2 Geothermal Resources in Chile……………………………………………………….…………………

List of contents

1. Introduction ...... 1 1.1 RENEWABLE VS NON-RENEWABLE RESOURCES ...... 1 1.2 GEOTHERMAL ENERGY AS RENEWABLE RESOURCE ...... 3 1.3 GEOTHERMAL RESOURCES IN LATIN AMERICA ….…………………………………………….…..…..… 5

2. Geothermal resources: an overview on systems, exploration, technology, environmental impact ..…………………………...... ………………………………………………..………….. 7 2.1 DIFFERENT TYPES OF GEOTHERMAL SYSTEMS ...... 7 2.2 EXPLORATION FOR GEOTHERMAL RESOURCES ...... 8 2.3 STATE OF THE ART OF EXPLOITATION TECHNOLOGY ………………….…………………………….. 12 2.3.1 Advanced power plants technology and unconventional geothermal resources ………………………………………………………………………………………..……….…….… 14 2.3.1.1 Self-superheating (SSH) and combined-cycle power plants …………………………… 15 2.3.1.2 Hybrid systems and combined heat and power production (CHP) ………………... 15 2.3.1.3 Hot dry rocks and enhanced geothermal systems (EGS) ………………….……………. 16 2.3.1.4 Closed-Loop Geothermal Wells ………………….…………………………………………………. 17 2.3.1.5 Coproduction of geothermal and Oil/Gas and geopressured geothermal systems ……………………………………………………………..…………………………………………. 18 2.3.1.6 Supercritical fluids and magmatic systems …………………………….……………………… 19 2.3.2 Direct Use ……………………………………………………………………………………………..…………. 21 2.4 ENVIRONMENTAL IMPACT………………………………………………………………………………………….22

3. Current status of power production from geothermal resources ………………………………. 24 3.1 THE WORLD SCENARIO ……………………………………………………………………………………………… 24 3.2 GEOTHERMAL RESOURCES IN CHILE……………………………………………………….…………………. 26

3.2.1 The Apacheta Geothermal project ………………………………………………………..…………. 27 3.3 GEOTHERMAL RESOURCES IN ECUADOR …………………………………………………………………… 29 3.3.1 The Tufiño-Chiles-Cerro Negro geothermal field ……………………………..………………. 30 3.3.2 The Chachimbiro geothermal field …………….……………………………………………………. 31

4. The application of amphibole thermobarometry to constrain the heat source of geothermal systems in geothermal areas of Chachimbiro (Ecuador), Apacheta and La Torta (Chile) ……………………………………………………………………………………………………….……. 34 I

4.1 INTRODUCTION …………………………………………………………………………………………………….…. 34 4.2 VOLCANOLOGICAL, GEOPHYSICAL, GEOCHEMICAL AND GEOTHERMAL BACKGROUND ………………………………………………………………………………………………………….. 35 4.2.1 The Apacheta and La Torta geothermal areas (Chile) ……………………………………… 35 4.2.2 The Chachimbiro Geothermal Area (Ecuador) …………………………………….……..…… 38

4.3 SAMPLES AND ANALYTICAL METHODS ……………………………………………………………………. 40 4.4 RESULTS ……………………………………………………………………………………………………………..…… 40 4.4.1 Mineral chemistry and petrography ……………………………………….…………..…………. 40 4.4.2 Amphibole texture and classification ……………………………………………………………… 48 4.5 DISCUSSION …………………………………………………………………………………………………………….. 50 4.5.1 Compositional-textural relationship of amphiboles ………………………………………… 50 4.5.2 Amphibole thermobarometry application. The P-T-h diagram ………………………… 55 4.5.3 Comparison between thermobarometric and geophysical constraints ……………. 59

4.6 CONCLUSIONS …………………………………………………………………………………………………………….62

5. Origin of geothermal fluids along active to semi-dormant volcanoes of Northern Ecuador (1°S to 1°N) as inferred from chemical and isotopic composition ………………… 64 5.1 INTRODUCTION ………………………………………………………………………………………………………… 64 5.2 GEODYNAMIC, GEOLOGICAL AND VOLCANOLOGICAL SETTINGS ………………………………. 66 5.2.1 Chiles-Cerro Negro Volcanic Complex ……………………………………………………………… 67 5.2.2 Chachimbiro Volcanic Complex and Cuicocha caldera …………………………………..… 68 5.2.3 Chacana Caldera Complex …………………………………………………………………………..….. 69 5.2.4 Quilotoa volcano …………………………………………………………………………………………..… 70

5.3 SAMPLING AND ANALYTICAL METHODS …………………………………………………………………… 71 5.4 RESULTS …………………………………………………………………………………………………………………… 72 5.4.1 Chemical composition of thermal waters ………………………………………………………… 72 5.4.2 Chemical composition of gases ……………………………………………………………………..… 74 18 13 5.4.3 Isotopic composition of waters (δD, δ O) and gases (δ CCO2 and R/Ra) …………. 76

5.5 DISCUSSION ……………………………………………………………………………………………………………… 77 5.5.1 Thermal water discharges ……………………………………………………………………………….. 77 5.5.2 Gas discharges ………………………………………………………………………………………………… 78

5.5.3 RH and T geoindicator …………………………………………………………………………………..… 80

5.6 CONCLUSIONS …………………………………………………………………………………………………………….83

INTRODUCTION CHAPTER 1

1 Introduction

1.1 RENEWABLE VS NON-RENEWABLE access to fossil fuels greatly affects their RESOURCES consumption rate and depletion times of reserves (Shafiee and Topal, 2009). The energy supply issue has always played an Furthermore most of the fossil fuels are still important role in the development of society, classified as resources and not yet as but with the advent of modern highly reserves, which means they are not fully industrialized and technological realities this characterised or are costly to extract, so they role has become strategic. The world’s cannot be brought to the market (IEA, largest economies and the rapid ascent of 2013a). Using advanced technology to move developing countries need of affordable and from resources to reserves may tend to make reliable energy supply to plan long-term fossil fuels availability last longer but at strategies for meeting social and economic present, the reserves of oil, gas and coal are growth objectives. By far the energy policies expected will last a further 51, 53 and 114 have been based almost entirely on non- years, respectively (British Petroleum, 2016). renewable resources, mainly fossil fuels. Another risk factor is the complex Around 45% more oil, gas and coal is used geopolitical contexts of oil and gas sector. today than twenty years ago (IEA, 2013a) and The major production areas are historically today fossil fuels meet more than 80% of characterized by political instability and total primary energy demand (IEA, 2015a). diplomatic tensions that often result in The current models of economic growth and armed conflicts as happened recently in development are affected by the availability Afghanistan, Iraq, Libya, Syria, Yemen, Russia of raw materials, their cost and their and Ukraine. These scenarios have serious commercial use on the market. The repercussion on production quotas, threaten conventional fuels such as oil, gas, coal and the production, transportation and supply uranium in this respect offer clear infrastructures and thus, ultimately, affect advantages since they are abundant in many the sale price and the availability of regions of the world, the technologies and hydrocarbons. As result oil and gas-importing exploitation techniques are proven and fully countries could become a “hostage” of the available, they have a high calorific value, exporting countries and the changing they are tradable and their by-product are geopolitical situation, taking risk of energy widely used in other manufacturing sectors. crisis as in the past during the conflicts in However, no doubt that this type of energy Middle East in the 1970s. supply suffers from numerous weak points. Last but not least problem is the First of all the availability of non-renewable environmental impact. The production and energy resources will go running out in the supply chain of conventional fuels is highly future. The heavily dependence on continued risky. The potential hazards for hydrocarbon may manifest as crude oil spills at the

INTRODUCTION CHAPTER 1 production site or during the transport to the countries need access to modern, reliable, consumer. For the nuclear sector the main affordable and clean energy infrastructure to challenges comes from the storage of meet rising energy demands and support radioactive waste or the release of their economic development. radionuclides into the environment during These challenges can be addressed by energy the rare, but highly impactful accidents, at efficiency policies and increasing use of the nuclear power plants. On global scale the renewable and sustainable energies which main concern comes from climatic change are attractive for many reasons, including caused by the emission of greenhouse gases supply diversification, increased energy (GHG), predominantly carbon dioxide and security, fostering new industries and skill methane. Suffice it to say that the energy diversification, reducing air and water sector alone represents roughly two-thirds of pollution, and contributing to global efforts all anthropogenic greenhouse-gas emissions to reduce GHG emissions (ESMAP, 2012). and over 90% of energy-related emissions are For some time the international community

CO2 from fossil fuel combustion (IEA, 2015a). has moved concrete steps in order to achieve Projection of long-term energy trends by the a more sustainable development. The Kyoto International Energy Agency in 2015 (New Protocol in the 1990s, the recent Paris Policies Scenario: IEA, 2015b), outline an Agreement (2015) and the UN’s 2030 Agenda energy demand growth by nearly one-third for Sustainable Development are a global between 2013 and 2040 driven, in the same effort to combat climate change and ensure period, by gross domestic product (GDP) access to modern energy services for all growth at an average annual rate of 3.5% and countries by encouraging the use of by world population increase of 0.9% per renewable energies. Since the introduction of year, from 7.1 to 9 billion of people. The sustainable energy policies, the global same projection shows that growth in energy investment in clean energy continues to demand will continue to be met grow. Renewable energy sources have grown overwhelmingly by fossil fuels, with energy- at an average annual rate of 2.2% in the related CO2 emissions 16% higher by 2040. It period 1990-2014 and, in 2014, 13.8% of seems clear that a model of economic growth total primary energy supply was produced fully dependent on fossil fuels poses serious from renewable sources (IEA, 2016). In the problems concerning environmental impact near future low-carbon fuels will continue to and energy security, pointing out how increase as outlined in the New Policies necessary is a transformation of the energy Scenario where demand for energy from sector toward a more efficient use and renewable sources grows by nearly 80% and sustainable supply of energy. its share to total primary energy reach 18% in This goal is even more urgent for low-middle- 2035 (IEA, 2013b). Despite lower fossil-fuel income countries which suffer the high prices prices, there were no signs of weakening of fossil fuels and cannot rely on alternative appetite for renewables: for example in the energy supply such as nuclear since they lack electricity generation sector, renewables- of technology and economic resources. These based power production is estimated to have

INTRODUCTION CHAPTER 1 increased by 128 GW in 2014 which amount of energy that could reasonably be amounted to more than 45% of world power extracted at costs competitive with other generation capacity additions in 2014 (IEA, forms of energy constitutes what is called a 2015a). geothermal resource (Muffler and Cataldi, 1978). Geothermal resources have been classified 1.2 GEOTHERMAL ENERGY AS according to their reservoir fluid RENEWABLE RESOURCE temperatures so that we can distinguish between high (T>180°C), medium Geothermal energy is the heat stored in the (100°C

INTRODUCTION CHAPTER 1 the fluid extraction rate exceeds the reservoir fracturing of hot dry rocks) plants, unless replenishment rate. A long-term sustainable seismicity can be also correlated with production relies upon an exhaustive model operations of hydrothermal power plants of the resource characteristic and a strategic (Sigfússon and Uihlein, 2015). positioning of production and re-injection Among renewable energies, geothermics is wells to minimize the effects of pressure and one of the most versatile because it can be temperature decrease. More than 95% of the used both for heating and power generation, fluid produced is often reinjected into the provides an opportunity to be exploited by reservoir contributing to limit pressure losses cascade utilisation (stepwise usage at and to replace at least part of the fluid progressively lower temperature) increasing extracted (Barbier, 2002). The extraction and the total efficiency and results in economic re-injection processes should be frequently benefits (Sigfússon and Uihlein, 2015), is monitored by geochemical tracers, seismicity, available continuously and is not affected by reservoir pressure and temperature, micro- daily or seasonal variations compared to gravity and results from monitoring tools are other renewable resources (e.g. solar or wind then fed into geothermal system model power). It is considered a base-load energy which aid in planning the exploitation of the since it provide a stable and reliable power resource and predicting its behaviour in the around the clock at a relatively low cost and future (Sigfússon and Uihlein, 2015). with few operational or technological risks. From an environmental perspective, the As reported by Gehringer and Loksha (2012), greenhouse gas emissions of a geothermal once a geothermal power plant is power plant are much lower than those of operational, it will produce a steady output fossil fuel-fired power plants, not to mention usually for several decades, at costs the reduction of emissions due to direct use competitive with other base-load generation of geothermal energy for heating and options, such as coal. Technological risks industrial processes. Geothermal plants emit involved are relatively low because the approximately 5% of carbon dioxide, 1% of geothermal power generation from sulphur dioxide equivalents and less than 1% hydrothermal resources is a mature of nitrous oxide emitted by a coal power technology. Levelized costs of generation are plant of equal size (Sigfússon and Uihlein, typically between US$ 0.04 and 0.10 per kWh 2015). Anyway like any infrastructure (for medium sized plants around 50 MW), development, geothermal installations have offering the potential for an economically a social and environmental impact that have attractive power operation. to be managed during the project Therefore geothermal energy has many preparation and development. The impacts attractive qualities stemming from its various are usually highly localized concerning forms of high, medium and low enthalpy hydrogen sulphide emissions from high largely distributed worldwide, many uses enthalpy power plants and induced seismicity ranging from direct utilization of heat to low during reservoir stimulation with Enhanced cost base-load power production and a Geothermal Systems (EGS; hydraulic mature technology in hydrothermal systems

INTRODUCTION CHAPTER 1 sector and broad prospect of development in types of energy resources for the non-conventional geothermal resources such stakeholders. as the EGS. These characteristics make The countries located along the western geothermal energy one of the most active continental margin of Latin America, important in the panorama of renewable from Mexico to Tierra del Fuego, stand out energies since that it has the potential to among the other countries for the intense offer a strategic alternative to the energy geodynamic processes which have created supply diversification in the key of the ideal geological conditions for one of the renewability and sustainability as well as major exploitation potential of geothermal ensure a lower dependency on the use of energy of the entire planet. Anyway, unless conventional fuels and reduce the some countries of Latin America such as environmental problems and the risk Mexico, Costa Rica and Guatemala have associated with geopolitical framework and already built geothermal power plants, the price volatility. enormous geothermal potential of South America has not yet been exploited. In the last decades, numerous investigation and 1.3 GEOTHERMAL RESOURCES IN exploration programs have been carried out, LATIN AMERICA but as of 2016 only one geothermal power plant, in the Apacheta geothermal field in The best high enthalpy geothermal fields are Chile, has been approved and it is generally found in tectonically and developing. For this reason is crucial to volcanically active areas where the heat flux improve knowledge about resources is high. Geothermal resources have been endowments with the aim to identify the identified in nearly 90 countries and more least risky projects that may attract new than 70 countries already have some investment in the geothermoelectric sector. experience utilizing geothermal energy This PhD thesis maintains a consistent focus (Gehringer and Loksha, 2012). Numerous on high-medium temperature hydrothermal countries worldwide possess a geothermal resources exploitable for power production potential that could meet a substantial part in geothermal areas of Ecuador and Chile of their energy demands. Anyway, unless the through the study of the heat sources of technologies for a commercially use of hydrothermal systems and the thermal fluids geothermal power are fully available and discharged by the reservoir. proven, the share of geothermoelectric In Chapter 2, a briefly overview on different production in the overall renewable power types of geothermal systems, exploration capacity is still quite small at about 0.7% techniques, and exploitation technology is (REN21, 2016). This factor is mostly related given. to the high upfront costs and the risk In Chapter 3, the worldwide current involved in exploration, especially in the utilization of geothermal resources for power drilling phase, making geothermal projects a production is discussed, with special risky investment less attractive than other emphasis to Ecuador and Chile.

INTRODUCTION CHAPTER 1

In Chapter 4, thermobarometric applications are applied to amphiboles. Mineral analyses on crystal from Apacheta and La Torta geothermal concessions (Chile) and Chachimbiro geothermal area (Ecuador) are used to retrieve the pressure-temperature conditions during the crystallization of amphibole with the aim to constrain the depth of magma chambers that could represent the heat source for the geothermal systems. In Chapter 5, geochemical technique, isotopic geochemistry and fluid geothermometry are applied to fluids discharged by thermal springs in geothermal areas of northern Ecuador.

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2

2 Geothermal resources: an overview on systems, exploration, technology, environmental impact

2.1 DIFFERENT TYPES OF Sedimentary geothermal systems: are found GEOTHERMAL SYSTEMS in many of the major sedimentary basins of the world. These systems owe their existence The current commercially exploitable to the occurrence of permeable sedimentary geothermal resource relies upon geothermal layers at great depths (> 1 km) and above systems with availability of hydrothermal average geothermal gradients (> 30°C/km). resources which consist of three natural main These systems are conductive in nature elements: a heat source, a reservoir and a rather than convective, even though fluid. As summarized by Sigfússon and Uihlein fractures and faults play a role in some cases. (2015), a geothermal system is called Geo-pressured systems: are analogous to hydrothermal when a natural aquifer (fluid), geo-pressured oil and gas reservoirs where usually with fracture permeability (reservoir), fluid caught in stratigraphic traps may have coincides with elevated temperatures in the pressures close to lithostatic values. Such crust (heat source, generally represented by systems are generally fairly deep. a magmatic intrusion) and the energy stored Hot dry rock (HDR) or enhanced geothermal in the reservoir is accessible since systems (EGS): consist of volumes of rock groundwater transfers the heat from rocks to that have been heated to useful the surface either through bore holes or temperatures by volcanism or abnormally natural cracks and faults (Fig. 2.1). high heat flow, but have low permeability or A large amount of geothermal resources have are virtually impermeable. Physical and been identified in other types of geothermal chemical treatments are used to fracture the systems which store or are able to produce, rocks creating permeability allowing the fluid with artificial intervention, hot fluids or to circulate throughout the fractured rock steam exploitable for direct utilization or and to transport heat to the surface. power production. According to Shallow resources refer to the normal heat Saemundsson et al. (2009) we can flux through near surface formations and the distinguish: thermal energy stored in the rocks and warm Convective systems: the heat source is the groundwater systems near the surface of the hot crust at depth in tectonically active areas, Earth’s crust. Heat is used directly and with above average heat-flow. Here the extracted with the aid of ground source heat geothermal water has circulated to pumps (home heating). considerable depth (> 1 km), through mostly Another type of unconventional geothermal vertical fractures, to mine the heat from the resource is hydrothermal fluids in rocks. supercritical conditions. Three high- temperature geothermal fields were selected

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 in Iceland as sites for deep drilling operation 2.2 EXPLORATION FOR GEOTHERMAL (Iceland Deep Drilling Project, IDDP) in order RESOURCES to reach natural fluid in supercritical conditions to a minimum depth of some 3.5– Geothermal development is essentially a 5 km, where temperature can be expected to sequential and a systematic process of range between 400 and 600 °C (Friðleifsson exploring of productive sites with the and Elders, 2005; Friðleifsson et al., 2014). ultimate aim of geothermal power With the exception of shallow resources, for production. The development phases begin which the recent developments in the with reconnaissance and exploration, pre- application of ground source heat pumps feasibility, feasibility and finally power plant have opened up a new dimension in their construction (Ochieng, 2013). utilization, the other geothermal systems are During the early stages of a geothermal still in development stage or under study. project set up, a successful surface Anyway, with regard to HDR, they are much exploration could avoid unnecessary more widespread than hydrothermal expenditure when the project enters in the resources and successful EGS have the development phase (Shah et al., 2015). potential to produce power and or heat on According to Shah et al. (2015), geothermal much larger scale than the hydrothermal exploration is a multidisciplinary task which resources.

Fig. 2.1. Schematic view of an ideal geothermal system (source: http://www.bgs.ac.uk/research/energy/geothermal/).

