Geological and Geochemical Reconnaissance of a Non-Volcanic

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. Repeat homogenization separated from the Jacalteco Terrane to the north by the Motagua and melting measurements were conducted on all inclusions, and fault system. The lithologies of the Sula-Tambor Terrane and as- measurements were calibrated to standards of critical-point pure sociated overlap sequences are documented by Flores et al. (2012), water inclusions in quartz. The bulk rock compositions of the and include: greenschist to granulite facies metasedimentary and samples were analyzed by Acme Analytical Laboratories Ltd., metavolcanic basement lithologies of the San Diego Phyllite Vancouver, Canada. and Las Ovejas/Banaderos Complexes; Late-Jurassic to Early- Sulfide minerals (dominantly arsenopyrite) from crushed Cretaceous southwardly-thrusting tectonic sheets of greenschist downhole samples were analyzed for their sulfur isotope ratios at to amphibolite facies metabasalts and oceanic metasediments of the Queen’s Facility for Isotope Research, Queen’s University. The the El Tambor Complex; Late Cretaceous sediments, volcanics, sulfide minerals were separated by hand picking and comprised and granites; Tertiary pyroclastic units of the Padre Miguel Group approximately 45-50% by volume of each sample, with gangue and associated lavas; and possible Quaternary tuffs. minerals (quartz, calcite, graphite, feldspars, biotite) comprising The rock units intersected by mining exploration boreholes the rest. None of the gangue minerals contains significant sulfur. at Joaquina comprise biotite- and plagioclase-phyric, welded to non-welded lithic-crystal tuffs of the Padre Miguel Group and Results and Discussion graphitic-sericitic metasiltstones of the El Tambor Group. The maximum drilled vertical thickness of the pyroclastic units is Local Geology 52.7 m. All boreholes terminated in the El Tambor metasiltstone, which occurs to the maximum drilled depth of 124.8 vertical me- The Joaquina geothermal prospect is located approximately ters. Coarse recent river alluvium and unwelded tuff of assumed 10 km south of the main expression of the Motagua Fault System Quaternary age occur in the first few meters in some boreholes. (Fig. 1), an arcuate, crustal-scale, left-lateral structure that marks The most prominent structures in the Joaquina area are steeply the active boundary between the Caribbean and North American dipping NE striking left-lateral faults and moderately dipping plates (the Chortis and Maya Blocks, respectively). Joaquina is ~E-W striking normal faults. Active manifestations and native sulfur mineralization along and above the fault traces attest to the importance of these structures in transmitting deep geothermal fluids to the surface. The most active areas of thermal manifestations seem to be located at the intersection of these structures. The most prominent upflow zone at Joaquina occurs at the intersection of the El Puente Fault (040/~90) and the ESE-oriented Rio Las Cañas Fault (RLC Fault). Fault damage zones created at the intersection of these two structures likely control a potential deep reservoir. Surface Manifestations There are 72 mapped hotsprings (some of which include multiple vents) in the Joaquina area, the majority of which occur along the WNW-ESE segment of the Rio Las Cañas at elevations between 685 and 705 m (Figures 2 and 3). Many of these vents are beneath the river water level in

Figure 2. (a) Looking east along Rio Las Cañas with vapor plumes rising from two vigorously boiling hotsprings. (b) A hot water and steam jet issuing from borehole BRRC-01. (c) Travertine micro-terraces precipitating from a boiling hotspring vent. (d) Bladed calcite and quartz (BDH-1S, 21 m depth). (e) A calcite-arsenopyrite vein crosscutting graphitic metasiltstone (BRRC-02, 96 m depth). (f) Pyrite, chalcopyrite, and sphalerite occurring in altered crystal tuff (BDH-1S, 23 m depth).

