Application of Minerals for the Characterization of Geothermal Reservoirs and Cap Rock in Intracontinental Extensional Basins and Volcanic Islands in the Context of Subduction

Abstract

Whether from the near-surface or at great depths, geothermal energy aims to harness the heat of the Earth to produce energy. Herein, emphasis is put on geothermal reservoirs and their cap rock in crystalline rocks, in particular, the basements of sedimentary basins and volcanic islands in the context of subduction. This study is based on a case study of three examples from around the world. The aim of this paper is to show how the study of newly formed minerals can help the exploration of geothermal reservoirs. The key parameters to define are the temperature (maximum temperature reached formerly), fluid pathways, and the duration of geothermal events. To define these parameters, numerous methods are used, including optical and electronic microscopy, X-ray diffraction, microthermometry on fluid inclusions, chlorite geothermometry, and geochemistry analysis, including that of isotopes. The key minerals that are studied herein are phyllosilicates and, in particular, clay minerals, quartz, and carbonates. They are formed because of hydrothermal alterations in fracture networks. These minerals can have temperatures of up to 300 °C (and they can cool down to 50 °C), and sometimes, they allow for one to estimate the cooling rate (e.g., 150 °C/200 ka). The duration of a hydrothermal event (e.g., at least 63 Ma or 650 ka, depending on the site) can also be established based on phyllosilicates.

1. Introduction

Geological energy resource reservoirs, including fossil fuels and geothermal energy, need to be thoroughly characterized before exploitation. In the global context of the energy transition, the use of the heat of the earth to provide energy is of crucial importance. Deep geothermal exploitation requires three components to be found together: heat, water, and flow pathways. Flow pathways are rather obvious in rocks that present matrix permeability and porosity like limestones and sandstones in sedimentary basins. However, in geothermal reservoirs located in basement and volcanic areas, for heat or electricity production, the recognition of fluid flow pathways is of vital importance. In those cases, the flow pathways are mainly discontinuities (e.g., joints between lava flows or debris flow layers in volcanic areas [1,2]), faults (e.g., in volcanic [3,4] and granitic environments [5,6,7,8,9,10,11]), and the damage zones around them. In all of these zones, the natural circulation of hydrothermal fluids generates fluid–rock interactions [4,12] that can produce an increase in porosity and permeability [13] and hence improve the ability of fluids to circulate within the rock and create a geothermal reservoir. The fluid–rock interactions that occur in such cases are responsible for the dissolution of preexisting minerals [13] and the crystallization of newly formed ones [2,14,15] in the fractures and their wall rocks, which can help in determining the physico-chemical conditions reached by the reservoir [16] and the zones that produce hot water. In addition, they help locate the cap rock responsible for the coverage of geothermal reservoirs, which prevents too much hot water from leaking to the surface [17,18,19,20,21,22,23]. Several kinds of newly formed minerals have already proven to be good indicators and are found in both the geological contexts presented here. Quartz is able to trap the fluids responsible for hydrothermal alteration into fluid inclusions, which can be studied by microthermometry [15,24,25,26,27] or crush–leach analyses [28,29]. Microthermometry is based on the phase changes that occur in fluid inclusions while cooling and heating rock samples under an optical microscope. Carbonates, which are generally not found in fresh granite or volcanic rocks like andesite, are also produced during hydrothermal alteration in these environments, mostly by the dissolution of plagioclase [6,10,30]. They sometimes also contain fluid inclusions. Clay minerals are very sensitive to the conditions of fluid–rock interactions and hence might provide an idea of the temperature reached by the reservoir [2,14,15]. For example, chlorite can be used as a geothermometer, and several examples of this can be found in the literature [31,32]. Their use, combined with fluid inclusions (for example, in quartz), can help show the cooling of a paleo-hydrothermal reservoir [15] and hence provide a model for the evolution of present-day geothermal reservoirs. In addition, the porosity that is created between the clay crystallites promotes matrix permeability [13,14,27] and is favorable to geothermal projects. Finally, carbonates and some clay mineral species can be dissolved by acid and other chemical injections during the chemical stimulation of geothermal wells to enhance the permeability around them. Hence, these three types of minerals are actively searched for during geothermal exploration. This paper focuses on three examples from all over the world (Figure 1), including a crystalline basement of sedimentary basins, namely, Soultz-sous-Forêts granite (Upper Rhine Graben, URG), another granite body that can be considered a reasonable surface analog (Noble Hills, CA, USA), and andesitic volcanic environments in Guadeloupe (Bouillante’s harnessed geothermal field and its Terre-de-Haut surface analog). Despite these two environments differing heavily with regard to geology and geodynamics, similarities exist in terms of the newly formed minerals encountered in these environments; hence, they both deserve to be studied here. This paper will pinpoint the use of some mineral species for a better exploration of geothermal reservoirs and discuss their relevance to the study of analogous surface systems [7,8,33,34] when little information is available from geothermal boreholes or when none have been drilled. In particular, it will show that similar minerals, like clay minerals, provide information on both geothermal reservoirs and the cap rock of geothermal reservoirs, which are generally studied separately. This paper is composed of seven main sections. Section 2 discusses the geological contexts of the two major geological environments, presenting information on intra-continental extension basins with examples from the Upper Rhine Graben (URG, France) and the Basin and Range section in Death Valley (DV, CA, USA) with a focus on the Noble Hills (used as a surface analog) and a volcanic setting in the Lesser Antilles (Guadeloupe, France). Then, the next three sections are dedicated to mineral species: clay minerals (Section 3), carbonates (Section 4), and, finally, quartz (Section 5). The last two sections (Section 6 and Section 7) present a discussion of this study’s results and the conclusions that can be drawn from them, respectively.

2. Geological Settings of the Chosen Sites

Even though they did not develop in the same geological context, all three studied sites are crosscut by numerous normal and strike-slip faults that conducted—and sometimes still conduct—hydrothermal fluids responsible for the formation of (paleo)geothermal reservoirs.