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 entails activities like geological, geochemical radiation that is emitted or reflected by the and geophysical surveys. The main goal of terrestrial surface. These techniques are exploration is to determine the existence of a useful surveying tools for geological mapping, geothermal resource and subsequently identification of thermal anomalies and identify suitable drilling sites based on recognition of hydrothermally altered integrated data obtained by all exploration products like sinter and tuff in prospective methods. geothermal areas (Calvin et al., 2005). Geological survey is of paramount Contextually with geological studies is importance to provide information on the necessary to carry out a detailed hydrologic stratigraphic and structural framework of the survey. The objective is to learn as much as area (Barbier, 2002). A preliminary mapping possible about the fluids in the system, of the area is performed with the including their age, physical properties, identification of the lithologic units and their abundance, flow paths, and recharge modes chronological sequence, structural and (DiPippo, 2012). tectonic setting and mapping of thermal Geochemical survey plays a major role in surface manifestations. Petrological studies preliminary prospecting of geothermal are performed to identify the types of rocks resources and it is an essential tool for and structural mapping is done in the effort monitoring the geothermal system during the to distinguish the types of faults and fissures exploitation phase. Furthermore, that transect the geothermal prospect and be geochemical methods are relatively able to discern the structural controls of the inexpensive when compared to geophysical system (Ochieng, 2013). Many geothermal survey therefore it is used in all stages of fields are related to past, recent or ongoing geothermal exploration and development. volcanism so that in cases of volcanic The major goals of geochemical exploration settings, the eruptive structures, the timing are to obtain the composition of geothermal of events, the nature and volume of the fluids and use this to obtain information on erupted material have to be evaluated in temperature, origin and flow direction with order to understand the volcanological the aim to localize the subsurface reservoir, evolution that could offer insights into the interpret the subsurface conditions and the present, unseen state of the deep formation processes controlling the water chemistry (DiPippo, 2012). An exhaustive mapping of and contribute to conceptual model of the thermal manifestations is performed and the geothermal system (Ármannsson, 2007). physical properties of surface manifestations Geochemical studies of fluids involve are measured and recorded, including sampling, analysis and data interpretation of temperature, flow rate and conductivity water samples from hot spring, gas samples (Árnason and Gíslason, 2009). Geological from fumaroles, bubbling gases from hot survey also makes use of remote sensing pools (Barbier, 2002). technique, that is satellites and/or airborne The fundamental contribution of based multi-spectral or hyperspectral sensors geochemistry to geothermal exploration is used to detect natural electromagnetic provided by interpretation of the behaviour

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 of conservative constituents and the use of Mg) and temperature-dependent fluid- geothermometers and isotopes. The mineral and gas-gas equilibria (Marini, 2000). knowledge of the origin of geothermal As reported by Manzella (1973), a waters is important to discriminate their geophysical exploration play a key role in chemical properties and their source of geothermal exploration since it is direct at recharge. As reported by Ármannsson, obtaining indirectly, from the surface or from (2007), stable isotope ratios studies shallow depth, the physical parameters of (especially 2H and 18O) and conservative the geothermal system. Physical properties constituents ratio (e.g. Cl and B) play an of the subsurface are variably affected by important role in hydrogeological temperature gradient, porosity, saturation, investigation. The isotopes carry imprints of salinity, faulting, and difference in the density the origin of the waters and Cl and B ratio and elastic properties of rocks. The presence can be used to trace origin, mixing and flow of a geothermal system generally causes of geothermal fluids. Similarly, the origins of inhomogeneities in the physical properties of geothermal gases can be traced through the subsurface, which can be observed as 13 isotope ratio studies (e.g. C in CO2 and CH4 anomalies measurable from the surface. 34 and S in H2S). Geothermometry is the Thermal, seismic, gravity, electric, application of geochemistry to infer reservoir electromagnetic and magnetic surveys can be temperatures from the composition of used to detect variations of subsurface geothermal fluids. Many reaction in characteristics, providing valuable geothermal systems are temperature information on the geometry (shape, size and dependent but their kinetics are not fast at depth) of the heat sources, reservoir and cap lower temperature so that their equilibrium rock. It also aims at imaging structures that characteristics are preserved even though are responsible for the geothermal system, the waters rise to the surface and cool down and delineating the areal extent of the (Ármannsson, 2007). Many chemical and geothermal resource (Ochieng, 2013). The isotopic geothermometers are used to following brief description of these methods, estimate the aquifer temperatures beyond unless otherwise specified, is based on the zone of secondary processes like boiling, Barbier (2002): cooling and mixing on the basic assumptions Thermal survey: provides information on the that the sampled fluids are representative of thermal condition of the subsurface, the the undisturbed aquifers where local areal distribution of the heat flow and equilibrium conditions are achieved location and intensity of thermal anomalies. (Ochieng, 2013). The most important and Soil temperature and heat flow used geothermometers are based on measurements can often map hidden temperature-dependent solubility of structures, such as faults or fissures which individual minerals (e.g. SiO2), exchange control flow of geothermal fluid (Ochieng, reactions involving at least two minerals and 2013). the aqueous solution (e.g. Na-K, K-Mg, Na-K- Seismic survey: elastic properties of rocks influence the propagation velocity of seismic

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 waves. Interpretation of seismic information magnetic fields. These methods are more can provide data on the location of active suitable than the above-mentioned electrical- faults that can channel hot fluids towards the resistivity methods since it has a much surface and some indication of the location deeper penetration into the ground, and physical characteristics of heat source. mitigating the screening effect of very Gravity survey: variations in the Earth’s resistive surface rocks. The electrical and gravity field are caused by changes in the magnetic fields are measured at several density of subsurface rocks. They give points and comparison between these fields valuable information on the type of rocks at allows to obtain the resistivities of the depth and their distribution and geometric underlying formations. Magnetotelluric characteristics. Positive gravity anomalies method use natural oscillations of the Earth’s usually imply higher density values which are electromagnetic field due to interaction of normally associated with plutonic intrusions the solar wind with the earth’s magnetic field and dykes while negative gravity anomalies or interaction with electromagnetic signals implying lower densities values caused by originated during electric storm. Part of this higher porosities or by highly fractured parts low frequencies energy penetrates into the of a rock and alteration minerals produced by earth and is used to determine the resistivity circulation of hot water (Pipan, 2009). structure of the sub-surface ranging from a Electrical-resistivity surveys: it quantifies how few tens of meters to several hundreds of strongly a given material opposes the flow of kilometres (Shah et al., 2015). Due to very electric current. Resistivity is largely affected low frequencies used in this method, it is by electrical conduction within waters often possible to achieve a penetration as occupying the pore spaces in the rock. great as 3–5 km with a reasonable degree of Temperature and salinity of interstitial fluids precision. tend to be higher in geothermal reservoirs Magnetic survey: The Earth has a primary than in the surrounding rocks. Consequently, magnetic field which induces a magnetic the resistivity of geothermal reservoirs is response in certain minerals at and near the generally relatively low. It is this contrast in Earth’s surface. By detecting spatial changes resistivity between hot water-saturated rocks of the magnetic field, the variations in and the surrounding colder rocks that is used distribution of magnetic minerals may be in resistivity surveys. This technique is based deduced and related to geologic structure. In on injection of current into the ground and the exploration, magnetic measurements measurements of voltage differences generally aim mainly at locating hidden produced as a consequence at the ground intrusives and possibly estimating their surface. depth, or at tracing individual buried dykes Electromagnetic survey: induction or and faults. On the other hand circulation of electromagnetic methods are a tool for hydrothermal fluids causes alterations in the determining the electrical resistivity rock which lead to a reduction of magnetic distribution in the subsurface by means of susceptibility as a consequence of the measurements of transient electric and destruction of the magnetite contained in the

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 rocks (Ochieng, 2013). The magnetic method flashed into steam, while the greater part may also be used to constrain temperatures remains as boiling water. in the crust using the Curie isotherm (the Vapour-dominated reservoir produces dry temperatures at which minerals lose their saturated or superheated steam at pressure magnetic properties) (Nabighian, 2005) and above atmospheric and are related to high- to confirm the existence of a hot rock (or enthalpy resources. The steam is the near the melting point) mass in the crust continuous predominant phase, regulating (Lichoro, 2014). the pressure in the reservoir. The presence of the cap-rock is of primary importance because prevents the leakage of fluids and 2.3 STATE OF THE ART OF keeps the system under pressure. The following description of technological EXPLOITATION TECHNOLOGY status and development of geothermal

resources for power production is based on Hydrothermal resources of extractible hot the 2014 JRC Geothermal Energy Status water or steam are the most used Report written by Sigfússon and Uihlein geothermal resources for power production (2015). Fluids production from a geothermal and their reservoirs are categorised as: field are derived from oil and gas industry

and conversion systems are based on • liquid -dominated traditional steam-electric power generation • vapour-dominated technology. A good understanding of the

type and temperature of geothermal As reported by Barbier (2002) liquid- resources is of fundamental importance to dominated reservoirs contain water in liquid determine the design of the power plants phase and can be subdivided in hot water and to maximize the efficiency of converting reservoir and wet steam reservoir. Hot water fluid energy into electrical output. reservoir produces liquid at temperatures up Geothermal power plants belong to the to 100 °C which remain below the boiling following categories: point of water at any pressure. Impermeable cap-rocks may be not present. Wet steam • Direct dry steam reservoir contains pressurised water at • Flash cycle and dual flash temperatures exceeding 100 °C with little • Binary amount of steam, generally as discrete bubbles. Water represents the dominant Direct dry steam technology (Fig. 2.2a) is phase and controls the pressure inside the used for exploitation of vapour-dominated reservoir. An impermeable cap-rock generally resources. The steam from production wells exists to prevent the fluid from escaping to (PW) is piped towards a particulate remover the surface, thus keep in it under pressure. (PR) and moisture remover (MR) and finally When fluid is brought to the surface and its transferred into the turbine attached to the pressure decrease, a fraction of the fluid is generator (T/G). The exhausted steam

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 condenses in a condenser (C) and is then flash. The single-flash and double-flash pumped (CP) towards the cooling tower (CT). technology reach efficiencies between 30- The cooling fluid is recirculated by pumps 35% and 35-45%, respectively when (CWP) towards the condenser again or re- electricity is the sole product but overall injected to the reservoir through re-injection efficiency is increased by producing hot wells (IW). The amount of non-condensable water through heat-exchangers. Flash steam gases (mainly CO2 and H2S) is typically power plants are the most common type of between 0.5-10% by weight of steam and geothermal power plants, making up about requires a gas extraction system to be two thirds of geothermal installed capacity. installed. Dry steam power plants reach Binary cycles technology is able to use low- to efficiency of 50-70% (DiPippo, 2012), are medium-temperature resources which are commercially proven, simple to operate and more prevalent compared to high enthalpy require low capital costs and represent fluids and today have an 11% share of the almost a quarter of geothermal power installed worldwide generating capacity and capacity today. However they are only a 45% share in terms of numbers of plants suitable for dry steam reservoirs which are (Bertani, 2012). Binary cycle power plants, not common and unevenly distributed employing organic rankine cycle (ORC) or a resources and almost all the known kalina cycle, operate at lower water geothermal potential have already been put temperatures of about 74-180 °C using the into operation. heat from the hot water to boil a working The flash steam technology (Fig. 2.2b) makes fluid, usually an organic compound with a use of liquid-dominated hydrothermal low boiling point. In ORC plants (Fig. 2.2d), resources with a temperature above 180 °C. geothermal fluid is usually pumped from In the reservoir the liquid water boils, or production wells (PW) towards an evaporator “flashes” as pressure drops. The hot water or (E), and passes a pre-heater (PH) before it is liquid-vapor mixture coming from the pumped back into injection wells (IW). In the production wells (PW) flows through cyclone evaporator a preheated working fluid from a separators (CS), where the steam is pre-heater is boiled prior to entering a separated from the liquid, and moisture turbine unit (T/G). The working fluid is remover (MR) before being directed towards condensed in a condenser (C) and pumped the turbine and generator. The exhausted back (CP) to the pre-heater in a closed loop. steam is condensed in a condenser (C) with Cooling water is pumped (CWP) from a the same flow path of dry steam power cooling tower (CT) towards the condenser plants prior its discharge in re-injection wells and make-up water (M) is pumped into the (IW). In a single flash plant the separated cooling tower to compensate for losses by water from the cyclone separators is pumped evaporation. Kalina plants operate with a towards injection wells. The separated water mixture of ammonia and water and the may also be flashed in a flasher (F) (double- chemical composition of the working fluid is flash) (Fig. 2.2c) where additional steam is adjusted to the temperature of the generated at lower pressure than the first geothermal fluid. In binary plants, the fluid

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 from production wells and the working fluid dry steam, single or double flash and binary are kept separated during the whole process, systems. so there are little or no air emissions. The However the necessity to overcome some ORC can reach efficiencies between 25% and limitations intrinsic to the resource itself has 45% while the kalina cycle can operate up to led to the development of new strategies of efficiencies of 65% (Emerging Energy exploitation and exploration. The main issue Research 2009). related to conventional resources (i.e. hydrothermal systems) concerns the limited 2.3.1 Advanced power plants distribution of natural permeable reservoir technology and unconventional from which to extract high enthalpy hot geothermal resources fluids. Consequently the efficiency of the production facilities is often low since most

of the resources have relatively low Today geothermoelectric production relies temperatures. Another critical point concerns primarily on the availability of conventional the need to install the re-injection wells to geothermal resources such as hydrothermal balance the production rate of geofluids and systems, where hot water or steam extracted the inflow toward the reservoir in order to from a permeable, natural reservoir are maintain a long-term sustainability of the converted into electricity by means of mature geothermal field. This result in higher costs of and widely applied technology that is direct power plant installation since the drilling

Fig. 2.2. Simplified flow diagrams for: a) direct steam geothermal power plant. b) single flash geothermal power plant. c) double flash geothermal power plant. d) organic rankine cycle (ORC) geothermal power plant. Red and yellow line: flow of hot geofluid toward the turbine and condenser or toward the pre-heater (ORC plant); blue line: path of the cooling fluid; green line: path of working fluid in ORC plant. See text for details and acronyms (modified from Sigfusson and Uihlein, 2015).

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 phase frequently constitutes more than 50% double flash type resulting in an increase of of the capital expenditure (Sigfússon and the specific output approximately 5% Uihlein, 2015). Furthermore, from an (Mathieu-Potvin, 2013). environmental point of view, the main issue Another option to increase the exergy of the is represented by emissions of hydrogen resource to useful output is to use a sulphide. Innovative approaches, concerning combination of dry/wet steam and binary technological application to use in power technology, called combined cycle. The heat plants to increase performance of from the low pressure steam coming out of geothermal energy utilization and the the turbine (dry steam cycle) (Budiarto et al. exploration of unconventional geothermal 2014) or the heat from separated geothermal resources, have been conducted with the aim brine (wet steam cycle) are used to run a to improve the use of geothermal energy and binary system (Sigfússon and Uihlein, 2015) to reduce the problems related to it. to produce additional power prior to re- injection. 2.3.1.1 Self-superheating (SSH) and combined-cycle power plants 2.3.1.2 Hybrid systems and combined heat and power production The efficiency of geothermal plants have low (CHP) values due to the low temperature of the steam, which is generally below 250°C, and Exploitation of geothermal energy can be results about three time lower than the enhanced integrating another resource into a efficiency of nuclear or fossil-fuelled plants geothermal power plant. This is referred to (Barbier, 2002). As reported by Budiarto et al. hybrid systems, where the steam originated (2014), the self-superheating (SSH) from the geofluid can be superheated geothermal power plant is a modification of through any other source (e.g. biomass or the geofluid pathway in the single and double coal) (Kagel, 2008). This combination offers flash cycles to increase the power output of the flexibility of determining the optimal the plant. In the SSH system the hot fluid at steam temperature independently of the the wellhead is used, before entering the geothermal fluid temperature and increase separator, to superheat the steam leaving efficiency, and therefore creates more the separator through a heat exchanger. In electricity without expanding the use of the double flash type the self-superheating geothermal resource. Power plants in the approach can be applied at each stage of the order of 5 MWe could become conceivable double flash cycle. In this design the even in areas with a relative low geothermal saturated liquid leaving the first separator is gradient (Kohl and Speck, 2004). A similar brought to the second heat exchanger where approach can be extended by connecting it transfers its heat to the steam leaving the geothermal power plants to industries that second separator (Mathieu-Potvin, 2013). In produce waste heat, such as steel mills or this way the temperature of the steam at the waste incinerators, to enhance the turbine inlet is higher than that for single or temperature of the geofluid and increase

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 power production (Gehringer and Loksha, known as enhanced geothermal systems 2012). (EGS) (Fig. 2.3). Adoption of flash or binary Geothermal energy can be more efficiently technologies may be used with EGS exploited to produce both electricity and also depending on the temperature of geothermal for direct use purposes, in combined heat fluid extracted from the artificial reservoir and power plants (CHP). The geothermal created by hydraulic stimulation. fluid, after being used for power production, EGS technologies have the potential to can be exploited by cascading the residual produce large amounts of electricity almost heat to successively lower temperature anywhere in the world since temperatures of applications including space or district 150°C can be found at depth of 5 km in areas heating, greenhouse heating, aquaculture with normal geothermal gradient of 30- pond and swimming poll heating (Lund and 35°C/km. Thus any convenient volume of hot Chiasson, 2007). As a result the energy dry rock in the earth’s crust, at accessible contained in the fluids is almost completely depth, can become an artificial reservoir used (Gehringer and Loksha, 2012) and the (Barbier, 2002). At the moment, several pilot economics of the entire system is improved projects are being conducted in Australia, (Lund and Chiasson, 2007). Europe, Japan and the US (Sigfússon and Uihlein, 2015). One of these pilot geothermal 2.3.1.3 Hot dry rocks and enhanced power plants has been built by a French- geothermal systems (EGS) Germany industrial consortium in Soultz, France. Three 5 km-depth boreholes (two The exploration and exploitation of production wells and one re-injection well) unconventional geothermal resources could have been drilled and stimulated by hydraulic further expand the diffusion and the benefits and chemical methods. On surface, an ORC of geothermics. Hot dry rocks (HDR) are binary power plant with a net capacity of 1.5 volumes of subsurface rocks abnormally MWe has been installed and put into heated but lacking of permeable fracture and operation in 2008 (Genter et al., 2010). circulating natural fluid which can be However the experimental projects carried exploited through hydraulic fracturing. As out so far demonstrate that there are some reported by Sigfússon and Uihlein, (2015), significant problems in energy production the basic concept is to drill two wells into a with the EGS system and the commercially hot dry rock with limited permeability and viability of the technology has not been fluid content to create permeability in the successfully proven yet (Gehringer and rock by hydro-fracturing the reservoir with Loksha, 2012). As reported by Barla (2008), cold water pumped into the first well (the the hydraulic fracturing in HDR systems must injection well) at high pressure. The second be pushed at a considerable distance from well (the production well) intersects the the injection well, in order to create stimulated fracture system and returns the continuous and stable fractures in the rock hot water to the surface where electricity can through which fluids can circulate with be generated. This technical solution is sufficient flow rate extracting heat from the

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2

from an economic point of view, experts agree that the following key parameters, representing the lower end of the range for each, are required for a commercially-viable HDR reservoir: production flow rate 50-75 kg/s, effective heat transfer area >2 million square meters, rock volume accessed >200 million cubic meters and flow losses (% of injection flow) <10% (Garnish et al., 1992).

2.3.1.4 Closed-Loop Geothermal Wells

An innovative approach to overcome the limitations associated with EGS systems consists in positioning to a proper depth a closed loop heat exchanger to mine heat from the hot rocks. As reported by (Riahi et al., 2017) this systems works with a pressurized working fluid injected from the Fig. 2.3. Schematic view of HDR-EGS system (source: surface into a coaxial-designed wellbore. The http://lifefreeenergy.com/g/geothermal-energy- fluid flows downward through the outer tube home.html). extracting heat from the surrounding hot rocks and rises upward through the thermally surrounding rocks. This implies that high insulated inner tube. In closed-loop system, pressures have to be applied to increase and compared with traditional EGS application, maintain permeability with the risk of no geothermal fluids production take place induced seismicity. Loss of circulating water, and there is no contact between the working progressive scaling of underground fluid fluid and rock formation. Consequently, the pathway, and the limited heat capacity of the hydraulic fracturing is not requested thus artificial reservoir, represent other issues mitigating the risk of induced seismicity, and related to EGS systems. Moreover, as well as pipe corrosion due to aggressive fluids, conventional hydrothermal resources, the scaling of fluid pathway due to minerals geothermal fluids should be treated and re- precipitation or release to the atmosphere of injected underground because of its polluting pollutants from the artificial reservoir, are properties. These operations entail high also avoided. economic costs since they require the drilling A virtual-modelled pilot project of a 1 MWe and maintenance of additional wells, the binary power plant was tested with good treatment and the pumping of the fluids results in Italy (Santucci, 2013) while a (Alimonti and Soldo, 2016). Furthermore, successful demonstration of the world’s first

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2

New Geothermal Power system was carried them by using WellBore Heat Exchangers out in Kokonoe, Japan, in October 2016 (WBHX) in which a heat carrier fluid circulate (Kyoto University, 2016). in a closed loop, in a similar way to what happens in closed-loop geothermal wells 2.3.1.5 Coproduction of (cap. 2.3.1.4). An example of this application geothermal and Oil/Gas and geo- was carried out by Alimonti and Soldo (2016) pressured geothermal systems who evaluated the implementation of the WBHX on the Villafortuna Trecate oilfields in Heated water is a natural byproduct of Italy, one of the largest European oil fields in oilfield production processes that has long which only 8 wells are still producing against been considered unusable (Blodgett, 2014). 50 wells drilled. The authors developed a Field operators in oil and gas industry need to model of heat extraction from a reservoir handle hot fluid with pumping and between 5800 and 6100 m depth with reinjection and this operation requires extra temperature of about 160-170 °C. They set cost (Budiarto, 2014). The idea behind the up a simulation of WBHX considering two coproduction is that the hot water, upon different heat transfer fluids (water and separation from the hydrocarbons in a diathermic oil) and established that optimum separator located on the surface, would pass condition is obtained with water at a 3 through a binary power plant then disposed flowrate of 15 m /h. The thermal power of or used for other purposes (Milliken, output is 1.5 MW and the net electrical 2007). The potential for power production power, generated through an ORC binary from oil fields is substantial. By considering plant, is 134 kW (Fig. 2.4). Another example only the United States, there are 823,000 oil in this regard was proposed by Wight and and gas wells that co-produce hot water Bennet (2015). The authors evaluated the concurrent to the oil and gas production; this electricity generation via a binary cycle equates to approximately 25 billion barrels power plants using water as circulating fluid annually of water which could be used as fuel and abandoned oil fields wellbore as a heat to produce up to 3 GW of clean power exchanger. They established that a power of (ElectraTherm, 2012). In the United States, at 109 kW could be generated using a wellbore the Rocky Mountain Oil Test Center, depth of 4200 m with a mass flow rate of 2.5 Wyoming, geothermal company Ormat kg/s and a given geothermal gradient of Technologies built a successful 0.25 MW 0.0311 °C/m. Well abandonment at present coproduction demo unit which first ran in remains the final outcome for every oil and 2008 (Blodgett, 2014). gas well drilled so that they could represent a Moreover, there are currently many oil and significant source of renewable energy gas wells which are neglected or abandoned (Wright and Bennet, 2015). Furthermore the (Budiarto, 2014). These wells are either deep use of abandoned oil wells has many enough to encounter hot water, or could be advantages ranging from economic to deepened into hot zones (Kagel, 2008). environmental point of view. Dismissing oil Geothermal heat could be extracted from fields costs are reduced and economic