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the wet season (May to October), and reconnaissance during the Values of alkalinity/SO4 and Cl/alkalinity for the Joaquina dry season confirmed that additional unmapped vents issue from hotsprings suggest that the most direct upflow of geothermal fluid beneath the river surface even when the river level is at its lowest. from the subsurface is situated around the intersection of the RLC A second localized yet vigorous area of hotspring activity occurs to and El Puente Faults. This area is also in the vicinity of the site’s the SE of the map area, and a warm spring occurs in the NE at an main fumarolic features. elevation of 733 m. Some of the boiling springs deposit cm-scale travertine terraces directly around their vent locations (Figure 2c). Vapor-saturated fumaroles and steam-heated features such as boiling mudpots, advanced argillic alteration and acid-sulfate springs are also present at Joaquina, and typically occur at higher elevations (710 and 720 m) to the south of the main hotspring district. The absence of these features to the north of the main hotspring district suggests a southerly-dipping upflow path. The discharge of some steam-heated features is noticeably less during the dry season than the wet season as a result of the decreased meteoric water recharge to the shallow oxidized groundwater system. The hotsprings at Joaquina range in temperature from 44 to 100°C and have measured flowrates of 0.1 to 6.25 L/s. The total combined outflow of hotsprings from the Joaquina field is estimated to be 67.6 L/s, equating to a convective energy transfer of 17.7 MWth (following methods outlined by Mwawongo, 2007). This estimate does not consider the potentially significant contribu- tion from unmapped manifestations that are covered by the river Figure 4. Piper diagram comparing the Joaquina fluids with those from in both wet and dry seasons. The largest individual contributor to deep-circulation geothermal systems of Honduras. Fields in the anion the convective energy budget at Joaquina is a vigorously boiling ternary diagram represent the typical composition of mature (M), volcanic spring on the north side of Rio Las Cañas, which has an estimated (V), steam-heated (S), and peripheral (P) waters (modified after Giggen- bach, 1988). energy output of 1.9 MWth. The waters issuing from hotsprings at Joaquina are Na- bicarbonate(-sulfate)-rich (Figure 4) and are low in chloride (41 In Cl-HCO3-SO4 space (Figure 4), the Joaquina fluids plot - 54.5 ppm). A varying degree of dilution from cool near-surface close to the SO4-HCO3 tie line. The labels developed for this waters has affected the hot springs, as shown by the inverse rela- diagram group waters into compositions that reflect geothermal tionship between outflow temperature and the Mg concentration fluids rising adiabatically from depth (‘mature waters’), geo- of the fluid; the most diluted springs are at the margins of the thermal fluids that have a component of magmatically derived field. A few hotsprings appear to have been affected by conductive sulfate (‘volcanic waters’), ground waters that have condensed cooling and minor dilution. the CO2 and H2S of boiled geothermal fluids (‘steam-heated waters’), and HCO3-rich waters that are commonly present in the outflow regions of magmatically-driven geothermal systems (‘peripheral waters’). As illustrated by Gokgoz (1998) for the Kizildere and Tek- kehamam geothermal fields in Turkey, the labels on this diagram are not instructive for most deep-circulation-type geothermal systems situated in metamorphic rocks. In these environments, the host rocks would have likely lost the bulk of their Cl during metamorphism, on account of the high mobility of Cl in aqueous solutions (e.g., metamorphic brines). The absolute anion and cation concentrations of the Joaquina fluids are very similar to those of the Platanares and Azacualpa geothermal fields in Honduras (Janik et al., 1991; Barberi et al., 2013), and the Beowawe field in Nevada (Cole Figure 3. Nearest neighbor contoured maps of (a) soil temperature (°C) at 1 m depth and (b) soil CO2 concentration (wt.%). Thermal manifestations and measurement locations (black crosses) are shown. and Ravinsky, 1984). The higher sulfate Hotsprings (circles) are colored in a gradient according to their discharge temperature (red = 90 – 100°C, content of the Azacualpa fluids is a result light blue = 40 – 60°C). of the presence of evaporitic units in the

