Paipa Geothermal System, Boyacá

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https://doi.org/10.32685/pub.esp.38.2019.04 Paipa Geothermal System, Boyacá: Review Published online 20 May 2020 of Exploration Studies and Conceptual Model Paleogene Claudia María ALFARO–VALERO1* , Jesús Bernardo RUEDA–GUTIÉRREZ2 , Jhon Camilo MATIZ–LEÓN3 , Miguel Angel BELTRÁN–LUQUE4 , 1 [email protected] Gilbert Fabián RODRÍGUEZ–RODRÍGUEZ5 , Gina Z. RODRÍGUEZ–OSPINA6 , Servicio Geológico Colombiano 7 8 Dirección de Geociencias Básicas Carlos Eduardo GONZÁLEZ–IDÁRRAGA , and Jaison Elías MALO–LÁZARO Diagonal 53 n.° 34–53 Cretaceous Bogotá, Colombia Abstract The Paipa geothermal system is located at 2525 masl in a terrain tilted from 2 [email protected] Servicio Geológico Colombiano south to north towards the Chicamocha River, where the geology is dominated by Dirección de Geociencias Básicas sedimentary rocks intruded by felsic magmas. The Miocene and Pleistocene volcanic Diagonal 53 n.° 34–53 Bogotá, Colombia activity produced pyroclastic deposits and dome complexes. A deep saline sodium Jurassic 3 [email protected] sulfate water, presumably originating from the infiltration of meteoric water followed Servicio Geológico Colombiano Dirección de Geociencias Básicas by dissolution of an evaporite, mixes with geothermal fluid. This process masks the Diagonal 53 n.° 34–53 chemical and isotopic composition of the geothermal component of fluid discharge in Bogotá, Colombia 4 [email protected] hot springs, the temperatures of which reach 76 °C. Organic and magmatic/mantle con- Servicio Geológico Colombiano Triassic tributions also affect the composition of the gas phase discharges. High concentrations Dirección de Geociencias Básicas Diagonal 53 n.° 34–53 of radioactive elements are found in the area, mainly in El Durazno Intrusive, a highly Bogotá, Colombia altered intrusion located to the west. Extensive outcrops of the Une Formation at high 5 [email protected] Servicio Geológico Colombiano elevation (2900 masl) in the Tibasosa–Toledo Anticline represent the main recharge Dirección de Geociencias Básicas zone. High angle faults (Las Peñas, Paipa–Iza, and Agua Tibia Faults) and the contacts Diagonal 53 n.° 34–53 Permian Bogotá, Colombia between the magmatic intrusions and the surrounding metamorphic and sedimentary 6 [email protected] rocks control the permeability of the fractured reservoir. Northward, rising hot water Servicio Geológico Colombiano Dirección de Geociencias Básicas encounters elevated permeability in sedimentary rocks, mainly in the Une Formation, Diagonal 53 n.° 34–53 forming a sedimentary reservoir. The subsequent fluid outflow is facilitated by a nor- Bogotá, Colombia 7 [email protected] Carboniferous mal subvertical NW fault (Cerro Plateado Fault). Two main discharge zones, Instituto Servicio Geológico Colombiano de Turismo de Paipa–Lanceros and La Playa, are controlled by intersections between Dirección de Geociencias Básicas Diagonal 53 n.° 34–53 faults and permeable rocks. Bogotá, Colombia Keywords: geothermal exploration, hot springs, geothermal conceptual model, Paipa 8 [email protected] Servicio Geológico Colombiano Devonian geothermal system. Dirección de Geociencias Básicas Diagonal 53 n.° 34–53 Bogotá, Colombia Resumen El sistema geotérmico de Paipa se encuentra a 2525 m s. n. m. en un terreno * Corresponding author inclinado de sur a norte hacia el río Chicamocha, donde la geología está domina- da por rocas sedimentarias intruidas por rocas magmáticas félsicas. La actividad Silurian volcánica del Mioceno y Pleistoceno produjo depósitos piroclásticos y complejos de domos. Un circuito profundo de agua salina sulfatada sódica, presumiblemente originado por la infiltración de agua meteórica seguida por la disolución de una evaporita, se mezcla con fluido geotérmico. Este proceso de mezcla enmascara la composición química e isotópica de la descarga acuosa de los manantiales termales, Ordovician

Citation: Alfaro–Valero, C.M., Rueda–Gutiérrez, J.B., Matiz–León, J.C., Beltrán–Luque, M.A., Ro- dríguez–Rodríguez, G.F., Rodríguez–Ospina, G.Z., González–Idárraga, C.E. & Malo–Lázaro, J.E. 2020. Paipa geothermal system, Boyacá: Review of exploration studies and conceptual model. In: Gómez, J. & Pinilla–Pachon, A.O. (editors), The Geology of Colombia, Volume 4 Quaternary. Cambrian Servicio Geológico Colombiano, Publicaciones Geológicas Especiales 38, p. 161–196. Bogotá. https://doi.org/10.32685/pub.esp.38.2019.04

161 Proterozoic ALFARO–VALERO et al.

cuya temperatura alcanza los 76 °C. Aportes de fuente orgánica y magmática/man- télica también afectan la composición de las descargas en fase gaseosa. Concentra- ciones altas de elementos radiactivos se encuentran en el área, principalmente en El Durazno, una intrusión con alteración intensa ubicada al oeste. Extensos aflora- mientos de la Formación Une localizados a mayor elevación (2900 m s. n. m.) en el Anticlinal de Tibasosa–Toledo conforman la principal zona de recarga. Fallas de alto ángulo (fallas de Las Peñas, Paipa–Iza y Agua Tibia) y los planos de contacto entre intrusiones magmáticas y las rocas metamórficas y sedimentarias que las rodean controlan la permeabilidad del reservorio fracturado. Más al norte, el flujo ascen- dente causado por flotabilidad del agua caliente encuentra alta permeabilidad en rocas sedimentarias, principalmente de la Formación Une, formando un reservorio sedimentario. La circulación lateral posterior del fluido es favorecida por una falla normal subvertical NW (Falla de Cerro Plateado). Dos zonas de descarga principales, Instituto de Turismo de Paipa–Lanceros y La Playa, son controladas por la intersec- ción entre fallas y rocas permeables. Palabras clave: exploración geotérmica, fuentes termales, modelo conceptual geotérmico, sistema geotérmico de Paipa.

1. Introduction The Paipa geothermal system has had an intermediate– high exploration priority since the national assessment studies The Colombian convective geothermal systems are located in of geothermal resources in Colombia (Organización Latino- the Andean Region. At least 12 geothermal areas have been americana de Energía, Instituto Colombiano de Energía Eléc- identified (Figure 1a), which include (1) the Cerro Bravo–Cerro trica, Consultoría Tecnológica Colombiana & Geotérmica Machín Volcanic Complex, which comprises the zones of Ne- Italiana, 1982). reidas–Botero Londoño and the Villa María Fault in the Nevado The Grupo de Investigación y Exploración de Recursos del Ruiz Volcano, Laguna del Otún in the Santa Isabel Volcano, Geotérmicos of the SGC has carried out several exploration Santa Rosa–San Vicente in the vicinity of the Paramillo de Santa studies in geology, geochemistry, geophysics, and modeling. Rosa Volcano, and the Cerro Bravo Volcano and Cerro Machín These results are integrated into a conceptual model presented Volcano (Geocónsul, 1992); (2) the Chiles–Cerro Negro Volcanic in this chapter. Complex; (3) the Azufral Volcano; (4) the Paipa–Iza geothermal area; (5) the Cumbal Volcano; (6) the Galeras Volcano; (7) the 2. Background Puracé Volcano; (8) the Sotará Volcano; (9) the Doña Juana Vol- cano; (10) and the Nevado del Huila Volcano. The geothermal The Paipa geothermal system is located on the high plateau of areas numbered 2 to 10 were identified through a national assess- the Eastern Andean Cordillera, south of the town of the same ment study of geothermal resources in Colombia (Organización name and from the Chicamocha River to the west of the Tiba- Latinoamericana de Energía, Instituto Colombiano de Energía sosa–Toledo Anticline (Figure 2). Eléctrica, Consultoría Tecnológica Colombiana & Geotérmica The first studies of this system included fluid geochemis- Italiana, 1982). In addition, there are the (11) San Diego geother- try and geology (Bertrami et al., 1992; Boussingault & Rolin, mal area and (12) Sibundoy Volcano, the geothermal systems of 1849; Empresa Nacional de Uranio S.A. & Instituto de Asuntos which were identified from the hot springs national inventory Nucleares, 1979, 1980; Ferreira & Hernández, 1988; Garzón, project (Servicio Geológico Colombiano, 2015). Currently, five 2003; Hernández & Osorio, 1990; Navia & Barriga, 1929; Or- of those systems are being investigated for geothermal explora- ganización Latinoamericana de Energía, Instituto Colombiano tion: Nevado del Ruiz by CHEC–EPM, ISAGEN, and the Servi- de Energía Eléctrica, Consultoría Tecnológica Colombiana & cio Geológico Colombiano (SGC) (Alfaro, 2015); Chiles–Cerro Geotérmica Italiana, 1982;Renzoni & Rosas, 1967; Renzoni Negro by ISAGEN–CELEC (Alfaro, 2015); and the Azufral Vol- et al., 1998). cano, San Diego, and Paipa areas by the SGC (Alfaro et al., 2015, At least three conceptual models of the Paipa geothermal 2017; Rueda & Rodríguez, 2016). Other conductive geothermal system have been proposed in the past. Based on geological ob- systems could be hosted in sedimentary basins having high con- servations and fluid geochemistry, Ferreira & Hernández (1988) ductive heat flows, particularly in the Eastern Llanos, Eastern formulated a model with a magmatic heat source hosted in the Cordillera, Caguán–Putumayo, Catatumbo, and Cesar Ranchería sedimentary sequence, inferred from the Olitas, Pan de Azúcar, Basins (Figure 1b; Alvarado et al., 2008). and El Durazno volcanic bodies. A reservoir of primary perme-

162 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model Quaternary °C/km 39.3 35.3 32.6 30.5 29.0 27.6 26.6 25.7 24.9 24.1 23.3 22.5 21.8 21.0 20.1 19.4 18.3 16.9 15.1 Gradient Llanos

Vaupés–Amazonas Orientales La Guajira La

Catatumbo

Cesar–Ranchería La Guajira Offshore Guajira La

Eastern Cordillera Eastern Middle Magdalena Valley Magdalena Middle Lower Valley Magdalena

Caguán–Putumayo Upper Magdalena Valley Magdalena Upper Amagá

Sinú Sinú Offshore Sinú San Jacinto Cauca–Patía Urabá Basin

Chocó Chocó Offshore Chocó Tumaco b Volcanoes Orinoquía Amazonía Andina Caribe Pacíﬁco Region Guanía Vichada Vaupés Arauca Amazonas Casanare Guaviare Meta 4 Norte de La Guajira Santander Boyacá Cesar Santander Caquetá Bogotá Bolívar Magdalena Cundinamarca 1 1 Sucre Huila Tolima 1 Caldas Antioquia Atlántico 0 Córdoba 1 NN Quindío Risaralda 7 Putumayo 2 8 Cauca 1 0–20 Valle del 80–95 20–30 30–40 40–50 50–60 60–70 70–80 Chocó Cauca 6 9 3 5 2 Nariño Temperature °C a Convective hydrothermal systems (Geocónsul, 1992; Organización Latinoamericana de Energía, Instituto Colombiano de Energía de Energía Colombiano Instituto de Energía, Latinoamericana Organización 1992; (Geocónsul, systems hydrothermal (a) Convective Colombia. from resources 1. Geothermal Figure Colombiano, Geológico (Servicio springs hot of inventory and 2015) Colombiano, Servicio Geológico 1982; Italiana, Geotérmica & Colombiana Tecnológica Consultoría Eléctrica, geothermal Paipa–Iza the (4) Volcano, Azufral the (3) Complex, Volcanic Negro Chiles–Cerro the (2) Complex, Volcanic Machín Bravo–Cerro Cerro the (1) areas: Geothermal 2015). geothermal Diego San (11) Volcano, Huila the (10) Volcano, Juana Doña the (9) Volcano, Sotará the (8) Volcano, Puracé the (7) Volcano, Galeras the (6) Volcano, Cumbal the (5) area, 2008). et al., map (Alvarado gradient (b) Geothermal Volcano. Sibundoy (12) area,

