GEOCHEMICAL DATA INTERPRETATION OF THERMAL SPRINGS IN COLOMBIA

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1 PROCEEDINGS, Thirty-First Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 3-February, 26 SGP-TR-79 GEOCHEMICAL DATA INTERPRETATION OF THERMAL SPRINGS IN COLOMBIA A. Aguirre & R. M. Barragán R. 2 Geocol Consultores Ltda. Carrera 4 No. 9-3, Oficina 74 Bogota, Colombia. alcidesaguirre@yahoo.com.ar 2 Instituto de Investigaciones Eléctricas, Gerencia de Geotermia, Reforma 3, Col. Palmira Cuernavaca, Morelos, C.P. 6249, México rmb@iie.org.mx ABSTRACT A preliminary geochemical data interpretation of, and the N-NE area of Province (Republic of Colombia) geothermal systems, is presented. The geothermal area is very dry and strong evaporation process for samples has been recognized. Thermal waters from were classified as sodium-sulphate type waters with ph between 6 and 7.4 and a reservoir temperature of 75 C was estimated. Isotopic results for relate the samples to primary magmatic waters. Samples from are classified as sodium chloride and magnesium chloride type waters with ph between 6.2 and 6.7, and isotopic results relate them to andesitic waters. The reservoir temperature was estimated around 22 C. The N-NE area of Province is characterized by thermal springs emerging from sedimentary rocks, which are related to three types of water: Sodium bicarbonate, sodium chloride and calcium bicarbonate, with ph between 6.6 and 8.. The reservoir temperature varies from low to moderate (4 C). Most of the samples from Province show high content of meteoric waters. INTRODUCTION Preliminary geochemical studies on geothermal resources in Colombia were developed by OLADE, (982). Results showed that and were the most interesting areas. As more data has become available, three areas have been identified by INGEOMINAS as promissory to carry out exploration (Figure ): ) Geothermal System that shows potential for electric energy generation in the Nariño Province and was classified as a medium to high enthalpy system (Alfaro, 2a); 2) The N-NE area of Province, which offers an alternative for direct uses; and 3) The - Iza complex area which is under evaluation to define its suitable use. In this area there is shallow sodium sulphate deposit, which masks the geochemical interpretation of fluids, precipitating sodium sulphates (Alfaro, 2b). Figure.. Location map of and geothermal systems, and Province in Colombia, South America. The objective of this work is to estimate geochemical features of the mentioned systems through interpretation of chemical and isotopic composition of springs. Data were provided by INGEOMINAS (Colombian Geological Survey). Samples were collected during between 22 and 23. GEOLOGICAL SETTING Colombia is located on the confluence of three tectonic plates, where Nazca and Caribbean plates are subducting under the South American one. In

2 Colombia, the Andes are divided in three cordilleras: Western, Central and Eastern. Geothemal System is located in the South of the Western Cordillera, whereas Geothermal System and Province are located in the meridional zone of the Eastern Cordillera, which has a wide deformation zone, as a result of the interaction among the plates (Sarmiento, 2). According to Calvache et al., (2), is a strata volcano located in an area, which is constituted by tectonic blocks of metasedimentary and metavolcanic rocks. In addition, there are interlayered basaltic rocks and Cretaceous marine sedimentary rocks. All of this sequence is intruded by Paleogene rocks and covered by Neogene explosive and effusive rocks. The Eastern Cordillera is characterized by sedimentary rocks, which have a range of age between Precambrian and Recent. Some places of this cordillera show volcanism as in the case of and Iza, where the sedimentary sequence is intruded by Neogene rocks. Acosta et al., (23), suggests that and Iza show the same geological environment, except by the saline deposit affecting. The distance between their volcanic centers is about 7 km and they would be connected by a regional fault. These rocks are under study to be classified. The same authors consider these magmatic