1 Introduction

During geothermal exploration, the geochemical methods are extensively used and play a major role in both exploration and exploitation phases (Nicholson 1993). Geochemical methods are particularly useful to assess the subsurface temperatures in the reservoir, the origin of the fluid and flow directions within the reservoir (Ármannsson and Fridriksson 2009). The geochemical exploration is based on the assumption that fluids on the surface reflect physico-chemical and thermal conditions in the geothermal reservoir at depth. However, on some occasions, there is not any evidence of endogenous fluids manifestations at surface that traditionally provides evidence of the presence of an active geothermal system, in the form of aqueous solutions (hot springs, mud pools, geysers), hydrothermal alteration deposits (travertine, phreatic craters, explosion breccias) or gas mixtures (fumaroles). Discovery of new geothermal systems will therefore require exploration of areas where the resources are either hidden or lie at great depths (i.e., Dixie Valley, Nevada, U.S.A., Lewicki and Oldenburg 2004). Geochemical methods for geothermal exploration of these areas must include soil gas surveys, based on the detection of anomalously high concentrations of some hydrothermal gases in the soil atmosphere, generally between 40 cm and 1 m depth from the surface (Bertrami et al. 1990; Finlayson 1992; Voltattorni et al. 2010). The observed enrichments of some chemical species or chemical ratios in the soil environment may indicate the presence of enhanced vertical permeability areas related to high-temperature hydrothermal activity at depth. Mapping these soil gas enrichments relative to background concentrations and/or their fluxes can be useful in delineating main upflow regions and areas of increased subsurface permeability related to high-temperature hydrothermal activity at depth (Werner and Cardellini 2006; Chiodini et al. 2007; Hernández et al. 2012; Barberi et al. 2013).

One of the most studied gases in the soil atmosphere for geothermal exploration purposes has been CO2, because its emission rate in active hydrothermal regions can be as high as those from active volcanoes (Mörner and Etiope 2002; Werner and Cardellini 2006). However, the occurrence of interfering processes affecting the reactive gases such as CO2 during its ascent from magmatic bodies or hydrothermal systems toward the surface environment hinders the interpretation of their enrichments in the soil atmosphere and fluxes for geothermal exploration purposes (Marini and Gambardella 2005; Agusto et al. 2013). These processes include gas scrubbing by groundwaters and interaction with rocks, decarbonatation processes, biogenic production, etc. Within the rest of the soil gases, particular interest has been addressed to nonreactive (noble) gases. They offer important advantages for the detection of vertical permeability structures, because their interaction with the surrounding rocks or fluids during the ascent toward the surface is minimum (Hernández et al. 2004; Padrón et al. 2012, 2013a). This is the case for helium (He). He has unique characteristics as a geochemical tracer: it is chemically inert and radioactively stable, non-biogenic, highly mobile and relatively insoluble in water (Reimer 1980; Ozima and Podosek 2002; Fu et al. 2005). There are two naturally occurring isotopes of helium: 4He and 3He, with an atmospheric 3He/4He ratio (RA) of 1.384 × 10−6 (Clarke et al. 1976). Radioactive decay of 238U, 235U and 232Th is the main source of 4He, while 3He is considered a primordial gas that is being released to the atmosphere by mantle degassing (Graham 2002). Soil helium surveys have been used extensively for many objectives: (1) as an indicator of tritium groundwater contamination (Olsen et al. 2006); (2) as an indicator of crustal leaks along faults (Fu et al. 2005; Lombardi and Voltattorni 2010); (3) to study seismicity along active faults (Reimer 1980; 1985); (4) to detect structures of enhanced permeability for deep gas migration and preferential routes for degassing in a volcanic system (Hernández et al. 2004; Padrón et al. 2012, 2013a); (5) to select areas for exploratory drilling for uranium deposits (Reimer and Bowles 1979; Reimer 1986); and (6) for volcano monitoring (Padrón et al. 2013b). With regard to geothermal exploration, although few works have been reported to date on its application, soil He surveys have been revealed as a quick, inexpensive and promising tool for this purpose (McCarthy 1982, 1983; Di Filippo et al. 1999).

Hydrogen (H2) is the second most abundant reduced gas in the atmosphere (after methane), with a concentration ~530 ppb (Novelli et al. 1999). It is one of the most abundant trace species in volcano-hydrothermal systems and is a key participant in many redox reactions occurring in the hydrothermal reservoir gas (Giggenbach 1987; Chiodini and Marini 1998). Although H2 can be produced in soils by N2-fixing and fertilizing bacteria (Conrad 1996), soils are considered nowadays as sinks of molecular hydrogen (Smith-Downey et al. 2006). Oxidation by soil hydrogenases and methanogens, sulfate-reducing and ferric iron-reducing bacteria are the main causes for this uptake of molecular H2 by soils (Trevors 1985; Conrad 1996; Gödde et al. 2000). Because of its chemical and physical characteristics, H2 generated within the crust moves rapidly and escapes to the atmosphere. These characteristics make H2 one of the best geochemical indicators of magmatic and geothermal activity at depth.