384 Libbey, et al. host rock stratigraphy. Fluids from the Kizildere and Tekkehamam temperature is below 210°C. At temperatures above this, the geothermal fields in Turkey share similar anion and cation ratios thermodynamically-predicted equilibrium fluid concentration of with those at Joaquina, however, the Turkish fluids have much silica (>~380 ppm SiO2, in accordance with quartz saturation) higher absolute concentrations of bicarbonate, Cl, SO4, Na, and K. exceeds saturation of the fluid with amorphous silica at 100°C, and The low Cl content of fluids from Joaquina offers additional will lead to precipitation of silica sinter at the vent site (Moore, support for the hypothesis that the heat source is related to deep- pers. comm.; Fournier and Rowe, 1966). circulation rather than a magmatic body. Geothermal systems Disequilibrium is always a possibility that must be considered related to plutonic igneous bodies undergo significant addition when interpreting the chemistry of geothermal fluids. For example, of magmatically-exsolved HCl, which dissolves in the convect- Barberi et al. (2013), in a study of the Platanares geothermal sys- ing fluids and increases their chloride concentration (a signature tem, found that the compositions of downhole fluid samples were that is clearly not present at Joaquina). Furthermore, the low As/ out of equilibrium with respect to the thermodynamically predicted SiO2 ratios of the Joaquina fluids are consistent with those of mineral assemblage. They concluded that the silica and K-Mg other non-magmatic, deep-circulation geothermal systems (e.g., geothermometers provide the most reliable temperature estimates Arehart et al., 2003). for this system, whereas temperatures are overestimated by the Oxygen and hydrogen isotopic compositions of the hot spring Na-K geothermometer. Similarly, Simmons (2013) found that the fluids at Joaquina range from -7.4 to -7.6‰ δ18O and -52 to -53 silica geothermometer was the most reliable geothermometer for δD, respectively. These values plot very close to the Guatemala- magmatic and non-magmatic geothermal systems of the Great Belize Meteoric Water Line and the Global Meteoric Water Line, Basin, USA, and that the Na-K geothermometer overestimated and are consistent with isotopic values of precipitation in central the reservoir temperature. Given the similarities of the Joaquina Guatemala (e.g., Lachniet and Patterson, 2009). There are positive system to non-magmatic basinal systems of Honduras and the O-isotope shifts (characteristic of geothermal fluids) of 0.1 to 0.3 United States, the reservoir temperature at Joaquina is likely to be δ18O. These shifts indicate that the geothermal system is meteroric most reliably indicated by the silica geothermometric temperature water-dominated. The very small shift in oxygen isotopic values of ~180°C. indicates either a low degree of water-rock isotopic exchange at depth, a high water-rock ratio, and/or a mature geothermal system. One-Meter Soil Temperature Low O-isotopic shifts have been observed in other intermediate- Soil temperature measurements at a depth of one meter ranged high temperature deep-circulation geothermal systems (e.g., from ambient ~28 °C to 97 °C (Fig. 3a). Contouring of these Beowawe, Nevada; John et al., 2003) as well as magmatically- measurements reveals a maximum around 786605E, 1636800N heated systems (e.g., Wairakei, New Zealand - δ18O shift = 0.8; (15P, WGS84), encompassing a region of fumaroles, boiling Krafla, Iceland - 18δ O shift = 0.5-1.0; Cole, 1994). mudpots, advanced argillic alteration, and an acid-sulfate spring Fluid samples from Joaquina plot on a very well-defined in the general vicinity of the intersection between the RLC and El trend on the Na-K-Mg geo-indicator diagram (Figure 5), corre- Puente faults. Temperatures decrease over the ~600 m westward- sponding to deep fluid temperature between 180 and 200°C. The trending outflow path. Deviations from this trend appear to be the quartzadiabatic geothermometer (Fournier and Potter, 1982) yielded result of seepage along cross-cutting NE-trending structures. Us- similar equilibrium temperatures, 177 to 180°C, for least-diluted ing methods outlined by Mwawongo (2007), the conductive heat samples. The lack of silica sinter around the boiling hotspring transfer through the soil at Joaquina is estimated to be 11.7 MWth. vents is