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2

dissolve methane. These characteristics make them attractive for geothermal exploitation allowing the use of mechanical energy stored in the form of high pressure and thermal energy. Furthermore, dissolved natural gases can be extracted and used for further power production or for sale to enhance the economics of a development project. The geopressured formations in the Gulf of Mexico (especially Texas and Louisiana) are estimated to hold tens of thousands of megawatts of geothermal energy, and a hundred year supply of natural gas for the Fig. 2.4. Simplified schematic explanation of WellBore United States (Muffler, 1979). As reported by Heat eXchanger (WBHX) and associated ORC power Taylor (2007), one major test of this plant. P and CP=pumps; E=evaporator; T=turbine; technology has been carried-out at the G=generator; C=condenser (Alimonti and Soldo, Pleasant Bayou site, near Houston, United 2016). States, where a demonstration 1MW binary feasibility of the geothermal plant are power plant powered by a geopressured improved, since drilling cost are avoided, system and extracted natural gas, was put while environmental impact is reduced since into operation from January to May of 1990 no geothermal fluids production take place (Campbell, 2006). Anyway, despite the (Alimonti and Soldo, 2016). advantage of being able to co-produce Some sedimentary basins contain natural gases and electricity through sedimentary rocks with pore pressure geothermal resource, geopressured fields exceeding the normal hydrostatic pressure have not yet been completely explored and gradient. These systems are classified as geo- the resource has yet to achieve commercial pressured geothermal systems. They are status (DiPippo, 2012). confined and analogous to geo-pressured oil and gas reservoirs where fluid caught in 2.3.1.6 Supercritical fluids and stratigraphic traps may have pressures close magmatic systems to lithostatic values. The known geo-pressure systems are found in conjunction with oil When a fluid reaches a temperature and exploration (Saemundsson et al., 2009) and pressure higher than its critical point, its the most intensively explored geo-pressured behaviour became supercritical and the geothermal sedimentary basin is in the distinction between the liquid and the vapour northern part of the Gulf of Mexico and in phases disappears. Pure water passes into its Europe in Hungary. According to DiPippo supercritical behaviour at temperature of (2012), the main properties of these systems 373.95 °C and pressure of 22.064 MPa (Pioro are high pressure, high temperature and and Mokry, 2011). Temperatures in the

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 immediate vicinity of magmatic intrusions bottom of the well reached fluids at exceed the critical temperature of water, supercritical conditions (http://iddp.is/). A implying the possible occurrence of deep well producing from a reservoir with a geothermal water as a single-phase, temperature significantly above 450 °C supercritical fluid (Scott et al., 2015). might, under favorable conditions, yield These supercritical geothermal resources enough high-enthalpy steam to generate 40 - may be very useful for power production 50 MW of electric power. This exceeds, by an since thermodynamic constrains indicate that order of magnitude, the power typically an aqueous hydrothermal fluid at obtained from a conventional geothermal supercritical conditions with a temperature well (Friðleifsson, 2003). As reported by of 400 °C and a pressure of 25 MPa has more Elders et al. (2014), the potential advantages than five times the power-producing of exploitation of supercritical geothermal potential of liquid water at a temperature of resources are a general improvement in the 225 °C (Tester et al., 2007). economics of geothermal power production These kinds of resources are being actually with increase ratio of drilling costs to power investigated by Iceland Deep Drilling Project output per well and increase in the power (IDDP). This is a long-term program to output of existing geothermal fields without improve the economics of geothermal energy increasing their environmental footprints. by producing supercritical hydrous fluids Significant benefits in the life time of existing from drillable depths. Producing supercritical geothermal fields are also obtained by fluids will require the drilling of wells and the increasing the size of the producible resource sampling of fluids and rocks to depths of 3.5– by extending it downwards. 5 km, and at temperatures of 450–600 °C The thermal energy stored in magma bodies, (Friðleifsson and Elders, 2005; Friðleifsson et relatively close to the surface of the Earth, al., 2014). Three geothermal fields, Krafla, represents a huge potential resource that Hengill (Nesjavellir) and Reykjanes, were could be directly tapped and exploited selected as being the most suitable to drill (Barbier, 2002). According to DiPippo (2012), deeper to develop supercritical geothermal the concept is to drill a well until reach the resources (Friðleifsson et al.,2003). In 2009, magma, insert an injection pipe and pump the first drilled well at Krafla, in north-east down cold water at high pressure. The Iceland failed to reach its target as drilling thermal stress induced by the contact had to be terminated at a depth of only between cold water and magma will induce 2.1km when magma unexpectedly entered cracking of the solidifying rock, making it the borehole (Hólmgeirsson et al., 2010; permeable. If the water can be made to Pálsson et al., 2014). Conversely, the drilling return to the surface by passing upward of second well, at the Reykjanes Peninsula, through the cracked, extremely hot glassy was successfully completed on January 2017. material, it would reach the surface hot and Temperature at the bottom of the well has ready for use in a binary power plant. Two already been measured at 427°C, with fluid research projects were conducted by the U.S. pressure of 340 bars making it clear that the Department of Energy in the 1970s and

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2

1980s. The first one was carried out at the to the user as the geothermal fluid usually is lava lake within the crater of Kilauea Iki, unsuitable for direct use due to fluid Hawaii (Hardee et al., 1981). It succeeded in chemistry (Karlsson and Ragnarsson 1995) drilling to the molten lava and several (Fig. 2.5). The main components of a direct experiments were run to understand the use system are the down hole and circulation mechanism of energy extraction from a lava pumps, pipelines, and the heat extraction body. The second project took place at Long and exchange parts (Lund 1998). Valley caldera in Central California with the aim to better understand the existence and behaviour of large magma bodies within calderas (Chu et al., 1990) but it was suspended for lack of funds. Anyway it produced scientific information that led to a better understanding of the Long Valley caldera.

2.3.2 Direct Use

The following discussion is taken from Sigfússon and Uihlein (2015).

Direct use is the oldest form of geothermal energy exploitation by mankind Fig. 2.5. Schematic process flow diagram for a direct (Gudmundsson, 1985) and it is used for use system with heat exchanger (Sigfússon and Uihlein, various purpouse: space and district heating, 2015). greenhouse heating, aquaculture pond A technology that is expanding very quickly is heating, agricultural drying, industrial uses, the ground source heat pumps (GSHP). They cooling, snow melting, bathing and swimming use shallow geothermal energy which is (Lund, 2011). Most of the direct uses of available almost everywhere converting the geothermal energy occur with the low temperature geothermal energy of the exploitation of fluids with temperatures ground to thermal energy at a higher below 150 °C (Blanco Ilzarbe et al. 2013). temperature which can be used for space or Advantages of direct use are a widespread water heating (Ochsner 2008). Usually, a resource available at economic drilling refrigerant is used as the working fluid in a depths and the use of conventional well closed cycle (Self et al. 2013) and electric drilling, heating and cooling equipment (Lund energy is used to drive the compressor (Fig. 2009). 9). The GSHP can be used with other energy A basic direct use system consists in heat sources, such as solar thermal collectors, exchanger that extracts the heat from the whose energy can be added to the GSHP’s geothermal fluid and transfer it to a ground loop to increase efficiency of the secondary clean fluid which carries the heat

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 system. The installed capacity of GSHP has 2.4 ENVIRONMENTAL IMPACT seen a dramatic increase with annual growth rates of 10 % since 1994 with a main focus on Local environmental impacts from Europe and the United States (Self et al. geothermal power replacing the use of fossil 2013). fuels also tend to be positive on balance, due to avoided impacts of fuel combustion on air quality, the hazards of fuel transportation and handling. Nevertheless, like any infrastructure development, geothermal power has its own environmental impacts and risks that have to be assessed, mitigated, and managed (Gehringer and Loksha, 2012). The environmental impact of geothermal activities has been classified by Mannvit hf (2013):

Surface disturbance (access roads, pipe and power lines, land use); Physical effects (effect of fluid withdrawal on surface manifestations, land subsidence, induced seismicity, visual effects due to Fig. 2.6. Schematic process flow diagram for a ground structures); source heat pump system (Sigfússon and Uihlein, Noise (during drilling, construction and 2015). operation);

Thermal pollution (hot liquids and steam According to REN21 (2016), geothermal heat released from discharging boreholes and the represents the 2% share of modern power plant); renewable heat generation and account for Chemical pollution (disposal of liquids and one-half of total final geothermal output (75 solid waste, gaseous emissions); TWh). Over the past few years, direct use of Impact on protected faunas and floras; geothermal heat, excluding heat pumps, has grown by over 3% annually on average, with The most relevant environmental impact geothermal space heating growing around arises during plant operation. Geothermal 7% annually. China, Turkey, Japan and fluids usually contain gases, such as CO , H S, Iceland lead in terms of heat energy 2 2 NH , and CH (called non-condensable gases, generated by direct use of geothermal. 3 4 NCG), which can contribute to global

warming, acid rain or noxious smell if

released into the atmosphere (Gehringer and

Loksha, 2012). Flash and direct steam power

plants direct the geothermal fluids through

GEOTHERMAL RESOURCES: AN OVERVIEW ON SYSTEMS, EXPLORATION, TECHNOLOGY, ENVIRONMENTAL IMPACT CHAPTER 2 the turbines and gases are extracted from water from cooling towers has a higher condensers. Conversely, binary power plants temperature than ambient water, therefore direct the geothermal fluids in a closed loop constituting a potential thermal pollutant through heat exchangers prior to re-injection when discharged to nearby streams or lakes. therefore facilitating nearly emission free The withdrawal and/or reinjection of operation Sigfússon and Uihlein (2015). geothermal fluids may cause ground

Although CO2 constitutes 90% of the NCG subsidence at the surface. In certain areas, (Bertani and Thain, 2002), emissions from this may trigger or increase the frequency of geothermal power generation, while not micro seismic events, unless the distinction exactly zero, are far lower than those between induced seismicity and natural produced by power generation based on seismicity is not easy in hydrothermal fields burning fossil fuels. Data from 85 geothermal Sigfússon and Uihlein (2015). The induced plants, representing 85 percent of global seismicity is primarily associated to EGS due geothermal capacity in 2001, indicate a to the use of rocks fracturing techniques to weighted average of CO2 emissions of 122 create the artificial reservoir and earthquake g/kWh (Friðleifsson et al., 2008). up to M 3.4 were detected in Basel Of the gases emitted from high temperature (Deichmann and Giardini 2009, Zang et al. power plants, H2S is of primary concern due 2014) to its toxicity and because it may constitute a large share of emission (up to 20-35 vol % of NCG) Sigfússon and Uihlein (2015). Although is not present in every geothermal resource, mercury may be emitted during power production both in gaseous and in particulate form. As reported by Gehringer and Loksha (2012) discharge of waste fluids is also a potential source of chemical pollution. After having passed the turbine, geothermal fluids with high concentrations of chemicals, such as sodium chloride (NaCl), boron (B), fluoride (F), or heavy metals such as mercury (Hg) and arsenic (As), should either be treated or reinjected into the reservoir. Fluids coming from low to medium temperature geothermal fields, as used in most direct-use applications, generally contain low levels of chemicals. Furthermore the pollution it is not only limited to the chemical composition of waters but also to the residual heat. Waste

CURRENT STATUS OF POWER PRODUCTION FROM GEOTHERMAL RESOURCES CHAPTER 3

3 Current status of power production from geothermal resources

3.1 THE WORLD SCENARIO sources, the share of geothermoelectric production in the overall renewable power The exploitable geothermal energy potential capacity represents only 0.7% of the total in several parts of the world is far greater (REN21, 2016). The underutilization of than the current utilization and geothermal geothermal resources can be explained by power has an important role to play within considering that the current commercially the energy systems of many countries since available geothermal power technology relies that it can deliver a clean, reliable and locally upon the availability of hydrothermal produced baseload power. It has been resources (i.e. underground sources of estimated that nearly 40 countries worldwide extractible hot fluids or steam) to energize possess enough geothermal potential that the power plant and estimates are that could, from a purely technical perspective, geothermal resources, in the form of hot satisfy their entire electricity demand. steam or fluids, are only available on 1/4 to Geothermal resources have been identified 1/3 of the planet’s surface (Gehringer and in nearly 90 countries and more than 70 Loksha, 2012). Other issues are related to countries already have some experience high upfront investments needed for the pre- utilizing geothermal energy (Gehringer and survey, exploration and test drilling phases Loksha, 2012). Currently, electricity from and to high resource risks at initial stages of geothermal energy is produced in 23 development of a geothermal project (Fig. countries (Bertani, 2015) (Fig. 3.1) but, as of 3.2). 2011, only 0.3 percent of the world’s total However, even if slowly, the use of power generation come from geothermal geothermal resources has been growing for resources (ESMPA, 2012) and, also when decades (Fig. 3.3). Short term forecast compared with other renewable energy highlights a power capacity increase by 70%

Fig. 3.1. Installed capacity (GWe) in 2015 worldwide from geothermal resources (Bertani, 2015).

CURRENT STATUS OF POWER PRODUCTION FROM GEOTHERMAL RESOURCES CHAPTER 3

example. Central and South America countries, especially the western regions, are located along an active convergent plate margin characterized by Quaternary volcanism which ensures the presence of diffuse heat flow anomalies. Geothermal exploration in Latin America started with the study and exploration of the El Tatio area in Chile in the’20s (Tocchi, 1923), but it is only since the ‘70s and ‘80s that the governments of the main countries Fig. 3.2. Cumulative cost and risk profile at various in the Region started an extensive campaign stages of development of geothermal project of investigation in search of geothermal (Gehringer and Loksha, 2012). resources. Mexico was the first country to to 2020 (Bertani, 2015) (Fig. 3.3) and the generate electricity from geothermal sources share of geothermal power in the overall in 1973 then El Salvador in 1975, which were energy balance of the world is expected to followed by Nicaragua in 1983 and Costa Rica grow to 0.5% by 2030 in the International and Guatemala in the ‘90s. As of 2015 the Energy Agency’s conservative Current Policies geothermoelectric capacity in Latin America Scenario or to about 1.0% in the aggressive is 1,629 MW but it represents just 13% of 450 Scenario (Gehringer and Loksha, 2012). global electric power generation from As of 2015 the global total of geothermal geothermal resources and moreover it is power has been estimate to be 13.2 GW with generated only in Central America since that about 315 MW of new geothermal power do not exist operating geothermal power completed in 2015 (+2.3% compared to 2014) plants in South America (Bona and Coviello, (REN21, 2016). The United States and 2016). Philippines have the biggest installed capacity of geothermal power plants, 3,450 MW and 1,870 MW, respectively (Fig. 3.1) and countries that added capacity during the year 2015 were Turkey, the United States, Mexico, Kenya, Japan and Germany (REN21, 2016). With sufficient investment in drilling and improved knowledge about resource endowmnents, geothermal energy could have a much larger role in Central and South America, the Caribbean, East Africa and Southeast Asia (ESMPA, 2012). The Fig. 3.3. Installed capacity (MWe) from 1950 up to 2015 (modified from Bertani, 2015). unexpressed potential of geothermal resources in America Latina is a striking

CURRENT STATUS OF POWER PRODUCTION FROM GEOTHERMAL RESOURCES CHAPTER 3

3.2 GEOTHERMAL RESOURCES IN CHILE

Early geothermal exploration in Chile began in ‘20s with drilling of the El Tatio geothermal field (Tocchi, 1923). At the end of ‘60s a new program of geologic, geophysical and geochemical investigation was carried out by the Chilean Development Corporation (Corporación de Fomento de la Producción, CORFO) and the United Nations Development Program (UNDP) (Lahsen, 1976). Between the ‘70s and the ‘80s numerous exploration, drilling and feasibility studies have been conducted (Lahsen 1988; Hauser, 1997; Peréz, 1999) until closure of exploration programs in 1982. By early 2000, a new impulse was given by the enactment of a law providing the framework for the exploration and development of geothermal energy in Chile. The high-temperature areas in Chile are located along the Andean Cordillera in close spatial relationship with active volcanism which is primarily controlled by the convergence of the Nazca and South Fig. 3.4. Quaternary volcanic zones of South America American Plates. The main geothermal controlled by subduction of Nazca Plate and Antartic systems occur in the extreme northern (17- Plate beneath South American Plate. The volcanic arc 28 °S) and central-southern part (33-46 °S) of is subdivided in Northern (NVZ), Central (CVZ) and Southern Volcanic Zone (SVZ) (modified from Thorpe Chile (Fig. 3.4). Flat slab geometry of and Francis, 1979; Thorpe et al., 1984). subducting oceanic crust generates areas of volcanic gaps (Barazangi and Isacks, 1976) (Fig. 3.5a) has about 90 hot-spring areas (28-33 °S and 46-48 °S) where thermal (Hauser, 1997) and 45 exploration manifestation, as well as in the Coastal concessions are being surveyed; the most Range, are scarce and their temperature are advanced exploration programs have been usually lower than 30 °S (Lahsen et al., 2010). conducted in the Colpitas, Apacheta, Pampa In the country there are more than 300 Lirima and El Tatio-La Torta geothermal geothermal areas associated with Quaternary prospects (e.g. Urzúa et al., 2002; Aguirre et volcanism. As reported by Lahsen et al. al., 2011; Sofia and Clavero, 2010). (2015a), the Northern Chile Geothermal Zone Exploratory wells have been drilled in all of

CURRENT STATUS OF POWER PRODUCTION FROM GEOTHERMAL RESOURCES CHAPTER 3 these areas, and the estimated combined studies have been completed at the power potential of exploitable geothermal Tinguiririca, Calabozos, Laguna del Maule, energy of these four prospects is between Chillán and Tolhuaca geothermal areas (e.g., 400 and 1,000 MWe. There are currently 3 Clavero et al., 2011; Sofia and Clavero, 2010; geothermal systems in the country with Melosh et al., 2010, 2012; Hickson et al., available measured wellhead resource 2011). Exploratory wells have been drilled in values: Apacheta (9 MWe); El Tatio (23 these prospects and the estimated combined MWe); and Tolhuaca (13 MWe) (Aravena et power potential for the five areas ranges al., 2016). These wells yield a total confirmed from 650 to 950 MWe. Exploitation power potential of 45 MWe (Aravena et al., concessions were granted for the Laguna del 2016). Exploitation concessions have been Maule (Mariposa) and Tolhuaca (San granted for the Apacheta and El Tatio Gregorio) projects, where production size geothermal fields and the environmental wells have been drilled. assessment for the installation of a 50 MWe power plant has been approved for the 3.2.1 The Apacheta Geothermal project Apacheta field. In the Central-Southern Zone (Fig. 3.5b) there The Apacheta geothermal fields is located in are about 200 hot-spring areas (Hauser, the Antofagasta Region, Northern Chile about 1997), and 31 exploration concessions have 100 km NE from city of Calama, at about been granted; the most advanced exploration 4500 meters a.s.l. The geothermal system is

a b

Fig. 3.5. Geothermal prospect of a) Northern Chile and b) Central- Southern Chile (Lahsen et al., 2015b)

CURRENT STATUS OF POWER PRODUCTION FROM GEOTHERMAL RESOURCES CHAPTER 3 build up upon a large zone of silicic volcanism et al., 2002) (Fig. 3.6a, b). Temperature >200 occupying the 21°-24° south segment of the °C was successively measured at depth > 500 Andean Central Volcanic Zone (AVCZ; de m in a 550 m-deep core-hole carried out by Silva, 1989; de Silva et al., 1994) and it is the Empresa Nacional de Geotermia in hosted by a 3 to 5 km wide NW tranding October 2007. Four commercial-diameter graben nesting the Pliocene-Pleistocene lavas exploratory 2000 m deep wells, drilled by and pyroclastic products of Apacheta- Geotermica del Norte S.A. between August Aguilucho Volcanic Complex (AAVC). 2009 and October 2010, allowed to detect The Chilean mining company CODELCO the presence of water-dominate reservoir discovered the geothermal field in 1999, with temperature of about 260 °C. during a shallow water exploration well On September 2015 was announced the (Salgado and Raasch, 2002). MT and TDEM construction, in the Apacheta geothermal survey detected a low resistivity boundary field, of the first power plant powered by (<10 ohm-m) extending over an area of 25 geothermal energy of South America. The km2 and geochemical survey indicated ORC binary plant will comprise two 24 MW reservoir temperature of 250-260 °C (Urzua units which, when fully working, will be able

Fig. 3.6. a) Map of the Apacheta prospect. Thick black line indicates location of cross section depicted in b). b) Conceptual cross section of temperature and MT resistivity (Urzua et al., 2002). c) Production and re-injection platforms of power plant. Red squares and lines: production platforms and pipeline system of hot fluid toward the plant. Blue squares and line: re-injection platforms and pipeline system to re-injection wells. Green squares: platforms

that have already been constructed in the context of the exploration stage (Enel Latinoamérica (Chile) Ltda.).

CURRENT STATUS OF POWER PRODUCTION FROM GEOTHERMAL RESOURCES CHAPTER 3 to produce nearly 340 GWh per year, 3.3 GEOTHERMAL RESOURCES IN equivalent to the needs of consumption of ECUADOR 165,000 Chilean homes, and avoiding the emissions of more than 166,000 tons of CO2 Ecuador is located on the active convergent per year (Rojas, 2015). The project margin of South America, in the Andean contemplates the construction and operation Northern Volcanic Zone (Fig. 3.4). It is of a maximum of 20 geothermal wells, being characterized by Quaternary volcanism either production or reinjection, located in 11 resulting from the eastward subduction of individualized platforms (Fig. 3.6c). In each the Nazca Plate beneath the South American platform, a maximum of 4 wells can be Plate. In Ecuador there are more than 50 perforated. The wells will be perforated until volcanoes, many of which are of Holocene they reach the geothermal reservoir, with an age and still active (Hall et al., 2008). The estimated depth of 1,900 to 2,700 meters intense and widespread volcanic activity (Dixon and Nakagawa, 2016). ensures the presence of important The commissioning of the first 24 MW unit is exploitable crustal thermal anomalies. expected to be in early 2017, while that of Earlier geothermal exploration in Ecuador the second unit in the first months of 2018. began in 1979 with the “Proyecto de

Fig. 3.7. Geothermal areas of Ecuador and location of main geothermal prospect for electric generation (modified from INER, 2015).