2.1. Extensional Basins in Continental Domains: Examples of the Upper Rhine Graben (France) and Death Valley (USA)

2.1.1. Upper Rhine Graben (URG)

The Upper Rhine Graben is the central part of the European Cenozoic Rift System [34]. It is 300 km in length about 50 km in width. It extends from Basel (Switzerland) to Mainz (Germany) in an NNE-SSW direction and covers the eastern part of France (Figure 2). It hosts several thermal anomalies [34,35,36] associated with natural brine circulation through a nearly vertical fracture network, which crosscuts Triassic sediments overlying the Paleozoic crystalline basement [37,38,39]. Fifteen deep wells were drilled in the URG over more than 30 years to harness thermal anomalies for geothermal purposes [12,40,41,42]. The Paleozoic granitic basement is made of several types of granite, mainly, a porphyritic monzo-granite locally affected by intense fracturing and vein alteration and a two-mica granite crosscut by Soultz-sous-Forêts (called Soultz in the following) and Basel deep drill holes [12] (Figure 2). The enhanced geothermal system (EGS) project at Soultz was initiated in the 1980s to develop a deep fractured granitic reservoir [43]. Crystalline rocks are generally characterized by low matrix porosity, and the main flow occurs in permeable and connected fractures either in the basement or its sedimentary cover [44,45]. Thus, projects based on EGS technology require a good knowledge of the fracture network to understand flow distribution at depth and to design borehole trajectories according to the geometrical properties of the fracture network [46]. The deep geothermal site at Soultz is based on several boreholes. EPS-1 is an entirely cored well, drilled for scientific purposes down to ~2200 m True Vertical Depth (TVD). Well GPK-1 was drilled down to ~3500 m TVD, with some sore sections for both scientific and pre-industrial purposes. The most important wells, GPK-2, GPK-3, and GPK-4, drilled for industrial purposes, go down to ~5000 m TVD. This triplet harnesses the deep reservoir located between ~4000 m TVD and ~5000 m TVD. It has been entirely drilled in destructive conditions and provides only cuttings. Drillers collected cuttings all along the drilling process and provided raw samples of nearly 1 kg for 3 m to 6 m sections. These samples sometimes showed anomalously high concentrations of biotite related to buoyancy [47] or total loss in fault zones that did not allow for samples to be collected. However, their study all along the 3 deep wells might be considered as having statistical validity. In the Soultz granite, the porosity of 25% measured in the rock samples in a core section of the EPS-1 well in an altered halo around a fracture [27] was associated with high permeability [13]. In this zone, the primary crystals of the granite were totally replaced by an assemblage of carbonates, clay minerals, and newly formed quartz, originating likely from a single hydrothermal alteration event. Hence, a specific mineralogical signature of paleo-fluid circulations is encountered. The vein alteration was quantitatively characterized thanks to the mineralogy of newly formed deposits [48,49]. At Rittershoffen, 10 km away from Soultz, a high-temperature anomaly is concentrated around the Rittershoffen fault, which probably hosts the main hydrothermal circulation [36,50]. The surface installations produce heat at Rittershoffen (24 MWth) and electricity at Soultz (1.5 Mwe). Other geothermal plants are located in Germany in Insheim (4.8 MWe), Landau (2.9 MWe and 3 MWth), Bruchsal (0.5 MWe), and Brühl (Figure 2).

2.1.2. Death Valley with a Focus on the Noble Hills

The Death Valley (DV; Figure 3) is also located in a Cenozoic system, about 700 km long with dextral strike-slip and extension [52,53,54,55,56]. This northwest-trending system is located between the Basin and Range region to the east and the Sierra Nevada batholith to the west [54]. Today, it accommodates ~25% of the Pacific-North America relative motion [52,56,57]. It was formed by a right-lateral movement giving a pull-apart structure [58]. The Noble Hills (NH), located in the southern part of the DV region, trend parallel to the Southern DV Fault Zone (SDVFZ). Geological markers along the SDVFZ trace [59] suggest that the NH correspond to a transported fragment of the frontal part of the Owlshead Mountains (OM), a Cretaceous (~95 Ma, [60]) granitic pluton at a 40–41 km distance to the SE. The NH are composed of Proterozoic sediments (quartzite, dolomite, detrital flysh, and carbonate sequences) crosscut by 1.1 Ga diabase sills, the whole being intruded by Mesozoic granite [61] in which important signs of hydrothermal alteration were discovered [7] along the fracture patterns characterized by [8,62]. In [7,8,62,63], the Noble Hills granite is considered as a surface analog to the Soultz site as it occurs also in a transtensive geological context and, in addition, in a desert environment that favors observation. It allows for a 3D study of fracture systems and their related hydrothermal alteration, which is hardly possible in Soultz deep geothermal wells since they were drilled in destructive conditions.

2.2. Volcanic Islands: Two Examples in the Guadeloupe Archipelago (Bouillante Geothermal Power Plant and Its Surface Analog in Terre de Haut, Les Saintes)

The Guadeloupe Archipelago

• Bouillante

The Bouillante geothermal power plant (Figure 4) is located on the west coast of Basse-Terre island, in the inner active part of the Lesser Antilles arc [64] that results from the subduction of the North American plate under the Caribbean plate at a velocity of approximately 2 cm.yr⁻¹ [65,66,67]. The Bouillante geothermal field was brought into production in 1986 and increased its production capacity up to 15 Mwe in 2005, representing about 7% of the island’s annual electricity needs [68]. In 2019, the production of electricity was nearly 110.000 MWh [69]. The Bouillante field is located at the western end of a volcano-tectonic depression belonging to the Marie-Galante graben system limited by the N090-N120 Bouillante–Capesterre regional fault that is found immediately north of Bouillante [70]. This extensive structure is bounded offshore to the west by a major N140–170 fault linking the normal-sinistral Montserrat–Bouillante system to the north with the Les Saintes system to the south [71]. The Bouillante field is contained within an andesitic volcanic substratum. Its high-enthalpy hydrothermal system is emplaced in submarine volcanoclastic formations (mostly hyaloclastites) and subaerial formations (andesitic lava flows, pyroclastites, and lahars) and is highly dependent on the fracture network. Depositional conditions created porosity between the clasts in volcanoclastic flows and discontinuities between the various lava flows. Later tectonic activity was responsible for fractures and faults. Porosity and fractures promoted fluid circulations resulting in hydrothermal activities of several types from high-temperature, up to 245 °C [70], to low-temperature episodes (<100 °C, [72]). The Bouillante field is related to the volcanic activity of the Axial Chain complex (1.023 to 0.445 Ma volcanic deposits; [73]) the Bouillante Volcanic Chain (c. 0.850–0.250 Ma ago after adularia dating; [74]) and the presently active Grande Découverte–Soufrière system (0.200 Ma ago–present day). The Bouillante hydrothermal event is related to changes in the magmatic history of the studied area, with at least two volcanic pulses [74].