163 ALFARO–VALERO et al. (masl) Altitude 3201 3151 2631 2491 2471 2916 2856 3262 2599 2995 2835 2795 2745 2705 2890 2680 2650 2770 2560 3048 2658 2548 2528 3107 3077 3027 2807 2697 2607 3464 2724 2584 3353 3023 2953 2753 2503 2453

Anticline 73° 04' 30" W 30" 04' 73° Tibasosa–Toledo Pantano de Vargas Alto

Los Godos 73° 06' 00" W 00" 06' 73° Creek valley Honda Grande Lake

Sochagota 73° 07' 30" W 30" 07' 73° 3 km Alto 2 Los Volcanes

1 73° 09' 00" W 00" 09' 73° 0

5° 45' 00" N 5° 43' 30" N 5° 42' 00" N 5° 40' 30" N 73° 00' W 00' 73°

Boyacá 73° 06' W 06' 73° Anticline Chicamocha River Tibasosa–Toledo Sochagota Lake 10 km Colombia 8

6 73° 12' W 12' 73° 4 2 South America 0 5° 48' N 5° 42' N 5° 36' N Location of the Paipa geothermal area. geothermal of the Paipa 2. Location Figure

164 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

ability occurs in the white sandstones of the Une Formation, mal alteration, isotopic investigation of groundwater (Alfaro, Quaternary and shallow aquifers occur in the Labor y Tierna Formations. 2012), potential field surveys (Vásquez, 2012), a geoelectrical A clay layer from the Churuvita Group forms the cap rock, and survey (Franco, 2012), and a magnetotelluric study (González– a recharge zone infiltrates through the Une Formation. The dis- Idárraga & Rodríguez–Rodríguez, 2017, Anexo A). From these charge zone is fed by deep fluids transmitted through NE–SW new studies, an updated conceptual model was formulated deep normal faults, and the reservoir temperature is above 200 (Alfaro, 2015). The regional recharge likely comes from the °C based on cation aqueous geothermometers. western flank of the Tibasosa–Toledo Anticline through the According to Lozano (1990), from the integration of geo- Une Formation. El Durazno Intrusive is a magmatic intrusion, logical information and a resistivity survey performed by the and it could contribute some of the heat from radiogenic heat Instituto Colombiano de Energía Eléctrica (ICEL) and the due to the relatively high concentrations of 238U, 232Th, and 40K. Electrificadora de Boyacá, a magmatic heat source is located The most significant hydrothermal alteration on surface is of between 3 and 5 km depth, which is related to volcanism; the the argillic and advanced argillic type in El Durazno Intrusive. system also has two reservoirs. The first is shallow and located Geophysical contrasts in the complete Bouguer anomaly define within the Guadalupe Group and its impermeable layer would the Paipa–Toca Fault, which has a N–S direction, and a linea- be the Guaduas Formation. The second reservoir is deeper (at ment called the Firavitoba Fault, as previously identified by 1000 to 1200 m) and thicker, and it is hosted in the Churuvita Velandia (2003). The low resistivity anomalies from the shal- Group and the Une Formation; its impermeable layer would be low surveys are attributed to saline pore waters in the Churuvi- the Conejo Formation. According to this work, the most inter- ta Formation, south of the intersection between the Buenavista esting area in the resource’s location has been identified from and Rancho Grande Faults. Lastly, three resistivity structures the resistivity survey, which is located in the depression of So- were identified: a very high resistivity zone that corresponds chagota Lake, where it is elongated in a NE direction and has a to the center of the Tibasosa–Toledo Anticline, a shallower resistivity between 10 and 20 Ω·m. low resistivity zone possibly related to a clay–rich layer or an In the last 15 years, the SGC accomplished the following aquifer filled with saline water, and an intermediate resistivity studies: (1) geological mapping at a scale of 1:25 000 and a zone characteristic of the sedimentary rocks that could host a structural model (Velandia, 2003), including a map of volcanic geothermal reservoir. units (Cepeda & Pardo, 2004); (2) chemical and isotopic char- acterization of hot spring waters (Alfaro, 2002a, 2002b); (3) a 3. Materials and Methods preliminary resistivity survey (Vásquez, 2002); (4) a radon survey (Alfaro & Espinosa, 2004); and (5) preliminary hydrother- This work is directed at developing a conceptual model based mal alteration and gas studies (Alfaro, 2005a, 2005b). From the on the integration of a 3D geological model that is constrained integration of those studies, a conceptual model was proposed by gravity and magnetic surveys, with magnetotelluric mod- (Alfaro et al., 2005), in which the system is hosted in a volcanic els. These were used to determine the lithologic and structural caldera and the heat source comprises cooling plutons that are controls on the localization of reservoirs of hot water and their 2.1–2.5 Ma. A deep reservoir likely occurs in fractured rocks relation to igneous intrusions, which might be heat sources and associated with basement faults (Paipa–Iza and Cerro Platea- influence fluid flow through the hydrothermal system. Fluid do Faults), which were also conduits for magmas. Shallower geochemistry, vertical electrical sounding, rock radioactive reservoirs are potentially hosted in the sedimentary sequence, elements analysis, soil temperature measurements, and geo- which because of their lateral extension allow the geothermal chronologic data were integrated as part of the model. The in- water to flow northwards up to the surface discharge zone. The tegration of the results was performed through workshops with geothermal fluid has been heated to more than 300 °C by deep participation from the scientific team comprising coauthors of circulation. The fluid discharge zone is structurally controlled this chapter. The methods for the specific surveys can be found by fault intersections related to the rotation of blocks defined in the references cited. by Velandia (2003). The outflow goes from south to north up to the main discharge zone in the ITP–Lanceros sector. The hot 4. Results spring water is not representative of the reservoir fluid because its chemical and isotopic composition is masked by mixing with 4.1. Geology a low temperature saline sodium sulfate source and by a contribution of an organic gas. The regional geology is represented in maps at a scale of Complementary studies were performed by the SGC, most 1:100 000, which show the predominance of Jurassic, Cre- of which were related to the geothermal exploration, including taceous, and Paleogene sedimentary units, with evidence of uranium exploration (González et al., 2008), a cold and hot volcanism represented by andesite rocks (Renzoni & Rosas, springs inventory (Ortiz & Alfaro, 2010), surface hydrother- 1967; Renzoni et al., 1998).

165 ALFARO–VALERO et al.

The basement comprises metamorphic rocks (phyllites, they are classified as volcanic arc granites (VAG) in the Pearce schists, and gneisses) intruded, at least in the sector where the et al. (1984) diagram (Figure 4c–e). Anorthoclase, sanidine, al- basement is exposed, by Paleozoic and Mesozoic plutons in- bite, and subordinate biotite and hornblende make up the phe- cluding Chuscales, Otengá, Santa Rosita, and Aguachica. These nocrysts. The matrix is mostly cryptocrystalline, with varying units are presumably overlain in the study area by the Devonian amounts of glass replacing the feldspar cores. Cepeda & Pardo – Carboniferous sedimentary sequences of the Tibet, Floresta, (2004) reported ages of 2.13 ± 0.39 to 2.29 ± 0.17 Ma for one and Cuche Formations (Mojica & Villaroel, 1984). of the domes of the alto Los Volcanes. Ar–Ar ages indicate The Paipa area is locally covered by sedimentary sequences effusive activity at 1.7 Ma for the alto Los Volcanes domes, 1.8 of Cretaceous and Paleogene ages over a basement that does Ma for the Honda Grande Creek dome, and 2.7 Ma for the alto not crop out in the studied area (Figure 3; Velandia, 2003). The Los Godos (Rueda, 2017). Fission track data in zircon show Tibasosa unit in the SE area comprises gray shales, limestones, evidence of volcanic activity between 5.9 and 1.8 Ma based on and sandstones. The Une Formation consists principally of the average of all grain ages (Bernet et al., 2016). sandstones that are fine to coarse grained with conglomerate The volcanic caldera hypothesis (Cepeda & Pardo, 2004) lenses and intercalations of gray shales. The Churuvita Group is now discarded because the basal unit (vitreous ignimbrite) comprises shales and quartz sandstone. The Conejo Formation actually corresponds to emplacements of the dome structures. comprises thick layers of black shales with layers of fine– Their geomorphology is not due to a caldera collapse but rath- grained quartz sandstones. Thin layers of siliceous siltstones er to the eruption of two dome complexes (alto Los Volcanes and mudstones with phosphorite–rich layers that show intense and alto Los Godos) on either side of the Honda Grande Creek fracturing in outcrops compose the Plaeners Formation. Los valley, which forms a depression between them (Rueda, 2017). Pinos Formation comprises black and green siltstones and thin to medium interlayers of sandstones. The Labor y Tierna 4.2. Structural Geology Formation comprises fine and coarse–grained sandstones. The Guaduas Formation is made of interbedded claystones, quartz The Paipa geothermal area is located in the axial zone of the sandstones, and coal layers. Quartz sandstones with layers of Eastern Cordillera of Colombia, which has been subject to tec- claystone compose the Bogotá Formation. The Tilatá Forma- tonic inversion and extension since the Cretaceous (Colletta et tion is the youngest unit and comprises unconsolidated sandy al., 1990; Cooper et al., 1995; Dengo & Covey, 1993; Fabre, sedimentary deposits. Overlying the Tilatá Formation, there are 1983). Velandia (2003) showed that the reactivation of old and Quaternary alluvial deposits. new structures is related to the compressive regime during the Several investigations of the Paipa Volcano (Ferreira & Andean Orogeny. In the area of interest, there are traces of a Hernández, 1988; Garzón, 2003; Hernández & Osorio, 1990; transpressive tectonics in the Boyacá and Soapaga Faults. Ac- Navia & Barriga, 1929; Renzoni & Rosas, 1967) describe the cording to Velandia (2003), there are two structural styles in geology and geochemistry of this atypical volcanic center in the area (Figure 5). Thick skin style normal faults such as the the Eastern Cordillera. Cerro Plateado, Paipa–Iza, Las Peñas, and Agua Tibia Faults cut Cepeda & Pardo (2004) proposed the existence of a volca- the basement. Thin skin style structures comprise thrust faults nic caldera and associated deposits comprising air fall, a debris in the northwest area that include El Bizcocho, El Batán, El flow, pyroclastic flows, and rhyolitic flow domes (Figure 3). Hornito, Canocas, Santa Rita, El Tuno, Rancho Grande, and The first episode involved the construction of a volcanic edi- Buenavista Faults. All of them are reverse type structures. fice, which was followed by caldera collapse (Cepeda & Pardo, The steep subvertical faults (e.g., Paipa–Iza and Cerro 2004). The products include block and ash flows, lavas, and Plateado) provide permeability for hydrothermal circulation pyroclastic deposits. and magma ascent. Stratigraphic units (Plaeners Formation) Rueda (2017), based on Ferreira & Hernández (1988), and fault intersections also form channels for fluid flow (Ve- Garzón (2003), Hernández & Osorio (1990), and new field ob- landia, 2003). servations, added the domes unit to the geological map of the Paipa area (Figure 3, red polygons) and divided it into three 4.3. Hydrothermal Alteration areas, which are the alto Los Volcanes, Honda Grande Creek, and alto Los Godos (Figures 3, 4a, 4b). The rocks from the alto In the Paipa geothermal area, the surface hydrothermal al- Los Godos and some from the alto Los Volcanes are interpreted teration is mainly related to water–rock interaction involving as ignimbrites and were produced in the first volcanic episode acidic low temperature waters. Intense acid alteration in El Du- of the area (Cepeda & Pardo, 2004). However, the geomorpho- razno Intrusive is described by González et al. (2008). X–ray logical, petrographic, geochemical, and geochronological data diffraction analyses indicate igneous phases made of sanidine indicate that these units comprise rhyolites and trachydacites, and quartz and hydrothermal minerals made of kaolinite and with a calc–alkaline signature (Peccerillo & Taylor, 1976), and alunite. The acid hydrothermal alteration extends to shallow