CURRENT STATUS OF POWER PRODUCTION FROM GEOTHERMAL RESOURCES CHAPTER 3

Investigación Geotérmica de la Repubblica de survey studies (Chimborazo, Oyacachi, Ecuador” (INECEL, OLADE, BRGM, Aquater) Salinas de Bolivar, Cuicocha, Tungurahua, San with the aim to find high-temperature Vicente, Portovelo) while others are only hydrothermal systems along areas of recent identified by vulcanological signs and by the volcanism. The report summarized the areas presence of hot springs (Cayambe, Pululahua, of interest in two main groups: a high Guagua Pichincha, Imbabura, Mojanda, temperature group, Tufiño, Chachimbiro and Iguán, Soche and Reventador). Among the Chalupas and a low-temperature group, Ilaló, most advanced projects pre-feasibility Chimborazo and Cuenca. Between 1981 and studies provided favorable results for high- 1992, INECEL with the collaboration of temperature resources at Tufiño-Chiles-Cerro Colombian ICEl carried out prefeasibility Negro, Chachimbiro and Chacana while in the studies on bi-national geothermal prospect of case of Chalpatán have determined the Tufiño-Chiles-Cerro Negro located at the existence of a resource of low temperature. border between Ecuador and Colombia. In Tufiño-Chiles-Cerro Negro, Chachimbiro and the same period other investigation through Chacana are therefore the only projects in geochemical and isotopic technique were Ecuador that currently have clear carried out in various geothermal areas. development prospects for electricity Anyway the geothermal investigation generation. program ended in 1993. In 2008, the Ecuadorian government through MEER, re- 3.3.1 The Tufiño-Chiles-Cerro Negro starts geothermal exploration, aiming to geothermal field develop the former geothermal prospects for power generation. In 2010 MEER launched This prospect is located in Ecuador, on the the Geothermal Plan for electricity Western Cordillera, at 35 km west of the city generation, which described and priorized 11 of Tulcán, 7 km west of the villages of Tufiño geothermal prospects countrywide. These and Chiles, in the province of Carchi prospects were: Chachimbiro, Chalpatán, (Ecuador) and Nariño department Chacana-Jamanco, Chalupas, Guapán, (Colombia). The development area lies across Chacana-Cachiyacu, Tufiño, Chimborazo, the Ecuador-Colombia border. Late Chacana-Oyacachi, Baños de Cuenca and Pleistocene andesitic to dacitic Chiles volcano Alcedo. and the adjacent to the west Cerro Negro de According to INER (2015) (Fig. 3.7), there are Mayasquer constitute the main heat source four prospects classified as high enthalpy of the area. The tectonic framework is resources (Tufiño-Chiles-Cerro Negro, characterized by active NNE trending regional Chacana, Chachimbiro and Chalupas) in strike-slip faults (Cepeda et al., 1987) and Ecuador, three medium-low enthalpies local NNE and NW trending fault systems (Chalpatán, Baños de Cuenca and Ilaló) and cutting the volcanic complex (ICEL, 1983). other prospects with not enough information As reported by Coviello (2000), in 1978 to be cataloged. Some of these not cataloged INECEL (Instituto Ecuatoriano de prospects already have certain preliminary Electrificación) carried out the first

CURRENT STATUS OF POWER PRODUCTION FROM GEOTHERMAL RESOURCES CHAPTER 3 geothermal reconnaissance activities during 200-300 m deeper, indicating a drilling target which identified the presence of several for production at 1,000 to 1,500 m depth; relatively high temperature thermal sources this is indicated by the presence of a thick and volcanic structures of recent age. In conductive layer, which is shallow (about 100 1979, the OLADE (Organización m) below the acid springs, but deepens 400 Latinoamericana de Energía) with the to 500 m towards the east (outflow). technical support of Italian (Aquater) and In 2009, a shallow (500 m), small gradient French (Brgm) companies operating in the bore holes was drilled in Ecuadorian side of energy sector, carried out a preliminary the prospect. It was the very first geothermal geological and geochemical study on the hole to be drilled in Ecuador and reached a most promising areas for the existence of total depth of 554 m. Lithology comprises of geothermal resources. In 1982 the till, lavas and thick sequence of rather low governments of Ecuador and Colombia permeability volcaniclastics (microbreccias) signed an agreement to develop a joint and associated sediments as sandstone and geothermal exploration that marked the siltstone. beginning of the “Binational Geothermal Recently, in 2013, new fundings allowed to Project Tufiño-Chiles-Cerro Negro. During the carry out complementary studies, comprising 1980’s were carried out pre-feasibility studies geology (mainly 40Ar/39Ar dating of rocks and on behalf of OLADE, INECEL and ICEL defining alteration mineralogy), geochemistry (Instituto Colombiano de Energía Eléctrica) (focused on gas analyses and isotope which detected the presence of a 2000 determination, D/18O, He, 13C and 34S) and meters-deep reservoir with temperature geophysics based on about 100 MT/TDEM higher than 200 °C and a shallower reservoir stations. This international contract was won between 500 and 1000 meters with by SYR in late 2013 and field work on geology temperatures up to 150 °C. and geochemistry started February 2014 on As reported by Beate and Urquizo (2015), the Ecuadorian side. acid hot springs, up to 55°C occur 2 – 3 km to the east of Volcan Chiles, along E-W faulting 3.3.2 The Chachimbiro geothermal field and extensive areas of hydrothermally altered rocks are found few kilometers south The following discussion is based on Beate of Volcán Chiles indicating likely that the and Urquizo (2015). shallow part of the system has sealed up. Gas This prospect is located in Ecuador, on the geothermometers indicate reservoir east slopes of the Western Cordillera about temperatures as high as 230°C. Resistivity 70 km NNE from Quito and 17 km NE from data suggest the existence of a geothermal Ibarra. Long-lived Quaternary andesitic- reservoir with a fault controlled eastwards dacitic Chachimbiro volcanic complex lateral outflow on the east flank (INECEL- represents the heat source for the system OLADE-AQUATER, 1987). Elevation of top of and several mixed chloride-bicarbonate hot reservoir is below 3,100 masl with 100°C and warm springs, with temperatures up to waters, but exploitable temperatures are 61 °C, are located on the E and SE slopes of

CURRENT STATUS OF POWER PRODUCTION FROM GEOTHERMAL RESOURCES CHAPTER 3

volcanic structure. The subsurface rocks are However, although generally encouraging, formed by thin layer of late Tertiary volcanic the relatively high values of resistivity in products and by a folded and faulted late some parts of the clay seal appear to indicate Cretaceous volcanic and sedimentary potentially permeable gaps that complicate basement of accreted oceanic affinity. The the conceptual model and increase the risk tectonic setting is dominated by regional NE- that a reservoir could not be present. SW trending dextral strike-slip faults with Integration of the geology, fluid geochemistry extensional component providing both the and geophysics survey results yields three conduit for the magmatic intrusion as well as possible conceptual models for the resource: the pathways for the ascent of deep thermal model 1, moderate to high temperature fluids and ensuring a favourable environment geothermal resource are associated with the for the development of a high permeability Azufral fault zone and an optimistic fracture network. Hydrothermal alteration interpretation of the resistivity anomaly occurs and smectite-chlorite haloes around provides an upside areal extent of 12 km2. By advanced argillitic alteration might indicate applying a range of power densities from 10 that the temperature of the thermal to 20 MWe per km2, probabilistic manifestations was higher in the past. The assessments of resource capacity provide a geochemistry of prospect waters and gas is range of 13 to 178 MWe with a mean complex and does not lead to a single resource size of 81 MWe. Model 2, the high interpretation. Solute and gases temperature system has peaked and is now geothermometry indicates temperatures up in a cooling and waning phase. Model 3, the to 235°C and 206°C, respectively. Geophysical system is immature and the fluids have failed surveys (MT) resolves an extensive, low to achieve chemical equilibrium. This latter is resistivity, smectite clay alteration zone the most pessimistic model and would imply consistent with the clay alteration that caps that a commercial resource does not exist at almost all developed geothermal reservoirs. Chachimbiro. A conceptual model of

Fig. 3.8. NW-SE cross section of MT resistivity. Conceptual model of Chachimbiro geothermal system (SYR: Chachimbiro Pre-Feasibility Study presentation: informe final). 32

CURRENT STATUS OF POWER PRODUCTION FROM GEOTHERMAL RESOURCES CHAPTER 3

Chachimbiro prospect is shown in Figure 3.8 (SYR: Chachimbiro Pre-Feasibility Study presentation: informe final). Ecuadorian utility Corporacion Electrica del Ecuador (Celec) will start initial preparation work for exploration work at the Chachimbiro geothermal prospect. Celec is in discussion with “Japan International Cooperation Agency (JICA)” on technical cooperation for advanced stage of pre-feasibility studies, including the first deep geothermal drilling (Hristova, 2015).

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THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

4 The application of amphibole thermobarometry to constrain the heat source of geothermal systems in geothermal areas of Chachimbiro (Ecuador), Apacheta and La Torta (Chile)

4.1 INTRODUCTION Renzulli 2012; Simakin et al. 2012; Molina et al. 2015). The high sensitivity of amphibole to Knowledge of physico-chemical parameters physical-chemical changes makes it a good of magmatic systems is crucial to describe tracer of sub-volcanic processes such as the processes of cooling and differentiation magma storage, mixing and ascent. In and to formulate petrogenetic intepretations. literature there are many works concerning Thermobarometric models based on the use of amphibole compositional data to chemical equilibria among coexisting unravel magmatic processes and pre-eruptive mineral-mineral or mineral-melts pairs conditions of subduction-related volcanic (Andersen and Lindsley, 1988; Beattie, 1993; systems (Rutherford and Hill, 1993; Holland and Blundy, 1994; Putirka et al., Rutherford and Devine, 2003; Humphreys et 2003; Putirka, 2005) are useful tools widely al., 2006; Ridolfi et al., 2008; Rutherford and used to estimate the P-T path and chemical Devine 2008; Thornber et al., 2008; Ridolfi et evolution during igneous processes (Civetta al., 2010a; Ridolfi and Renzulli 2012; Costa et et al., 1998; Scaillet and Evans, 1999; Lindsay al., 2013; De Angelis et al., 2013; Shane and et al., 2001; Bachmann and Dungan, 2002; Smith, 2013; Erdmann et al., 2014; Kiss et al., Thornber et al., 2003; Blundy et al., 2006; 2014). The good reliability achieved with Herzberg et al., 2007; Martins et al., 2008; amphibole thermobarometry to constrain the Savov et al., 2008; Stroncik et al., 2009; sub-surface P-T conditions (Ridolfi et al., Turner et al., 2013; Yücel et al., 2014; Ridolfi 2010a; Ridolfi and Renzulli, 2012), makes it et al., 2015). possible to extend this application to high An important contribution to enthalpy geothermal energy studies for thermobarometry comes from calcic which the preliminary prospecting stage amphibole since it crystallizes in a wide range needs tools able to identify the depth and of physico-chemical and compositional chemical-physical conditions of magma body conditions (Anderson, 1980; Niida and Green, which represents the heat source of the 1999; Féménias et al., 2006; Evans, 2007; geothermal system. So far there are not Ishimaru and Arai, 2008; Ridolfi et al., 2010a; examples from literature in this regard. Ridolfi and Renzulli, 2012) and was recently Anyway, the high sensitivity of amphibole used by several authors to relaunch old composition to several and somehow mutual (Anderson et al. 2008) and develop new factors such as temperature (T), pressure (P), thermobarometric applications (Ridolfi et al. oxygen fugacity (fO2), melt composition and 2010a; Krawczynski et al. 2012; Ridolfi and volatile content (H2Omelt), make the

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 application of amphibole-related approaching, as much as possible, total thermobarometers difficult any time a equilibrium conditions in both textural and system depart from chemical equilibrium. chemical terms. Since this thermobarometric Indeed rapid or significant variations in these model was obtained at “perfect” equilibrium parameters can lead to amphibole conditions, a fundamental aspect is to avoid disequilibrium texture such as reverse and its application to amphiboles showing clear oscillatory zoning, fast crystal growth and evidence of crystallization at disequilibrium quenching (Rutherford and Hill, 1993; Scaillet conditions. In this work we suggest a method and Evans, 1999; Kuşcu and Floyd, 2001; Sato to avoid as much as possible the application et al., 2005; Browne and Gardner, 2006; of amphibole thermobarometry to Ridolfi et al., 2008; Humphreys et al., 2009; compositions resulting from disequilibrium De Angelis et al., 2013; Shea and Hammer, crystallization. Furthermore we test the 2013). These disequilibrium states, if not reliability of the barometric model to high properly considered, can result in misleading, pressures comparing our results to a model anomalous and even erroneous of seismic tomography for the AGA system thermobarometric applications such as the (Ward et al., 2014). use of granitoid amphibole barometers to high temperature crystallizing conditions (e.g. Kiss et al. 2014). 4.2 VOLCANOLOGICAL, GEOPHYSICAL, This work tests the capability of amphibole GEOCHEMICAL AND GEOTHERMAL thermobarometry to characterize the heat BACKGROUND sources of the Chachimbiro Geothermal Area (CGA), located in the northern sector of 4.2.1 The Apacheta and La Torta Ecuador, Apacheta Geothermal Area (AGA) geothermal areas (Chile) and La Torta Geothermal Area (TGA), located in the northern sectors of Chile. We used the The Apacheta geothermal area (AGA; about application of Ridolfi and Renzulli (2012) 100 km NE of the city of Calama) and La Torta (hereafter R&R2012) allowing to estimate the geothermal area (TGA; about 100 km E of the intrinsic physico-chemical parameters with city of Calama) are located in the reasonably low uncertainties (T±24 °C, northernmost sectors of the Altiplano Puna P±12%, fO ±0.4 log units) and the 2 Volcanic Complex (APVC) close to the Chile- composition of the melt in equilibrium with Bolivia border, above an exceptionally thick Mg-rich calcic amphibole in a wide range of continental crust (>70 km, Schmitz et al., conditions, up to 1,130°C and 2.2 GPa. We 1999) in a large area of silicic volcanism have chosen this occupying the 21-24°S segment of the thermobarometric/chemometric model Andean Central Volcanic Zone (ACVZ; de because as far as we know it is the only Silva, 1989; de Silva et al., 1994; Stern, 2004) amphibole application calibrated with (Fig. 4.1 a, b, c). The ignimbrite flare up that accurately selected experimental amphiboles produced the APVC is characterized by

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 episodic volcanism over the last 11 Ma time- Zandt, 1999; Zandt et al., 2003; Ward et al., span that climaxed at about 4 Ma (de Silva, 2014), indicate that the magmatic system of 1989). Since peak activity, the temporal and the APVC is currently active (de Silva, 1989). spatial record of volcanism suggests a waning The AGA extends within a NW-SE oriented of the ignimbrite activity over the past 2 graben (Inacaliri Graben) between the million years (Salisbury et al., 2011) with only Quatenary Azufre volcano toward NW and one other super-volcanic eruption at 2.6 Ma the Pliocene Lailay volcano to the south. The (Casey, 2011). The crystal-rich ignimbrites are geothermal system is associated with the distribuited over an area of about 70,000 km2 basaltic andesite to rhyolite Plio-Pleistocene and are characterized by predominantly calc- Apacheta-Aguilucho Volcanic Complex alkaline dacite with rare rhyolite and (AAVC) (Mercado et al., 2009) including the andesites (de Silva et al., 2006). The associated Apacheta (also known as occurrence of dacite to rhyolite lavas and Pabellòncito) and Chac-Inca domes (Fig. 4.1 domes (tortas) erupted in the past 100 ka c) dated at 50 ± 10 (K/Ar method; Urzua et and the existence of a partially melted zone al., 2002), 80-130 Ka (Ar-Ar method; Renzulli in the upper crust of the APVC, i.e. Altiplano- et al., 2006) and 140±80 Ka (Ar/Ar method Puna Magma Body (APMB; Chmielowsky and (report ENG, Empresa Nacional de Geotermia

Fig. 4.1. a) Image showing the subduction zone of South America with the subdivision in Andean Northern, Central, Southern and Austral Volcanic Zone (modified by Cordani et al. 2000); the red rectangles indicate the location of CGA, AGA, TGA geothermal areas. b) image showing location of APVC, AGA and TGA (modified by Piscaglia 2011) and c) enlarged view of AGA and TGA with the position of studied volcanic structures. Location of El Tatio geothermal area is also reported. d) enlarged view of CGA with the border of Chacimbiro Volcanic Complex inside which are shown the location of Pucarà dome (red circles) and the other main volcanic structures (yellow circles); toward NNW are located other two important volcanoes of the area, Pilavo and Yanaurcu.

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

S.A., Chilean National Geothermy Company), belonging to the zeolite (shallower depths) respectively. The 1.5 Ma-years-old Chanka and argillitic facies (Piscaglia, 2011). Thermal dome (Roobol et al., 1974) (Fig. 4.1 c) lies fluid discharges of the Apacheta fumarolic about 13 km toward NW, on the system have outlet temperatures varying prolongation of the southern fault of Inacaliri between 83.2 and 84.3 °C. The chemical Graben. According to Piscaglia (2011) AGA composition of dry gases is dominated by the products are classified as high-K calc-alkaline presence of CO2 and N2 (Tassi et al., 2010). andesites, dacites and rhyolites [57.7-69.9 Geochemical surveys and related gas wt% SiO2; 5.5-7.9 wt% (Na2O+K2O); 2-4.5 K2O, thermometers (CO2/Ar and H2/Ar) of the AGA normalized to an anhydrous basis] (Fig. 4.2). indicate reservoir temperatures of ca 250°C A temperature >200°C was found at depths (Urzua et al., 2002). Additional reservoir

>500 m in a 550 m-deep core-hole recently temperatures, estimated with the H2/Ar carried out on October 2007 [ENEL (Ente (Chiodini et al., 2001) and organic gas Nazionale per l’Energia Elettrica, Italian geothermometers (Capaccioni and Mangani, National Electric Energy Company), personal 2001; Tassi et al., 2005) are particularly high communication)] whereas MT (>330°C), possibly in relation to the presence (Magnetotelluric) and TDEM (Time Domain of a deep magmatic system still active in the Electromagnetic) geophysical survey area, as also indicated by the relatively high detected a low resistivity boundary (< 10 contents of light alkenes, HCl and SO2 (Tassi Ω·m) extending over an area of 25 km2 et al., 2010). (Urzua et al., 2002). The ENG has recently La Torta geothermal area is dominated by the obtained the approval of the environmental tabular Pleistocene La Torta dome dated impact assessment for the installation of a 50 34±7 Ka (Ar/Ar method, Renzulli et al., 2006) MWe geothermal power plant in the AGA. (Fig. 4.1 c) with a high-K calc-alkaline rhyolite

Prominent hydrothermal alteration is present composition [71.7 wt% SiO2; 7.5 (Na2O+K2O); in the north, west, southwest and eastern 4.1 K2O, according to Piscaglia (2011)] (Fig. flanks of Apacheta and Aguilucho volcanoes 4.2). Hot springs and gas seeps near 4800 m related to past and present fumarolic activity. elevation at La Torta discovered by ENAP The structural weakening of the Apacheta (Empresa Nacional del Petróleo, Chilean and Aguilucho volcanoes by hydrothermal National Oil Company) in 1998, the presence alteration is confirmed by the presence of a of Sol de Mañana geothermal field 20 km to debris avalanche deposit morphologically the southeast in Bolivia and the El Tatio emphasized by small hummocks in the geothermal field 10 km to the northeast (Fig. eastern flank of the volcanoes and mainly 4.1 c) suggests a large scale alignment of consisting of hydrothermally altered lava hydrothermal manifestations along regional fragments (Mercado et al., 2009; Aguilera et extensional structural environment al., 2008). The study of the drilled cap-rock (Cumming et al., 2002). La Torta geothermal samples confirm the presence of field is located at the southern extension of hydrothermally altered volcanic rocks the eastern edge of El Tatio graben and the

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 geology of La Torta area is assumed to be by lava dome extrusions (Von Hillebrandt similar to that of El Tatio, dominated by thick 1989; Hall and Mothes 2008; Hidalgo et al., Tertiary ignimbrite flows (Lahsen and Trujillo, 2008; Robin et al., 2008). The CGA is 1976b; Hauser, 1997). According to Cumming underlain by mid-Tertiary volcanoclastic et al., (2002), MT-TDEM survey identified a sediments and strongly tectonized low resistivity hydrothermal smectite clay Cretaceous oceanic basalts and associated zone extending more than 15 km southeast sediments (Pallatanga Terrain). The inferred of El Tatio, continuing beneath the La Torta thickness of the crust is about 50-60 km dome and covering more than 50 km2. Some (Feininger and Seguin, 1983; Guillier et al., wells drilled at El Tatio show temperature 2001). reversals indicating that they had The heat source of the CGA is represented by encountered a tabular outflow flowing to the the Quaternary Chachimbiro Volcanic west and north from an area to the east or Complex (CVC), which includes the collapsed south of the wells (i.e La Torta area). Middle Pleistocene andesite Huanguillaro

Consistently, N2/Ar and He isotopes and volcano, a series of caldera-filling rhyodacite geothermometry of 250 to 280°C suggest domes and the Late Pleistocene-Holocene that the gas seep at La Torta originates from Chachimbiro – Pucará NNE line of dacite the same hydrothermal system as the gases (Beate and Salgado, 2010) and andesite from the El Tatio fumaroles. A progressive domes (Fig. 4.1 d). The most recent volcanic southeast (La Torta) to northwest (El Tatio) products of CVC are pyroclastic deposits depletion of less water-soluble gases relative related to volcanic activity of Pucarà dome. to CO2 is consistent with an upflow near the This dome suffered south-east sector La Torta gas seep. Geophysical and collapse ascribed to a major lateral blast, geochemistry evidence suggest that El Tatio followed by a period of quiescence and a and La Torta share the same deep reservoir second phase of dome building. Finally a new with a magma body centered near the Cerros eruptive phase marked the end of volcanic del Tatio and La Torta dome. activity of Pucarà and the entire volcanic complex. Radiocarbon dating of soils 4.2.2 The Chachimbiro Geothermal Area underlying the pyroclastic deposits, constrain (Ecuador) the first eruption (called Lower Event) between 7,250-4,420 years BP while the The CGA is located in the Andean Northern second eruption (called Upper Event) is Volcanic Zone in the northern part of the younger than 4150 years BP (Comida, 2012). Ecuadorian Volcanic Front (EVF) (ANVZ, CVC domes are classified as andesites and Stern, 2004) (Fig. 4.1 a), about 70 km NNE dacites of calc-alkaline and subordinately, from Quito and 17 km NW from Ibarra. The tholeiitic series [58.6-69 wt% SiO2; 4-6.5 wt% Holocene volcanic activity of the EVF is (Na2O+K2O); 0.5-2 wt% K2O, according to characterized by powerful but low-frequency Comida (2012)] (Fig. 4.2). Bernard et al. explosive eruptions of andesite to dacite, and (2014) pointed out as the source of the most