2.

Les Saintes, Terre-de-Haut island

Terre-de-Haut island (Figure 4) belongs to the Les Saintes sub-archipelago located southeast of Basse-Terre of Guadeloupe. It is considered an exhumed fossil analog system to the buried Bouillante’s geothermal field and displays the same kind of volcanic rocks, porosity, and fluid pathways. Verati et al. (2016) [3] recognized four families of fault systems, active from 3 to 2 Ma, displaying normal-slip movements, and three of them (excluding the N090-N110 direction) also show a strike-slip movement. These four families of fault systems are consistent with the global tectonic framework of the entire Guadeloupe archipelago (see above about Bouillante and in [3]).

New structural, petrographic, and petrophysical (porosity, density, thermal conductivity, and P-wave velocity) data were published recently [1,3,14,75]. Geochronological data [75] revealed the presence of three main subaerial volcanic phases on Terre-de-Haut, from 2.98 ± 0.04 Ma to 2.00 ± 0.03 Ma. The occurrence of a hydrothermally altered zone was recently accurately documented in terms of the nature of hydrothermalism, duration of the event, and cooling of the system [2,3,14,15]. According to [3], the intersection of the two major normal fault systems (N090–N110 and N130–N140) is responsible for the development of this hydrothermally altered area, which is also the case in the Bouillante geothermal field [70]. This hydrothermally altered zone displays a succession of parageneses that are characteristic of high-temperature hydrothermal alteration in epithermal settings and its retrogression during cooling [2,3,14,15]. In their study of clay mineral assemblages in the hydrothermal zone of Terre-de-Haut, [14] showed a horizontal zonation with illite and chlorite isograds. This clay mineral distribution is commonly observed in geothermal contexts and is similar to that observed vertically in Bouillante’s boreholes [70].

3. Clay Minerals

Clay minerals are among the most ubiquitous minerals found in the volcanic and crystalline basement environments discussed in this paper, in which they are issued from hydrothermal or surface alteration processes. Their specific properties in terms of abundance, hydration, gravity, and electrical resistivity make them very useful for the exploration of geothermal resources. Their study is spread worldwide and for multiple applications, including the location of clay cap rock or the cap rock of geothermal systems, the characterization and better knowledge of geothermal reservoirs, and their evolution through time. In some geothermal systems, they might impair permeability and hence, they have to be studied carefully. Clay minerals are classically studied by X-ray diffraction (XRD) and optical and electron microscopy (scanning electron microscopy, SEM, and transmission electron microscopy, TEM).

3.1. Clay Cap Rock and Cap Rock: Electrical Resistivity, Gravimetry, and Magnetotelluric and Mineralogical Exploration

The underground circulation of hot water, of interest for geothermal energy production, is often indirectly inferred from the abundance of clay minerals formed by hydrothermal alteration at different temperatures. Clay minerals, such as smectite and chlorite, can be mapped from the surface using electrical soundings and give information about the structure of the geothermal system, for example, on Krafla volcano (Iceland) [20,21]. Their massive occurrence close to the surface shows the presence of a clay cap that forms an impermeable lid over the geothermal reservoir, as visible in [70] and reported by [72]. Strangway et al. (1973) [76] were among the first ones to use audio-frequency magnetotellurics to reveal low-resistivity bodies. It is now widely processed for geothermal exploration (e.g., [77,78,79,80]). Lévy et al. (2018, 2019) [20,21] compared frequency domain electrical properties, in boreholes and on samples in the laboratory, to investigate the specific role of smectite in the electrical response of igneous basaltic rocks. They evaluated what physical processes make smectite a better electrical conductor than surrounding minerals in the active hydrothermal system at the Krafla volcano, Iceland. They showed that cation exchange capacity (CEC) is found to be a reliable measure of the smectite weight fraction in the volcanic samples: the higher the bulk electrical conductivity, the higher the fluid conductivity for samples with a high smectite content, with a non-linear relationship [20]. They compared this value with an independent quantification of the smectite content using X-ray diffraction. In Guadeloupe at Bouillante, Ref. [81] interpreted the resistivity distribution from electromagnetic surveys in terms of the water saturation of rocks, hydrothermal alteration, and the presence of hydrated minerals, likely resulting from hydrothermal alteration in that volcanic context. They also used gravity to distinguish and characterize the denser formations from the low-density areas. Combining both these approaches, they showed a layering of the geological formations in the geothermal system. The shallow resistive layer they observed is explained in terms of recent massive volcanic formations, while the conductive intermediate layer marks the low-density, demagnetized clay cap of the altered geothermal system. The methods indicated above allow for the location of the clay cap rock based on differences in its physical properties compared with the surrounding rocks, with a slight uncertainty in its exact dimensions. Other newly formed minerals like carbonates or quartz do not contribute, or barely, to the physical properties of the cap rock.