consistent with the observation that subsurface reservoir Soil Chemistry Surveys of carbon dioxide, mercury (Hg), arsenic (As), and antimony (Sb) concentrations in soil can be effective tools for tracing buried fault structures during geothermal exploration. The low cost of these surveys makes them ideal for early stage regional reconnaissance, and the results can be used in conjunction with geological and geophysical surveys to target areas for further detailed study and drilling. Soil CO2 contents ranged from below the detection limit (~0.005%) to 26.9%. Contoured values of soil CO2 produce a similar pattern to that of soil temperature, albeit with slightly greater complexity (Figure 3b). The largest area of high soil CO2 coincides with the highest soil temperatures and the area of most active thermal manifestations. Maps of As, Hg, and Sb contents in soil display patterns that broadly correspond to those of soil temperature and CO2, and indicate that the primary upflow occurs in the northern part of the study area apparently at the intersection of the RLC and El Puente faults (Figure 6a-c). Figure 5. Na-K-Mg geoindicator diagram (Giggenbach, 1991) showing Na-K temperatures (deep equilibration) and K-Mg temperatures (shal- Potential sources of soil CO2 are mantle degassing, decar- low re-equilibration). The Joaquina data were plotted with a spreadsheet bonation, and respiration. Potential sources of CH4 are bacterial provided by Powell and Cumming (2010). decomposition of organic matter, thermal decomposition of or-

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13 Values of δ C of soil CH4 range from -36.7 to -37.1 ‰, and may also be the result of hydrothermal interaction with graphitic carbon. Methane carbon produced together with CO2 in such oxidation-hydrolysis reactions is expected to be -34.5 to -40.5 ‰ lighter than CO2 carbon, for temperatures between 200 and 150°C (Bottinga, 1969; Ohmoto, 1972). This would produce methane that is isotopically lighter than that observed at Joaquina and may indicate that graphite alteration and methane produc- Figure 6. Nearest neighbor contoured maps of (a) soil Hg, (b) As, and (c) Sb in the Joaquina study area. tion occurs at a temperature of > 200°C deep within the Joaquina system, or that there is disequilibrium between coexist- ganic matter (catagenesis), inorganic synthesis (CO2 + 4H2 = ing CO2 and CH4. Some authors suggest that gas-gas isotopic CH4 + 2H2O), and degassing from the mantle (controversial; e.g. disequilibrium is to be expected in geothermal fluids as different Etiope and Klusman, 2002). Carbon isotope fractionation during fluid-mineral reactions buffer their concentrations (Giggenbach, devolatilization of carbonates can result in the creation of CO2 1981; Arnorsson and Gunnlaugsson, 1985). that is 2 ‰ depleted with respect to the parent rock carbon (Val- ley, 1986). Mineral-vapor (CaCO3-CO2(V)) isotope fractionation of Hydrothermal Alteration carbon in boiling hydrothermal fluids above 100°C is less than± 3 Hydrothermal alteration is prevalent in downhole rock samples ‰ (Bottinga, 1969), and the isotopic fractionation between vapor from Joaquina (Figure 2d-f). The alteration cuts the metamorphic and liquid is inferred to be less than this - a notion supported by the fabric of the El Tambor phyllites and is also observed in the overly- similar carbon-isotope values of super-heated volcanic gases and ing tuffs and recent alluvium. It is thus inferred that the alteration is moderate- to low-temperature fumaroles (Sano and Marty, 1995). related to the present hydrothermal activity. Hydrothermal miner- 13 The δ C values of soil CO2 at Joaquina range from -5.2 to -7.2 als present in the downhole samples include pyrite, arsenopyrite, ‰. These values are broadly similar to albeit somewhat higher pyrrhotite, sphalerite, quartz, chalcedony, illite-smectite, chlorite, 13 than the δ C values of CO2 at Platanares and Azacualpa, which and calcite. Bladed calcite is a common occurrence, indicating pre- range from -8.40 to -11.9 ‰ and -7.21 to -7.59 ‰, respectively. cipitation from boiling hydrothermal fluids. This variety of calcite In the Honduran systems, the CO2 is interpreted to be the product is present in the deepest rock samples available from Joaquina of thermal breakdown of marine carbonates (0 to -4 ‰) and oxi- (vertical depth of 127 m in BRRC-03, corresponding to a depth dation of organic rich rocks (<-15 ‰; Janik