4,720-4,855 years BP, was a moderate-sized Mwe. Sodium-chloride to Na-Cl-HCO3 type dome, destroyed by the eruption, emplaced hot springs are present, with temperatures in a valley at the foot of Cerro La Viuda about ranging from 40 to 55 °C (Beate and Salgado, 3 km N of Pucarà dome. Juvenile samples 2010) probably originated through dilution of collected by Bernard et al. (2014) are calc- the parent geothermal fluids with different alkaline rhyodacites [67.3-68.5 wt% SiO2] degrees of re-equilibration with country (Fig. 4.2). rocks at lower temperatures (Aguilera el al., Older Yanaurcu volcano (andesite to 2005). Possible deep-temperatures of the rhyodacite) as well as late Pleistocene Pilavo geothermal reservoir are 225-235°C, volcano (mainly basaltic andesites), are although temperatures at the base of the located to the W of Huanguillaro volcano geothermal system are inferred to be higher (Chiaradia et al., 2011; Gorini, 2011) (Fig. 4.1 than 260°C (Aguilera et al., 2005). Deep d). equilibrium temperatures of the reservoir The geothermal reservoir could be composed around 260°C were also estimated by by fractured volcanic rocks related to dome Gherardy and Spycher (2014) through the activity and older proximal lava flow facies application of the integrated related to early volcanic activity covering the multicomponent geothermometry (Spycher crustal basement, as well as locally fractured et al., 2011, 2014) based on the mineral facies of the Pallatanga Terrain. On the basis saturation index method (Reed and Spycher, of surface exploration data Almeida (1990) 1984), coupled with numerical optimization

Fig. 4.2. K2O vs SiO2 diagram (Rollinson, 1993). All compositions recalculated to 100% on a LOI-free basis. Data for Apacheta Geothermal Area (AGA), La Torta Geothermal Area (TGA), Chachimbiro Volcanic Complex (CVC) and Pucarà domes are unpublished. 39

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 to reconstruct the deep fluid chemistry. As emission gun and an energy dispersive reported by Beate and Salgado (2010) fault system (EDS). The ESEM analyses were controlled areas of hydrothermally altered performed on carbon-coated thin sections at rocks with no anomalous temperature, crop vacuum conditions (0.83 mbar) with an out in the central part of the CGA, probably accelerating voltage of 30 kV and a beam indicating a self-sealing of the upper part of current of 264 mA. the system. Resistivity soundings on the E Electron microprobe analyses of the minerals part of the CGA reveal a lateral outflow to (Table 4.1a, b) were performed at the the E, whereas there are no deep-reaching “Istituto di Geoscienze e Georisorse” - resistivity surveys in the centre of the area. National Research Council (IGG-CNR), Padua research centre (Italy), using a Cameca SX-50 microprobe equipped with four vertical 4.3 SAMPLES AND ANALYTICAL wavelenght-dispersive spectrometers. METHODS Analyses were carried out in spot mode (beam size ~ 1 μm, 15 keV, 15 nA) and using Representative volcanic products from AGA, natural silicate standards for major elements TGA and CGA for petrographic observation calibration. Analyses were focused on and electron microprobe analyses were amphibole crystals (Table) and comprise selected. For AGA was taken into account amphiboles from abundant intermediate- samples of andesite (AA010) and rhyolite basic microcrystalline enclaves hosted in (AA064) lava flows along the Apacheta- Apacheta, Chac-Inca and Chanka domes. We Aguilucho volcanic edifice and associated selected 152 amphibole crystals for a total of lava domes of Apacheta (CPBa) and Chac-Inca 462 data (among single point analyses and (CINKA). Sampling has been extended to the core-rim transects, 50). Chanka dome (CKA1, CKA1a, CKA1b). For TGA we considered the lava dome of La Torta (TA12, CT1a). 4.4 RESULTS Sample collection from CGA was focused on the most recent volcanic products of CVC, i.e. Petrography and mineral chemistry of Pucarà lava dome (CH17) and juvenile ejecta representative thin sections are briefly from related pyroclastic deposits of lower described below except the amphibole, event (CH22, CH23, CH24) and upper event which is discussed in detail in section 3.4.2. (CH27) (Comida, 2012) (see section 3.2.2). Polarized light optical microscopy was used 4.4.1 Mineral chemistry and for textural and modal analyses. Qualitative petrography compositional analyses were performed at the University of Urbino (Italy) with an Among the andesite lava of AAVC, the AA010

Environmental Scanning Electron Microscope sample (62.1 wt% SiO2, Piscaglia, 2011) (Fig. (ESEM, Quanta 200 FEI) equipped with a field 4.3 a) contains both clino- and orthopyroxene

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

Chemical compositions of representative minerals of AGA and TGA. a. 1 Table 4.

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

Table 4.1a: continued

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

1b. Chemical compositions of representative minerals of CGA. 4. Table

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 and has a partially vesiculated and seriated oscillatory zoning. Quartz crystals have porphyritic texture (porphyritic index, P.I.=20 rounded edges and sometimes are embayed. vol.%). The phenocrysts are, in order of Clinopyroxenes have augitic compositions abundance, plagioclase, orthopyroxene, (Wo32-46 En38-53 Fs10-15) and represent the clinopyroxene, amphibole, biotite and Fe-Ti most abundant mafic phase. Orthopyroxenes oxides. The groundmass is micro- have generally enstatitic compositions (Wo3 cryptocrystalline with the same mineralogical En79 Fs18). Fe-Ti oxides are magnetite (Mgn87- paragenesis of phenocrysts. Plagioclase 95 Usp3-11 Spn2-3) with chromite content less phenocrysts compositions range from An36 to than 0.12% (Table 4.1a). Hydrated minerals

An73. Bigger sized crystals show resorption are always affected by breakdown textures and sieve textures with intense direct, or secondary alteration processes. inverse and oscillatory zoning, while smaller Chac-Inca and Apacheta domes are classified crystals are generally lacking of such textures as dacites (66.7-67.6 wt% SiO2, Piscaglia, and zoning. Clinopyroxenes have augitic 2011). The CPBa (Fig. 4.3 c) and CINKA compositions (Wo43-45 En40-41 Fs13-15) and sections have porphyritic and partially represent the most abundant mafic phase, seriated texture (P.I.=30-35 vol.%). The while orthopyroxenes have generally mineral assemblage is composed of enstatitic compositions (Wo2-4 En65-72 Fs25-32). plagioclase, biotite, amphibole, quartz, Fe-Ti oxides are magnetite with nearly clinopyroxene, Fe-Ti oxides, ±apatite and constant composition (Mgn79 Usp14 Spn6-7) zircon as accessory phases. The groundmass with chromite content less than 0.14% (Table of the Chac-Inca dacite is micro and crypto- 4.1a). crystalline locally glassy with interstitial AA064 rhyolite lava (Fig. 4.3 b) represents the devitrified glass . The Apacheta sample show most evolved magma erupted in the AAVC micro-cryptocrystalline and highly

(69.6 wt% SiO2, Piscaglia, 2011). It has a vesiculated groundmass. Both rocks show the porphyritic texture (P.I.=20 vol.%) with same mineralogical paragenesis of convoluted banded textures and light- phenocrysts. Plagioclase phenocrysts are coloured zones surrounding highly variably sized (from 0.3 mm up to 2 mm) with vesiculated darker zones containing more oscillatory zoning and composition range basic material, probably due to mingling from An33 to An48. Quartz crystals have processes. The phenocrysts are, in order of rounded edges. Fe-Ti oxides are magnetite abundance, plagioclase, sanidine, amphibole, (Mgn87-90 Usp6-8 Spn4) (Table 4.1a). Accessory quartz, biotite, orthopyroxene, phases are hosted as inclusions in hydrates clinopyroxene, Fe-Ti oxides and titanite minerals. Both lava domes contain frequent (accessory phase). The groundmass is micro- micro-vesiculated intermediate-basic cryptocrystalline with interstitial devitrified enclaves composed by micro-phenocrysts of glass and locally pilotaxitic texture. plagioclase, orthopyroxene, amphibole, Plagioclase phenocrysts compositions range biotite and Fe-Ti oxides (Fig. 4.3 c). Small- from An28 to An68 with direct, inverse and sized and acicular micro-phenocrysts habits

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 of Chach-Inca enclaves compared with pseudomorph crystal composed by relatively bigger sized and well developed plagioclase and pyroxene assemblage. micro-phenocrysts habits in Apacheta Holocrystalline, fine-grained and micro- enclaves, suggest a different rate of cooling vesiculate-textured intermediate-basic for both domes during the interaction enclaves (Fig. 4.3 e) are common in the between two different magmas. Bigger-sized dacite lavas (e.g. CKA1a and CKA1b samples). plagioclases are also present with a They have a mineral assemblage composed composition range from An37 to An62 (Table of quench and acicular phases such as

4.1a). They show sieve and resorption plagioclase (An35 to An42), a more ferric textures with overgrowth rims and inclusions pyroxene (Wo1-3 En69-78 Fs19-30), compared to of amphibole, biotite and Fe-Ti oxides hosting dacite, and amphibole. Fe-Ti oxides sometimes with poikilitic textures. Probably (Mgn85-91 Usp6-11 Spn3-5; with chromite they belonged to the dacite and content less than 0.3%) (Table 4.1a) and subsequently they have been embedded by traces of biotite and quartz are present. The the enclave during the mingling processes. plagioclases are affected by sieve and

Chanka dome is a dacite (66.2 wt% SiO2, resorption textures with overgrowth rims. Piscaglia, 2011). CKA sample (Fig. 4.3 d) has Their habits, size and textural relations with porphyritic and partially seriate texture other crystals, suggest they belong to the (P.I.=30-35 vol.%). The phenocrysts are, in hosting dacite. These enclaves represent, order of abundance, plagioclase, biotite, together with convoluted banded textures of amphibole, quartz, pyroxene and Fe-Ti AAVC rhyolite lava, mixing/mingling episodes oxides. The groundmass is micro- between magmas with different cryptocrystalline, locally glassy with perlitic compositions and physical parameters that cracks. Plagioclase phenocrysts are variable could have generated, to some extent, sized (from 0.3 mm up to 4 mm) with disequilibrium conditions. Micro-vesiculate composition ranging from An35 to An54. texture are evidence of gas exsolution, rapid Generally, they have a fresh appearance undercooling and high nucleation rate due to although weak coarse-sieve texture and the intrusion in a silicic body of a hotter basic smooth edges, probably linked to resorption magma coming from deeper zones. processes, are locally observed. Sometimes La Torta dome is classified as rhyolite [71.7 they are poikilitic enclosing crystals of biotite. wt% SiO2, Piscaglia, 2011). CT1a (Fig. 4.3 f) Quartz crystals have rounded edges. and TA12 samples have a porphyritic texture

Pyroxenes have enstatic compositions (Wo2-3 (P.I.=30-35%) with a mineral assemblage

En80-81 Fs16-17). Fe-Ti oxides are magnetite composed by plagioclase, quartz, biotite,

(Mgn85-88 Usp6-8 Spn4) with chromite content amphibole and Fe-Ti oxides, apatite, zircon up to 5% (Table 4.1a). Euhedral to subhedral and titanite as accessory phases. It is mainly large sized-hydrate minerals (up to 2 mm) characterized by a glassy groundmass with can be affected by intense breakdown that perlitic cracks but locally show an oxidized sometimes replace the entire mineral with cryptocrystalline matrix, in which are

Plagioclase phenocrysts range from An37 to Pyroclastic products (Fig. 4.3 h) (CH22, CH23,

An66. They are characterized by sub-euhedral CH24, CH27 samples) are classified as to anhedral habits with size up to 2 mm and andesites on petrographic and modal are affected by sieve and resorption textures mineralogy bases. They show similar with direct, inverse and oscillatory zoning. petrographic characteristics, regardless of Fe-Ti oxides, zircon, biotite and amphibole sampling location (see section 3.2.2). They appear as common inclusions in plagioclase. have similar porphyritic indexes (10-13 vol%) Quartz crystals exhibit anhedral habits with and a phenocrystic assemblage mainly common rounded edges and embayed consisting of plagioclase, amphibole and Fe-Ti contours. Fe-Ti oxides exhibit massive and oxides. The micro-cryptocrystalline and elongated habits related, respectively, to micro-vesiculated groundmass is composed magnetite (Mgn92-94 Usp2-5 Spn3-4; with by the above-mentioned phases, traces of chromite content less than 0.4%) and biotite and apatite and SiO2 patches, ilmenite (Table 4.1a). Biotite is a common probably tridymite. Only one section from phase and some of them have Fe-Ti oxides the lower event is characterized by higher and plagioclase reaction rims. porphyritic index, up to 20 vol%, and traces In CGA, the Pucarà dome is a calc-alkaline of clinopyroxene micro-phenocrysts. andesite (60 wt% SiO2, Comida, 2012) (Fig. Plagioclase phenocrysts compositions range

4.2). CH17 sample (Fig. 4.3 g) has a from An26 to An56. They are variable-sized, porphyritic texture (P.I.=25 vol%) and a from 0.2 mm up to 2.5 mm, with subhedral phenocrystic assemblage composed by to euhedral habits and rare sponge textures. amphibole, plagioclase, Fe-Ti oxides, traces of All the samples are generally characterized clinopyroxene and orthopyroxene (as by direct, inverse and oscillatory zoning. microphenocrysts). The groundmass is Small inclusions of apatite and biotite are micro/criptocrystalline with rare interstitial occasionally found. Fe-Ti oxides are glass and locally pilotassitic texture. It is magnetite and Ti-magnetite (Mgn19-87 Usp8-77 composed by the same mineralogical Spn4-7) with chromite content less than 0.12% paragenesis of phenocrysts and SiO2 patches, (Table 4.1b). The lack of pyroxenes, the probably tridymite. Apatite is found as micro- appearance of biotite, although rare and not inclusions in amphibole. Plagioclase easily distinguishable from amphibole phenocrysts compositions range from An60 to microlites and the lower Ca content in

An78. They are less than 1mm in size, show plagioclase phenocrysts, suggest that these euhedral to anhedral habits and rare sponge products are more evolved compared to textures. Pyroxenes have augitic (Wo37-41 Pucarà dome andesite and could have a

En43-48 Fs15-16) and enstatitic compositions chemistry composition similar to the

(Wo3 En71 Fs26). Fe-Ti oxides are magnetite pyroclastic products studied by Bernard et al. and Ti-magnetite (Mgn52-71 Usp17-39 Spn8-11) (2014).

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

Fig. 4.3. Photomicrograph of selected thin section textures. a) andesite lava of AAVC; b) rhyolite lava of AAVC with banded texture; c) Apacheta dome with contact between intermediate -basic enclave and hosting dacite; d) Chanka dome; e) holocrystalline, fine-grained and micro-vesiculate-textured intermediate-basic enclave; f) La Torta dome; g) Pucarà dome; h) typical texture of juvenile ejecta of Pucarà dome pyroclastic deposits.

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

4.4.2 Amphibole texture and probably the result of syn/post-eruptive classification oxidation which commonly induced the formation of thin opacitic rims of breakdown Amphibole phenocrysts from AAVC andesite (thickness <25 µm) (Fig. 4.4 d, e) (AA010) and rhyolite (AA064) lava flows are characterized by fine-grained intergrowth of affected by breakdown texture (composed of clinopyroxene, plagioclase and Ti-magnetite. plagioclase, pyroxene and magnetite) or Broader opacitic rims of breakdown coarse-grained reaction rim (composed of (thickness >25 µm) or pervasive breakdown pyroxene). Pervasive fine-grained resorption textures in the crystal core can also occur. textures can also be present (Fig. 4.4 a, b). Amphibole phenocrysts in the pyroclastic Zoning textures are not frequent. Dome- deposit look quite different from the dome- related crystals (Apacheta, Chac-Inca, Chanka related amphiboles (Fig. 4.4 f). They are and La Torta) are variable-sized from 0.2 to variable-sized, ranging from 0.3 to 1 mm and 1.5 mm but they can exceed 2mm, with sometimes up to 2 mm. They show subhedral subhedral to euhedral habits. They show a to euhedral habits with pale yellow-green green-brownish pleochroism and usually, are pleochroism. Breakdown textures are not not affected by evident reaction or present. “Cycling” zoning with alternating breakdown textures. Large poikilitic crystals dark and light greenish layers is common (Fig. are present. Plagioclase, biotite, Fe-Ti oxides 4.4 f). Sometimes the zoning can be and zircon can occur as inclusions. described as “patchy”. Amphiboles hosted in the intermediate-basic Mineral formulas were calculated with the enclaves are affected by alteration or can be Amp-TB-xls spreadsheet (Ridolfi et al., 2010a) totally replaced by pseudomorph minerals. using the 13 cations method, also known as Zoning texture characterized by a dark the 13eCNK method (Leake et al., 1997) (Tab brownish core and a pale brownish rim can 4.2a, b). AGA, TGA and CGA amphiboles be present (Fig. 4.4 c). belong to the group of calcic amphiboles (i.e. B B 2+ CGA dome-related amphiboles are variable- Ca>1.5) with Na<0.5, Mg/(Mg+Fe )>0.5 sized, commonly ranging from 1 to 3 mm but and Ti<0.5 apfu (i.e. atoms per formula unit). in some cases up to 6 mm (Fig. 4.4 d) with Figure 4.5 (a, b) shows the classification of euhedral and anhedral habits characterized amphiboles according to Leake et al. (1997). by rounded edges. They typically show pale CGA amphiboles belong almost entirely to [4] yellow to brown-reddish and brown tschermakite group ( Al>1.5, Ti<0.5 and A pleochroism with no evident optical zoning. (Na+K)≤0.5 apfu) (83% of dataset) while the The larger crystals show subhedral-elongated remaining amphiboles are Mg-horneblende 4] shapes (Fig. 4.4 d) with a sort of hopper ( Al≤1.5 and Ti<0.5) (17% of dataset). The morphology (with a negative crystal cavity) situation is the opposite for AGA amphiboles suggesting undercooling and fast growth with the most consistent group represented conditions (Shea and Hammer, 2013). The by Mg-horneblende (81% of dataset) then [4] reddish colours of the crystals is most Mg-hastingsite group ( Al>1.5, Ti<0.5,

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

Fig. 4.4. Representative photomicrographs of amphibole textures . a) Coarse-grained reaction rim formed mainly by pyroxene and b) fine-grained pervasive resorption textures in amphiboles of rhyolite lava of AGA. c) Typical zoned crystal with brownish dark core and brownish pale rim found in intermediate-basic enclaves of Apacheta dome (AGA). d) and e) large-sized reddish coloured amphiboles with elongated shape of Pucarà dome (CGA). Breakdown texture is evident. f) greenish coloured amphibole in the pyroclastic deposit of Pucarà dome. Breakdown texture is absent but concentric optical zoning is evident.

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

A(Na+K)>0.5 apfu and [6]Al

Fig. 4.5. a) Si vs Mg/(Mg+Fe2+) and b) [4]Al vs A[Na+K] classification diagrams of AGA, TGA and CGA amphiboles according to Leake et al. (1997). The horizontal and vertical black bar indicates the average values of standard deviation of amphiboles analysis.