In Italy, the authors of Ref. [18] performed mineralogical investigations on the cap rock of the geothermal system located close to the Vico volcano to assess its effectiveness and degree of interaction with fluids. In this system, a low permeability siliciclastic cap rock overlies a permeable carbonate reservoir. Where it is unfractured, the cap rock shows maximum paleo-temperatures interpreted as the thermal signature of the original cold sedimentary basin. Where it is fractured, it is characterized by an assemblage of kaolinite, calcite, and mixed layers of illite–smectite (I/S). The temperature assessed in those fractures is between 85 and 140 °C, indicating a strong interaction with hot fluids from the carbonate reservoir. Clay recrystallization occurred along the walls of tensile fractures pre-dating the active hydrothermal system, which acted as passive anisotropies that focused on localized alteration. The compositional and structural changes in mixed layers of I/S were shown to be a function of hydrothermal fluid temperature.

3.2. Geothermal Reservoirs

Clay minerals are renowned for resulting from the hydrolysis of preexisting minerals in various geological environments, hence including the nature of rocks and the temperature of the environment in which they formed, from the soil [82] to high-enthalpy geothermal systems [72,83]. Recently, [84] showed how clay mineralogy can be used as a signature of granitic geothermal reservoirs of the central Upper Rhine Graben (URG) at Soultz and Rittershoffen. Before that, [10,27,85] showed the abundance and variety of clay minerals in hydrothermally transformed zones of the Soultz granite, while [48] indicated the significance of hydrothermal alteration zones for the mechanical behavior of a geothermal reservoir.

3.2.1. Types of Hydrothermal Alteration and Methods of Characterization

Several types of hydrothermal alterations are recognized, among which the most common are propylitic and argillic types in geothermal environments. They are evidenced in the field by the change in color and mineralogy of the rock (e.g., Figure 4). They are characterized in the laboratory by petrography including the quantification of carbonate content by manocalcimetry, mineralogy, and geochemistry; see, for example, [4,7,23,27,48,49,63]. Chemical modeling is also frequently performed, in which pH, redox potential, ionic strength, and kinetic rates of mineral crystallization and dissolution are taken into account. For example, EQ3/6 or PHREEQC (see, for example, [86]) software packages allow for modeling geochemical interactions among aqueous solutions, solids, and gases by implementing the principles of both chemical thermodynamics and kinetics. They are useful for interpreting the chemical composition of aqueous solutions and for calculating the results of their reaction with gases and various solids including minerals. Similar to every type of modeling, they show limitations. For example, they are adequate at only low ionic strength. However, in sodium chloride-dominated systems, which are common in deep geothermal environments, the PHREEQC model may be reliable at higher ionic strengths as special effort was focused on these elements. Another limitation concerns ion exchange modeling, which generally requires experimental data on samples from the studied site to obtain appropriate modeling.

• Propylitic alteration

In the Noble Hills, away from the fractures, the granite underwent pervasive propylitic alteration [63]. This type of alteration is described as isochemical by [86] and, hence, occurs in rock-dominated systems. It is characterized by a calcite–corrensite–epidote–K–white mica assemblage in the Noble Hills [63], while amphibole and biotite are partly transformed into chlorite + carbonate ± epidote or hydrogrossular and plagioclase into calcite + corrensite or illite at Soultz [27]. At Bouillante, it affects all parts of the system and consists of the crystallization of trioctahedral phyllosilicates (chlorite or corrensite), ca-silicates (heulandite–clinoptilolite, prehnite, pumpelleyite, wairakite, and epidote), quartz, and minor calcite in replacement of most of the primary minerals of the intersected volcanic or volcanoclastic formations [83]. These assemblages are classical in propylitic alteration. Indeed, Ref. [86] indicated a limited number of phases: K- and Na-feldspars, muscovite, quartz, clays (including chlorite), epidote, prehnite, or calcite, the proportion of which depended on the initial chemical composition of the rock but was independent of the initial fluid composition. Berger and Velde (1992) [86] determined the parameters controlling the propylitic and argillic alteration process using the EQ3/6 software 3230 package.

2.

Argillic alteration

BergerVelde (1992) [86] obtained the extreme argillic facies parageneses (quartz, kaolinite) by modeling using the EQ3/6 software package at temperatures < 200 °C, at water/rock ratios > 10³–10², and with acid solutions, by reacting propylitic mineral assemblages and their coexisting fluids with imposed chemical variables (ƒCO₂, ƒS₂, salinity). In the field, argillic facies are found worldwide.

At Soultz, the granite underwent intense hydrothermal alteration, as pinpointed by the local total dissolution of plagioclase (oligoclase), biotite, amphibole, and quartz and the crystallization of mineral assemblages made of illite, calcite and other carbonates, quartz, and tosudite (see “2. Argillic alteration” in the Section 3.2.1 and [27]). This vein argillic alteration was compared with the reference facies (propylitized granite) in order to show gains and losses in elements during alteration, which deduced that it occurred in an open system. The geochemistry of the brine circulating at present [87,88] is characterized by high salinity (ca. 100 g/L) due to a great abundance of Na, K, and Ca as major cations and Cl and SO₄ as major anions. Li, Zn, Ba, and other minor elements are also present in significant abundance (173 mg/L, 2168 µg/L, and 5070 µg/L, respectively, [89]). In addition, the geothermal fluid is characterized by a high CO₂ content and natural anoxic conditions [88,90]. Geochemical modeling of the fluid–rock interactions based on thermodynamics and kinetics of reactions with the present-day fluid collected in the Soultz geothermal reservoir with a temperature of 140 °C showed that this brine is indeed able to precipitate illite from the alteration of plagioclase [91]. Where it is intensely altered and characterized by a great abundance of clay minerals, the mechanical properties of the Soultz granite are thoroughly modified, leading to a likely a-seismic behavior [48,49]. No such or opposite relationships have so far been quantified at Soultz or, to our knowledge, in any other context for specific secondary quartz and carbonate contents.

In the Noble Hills granite, close to the fractures within which hydrothermal fluids circulated, illite, kaolinite, illite/smectite, calcite, and oxides, characteristic of the argillic alteration, were encountered, which overprinted the propylitic alteration [7]. The alteration was also outlined by the correlation between the loss on ignition, representing the hydration rate, and porosity, calcite content, and chemical composition such as Na depletion due to plagioclase alteration and a K enrichment associated with illite precipitation. Moreover, the Noble Hills granite shows signs of several generations of fluid circulations resulting in successive veins of various mineralization (quartz, carbonates, barite). The porosity, the calcite content, and the temperature indicated by the Kübler Index calculated from the crystallinity of illite increase together near fracture zones.