166 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model Quaternary Domes geometry Fault Concealed fault Thrust fault Concealed thrust fault Photogeological lineament Qhb NgQv Volcanic Rock (Surge, ash fall) Ash and blocks flow Low density currents metamorphic fragments Poorly selected layers of domes (Poorly selected layers of ash and dome blocks with Rhyolitic and coal in silky clay matrix Volcanic rocks and deposits Ash, pumice, and lithic flows. Faults and lineaments Ash fall with pumice gradation El Durazno Intrusive and breccia angular fragments of sandstones metamorphic blocks and pumice) Geological legend Q Kt Kc Ku Klt Kpl Klp Kch Pgb NgQt KPgg and deposits Une Formation Tilatá Formation Churuvita Group Bogotá Formation and well selected. Conejo Formation Sedimentary rocks quartz sandstones. Plaeners Formation Guaduas Formation Tibasosa Formation shales intercalations shales intercalations Los Pinos Formation with siltstones levels. of siliceous siltstones and quartz sandstones. Quartz sandstones with fluviolacustrine deposits. Labor y Tierna Formation of alluvial, lacustrine, and Quartz sandstones, friable downward and ash upward. intercalations with glauconite Quartz and lithic sandstones, fine sandstones intercalations Limestones, black shales, and with some of phosphorites and Arcillolites, siltstones, coal beds, Black and gray shales with some Black and gray shales with quartz Sands with silts and conglomerates Sand, silt, clays, and conglomerates Black shales with quartz sandstones intercalations. Alluvial conglomerates sandstones intercalations and thin layers Siliceous siltstone in thin or medium layers 2 km

Kt 73° 04' W 04' 73° 1.5 Ku 1 Q 0.5 s s e

o v x Lanceros Fault Lanceros d Q 0 e l g o

NgQv N Kc 73° 05' W 05' 73° G m s o o c L e o t m l

o NgQt A Alto Los Godos Kch Fault Buenavista d dome complexes Kch

Kpl 73° 06' W 06' 73° Klp

Klt KPgg Fault Tibia Agua

Q Paipa–Iza Fault NgQt NgQt Creek valley

Honda Grande El Batán Fault Batán El Cerro Plateado Fault Kc

Kch 73° 07' W 07' 73° Kpl

Paipa–Iza Fault El Hornito Fault Hornito El Lake Klp

Sochagota El Bizcocho Fault Bizcocho El NgQt

Las Peñas Fault

NgQt

73° 08' W 08' 73°

Canocas Fault Canocas Santa Rita Fault Rita Santa dome complexes Pgb Alto Los Volcanes

Kpl El Tuno Fault Tuno El Paipa–Iza Fault KPgg Klt Intrusive

El Durazno Klp Boyacá Fault Boyacá

Pgb 73° 09' W 09' 73° Qhb NgQt 5° 45' 30" N 5° 44' 30" N 5° 43' 30" N 5° 42' 30" N 5° 41' 30" N 5° 40' 30" N 5° 39' 30" N Geological map edited from Velandia (2003), Cepeda & Pardo (2004), and Rueda (2017). & Pardo Cepeda (2003), Velandia from map edited 3. Geological Figure

167 ALFARO–VALERO et al.

Alto Los Volcanes domes(Olitas)

Volcanic deposits

SE dome Central dome NW dome

SW dome

1000 15 6 Phonolite c d e

Trachyte 5 Foidite Tephri- (Q < 20) Shoshonitic series phonolite WPG 4 10 Trachydacite 100 (Q > 20) Hight–K PhonotephritePhonotephrite Trachy Rhyolite Calk–alkaline O (wt %)

2 andesite 3 series Basaltic

Tephrite O (wt %) 2

trachy Nb (ppm) K

O + K Basanite andesite 2 Traqui 2 basalt 10

Na 5 Calk–alkaline series ORG 1 Picro Basalt Basaltic Andesite Dacite VAG + basalt Andesite syn–COLG Tholeiitic series (Low–K) 0

40 50 60 70 80 50 55 60 65 70 75 1.0 10 100 1000

SiO2 (wt %) SiO2 (wt %) Y (ppm)

Figure 4. Expression and composition of the dome complex located in the geothermal area. (a) Alto Los Volcanes domes. (b) Alto Los

Godos domes. (c) TAS classification of rock composition.(d) K2O vs. SiO2 concentrations (Peccerillo & Taylor, 1976). (e) Tectonic origin (Pearce et al., 1984). VAG: Volcanic arc granites (Rueda, 2017).

168 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

Quaternary

Rancho Grande Fault Grande Rancho 73° 06' W 06' 73°

Cerro Plateado Fault

El Batán Fault Batán El Buenavista Fault Buenavista Lake

Fault Sochagota El Bizcocho Fault Bizcocho El Las Peñas 2 km

Fault Canocas Fault Canocas

Santa Rita Fault

El Hornito Fault Hornito El Paipa–Iza

1 73° 09' W 09' 73° 0 b 5° 45' N 5° 42' N Anticline Town Study area Volcanic complex Syncline Basin Lineament Fault Flection hook Corrales Sogamoso Tota Lake

Iza

Santa Rosa Soapaga Fault Soapaga Firavitoba 73° 00' W 00' 73° Duitama Pesca

Paipa Boyacá Fault Boyacá Toca

Tuta Los Medios Syncline Medios Los

Sotaquirá

Chivatá Fault Chivatá Arcabuco Anticline Arcabuco

Tunja

Tunja Syncline Tunja Tunja Fault Tunja

Arcabuco Santander 73° 30' W 30' 73°

a 5° 30' N 30' 5° Detailed map of the study area. The red arrows represent the direction of block rotation rotation of block the direction represent arrows The red map of the study area. (b) Detailed framework. (a) Regional area. geothermal of the Paipa geology 5. Structural Figure 2003) (Velandia,

169 ALFARO–VALERO et al. depths (50 and 100 m) in core samples of 4 boreholes (Figure 4.5. Fluid Geochemistry 6), and X–ray diffraction analysis has proven the existence of alunite and kaolinite plus additional secondary minerals such The Paipa geothermal area is characterized by abundant water as muscovite, barite, dickite, pyrite, and tridymite (Rodríguez and gas discharges from hot springs (Figure 9); the summary & Alfaro, 2015). that follows of the geochemistry and isotope composition of X–ray diffraction of the volcanic deposits characterize ox- hydrothermal fluids is based on Alfaro (2002a, 2002b, 2005b). idation and weathering that overprint hydrothermal alteration. These manifestations are distributed in two main discharge These comprise kaolinite veneer that covers the surface of py- zones: the ITP–Lanceros and La Playa sectors. The ITP– roclastic deposits and hematite–limonite that forms yellow and Lanceros sector is located near the intersection of El Bizcocho reddish staining. and El Hornito Faults in Quaternary deposits, and this area has Xenoliths in the alto Los Volcanes sector show low intensi- the largest number of hot springs and the highest flow rates. ty, high temperature alteration (Alfaro, 2005a). Some xenoliths The most representative of these is Pozo Azul, which is 56 °C comprise sedimentary and rhyolitic clasts in which plagioclase and flows at 6 L/s (Fonseca, 2018). In La Playa sector, approx- is replaced by epidote and cross cutting veins are filled with imately 3.5 km to the south of the main discharge zone in the chlorite and albite, indicating temperatures exceeding 220 °C Plaeners Formation, there are fewer springs, but they have the (Figure 7a–h). Metamorphic xenoliths containing biotite, adu- highest temperatures, which are up to 76 °C; there is a low– laria, and quartz filled veins suggest hotter alteration tempera- pressure steam vent at 75 °C. tures up to 320 °C (Figure 7i–l; Alfaro, 2005a). The Salpa springs (named after a salt plant) have temperatures of 21 °C and coincide with the Quaternary deposits 4.4. Radiogenic Heat Production northeast of the ITP–Lanceros sector. El Hervidero spring is a bubbling intermittent spring, which at 21 °C emerges through From the concentrations of 238U, 232Th, and 40K measured by deposits 1.5 km south of La Playa. The Olitas spring (23 °C) is gamma spectrometry in core samples from boreholes drilled located in the alto Los Volcanes dome complex. The pH of most by SGC at El Durazno Intrusive (González et al., 2008), the waters is neutral, and the high TDS of the ITP–Lanceros, La average heat production was estimated. The highest concen- Playa, and Salpa buffers the pH. The exception is El Hervidero trations of 238U are found in boreholes 2 and 3, with values spring, which is moderately acidic at pH = 3.7 (Table 1). of up to 159 mg/kg and 374 mg/kg, respectively. The highest According to their chemical classification (Figure 10a), 232Th concentration was measured in borehole 2, with values the dominant thermal water type, including those from ITP– of 133 ppm. 40K concentrations are similar in boreholes 1, 3, Lanceros, La Playa, and Salpa springs, is sodium sulfate, with and 4, with concentrations up to 7%, but in borehole 2, the 40K high salinity (20 000 to 60 000 mg/l of total dissolved solids). concentration is significantly lower (up to 1.16%) (Rodríguez El Hervidero and Olitas are low salinity springs (<150 mg/l of & Alfaro, 2015). total dissolved solids) and are classified as sodium sulfate and The radiogenic heat contribution of the boreholes was es- sodium bicarbonate types, respectively. timated using equation 1 (Beardsmore & Cull, 2001), which The pattern of dissolved species of Salpa springs is similar establishes that the radiogenic heat rate generated is related to to that for the hot springs in the ITP–Lanceros and La Playa radioactive decay rate and particle emission energy: sectors (Figure 10b). The evidence of a mixing process is confirmed by linear trends in the Na–K–Mg diagram (Figure 11a) A = abundance in the rock (ppm of 238U, 232Th or and the stable isotope plots (Figure 11b). The highest tempera- %40K) × ρ × A’(μW/kg), ture spring (El Batán) is the least affected by mixing. (Equation 1) The isotope composition of El Hervidero deviates from the mixing trend (Figure 11b) and shows a very significant oxy- where A is the rate of heat generation, ρ the density of the gen–18 depletion that is possibly due to the 18O fractionation rock, and A’ is the constant of heat generated by the decay of between CO2 and H2O (Bertrami et al., 1992). 238U, 232Th, and 40K. The highest oxygen–18 content corresponds to the Salpa The results are scattered, and, for that reason, the box dia- springs, and there is a direct correlation between sodium sulfate gram was used to represent these data (Figure 8). Borehole 2 concentration and 18O enrichment. This results not from high has the highest average heat generation, with a value of 12.52 temperature water–rock interaction but mixing (Alfaro, 2002b). μWm–3, which is of the same order of magnitude as that for The end–member meteoric water composition is δD –75% and the Copper Basin intrusion (10.3 μWm–3) (Middleton, 1979). δ18O –11%. The estimated recharge elevation is approximately However, although the dimensions of El Durazno Intrusive are 2800–2900 masl. From this, and considering that the Une For- unknown at depth, it is clearly much smaller. mation has good permeability, the Tibasosa–Toledo Anticline,

