GRC Transactions, Vol. 38, 2014 Geological and Geochemical Reconnaissance of a Non-Volcanic Geothermal Prospect in Guatemala — Joaquina Geothermal Field R. B. Libbey1,2, A. E. Williams-Jones1, B. L. Melosh1, and N. R. Backeberg1 1McGill University, Montreal, QC, Canada 2Adage Ventures Inc., Toronto, ON, Canada Keywords Introduction Geochemistry, soil CO2, shallow temperature, soil chemistry, Regional Geology bulk rock chemistry, fluid inclusions, Motagua, Guatemala, deep-circulation Based on the morphotectonic zones of Burkart and Self (1985), the Joaquina project area is situated in the northern section of Guatemala’s Zone III. This zone is characterized Abstract by monogenetic volcanism related to East-West extension and decompressive generation of relatively mafic, olivine- to neph- The results of a geological and geochemical survey of the Joa- eline-normative magmas. Quaternary, behind-the-volcanic-front quina geothermal field in Guatemala are summarized. Structural (BFV) volcanism has a clear association with Holocene-Mio- mapping, soil chemistry including CO2 (soil gas), and shallow temperature measurements were employed in conjunction with petrographic and bulk rock chemical analyses of drill cutting samples to identify regions of hydrothermal fluid upwelling and outflow in the Joaquina system. A chemical reconnaissance of thermal manifestations provided evidence for the presence of a meteorically-derived Na-bicarbonate(-sulfate) geothermal fluid, similar to those in neighboring geothermal systems in Honduras. Geo- thermometric analyses of the Joaquina fluids yielded reservoir temperature estimates (Tsilica-adiabatic) of ~180°C. Sulphur and carbon isotopic analyses of soil gas and sulfide minerals in drill core indicate that the sulfur, CO2, and CH were likely derived from Figure 1. (a) Generalized tectonic map of Guatemala and surrounding regions showing the location of the 4 Joaquina geothermal system (red circle; Adapted from Walker et al., 2011). (b) Perspective satellite image hydrothermal alteration of organic- (3x vertical exaggeration) of southern Guatemala and neighboring regions looking north, displaying location rich metasediments of the El Tambor of study area (red circle), developed geothermal systems (blue circles), volcanic centers, and other land- Complex. The natural thermal output forms. A: Agua; Ac: Acatenango; AB: Bahia de Amatique; Al: Almolonga; AR: Apaneca Range; At: Atitlan; C: of the Joaquina system is estimated Chiquimula; Ch: Chingo; CB: Cuilapa-Barbarena; CC: Coatepeque Caldera; CS: Cerro Santiago; F: Fuego; conservatively at 29.4 MW . This Fl: Flores; G: Guatemala City; I: Ipala; Ix: Ixtepeque; Iz: Izalco; LI: Lago de Izabal; M: Moyuta; MV: Motagua th Valley; P: Pacaya; PV: Polochic Valley; Q: Quezaltepeque; S: Suchitan; SA: Santa Ana; SD: San Diego; SDC: study represents the first detailed field Sierra de Chuacus; SDLM: Sierra de las Minas; SM: Santa Maria; T: Tecuamburro; Ta: Tahual; Tac: Tacana; Taj: investigation of a non-volcanic geo- Tajumulco; To: Toliman. The satellite imagery was provided by Google Earth. Locations of volcanic centers thermal system in Guatemala. were provided by the Smithsonian Institution Global Volcanism Program. 381 Libbey, et al. cene rifting south of the Motagua Fault System (Walker et al., Geothermal Exploration at Joaquina 2011), a region that is thought to be experiencing ~5-10 mm/ yr of East-West extension (Guzmán-Speziale, 2001; Lyon-Caen Interest in the Joaquina geothermal prospect developed after et al., 2006; Álvarez-Gómez et al., 2008). The BVF volcanism many mining exploration boreholes drilled to vertical depths of in Guatemala was initiated after the left-lateral Motagua Fault tens of metres to >200 m (between 2001 to 2007) intersected pres- replaced the Polochic Fault as the major boundary between the surized hot water and steam. Core and rock chip samples from Caribbean and North American plates at around 4 Ma. This these holes contain abundant evidence of hydrothermal alteration, switch to the more arcuate Motagua Fault increased trans- and although collapsed and inaccessible, many of these boreholes tensional deformation along the plate boundary (Rogers and still quiescently vent hot geothermal gases. One of these boreholes, Mann, 2007). BRRC-01 is reported to have discharged a ~10 m-long spray of The tectonic and geothermal regime of the BVF region of hot water and steam, known locally as the ‘Joaquina Geyser’, for Zone III shares some similarities with the Great Basin region in an undetermined period of time (Figure 2b). the Western United States. In the latter region, the right-lateral This study represents the first detailed investigation of a non- Walker Lane Fault System strikes NW-SE and a series of exten- volcanic geothermal system in Guatemala. Prior to this study, very sional faults occurs approximately normal to this. Geothermal limited geothermal exploration had been conducted at Joaquina. activity in the Great Basin appears to be largely amagmatic in The only literature on the field is an exploration plan developed by origin (with a few disputed exceptions, e.g., Steamboat