The Bouillante geothermal wells go down to a depth of 1000 m where temperatures exceed 250 °C [83]. These authors paid special attention to the clay content of fractured zones that channel the present-day geothermal fluids. They identified three successive zones, dominated, respectively, by dioctahedral smectite, illite, and chlorite at increasing depths. Bouchot et al. (2010) [70] indicated that the temperature of crystallization of those clay minerals is <180 °C for smectite, 180–240 °C for illite, and >240 °C for chlorite. The zones described by [81] result from the superimposition of at least two successive alteration stages: the first one of the propylitic type (see “1. Propylitic alteration” in Section 3.2.1) and the second one related to the circulation of the present geothermal fluids and assimilated to argillic alteration, resulting in the crystallization of aluminous clay phases (smectite, illite ± I–S mixed layers, and accessory kaolinite). These authors [81] indicate that the present geothermal fluid circulates within the natural fracture network.

In all of these cases, a clear link has been established between the argillic alteration and the fracture system, which has generally been thoroughly studied similarly to Soultz [10,11,27,92], the Noble Hills [7,8,62], and Guadeloupe [1,2,3,14]. As an example, a compiled map is provided here (Figure 5) on which the mineralogy from [14] was superimposed over the location of the hydrothermally altered zone and structural data from [3]. It shows a clear relationship between the fracture network and the distribution of clay minerals. The chloritic hot core is located at the intersection between two fractures oriented N100 and N140. The southernmost isograd of illite appearance (Ilt+) is clearly modeled on fractures and normal faults.

3.2.2. Tosudite, a Specific Lithium-Rich Clay Mineral

In the Soultz granite harnessed for geothermal energy, tosudite was found in highly altered zones of the granite, close to faults in which quartz veins formed [27,93]. Tosudite, a di-dichlorite–di-dismectite mixed layer clay mineral [93], indicates the presence of lithium in the brine from which it crystallizes (see “2. Argillic alteration” in Section 3.2.1). It is a rather rare mineral even though several occurrences have already been encountered worldwide in various environments like granites [93,94,95], rhyolitic rocks [96], altered wall rocks of gold-bearing veins developed in tuffs [97], or very low-grade metamorphic grauwackes [98]. At Soultz, chemical analyses of tosudite pseudomorphous after plagioclase show 1.02 wt% Li₂O [27,85]. In Li-rich environments, it is found instead of kaolinite in argillic alteration [27] and hence, is a good indicator of the chemical content of fossil brines when geothermal reservoirs are no longer active. Where they are still active, Li, which is a high-value element for electric batteries, can be extracted together with geothermal energy [99,100]. In addition, tosudite is responsible for the locally important porosity of the rock matrix, as shown in Figure 5, and hence, permeability [13], together with illite (Figure 6) as also shown for other geological environments like in Terre-de-Haut [14].

3.2.3. Chlorite Geothermometry

Chlorite crystals can be used for geothermometry in order to determine the temperature reached by geothermal reservoirs, provided they are found in association with quartz. Beauchamps et al. (2021) [15] applied this method to Terre-de-Haut to estimate the crystallization temperature of chlorite using the geothermometer developed by [31] specifically for low-T and low-pressure contexts (T < 350 °C and pressures below 4 kbar). This semi-empirical thermometer is based on a ratio of end-member activities and directly links the temperature to a K constant for a chlorite + quartz equilibrium. This choice was driven by the low P-T conditions prevailing in the studied area, and the Si-rich composition of the Terre-de-Haut chlorites, preventing the use of thermodynamic models such as that developed by [101]. In addition, the geothermometer developed by [31] does not require a prior quantification of the Fe³⁺ content, assuming that all iron is divalent. The thermometer developed by [32] can also be used provided the Fe³⁺ proportion is quantified for each chlorite analysis, which might be difficult to obtain. In Terre-de-Haut, the thermometer developed by [31] was used by [15] to determine cooling conditions of the paleo-geothermal reservoir and it was coupled with microthermometry of fluid inclusions in a euhedral quartz crystal, indicating as a whole (i) a temperature of at least 240–270 °C in fluid inclusions trapped in the core of the quartz crystal, (ii) chlorite formation at about 120 °C, and (iii) temperatures of ca. 50 °C in fluid inclusions of the quartz outer growth zones.

3.2.4. Use of Illite (and Muscovite)

• Determination of temperature conditions using the Kübler Index

The Kübler index allows us to determine the temperature conditions reached by geological systems, in particular sedimentary, and hence, if they reached diagenesis only or metamorphic conditions. Its calculation is based on the mean thickness of the coherent scattering domain of illite crystals from X-ray diffraction full-width data at half maximum (FWHM) intensity [102].

For example, in the Noble Hills, the Kübler Index was calculated from illite crystallinity and allowed the identification of an NW-SE temperature gradient related to propylitic alteration and the thermal and tectonic history of this zone [63].

2.

Dating of hydrothermal or metamorphic events

Among clay minerals, illite contains large amounts of potassium (K), which can be used for dating thanks to the K/Ar or Ar/Ar method. Even though it is not a clay mineral species, muscovite is another type of phyllosilicate that contains K and can also be dated.

For examples in the geological contexts considered here, Ref. [103] dated illite for the reconstruction of the thermal history of the Lower Triassic sedimentary rocks that crop out in the western shoulder of the URG or are deeply buried and overlain the granite basement in the URG. Two episodes of crystallization could be identified at Soultz at ~95 and ~70 Ma, hence, before the Miocene rifting. Dating performed on illite fractions extracted from the granite in the highly altered zone where tosudite was first described at Soultz indicates that illite crystallized during two episodes at ~63 and ~18 Ma [93], hence, before rifting for the first episode and during it for the second one. The geochemical simulations performed by [91] indicate that the brine circulating at present in the fracture network of the Soultz granite is able to promote the crystallization of illite from the alteration of plagioclase. Hence, illite precipitation might continue at present.