170 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model Quaternary 9 3.5 9.7 9.4 16.1 43.5 37.8 11.6 Pyrite % Tridymite % 4 3 6.1 4.2 2.5 4.8 8.8 7.6 62.6 72.9 14.6 10.4 17.4 Pyrite % Dickite % Orthoclase % <2 <2 <2 <2 2.1 3.1 5.2 4.6 2.7 2.3 4.4 3.9 9.4 48.2 26.3 14.7 10.9 Barite % Barite % Dickite % Orthoclase % 2 7 <2 <2 2.2 2.2 8.2 8.2 8.5 8.5 2.4 8.8 24.7 13.6 16.9 12.9 Barite % Dickite % Kaolinite % Kaolinite % 5 <2 <2 2.1 4.1 9.1 5.5 3.5 4.9 8.8 5.7 5.9 5.9 3.9 34.5 35.7 17.5 10.5 18.3 12.3 14.7 Muscovite % Muscovite % Muscovite % Muscovite % 6 20 12 4.1 3.1 3.5 5.8 2.7 4.7 7.8 3.3 3.6 6.4 7.4 7.9 26.8 30.5 20.3 51.3 56.9 Pyrite % Alunite % Alunite % Alunite % 31 31 29 56 <2 4.9 9.5 7.6 25.1 36.1 14.1 18.1 15.1 44.5 47.5 43.3 48.3 28.7 21.6 80.2 27.6 27.4 27.9 50.9 31.7 39.4 61.3 60.3 80.9 11.5 11.2 11.3 13.4 Quartz % Quartz % Quartz % Quartz % 45 <2 <2 2.5 5.7 3.3 7.7 54.1 71.1 61.1 63.1 17.1 65.2 46.7 70.5 52.3 62.8 57.9 54.9 31.9 67.3 82.6 88.9 17.5 18.6 16.9 Sanidine % Sanidine % Sanidine % Sanidine % 1 7 6 21 42 42 28 48 35 32 32 23 40 23 40 85 85 49 95 95 24 50 50 56 30 36 60 60 12 12 12 18 14 100 100 Depth (m) Depth (m) Depth (m) Depth (m) P1–1 P2–1 P3–1 P4–1 P1–5 P2–5 P3–5 P4–5 P1–2 P2–2 P3–2 P4–2 P1–8 P2–8 P3–8 P4–8 P1–3 P1–7 P2–3 P2–7 P3–3 P3–7 P4–3 P4–7 P1–6 P1–9 P2–6 P2–9 P3–6 P4–6 P1–4 P2–4 P3–4 P4–4 P2–10 Borehole–1 Borehole–2 Borehole–3 Borehole–4 Cores samples from boreholes drilled in El Durazno Intrusive. The mineral composition was determined from XRD analyses (Rodríguez & Alfaro, 2015). & Alfaro, (Rodríguez XRD analyses from determined was composition The mineral Intrusive. Durazno in El drilled boreholes from samples 6. Cores Figure

171 ALFARO–VALERO et al. . (Qtz) (d) . Ab Ab Chl Qtz Qtz l d h Chl Sme Ab c g k Pl Chl Qtz Ms Ill j f b Ser Ep Bt (Ep) Epidote replacing (Pl) Plagioclase and (Ser) Sericite. and (Ser) Sericite. Plagioclase (Pl) replacing Epidote (i) (Ep) Volcanes. Los in alto in xenoliths Petrography (f)(e) (Adl) Adularia. Chlorite. (h) (Chl) (g) (Sme) Smectite. (Ill) Illite. Pl Adl i a e (Ab) Albite in veins. Figure 7i–l in crossed nicols (Alfaro, 2005a). (Alfaro, nicols 7i–l in crossed Figure in veins. matrix. (l) (Ab) Albite feldspathic replacing Chlorite (j) (Chl) (k) Radial in quartz vein. Chlorite (Chl) (Ab) Albite. (Ab) Albite. (c) (b) (Ms) Muscovite. (a) (Bt) Biotite. minerals. vein filling of images microscopy electron scanning xenoliths and identified in minerals 7. Secondary Figure Quartz.

172 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

80 Quaternary

60 ) –3

40 Heat generation (µWm

12.52 9.62 9.70 8.28

0 Borehole–1 Borehole–2 Borehole–3 Borehole–4

Figure 8. Box diagram representing the estimated heat production for boreholes drilled at El Durazno Intrusive (Rodríguez & Alfaro, 2015).

13 located east of the study area, is likely the main recharge zone which is the component of natural gas in which the δ CCH4 (Alfaro, 2012; Malo & Alfaro, 2017). values range from –25 to –55‰ (Schoell, 1980). This sug- Hot springs in ITP–Lanceros and springs in Salpa and El gests that gases of two different origins have mixed (Alfaro, Hervidero are characterized by continuous strong gas dis- 2005b). Based on recent results for 3He/4He and 4He/20Ne, the charge. The gas composition of some springs (Table 2) is contribution from a mantle source is estimated to be between dominated by CO2, with a lower CH4 concentration than ex- 35 to 55% (Figure 13). pected for geothermal fluids (CH4/CO2 < 0.0001) (Figure 12a), Aqueous and gas geothermometers seem to be unreliable which could be related to the input of low methane magmatic due to the lack of equilibrated deep fluid in hot spring discharg- gas or to oxidizing environments associated with protracted es. The alkali geothermometers are not applicable, as the contri- 13 interaction with aerated groundwater. The δ CCO2 ranges be- bution of the saline source is dominant, which is evident in the tween –4.7 and –4.9‰ in ITP–Lanceros hot springs, and it is triangular Na–K–Mg plot (Figure 11a). The source of elevated –6.7‰ in Salpa and –3.1‰ in El Hervidero. From this, the concentrations of sodium, sulfate, and other ions species are most likely source of the CO2 is magmatic given that the man- likely due the dissolution of an evaporite deposit. The SiO2 geo- tle–derived values are –5 to –7‰ (Hoefs, 1978). The variation thermometers indicate temperatures up to 120 °C (Table 3), and 13 13 of δ CCH4 versus δ CCO2 (Figure 12b) found in the Pozo Inun- using a simple enthalpy–silica model (Truesdell & Fournier, dado spring from the ITP–Lanceros sector and El Hervidero 1977), the highest possible equilibration temperature is close shows that the origin of CO2 is magmatic and characteristic to 230 °C. of geothermal gases which ranges between –2 and –9‰ (Ono The results of the calculation of geothermometers based on 13 13 et al., 1993). CCH4 ranges from –35.85 and –33.49‰; it does chemical composition and C in CH4/CO2 are shown in Ta- not reflect the typical geothermal gases, for which the range ble 3. The calculated equilibration temperatures range widely is between –30 to –24‰ (Ono et al., 1993). This composi- between 41–92 °C (CO2/H2 geothermometer) to 340–522 °C tion could be due to the contribution of thermogenic methane, (CH4/CO2 geothermometer) and none of them seem reliable.

173 ALFARO–VALERO et al.

Na Cl Sochagota Na Cl Lake Ca HCO3 Ca HCO3 ITP Mg Laceros SO 5° 45' 30" N 4 Mg Salpa SO4 600 480 360 240 120 120 240 360 480 600 (meq/l) 600 480 360 240 120 120 240 360 480 600 (meq/l) El Hornito Fault

Boyacá Fault

5° 44' 30" N El Batán Fault Buenavista Fault

Rancho Grande Fault

Canocas Fault Na Cl

5° 43' 30" N Ca HCO3

La Playa Steam Mg SO4 Lanceros Fault

Vent 300 240 180 120 60 60 120 180 240 300 (meq/l)

Na Cl 5° 42' 30" N Santa Rita Fault Agua Tibia Fault Paipa–Iza Fault Ca HCO3 Cerro Plateado Fault El

Mg Hervidero SO4

0.5 0.4 0.3 0.2 0.1 0.1 0.2 0.3 0.4 0.5 (meq/l)

5° 41' 30" N Paipa–Iza Fault Na Cl

Ca HCO3

Las Peñas Fault Mg Olitas SO4 Paipa–Iza Fault

1.1 0.88 0.66 0.44 0.22 0.22 0.44 0.66 0.88 1.1 (meq/l) 5° 40' 30" N

Warm springs (20–50 °C)

Hot springs (>50 °C) 73° 08' 30" W 73° 07' 30" W 73° 06' 30" W 73° 05' 30" W 73° 04' 30" W 73° 03' 30" W

Figure 9. Locations of hot springs. The Stiff diagrams show the predominance of sodium sulfate waters (Alfaro, 2002a).

4.6. Shallow Temperature Survey tracting the average ambient temperature, and the results are presented in Figure 14. A shallow temperature survey was conducted in the Paipa geo- The average normalized temperature is 4.9 °C. Two positive thermal area. A total of 141 temperature measurements were anomalies (8.4 and 8.6 °C), well–defined by data, are observed, recorded at the surface and to 20 and 150 cm depths away from they occur around the intersection of El Bizcocho, Santa Rita, the immediate zones of influence of the hot springs (Rodríguez El Tuno, Paipa–Iza, and El Batán Faults, and another in the cor- & Vallejo, 2013). The measurements were normalized by sub- ridor bounded by El Hornito and Canocas Faults. In the same

174 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

Table 1. Chemical and isotopic composition of hot springs from the Paipa geothermal system (Alfaro, 2002a, 2002b). Quaternary

Electrical S Cat- S An- 18 Na K Ca Mg Li Sr B Fe Mn Cl SO4 HCO3 F Si SiO2 TDS d O dD T conduc- iones iones ID Hot Spring pH °C tivity (mS/ (‰) V– Concentration (mg/l) cm) SMOW

ITP–Lanceros

1 Pozo Azul 53.7 7.0 44500 13525 1500 90 18.0 18 4.8 5.20 0.3 1.8 5351 20187 2700 15.2 30 64 632.51 616.57 38652 –6.935 –67.05

Pozo Inun- 2 43.5 7.3 41400 11750 1400 77 17.0 18 4.4 5.30 0.4 1.5 4890 19687 2475 14 27 58 552.03 589.40 36094 –7.025 –66.5 dado

Manantial 3 Pozo Inun- 52.7 6.8 42900 12250 1500 77 18.0 18 4.6 4.80 0.4 1.7 5253 19750 2580 14.8 29 62 576.42 602.71 38024 –7.035 –68.15 dado

Pozo H. 4 63.4 7.2 42500 12500 1400 87 17.0 18 4.6 4.90 0.3 1.7 5138 19375 2520 14.7 30 64 585.14 590.67 36680 –7.46 –70.15 Lanceros

5 Ojo del Diablo 68.1 7.0 43200 12375 1562 175 21.2 18.2 5.7 5.10 0.2 1.6 5103 19250 2606 15.5 31.5 67.5 588.61 588.53 38848 –7.205 –68.2

6 Pozo Blanco 62.2 7.0 44100 13000 1500 100 17.0 20 4.6 4.90 0.4 1.8 5333 20937 2640 15.3 30 64 610.10 630.71 38218 –7.215 –67.9

Cuarto Máqui- 7 nas Tobogan 22.9 6.0 8320 1550 160 40 8.0 1.6 0.4 0.05 <0.2 <0.1 728 2870 298 0.48 7 15 74.16 85.24 5004 –9.56 –71.55 ITP

Fondo piscina 8 25.1 6.0 11130 2450 280 48 10.0 3.4 0.6 0.09 <0.2 0.6 1154 4187 479 0.29 7 15 116.93 127.65 8042 –9.365 –70.2 olimpica

Fondo piscina 9 30.2 6.3 27600 6883 810 100 20.0 10 1.6 2.10 <0.2 2.9 3009 13062 1402 1.4 10 21 326.70 380.06 21344 –8.49 –70.45 olimpica

