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GRC Transactions, Vol. 35, 2011

Surface Exploration at Pampa Lirima Geothermal Project, Central of Northern

R. Arcos1, J. Clavero1, A. Giavelli1, S. Simmons2, I. Aguirre1, S. Martini1, C. Mayorga1, G. Pineda1, J. Parra1, J. Soffia1 1Energía Andina, Chile 2Hot Solutions, New Zealand

Keywords low mixing degree. Minimum temperatures from water and gas Central Andes, Chile, geothermal project, surface exploration, geothermometers range from 200-240°C. Pampa Lirima MT/TDEM geophysical survey carried out in the area of the project (82 sites) revealing a large conductivity anomaly inter- preted as being associated to an important geothermal system at ABSTRACT depth, consistent with the geochemical data at surface. Slim holes drilling will be carried out during 2011 for proving the existence Pampa Lirima geothermal project is located in the of an exploitable geothermal system. of northern Chile, ca. 1,700 km north from , within the Central Andes Volcanic chain. Introduction Energía Andina, a Chilean geothermal company, obtained the exploration concession through a bidding process in June 2009. Pampa Lirima geothermal project is located in the Altiplano Since then, Energía Andina has developed an extensive and rapid of northern Chile, ca. 1,700 km north from Santiago, within the surface exploration program, together with a successful com- Central Andes Volcanic chain. munication program with local communities in the area. Surface Energía Andina, a Chilean geothermal company, obtained the exploration has been focused in understanding the geology of exploration concession through a bidding process in June 2009. the area and generating geochemical and geophysical models. Since then, Energía Andina has developed an extensive and rapid The superb results found through this rapid program have led the surface exploration program, together with a successful com- company to the next step, and a series of slim-hole wells have munication program with local communities in the area. Surface been programmed for 2010. Local communities have been in- exploration has been focused in understanding the geology of volved from the beginning of the project in the acquisition of the the area and generating geochemical and geophysical models. different data, as well as in the environmental issues associated The superb results found through this rapid program have led the to the development of a possible future geothermal power plant. company to the next step, and a series of slim-hole wells have Pampa Lirima project is located on the western edge of this been programmed for 2011. Local communities have been in- high plateau, with an average elevation of 4000 m asl. Geothermal volved from the beginning of the project in the acquisition of the features known for many years consist essentially on hotsprings different data, as well as in the environmental issues associated located in the lower southwestern part of the Lirima basin (Baños to the development of a possible future geothermal power plant. Lirima) and at the lower flank of a Plio- volcanic chain (Baños Andrés Jiguata). The geology of the area is constituted by Geology Jurassic to Paleogene volcanoclastic sequences, partially covered by Middle sheets as well as eroded volcanic The geology of the area of the conces- edifices of the same age. The youngest units cropping out in the sions Pampa Lirima 1, 2, 3 and 4, and its immediate surroundings area consist of volcanic complexes of to Pleistocene in especially west of them is characterized by the following (Arcos, age, located on the northern part of the Lirima basin. These vol- 2010): 1) basement rocks, 2) coverage of volcanic canic complexes have been interpreted originally as the possible and sedimentary rocks of -Miocene, and 3) middle heat source for the geothermal system in the area. Thermal waters Miocene to Pleistocene volcanic edifices. (Figure 1: Geological from Baños Lirima are characterized by high Cl and B concen- Map by cycles.) trations and δO18 enriched, and relatively low Mg concentration, The Mesozoic basement recognized to the west of concessions consistent with deep circulation from a geothermal reservoir, and Pampa Lirima 1 and 2 consists of clastic-carbonate sequences