Favier et al. (2021) [104] performed Ar/Ar dating of hydrothermal muscovite developed in Terre-de-Haut island during the pseudomorphic transformation of pyroxenes within altered rhyodacites at temperatures above 300 °C, probably corresponding to the high-temperature event also described thanks to fluid inclusions in quartz (see Section 5). The white micas display an age of ~2.59 Ma corresponding to a high-temperature fluid circulation in the Terre-de-Haut geothermal paleo-reservoir. The whole temporal dataset reported by [104] implies a fast-cooling rate (>150 °C/200 ka) and a maximal lifetime of 650 ka for the Terre-de-Haut hydrothermal system.

3.2.5. Deciphering Meteoric and Hydrothermal Alteration Based on Clay Minerals

Surficial clay minerals as indicators of present-day geothermal activity were studied around Bouillante by [82] on >100 samples collected over and around the Bouillante geothermal system. Among the three types of mineral associations, that composed of kaolinite–smectite mixed-layered clays ± halloysite ± kaolinite ± smectite ± silica was found both within and beyond the bounds of the known geothermal area, suggesting a pedogenic origin. Hence, it is interpreted as a background of clayey tropical weathering (kaolinite–smectite) with a hydrothermal overprint characterized by dioctahedral smectites (beidellite) and illite–smectite. In addition, in the wells drilled in the Bouillante geothermal field [105], scanning electron microscopy, electron microprobe analysis, X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), and oxygen-isotope analysis were used to study the montmorillonite–beidellite transition. They showed that beidellite precipitated from the hot geothermal fluid while montmorillonite precipitated from reacting solutions originating from the phreatic water table (±seawater) with minor contribution from the boiling zones. It appears that the montmorillonite vs. beidellite ratio of the smectite material is due to the mixing rate of geothermal fluid with meteoric waters rather than depth.

In Death Valley, weathering was evidenced in the Owlshead Mountains only (and not in the NH) by the presence of montmorillonite [63], but it is considered negligible in both Owlshead and Noble Hills granites (which is in agreement with the arid climate and desertic environment) compared with hydrothermal activity.

At Soultz, Reference [106] identified paleo-weathering at the top of the granite, between around 1400 and 1550 m depth, hence, on a 150 m thickness. There, standard porphyritic granite is red-colored as a result of paleo-weathering, because most of the primary iron-bearing minerals (biotite, magnetite, amphibole) are partly altered into iron-hydroxide or hematite. Seven kilometers away from Soultz, at Rittershoffen, in the same URG context, the top of the granitic basement between 2212 and 2269 m (measured depth, MD) consists of a 56 m thick reddish granite [107] that was affected by a paleo-weathering alteration event because of paleo-emersion before and during the Permian [107]. It is characterized by a decreased magnetic content, as already measured on a continuous core at Soultz [12]. Gamma-ray values encountered during well logging are high and close to 300 API units on average, a value that is also encountered for hydrothermal clay-rich sections with a great abundance of illite. This paleo-weathering effect is also measurable through both the decreasing rate of the penetration value during drilling associated with the declining influence of paleo-weathering and some of the well log responses such as the magnetic susceptibility log [106].

4. Carbonates

In addition to the nature and abundance of clay minerals, [30,47] proved that the carbonate content is a good proxy for the hydrothermal alteration of granite bodies, in particular, in the Rhine graben. It helps locate zones that have to be stimulated by acid treatment when it is necessary to improve the permeability of the reservoir through better connectivity between wells and fractures. Indeed, in an Enhanced (or Engineered) Geothermal System (EGS), acid treatments dissolve carbonates that were naturally deposited in the fracture system and within its wall rocks. This helps increase the permeability and enhance the capacity of geothermal brines to circulate within the granitic reservoir and to be produced at the surface. Indeed, [10,30,47] were the first authors to systematically analyze the carbonate content of granite in the deep geothermal boreholes at Soultz by manocalcimetry. The average calcite content of fresh granite is 0.252 wt.% and does not exceed 1.8 wt.% worldwide [108]. As a consequence, measurements above this latter value can be regarded as a calcite anomaly [47] due to hydrothermal alteration and likely fluid transfers from the overlying Muschelkalk carbonate layers in the URG. Contents as high as 18 wt% in GPK-4 at Soultz were reported by [6,47] always in relation to fracture zones and correlated with fluid flow data obtained by flowmeter in the wells. These authors concluded that maximum calcite contents are similar and over 10 wt% in the three deep boreholes GPK-2, GPK-3, and GPK-4. However, GPK-3 and GPK-4 have similar behavior regarding relationships between fracture zones, fluid flow, and calcite content (the lower the calcite content, the better the fluid flow), which is opposite to that of GPK-2. This suggests that the fracture zones of GPK-3 and GPK-4 are localized and discrete, while those of GPK-2 are a well-structured network of medium-scale fractures. The connectivity of these three wells to the fracture network may be different too. This difference in behavior between the three deep wells was also illustrated by [109], who studied induced microseismicity. Hence, it might be very challenging to generalize what is learned from a given well to the whole geothermal reservoir and other reservoirs even in the same geological context. Each borehole must be studied independently in order to compare all of them and obtain a global view of a given geothermal reservoir. Subsequently, the calcite content was also included in the reservoir assessment of the Rittershoffen and Illkirch geothermal sites, also located in the URG [42,110]. In the NH granite, [7] indicated that the argillic alteration is highlighted by calcite content that increases together with porosity and temperature near fractures. They also noted an increase in the loss on ignition during bulk rock chemical analyses, which indicates that the degree of hydration of the rock is correlated with Na depletion due to plagioclase alteration and a K enrichment associated with illite crystallization.