Fondo piscina 10 25.9 6.1 9750 1975 230 50 10.0 2.2 0.4 0.60 <0.2 <0.1 985 3050 373 0.13 6 13 95.10 97.45 6922 –9.475 –71.5 olimpica

Motobomba 11 69.8 7.1 43300 12375 1550 175 22.5 18.1 5.8 5.10 0.3 1.6 5065 19812 2606 15.30 30.0 64.3 588.41 599.15 38750 –7.335 –69.05 Lanceros

La Playa

El Batán–Pis- 12 75.5 7.4 22000 5500 725 100 22.5 10.6 2.4 2.40 <0.2 0.2 2378 8250 1420 18.7 36.0 77.1 264.60 263.22 13500 –8.62 –68.95 cina La Playa

Vía Escuela 13 53.5 6.8 52300 5500 685 40 20.0 10 1.8 6.00 <0.2 0.2 6807 8750 2699 20 35 75 260.36 419.61 41813 –6.685 –67.6 La Playa

Salpa

Pozo Las 14 Marismas 21.5 6.3 55800 16926 1757 47.3 14.6 22.9 5.2 7.40 4.67 2.00 7095 26652 2989 5.76 31.6 784.55 804.69 63664 –6.36 –67.1 Salpa

Pozo principal 15 21 6.5 46000 13250 1637 175 22.5 18.0 4.7 5.30 0.2 1.7 5455 20812 2620 6.10 19.5 41.8 628.69 630.73 41612 –7.275 –69.65 Salpa

Olitas

16 Piscina Olitas 23.2 5.7 172.2 23.5 14.5 11.5 0.9 <0.1 0.6 0.03 <0.2 <0.1 1.3 17.5 93.7 3.2 46.0 98.6 2.04 2.11 92 –10.185 –73.35

El Hervidero 17 El Hervidero 21.8 3.7 103.2 4.0 5.0 2.5 0.4 <0.1 <0.2 <0.05 1.0 <0.1 3.7 22.5 0.0 0.1 9.0 19.3 0.46 0.58 55 –14.52 –62.9

corridor, a groundwater well of approximately 97 m depth dis- m (Franco, 2016). The survey area is characterized by low charges sodium bicarbonate water at 34.8 °C, which is consis- (between 12 and 58 Ω·m) and very low resistivities (<12 tent with the temperature anomaly. Negative anomalies (low Ω·m), which is illustrated in three plan maps, at 2500, 2350, temperature) occur in the Honda Grande Creek valley, which and 2200 masl. (Figure 15). The lowest resistivity anomaly extends around the alto Los Volcanes dome complexes. This is found to the east of Sochagota Lake in the proximity of thermal pattern is possibly related to the permeability of the the Salpa sector, which is the zone of the highest sodium sul- soil, which is lower in the valley, where clay layers from the fate concentration water (Alfaro, 2002a). An extensive NNW weathering of pyroclastic deposits form an insulating barrier. trending anomaly occurs between El Hornito and Canocas Faults (at 2500 masl) and extends to El Tuno Fault at 2200 4.7. Vertical Electrical Soundings (VES) masl. Other anomalies appear to be controlled by the Cerro Plateado Fault and the corridor between El Batán and Bue- The geoelectrical study, which employed a Schlumberger navista Faults. At 2350 and 2200 masl, the anomaly related array and 152 VES, penetrated to a maximum depth of 300 to the Cerro Plateado Fault seems to extend laterally to the

175 ALFARO–VALERO et al.

1000 a b Hot springs (>50 °C) Salpa Springs Warm springs (19–50 °C) 80 80 Sodium bicarbonate Warm springs (19–50 °C) 60 60 100

40 40

20 20 10

Mg So4 1 80 80

Concentration (meq/l)

60 60 0 40 40

20 20

Na K Ca Mg Cl So4 HCO3 80 60 40 20 20 40 60 80 Ca Na + K HCO3 Cl Major Ions

Figure 10. Chemical classification of the spring waters.(a) Piper diagram. Most of the springs are of the sodium sulfate water type, except for Olitas spring (blue circle), which is a sodium bicarbonate spring. (b) Schoeller diagram. The dominant sodium sulfate pattern followed by the hot springs is clearly defined by the saline springs of Salpa (Alfaro, 2002a).

Na/1000 a b 0 100 –40 –40

–50

–60 17 25 75

–50 V–SMOW) (‰ D El Hervidero –70 O*8 + 10 Full equilibrium line 18 Global meteoric line 160° 140° 120° dD=d Salpa 180° –80 springs 200° 100° –16 –14 –12 –10 –8 –6 50 50 18 220° O (‰ V–SMOW) 14 80° 240° ITP–Lanceros –60 260° Partial equilibrium 9 280° El Batán 12 V–SMOW) (‰ D 1 300° 8 IPT–Lanceros 3 13 10 6 320° 76 °C 7 12 5 75 25 11 14 340° –70 4 15 Salpa 8 9 springs 7 10 Immature waters Olitas 16 16 Y = 1.37576*X ± 58.3197 100 El Hervidero 17 0 R = 0.8785

K/100 0 25 50 75 100 Mg –80 –11 –10 –9 –8 –7 –6 18O (‰ V–SMOW)

Figure 11. Evidence of a mixing process of hot springs with saline sodium sulfate water. (a) Na–K–Mg relative composition (Giggenbach, 1988; Alfaro, 2002a). (b) Environmental stable isotopes (Alfaro, 2002b). west up to El Tuno Fault and to the east up to the Agua Tibia 4.8. Gravity and Magnetic Data Fault. The low resistivities could be related to the circulation of saline waters resulting from a high concentration of The acquisition of gravity and magnetic data was carried out in sodium and sulfate water, to the circulation of hydrothermal three field campaigns compiled by Beltrán (2015). An additional waters, or to clay layers in the sedimentary sequence (Fran- campaign was performed to unify the base stations (data levelling) co, 2016). in order to calculate the gravity fields (Vásquez, 2012). After apply-

176 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

Table 2. Chemical and isotopic composition of gases discharged by hot springs in the Paipa geothermal system. Quaternary

Concentration (molar percentage in dry gas) d13 C d13C 3He/4He CO2 CH4 He/Ne ID Hot Spring % (1) (‰) (‰) (R/Ra c) (2) (1) (1) (2) CO2 H2S NH3 He H2 Ar O2 N2 CH4 ITP–Lanceros 1 Pozo Azul 97.95 0.208 0.00175 0.00099 0.02159 0.90350 0.90768 0.00298 –4.7 1.12 3.39 2 Pozo Inundado 98.43 0.104 0.00286 0.00007 0.00095 0.02886 0.34299 1.07925 0.00688 –4.7 –35.85 4 Pozo Lanceros 92.26 0.280 0.00118 0.00030 0.00015 0.06592 1.53176 5.85692 0.00356 –4.9 5 Ojo del Diablo 99.15 0.354 0.00106 0.00025 0.00132 0.00944 0.00435 0.45406 0.02100 –4.8 3.14 3.32 Salpa Pozo La Marismas 14 99.73 0.129 0.00071 0.00014 0.00011 0.00191 0.02590 0.11027 0.00259 –6.7 6.76 3.56 Salpa El Hervidero 17 El Hervidero 99.25 0.136 0.00046 0.00411 0.00015 0.00471 0.02825 0.43606 0.13954 –3.1 –33.49 1125.05 2.73

(1) Alfaro (2005b) (2) This work. Analyses by the Istituto Nazionale di Geofisica e vulcanologia (INGV), from Italy. Agreement SGC–INGV No. 08, 2018.

a CO2/100 b 0 Marine 0 100 carbonate 14 –4 CO2 0% –8 Magmatic 20% carbon –12 40%

Low methane magmatic or oxidised gases 25 75 –16 60% 80% –20 100% Residual kerogen

(‰) –24

5 4 Kerogen CO2

CH 0%

C –28 gas from 20% 50 50 13 refractory –32 40% El Hervidero ER kerogen 17

Craking 350 °C RI 2 Pozo Inundado –36 60% 300 °C 17 SR 250 °C –40 80% reduction 200 °C 100% 75 LN 25 –44 BS gas from 150 °C RG labile NE BL Bs2 –48 kerogen BP AC 100 °C NE LN –52 SD oxidation Hydrothermal gases HC 100 RE RE 0 –30 –26 –22 –18 –14 –10 –6 –2 2

10CH4 0 25 50 75 100 N2 13 CCO2 (‰)

Figure 12. Gas discharge compositions. (a) Relative concentrations of CO2–CH4–N2, for which the blue circles correspond to the Nevado del Ruiz as a reference of the typical composition of hydrothermal and magmatic (ER and RI) gases. The red circles 5, 14, and 17 correspond 13 13 to Ojo del Diablo, Salpa, and El Hervidero hot springs, respectively. Modified from Giggenbach et al. (1990).(b) CCH4 vs. CCO2 (Alfaro, 2005b). Circles: geothermal wells from New Zealand. Squares: natural gas wells from New Zealand and Thailand. Diamonds: natural gas wells in the Gulf of Thailand. Modified from Giggenbach (1997). ing variable density gridding, adjusting the reduction of data, and The residual gravity map (Figure 16) shows several highs calculating the power spectrum, the residual anomaly grids were associated with the Paipa Volcano and Las Peñas and San- obtained by the application of a matched filter over the total Bou- ta Rita Faults; other gravity anomalies are associated with guer anomaly and over the total field magnetic anomaly (Llanos alto Los Godos, which are controlled by the Cerro Plateado et al., 2015). Fault.

177 ALFARO–VALERO et al.

100

70 R/Ra = 2.73 R/Ra = 2.81

60 R/Ra = 3.56 R/Ra = 3.39 R/Ra = 3.32

He Source contribution (%) contribution Source He 30 4

He / / He 3 10

0 El Hervidero Salpa Pozo Azul Steam Vent Ojo del Diablo

Crust source Mantelic source

Figure 13. 3He/4He ratios indicating a mantle contribution to the gas compositions.

Table 3. Temperature estimation of the reservoir fluid at Paipa’s geothermal system (Alfaro, 2002a, 2005b).

