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Proceedings 26th NZ Geothermal Workshop 2004 THE CASITA GEOTHERMAL FIELD, NICARAGUA IAN BOGIE1, GREG N USSHER1 & JAMES V LAWLESS1 1Sinclair Knight Merz Ltd, PO Box 9086, Newmarket, Auckland. ([email protected]) SUMMARY – The Casita geothermal field is held under a concession by Triton Power S.A., a 100% owned subsidiary of Polaris Energy Corporation. It is located in the eastern portion of the San Cristobal volcanic complex that is part of the Marabios Range of northwestern Nicaragua. San Cristobal located in the northwest is currently volcanically active, but Casita has no clear history of eruption. It appears erosionally older and was the site of a major catastrophic mudslide from its southern slopes during Hurricane Mitch in 1998. An even older volcanic centre occurs to the east of Casita in the form of the La Pelona caldera. Casita is a ridge, with volcanic craters along it, which is made up of pyroxene andesites with lavas predominating on its northern slopes and pyroclastics on its southern slopes as a result of the prevailing winds sorting the eruptive products. In addition to the predominance of pyroclastics in the south a further contributory cause of the 1998 mudslide was the presence of widespread hydrothermal alteration on Casita. There is also a very large area of hot and steaming ground, although actual fumarolic activity is relatively feeble. Repeated sampling and analysis of fumarole gases over time indicates the presence of a vapour-dominated reservoir at a temperature of approximately 235°C. The presence of Cl in warm surrounding ground water wells at lower elevations suggest a possible underlying neutral-Cl reservoir. Further afield, hot neutral-sulphate springs at El Bonete approximately 8 km to the north-east have an associated resistivity anomaly orientated towards Casita and have been interpreted to be an outflow of condensate from above the vapour-dominated reservoir at Casita. This interpretation has been reinforced by a recent MT survey over Casita, which shows a possible upflow zone under the Casita ridge and the link between the Casita and El Bonete resistivity anomalies. The presence of this significant condensate outflow, the extent of the thermal ground on Casita and the possibility of a major vapour-dominated zone indicate the presence of a significant geothermal resource at Casita. The concession holder has made preparations for exploration drilling. 1. INTRODUCTION range, breaks this pattern. The developed Momotombo geothermal field is located beneath The Casita geothermal field is located in its southern flanks, but is likely to be older than northwestern Nicaragua. It is held under the volcano itself which is considered to have concession by Triton Power S.A., a 100% owned began forming 4500 years ago (Smithsonian, subsidiary of Polaris Energy Corporation (of 2004). Panama). The field is hosted by the San Cristobal volcanic complex (Figure 1), one of a chain of The volcanic rocks of the Marabios range and volcanoes from San Cristobal to Momotombo that gases from the Momotombo geothermal field makes up the Marabios Range. having geochemical signatures indicative of receiving the greatest contribution from sediments 2. GEOLOGY and altered oceanic slab of any of the rocks or gases of the Central American volcanic arc Three of the volcanic complexes which form most (Snyder et al. (2001). The northeast subduction of the Marabios Range follow a similar general of the Cocos plate beneath the Caribbean plate, pattern with an active volcano in the northwest of which is producing this volcanism, is at a steep the complex, with increasingly older volcanic angle. This has produced compression and centres to the southeast, within which geothermal densification of the leading part of the Caribbean fields are found. Other volcanic centres of plate upon which the volcanic front is located. varying age, including andesitic cones, dacite This, in combination with the high flux of dome complexes, maars and scoria cones, volatiles from the slab may be responsible for surround the main aligned eruptive centres. The voluminous magmatism with many shallow Telica volcanic complex to the southeast of the intrusions as a result of a high rate of magma San Cristobal complex (Figure 1) has Telica as generation and subsequent buoyancy within the the active volcano and San Jacinto as the crust, ultimately producing this concentration of geothermal field. The El Hoyo complex to the volcanic centres and geothermal fields. A southeast of Telica has Cerro Negro as the active satisfactory mechanism for the repeated pattern of volcano and El-Hoyo-Cerro Colorado as the younging of volcanic centres to the northwest of geothermal field. Momotombo, the most the three volcanic complexes is yet to be put southeasterly active volcano of the Marrabios forward. 