Furthermore, for carbonates trapped in small cavities, the fluid in which they crystallized allows for the characterization of salinity and trapping temperature thanks to the microthermometry of those fluid inclusions. This technique was implemented by [26] to study the argillic alteration related to a cataclase zone of a sample collected at a depth of 1641.91 m in the EPS-1 drill hole at Soultz. At least three alteration stages were superimposed. The sample included, from the oldest to the youngest (as indicated by crosscutting relationships) a 3 mm wide quartz vein, a 1 mm wide ankerite vein, and a thin vuggy quartz vein. The ankerite crystals clearly showed a zonation of at least three crystallization events. The inclusions showed two phases (liquid + vapor) and had temperatures of ice melting between –9.7 and +1 °C, where values higher than 0 °C are attributed to a metastable behavior. Two groups of salinities were evidenced: salinity of about 10 wt% eq. NaCl and salinity of less than 1 wt% eq. NaCl. A salinity of 10 wt% eq. NaCl is the present-day salinity of the fluids sampled in the granite and its sedimentary cover [111]. Very low salinities might correspond to meteoric water penetrating deeply due to rapidly opened fractures caused by seismic activity [26]. These variations could correspond to the mixing, at constant temperature, of sedimentary brines and meteoric waters [24,112]. Homogenization temperatures lie in a very narrow range around 145 °C (137.5 °C to 158 °C), suggesting that the inclusions have not been altered since their formation. Corrected from the effect of pressure, they indicate a temperature of fluid between 144 °C and 159° C [26], intermediate between the present-day temperature of the fluid collected at the base of Triassic (120 °C) and that harnessed in the deep reservoir at 5000 m depth (200 °C) at Soultz [51].

5. Quartz

The partial sealing of fractures by secondary drusy quartz is a frequent sign of hydrothermal alteration in granitic [7,27,42,110,113] and andesitic [15] environments related to the dissolution of primary silicates and their secondary precipitation in fracture systems. Quartz crystals are easily detectable in rock chips, also called cuttings, recovered from drilling operations by visual inspection and in the laboratory by optical microscopy. In the laboratory, fluid inclusion studies and isotopic measurements can also be performed [24,25,26,27,114]. Sometimes, drilling operations provide poor-quality cuttings because of gravity sorting of biotite, for example, of total loss in fractured zones. When cuttings are not available or reliable, gamma-ray (GR) logging, which measures the natural radioactivity of a rock, is a good indicator of hydrothermal alteration. At Soultz and Rittershoffen, the occurrence of geodic quartz is associated with sharp localized GR-negative anomalies. In contrast, K-bearing clay minerals like illite are associated with positive anomalies that can extend over several meters [106]. One of the major permeable fracture zones of the GPK-1 drill hole at Soultz (3489 to 3496 m MD [115], hence, 7 m wide) is associated with mud losses, indicating permeability, and also with the occurrence of CH₄, CO₂, and radon [12]. In this zone, GR presents a negative anomaly that corresponds to a quartz vein associated with the fracture [115] and a positive anomaly related to illite-bearing altered rock. This fracture zone is also visible on flow logs and borehole acoustic logging imagery. Euhedral quartz fragments were also found in the rock chips recovered during drilling at Rittershoffen at the same depths as the temperature anomalies [42,113,116], indicating fluid flow and, hence, permeable zones. At both Soultz and Rittershoffen, the permeability of the fracture zones seems to be influenced by the crystal growth of secondary quartz in fracture voids [117]. Indeed, these authors demonstrated by numerical simulations that the tortuosity related to the needle quartz shape induces a drop in fracture permeability higher than for the quartz compact feature.

Similar to carbonates, quartz is able to trap the fluid it crystallizes from in the form of fluid inclusions. At Soultz, several microthermometry studies were performed on quartz samples [24,26,27]. Most of the fluid inclusions in secondary quartz crystals contain two phases (liquid + vapor) at room temperature, similar to those in ankerite [26]. All the salinity and temperature values are rather consistent even though they sampled quartz veins at different depths in the EPS-1 borehole. They are also consistent with data from ankerite [26]. Hence, the temperature registered by quartz and ankerite corresponds to the present-day temperature of the fluid circulating at present at the same depths in the granite body. Similar to illite (see Section 3.2.4), it is likely that quartz and carbonates continue precipitating at present in veins in the Soultz granite [26]. In order to provide additional data on flow paths and the duration of interactions between the fluid and granite, other analyses can be performed like the determination of the isotopic signature of quartz [114].

In Bouillante, [72] identified an early alteration facies typical of a high-temperature geothermal system. It is characterized by an assemblage of newly formed K-feldspar/adularia, quartz, and pyrite. The textural properties of quartz indicated fracturing associated with boiling. In Terre-de-Haut, the study of two quartz generations was performed by microthermometry [15]. The core of a geodic euhedral quartz crystal recorded a rather high temperature (240–270 °C). The outer growth zone of the same crystal recorded a lower temperature (ca. 50 °C). Both events were associated with a low-salinity fluid (2 wt% NaCl). Hence, the hydrothermal fluid was probably of meteoric and seawater origin and in its outer growth zone, hence registering the cooling of the hydrothermal system. Additionally, a second type of fluid inclusions observed in a banded quartz vein indicated at least a hydrothermal episode with a CO₂-(H₂O) fluid, with traces of H₂S. Hence, some similarities can be highlighted between Terre-de-Haut and the Bouillante active geothermal systems in terms of the highest temperatures of circulation events and gas composition, despite a difference in fluid origin (mostly meteoric in Terre-de-Haut [15], and combined magmatic, marine, and meteoric in Bouillante [70]).

6. Discussion

Following the examples presented before, newly formed minerals related to hydrothermal alteration are indeed very useful worldwide for the characterization of geothermal reservoirs (either active or fossil) and their cap rock. Indeed, they recorded the conditions and duration of the hydrothermal events. They might also impair the permeability of geothermal reservoirs, but some of them can be dissolved during chemical stimulations of EGS.