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 can be rapid, i.e. characterized by spikes and ΔNNO and SiO2 melt show an inverse drops (e.g. Kiss et al. 2014) or gradual such as correlation with pressure. the intra-crystalline chemical variations of Large chemical variations within the crystals Fig. 4.6 a. A normal zoningis typically and rapid changes of physico-chemical characterized by a stage of crystallization at parameters point out a crystallization of constant conditions followed by a SiO2 amphiboles at disequilibrium conditions. This increase (and concomitant Al2O3 decrease) to is in agreement with petrographic the rim. An example of normal zoning is observation of micro-vesiculated reported in Fig. 4.6 b also showing intermediate-basic enclaves within dome- disequilibrium conditions during the very last related samples, convoluted banded textures crystallization stage, characterized by a rapid in rhyolitic lava (i.e. AA064), zoned crystals

Al2O3 increase at the rim (most probably a and frequent sieve textures in plagioclase, syn-eruptive phenomenon). In theory, a which point out disequilibrium conditions reverse zoning should show the opposite of a ascribed to magma mixing/mingling normal zoning. The first 5 analytical spots of processes. the crystal in Fig. 4.6 c show a similar These disequilibrium conditions are in phenomenon, characterized by an increase of contrast with R&R2012 model since that the

Al2O3 (and SiO2 decrease) toward the rim. equations are calibrated on amphiboles However, this crystal should be referred as a crystallized at equilibrium conditions which in complex or oscillatory zonings because the turn mean homogeneous amphiboles

SiO2 content increases at the last stage of crystallized at steady-state conditions. For crystallization. By contrast, homogeneous this reason the method should be applied to amphiboles show no significant intra- crystal showing homogeneous composition crystalline variation for both chemical or micro-domains with homogeneous composition and calculated intensive composition of zoned amphiboles (Ridolfi et parameters (Fig. 4.6 d). al., 2016). For this reason we compared the Indeed, Figure 4.6 also report the physico- intra-crystal compositional variation with the chemical (P, T, ΔNNO) and compositional standard error of homogeneous amphibole conditions (SiO2 melt, H2Omelt) of amphiboles EMP analyses. As error reference we crystallization calculated with the R&R2012 considered the average standard deviations method. P is closely related to major element (σ) of element oxides reported for the 61 variations and therefore to zoning of the experimental amphiboles used to retrieve amphibole with direct proportionality to the thermobarometric model (see Table 4.2

Al2O3 and FeO and indirect proportionality to in Ridolfi and Renzulli 2012). Comparison of

SiO2 and MgO (Fig. 4.6 a, b, c). Temperature these values with the element oxide σ of changes in a similar way to the pressure but homogeneous micro-domains (Ridolfi et al., with less pronounced variations. H2Omelt 2010b; Ridolfi et al. 2016) indicate that they changes accordingly to the pressure while approximate well the typical uncertainty of

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

EMP amphibole analyses and thus can be ΔNNO +2.0 (Fig. 4.6 b, blue rectangles). The used to evaluate compositional zonings. On outer rim of the crystal, which is in contact this basis, EMP analyses performed on with the groundmass, shows once again a amphiboles showing chemical variability rapid spike of pressure to 370 MPa, a falling within R&R2012 average σ, have been significant increase of temperature to 884°C regarded as compositionally homogeneous and a decrease of fO2 (Fig. 4.6 b). The first (i.e. crystallized at chemical equilibrium pressure drop can be interpreted as condition) and have been taken as one depressurization condition due to magma through the calculation of their average ascent toward the surface which is consistent value. These average compositions were then with a H2O decrease and an enrichment of used for the calculation of the physico- SiO2 content in coexisting melt as shown in chemical and compositional parameters with Figure 4.6b. Conversely, the sudden increase the equations of R&R2012 (Table 4.2a, b). of pressure of about 170 MPa near the rim is The values outside R&R2012 average σ simply due to sin-eruptive disequilibrium ranges are generically ascribed to phenomena since that a pre-eruptive sink of crystallization at disequilibrium conditions the crystal of about 6 Km in the crust and were discarded. (considering a crustal density of 2.7g/cm3) is In Figure 4.6 are reported the average σ of not reliable. Crystal rims with P-T increases

SiO2, Na2O, TiO2, MgO, FeO, Al2O3 of the are commonly found in our amphiboles physico-chemical uncertainties of the especially in micro-vesiculated enclaves of R&R2012 method. In oscillatory zoned AGA volcanic products and represent amphibole the chemical variability is larger evidence of disequilibrium conditions than the average σ of R&R2012 and it is not suffered by crystals during mixing/mingling easy to distinguish clear homogeneous processes. The amphibole with micro-domains (Fig. 4.6 a) and the physico- complex/oscillatory zoning chcraterized by chemical parameters calculated with this reverse zoning at main core micro-domain kind of data cannot be considered reliable. In (Fig. 4.6 c) shows the most reliable normally zoned amphiboles the compositional values at the core. The compositional variation is less pronounced average composition is then considered showing element oxides variation lower than valulable for thermobarometric constraints. average σ of R&R2012 so that a The homogeneous amphibole (Fig. 4.6 d) homogeneous dark-greenish core with an show compositional variations within the average pressure of 313 MPa, average average σ of R&R2012. However, the outer temperature of 843°C and ΔNNO +0.7 can be rim shows a significant compositional change recognized (Fig. 4.6 b, red rectangles). At the and a slight pressure drop that can be optical zonation transition another interpreted as a magma ascent phenomenon homogeneous micro-domain can be toward the surface, similar to the normal observed with an average pressure of 195 zoned amphibole in Figure 4.6 b. MPa, average temperature of 842°C and

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

Another disequilibrium condition to avoid is be discarded because if used for represented by fast growing textures which thermobarometric calculations they could are present both in AGA and CGA amphiboles lead to large overestimations of P and (Fig. 4.4 d). Recent undercooling experiments crystallization depth. at 150 MPa carried out by Shea and Hammer (2013) make it possible to evaluate the 4.5.2 Amphibole thermobarometry behaviour of a model calibrated at conditions application. The P-T-h diagram of physico-chemical equilibrium (i.e. R&R2012) in disequilibrium conditions. The Once we have cleaned up the database experimental amphiboles, calculated with analyses from data considered synonymous R&R2012 model, show slight temperature with disequilibrium, we compared cation overestimations (6-27 °C, within the margin variation diagrams for CGA and AGA of error reported in R&R2012, ±24 °C) but amphiboles with natural and experimental pronounced pressure overestimations amphiboles used by R&R2012; they are in increasing with the cooling rate. In such a agreement and thus CGA and AGA data are non-steady state context dominated an suitable for the application of elevated thermal gradients, the crystallized thermobarometer. amphibole show disequilibrium textures (e.g. The calculated data were plotted in a acicular, elongate and hopper morphologies). pressure-temperature-depth (P-T-h) diagram Most probably these amphiboles record the which represents the storage and equilibrium compositional information of the liquid that crystallization conditions of amphibole in the was in equilibrium with the high pressure different volcanic products (Fig. 4.7 a, b). phases (i.e. at the temperature and pressure Most of the data of the dacitic domes of the deep source). For these reasons, the Chanka, Chac-Inca, Apacheta and La Torta are application of the R&R2012 model to natural grouped in a pressure range between 97 and amphiboles showing similar morphologies 144 MPa and temperature between 755 and should be evaluated carefully. 818°C (Fig. 4.7 a). The depth corresponding Substantially, during the application of the to the P values are between 3.8 and 5.6 km thermobarometric model, the chemical (the depth is calculated from average height variations inside the crystals should be of AGA and TGA, 4.63 Km; the average crustal compared with the average σ in R&R2012 in density is 2.6 g/cm3; Leidig & Zandt, 2003) order to identify homogeneous micro- indicating the presence of shallow magmatic domain indicating equilibrium condition. The chambers in the upper crust. Oxygen fugacity intensive parameters calculated on the basis ranges between ΔNNO -0.1 to +1.4. Melt in of chemical values should be evaluated on equilibrium with amphiboles shows a H2O the basis of their reliability, as in the case of content between 5.1-6.8 wt% and a SiO2 pressure drops due to ascent of magma. Data content between 74.3-78.5 wt%. However, outside average σ or associated with zoned some amphiboles of Apacheta dome, show or fast-growing textured amphiboles should growth conditions slightly deeper with P

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

Fig. 4.6 a, b. (see caption ahead).

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

Fig. 4.6 c, d. (see caption ahead).

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

Fig. 4.6 a, b, c, d. Core to rim compositional profiles of selected amphiboles. a) oscillatory zoning, b) normally zoning with disequilibrium rim. Red and blue rectangle show homogeneous micro-domain within the amphibole. c) Inverse zoning with disequilibrium rim. d) Un-zoned amphibole. Optical zoning is correlated with element oxide variations. Pressure and temperature calculated with R&R2012 method change accordingly to chemical variation. R&R2012 method requires homogeneous crystals to retrieve reliable physico-chemical parameters. Average standard deviation (σ) of amphibole used in R&R2012 equation should be used to assess compositional variation within the studied crystals. Compositional variations falling outside the σ can be ascribed to disequilibrium condition suffered by the crystal and should be not used to retrieve P, T and chem ometric parameters. Vertical black bars of SiO2, Na 2O, TiO2, MgO, FeO, Al2O3 indicate the average standard deviation (σ) of the experimental amphiboles in R&R2012. Vertical black bars if P, T, ΔNNO, SiO2melt and H2O melt, indicate the uncertainties (i.e. standard error of estimate) of R&R2012 method. between 165 and 189 MPa (6.5-7.4 Km), T in (Fig. 4.7 a). Oxygen fugacity shows the higher the range 820-828 °C (Fig. 4.7 a), ΔNNO +0.9 values with ΔNNO +1.4 to 2.1. Coexisting to +0.3. They arein equilibrium with a melt has a SiO2 content between 61.8-65.6 rhyolitic melt containing 6.0 wt% H2O and wt% and H2O content between 5.0-6.1 wt%.

74.5-76.7 wt% SiO2. At greater depth are Black arrow in Figure 4.7a highlight the core- some amphiboles of La Torta dome with a rim path of normal zoned amphibole with a 248-267 MPa pressure range (9.7-10.5 Km), T core in equilibrium at 189 MPa (about 7.4 between 850 and 872 °C and redox condition km) and a rim at 125 MPa (about 4.9 km). of ΔNNO +0.6 to -0.5. The coexisting liquid Amphiboles of the basic-intermediate shows a higher H2O content (up to 6.6 wt%) enclaves in the Chanka and Apacheta domes and a less evolved composition (68.4-70.6 overlaps the P-T path of dome-related wt%). amphiboles of Apacheta, Chac-Inca and Amphiboles belonging to lava erupted by Chanka domes with P range between 92-194 AAVC are distributed in two separate groups. MPa (depth 3.6-7.6 km), T range of 742-837 Not surprisingly, evolved product such as °C and similar oxygen fugacity (ΔNNO -0.5 to rhyolite lava (AA-064) includes amphiboles +1.6). SiO2 and H2O content in coexisting melt with low P (114-150 MPa; depth 4.5-5.9 km) is 72.1-79.2 wt% and 5.6-7.1 wt%, and T (778-808 °C) (Fig. 4.7 a), and redox respectively. These amphiboles most conditions in the range 0.8≤ΔNNO≤1.9. probably crystallized in a silicic magma body Coexisting melt show the most evolved and fell into the enclave during ma composition with 78.3-79.8 wt% SiO2 content interaction with the basic magma. Indeed, a and 5.3-5.9 H2O content. High silica andesite group of crystals in the enclaves is plotted at lava (AA-010) contains amphiboles with greater P-T condition between 375-392 MPa higher P-T values that can be further (14.7-15.4 km) and 948°C in higher oxidation subdivided in two sub-groups: the first one condition (ΔNNO +1.8). SiO2 and H2O content with P-T values between 383-435 MPa (15-17 in coexisting melt is 62.1-63.4 wt% and 5.5- Km) and 938-959 °C, the second one is in 6.1 wt%, respectively. One crystal show even equilibrium with lower values in the range higher P-T conditions, 505 MPa (about 20 318-323 MPa (12.5-12.7 km) and 927-936°C km) and 1011 °C (Fig. 4.7 a), with ΔNNO +0.8

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 and coexisting melt with 54.3 wt% SiO2 and 4.5.3 Comparison between 4.6 wt% H2O. thermobarometric and These data indicate that the dome-related geophysical constraints magmas were stored in different magma chambers placed at different depths in the In order to test the reliability of R&R2012 crust. method, it was carried out a comparison P-T data for CGA show a more scattered between thermobarometric data of AGA and pattern (Fig. 4.7 b). Most of the amphiboles TGA with the geophysical results for the belonging to pyroclastic products show APVC. The APVC is characterized by a crystallization pressure between 187-364 laterally extensive seismic low-velocity zone MPa corresponding to depth of about 6.6-13 (LVZ) located at depth of about 17-19 km Km (the depth is calculated from the peak of below sea level referred to as Altiplano-Puna Pucarà dome, 3.041 Km; the average crustal Magma Body (APMB) (Chmielowsky et al., 3 density is 2.9 g/cm ; Feininger and Seguin, 1999; Leidig and Zandt, 2003; Zandt et al., 1983: unlike the AGA and TGA, the basement 2003) with an inferred percentage of melt of rocks of CGA are formed by oceanic crust, 5-10%, temperature below 1000°C and felsic therefore the average crustal density is composition (Beck and Zandt, 2002). The probably higher than AGA) and temperature extension of APBM, its spatial relations with between 819-912 °C. Oxygen fugacity ranges the large caldera forming ignimbrites and its between ΔNNO +0.2 to 3.0. Melt in chemical-physical conditions, suggest it can equilibrium with amphibole has SiO2 content be considered the intrusive equivalent of the between 65.5-76.3 wt% and H2O content APVC volcanic products (de Silva et al., 2006). between 5.5-9.7 wt%. The black arrow in Recently, Ward et al. (2014), presented a Figure 4.7 b shows the general path of P-T new 3-D seismic images of the APVC crust variations on core-rim transect of zoned considerably improving the extension and amphiboles. The lack of breakdown textures morphology of the APMB. They identified may be linked to the eruptive style of such a ̴200 km diameter large and ̴11 km thick pyroclastic products. They were ejected LVZ below the APVC, with an inferred during a later blast and may have reached percentage of partial melt up to 25% and a the surface as quickly as not to allow the much larger volume ( ̴500,000 Km3) than formation of breakdown textures. previous estimates. We plotted our All the amphibole crystals from Pucará lava thermobarometric data in a cross-section of dome are affected by fast-growing textures the joint ambient noise-receiver function ascribed to rapid undercooling (Shea and inversion S-velocity model performed by Hammer, 2013) (Fig. 4.1.1) and thus do not Ward et al. (2014) that show the depth and represent crystallization at total equilibrium lateral extent of 2.9 km/s and 3.2 km/s conditions, thereby related velocity contour with an estimated thermobarometric and chemometric results percentage of melts respectively of 10%̴ were discarded. and ̴4% (Fig. 4.8a). Since that AGA and TGA

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4

Fig. 4.7 a) and b) P-T-h diagrams for AGA, TGA and CGA amphiboles. The black dotted lines represent the stability field of Mg-rich amphibole. The solid black lines represent the SiO2 content of melts in equilibrium with amphiboles used to derived the thermobarometric equations. Red bars indicate the P-T uncertainties of R&R2012 method. The blue bars indicate the average value of SD for the entire dataset of AGA, TGA and CGA anphiboles Black arrows show the general path of P-T variation from core to rim of zoned amphiboles.

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 are not aligned with the cross-section of seismic profile depth to sea level, we correct Ward et al. (2014) (cross-section lies our amphiboles depth to sea level]. As we approximately 30 km NE from AGA domes) can see in Figure 4.7a and b, at depth of 5 km we correct this misalignment plotting our the data from AGA with low depth of data also into the map slices at different crystallization fall within the 3.2 km/s velocity depth of Ward et al. (2014) model (Fig. 4.8b, contour. Between the 10 and 15 km map c, d), obtaining a more realistic qualitative slices (Fig 3.8c, d) the data from Apacheta contextualization of our crystallization depths dome with the highest depth of [since that Ward et al. (2014) referred their crystallization (andesite lava and

Fig. 4.8 a) Seismic tomography cross-section of APMB (Ward et al., 2014) with crystallization depth of amphiboles obtained with the R&R2012 method [(referred to sea level (red dotted line)]. b), c), d) map slices, at different depth, of APMB as constrained through joint ambient noise-receiver function inversion S-velocity model by Ward et al., (2014). The location of AGA and TGA domes is highlighted by the black boxes in b, c and d.

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 intermediate-basic enclaves) fall within the 4.6 CONCLUSIONS 2.9 km/s velocity contour and in proximity of the volume of rocks with velocity of 2.5 km/s In the framework of the geothermal (red coloured area in Fig 3.8a, c, d). Crystals exploration, a fundamental aspect is to from La Torta dome are located above the constrain the heat source of geothermal lowest velocity contour (1.9 km/s; white system. In this study we selected amphiboles coloured area) and presumably at the border from geothermal areas of Chachimbiro between 2.9 and 3.2 km/s velocity contour. (northern Ecuador), Apacheta and La Torta In fact in the 5 km map slice La Torta dome is (northern Chile). We applied the Ridolfi and located between the 2.9 km/s and 3.2 km/s Renzulli (2012) amphibole thermobarometer velocity contours while at depth of 10 km, to retrieve the P-T conditions during corresponding to the deepest crystal of La amphibole crystallization trying to Torta, the 2.9 km/s velocity contour enclose reconstruct the magma path during its ascent the crystals (Fig. 4.8 b, c). Data from Chanka toward the surface. The studied amphiboles dome always lies outside the 2.9 km/s are frequently affected by chemical zoning, contour (Fig. 4.8b, c, d). breakdown and fast-growing textures, Overall the crystallization depths of indicating disequilibrium conditions suffered amphiboles are coherent with the APMB of by crystals. Since that the R&R2012 Ward et al. (2014). Indeed the deepest values thermobarometer requires homogeneous of Apacheta dome, represented by less amphibole composition (i.e. equilibrium differentiated products, are located within condition) to calculate reliable P and T the lowest velocity contour which is linked to values, we have compared the intra-crystal more elevated temperatures of rocks or to compositional variation with average larger volume of silicate melts. Moreover standard deviation (σ) of element oxide of with increasing age of the volcanic structures experimental amphibole used to retrieve the from south [La Torta dome, 34±7 Ka (Renzulli thermobarometric model to identify et al., 2006)] to north [Chanka dome, 1.5 Ma- homogeneous micro-domains in the crystal years-old (Roobol et al., 1974)] also increases with compositional variation in the range of the distance from the central area of APMB σ. These micro-domains approximate the where much more volume of melt is typical uncertainty of EMP analyses and can expected due to the low velocity (1.9 km/s) be considered crystallized at equilibrium (Fig. 4.8b, c, d). The younger eruptive centre condition. Thus, these compositions were (i.e. La Torta dome) seems to be located used to retrieve P-T values with R&R2012. where the inferred volume of magma is Element oxide variation outside the σ range higher while the older eruptive centre (i.e. can be ascribed to disequilibrium condition of Chanka dome) is located at the border of amphibole crystallization and should be APMB where higher velocity contour (3.2 discarded together with fast-growing texture km/s) probably indicates the presence of (non-steady state condition) to avoid large colder rocks or less volume of melt.

THE APPLICATION OF AMPHIBOLE THERMOBAROMETRY TO CONSTRAIN THE HEAT SOURCE OF GEOTHERMAL SYSTEMS IN GEOTHERMAL AREAS OF CHACHIMBIRO (ECUADOR), APACHETA AND LA TORTA (CHILE) CHAPTER 4 errors in the calculation of intensive amphibole points out a polybaric parameters (e.g. overestimation of pressure). differentiation of magma below the This procedure allowed us to obtain reliable geothermal area. P-T value of magma storage and crystallization below the geothermal areas. The feeding systems of AGA and TGA are composed by shallow magma chambers with crystallization of evolved magma between depth of 4-7.5 km (T=755-828 °C) and 4-10 km (T=755-872 °C), respectively. They represent the primary source of heat for the hydrothermal reservoir. In the case of AGA, the shallow magma chamber is fed by basic to intermediate magmas from deeper crustal levels (about 15-19 km, T=948-1011 °C). We therefore suggest polybaric differentiation of magma in the upper crust (20 km) beneath an area of APVC from Chanka (north) to La Torta (south) domes. This petrological evidence is in agreement with the existence of a partially melted zone in the upper crust of the APVC, the so called Altiplano-Puna Magma Body (APMB) which was detected by geophysical data. Crystallization depths of amphibole obtained with the R&R2012 method is thus consistent with the seismic tomography model for the APMB recently carried out by Ward et al. (2014), pointing out a good accuracy and reliability of R&R2012 method. Pre-eruptive conditions of amphiboles in the feeding system of CGA constrain the presence of shallow magma chamber at depth of about 6.5 km (T between 800- 900°C) which represent the heating source of the geothermal system. Deeper magma sources are present up to depth of 19 km with temperature higher than 900°C. Also in this case the presence of normally zoned

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

5 Origin of geothermal fluids along active to semi-dormant volcanoes of Northern Ecuador (1°S to 1°N) as inferred from chemical and isotopic composition

5.1 INTRODUCTION acting on the recharging meteoric water were recognized to occur within the fluid Late-Pliocene to present-day volcanism in reservoirs. Isotopic tracers suggested the Ecuador is characterized by large occurrence of significant mantle-related fluid stratovolcanoes, caldera complexes and lava contribution. Inguaggiato et al. (2010) also domes many of which active and potentially emphasized the different isotopic active (Hall et al., 2008). Volcanic edifices are characteristics of fluids from the Ecuadorian distributed along three NNE-trending Quaternary active volcanism (north of 2°S), alignments and built up over basements of with respect to those from the inactive arc different age, thickness and composition in a (south of 2°S). However, they did not find complex geodynamic setting characterized by differences between the hydrothermal a flat-slab subduction (Gutscher et al., 1999, systems through the frontal arc, the Inter- 2000) between the Nazca and the South Andean Depression and the main volcanic arc American Plates (Fig. 5.1a). A widespread despite their different basement terranes. geothermal activity which could be exploited The present work provides a description of is present in northern Ecuador (ca. between the geology and fluids geochemistry of 1°S and 1°N) closely associated to Quaternary geothermal areas located between the volcanism. Despite this great geothermal frontal and main arcs (including the Inter- potential, the exploitation of geothermal Andean Depression) of Northern Ecuador energy is still restricted to direct use of low (1°S to 1°N) and directly associated to the and medium temperature fluids for bathing, Late-Pliocene and the ongoing volcanic balneology and swimming pool (Beate and activity (Hall et al., 2008). The thermal fluids, Urquizo, 2015) although several high- with the exception of those from the enthalpy resources for electric-power Nangulvì and Cristobal Colon systems, are generation have been identified (Almeida et likely related to the following volcanoes (Fig. al., 1992; Beate and Urquizo, 2015). 5.1b; Table 5.1): i) Chiles-Cerro Negro The first systematic characterization of Volcanic Complex, ii) Chachimbiro Volcanic thermal and cold waters from the whole Complex and Cuicocha caldera, iii) Chacana Ecuador as well as dissolved and bubbling Caldera Complex and iv) Quilotoa volcano. gases was performed by Inguaggiato et al Seven thermal water samples (from Chiles- (2010). On the basis of the chemical and Cerro Negro, Chachimbiro, Cristobal Colon isotopic features of the thermal discharges, and Nangulvi) and two bubbling gas samples strong gas–water interaction processes

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

Fig. 5.1a, b. (see caption ahead)

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

Fig. 5.1. a) Geodynamic setting of the Ecuadorian arc, including the Carnegie Ridge Plateau and other main oceanic features (Grijalva F.Z. = Grijalva Fault Zone; GSC = Galápagos Spreading Centre). The Andes are delineated by the 2000 m contour (dark grey) and the main Quaternary volcanoes by the small white triangles. Depth contours of the

Wadati-Benioff Zone indicated as dotted line (modified from Bourdon et al., 2003). The thick solid-dotted line

represents the regional boundary between the oceanic terranes domain to the west and continental terranes to

the east (from Spiking et al., 2005) (approximate location). b) Distribution of Ecuadorian volcanoes with Holocene

activity and subdivisions of the volcanic arc (modified from Hall and Mothes, 2008a).

Table 5.1 Summary table for sampling sites and related volcanic/geothermal areas.