1 Figure 1: The volcanoes of Nicaragua (from INETER), North at top. San Cristobal volcano forms a broad cone and is produced a major dacitic ignimbrite eruption the highest volcano in Nicaragua at 1745 m. It associated with the caldera formation. A last erupted in 2003 and a steady vapour plume is corresponding large associated intrusive is emitted from the volcano. Recorded eruptions go inferred at depth. The relationship between the back to 1522. There is also a possibility of an three volcanic centres is illustrated in Figure 2. eruption from Casita in the 16th Century, but this may have been confused with San Cristobal as Of note at Casita was the occurrence of a debris radiometric dating (Henneberger, et al. 2003) avalanche and resulting lahar runout flow from indicates eruptions within 12, 000 BP at Casita. the southern flank of Casita that occurred during high rainfall associated with Hurricane Mitch in Casita forms a ridge emerging from the San 1998. Approximately 2500 people lost their lives Cristobal cone upon which a set of craters is when the townships of El Porvenir and Rolando found the largest of which is Ollade. The ridge Rodriguez were buried. The area of the disaster is runs onto and overlaps the arcuate depression of now designated as a national monument. the La Pelona Caldera, which is regarded to be Plio-Pleistocene in age and is notable for having Figure 2; Geological cross section from San Cristobal to La Pelona. Vertical scale equal to horizontal scale. 2 The immediate trigger of the slide was the heavy Henneberger et al. (2003) report a magmatic precipitation from Hurricane Mitch with 500 mm helium isotope signature. This does not imply the on the day of the slide. In comparison the normal presence of acidic magmatic volatiles. This is monthly rainfall is 328 mm. The contributing because such a signature is typical for andesitic factors towards the slide are the predominance of stratovolcano geothermal systems with neutral pyroclastics on the southern flanks of Casita as a reservoirs (eg Momotombo, Snyder et al., 2001). consequence of the prevailing winds sorting There are no other such volatiles present and the eruptive products (van Wyk de Vries and Borgia, gases from Ollade do not have the quenched 1996) and the presence of the geothermal system chemistry that is indicative of directly derived that has hydrothermally altered them. This has magmatic gases in which acid producing volatiles reduced their strength allowing the volcano to have been scrubbed out (Bogie and Lovelock, deform under its own weight to an unstable 1999). profile (Kerle and van Wyk de Vries, 2001). Deformation is also evident on Casita’s eastern The warm wells northeast of Casita have neutral flank and van Wyk de Vries et al. (2000) predict bicarbonate-sulphate chemistries, but have Cl failure of this slope into the La Pelona caldera. concentrations of up to 120 mg/kg. The El Bonete hot springs have neutral sulphate 2. THERMAL ACTIVITY chemistries but with only up to 59 mg/kg Cl. The failure of the waters to lie on clear mixing lines Fumarolic activity with steam at the local boiling on Cl-enthalpy diagrams suggests that at least point of 96°C was first documented in the north three waters are involved and that solute of the Ollade crater at Casita in 1913 (Hazlett, geothermometry for the waters is moot. The first 1987) and this activity, although relatively weak, water is a steam-heated ground water containing continues to the current day. Additionally, there gas condensates and their reaction products. The are widespread areas of steaming ground east and second is a cold unmineralised ground water and southeast of the crater, the activity varies in third and of most significance is a possible intensity with the seasons, but reaches an area of neutral-Cl water. The latter could be leakage up to one km2. Further outlying steam emissions from a deep reservoir underlying the vapour- are found on the northern rim of the La Pelona dominated zone on Casita. caldera. There is an even larger area of diffuse 4 hot ground increasing the area of surface thermal 150 200°C 250°C 300°C 2 manifestation on Casita to approximately 7 km . Vapour Equilibrium Line Ten kilometres to the northeast of the Ollade 3 crater ground water wells at Las Grietas show UNOCAL CAS-16B ) 350°C elevated temperatures of up to 50°C. Continuing r /A 2 2 another 8 km to the northeast hot springs with H ( g 300°C temperatures up to 82°C are found at Monte lo Largo and 20 km from the Ollade crater the El Liquid Equilibrium Line Bonete hot springs have temperatures of up to 1 250°C 72°C. These could possibly be related to warm water found in the El Limon underground gold 200°C 0 mine. Other warm wells are found 11 km 23456 northwest of the Ollade crater at Sta. Carlota. log(CO2/Ar) Thus thermal manifestations are found over a very Figure 3: Gas geothermometry and determination wide area, but as the distance increases from of liquid-vapour conditions utilising the method Casita so does the uncertainty of a clear link to a of Giggenbach (1992).

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