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Oligocene to Miocene units consist of pyroclastic rocks with interbedded sedimentary units grouped in the Altos de Pica Fm. (“Estratos de Cultane” together with the Huasco Ignimbrite aged Oligocene to Middle Miocene), reaching a thickness between 500 and 700 m, including as well the Cordon Saitoco and Loma Chislaca volcanic complexes, both from lower Mio- cene. The units that form this structural domain are found in angular unconformity overlying Mesozoic basement units, and also are partially covered by the volcanic edifices and their products ranging in age from the middle Miocene to Recent. Such unconformity covers a wide timespan from the Paleocene (65 Ma) to Early Oligocene (28 ma). The set of , sediments and of the Oligo-Miocene coverage present a structural style characterized by a succession of anticlinal and synclinal folds with submeridional axis, some of them with greater wave amplitude than others, with their hinges either plunging to the N or double plunging. It is also observed that the folding axes are in turn folded, varying from NE direction to NS or even folded up to a NW orientation in their southern ends. In the structural model of the Pampa Lirima area (Radic, 2010; Fig. 2), the deformation style that shows this Oligo - Miocene sequence has been interpreted as the surface expression of a strike slip dextral fault that would affect in depth at least the Mesozoic basement rocks and developing a positive “flower-like” structure in the volcanic coverage. From the surface distribution of this fold and thrust belt in the southern area (S of Lagunillas Pampa), we infer that the possible trace of this strike slip fault south of 20° S, could be aligned or connected with the Collacagua river valley and the depression of the Huasco lagoons, whose NS features lengthen this path to the south until ​​Pampa Caya in Figure 1. Geological map of the Lirima area, showing (red circle) the area of the geophysi- Guatacondo valley, where this structure could be linked cal anomaly found by the MT survey. with the N-terminus of the West Fault System (Arcos et al, 2009). from the Jurassic to Early Cretaceous, forming a folded and Regarding the structural domain formed by the wide range of thrusted belt, and a Cretaceous volcanic sequence, covering the middle Miocene to Pleistocene volcanoes, they are all overlying older units in angular unconformity (Figure 2: Structural Model in angular unconformity the rocks of the Oligo-Miocene coverage Profile Radic, 2010). and distributed in 3 NW-SE strips. Miocene complexes occupy the southwestern strip, the upper Miocene corresponds to the immediate NE strip (central strip), whereas the Pliocene units occupy the north-eastern strip (Polanco and Gardeweg, 2000, Arcos, 2010, Polanco, 2011, Martini, 2011). Only Porquesa dome complex (Pleistocene), is located outside of this disposition in NW-SE strips, not only due to its geographical position but also because its morphology shows NE- SW elongation axes, indicating different Figure 2. Deformation style in Pampa Lirima area (Radic, 2010). 1) Jurassic to Lower Cretacic sedimen- tary rocks (fold and thrust belt); 2) Upper Cretacic volcanic rocks Oligocene to Lower Miocene Cover; structural control than the older ones. 3) Ignimbrites and epiclastic (=Altos de Pica Fm.), 4) Huasco Ignimbrite Middle Miocene to Pliocene The structural style of this domain is Volcanism; 5) Middle Miocene volcanism; 6) Upper Miocene volcanic edifices; 7) Pliocene volcanic characterized solely by the morphology of edifices ; 8) Intrusives. the volcanic edifices, some reaching great