Discharge T –no T Tempera- Qtz T CO2–H2S– T T Td13 ID Hot Spring Sector steam CH4/CO2 T °C CO2/Ar CO2/H2 CH4/CO2 ture °C H2–CH4 H2/Ar °C °C °C loss °C (°C) (1) (2) (3) (4) (5) (6) (7) (8) (9) ITP–Lanceros 1 Pozo Azul ITP–Lanceros 54 114 513 108 85 2 Pozo Inundado ITP–Lanceros 43 109 468 98 84 220 226 159 4 Pozo Lanceros ITP–Lanceros 63 114 500 83 43 5 Ojo del Diablo ITP–Lanceros 68 117 415 106 116 285 92 La Playa 12 El Batán La Playa 76 123 Salpa Pozo La Marismas 14 Salpa 22 522 74 335 34 Salpa El Hervidero 17 El Hervidero 22 340 62 79 305 41 228 233 166

(1) Fournier (1977) (6) Koga (1987) in Koga (2000) (2) Giggenbach (1991) (8) Panichi & Gonfiantini (1978) (3) D’Amore & Panichi (1980) (7) Lyon & Hulston (1984) (4) Giggenbach (1987) (9) D’Amore & Panichi (1987) (5) Giggenbach & Goguel (1989)

178 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

4.2 5.4 3.1

Sochagota Quaternary 4.4 73° 09' W Lake 73° 06' W 4.7 4.8 5.8 6.1 5.1 4.9 5.3 Boyacá Fault El Hornito Fault 4.7 3.6 5° 45' N 5.8 5.3 5.9 4.2 5.2 5.5

5.6 6.7 5.6 ITA well 4.1 3.2 4.3 El Batán Fault 4.9 4.9 4.2 Buenavista Fault 3.8 5.3 4.3 5 4.9 5.8 Rancho Grande Fault 6 4.4 5.5 4.8 8.6 4.6 5.9 Canocas Fault 7 6.2 5.3 El Bizcocho Fault

5.9 4.7 5.4 4.7 8.1 Paipa–Iza Fault

5.7 5 6.1

Lanceros Fault Agua Tibia Fault 5.3 5.9 5

Cerro5 Plateado Fault 5.3 6.3 6.1 6.2 6.3 4.7 5.1 El Tuno Fault 6 4.1 5.1 Santa Rita Fault 4.8 5.6 6.1 5.4 4.9 Positive anomalies 4.8 Paipa–Iza Fault 8.4 4.2 Negative anomalies 7.5 5.5 Measurement stations 3.8 4 3.5 3.3 5° 42' N 6.2 7.1 5.6 ITA groundwater well 5.2 3.9 6.9 4.2 2.3 3.8 El Durazno Intrusive 4.7 3.8 4.7 2 4.8 4.6 Domes 5.1 4.9 1.9 2.3 4 5.2 4.6 Sochagota Lake 3.9 5.2 Paipa–Iza Fault 6.4 Temperature values °C 5 5.4 1.8–2.8 3.9 2.8–3.4 4.5 Las Peñas Fault 3.4–4.0 3.5 5.7 Paipa–Iza Fault 2 3.8 3.2 4.0–4.4 4.4–4.7 4.7–4.9 5.4 4.9–5.1 5.1–5.3 4.2 4.5 5.3–5.4 5.4–5.7 4.9 5.7–6.0 4.2 6.0–6.4 6.4–6.9 6.9–7.6 7.6–8.6

Figure 14. Map of shallow temperatures at 150 cm depth (Rodríguez & Vallejo, 2013).

179 ALFARO–VALERO et al.

a Sochagota 2500 masl b Sochagota 2350 masl Lake Lake Salpa Salpa

Boyacá Fault Boyacá Fault El Hornito Fault El Hornito Fault 5° 45' N 5° 45' N

El Batán Fault Buenavista Fault El Batán Fault Buenavista Fault

Rancho Grande Fault Rancho Grande Fault

Canocas Fault Canocas Fault

Paipa–Iza Fault Paipa–Iza Fault

Lanceros Fault Lanceros Fault

El Tuno Fault El Bizcocho Fault Cerro Plateado Fault El Tuno Fault El Bizcocho Fault Cerro Plateado Fault Santa Rita Fault Agua Tibia Fault Santa Rita Fault Agua Tibia Fault

5° 42' N 5° 42' N

Paipa–Iza Fault Paipa–Iza Fault

Las Peñas Fault Las Peñas Fault Paipa–Iza Fault Paipa–Iza Fault

Agua Tibia Fault Agua Tibia Fault 73° 09' W 73° 06' W 73° 09' W 73° 06' W

Resistivity c Sochagota 2200 masl Lake [Ω·m] Salpa <–1 1.5–2.3

Boyacá Fault El Hornito Fault 2.3–3.4 5° 45' N 3.4–5.1 5.1–7.7

El Batán Fault Buenavista Fault 7.7–11.6 11.6–17.3 Rancho Grande Fault 17.3–26.0 Canocas Fault 26.0–39.0 39.0–58.5 58.5–87.8 Paipa–Iza Fault

Lanceros Fault 87.8–132 132–198 El Tuno Fault El Bizcocho Fault Cerro Plateado Fault

Agua Tibia Fault 198–296 Santa Rita Fault 296–445 445–667 5° 42' N 667–1000 Paipa–Iza Fault 1000–1501 1501–2251 2251–3376 3376–5064 Las Peñas Fault Paipa–Iza Fault Agua Tibia Fault 5064–7597 >7597 Stations 73° 09' W 73° 06' W

Figure 15. Resistivity maps at (a) 2500, (b) 2350, and (c) 2200 masl (Franco, 2016).

180 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model Quaternary 73° 09' 00" W 73° 07' 30" W 73° 06' 00" W 73° 04' 30" W mGal

12.1 10.7 El Hornito Fault 9.2 5° 45' 00" N Boyacá Fault 7.9 6.8 6.0

El Batán Fault Buenavista Fault 5.3 4.6 Rancho Grande Fault 3.9

Canocas Fault 3.1 2.4

5° 43' 30" N 1.8 1.2 0.8 Lanceros Fault 0.3

Cerro Plateado Fault –0.2 –0.6 El Tuno Fault Santa Rita Fault –1.0 Paipa–Iza Fault Agua Tibia Fault –1.3 –1.7

5° 42' 00" N –2.1 –2.5 –2.9 –3.3 –3.7 –4.0 –4.4 Las Peñas Fault –4.8 Paipa–Iza Fault –5.2 5° 40' 30" N –5.7 –6.2 –6.8 –7.4 –8.1 Lava domes –8.8 –9.6

1000 0 1000 2000 3000 m –10.3 –12.3

Figure 16. Residual gravity anomaly map. The red color represents gravity highs, and the blue color represents gravity lows (Llanos et al., 2015).

To the south of the Paipa geothermal area, there is an- sif or igneous intrusions exposed at the surface north of the other gravity anomaly that could be associated with an in- geothermal area. trusive body that lacks surface expression. Westward, in The total magnetic intensity map (Figure 17; Llanos et El Durazno Intrusive, intermediate values of gravity are al., 2015) shows a magnetic high associated with the Paipa observed. To the northeast, a gravity high is possibly as- Volcano, which resembles the shape of the gravity anomaly sociated with metamorphic rocks from the Floresta Mas- and seems to also be controlled by Las Peñas and Santa

181 ALFARO–VALERO et al. 73° 09' 00" W 73° 07' 30" W 73° 06' 00" W 73° 04' 30" W nT

140.3 104.0 El Hornito Fault 90.1 5° 45' 00" N Boyacá Fault 81.9 74.5 68.1

El Batán Fault Buenavista Fault 62.1 56.0 Rancho Grande Fault 50.2

Canocas Fault 43.8 38.1

5° 43' 30" N 32.1 27.9 24.7 Lanceros Fault 22.4

Cerro Plateado Fault 20.1 18.1 El Tuno Fault Santa Rita Fault 16.1 Paipa–Iza Fault Agua Tibia Fault 14.1 11.9

5° 42' 00" N 9.9 7.7 5.8 3.7 1.6 –0.8 –3.1 Las Peñas Fault –5.6 Paipa–Iza Fault –8.0 5° 40' 30" N –10.7 –13.5 –16.7 –20.7 –26.7 Lava domes –36.5 –52.3

1000 0 1000 2000 3000 m –105.0 –393.9

Figure 17. Total intensity magnetic map (Llanos et al., 2015).

Rita Faults. An asymmetric magnetic dipole is associated 4.9. 3D Geological Model Constrained by with the alto Los Godos dome complex. The two highest Geophysics of Potential Fields magnetic anomalies, located in the northwest and east of the area, are interpreted as intrusive bodies (Llanos et al., A geological–geophysical 3D model was obtained using Geo- 2015). To the south, a small dipole possibly reflects a con- modeller inversion software. The input data comprise geolog- cealed intrusion. ical maps from Velandia (2003) and Cepeda & Pardo (2004),

182 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

cross sections drawn from the 1:25 000 geological map and The reported thickness of the Cretaceous to Quaternary Quaternary gravity and magnetometry grids. After the physical properties sedimentary sequence previously described is approximately (density and magnetic susceptibility) were assigned to each 1500 to 2000 m. The Devonian – Carboniferous sedimenta- geological formation, a joint stochastic inversion was execut- ry units may comprise almost 1500 m of sand and clay–rich ed at a voxel resolution of 100 m × 100 m × 100 m (Llanos layers. From this, a sedimentary cover of greater than 3000 m et al., 2015). thickness can be assumed, which is consistent with the depths The 3D geological model (Figure 18) contains the sedi- of conductive anomalies. Within the sedimentary units, a more mentary and metamorphic units, domes (intrusive bodies) and conductive layer is identified by resistivities between 0 and 3 volcanic deposits. The most notable features of the geological Ω·m. This high conductivity is interpreted as the contribution model, as constrained by inversion with potential geophysical of saline pore waters in permeable intervals, as observed on the data, are represented by the 3D density model (Figure 19). surface and in the top 1000 m (Figure 22c). The 3D density model reveals positive anomalies that can The geoelectrical response of the basement, shaped by be related to attributes of the Paipa geothermal system (Figure metamorphic rocks intruded by granitic bodies, is resistive, 19). Intrusion 1 and 2 underlie the alto Los Volcanes and El as shown in the volumetric representation of the resistivi- Durazno Intrusive, respectively. Intrusions 3 and 4 do not have ty presented in Figure 23a. It is located near the surface to surface expressions, seem to be related to the alto Los Godos the east and southeast and deeper to the west and northwest, dome complex and are separated from intrusion 1 by the Cerro where it is below the modeled depth (i.e., >4 km). The most Plateado Fault. Intrusions 5 and 6 could be related to the high notorious features are the resistive bodies (500–1000 Ω·m) density basement in the Tibasosa–Toledo Anticline or to other that correlate with the intrusions and high density anomalies blind magmatic intrusions (Alfaro et al., 2017). (Figure 19) corresponding to El Durazno Intrusive and to the dome zones (Figure 23b). The intrusive structure of El Duraz- 4.10. Magnetotelluric Model and Geological no forms an oval–shaped resistive anomaly that narrows with Interpretation depth. The extensive high resistivity structure developed beneath the alto Los Volcanes domes extends laterally towards Figure 20 shows the locations of five profiles defined for 2D the alto Los Godos domes (Figure 23b); in other words, the modeling using WinGLink geophysical interpretation software same magmatic intrusion fed the two dome complexes, which by Geosystem SRL, owned by the SGC (González–Idárraga & correlates with the lack of a high density anomaly underneath Rodríguez–Rodríguez, 2017) and 88 MT soundings used for alto Los Godos (Figure 19). To the east, other resistive struc- 3D modeling by means of WSINV3DMT (González–Idárraga tures with shapes and thicknesses similar to those previously & Rodríguez–Rodríguez, 2017). As can be observed in Figure described (see the miniatures in Figure 23b) could be due to 21, the 2D models and the same profiles from the 3D model intrusions with no superficial expression or to rocks from the visualized using Paraview software show contrasts between anticline’s nucleus. very high, intermediate, and very low resistivities, which grosso modo are comparable in the 2D and 3D models. The observed 4.11. Conceptual Model differences are due to the resolving power of 3D anomalies in 2D modeling (Ledo, 2005; Wannamaker, 1999) and due to the The main results of the exploration studies are summarized in projection of anomalies that could appear as “hanging” resistiv- Table 4 and Figure 24. ity bodies in the profiles, which are called phantom anomalies The geothermal system arises and is hosted in terrain that (Ledo, 2005). is tilted from south to north towards the Chicamocha River. Comparing the magnetotelluric 3D model with the density The recharge occurs at an elevation of 2900 m through the model and bearing in mind some considerations based on sur- sandstones of the Une Formation in the Tibasosa–Toledo Anti- face geology and geological cross sections, a geological inter- cline, which dip towards the west. Once beneath the valley, the pretation was achieved. Two geological domains are identified groundwater flows downwards, following deep fault structures as crystalline rocks of high density and high resistivity and sedi- (Paipa–Iza, Las Peñas, and Agua Tibia Faults) and the intersec- mentary rocks of comparatively low density and low resistivity. tions between them. The sedimentary cover, including volcano–sedimentary de- The northward flow through the basement possibly follows posits, has low to intermediate resistivities between 0 and 80 secondary permeability channels opened by hydrostatic and hy- Ω·m. This cover, presented in Figure 22 in volumetric view drodynamic pressure or the contacts along intrusions. Heat is generated using Paraview software, is much thicker in the west swept by the fluid as a result of the interaction with intrusions and northwest, where it reaches 3 to 4 km depth, probably due beneath the dome complexes’ zone. The heat is either related to to the greater accumulation and basin subsidence in the west, magmatic activity or is radioactively associated with the decay when compared to the thinner deposits in the east. of 238U, 232Th, and 40K. The geothermal fluid is fed by magmatic

183 ALFARO–VALERO et al.

73.14° W 5.75° N a 73.12° W 73.10° W

5.73° N 73.08° W

73.06° W 5.72° N 4000

5.70° N Faults 3000

5.68° N Agua Tibia 2000 Elevation (masl) Buenavista 5.66° N

Buenavista 1000 Canocas Cerro Plateado El Batán El Biscocho El Hornito El Tuno Lanceros Las Peñas Paipa–Iza Rancho Grande Santa Rita Boyacá

b 73.14° W 5.75° N 73.12° W 73.10° W 5.73° N 73.08° W

73.06° W 5.72° N 4000 Geological units 5.70° N 3000

Domes 5.68° N El Durazno 2000 5.66° N Quaternary Elevation (masl) 1000 Vulcanitas Tilatá Bogotá Guaduas Labor y Tierna Los Pinos Plaeners Conejo Churuvita

Une 5.68° N

5.66° N Tibasosa 73.10° W 73.08° W

Metamorphic 73.06° W Basement

Figure 18. Geological 3D model of the Paipa geothermal area. (a) View of the structural model. (b) View of the geological model (Llanos et al., 2015).