et al., 1991; Barberi of 40 m below the present water table in this borehole), providing et al., 2013). This may also be the case for Joaquina, although a evidence of vapor-saturated liquid at ~4 bars of hydrostatic head significant source of marine carbonates has not been identified in (143.6°C). This interpretation assumes that the calcite formed the basement lithologies. under the present hydrologic regime. The CO2 at Joaquina could originate from a degassing upwelling mantle, with leakage to the surface along deeply penetrating Fluid Inclusion Analyses faults (Rogers et al., 2002). This interpretation is supported, in the Fluid inclusions in calcite were analyzed to assess fluid salinity case of the Platanares geothermal system in Honduras (a system and temperature conditions in the subsurface at the time of min- that is in the same general tectonic setting as Joaquina; Barberi et al., 2013), by helium isotope values with a mantle signature. Mantle-derived volatiles, including CO2 and He, have also been shown to be present in fluids along segments of the San Andreas Fault system that are devoid of magmatic activity (Pili et al., 2011). Further studies of He-isotopes are needed to validate the presence of mantle-derived volatiles at Joaquina. The interaction of hydrothermal fluids with graphitic carbon has been demonstrated to produce CO2 with a negative (< -4 ‰) carbon isotope signature in gases from the Larderello geothermal system in Italy (Gherardi et al., 2005). Ohmoto and Goldhaber 13 (1997) showed that CO2 generated from graphite with a δ C of -15 ‰ at temperatures above 350°C has a δ13C of < -5 ‰. Thus, the oxidation and hydrolysis of organic carbon can easily produce CO2 with δ13C values of -5 to -10 ‰ (Gherardi et al., 2005). Interaction Figure 7. The depth below the water table of fluid inclusions at Joaquina vs. of hydrothermal fluids with graphite is a likely explanation for the their measured homogenization temperature. The present-day boiling point CO2 at Joaquina considering the large carbon reservoir available with depth curve (BDP) is shown together with BDP curves for the present- within the graphitic phyllites of the El Tambor Complex. day conditions at an additional hydrostatic head of + 45 and +105 m.

386 Author(s) Name(s) eralization. Samples were selected from BRRC-03 (204 m depth, tion of these large structures is a major control of fluid upflow at 28.4 m below the modern watertable), BRRC-04 (143 m depth, Joaquina. Moreover carbon (which is largely controlled by calcite 67.5 m below modern watertable), and BDH 1-s (21 m depth, 8 content), sulfur, and copper contents have downhole distributions m below modern watertable). A total of 22 fluid inclusions was consistent with a south dipping structure along the Rio Las Cañas analyzed for homogenization and melting temperatures. The fluid (Figure 8). inclusions were all two-phase, liquid-rich varieties (~10% vapor) with ovoid to negative crystal shapes and were hosted in rhombic Sulfur Isotope Compositions of Sulfide Minerals and bladed calcite crystals. Although there is no direct evidence During adiabatic upwelling of hydrothermal fluids and va- for primary origin, the delicate nature of the bladed calcite crystals por phase separation, no significant fractionation of S-isotopes makes it likely that inclusions hosted in them are primary. is thought to occur between dissolved and gaseous H2S (e.g., The fluid inclusion homogenization temperatures range from Ohmoto and Rye, 1979; Szaran, 1996). Furthermore, fractionation 146.75 to 206.5°C (mean = 157.87°C, median = 154°C), which is higher than expected based on the projected boiling point with depth curve (with current water table depth derived from borehole drilling logs). However, these temperatures are similar to temperatures estimated using the Na-K-Mg and Silica geothermometers (see above). Final melting temperature of frozen inclusions are between -0.07 and -0.1°C, indicating the very low salinity of the geothermal fluids at depth (in agreement with chemical studies of hotspring fluids). The positive departure of the measured homogenization temperatures from the modern day boiling point with depth curve indicates that the fluids were trapped during a time when there was a larger hydrostatic head above the geothermal system (i.e., when the water table was higher than at present). Erosion over the lifespan of the geothermal system, and naturally