Springs, GeothermEx (2011) for Centram Geothermal Inc., which included Nevada, Roosevelt, Utah, Coso and Long Valley, California; chemical analysis of a fluid sample from the ‘Joaquina Geyser’. Arehart et al., 2003; Faulds et al., 2012). Fault terminations and Exploration and development programs at Joaquina are currently fault intersections (e.g., the intersection of NW-SE striking faults being conducted through a joint venture between Adage Ventures of the Walker Lane system with NNE striking normal faults) seem Inc. and Centram Geothermal Inc. to be loci of enhanced geothermal fluid circulation in the Great Basin (Faulds et al., 2010). An analogous scenario of deep fluid Methods flow along structurally complex zones is likely occurring in the active tectonic environment south of the Motagua Fault system in Thermal manifestations were mapped, analyzed on site for Guatemala, resulting in localized thermal manifestations, such as temperature, pH, chlorinity, and flowrate, and sampled for de- those found at Joaquina. The presence of a shallow, decompres- tailed chemical analyses at ActLabs, Ontario, Canada. The use of sively-generated magmatic heat source is unlikely at Joaquina, a FLIR® infrared camera aided in the location of vent sites for as Quaternary behind-the-volcanic-front volcanism in Guatemala some of these manifestations. is seemingly related to fast ascent of basic magmas along deep One-meter deep, ~2 cm-diameter holes were created across structures, with negligible heat transfer to the surrounding rocks the study area using a carbon steel tile probe and slide hammer. (akin to the geothermal systems of Honduras – e.g., Barberi et A K-type thermocouple and digital thermometer were lowered to al., 2013). However, an elevated regional heat flow likely plays a the base of each hole, allowed to equilibrate and the temperature role in the generation of the non-volcanic hydrothermal systems measured. The same holes were utilized for measuring soil CO2 of Guatemala. concentrations. Soil gas CO2 concentrations were measured us- The Joaquina project is situated within the Central American ing a Vaisala GM-70 infrared CO2 meter. This instrument has an Plateau, a region that extends from east of the modern volcanic operational range of 0 - >20 wt.%CO2 and a sensitivity of ~500 arc to the Caribbean Sea and includes the Caribbean-North ppm, making it capable of detecting CO2 anomalies that are above American plate boundary region. Rogers et al. (2002) have atmospheric levels (i.e. >~400 ppm). At each sample location, proposed that the northern Central American plateau was up- a perforated steel probe was inserted into the hole, and soil gas lifted in response to mantle upwelling following detachment was pumped up the probe through a series of tygon tubes, into of the down-going Cocos slab. Tomographic P-wave images the Vaisala infrared meter. A total of 103 one-meter temperature display the 300-km-wide gap in the subducting slab that under- and 123 CO2 measurements were made. lies the highlands. Slab detachment of the downgoing Cocos Soil gas samples were collected in vacutainers via a syringe plate is estimated to have occurred between the end of the inserted into a rubber bulb along the pumped path of fluids to silicic, subduction-related volcanism that produced the units the infrared CO2 meter. The proportions of CH4 and CO2 in the of the central American ignimbrite province (~20 - 10 Ma) and soil gas and their carbon isotopic composition were analyzed at a minimum age of 3.8 Ma derived from convergent rates and the University of Calgary. A total of 38 solid soil samples were slab geometry (Rogers et al., 2002). Upon slab detachment, up- collected from the study area. The samples were taken at a depth welling hot asthenospheric mantle flowed into the gap between of approximately 15 – 20 cm in the ‘A’ horizon, following the the two plates, which likely generated a >500°C heating of the methodology of Murray (1997), and sealed in glass containers base of the overriding plate for a timespan of several million with HDPE lids. All soil samples were analyzed chemically at years (van der Zedde and Wortel, 2001; Rogers et al., 2002). ActLabs, Ontario, Canada. Asthenospheric upwelling is consistent with the behind-the-arc A total of 29 samples of well cuttings were collected from four basaltic volcanism in Guatemala and Honduras (Walker et al., boreholes (BRRC-01, -02, -03, and -04). Composite samples rep- 2002). It is conceivable that remnant heat from this upwelling resenting a 9 m interval were collected every 30.5 m. Fifteen core may also be augmenting geothermal systems situated near the samples were collected for intervals of interest from BDH series Motagua Valley (Figure 1). wells. All samples were analyzed petrographically and doubly- 382 Libbey, et al. polished thin sections were created for the purposes of analyzing situated within the Sula-Tambor terrane (and the associated ‘Tam- fluid inclusions in calcite microthermometrically on a Linkam bor Project gold district’), of the Guatemala Suture zone, which is THMS600 stage at McGill University.