184 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

73.14° W Quaternary 5.75° N 73.12° W 5.73° N 73.10° W 73.08° W 5.72° N 73.06° W

5.70° N

5.68° N 4000

5.66° N 2 3000

2000 3 5 Elevation (masl) 1 1000

4 6

Density (g/cm)3 2.90 2.80 2.70 2.60 2.50 2.40 2.30 2.20 2.10 2.00 1.90 1.80

Figure 19. 3D density model of the Paipa geothermal area. The positive anomalies labeled with numbers are related to igneous intrusions (Alfaro et al., 2017).

3 4 CO2 and mantle He ( He/ He). Due to buoyancy forces, hot The resistivity structure suggests that the saline source orig- water rises along fault controlled conduits (such as the Cerro inates to the west due to the partial dissolution of evaporites Plateado Fault) or through porous sedimentary rocks, which hosted deep in the sedimentary sequence. The main fluid flow creates a sedimentary hosted reservoir. near the surface follows a NE corridor bounded by El Hornito That low density and intermediate resistivity sedimentary and Canocas Faults up to the vicinity of Sochagota Lake, near reservoir is hosted in the Une Formation between the two dome the northern border of the area, where it reaches the surface complexes. The resistivity related to the sedimentary reservoir through the Quaternary deposits. At depth, the saline water and its cap rock does not show a contrast with the sedimentary circulation extends laterally to the east, making possible the cover; this electrical response is normal in sedimentary geo- mixing process with the geothermal fluid beneath the surface. thermal systems (van Leeuwen, 2016). From this reservoir, the At shallow depths in the mentioned NE corridor, warm wa- hot water initiates the outflow to the discharge zone following ter feeds the ITA groundwater well and forms a surface thermal the Cerro Plateado Fault in a NW direction. Along its path, the anomaly. The low salinity of this well water indicates that the fluid discharges to the surface in a low permeability zone in hot water circuit is separated from the deeper saline water, probably springs and a steam vent of low flow rate through the Plaeners by sedimentary clay layers, which are common in most of the Formation (La Playa discharge sector). The impermeable clay sedimentary sequence. The water would increase its tempera- layers from Churuvita Formation and even from the Une For- ture by interaction with igneous intrusive bodies enriched in mation and the weathered pyroclastic deposits would prevent radiogenic elements, around El Durazno Intrusive or by interac- the hot water from flowing to the surface, becoming the cap tion with warm rocks in contact with the Une Formation, which rock of the system. When the hot water reaches the western lim- acts as the conduit of the fluid from the hot geothermal system, it of the Une Formation, it flows NNE up to the intersection of as previously described. El Hornito and El Bizcocho Faults to form the main discharge zone (ITP–Lanceros sector). 5. Conclusions The chemical and isotopic composition of the hot water is modified by the mixing with the saline pore waters and input The Paipa geothermal system was developed in sedimentary of thermogenic methane. stratigraphy of the Eastern Colombian Cordillera. Here, an iso-

185 ALFARO–VALERO et al.

A 73° 10' W 73° 09' W 73° 08' W 73° 07' W 73° 06' W 73° 05' W 73° 04' W

5° 46' N B

El Hornito Fault 5° 45' N

C Buenavista Fault Canocas Fault Rancho Grande Fault 5° 44' N

5° 43' N El Bizcocho Fault

Lanceros Fault

Cerro Plateado Fault Santa Rita Fault A’

B’

5° 41' N Agua Tibia Fault Las Peñas Fault

5° 40' N C’

E’ D’

Figure 20. MT survey area showing stations and the locations of 2D profiles. At each sounding, AMT and MT were measured for a range of frequencies of 10^4–0.01 Hz, approximately (Geological map from Cepeda & Pardo, 2004; Rueda, 2017; and Velandia, 2003). lated volcanic center has been active for the last 1 to 10 Ma, and to Quaternary in the west and northwest and, (2) in the central sodium sulfate–rich groundwaters circulate near the hydrother- northeast, the developed volcanic intrusive complex with sur- mal system and mix with the geothermal fluid. face expression defined by rhyolitic domes. The geological setting is divided into two domains: (1) the The main recharge zone occurs at elevations of 2800–2900 thick sedimentary sequence that ranges from the Cretaceous masl, coincident with outcrops of the Une Formation and the Ti-

186 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

2D modeling profiles 2D profiles from 3D modeling Quaternary 5000 5000 A A' A A' 4000 4000 PA0156 PA0064 PA0081 PA0084 PA0051 PA0052 PA0082 PA0058 PA0053 PA0083 PA0123 3000 PA0002 PA0012 3000

2000 2000

1000 1000 Elevation (m)

0 0

–1000 –1000

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 m 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 m

5000 5000 B B' B B' 4000 4000 PA0161 PA0071 PA0137 PA0098 PA0096 PA0145 PA0141 PA0091 PA0055 PA0140 PA0139 PAR401 PA0155 PA0006 PA0007 PA0057 PA0130 PA0132 PA0043 PA0008 PA0095 3000 PA0143 3000

2000 2000

1000 1000 Elevation (m) 0 0

–1000 –1000

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 m 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 m

5000 5000 C C' C C' 0

4000 3 4000 0 0 PA0026 A PA0092 PA0032 PA0036 PA0135 PA0038 P PA0094 PA0086 PA0028 PAR207 PA0148 PA0034 PA0017 PA0016 PA0005 PA0015 3000 PA0088 3000

2000 2000

1000 1000 Elevation (m) 0 0

–1000 –1000

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 m 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 m

5000 5000 D D' D D' 4000 4000 PA0089 PA0077 PA0136 PA0162 PA0075 PA0087 PA0076 PA0151 PA0078 PA0020 PA0150 PA0138 3000 PA0018 3000

2000 2000

1000 1000 Elevation (m) 0 0

–1000 –1000

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 m 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 m

5000 5000 3.0 E E' 1000 E E' 631 4000 398 4000 2.5 251 PA0159 PA0160 PA0126 PA0021 3000 PA0115 158 3000 100 2.0 63 2000 2000 40 1.5 25 1000 16 1000 10 Elevation (m) 1.0 0 6 0 4 3 –1000 –1000 0.5 2 Resistivity 1 Log(Resistivity [Ωm]) [Ωm] 0.0 0 1000 2000 3000 4000 5000 6000 7000 8000 m 0 1000 2000 3000 4000 5000 6000 7000 8000 m

Figure 21. 2D MT profiles. A conductive domain near the surface is observed in the northwest, and a deep resistive domain occurs in the east and southwest of the area from both 2D and 3D modeling (González–Idárraga & Rodríguez–Rodríguez, 2017).

187 ALFARO–VALERO et al.

a Latitude Longitude Log (resistivity [Ωm])

3.00

2000 2.67

2.33

2.00 1000 1.670

1.33

1.00 0 0.667 Elevation (m) 0.333

–1000 0.00 Longitude 73.06° W Latitude 73.10° W 73.08° W 73.14° W 73.12° W 5.75° N 5.77° N 73.16° W 5.66° N 5.68° N 5.70° N 5.72° N 5.73° N

b c Log (resistivity Log (resistivity 73.08° W [Ωm]) 73.12° W 73.08° W [Ωm]) 73.16° W 73.12° W 73.16° W 2000 3.00 2000 3.00

2.67 2.67 0 2.33 0 2.33 Elevation (m)

Elevation (m) 5.77° N 5.77° N 2.00 2.00

1.670 1.670 Latitude Latitude 1.33 1.33

1.00 1.00

5.68° N Longitude 0.667 5.68° N Longitude 0.667 0.333 0.333

0.00 0.00

Figure 22. Volumetric view of the sedimentary cover with resistivity values that range between 0 and 80 Ω·m, (a) is the depth view and (b) the top view. This layer is thicker in the west and northwest. (c) is the top view of a layer in the sedimentary rocks that has high conductivity. This low resistivity (0–3 Ω·m) is probably due to the saline fluid circulation. basosa–Toledo Anticline. The Une Formation provides permea- Figure 23. Volumetric view of the basement based on resistivity. bility that influences fluid flow through the hydrothermal system. (a) Metamorphic rocks and magmatic intrusions confined from 80 Deep high angle faults (Paipa–Iza, Las Peñas, Agua Tibia, to 1000 Ω·m. (b) Intrusive rocks range from 500 to 1000 Ω·m. and Cerro Plateado Faults) and their intersections plus the contact zones between igneous intrusions and surrounding rocks represent controls that influence deep fluid circulation. contribution of organic components to the geothermal fluids. 13 Clay layers from the Churuvita or Une Formations and from The input of magmatic/mantle gases is confirmed by the δ CCO2 weathered pyroclastic deposits have low permeability and con- and 3He/4He ratios. fine fluid flow within the sedimentary reservoir. The origin of the sodium sulfate saline water is the dissolu- The magmatic heat source is located beneath the volcanic tion of a deep evaporite in the sedimentary sequence from the dome complexes and is associated with cooling and/or radio- west and northwest sectors. genic heat production. In El Durazno Intrusive, the heat pro- Secondary minerals, including epidote, adularia, albite, and duction is estimated from boreholes to be 12 μW/m3, which biotite plus local occurrences of dickite reflect water–rock inter- could heat the water circulating through the northwest area, action between 200 °C and ca. 320 °C. Although aqueous and where a groundwater well discharges low salinity water at gas geothermometers are highly affected by mixing with fluids 34.8 °C. Although the dimensions of this intrusion have of different origins, a simple enthalpy–silica model suggests a not been determined, the resistivity model shows that it is current resource temperature of ca. 230 °C. large at depth. The gravity and magnetotelluric results show that igneous The chemical and isotopic composition of the hot springs intrusions and metamorphic rocks in the basement are related to is not representative of the geothermal fluid due to mixing with high density and high resistivity anomalies. The location of an saline low temperature water. The existence of coal deposits igneous intrusion below the alto Los Volcanes appears to extend 13 and hydrocarbon seeps as well as the δ CCH4 values indicate laterally northeast, giving rise to the alto Los Godos volcanic

188 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

73.16° W 73.14° W 73.12° W 73.10° W 73.08° W 73.07° W a 5.77 ° N 2000 Quaternary

5.68 ° N 0 Elevation (m)

Log (resistivity [Ωm])

3.00

2.67

2.33

2.00

1.670

1.33

1.00

0.667

0.333

0.00

73.08° W 73.07° W b 73.12° W 73.10° W 5.77 ° N 73.16° W 73.14° W

2000

Elevation (m) 5.68 ° N

Log (resistivity [Ωm])

3.00

2.67

2.33

2.00

1.670

1.33

1.00

0.667 0.333 El Durazno Intrusive Alto Los Volcanes and alto NE intrusive 0.00 Los Godos dome complexes

189 ALFARO–VALERO et al.

Table 4. Summary of evidence supporting the conceptual model.