(Chiles-Cerro Negro and Chachimbiro), 5.2 GEODYNAMIC, GEOLOGICAL AND represent new data with respect to VOLCANOLOGICAL SETTINGS Inguaggiato et al. (2010). We provide geochemical and isotopic data on thermal The Quaternary Ecuadorian volcanic arcs waters and gases to evaluate the relationship results from the eastward subduction of the between active, semi-dormant or dormant Miocene Nazca oceanic Plate (<25 Ma; Hey, Ecuadorian volcanoes and geothermal areas 1977; Pennington, 1981; Lonsdale, 2005; and the characteristics of hydrothermal Barckhausen et al., 2008; Müller et al., 2008; resources in the framework of the high Seton et al., 2012) beneath the South geothermal potential of northern Ecuador. American Plate (Fig. 5.1a) at a convergence We also point out the role played by the rate of 5-7 cm/y at least north to 0° of different geological and geodynamic settings latitude (DeMets et al., 1990; Kellogg and of Ecuadorian arc-trench system on the Vega, 1995; Norabuena et al., 1999). The characteristics of thermal fluids and gases. subducting plate carries an aseismic volcanic plateau, namely the Carnegie Ridge (Fig. 5.1a), formed by the eastward motion of the Nazca Plate over the Galapagos hotspot (Hey, 1977; Lonsdale, 1978, Lonsdale and Klitgord, 1978). From west to the east, Ecuadorian

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5 volcanoes are distributed along three NNE- 2000) and the partial melting of the oceanic trending alignments: Western Cordillera slab with the change of geochemical (frontal arc), the Inter-Andean Depression signature of volcanic products and transition and Eastern Cordillera (main arc) and the from calc-alkaline to adakitic magmatism Sub-Andean Zone (back-arc region). A (Samaniego et al., 2002; Bourdon et al., 2003; tectonic boundary (Fig. 5.1a) separates the Samaniego et al., 2005; Hidalgo et al., 2007; Late Cretaceous- Eocene-accreted oceanic Robin et al., 2009). terranes and associated volcano-sedimentary arc underlying the Western Cordillera (Lebrat 5.2.1 Chiles-Cerro Negro Volcanic et al., 1987; Daly, 1989; Bourgois et al., 1990; Complex Reynaud et al., 1999; Spikings et al., 2001; Hughes and Pilatasig, 2002; Kerr et al., 2002; Chiles (0°49' N, 77°56' W) and Cerro Negro Vallejo et al., 2009) from the Paleozoic (0°46' N, 77°57' W) are two adjacent active continental metamorphic and igneous rocks stratovolcanoes with an elevation of 4,748 forming the basement of the Eastern and 4,470 m a.s.l. respectively, located in the Cordillera (Litherland et al., 1994; Pratt et al., northern sector of Western Cordillera at the 2005). The thickness of the crust is inferred border between the province of Carchi, in to be 25-30 km (Feininger and Seguin, 1983) Ecuador and the department of Nariño, in up to 40-50 km (Guillier et al., 2001) for the Colombia (Fig. 5.1b), about 7 km west of the Western Cordillera and about 50 km villages of Tufiño (Ecuador) and Chiles (Feininger and Seguin, 1983) up to 75 km (Colombia). The most significant (Guillier et al., 2001) for the Eastern morphological features are horseshoe- Cordillera. shaped collapsing structures affecting the The volcanic products consist of medium to northern slope of Chiles and the western high-K calc-alkaline basaltic andesites to slope of Cerro Negro with 1 km and 1.8 km rhyolites with predominance of andesites across respectively, and landscape forms and dacites (Barragan et al., 1998; Monzier et linked to glacial moraine deposits (Cortés y al., 1999; Bourdon et al., 2003; Samaniego et Calvache, 1996). The volcanic complex is built al., 2005; Hidalgo et al., 2008; Robin et al., up on top of a thick sequence of Pliocenic 2009; Chiaradia et al., 2011). In the Sub- andesitic lavas overlying volcano- Andean Zone smaller stratovolcanoes and sedimentary deposits of Nariño Formation scoria cones have erupted both medium-K and accreted oceanic crust of Early-Mid calc-alkaline lavas (Ridolfi et al., 2008) and K- Cretaceous age (Pallatanga Terrain), with an rich, silica undersaturated lavas with alkaline associated island arc (Río Cala Formation) affinity (Barragan et al., 1998; Bourdon et al., and trench sediments (Natividad Formation) 2003; Hoffer et al., 2008). It is worth to note of Mid-Late Cretaceous age (Beate and that a flat-slab geometry, due to the Carnegie Urquizo, 2015). The tectonic framework is Ridge subduction, could have led to a characterized by active NNE trending regional lithospheric slab-tears (Gutscher et al., 1999, strike-slip faults (Cepeda et al., 1987) and

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5 local NNE and NW trending fault systems basis of data from the Observatorio cutting the volcanic complex (ICEL, 1983). Vulcanológico and Sismológico de Pasto Lavas are mainly medium-K calc-alkaline two- (SGC-OVSP), a division of the Servicio pyroxenes and olivine-bearing andesites and Geologico Colombiano and the Instituto dacites (Droux and Delaloye, 1996) with Geofísico de la Escuela Politécnica Nacional predominantly effusive products for Chiles, (IGEPN), three seismic swarms have been while Cerro Negro was characterized by a detected. Most earthquakes had magnitude more explosive behaviour, since its products between 1.0 to 4.0 and are located about 1-4 consist of pyroclastic flow deposits and km south of Chiles volcano with shallow debris avalanche (Cortés and Calvache, depth (up to 14 km). A major 4 km-depth 1996). Petrographic and geochemical data event occurred on October 2014 with a local suggest that fractional crystallization, magma magnitude of 5.7. Anyway no evidence of mixing and crustal assimilation occurred in surface volcanic activity linked to the above the genesis of the erupted magmas (Droux recent seismic swarms has been documented and Delaloye, 1996). The volcanic complex or reported. activity started in Pleistocene and the last eruption of Chiles is dated 160 ky while some 5.2.2 Chachimbiro Volcanic Complex lavas of Cerro Negro are probably Holocenic. and Cuicocha caldera An eruption reported in 1936 is questionable because may have been from El Reventador The Chachimbiro Volcanic Complex (0°28' N, (Global Volcanism Program, 2013a) located 78°18' W, max altitude 4054 m a.s.l.) is 14 about 100 km toward ESE. C dating related located in the northern part of the Western to the most recent debris avalanche deposit Cordillera on the border of the Inter-Andean of Cerro Negro provided an age of 6065±130 Valley, about 70 km NNE of Quito and 17 km years BP (Cortés and Calvache, 1996). The NE of Ibarra city (Fig. 5.1b). The whole current activity is represented by thermal complex is underlain by Cretaceous oceanic discharges consisting of acidic hot (up to plateau basalt of the Pallatanga Terrain and 55°C) springs occurring 2-3 km east of Volcan associated with the Late Cretaceous volcano- Chiles and bicarbonate springs close to the sedimentary and volcanic deposits of Rio Cala villages of Tufiño and Chiles (Beate and Group (Rio Cala and Natividad Formations; Urquizo, 2015). The development of tourist Boland et al., 2000; Vallejo, 2007; Vallejo et thermal spring area of Aguas Hediondas has al., 2009), overlain by Late Maastrichtian- repeatedly failed because of the detected Early Paleocene continental sediments and high (deadly) levels of H2S emissions (Global volcanic products of the Silante Formation Volcanism Program, 2013a). Other hot water (Vallejo et al., 2007, Vallejo et al., 2009) and discharges located near the collapse finally by Miocene-Pliocene volcanoclastic structures of both volcanoes were also deposits (Boland et al., 2000). Two main NE- reported (Cortés and Calvache, 1996). Since SW and WNW-ESE striking tectonic trends April 2013, as reported by Ruiz (2015) on the are present (Aguilera et al., 2005). According

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5 to Bernard et al. (2011), the area of diameter of 3.2 km with steep side flank Chachimbiro is characterized by persistent (>45°) and host a 148 m-deep sodium- volcanic activity started with the eruption of bicarbonate lake with a surface altitude of calc-alkaline andesites of the Mid-Pleistocene 3072 m a.s.l., formed after the dome-building Huanguillaro Volcano, which subsequently phase and feed by the catchment area of suffered a gravity-driven collapse involving Cotacachi volcano, rainwater and some explosive events (Aguilera et al., 1998). hydrotermal water inflow (Gunkel et al., The volcanic activity continued with the 2009). Post-magmatic activity consists of building up of caldera-filling rhyodacitic regular gas emissions in the western part of Tumbatu and Hugà domes with related the lake and in the channel between the two pyroclastic deposits and the Late-Pleistocene domes. The gas, mainly CO2 and N2, is of Chachimbiro-Pucarà NNE line of andesitic- volcanic origin, due to the occurrence of CO dacitic domes (Beate and Salgado, 2010). and the accumulation of boron in the lake The most recent volcanic products consist of (Gunkel et al., 2008). pyroclastic flow deposits from one of these domes with an age younger than ca. 8000 5.2.3 Chacana Caldera Complex years BP (Aguilera, 1998; Beate, 2001; Comida, 2012; Bernard et al., 2014). Near- The active Chacana Caldera Complex (0°19'S, neutral chloride to bicarbonate hot springs, 78°10' W) is located at about 40 km ESE of with temperatures ranging from 40 to 55°C, Quito in the Eastern Cordillera (Fig. 5.1b). It were recognized to the east of the system represents the largest rhyolitic center of (Beate and Salgado, 2010). Other volcanoes Ecuador with a length of 35 km in N-S are located W and WNW of Chachimbiro direction and a width of 10-15 km through E- such as Yanaurcu de Piñán (andesite to W, with a maximum altitude of about 4500 m dacite), Pulumbura (andesite) and Pilavo a.s.l. (Beate et al., 2009). The basement (basaltic andesite) (Bernard et al., 2011). consists of Paleozoic-Mesozoic metamorphic The Cuicocha lake-filled caldera (0°18' N, rocks in the middle part and eastern side, 78°21' W) is located in the Western Mesozoic oceanic basalts in the western side Cordillera, about 20 km SSW of Chachimbiro and thick Miocene-Pliocene sequence of Volcanic Complex, at the southern foot of the andesite flow, breccias and tuffs of the inactive Pleistocene Cotacachi volcano that Pisayambo Formation (Beate et al., 2010). stands along the Otavalo-Umpala fracture Ongoing tectonic activity includes NE- zone (Hanuš, 1987). The caldera-forming trending transpressive faults and eastward- eruption is dated at 3 ky BP (Hall and Mothes, trending thrust faults (Hall and Mothes, 1988). Cuicocha volcano can be considered 2008b). Prior to the caldera-forming semi-dormant as the last post-caldera activity eruptions, a Pliocene andesite volcanic field is represented by four intracaldera lava 60 km long (N-S direction) and 28 km wide (E- domes (Gunkel et al., 2009) dated between W) was present (Beate et al., 2010). 1350 and 1230 years BP. The caldera has a According to Hall and Mothes (2008b), the

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

Chacana Caldera Complex began its activity al., 2000). According to Hall and Mothes at about 2.7 My BP, with evidence of fault- (2008c), the regional basement consists of related breccias ring dike injections along the folded Tertiary sediments overlying caldera fractures. A subsequent event Cretaceous oceanic basalts and sediments. consisted of deposition of a ca.1250 m thick The volcanic edifice was produced by three sequence of dacitic to rhyolitic ignimbrites Late-Pleistocene to Holocene caldera-forming and tuffs that covered the western outer eruptions and its present morphology flank of the structure, followed by the main consists of ca 3 km-wide caldera with interior caldera collapse occurred about 0.8 My BP. steep-sided walls rising about 400 m above The depression was partly filled by thick the lake. A series of dacitic domes aligned sequences of tuffs, breccias, andesitic lavas along and inside the highly irregular caldera and a succession of fluvial sediments. Post- rim were recognized. The outer flanks of the collapse resurgence began after 0.44 My with volcanic edifice have a more gently slope to a long period of andesite to dacite lavas the north and south compared to the eastern erupted by a series of vents aligned NNE in side that abruptly slope downward toward the western sectors of the caldera. Volcanic the Toachi canyon. activity continued until about 0.16 My BP, Hall and Mothes (2008c) recognized at least including ignimbrites, pumice fall deposit and eight major Quaternary eruptive cycles, each obsidian flow. More recent andesite to dacite one separated by a long (10-15,000 years) lava flow occurred from 30 to 20 ky BP, dormant interval. The last eruption occurred whereas the youngest lava flow occurred in about 800 years BP. The volcanic activity was the 1700’s. Alkaline chloride hot (up to 65°C) characterized by large plinian eruptions springs were recognized along the NE- (VEI=4-6) whose deposits mainly consists of trending fault in the southern-central sectors phreatomagmatic ash fall units followed by of the caldera (Beate et al., 2010), as well as pumice lapilli falls and repeated series of on the SSW and W outer flank and on the NE surge and ash flows. The final stage of (Oyacachi) inner caldera rim. eruptive cycles was occasionally marked by dome extrusions. The tephra products and 5.2.4 Quilotoa volcano dome rocks are medium-K calc-alkaline dacites with a mineral assemblage Quilotoa volcano (0°51' S, 78°54' W) is constituted by plagioclase, amphibole, located in the Western Cordillera, biotite, quartz, Fe-Ti oxides and apatite as approximately 35 km WNW of the city of accessory phases. The similarities between Latacunga (Fig. 5.1b). The volcanic edifice, volcanic products, eruptive styles and with a maximum elevation of 3915 m.a.s.l., is stratigraphic sequences throughout the cyclic truncated by a subcircular caldera containing eruptions, might be the results of the same a lake with a diameter of about 2 km, a petrogenetic processes involving maximum depth of 256 m and an estimated homogeneous magma bodies at shallow total volume of about 0.35 km3 (Aguilera et depth (Hall and Mothes, 2008c). Bubbling

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5 gases currently occur in the southwestern tube containing 2 mL of an ammonia– sector of the shoreline along the NNE- cadmium solution to favor CdS precipitation trending fault crossing the lake and the (Montegrossi et al., 2001). Total alkalinity caldera walls (Aguilera et al., 2000). was measured by titration using HCl 0.01N as titrating agent and methyl-orange as colorimetric indicator. Water samples were − 2 − 5.3 SAMPLING AND ANALYTICAL analyzed by ion chromatography (Cl , SO4 , − − − METHODS NO3 , Br , and F ), molecular + spectrophotometry (NH4 ), atomic absorption + + + 2+ 2+ Water samples are reported in Table 5.2. spectroscopy (Li , Na , K , Ca and Mg ) and Four water samples (AH1-2; AN3-4) and two inductively coupled plasma-optical emission gas samples (AH1 and AN3) were collected spectroscopy (H3BO3 and SiO2). Analytical from the Chiles-Cerro Negro Volcanic area, errors were <3% and <5% for the main and II− whereas the CHA6-9 waters and the CHA6, minor components, respectively. The ΣS CHA 7, and CHA9 gases were collected from concentrations were determined after Chachimbiro volcanic complex. One gas centrifugation to separate the CdS precipitate sample (CU10) was collected from the from the liquid phase. Then, CdS was Cuicocha caldera lake. Five water (OY12, oxidized by adding 5 mL of H2O2 and analyzed 2− EP13, JA14, TO16 and RI17) and five gas as SO4 by ion chromatography. The isotopic (OY12, EP13, JA14, TO16 and RI17) samples composition of oxygen and hydrogen in H2O were collected within and close to the values were determined by mass Chacana caldera. Water and gas samples spectrometry (Vaselli et al., 2006 and (QU15) were collected at Quilotoa caldera references therein). The analytical errors lake. NA11 and CC5 water samples were were ±0.05 and ±1, respectively. collected from Nangulvì and at ~16 km NNW Gas samples (Table 5.3) were collected from (Cristobal Colon) of the Chalpátan caldera, bubbling waters by using a plastic funnel respectively. positioned above the bubbles and connected Water temperature and pH were measured with a glass thorion-tapped flask. At each in the field using portable instruments. Two sampling point two different gas aliquots water aliquots were collected for the were collected, as follows: i) a pre-evacuated determination of anions and cations. One of 150 mL glass flask for the determination of 13 12 13 them was acidified by adding Suprapur the of the C/ C ratios of CO2 (δ C-CO2) and 3 4 hydrochloric acid (1% HCl). Both aliquots He/ He values; ii) a pre-evacuated 60 mL were filtered at 0.45 μm in the field. An glass flask filled with 20 mL of a 5N NaOH and unfiltered water aliquot (125 mL) was 0.15 M Cd(OH)2 suspension for the sampled for the analysis of the oxygen and determination of the chemical composition. Inorganic gases (N2, Ar+O2, H2, He and CO) hydrogen isotopes in H2O. For the analysis of reduced sulfur species (hereafter ΣSII−), 8 mL and CH4 stored in the soda flask headspace of water were transferred in the field to a were analyzed by using a Shimadzu 15A gas

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5 chromatographic system equipped with a 9 described by Inguaggiato and Rizzo (2004). m long molecular sieve column and thermal The analytical uncertainty was ±1%. conductivity detector (TCD). Argon and O2 were analyzed using a Thermo Focus gas chromatograph equipped with a 30 m long 5.4 RESULTS capillary molecular sieve column and a TCD. Light hydrocarbons were determined by a 5.4.1 Chemical composition of thermal Shimadzu 14A gas-chromatograph equipped waters with a 10 m long stainless steel column (ϕ = 2mm) packed with Chromosorb PAW 80/100 Outlet temperature, pH value and chemical mesh (coated with 23% SP1700) and FID composition of water samples are shown in detector. Hydrogen sulfide concentrations Table 5.2. The outlet temperatures range were determined by analyzing SO 2− in the 4 from 8.6 to 70.2 °C which correspond to soda solution by ion-chromatography as after Jamanco (JA14, Chacana) and cold spring oxidation with H O . Carbon dioxide, 2 2 discharge at Chiles-Cerro Negro (AH2), dissolved in the soda solution as CO 2−, was 3 respectively. The highest temperatures analyzed by acidimetric titration with 0.1N measured at Chiles-Cerro Negro and HCl. The analytical errors were ±5% for the Chachimbiro are 54.7 °C and 58.8 °C, main gas components and ±10% for minor respectively. Finally, Cristobal Colon with and trace gas compounds. T=36.6 °C, Nangulvì T=50 °C, and Quilotoa The δ13C-CO values (expressed as ‰ vs. V- 2 T=14 °C. TDS (Total Dissolved Solids) show a PDB) were analyzed by mass spectrometry wide range of values ranging from those (Finningan Delta S), after a two-step typical of groundwater up to 11.58 g/L extraction and purification procedure of the (Quilotoa). Chachimbiro and Chacana have gas mixtures by using liquid N2 and a solid– TDS values in the same range (0.89-5.20 and liquid mixture of liquid N and 2 1.44-5.47 g/L, respectively) while lower trichloroethylene. Internal (Carrara and San values were measured at Chiles-Cerro Negro Vincenzo marbles) and international (NBS18 (0.03-1.34 g/L), Cristobal Colon (2.05 g/L) and and NBS19) standards were used to estimate Nangulvì (2.85 g/L). Except for the water external precision. The analytical uncertainty samples from Nangulvì and Quilotoa , which and the reproducibility were ±0.05‰ and show near-neutral pH values, those from ±0.1‰, respectively. Chachimbiro, Chacana and Cristobal Colon The 3He/4He ratios (expressed as R/Ra, where are slightly acidic (6.02-6.29, 5.98-6.5 and R is the 3He/4He measured ratio and Ra is the 6.14, respectively), whereas those from 3He/4He ratio in the air: 1.39 × 10-6; Mamyrin Chiles-Cerro Negro show the lowest pH and Tolstikhin, 1984) were determined by values (from 4.52 to 5.77). using a double collector mass spectrometer As shown in the LL classification diagram (VG 5400-TFT) according to method (Langelier-Ludwig, 1942; Fig. 5.2), the compositions of waters from the

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

Quilotoa -2.32 QU15 LW Quilotoa 22/01/2015 14 13 7.25 432.4 785.2 2222.5 210.3 4074.7 2493.9 1362.5 11.58 3.42 0.49 12.65 83.4 815.4 nd 0.03 6.59 -37.91

Cerro Negro, - -13.14 RI17 TS Vertiente Lisco Rio 23/01/2014 31.2 -49 5.98 61.1 72.3 244.9 29.4 248.4 15.1 772.8 1.44 0.18 0.95 0.61 133.7 34.3 nd 0.08 0.93 -96.86

Cerro Negro, Chachimbiro, - TO16 TS Calera de Tolontag de Calera 23/01/2015 39 -160 6.18 294.2 66.2 1224.5 79.1 566.6 1455.4 1785.1 5.47 2.42 2.99 1.32 68.4 124.1 nd 2.70 0.26 -90.93 -11.12 Chacana

JA14 TS Jamanco 21/01/2015 70.2 -126 6.03 256.1 9.2 1246.1 57.9 2039.6 318.0 479.3 4.41 2.66 2.46 4.99 83.6 104.1 10.7 0.25 0.14 -83.85 -11.73

Quilotoa QU 15 Quilotoa 983 bdl 16 0.36 0.71 0.081 0.044 0.00026 0.0015 0.035 0.0026 0.00013 0.00005 -4.92 3.97 5.04

EP13 TS El Pisque 20/01/2015 38.2 -41 6.29 33.8 91.0 373.7 47.5 200.1 14.4 1226.3 1.99 0.33 0.84 0.55 118.3 13.2 nd bdl 0.10 -95.32 -13.23

OY12 TS Oyacachi 20/01/2015 46.2 -132 6.5 128.8 61.5 984.1 38.9 691.1 103.7 2106.9 4.12 2.75 0.64 1.74 129.2 106.4 0.1 0.15 0.14 -84.36 -10.77 RI 17 Vertiente Rio Lisco Rio Vertiente 341 bdl 623 14.5 21.8 bdl bdl 0.011 0.0039 0.011 bdl bdl bdl -10.56 5.56 5.58