fluctuating water table depths can easily explain the higher than expected homogenization temperatures. Indeed, an addition of <45 m of hydrostatic head can account for all except one of the homogenization temperatures measured. The exception, which is for a fluid inclusion in BRRC-04 would require a hydrostatic head 105 m above the present water table. Assuming modest erosion rates of 1mm/year (based on a mean local relief of 100 m; e.g. Summerfield and Hulton, 1994), 45 m could be eroded in 45,000 years, and 105 m in 105,000 years. Thus, if the fluid inclusion homogenization temperatures are representative of past subsurface fluids, the Joaquina geothermal system may be a minimum of 45,000 to 105,000 years old. Typical geothermal systems are thought to have lifespans of 105 to 106 years (Browne, 1979). A fluid inclusion study of samples from boreholes in the nearby Platanares geothermal system, Honduras, revealed a similar departure of homogenization temperatures from the boiling curve to that observed at Joaquina. There, a minimum of 80 m of erosion would be needed to explain the fluid inclusion homogenization temperatures at depth (Bargar, 1991). Bulk Rock Chemistry Bulk rock chemical data from borehole samples can assist in delineating zones of active and fossil hydrothermal upflow. For example, the concentration of bulk rock sulfide and some trace metals show distinct enrichments in the shallow subsurface above upflow zones in active geothermal environments due to the localization of H2S-enriched fluids to these regions (Libbey and Williams-Jones, 2013). At Joaquina, contents of carbon and sulfur are highest in the bulk rock at shallow depths proximal to Figure 8. Contoured bulk rock chemistry of borehole samples, showing the inferred intersection between the El Puente and RLC faults, select borehole traces and the inferred approximate trace and relative mo- providing further support that the damaged zone at the intersection of the RLC fault.

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Acknowledgments between aqueous H2S and most sulfide minerals is considered to be minor at hydrothermal conditions. However, any fractionation that occurs between H2S and pyrite, sphalerite, and pyrrhotite will This project was made possible by the support of Adage Ven- cause a minor enrichment of the heavy isotope into the mineral tures Inc. and Centram Geothermal Inc. Much logistical support phase (chalcopyrite will be very slightly depleted in the heavy was supplied by José Manuel Lemus Abal and fieldwork guidance isotope relative to coexisting H2S; e.g., Sakai, 1968; Seal, 2006). was provided by César Fernando Monterroso Rey. Aaron Libbey Arsenopyrite is expected to have similar fluid-mineral isotopic provided a significant amount of logistical and field assistance fractionation characteristics to pyrite. to this project. Dr. Joseph Moore provided valuable guidance For the three samples analyzed from Joaquina, bulk sample related to fieldwork strategies. Nick Hinz provided a very helpful δ34S values range from -0.9 to -20.0 ‰. The dominant sulfur- review of this manuscript. The study was supported financially bearing mineral in all the samples analyzed is arsenopyrite. Trace by a NSERC Discovery grant to AEW-J. sphalerite, pyrrhotite, and chalcopyrite, however, are also present in the samples. Samples from BRRC-03, 96 m, BDH 1-S, 18.5 m, References and BRRC-01, 96 m have sulfur isotopic values of -0.9, -20 and -7.9 ‰, respectively. The value for the sample from BRRC-03 Álvarez-Gómez, J. A., Meijer, P. T., Martínez-Díaz, J. J., Capote, R., 2008. is consistent with both mantle values (~0 ‰) and sedimentary “Constraints from finite element modeling on the active tectonics of 34 northern Central America and the Middle America Trench.” Tectonics, pyrite sources (negative δ S values; Seal, 2006). Sulfur isotopic v. 27. doi:10.1029/2007TC002162. values from BDH and BRRC-01 suggest sulfur sources that are Arehart, G. B., Coolbaugh, M. F., and Poulson, S. R., 2003. “Evidence for a sedimentary in origin The large spread of isotopic values between Magmatic Source of Heat for the Steamboat Springs Geothermal System borehole samples is likely because of the different proportions Using Trace Elements and Gas Geochemistry.” Geothermal Resources of sulfide phases present in the hand-picked samples and differ- Council Transactions, v. 27. ent precipitation temperatures. An additional