The Canary Islands, owing to its recent volcanism, are the only Spanish territory with potential high-enthalpy geothermal resources (European Commission 1999). From the 1970s to the 1990s, the Spanish Geological Survey (IGME, http://www.igme.es) performed intensive research on geothermal resources in the country, including studies at Lanzarote, Gran Canaria, Tenerife and La Palma islands in the Canaries: geochemical and isotopic analysis of hydrothermal discharges (fumaroles) of Teide volcano, volcano-structural and magnetotelluric studies at Las Cañadas caldera, and groundwater hydrochemistry studies (Instituto Geológico y Minero de España (IGME) 1977, 1979, 1993a, b, c, d, e, f, g, h; Albert-Beltrán et al. 1990; Valentín et al. 1990). As a result of these exploratory studies, one exploratory drilling in Tenerife and two in Gran Canaria were performed with unsatisfactory results (Instituto Geológico y Minero de España (IGME) 1993h). Recently, interest in the development of geothermal energy in Canaries has increased again (García-Yeguas et al. 2012; Rodríguez et al. 2015; Piña-Varas et al. 2014). In this paper, we present the results of five detailed soil He and H2 surveys carried out with geothermal exploration purposes in several areas of interest (mining licenses) on the islands of Tenerife and Gran Canaria.

The main limitation of geothermal energy with respect to other renewable energies is the investment required in the initial exploration phase. The primary objective is to sort geochemically the five different mining licenses based on its geothermal potential to reduce the uncertainty inherent to the selection of the area with the highest potential success in the selection of exploratory wells. The location of such exploratory drillings within the areas in order to detect the hydrothermal system that might support economically the development of a geothermal plant for energy generation is beyond the objective of this paper. Future additional geophysical and geological studies would be necessary within the studied area to reduce the uncertainty in their location.

2 Local Geology and Volcano-structural Features of the Selected Mining Licenses

The Canary Islands archipelago comprises a group of seven major volcanic islands forming a roughly east–west chain with the ages progressively younger to the west. It is located over ocean crust of 185–190 million years old off the passive continental margin of NW Africa (Schmincke and Sumita 2010). Gran Canaria and Tenerife are the central islands of this group (Fig. 1). The oldest subaerial rocks dated in both are 14.5 and 11.5 Ma, respectively (Ancochea et al. 1990; Schmincke and Sumita 1998). These two islands are unique with a central volcanic complex that started to grow at about 5 Ma in the case of Gran Canaria (Schmincke and Sumita 1998) and 3.5 Ma in the case of Tenerife (Ancochea et al. 1990). A sector of the central volcanic edifice in Gran Canaria suffered a great gravitational collapse 3.5 Ma ago, resulting in debris avalanches episodes: Roque Nublo Breccias (García-Cacho et al. 1994). Previously, Tejeda Caldera produced several differentiated magmas: syenites, trachytes and phonolites. The central volcanic complex in Tenerife is characterized by mafic, intermediate and differentiated phonolitic magmas (Martí and Gudmundsson 2000). Multiple vertical collapses of the central volcanic complex formed Las Cañadas caldera (16 × 9 km, Fig. 1a) partially filled by post-caldera volcanic activity. The last activity inside Cañadas Caldera constructed Pico Viejo and Teide stratovolcanoes. The central volcanic complex and old basaltic edifices are connected by a “Mercedes star” volcano-rift system (Carracedo 1994).

Fig. 1
figure 1

Geographic location of the Canary Islands and simplified geological maps (modified from IGME, 2011) of a Tenerife, with the location of the four mining licenses (Garehagua, Berolo, Guayanta and Abeque) studied for geothermal exploration purposes and b Gran Canaria with the location of Atidama, the mining license studied for geothermal exploration purposes on that island

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Since Tenerife and Gran Canaria are promising areas for geothermal resources, five mining licenses were acquired for geothermal exploration studies: four in Tenerife Island and one in Gran Canaria (see locations in Fig. 1). Four of the study areas are located in the three differentiated volcanic rifts of Tenerife: NE Rift, NW Rift and south Rift (Fig. 1a). The five mining licenses comprise the subaerial surface under which groundwater temperature anomalies and high contents of silica (SiO2) in water were measured during the geothermal exploration studies in the 1970–1990s (IGME 1977, 1979, 1993a, b, c, d, e, f, g, h; Valentín et al. 1990).

