the the development of the Carbet-Lamentin hydrothermal system temperature around 200°C. More advanced is the retrograde ther- could be old and related to a heat source associated with the first mal evolution of the system Lamentin which gradually cools since stage of magmatic activity of the volcano dated 1-0,6 My. Another 250 ky to reach 100°C today. The system of Montagne Pelée, as hypothesis could be a more local magmatic activity, expressed a still immature geothermal system, is likely to show a prograde about 0.6 My ago in the southern part of the Lamentin depression evolution in the future (next 100 ky?). by the volcanic stage of Rivière Salée. Montagne Pelée. The hydrothermal system seems to be mainly Magmatism as a Heat Source developed within, or close to, the central eruptive conduit. The for Geothermal Systems volcano started its activity around 127 ky and is still active with its latest eruption recorded in 1929. During a previous stage, it Recent magmatic activity has been recorded in Martinique seems that the location of the volcano was slightly northward. (Westercamp et al., 1990; Germa et al., 2011) and is thought to Gas collected with the thermal fluids of Sources Chaudes hot be providing the required heat sources for the development of springs show 3He/4He ratios (7.2 Ra; Ph. Jean Baptiste, personal geothermal systems. Relationship between magmatic activity and communication, 2014) which are similar to the ratios measured hydrothermal systems can be tentatively analyzed even if ages are in the fumaroles of the Guadeloupe Soufrière volcano (6.5-8.2 not well-constrained. Ra; Ruzié et al., 2014). Petite Anse. The area of active and fossil hydrothermal activity Bouillante. The volcanic activity observed in the area of Bouil- is located within a volcano-tectonic corridor extending from Pointe lante belongs to the so-called Bouillante volcanic Chain which

366 Bouchot, et al. has been active between 1.2-0.2 My (Gadalia et al., 1988). The at least 300 ky (plateau) and occupying the same volume most recent magmatic activity recorded at Bouillante is dated at of geothermal reservoir. This is the case of the system of 0.5My (500 ky): it is considered has the probable heat source of Bouillante. 3 4 the high temperature geothermal system. The He/ He ratios of –– A retrograde trend of the system could be characterized by gas collected in the thermal fluids (4-4.7 Ra) is consistent with a a slight cooling of the resource (250 °C to 200 °C) coupled rather long-lived degassing magma. with a decrease of hydrothermal activity, partial equilibrium Finally, comparison of the four hydrothermal systems shows: of the fluid and a contraction of the reservoir. This is the –– the age of the magmatic activity considered as the heat case of Petite Anse geothermal system. source varies: 600 ky (Lamentin), 500 ky (Bouillante), 350 –– Advanced retrograde stage is marked by a significant cool- ky (Petite Anse), actual (Montagne Pelée), ing of the reservoir to reach 140-100 °C (Lamentin), coupled –– the duration of the magmatic activity varies: 1,1 My (Petite with an important contraction of the system. In this case, Anse), 0.7-0.8 My (Lamentin, Bouillante), 130 ky (Mon- we can assume that the magmatic heat source is at the end tagne Pelée), of cooling because of its old age (> 600 ky) and that the –– a time lag exists between the latest magmatic activity and geothermal system is progressively dying. This is the case the development of the (mature) hydrothermal system. It is of geothermal system of the Lamentin. estimated at 200 ky (Bouillante), 100-200 ky (Petite Anse, Lamentin), and more than 100 ky because the mature stage is not yet reached (Montagne Pelée). Summary

Toward a Continuum Geothermal Model Such a continuum model should contribute to exploration in active volcanic arc setting such as the Lesser Antilles volcanic arc. From comparative parameters, summarized in Table 1, we It appears that a magmatic environment, both sustainable (700 propose a continuum model of geothermal system with three suc- ky to 1 Ma) and wedged in the time around 500 ky (magmatic cessive stages, prograde, peak and retrograde stages (Figure 4): peak), appears more favorable to the development of active high –– The early hydrothermal stage is characterized by a rise in temperature geothermal system (peak stage). temperature of the system (prograde conditions) that follows To conclude, this model allows characterization of different an intense magmatic activity. During this prograde stage, the geothermal systems, explored or exploited, according to their reservoir temperature reaches 220 °C (at Montagne Pelée) degree of evolution. Each stage of this development (prograde but the partial equilibrium of fluids with the rock is not yet to retrograde) is associated with a specific use of the geothermal reached (“immature waters”). At this stage, we can assume resource: production of cold or heat (around 100 °C), plus/or elec- that the extension of system is limited and it will tend to be tricity production by binary cycle for temperature <180 °C, plus/ expanding later (if permeability and recharge are effective). or power generation by flash turbine as in Bouillante geothermal This is the case of the system of Pelée stratovolcano; it is plant (> 200 °C). difficult to estimate the duration of this prograde stage but could be around 100 to 200 ky. References –– The thermal peak stage reach 250°C in Bouillante system. This high temperature stage should be sustainable during Bouchot V., Sanjuan B., Traineau H., Guillou-Frottier L., Thinon I., Baltas- sat J.M., Fabriol H., Bourgeois B., Lasne E. (2010) - Assessment of the Bouillante geothermal field (Guadeloupe, French West Indies): Toward a conceptual model of the high temperature geothermal sys- tem. Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010. Boudon, G., Villemant B., Le Friant, A., Paterne M., Cortijo E. (2013) - Role of large flank-collapse events on magma evolution of volcanoes. Insights from the Lesser Antilles Arc. Journal of Volcanology and Geothermal Research. vol. 263, pp. 224-237. Chovelon P. (1984) - Cadre structural des minéralisations hydrothermales des prospects du Lamentin et du Morne Rouge (Martinique). Report BRGM 84 SGN 326 GTH, 51p. Gadalia A., Gstalter N., Westercamp D. (1988) - La Chaîne volcanique de Bouillante, Basse-Terre de Guadeloupe (Petites Antilles) : Identité pé- trographique, volcanologique et géodynamique, Géologie de la France, n°2-3, 11, pp. 101-130. Gadalia A., Baltassat J.M., Bouchot V., Caritg S., Coppo N., Gal F., Girard J.F., Gutierrez A., Jacob T., Martelet G., Rad S., Tailame A.L, Traineau. H., Vittecoq B., Wawrzyniak P., Zammit C. (2014) - Compléments d’ex- ploration géothermique en Martinique : conclusions et recommandations Figure 4. Continuum geothermal system based on a diagram temperature pour les zones des Anses d’Arlet et de la Montagne Pelée. Report BRGM/ of the reservoir versus duration of the geothermal system. RP-63019-FR, 230 p.

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