contributor to this Arnorsson, S. and Gunnlaugsson, E., 1985. “New gas geothermometers for data spread may be the presence of minor amounts of anhydrite geothermal exploration – calibration and application.” Geochimica et (observed in BRRC-04, 35 m at Joaquina). Small amounts of this Cosmochimica Acta, v. 49, 1307-1325. phase in the analyzed samples could cause a positive bulk rock Barberi, F., Carapezza, M. L., Cioni, R., Lelli, M., Menichini, M., Ranaldi, isotopic shift. Considering these variables, the isotopic data are M., Ricci, T., Tarchini, L., 2013. “New geochemical investigations in best explained by a sedimentary origin for the sulfur in the Joa- Platanares and Azacualpa geothermal sites (Honduras).” Jounral of Volcanology and Geothermal Research, v. 237, 113-134. quina system. This sulfur is likely the product of devolatilization of low-grade metasedimentary rocks of the El Tambor group, or Bargar, K. E., 1991. “Fluid inclusions and preliminary studies of hydrothermal alteration in core hole PLTG-1, Platanares geothermal area, Honduras.” perhaps deeper underlying units. Journal of Volcanology and Geothermal Research, v. 45, 147-160. Bottinga, Y., 1969. “Calculated fractionation factors for carbon and hydrogen Conclusion isotope exchange in the system calcite-carbon dioxide-graphite-methane- hydrogen-water vapor.” Geochimica et Cosmochimica Acta, v. 33, 49-64. The evidence presented in this paper show that the geothermal Browne, P. R .L., 1979. “Minimum age of the Kawerau geothermal field, system at Joaquina is best classified as a non-magmatic, deep- north island, New Zealand.” Journal of Volcanology and Geothermal circulation-type, akin to those in Honduras (i.e., Platanares and Research, v. 6, 213-215. Azacualpa). Meteoric waters penetrate deep into the subsurface Burkart, B. and Self, S., 1985. “Extension and Rotation of Crustal Blocks and are heated to temperatures of ~180°C (and possibly higher). in Northern Central-America and Effect on the Volcanic Arc.” Geology, Primary fluid upflow is directed along a steep to moderately south- v. 13(1): 22-26. dipping intersection between the NE-trending left-lateral El Puente Cole, D. R. and Ravinsky, L. I., 1984. “Hydrothermal alteration zoning in fault and the inferred east-striking RLC normal fault. Primary the Beowawe geothermal system, Eureka and Lander counties, Nevada.” outflow is directed westward, along the trace of the RLC fault. The Economic Geology, v. 79, 759-767. inferred reservoir rocks of the Joaquina system are thought to be Cole, D. R., 1994. “Evidence for oxygen isotope disequilibrium in selected fractured and thrust-stacked sequences of greenschist to amphibo- geothermal and hydrothermal ore deposit systems.” Chemical Geology lite facies meta-mudstones, meta-siltstones and meta-volcanics (Isotope Geoscience Section), v. 111: 283-296. belonging to the El Tambor group, however, basement Chortis Etiope, G. and Klusman, R. W., 2002. “Geologic emissions of methane to the Block units may also occur at depths >1 km. Fluids at Joaquina atmosphere.” Chemosphere, v. 49, 777-789. are Na-bicarbonate(-sulfate)-rich with low Cl reflecting the lack Faulds, J., Coolbaugh, M., Bouchot, V., Moeck, I., and Oguz, Kerem, 2010, of input of magmatic HCl and the chemical composition of the “Characterizing structural controls of geothermal reservoirs in the Great metamorphic host rocks. Sulfur, CO , and CH in the Joaquina Basin, USA, and Western Turkey: Developing successful exploration 2 4 strategies in extended terranes.” Proceedings World Geothermal Con- system are likely derived from sedimentary sulfur sources. The gress 2010, 11 p. total estimated natural output of the Joaquina system is conserva- Faulds, J. E., Hinz, N., Kreemer, C., and Coolbaugh, M., 2012. “Regional tively estimated at 29.4 MWth. This study illuminates the presence patterns of geothermal activity in the Great Basin region, Western USA: and character of an intermediate temperature geothermal system Correlation with strain rates.” GRC Transactions, v. 36, 897-902. at Joaquina, which is conceivably suitable for development with Fournier R. O. and Potter R.W. II, 1982. “A revised and expanded silica binary geothermal technologies. However, further geophysical and (quartz) geothermometer,” Geothermal Resources Council Bulletin, v. drilling efforts are needed to delineate deep production targets. 11, 3-12.

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