NA11 TS Nangulvì 19/01/2015 50 -114 7.01 247.1 4.5 729.8 10.1 1122.6 653.1 83.6 2.85 1.41 1.88 2.13 46.0 82.9 11.6 0.12 0.10 -52.80 -7.94

of thermal and cold water discharges from Chiles Chacana TO 16 Calera de Tolontag de Calera 992 bdl 6.3 0.15 1.1 0.005 bdl 0.00011 0.00015 0.15 0.0016 0.00085 bdl -5.92 nd nd CHA9 TS Timbuyaco 18-19/01/2015 43.3 -100 6.29 237.3 146.1 620.1 66.1 853.0 11.9 1710.1 3.64 2.12 0.28 1.14 141.8 139.5 0.5 0.32 0.12 -72.82 -9.40 SMOW) - JA 14 Jamanco 840 bdl 146 3.2 9.6 0.004 bdl 0.0019 0.061 0.56 0.0009 0.00012 bdl -7.14 nd nd

CHA8 TS Cahuasquì 18/01/2015 26.8 92 6.15 31.6 29.2 198.3 17.5 181.0 11.0 423.8 0.89 0.54 0.56 0.24 nd nd nd bdl 0.10 -78.51 -10.88 O (‰V EP 13 El Pisque 625 bdl 351 7.6 15.2 0.006 bdl 0.0041 0.056 0.181 0.0019 0.00054 bdl -10.42 4.17 4.19 2 Chachimbiro DH δ OY 12 Oyacachi 291 bdl 661 15.5 32.3 0.004 bdl 0.011 0.009 0.051 0.0002 bdl bdl -8.05 4.57 4.68 CHA7 TS Chachimbiro 17/01/2015 58.8 -70 6.02 89.2 54.6 1113.9 145.7 1824.7 56.8 682.1 3.97 4.80 0.35 2.56 173.5 236.7 1.2 bdl 0.14 -70.61 -7.78

O and and O 2

Cuicocha Cotacachi- CU10 Cuicocha 514 bdl 456 6.5 21.8 bdl bdl 0.0061 0.0021 0.75 0.00092 0.00011 bdl -7.98 4.43 4.60 CHA6 TS Aguas Savia Aguas 17/01/2015 32.5 -56 6.09 146.1 90.4 1565.2 61.9 2006.9 8.0 1322.6 5.20 3.81 0.35 2.58 99.5 288.2 nd bdl 0.07 -64.28 -6.77 OH 18

δ CHA 9 Timbuyaco 993 bdl 5.6 0.13 0.96 0.003 bdl 0.00009 0.00011 0.11 0.0011 0.00051 bdl -5.24 nd nd CC5 TS Cristobal Colon 16/01/2015 36.6 -85 6.14 150.0 151.5 131.4 39.0 88.9 2.1 1482.2 2.05 0.13 0.55 0.40 117.6 56.0 nd bdl 0.07 -80.69 -11.45 PDB) andR/Ra of free gases emitted from the thermal discharges of Chiles - CHA 7 Chachimbiro 824 bdl 168 3.9 3.9 bdl bdl 0.0022 0.0009 1.3 0.0068 0.00087 0.00007 -4.89 nd nd Chachimbiro AN4 TP Aguas Negras Aguas 16/01/2015 19 -2.2 5.15 102.3 76.9 137.0 34.1 110.5 771.8 0.0 1.23 0.13 0.66 0.27 nd nd nd 0.02 0.02 nd nd (‰V

2 CO

C CHA 6 Aguas Savia Aguas 991 bdl 7.1 0.17 1.6 0.006 bdl 0.00012 0.00011 0.55 0.0012 0.00041 bdl -6.89 1.46 1.50 13 AN3 TS Aguas Negras Aguas 15-16/01/2015 35.7 -380 5.77 104.9 78.1 142.0 33.5 116.3 538.5 326.9 1.34 0.16 0.41 0.23 138.4 22.9 40.5 0.65 bdl nd nd

AN 3 Aguas Negras Aguas 961 11 26 0.62 0.66 0.061 0.036 0.00031 0.00015 0.041 0.00015 0.00007 0.00009 -6.22 4.60 5.88 Chiles-Cerro Negro AH2 CS Aguas Hediondas 15/01/2015 8.6 130 5.39 2.8 1.8 2.2 1.0 0.1 10.0 15.7 0.03 bdl 0.03 bdl nd nd bdl bdl bdl -81.30 -11.52

composition, δ Chiles-Cerro Negro

AH 1 Aguas Hediondas 964 13 21 0.48 0.72 0.075 0.031 0.00026 0.00013 0.036 0.00021 0.00006 0.00011 -5.28 2.40 3.13 AH1 TS Aguas Hediondas 15/01/2015 54.7 -236 4.52 86.4 53.0 195.9 45.1 109.5 668.4 13.7 1.17 0.26 2.98 0.29 98.2 32.0 26.5 1.40 bdl -88.08 -11.76

. Chemical

(V-PDB) 2 O ‰O (vs. V- 2

O ‰ (vs. V-SMOW) - 3 Geoth. Volc. Areas Volc. Geoth. 2 3 Table 5.3 and Quilotoa Chacana Cuicocha, Table 5.2. Outlet temperatures, pH, chemical composition, Quilotoa. and Chachana Nangulvì, Chachimbiro, Colon, Cristobal 6 8 6 - + 2 C-CO 2- 2 4 2+ 4 3 O-H S 2+ + 4 - H H H BO - - 2 + 3 2 2 2 2 3 6 13 D-H + - 18 TDS g/L Li d H Geoth. Volc. Areas ID Type* Sampling location Date of sampling T°C Eh pH Ca Mg Na K Cl SO HCO F Br SiO HS NH NO d detected not nd: limit; detection below bdl: mg/L; as expressed concentration TS=* thermal spring; CS=cold spring; TP=thermal pool; LW=lake water concentration expressed as mmol/mol; bdl: below detection limit; nd: not detected C R/Ra Rc/Ra Ar O CH C C ID location Sampling CO H N CO Ne He d H

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

Chachimbiro and Chachana areas ranged 5.4.2 Chemical composition of gases from Na-Cl to Na-HCO3. Thermal waters from Chiles-Cerro Negro have a Na(Ca)-SO4 The chemical compositions of inorganic and composition. Lake Quilotoa had a Na-Cl(SO4) organic (C2-C6 hydrocarbons) fractions in the composition, similar to what observed in free gases are reported in Table 5.3. The gas November 1993 by Aguilera et al. (2000). discharges of the investigated areas Waters from Nangulvì and Cristobal Colon consisted of bubbling pools, hence it is not are classified as Na-Cl and Ca-HCO3, surprising that acidic and high-soluble gases respectively. (i.e. SO2, HF, HCl) were not detected. Except Silica content ranges between 46 and 173.5 for gas samples from Vertiente Rio Lisco and mg/L, the highest value being recorded in the Oyacachi (Chacana), where N2 is the Chachimbiro area. dominant gas (623 and 661 mmol/mol,

respectively) followed by CO2 (341 and 291 mmol/mol, respectively), the gas chemical composition is dominated by CO2 (from 514 to 993 mmol/mol) followed by N2 (from 5.6 Chiles-Cerro Negro Nangulvì Chachimbiro Quilotoa Chacana + Cristobal Colon

Fig. 5.2. Chemical classification of cold and thermal waters from the investigated areas (square classification diagram; after Langelier and Ludwig, 1942. 74

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5 to 456 mmol/mol), O2 (from 0.66 to 32.3 He concentration and Ne concentrations are mmol/mol), Ar (from 0.13 to 15.5 <0.0009 and <0.0022 mmol/mol, mmol/mol). The highest values of O2 (21.8 respectively, at Chiles-Cerro Negro and and 32.3 mmol/mol) and Ar (14.5 and 15.5 Chachimbiro. Volcanic lakes Quilotoa and mmol/mol) were detected in Vertiente Rio Cuicocha show similar values of He (0.0015 Lisco and Oyacachi (Chacana). Detectable and 0.0021 mmol/mol, respectively) and concentrations of H2S were found only at different values in Ne concentrations Chiles-Cerro Negro (11 and 13 mmol/mol). (0.00026 and 0.0061 mmol/mol, CO shows appreciable values at Chiles-Cerro respectively). At Chacana He and Ne are Negro (0.031 and 0.036 mmol/mol) and <0.061 mmol/mol and <0.011 mmol/mol

Quilotoa (0.044 mmol/mol). H2 is <0.081 at respectively.

Chiles-Cerro Negro and Quilotoa and <0.006 The highest concentration of CH4 was mmol/mol at Chachimbiro and Chacana. measured at Chachimbiro (1.3 mmol/mol).

Fig. 5.3. δD versus δ18O. The isotopic compositions of sampled springs highlight a meteoric origin for all the samples. Significative isotopic shift are reported for Quilotoa and Chachimbiro samples and, to a lesser extent, for Chacana e Ciles-Cerro Negro. Local meteoric water-lines have been reported.

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

Chacana and Chiles-Cerro Negro show oxygen is observed for Chacana suggesting methane concentrations one or two orders of the occurrence of enhanced water-rock magnitude lower than Chachimbiro (<0.181 interactions. Much greater deviations from and <0.041 mmol/mol, respectively). CH4 at the LWL are recorded for Quilotoa and, to a Cuicocha is 0.75 mmol/mol. lesser extent, for Chachimbiro water

∑C2-C6 shows low values ranging from samples, suggesting evaporation and input of <0.00038 mmol/mol at Chiles-Cerro Negro to arc-type magmatic waters (Giggenbach, <0.00278 at Quilotoa, Chachimbiro, Chacana 1992). and Cuicocha. The isotopic compositions of gases are reported in Table 5.3. 13 5.4.3 Isotopic composition of waters The δ CCO2 values range from -5.3 to -6.2 V- 18 13 PDB ‰ at Chiles-Cerro Negro, from -4.9 to - (δD, δ O) and gases (δ CCO2 and R/Ra) 6.9 V-PDB ‰ at Chachimbiro, from -5.9 to - The isotopic compositions of thermal waters 10.6 V-PDB ‰ at Chacana. Volcanic lakes are reported in Table 5.2. Quilotoa and Cuicocha show values -4.92 and

δD and δ18O ranges from -96.9 to -37.9 ‰ -7.98 V-PDB ‰, respectively. and -13.23 to -2.3 ‰ versus V-SMOW, R/Ra values, corrected for air contamination, respectively. Data and local meteoric water- indicate a significant contribution of mantle line (LWL) are reported on δD vs δ18O at Chacana (5.58) and, to a lesser extent, at diagram (Fig. 5.3). Samples from Nangulvì, Cuicocha (4.60) and Quilotoa (5.04), whereas Chiles-Cerro Negro and Cristobal Colon plot Chachimbiro shows the lowest value (1.50). close to the LWL. A moderate isotopic shift of

+ Fig. 5.4. Li versus H3BO3 diagram.

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

Concerning minor components, Fig. 5.4 + + reports the Li vs H3BO3 distribution. Li only derives from leaching of the hosting rocks,

whereas H3BO3 can be also added by addition of a vapor phase. Samples are mainly plotted with moderate shift respect to the rock leaching line. Samples of Chachimbiro and Chachana areas define a positive correlation + between Li and H3BO3, pointing out processes of rock leaching. Conversely, samples from Quilotoa caldera (data from this work and literature, Aguilera et al., 2000)

only display very high contents of H3BO3, thus possible revealing the addition and accumulation within the active caldera lake of a boron-bearing vapor phase. Waters from Chiles -Cerro Negro and Cristobal Colon show

Fig. 5.5. log aNa+/ aH+ versus log aK+/ aH+ low contribution of both elements. The different interaction between waters and rocks is also shown in Fig. 5.5 where thermal 5.5 DISCUSSION waters display a different approach to equilibrium with alkali-feldspar in the hosting 5.5.1 Thermal water discharges rocks. Samples from Chiles-Cerro Negro are far from any possible equilibrium, thus further supporting the idea of shallow, Waters from Chiles-Cerro Negro with steam-heated waters. Conversely samples temperature up to 55 °C, have a Na(Ca)-SO4 collected at Chachimbiro and Chacana areas composition, low pH values (<5.77) and low plot closer to equilibrium conditions TDS values (<1.34 g/L) possibly identifying apparently at temperature in the range 150- relatively shallow, steam-heated acid- 200 °C, confirming the processes of rock sulphate waters. High temperature slightly leaching highlighted in Li+ vs H BO diagram acidic waters from Chachimbiro (T up to 60 3 3 (Fig. 5.4). °C; pH=6.02-6.29) and Chacana (T up to 70 °C; The isotopic composition of waters reveals a pH=5.98-6.5) with higher TDS values (from meteoric origin for all investigated areas. 0.89 to 5.20 g/L, from 1.44 to 5.47 g/L, Anyway significant isotopic shift for Quilotoa respectively) compared to Chiles-Cerro suggests a process of evaporation probably Negro, are consistent with a direct related to steam accumulation as shown in emergence of partially or fully equilibrated Fig. 5.4. Moderate isotopic shift related to a thermal waters from the main geothermal few samples from Chacana suggests the reservoir.

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5 occurrence of enhanced water-rocks interaction as previously shown in Fig. 5.4 e Fig. 5.5. Input of arc-type magmatic waters at Chachimbiro does not seem to be the only one to control the isotopic enrichment process. This observation is based on a larger number of samples from Aguilera et al. (2005). The extrapolation of the thermal water trend towards the δD and δ18O of arc- type magmatic water [δD= -20‰ and δ18O= +10‰ (Giggenbach, 1992)] does not pass through the arc-type magmatic water value. According to the authors, the isotopic shifting matches, most likely, with significant Fig. 5.6. N2/100-Ar-He*10 triangular diagram from the water/rock exchange of oxygen isotopes geothermal areas of Chiles-Cerro Negro, Chachimbiro, supporting the water/rock interaction shown Cuicocha, Chacana and Quilotoa. in Fig. 5.4 e 5.5).

5.5.2 Gas discharges ratios, from mantle- to limestone-derived compositions (Chacana area and Chiles-Cerro As reported in the N2-Ar-He triangular plot Negro and Chachimbiro areas, respectively) (Fig. 5.6), all the samples display a N2-excess passing throughout an apparent mixture of with respect to atmospheric value, which is the two (Quilotoa caldera). The observed typical of the so called “andesitic” gases of vertical distribution of the samples could also 3 Giggenbach (1978). The existence of a result from different CO2/ He fractionation significant input of non-atmosferic N2, due to the selective dissolution of CO2 and He together with the relatively high R/Ra values into the shallow aquifers. The slight shifting such as those observed at Chacana and of gas compositions with respect to the Quilotoa (5.58, 5.04) and the values of mantle-limestone mixing curve suggest a 13 δ CCO2 (ranging from -5 to -10‰) supports modest but homogeneous contribute of 3 3 4 the idea of a mantle-like signature for these biogenic carbon. The CO2/ He vs He/ He fluids (Fig. 5.7). Further suggestions (reported as Rc/Ra; Fig. 5.9) confirm an concerning the relative contributions of apparently prevailing crustal component in mantle, crustal carbonate and biogenic the northermost geothermal sites (Rc/Ra = carbon are provided by the relative amount 1.50 at Chachimbiro-Agua Savia and Rc/Ra = of CO2 and the inert noble gases (He and Ar) 3.13, 3.35 at Chiles-Cerro Negro), whereas a and their isotopic composition. According to prevailing mantle component in the Chacana Fig. 5.8, the investigated Ecuadorian gas caldera, Quilotoa and Cuicocha volcanoes. 3 samples cover a large spectrum of CO2/ He The He isotopic values show that mantle

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

13 Fig. 5.7. CCO2 of bubbling gas versus Rc ⁄ Ra; mid-ocean ridge basalts (MORB) and crust values are plotted as reference. Data from Inguaggiato et al., (2010) referred to Quilotoa, Chacana and Cuicocha are also reported for comparison.

13 3 Fig. 5.8. δ C versus CO / He diagram. CO2 2

3 3 4 Fig. 5.9. log CO2/ He versus He/ He diagram.

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5 contribution was higher in the geothermal value, that is in turns controlled by the redox- areas related to the active and semi-dormant controlling system. volcanoes located between + 0.3°N and -1°S The dependence on RH and temperature of

(Cuicocha, Chacana caldera and Quilotoa) the H2-Ar and CH4-CO2 geothermometers can with respect to that of the northernmost be expressed, as follows: geothermal systems (between 0.5-1°N, i.e. Chiles-Cerro Negro and Chachimbiro). This (3) spatial distribution of mantle He was possibly RH = -6.523 + 10.0492 – 0.014T + log H2/Ar driven by the slab-tears inferred at this latitude of Ecuador, due to the flat-slab (4) geometry linked to the Carnegie Ridge RH = 1/4(– 5182/T + 2.41 – 0.0034T + log subduction (Gutscher et al., 1999). (CH4/CO2)

5.5.3 RH and T geoindicator Substituting the measured values of log H2/Ar and log CH4/CO2 to equations (3) and (4), the RH = f(T) can be plotted in the RH vs T diagram Giggenbach (1991) proposed that the H2/Ar molar ratio could be representative of the (Fig. 5.10). Assuming the attainment of deep degassing liquid. This ratio in the liquid equilibrium at the same P, T and redox phase is basically controlled by dominating conditions, the convergence point of the two redox conditions. These latter are well curves directly provides T and RH values at which the two systems were simultaneously represented by the RH value (RH = log H2/H2O) equilibrated. In the RH vs T plots, the RH = f(T) due to the ability of the H2/H2O redox pair to instantaneously adjusts in response to of HM (Hematite-Magnetite), GT (FeO1.5- variation in redox conditions (Giggenbach, FeO) and DA (D’Amore and Panichi, 1980) redox buffer systems, were reported. 1980; 1987). Similarly, the CH4/CO2 ratio can be used to evaluate equilibrium In the RH vs T plot of Fig. 5.10a, the two gas temperatures according to: samples from Chachimbiro area plot at equilibrium temperatures and RH values in (1) the range 149-153 °C and -3.80 ÷ -3.60 respectively, the latter being consistent with CH4 + H2O = CO2 + H2 the DA redox buffer. Bubbling gases collected where at Chiles-Cerro Negro area display slightly higher equilibrium temperatures (180-190 °C) (2) at similar redox conditions (RH = -3.76 ÷ -3.70; Fig. 5.10b). Gases from Lake Quilotoa log Keq = f(T) = RH - log CH4/CO2 apparently attained equilibrium at 200°C under redox conditions more oxidizing with The CH4/CO2 ratio is controlled by i) the respect to those dictated by the DA buffer dependence on of log Keq, and iii) the RH (Fig. 5.10c). This could be related to loss of

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

Fig. 5.10a-c: RH versus T diagram.

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

Fig. 5.10d-f: R versus T diagram. H

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

which characterize gas samples from Oyacachi, Jamanco and El Pisque, typically suggest a bacterial origin of methane. Conversely, samples from Quilotoa and, at lesser extent, Chachimbiro, Calera de Tolontag and Chiles-Cerro Negro plot at

CH4/(C2H6+C3H8) < 500, thus suggesting a prevailing thermogenic origin for methane. Silica content ranges between 46 and 173 mg/L, the highest value being recorded in the Chachimbiro area. Temperatures calculated for equilibria of thermal waters with methane due to its oxidation at shallow chalcedony (Fournier, 1991) and quartz (no depths (metanotrophic bacteria) and/or, as steam loss) (Fournier, 1977) at Chachimbiro stated below, loss of hydrogen due to its provide 148 and 170 °C respectively, which conversion to methane (hydrogenotrophic fits very well with temperature estimation bacteria) at more reducing conditions. Within according to gas phase equilibria (149-153 the Chacana area, gas sample from Calera de °C). Temperature ranges of 67-129 °C and 97- Tolontag (Fig. 5.10d) displays temperature 154 °C have been recorded respectively for and RH condition very similar to those from chalcedony and quartz for water samples Chachimbiro, i.e. 155°C and RH = -3,70. collected in the Chachana area. Also in this Conversely, gas samples collected at Jamanco case, the recorded range of silica (Fig. 5.10e) and Oyacachi and El Pisque (Fig. temperatures well fits with those calculated 5.10f) provide apparent equilibration according to gas phase equilibria (53-155 °C). temperatures < 100°C and RH ≤ -4.20. Thermal waters collected in the Chiles-Cerro However, only considering the CH4-CO2 Negro area report silica equilibration geothermometer, these gases seem to have temperature for chalcedony in the range 109 been equilibrated at 150÷170 °C and - – 132 °C and for quartz in the range 136-156 3.70

CH4/(C2H6+C3H8) > 500 (Bernard et al., 1978), active to semi-dormant volcanoes of the

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5

Cordillera in the northern half of the country 154 °C thus confirming the previous (Inguaggiato et al. 2010). The surface temperature calculated with gas phase. manifestations consist of thermal springs at Conversely, the location of the other water medium to low temperature rarely discharges is consistent with the lower associated with bubbling gases. The chemical calculated equilibrium temperature (<100°C) features of waters reflect two different and higher oxidizing conditions (RH>-4). origins: emergences of shallow waters, Decoupled conditions between liquid and gas heated by the uprising steam and sites of phases were recognized at Chile-Cerro Negro, deeper reservoir partially or totally where the acid waters with Na-SO4 equilibrated with rock minerals and gas composition represent shallow, steam phases. Among the investigated samples heated waters at temperature close to the those pertaining to Chachimbiro and Chacana boiling point, whereas the gas phase appears geothermal systems represent thermal to be equilibrated at deeper PT conditions, waters partially equilibrated with the hosting i.e. 178÷190 °C and -3,76

3.80

ORIGIN OF GEOTHERMAL FLUIDS ALONG ACTIVE TO SEMI-DORMANT VOLCANOES OF NORTHERN ECUADOR (1°S to 1°N) AS INFERRED FROM CHEMICAL AND ISOTOPIC COMPOSITION CHAPTER 5 higher in the Cuicocha, Chacana and Quilotoa calderas which are active and semi-dormant volcanoes being located at a latitude associated to slab-tears (e.g. prosecution of the Grijalva Fault Zone) of the flat-slab geometry due to the Carnegie Ridge subduction, making easier the influx to the surface of mantle-derived fluids.

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