690 Arcos, et al. altitudes above their bases (Pliocene), all with varying degrees Its chloride content varies between 250 and 310 mg / l, sulfate of erosion (the more ancient and, therefore, less preserved their between 260 and 330 mg / l and pH levels between 6 and 7. original morphology) and large volume of accumulated material The Baños de San Andres hot springs, are waters that outcrop (op. cit., 2000, 2010 and 2011). in the upper basin (4380 m asl) show a sulphate calcium-sodium In this domain a series of alignments can be recognized (or chemical signature, temperatures between 40 ° C and 60 ° C and are reflected), trending NS, NW and NE, some of which may be electrical conductivity from 1800 to 2140 microS/ cm. The sulfate reflecting structures affecting the rocks of the underlying structural content varies between 670 and 700 mg / l, chloride 130 to 141 domains, either Oligo-Miocene coverage or even the Mesozoic mg / l and pH between 6 and 7. basement. Finally, Chalviri baths are located in the NE sector of the concession Pampa Lirima 4, in the Agua Caliente valley, close to Hydrothermal Alteration the town of Chalviri, and correlate with a number of creeks and streams that drain to SE, towards the Cancosa Basin. Geologically The study area has many sectors with distinct color anomaly as this geothermal source is associated with a NW-SE alignment that a product of hydrothermal alteration (Arcos, 2010; Martini, 2010). marks the contact or boundary between the Charcollo and Andres The more relevant ones for geothermal exploration purposes are Jiguata volcanic complexes (both from upper Miocene) in the the Andrés Jiguata Hydrothermal Alteration Zone (Zahaj) and the southwest, and the large Pleistocene volcanic edifice of Cordón Lirima Hydrothermal Alteration Zone (Zahl). on the northeast. In both areas the alteration processes have similar features, which together with its structural arrangement, inferred subsurface development and preliminary hydrogeochemical aspects, suggest that it would be a single hydrothermal system (Arcos, 2010). They Aeromagnetic Survey color anomaly shows white-yellow tones with local red staining due to oxidation to limonite. Alteration styles range from argillic An aeromagnetic survey was carried out over the complete alteration (illite-smectite), locally phyllic (illite-sericite (musco- concession areas, covering an approximate surface of 1400 km2. vite)) to advanced argillic (silica, fine-grained alunite, kaolinite), The survey was designed considering N-S flight direction, line representing an acid sulfate -locally steam-heated- epithermal spacing of 300 m and flight altitude of 300 m above the ground system (Arcos , 2010; Martini, 2010). In Andrés Jiguata, the mean level. altered rocks are covered by relatively fresh lavas of Miocene Figure 3 shows the Reduction to the Pole of the observed total age (Msvaj); with alteration fading out, so it is estimated that the magnetic intensity, and the main feature are the high intensity alteration develops at the expense of these lavas. The alteration anomalies observed in the northern sector, clearly related to the zone is also covered by unaltered Pliocene volcanic rocks (PVQ Miocene-Pliocene volcanic system. and PVL), and locally by till deposits. In the case of the Lirima Alteration Zone, the altered rocks are covered by lavas of Cordón Saitoco (Mivcs), Patalani (Mmvp) and Quinchanali (Mmvq) vol- canic complexes, at least in the latter case, the upper edge of the rocks shows a gradual change from altered to unaltered lavas, in these, the alteration is evidenced by the development of veinlets and fractures filled with botroidal cryptocrystalline silica, which comprises, laterally, a large area (Arcos, 2010).

Water Geochemistry In the area of ​​geothermal exploration concessions Pampa Lirima 1 to 4, so far, have been recognized three sites with springs of geothermal fluids. These are the hotsprings of Baños de Lirima, Baños de San Andres and Baños de Chalviri (see location in the geological map outside of text). The first is located in the SW corner of Pampa Lirima in Coscaya River´s upper part, while the second is in the middle of the Andres Jiguata valley, which is a tributary of the same river mentioned. Geologically, both seem to be interrelated through the same NE-SW alignment and each by itself is associated with large areas of hydrothermal alteration of an argillic to advanced argillic, steam-heated type: Zahl and Zahaj respectively. The Baños de Lirima hot springs are located in the lower basin (approx. 4005 m asl) sprouting at temperatures between 38°C Figure 3. Reduction to the Pole of the Total Magnetic Intensity (ellipse and and 80°C, with chlorate sulfated-sodium chemical composition segmented line shows the primary prospective zone and main magnetic and electrical conductivitiy between 1300 and 1830 microS/ cm. alignment).