rocks are unsaturated in silica and rich in alkali and foids (foidtrachyt to phonolite). RESULTS Thermal spring sampling was undertaken by INGEOMINAS, in the three areas; to obtain a physical-chemical and isotopic characterization. from (SO, EZ and PC) are located on the area corresponding to mature waters. Most of the thermal springs are close to volcanic waters, though their ph is almost neutral. A majority of samples from and are superficial high bicarbonate waters. But, QB sample from is found close to the mature waters region. Thermal Springs VOLCANIC WATERS SW Seawater Cl SO XBP BA VP MO EE SA V2 SM PC LV LL STEAM HEATED WATERS LC SO R 4 O ACN V HCO3 EZ QB LP PC 5 E2 TE T MAPT PERIPHERAL WATERS Figure. 2. Relative Cl, SO 4 and HCO 3 contents of thermal waters (after Giggenbach, 988). X represents the following thermal springs from : PA, PI, PB, MI, HL, OD, P, P2, ML, PM and PS. Figure 3 shows a Schoeller diagram (after Truesdell, 992) for the samples. According to the trends observed, waters show the same pattern of behaviour which means that they result from mixing or evaporation. hot springs are characterized by high concentration of solutes. Two springs from Iza (BI and E2) show almost the same tendency observed for between them, but sulphate concentration is very low compared with hot springs. SR 25 Water Classification Fluids were classified, according to Piper (944), by using the program CLASIF.FOR (Mercado et al., 98). The springs from are mainly sodiumsulphate waters, but some of them are sodiumbicarbonate, sodium-chloride and calcium-sulphate waters. Most of the samples have neutral to slightly acid ph (6. to 7.4). springs were classified as sodiumbicarbonate, sodium-chloride and calciumbicarbonate waters and the ph ranges between 6.6 and 8.. springs are magnesium-chloride and sodium-chloride. samples show homogeneous ph (6.2 to 6.7). LOG CONCENTRACIÓN (meq/kg) BP PA,PI,MI,HL,OD,PB,ML,PM,PS P and P2 O PC and EE SA BI (Iza) E2 (Iza) LP Li Na K Mg Ca F Cl HCO3 SO4 B Figure 2 shows the relative Cl-HCO 3- SO 4 composition for the spring samples on a triangular plot. As it is seen in the diagram only few samples Figure. 3. Schoeller diagram for spring samples from Geothermal System.

3 Figure 4 shows a Schoeller diagram for system. As it is seen, waters have the same tendency (among them), which reflects a dilution process. Mg and Ca concentrations are very high revealing a great contribution of meteoric waters. (Fournier and Potter II, 979). The CCG is very useful to determine whether a groundwater may be of geothermal origin, since it has four expressions to be used according to the relative Na,K,Ca and Mg contents of samples. LOG CONCENTRACIÓN (meq/kg) T (Tercán ) T2 (Tercán 2) QB (Qda Blanca) LV (Laguna Verde) MA (Malaver) LC (La Cabaña) SR (San Ramón) TE (Tenguetán) Li Na K Mg Ca F Cl HCO3 SO4 B Figure. 4. Schoeller diagram for spring samples from geothermal field. Figure 5 shows a Schoeller diagram for. As it is seen, the spring waters have similar behaviour and the main process is again dilution. However, PC, PT and VP show a small difference with respect to the others because of lower SO 4 concentration. LOG CONCENTRACIÓN (meq/kg) N (Nápoles) V (Los Volcanes, Machetá) AC (Agua Clara) R (Repetidora) PC (El Paraíso CODECAL) PT (El Paraíso Termal) VP (Vereda Peñas) EZ (El Zipa) AT (Aguas Calientes, Tabio) BA (Bavaria) LL (Los Lagartos) SO (Soratama) MO (Montecillo) V2 (Los Volcanes, Choachí) SM (Santa Mónica) Li Na K Mg Ca F Cl HCO3 SO4 B Reservoir Temperature Estimation Silica geothermometers (Fournier and Potter II (982); Giggenbach (99); and Fournier (973)) were calculated and their results are given in Table. According to silica geothermometers reservoir temperatures range from very low to moderate (from 28.3 to 84.7 C). The maximum temperature was calculated for the sample T from, with 84.7 C for quartz, assuming no-steam loss. For the system, the hottest temperatures are related to the sample T with around 85 C. For Province, low reservoir temperatures were calculated, and the highest ( C) is related to the sample PT. For system the highest temperature of 36.4 C corresponds to sample O; while for BP sample, a moderate temperature (24 C) was obtained in spite of its sampling temperature (75.5 C). Cation geothermometers were calculated as follows: the Cationic Composition Geothermometer (TCCG) (Nieva and Nieva, 987); Na/K (F) (Fournier, 979); Na-K-Ca (Fournier and Truesdell, 973); Na/K (G) and K/Mg (Giggenbach, 988) and Mg correction Figure. 