Geothermal system Evidences/signs Source of information features Terrain tilted from north to south Topography–geology Hydraulic gradient between Tibasosa–Toledo Anticline and the valley Topography–geology Regional recharge Sandstones outcrops from the Une Formation on the anticline Geology zone Dipping strata from the Une Formation on the anticline Structural geology Composition of the recharge water (ca. 2900 masl) Isotope hydrology Hot magmatic intrusions from residual magmatic heat Geology Radiogenic heat Radioactive isotope geochemistry Heat source Magnetic survey, magnetotellurics, geology, Possible recent intrusions without surface expression and isotopic helium geochemistry Contact between magmatic intrusions and surrounding rocks Geology, magnetotellurics Upflow zone Vertical faults Structural geology Deep reservoir lo- Basement xenoliths Hydrothermal alteration cation Secondary minerals (up to 320 °C) Hydrothermal alteration Reservoir temperature Enthalpy–silica model (up to 230 °C) Fluid geochemistry Sedimentary reservoir Availability of sandstones from the Une Formation between high density and high Geology, geological model, magnetotellurics location resistivity dome complexes Deep faults: Paipa–Iza, Las Peñas, and Aguatibia Faults; and intersection between Structural geology Deep infiltration them conduits Contact between magmatic intrusions and surrounding rocks Geology

Outflow (lateral circu- Lateral extension of the Une formation Geology and geological model lation) Occurrence of a discharge zone of limited permeability (very low flow rate) Geology and geochemistry Clayish levels within the sedimentary sequence (Churuvita and Une Formations) Geology Cap rock Weathered pyroclastic deposits Geology Hot springs sites Geology and geochemistry Discharge zone Steam vent Geology and geochemistry Affected by mixing with a saline source (sodium sulfate) Geochemistry Geochemistry, isotopic gas geochemistry, Affected by mixing with organic carbon Geothermal fluid geology (coal mantles and seeps) composition Contribution of magmatic CO2 Isotopic gas geochemistry Contribution of a high 3He/4He source Isotopic gas geochemistry

dome complexes. In the middle of the geothermal area, a low The characteristic resistivity of the intrusions suggests that density and intermediate resistivity volume surrounded by high they are mostly crystallized down to 4 km. However, the mag- 3 4 density and high resistivity anomalies represents the sedimen- matic CO2 source and He/ He ratio indicate a current mag- tary reservoir hosted by the Une Formation. matic/mantle contribution.

Figure 24. Conceptual model of the Paipa geothermal system. The main recharge zone is located in the western flank of the Tibasosa– Toledo Anticline, in the area where the Une Formation crops out. The deep circulation through high angle faults as well as the contacts of the magmatic intrusions, enriched in radioactive elements, causes the water temperature to increase. The upflow zone feeds the relatively shallow reservoir in the Une Formation. The outflow from this reservoir is favored by the Cerro Plateado Fault. The Une For- mation provides permeability and fluid flow to the north until the main discharge zone (ITP–Lanceros sector) where the high vertical permeability conditions reach the surface. Saline water is mixed with the geothermal fluid along the outflow, masking its chemical and isotope composition.

190 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model

Sochagota Lake Sochagota Lake Upﬂow Recharge zone Quaternary

Outﬂow Saline source input

El Durazno El Durazno Metamorphic Intrusive Intrusive Seal cap basement

Alto Los Godos Alto Los Godos domes domes Cerro Plateado Fault

Alto Los Volcanes Alto Los Volcanes Intrusive geoforms domes domes supposed at depth

Resistive intrusion modeled

Density map at a 2000 masl Geological map

Sochagota Lake ITP–Lanceros hot springs ITA well 30.2–70 °C El Durazno 33.8 °C Intrusive U238 La Playa hot springs 53.5–76 °C Alto Los Godos domes U238

238 238 U U U238 2000 238 U ? 1000 ? 0

–1000 ? U238

–2000 ? 75 °C

–3000

–4000 ? ? –5000

–6000

–7000 ? –8000 250 °C ? –9000

–10000

350 °C

191 ALFARO–VALERO et al.

Acknowledgments Beardsmore, G.R. & Cull, J.P. 2001. Crustal heat flow: A guide to measurement and modelling. Cambridge University The authors express a very special thanks to the reviewers for Press, 324 p. Cambridge, USA. https://doi.org/10.1017/ the hard work undertaken in the careful revision of the manu- CBO9780511606021 script of this chapter. Thanks to the professionals that during the Beltrán, M.A. 2015. Interpretación de anomalías magnetométricas y development of this study took part of the Grupo de Exploración gravimétricas en el área geotérmica de Paipa–Iza. Servicio de Recursos Geotérmicos: Francisco, Wilson, Luis Eduardo, Geológico Colombiano, 123 p. Bogotá. José Vicente, Natalia, Lina Fernanda, Héctor, María Teresa, Jai- Bernet, M., Urueña, C., Amaya, S. & Peña, M.L. 2016. New thermo me, Edwin, Iván Darío, María Luisa, and Javier. Thanks to the and geochronological constraints on the Pliocene – Pleistocene chemists and geologists from the laboratories of the SGC, who eruption history of the Paipa–Iza Volcanic Complex, Eastern participated in the analytical work. Additionally, thanks to the Cordillera, Colombia. Journal of Volcanology and Geother- experts who along several years guided us and provided techni- mal Research, 327: 299–309. https://doi.org/10.1016/j.jvol- cal contributions to this study, particularly regarding geophysical geores.2016.08.013 methods and to the participants in an international panel for their Bertrami, R., Camacho, A., De Stefanis, L., Medina, T. & Zuppi, G.M. comments, suggestions, and recommendations on the conceptual 1992. Geochemical and isotopic exploration of the geothermal model. Finally, thanks to the local community for allowing us to area of Paipa, cordillera Oriental, Colombia. Geothermal in- learn about the fascinating Paipa geothermal system. vestigations with isotope and geochemical techniques in Latin America. Proceedings of a Final Research Coordination Meet- References ing held in San José, Costa Rica. International Atomic Energy Agency, p. 169–199. Viena. Alfaro, C. 2002a. Geoquímica del sistema geotérmico de Paipa. In- Boussingault, M. & Roulin. 1849. Viajes científicos a los Andes geominas, 97 p. Bogotá. ecuatoriales ó colección de memorias sobre física, química e Alfaro, C. 2002b. Estudio isotópico de aguas del área geotérmica de historia natural de la Nueva Granada, Ecuador y Venezuela. Paipa. Ingeominas, 15 p. Bogotá. Lasserre Editorial, 322 p. Paris, France. Alfaro, C. 2005a. Alteración hidrotermal en el sistema geotérmico de Cepeda, H. & Pardo, N. 2004. Vulcanismo de Paipa. Ingeominas, 103 Paipa. Ingeominas, 53 p. Bogotá. p. Bogotá. Alfaro, C. 2005b. 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Explanation of Acronyms, Abbreviations, and Symbols:

AMT Audio–magnetotellurics TDS Total dissolved solids ITP Instituto de Turismo de Paipa VAG Volcanic arc granites MT Magnetotellurics VES Vertical electrical soundings SGC Servicio Geológico Colombiano XRD X–ray diffraction

194 Paipa Geothermal System, Boyacá: Review of Exploration Studies and Conceptual Model Quaternary

Jhon Camilo MATIZ–LEÓN3 has a BS in cadastral and geodetic engineer, and a MS in information sciences and geomatics from Uni- 7 2 versidad Distrital Francisco José de Caldas. He previously worked as a 4 3 remote sensing researcher for the executive secretary of the Comisión 8 1 6 5 Colombiana del Espacio (CCE), under the responsibility of Instituto Geográfico Agustín Codazzi (IGAC), and has served as technical co- ordinator and zone leader for private companies in projects of digital Authors’ Biographical Notes mapping, geographic information systems (GIS), and satellite image processing for earth observation. He is currently working as a research- Claudia María ALFARO–VALERO1 has a BS in chemistry, Univer- er in 3D modeling, in litho–constrained geophysical inversion and geo- sidad Nacional de Colombia, and a postgraduate in geothermal tech- matics for the Grupo de Exploración de Recursos Geotérmicos of the nology, from the University of Auckland, New Zealand. Her research Servicio Geológico Colombiano. focuses on the geochemistry of hydrothermal and volcanic fluids and on geothermal exploration. She currently works as chief of the Grupo Miguel Angel BELTRÁN–LUQUE4 has BS in geologist and a MS in de Exploración de Recursos Geotérmicos. geophysics from Universidad Nacional de Colombia with experience in the mining industry and in geologic exploration in areas such as the Jesús Bernardo RUEDA–GUTIÉRREZ2 has a BS geologist, Uni- northern Cauca valley, northern Colombia ultramafic bodies (part of versidad Industrial de Santander, with experience in geological map- the Western Cordillera), the Middle Magdalena Valley, and the Central ping, research on mineral exploration, volcanology, structural geology, and Eastern Cordilleras. He currently works at the Servicio Geológico and lithogeochemistry. He currently works as a geologist in the Grupo Colombiano in the Grupo de Exploración de Recursos Geotérmicos de Exploración de Recursos Geotérmicos of the Servicio Geológico processing magnetic/gravimetric data from geothermal areas and un- Colombiano. dertaking active gravity and magnetic acquisitions in the San Diego geothermal area.

195 ALFARO–VALERO et al.

Gilbert Fabián RODRÍGUEZ–RODRÍGUEZ5 has BS in cadastral Carlos Eduardo GONZÁLEZ–IDÁRRAGA7 has a BS in physical and geodetic engineer from Universidad Distrital Francisco José de engineer from Universidad Nacional de Colombia and a MS in Earth Caldas of Bogotá. He currently works in the Grupo de Exploración sciences from Institute of Geophysics of the Universidad Nacional de Recursos Geotérmicos of the Servicio Geológico Colombiano as Autónoma de México (UNAM), emphasizing the exploration of natu- a researcher in geophysical sciences, with emphasis in shallow tem- ral resources employing electromagnetic methods. He currently works perature surveys, magnetotelluric and TDEM surveys, and vertical as a geophysicist in the Grupo de Exploración de Recursos Geotér- electrical soundings. micos of the Servicio Geológico Colombiano in the acquisition, processing, and interpretation of resistive models from magnetotelluric Gina Z. RODRÍGUEZ–OSPINA6 has a BS in geologist from Uni- and TEM data. versidad Industrial de Santander, with experience in well logging, in- ventories of thermal springs, and cartography. She currently works in Jaison Elías MALO–LÁZARO8 has a BS in chemistry from Univer- the Grupo de Exploración de Recursos Geotérmicos of the Servicio sidad de Córdoba and a specialization degree in instrumental chemical Geológico Colombiano in the acquisition, processing, and interpreta- analysis at the Pontificia Universidad Javeriana. He currently works tion of structural data integrated into the geological mapping of geo- in the Grupo de Exploración de Recursos Geotérmicos of the Servi- thermal areas. cio Geológico Colombiano, supporting studies of fluid geochemistry, measurements of radon gas, and isotopy.

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