The most used analytical means for the characterization of those minerals found in reservoirs and their cap rock are as follows. The determination of clay minerals like smectite, illite, chlorite, and inter-layered minerals is widely performed with XRD [2,14,82,83]. The study of clay minerals can be performed using electron microscopy, either SEM [14,83] or TEM [93]. Calcite and quartz are first observed using optical microscopy [2,14,15,82,83] and then microthermometry of fluid inclusions is performed to determine the conditions that prevailed during hydrothermal alteration [15,24,26], in particular, the salinity and temperature of the brine. The knowledge about the temperature encountered in a geothermal reservoir or its cap rock can also be obtained with the use of chlorite geothermometry [15,31,32,118]. In addition, K/Ar or Ar/Ar dating of illite or muscovite provides information on the age of a given event and also allows for the determination of its duration [93,103,104]. Where boreholes are available, data obtained in the laboratory are coupled with well-log and flow data to point out the most permeable zones and those that have to be stimulated if needed.

In general, the detailed study of minerals is combined with other parameters such as petrophysical (like porosity, permeability, thermal conductivity) or geophysical (e.g., gamma ray) properties [1,2,49,70]. According to [1], the advanced hydrothermal alteration of volcanic rocks like those found in Guadeloupe (Basse-Terre and Terre-de-Haut) tends to increase fluid flow properties in massive rocks like lavas (by their increased porosity and permeability) and decreases them in volcano-sedimentary deposits by the precipitation of newly formed deposits in the porosity. Hence, hydrothermal alteration significantly reduces the differences in the thermo-physical properties observed between fresh facies in volcanics (lava flows and dykes, debris flows) and pyroclastics (ash, pumice, and scoria deposits). The progressive transformation of primary minerals by hydrothermal alteration homogenizes the porous network by a reduction in pore throat diameters. In addition, after total alteration, the magnetic behavior is fully diamagnetic or paramagnetic, leaving no remanent magnetization [1]. At Soultz, Ref. [48] showed the influence of hydrothermal alteration on the magnetic susceptibility of the granite and, in particular, the amount of clay minerals on its mechanical behavior.

Figure 7 shows a synthesis of the factors and phenomena occurring at depth in a geothermal field developed in volcanic islands like Guadeloupe (Basse-Terre and Terre-de-Haut) in a subduction context. The recharge of the system occurs through rain and seawater. Water circulates thanks to faults and fractures, and the topography certainly promotes vertical water movements, as evidenced, for example, by [119]. In addition, volcanic rock layers (lava flows, debris flows, pyroclastics) are superimposed on one another, some of them with rather high porosity, allowing for horizontal water movements [1,2], which, in addition to hydrothermal convection, promote hydrothermal alteration halos along fractures. The core of the geothermal field shows the highest temperature and the whole system is overlain by a rather impermeable clay cap mostly made of smectite [70,72], which ensures its perennity for a rather long period. Verati et al. (2016) and Favier et al. (2021) [3,104] assessed a maximal timescale of about 400–650 ky for Les Saintes hydrothermalism, in agreement with both the timescale (~100 ky) proposed for active geothermal provinces in subduction contexts such as both the Taupo volcanic zone (New Zealand) [120] and Bouillante geothermal field [74]. The geothermal system at Soultz, in the Upper Rhine Graben, seems to be much more stable, as the dating of illite showed several episodes of crystallization at ~95 Ma and ~70 Ma in the Triassic sediments (before the Cenozoic opening of the Graben) [103], while in the granitic basement, it occurred at ~63 Ma (also before the opening) and ~18 Ma, hence, during the opening [93]. Numerical simulations indicate that illite might also precipitate from the present-day fluid in the Soultz granite [91]. Hence, the more recent hydrothermal event responsible for the crystallization of illite in the granite basement at Soultz might have lasted for several millions of years, which is much longer than the <1 Ma duration of the hydrothermal event in the volcanic island of Terre-de-Haut. This might be due to the basin context of the URG, which provides wide geothermal reservoirs, and also because the geothermal activity in volcanic islands depends on the occurrence and duration of volcanic episodes.

In addition to the various phenomena exposed before about clay minerals in crystalline rocks, another one can be encountered in geothermal reservoirs developed in deep sedimentary environments overlying granitic basements, similar to that in the URG. In detrital geothermal reservoirs (sandstones, conglomerates), injectivity decline problems are often encountered during water reinjection into the subsurface, despite massive surface filtration. The reinjected water carries only nano-sized particles, like illite, at relatively low concentrations but causes severe permeability impairment due to pore clogging by the transported particles [121,122]. Nuclear Magnetic Resonance (NMR) is a powerful method used to evaluate the clogging phenomena [123,124]. Independent NMR measurements allow for estimating the mean distance between the deposited clay particles and the reduction in this distance during clay injection, reflecting the compaction of the deposit over time due to pore space particle accumulation [123]. Coupled NMR measurements and SEM imaging [123] allow us to show where deposition occurs, for example, on the surface of quartz grains or within pores, and the shape and orientation of the deposits, which contribute to permeability reduction and threaten geothermal projects. This is very useful, as [125] indicated in this issue that oil reservoir quality is also reduced by the accumulation of mica flakes in pore spaces.

7. Conclusions

Newly formed quartz, carbonates, and clay minerals formed during hydrothermal alteration are widely used for the characterization of active or fossil geothermal reservoirs and their cap rocks. Indeed, they have recorded the conditions of salinity and temperature of hydrothermal events, but also their age and duration. Hence, they provide crucial data for the exploration and subsequent exploitation of geothermal reservoirs. Those minerals are found inside the rock matrix modified by hydrothermal alteration and within veins that crosscut the hydrothermally altered rock. Within the rock matrix, they are generally associated with enhanced porosity and permeability, but they might also impair the permeability of geothermal reservoirs. Some of those newly formed minerals, like calcite and specific clay minerals, can be dissolved during chemical stimulations of EGS, and hence, they have to be located precisely in the drill holes to target the zones to be stimulated. Concerning cap rocks, massive clay mineral formation plays a major role in the creation of an impervious lid that prevents hot fluids from leaking out of the reservoirs. Their identification and location, thanks to geophysical methods, is another key point for the exploration of geothermal fields.