5. Schoeller diagram for spring samples from Province. Some samples show very high values for Na/K (F) and Na/K (G), mainly (see Figure 6 for Na/K (G)). However, these values are not reflected in other estimations as those for the Na-K-Ca with Mg correction geothermometer. In order to choose the better temperature estimations, consistency among geothermometers should exist. Thus, the outstanding reservoir temperatures belong to samples: BP and E2 in, with temperature of 228 C and 222 C, respectively; N in with a temperature of C and LV in with a temperature of 2.9 C. Figure 6 shows a Na-K-Ca-Mg plot (Giggenbach, 988) for the samples. It is used to obtain detailed information on possible processes affecting waters. According to this diagram, samples VP, SO, EZ, PC and PT from Province, are plotted on or close to the full equilibrium line indicating temperatures between 2 and 34 C. Samples for

4 system show very high reservoir temperature, over 34 C. With regard to system, samples BP, PA and PB display reservoir temperatures in the range of 28 to 35 C (t K/Na ). Samples O and SA from, and R, N, AC, V, N, and MO from are affected by dissolution of crustal rocks, according to Giggenbach (988). Table. Reservoir temperatures estimated by silica and cation geothermometers..8.6 Na.4 4 sea water 8 K-uptake in clays Mg O Thermal Springs t Mg/Ca R V AC SA MO N BI LV E2 LLBP PB PA SM PM BA V2 EE LP LC Rock T MA QB TE K No. T (nosteam loss) T spring disch. (G) TCCG Na/K (F) CORR. Mg BP PA PI MI HL OD P O ML PM PS EE SA E BI N V AC R PC NA 68 PT NA 7 VP NA 68 EZ NA 84 AT NA 82 BA LL SO NA 63 MO V SM T TR QB LV MA TR LC SR TR TE TR Full Equilibrium t K/Na 2 PT.2 PC 6 EZ VP SO.2.4 Ca.6.8 Figure. 6. Na-K-Mg-Ca equilibration temperatures. For symbols see Table. As shown in Figure 7, a mixing trend towards the range 22 to 24 C (T K/N and T K/M) is noticed among the samples from. Most of the hot samples from were affected by evaporation, but they keep the same proportion among ions. Also, there is another mixing trend towards 22 C, which corresponds to the samples. As most of the samples belong to several geothermal systems, low-temperatures, below 8 C, were estimated for all. Thermal Springs 24 PM 26 X Na/ full equilibrium line BP partially equilibrated or mixed PO PO T K/N T K/M 8SO 2 EZ E2 PC immature waters PT BI K/ O Mg Figure. 7. Na-K-Mg relative composition (Giggenbach, 988) of thermal springs. Mixing Models Figure 8 is a plot of SiO 2 vs. enthalpy showing the quartz solubility curve (Fournier and Potter II, 982) and points correspond to and samples. As is seen in the figure, the samples from seem to be constituted of a mixing of hot water at around 8 C with groundwaters. Samples from seem to result from a mixing of a boiled liquid with a 6 SW 4

5 silica concentration of 425 mg/kg that comes from an original fluid at around 22 C. After boiling, the hot member is mixed with local groundwater to produce the samples composition. SiO2 (mg/kg) ABRW Quartz solubility curve PRW ARW Enthalpy (J/g) V 3 Figure. 8. The silica vs. enthalpy mixing model for geothermal waters from, and areas. The points labeled ARW and PWR represent the reservoir water for and (22 and 74 C), respectively. While ABRW represents the boiled reservoir water for. V represents the point corresponding to separated steam. Both silica and chloride concentrations, and enthalpy of the springs were used to obtain silica and chloride concentrations for the hot member of Geothermal System. Reservoir temperatures were taken as 22 C for and 75 C for. LC sample was selected as the cold member for and SA for. Figure 9 is an enthalpy vs. chloride plot for the samples. According to the figure, shows a very marked evaporation process, because of the trend with negative slope observed for all the samples, which