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More related to the geothermal context, two main characteristics can be observed: a) The more prospective area (ellipse in Figure 3), defined by the surface geothermal features, correspond to an apparently demagnetized (or weakly magnetized) area, and b) A clear magnetic lineament, coinci- dent with some regional structural trends and the MT South-East limit of the resistivity anomaly. Microseismicity Twelve short period, three components and continuous recording seismic stations were installed in the Lirima area, over a six months observation period. During this time, 3571 events were recorded and processed (two phases in at least three Figure 5\. MT cross-Sections in NW-SE and NE-SW orientation. stations), and approximately 1200 of these present focal depth less than 25 km. Figure XX shows epicentral distribution of the recorded seis- considered good enough to be included in the final 3D inversion micity, where most of the activity is located out of the observation (using code Mackie and Madden 1993). network. It is interesting to note in the same Figure, that some of The south to south-west part of the survey area is marked by the seismic events tend to be aligned along the lineament already a shallow conductive layer (resistivity < 10 Ohm-m within the defined with the MT and Aeromag data, reinforcing the idea that upper 1000 m below surface), interrupted in the center by a higher this could represent a major structural control within the studied resistivity zone (see depth slice at the fixed elevation of 3500 area. m.a.s.l. in Figure XX). Considering the geothermal context, the south-west part of this conductivity anomaly, could be interpreted Magnetoteluric Survey as the clay cap (argyllic alteration) of the geothermal reservoir. MT data were acquired and processed on 82 sites and cover- The south-east part of this anomaly needs further studies to be ing the frequency range 0.003 – 10.000 Hz. Both, acquisition and fully understood. processing, including the full tensor 3D inversion, was carried out Is interesting to note that the center resistive zone, coincide by SWS-Geosystem, and data quality on all of the 82 sites was with the regional alignment identified by the regional geology and the aeromag data. On the south-west corner of the 3D inversion zone, below the upper conductive layer, a very distinctive conductive body can be observed. The geometry of this conductive anomaly suggests a massive body, with deep roots, but there is not other geologi- cal or geophysical data contributing to explain the nature of this anomaly. A possible explanation for this, could be the presence in depth of partially melted rock (intrusive body) being the heat source of the potential geothermal system.

Conclusions An extensive and multidisciplinary exploration program, including surface geology, hydrogeochemistry and several geo- physical methods resulted in finding a large conductive anomaly that could be associated at depth with an active potential geother- mal system, although with some peculiar features, different from classic volcanic-related geothermal systems. The heat source of the Lirima system could be more related to an intrusive body at depth rather that to an active volcanic system. The very interest- ing and promising geothermal potential of this anomaly will be Figure 4. Registered superficial (< 20 km) micro-seismicity (different colors tested with a series of deep and shallow slim-holes during the related to focal depth). Red triangles represent seismic stations locations. second half of 2011.

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References Martini, S. 2011. Geología 1:20.000 del sector N de la Pampa Lirima. Informe Arcos, R., Pino, H., Fanning, M., Gardeweg, M., Camacho, J., Sprohnle, Interno ENERGÍA ANDINA S.A. C., Sanhueza, A. y Mont, A., 2009. Nuevos antecedentes geológicos y geocronológicos en el area de Collahuasi, Región de Tarapacá, Chile. Polanco, E. y Gardeweg, M. 2000. Antecedentes Preliminares de la Estrati- XII Congreso Geológico Chileno, Santiago, Chile. grafia Volcánica del Cenozoico Superior en los Cuadrangulos Pampa Lirima y Cancosa, Altiplano de la I Región, Chile (19º45’–20º00’S Arcos, R. 2010. Estudio Geológico a escala 1:50.000 de las Concesiones Y 69º00–68º30’W. Actas IX Congreso Geológico Chileno, Simposio Pampa Lirima 1, Pampa Lirima 2, Pampa Lirima 3 y Pampa Lirima 4. Nacional Nº3, pp. 324-328. Puerto Varas, Chile. Comuna de Pica, Región de Tarapacá. Chile. Informe Interno ENERGÍA ANDINA S.A. Polanco, E. 2011. Geología 1:20.000 del sector centro sur de Pampa Lirima… Martini, S. 2010. Alteración hidrotermal en el campo geotermal activo de Radic, J.P., 2010. Estilo Estructural del área de las concesiones geotérmicas Pampa Lirima. Informe Interno ENERGÍA ANDINA S.A. Pampa Lirima 1 a 4. Informe interno para ENERGIA ANDINA S.A.

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