is seen by the high concentration of chloride but the low enthalpy of samples. The chloride concentration for the reservoir of is 8,48 mg/kg and for reservoir is 2846 mg/kg. As can be seen in the figures 8 and 9, there is correspondence with respect to boiled reservoir water for (ABRW) in both diagrams. In addition, Figure 9 shows an evaporation process, in which some of the coldest samples (MA and TE) present higher chloride concentration. Also, the mixing process can be seen. Isotopic Results Isotopic results (δ 8 O and δd in permil SMOW) of spring samples from, and the northeast part of Province are shown in the Figure on a δd vs. δ 8 O plot. Samples nonaffected by evaporation were selected for plotting on the diagram. The hot members for and were obtained starting from mass fraction and heat and mass balance equation (Barragán et al., 999) and those values were plotted on the diagram (Figure ). According to this diagram, all the samples have a high content of meteoric waters, which is seen by their proximity to the Global Meteoric Water Line; Geothermal System is associated with andesitic volcanism; while system corresponds to primary magmatic waters. Enthalpy (J/g) BP V ARW ABRW Thermal Springs Lines for Thermal Springs Lines for PRW MA TE SA PM Chloride (mg/kg) Figure. 9. The enthalpy vs. chloride mixing model for geothermal waters from and areas. DISCUSSION AND CONCLUSIONS The geothermal area of is very dry and close to the surface, it is affected by a sodium sulphate deposit (Alfaro, 2b), which turns into a difficult geothermal system. Due to this, most of the waters are sodium-sulphate. It was recognized that a strong evaporation process took place before most of the samples were collected. In such samples an increase in chemical species (Na, K, Cl, SO 4, HCO 3, Li, Deuterium and Oxygen-8, etc.) occurred while sodium sulphate minerals were deposited in the sampling sites. springs seem to be constituted by mixing of two end members the hot one (sample BP) with a temperature of 75 C, obtained from the silicaenthalpy mixing model, or higher, around 228 C, obtained from TCCG geothermometer, and a cold

6 member. Sample BP was selected to find the hot member, as it is the least affected by evaporation. The estimated composition for the hot member is as follows: J/g for the enthalpy, 848 mg/kg for chloride and 87.9 mg/kg for SiO 2. The springs from and Iza have a similar behavior, as is seen in the Schoeller diagram; and also, the feasible connection between these two similar volcanic areas by means of a regional fault, might involve the possibility of similar reservoir conditions for both. Isotopic results for Geothermal System relate the samples to primary magmatic waters, though they emerge from sedimentary rocks. δd ( vs SMOW) springs hot member springs hot member Xa Xpm springs GMWL andesitic waters (Giggenbach, 992) Xa primary magmatic waters (Taylor, 974) Xpm δ8ο ( vs SMOW) Figure.. Deuterium vs. oxygen-8 plot of spring waters from, and areas. Xa and Xpm lines represent fractions of andesitic and magmatic vapors (respectively) possibly contributing to the formation of the thermal waters. Samples from Geothermal System are classified as sodium chloride and magnesium chloride with ph between 6.2 and 6.7. However, all the samples contain a high meteoric component, show similar behavior pattern in Schoeller diagram and isotopic results relate them to andesitic waters. The estimated composition for the hot member is as follows: J/g for the enthalpy, 2846 mg/kg for chloride and 337 mg/kg for SiO 2. Although slightly, some springs are affected by evaporation (TE and MA, mainly). Geothermometry results indicate homogeneous temperatures and the reservoir temperature was estimated 22 C. The N-NE area of Province is characterized by thermal springs emerging from sedimentary rocks, which are related to three types of water: Sodium bicarbonate, sodium chloride and calcium bicarbonate; all of them with similar behavior pattern on the Schoeller diagram. According to geothermometry, the reservoir temperatures vary from low to moderate. It seems that samples located in Tibirita (PC, PT and VP) and Tabio (EZ and AT) show partial equilibrium, according to cationic geothermometers, though their estimated temperatures are moderate (around 4 C). Soratama sample (SO) is a sodium chloride sample, it shows the highest concentrations of chemical species and it is close to the full equilibrium line at relatively low temperature (8 C). According to isotopic results, and Na-K-Mg and Cl-SO 4 -HCO 3 triangular diagrams, most of the samples from Province have high content of meteoric waters. ACKNOWLEDGEMENTS This study was part of the training course on geochemical and isotopic interpretation, which was supported by IAEA. Authors acknowledge the INGEOMINAS for providing data information and allowing to be published. Also, thanks are given to the IIE, especially to the Geothermal Unit staff for the valuable support. REFERENCES Acosta, J., Alfaro, C., Bernal, N. F., Cepeda, H., and Velandia, F. J. (23), Sistema geotérmico de, Boyaca: Modelo conceptual preliminar. INGEOMINAS, in progress, Bogotá. Alfaro, C., (2a.), Geoquímica del sistema geotérmico del Volcán. INGEOMINAS, Internal report, Bogota, 48 pp. Alfaro, C., (2b), Geoquímica de las fuentes termales del área de -Iza. INGEOMINAS, internal report, Bogota, 6 pp. Barragán, R. M., Arellano, V. M., Birkle, P., Portugal, E., and Díaz, G. (999), Chemical description of spring waters from the Tutupaca and Río Calientes (Perú) geothermal zones. International Journal of Energy Research., 23, Calvache, M., Cepeda, H., Mosalve, M. L., Torres, M. P., Cortés, G. P., and Bernal, N. F. (2), Cartografía geológica del Volcán de Túquerres, Nariño, Escala :25. V.. Memoria Explicativa, internal report. INGEOMINAS, Bogotá, 6 pp. Fournier, R.O. (973), Silica in thermal waters: Laboratory and field investigations. In: Proceedings,

7 International Symposium on Hydrogeochemistry and Biogeochemistry, Tokyo, 97, Vol, Hydrogeochemistry, Washington, D. C., Clark, pp Fournier, R.O. (979), A revised equation for NA/K geothermometer, Geoterm. Resourc. Counc. Trans., 3, Fournier, R.O. and Potter II, R.W. (979), Magnesium correction to the Na-K-Ca geothermometer for natural waters. Geochim. Cosmochim. Acta, 43, Fournier, R.O. and Potter Ii, R.W. (982), A revised and expanded silica quartz geothermometer. Geotherm. Resourc. Council Bull., (), 3-2. Fournier R.O. and Truesdell A. H. (973), An empirical Na-K-Ca geothermometer for natural waters. Geochim. Cosmochim. Acta, 37, Giggenbach, W. F. (988), Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators. Geochim. Cosmochim. Acta, 52, Piper, A. M. (944), A graphic procedure in the geochemical interpretation of water analyses. American Geophysical Union, Transactions 25, Sarmiento, L. F. (2), Mesozoic rifting and Cenozoic basin inversion history of the Eastern Cordillera, Colombian Andes: Inferences from tectonic models. Netherlands Research School of Sedimentary Geology (NSG), publication no pp. Taylor, H. P. (974), The application of oxygen and hydrogen isotope studies to problems of hydrothermal alteration and ore deposition. In: Econ. Geol., 69, Truesdell, A. H. (992), Effects of physical processes on geothermal fluids. In: D Amore, F. (Coordinator), Application of Geochemistry in Geothermal Reservoir Development, UNITAR/UNDP, Rome, pp Giggenbach, W. F. (99), Chemical techniques in geothermal exploration. In: Application of Geochemistry in Geothermal Reservoir Development, D Amore, F. (Coordinator)., UNITAR/UNDP, Rome, pp Giggenbach, W. F. (992), Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin. In: Earth and Planetary Science Letters, 3, pp Mercado, G. S., Nieva, G. D., and Barragán, R. M. (98), Interpretación geoquímica de zonas de alteración hidrotermal de la República de Colombia. Report IIE/3662/FE-G25/4, Instituto de Investigaciones Eléctricas, Temixco, Mor., for OLADE, p. 6. Nieva, D. and Nieva, R. (987), Developments of geothermal energy in Mexico, part twelve-a cationic composition geothermometer for prospection of geothermal resources. Heat Recovery Systems and CHP, 7, Olade, Icel, Geotérmica Italiana and Contecol. (982), Estudio de reconocimiento de los recursos geotérmicos de la República de Colombia. Informe final, Pisa, 455 pp.