Garehagua investigation license (Fig. 2a) comprises a large part of the southern volcanic rift of Tenerife. Its main structural characteristic is an apparent absence of a distinct ridge and a fan-shaped distribution of monogenetic cones (Kröchert and Buchner 2008). Basaltic and trachybasaltic rocks predominate in this study area (Fig. 1a). It is important to note that, inside this study area, a bubbling 3He- and CO2-rich gas spot, located at 2,850 m of horizontal depth inside Fuente del Valle water gallery, has been identified and characterized by previous studies (Pérez et al. 1996, 2007). Chemical and isotopic composition of this bubbling gas exhibits a significant magmatic component with: CO2 ~ 85 %, He ~ 10 ppm; 3He/4He ~ 7.0 RA; 222Rn ~ 100 KBq/m3 (Pérez et al. 1996, 2007). This sampling point is probably related to an upward migration of volcano-hydrothermal gases through a highly permeable pathway (Pérez et al. 2007; Marrero 2010).

Fig. 2
figure 2

Volcano-structural map and sample sites location of Garehagua (a), Berolo (b), Guayafanta (c), Abeque (d) and Atidama (e) mining licenses. Surfaces covered by historic lava flows are also displayed. Stars indicate the locations of the exploratory drillings carried out in Tenerife and Gran Canaria by IGME (1993h)

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Berolo (Fig. 2b) and Guayafanta (Fig. 2c) mining licenses comprise a large part of the northeast volcanic rift of Tenerife. The northeast rift is more complex than NW or NS rifts due the existence of Pedro Gil stratovolcano that broke the main NE–SW structure. Pedro Gil caldera was formed ~0.8 Ma ago by a vertical collapse of this stratovolcano (Ancochea et al. 1990) (Fig. 2c). A fissural eruption (Arafo–Fasnia–Siete Fuentes) along ~13 km took place and affected both mining licenses in 1704–1705 AD (Romero 1991), following a similar direction of the main volcanic rift alignment (N45E). The northern part of Berolo includes the location of two of the three surface thermal anomalies reported by IGME (1993b). Several different lithotypes can be distinguished in Berolo: inside Cañadas caldera, basaltic, trachybasaltic and phonolite rocks; in the rest of the study area, basaltic rocks predominate in the northern part and phonolites at the southern (Fig. 1a). In the case of Guayafanta, most of the area is covered by basaltic lava flows (Fig. 1a).

Abeque (Fig. 2d) investigation license is located inside of the northwest volcanic rift of Tenerife. The rift is formed by alignment of cones along a main NW–SE direction, including the last volcanic eruption that has taken place in Tenerife (Chinyero eruption, 1909 AD). This mining license includes one exploratory drilling carried out in Tenerife by IGME (1993h). This area shows mainly three different lithotypes: basalts, trachybasalts and phonolites (Fig. 1a).

Fig. 3
figure 3

Cumulative probability plots of soil He measured at Garehagua (a), Berolo (b), Guayafanta (c), Abeque (d) and Atidama (e) mining licenses. Solid black lines in the probability plots indicate different log-normal geochemical population in the original data. Solid gray lines indicate the separated background and peak log-normal populations

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Atidama mining license (Fig. 2e), the only license located in Gran Canaria, was selected owing to the presence of several groundwater temperature anomalies (up to 42 °C) and the results of hydrogeological studies carried out (Gasparini et al. 1987, 1990). For these reasons, two drilling exploratory wells were carried out in this area. Alignments of volcanic cones along NW–SE can be observed in the northern part of the grid, as occurred in other parts of Gran Canaria. Atidama comprises three predominant lithotypes: phonolites at southern part, mostly basanites at northern part and a great area of sedimentary deposits at southeast (Fig. 1b).

3 Methods

Soil gas samples were collected at 557, 577, 600, 406 and 541 sites selected in July–August, 2011, July–September, 2012 and March–April, 2014, in Garehagua and Berolo, Atidama and Abeque and Guayafanta, respectively (Fig. 2). The sampling sites were selected based on their accessibility and geological criteria. Soil gases were sampled at ~40 cm depth using a metallic probe with a 60 cc hypodermic syringes and stored in 10 cc glass vials for later laboratory analysis. Helium and Argon contents were analyzed at each sample by means of a Quadropole Mass Spectrometer (QMS; Pfeiffer Omnistar 422) and hydrogen, nitrogen and neon concentrations by a VARIAN CP4900 micro-gas chromatograph. The accuracy and detection limit of the instruments were estimated to be ±0.3 and 3 and ±0.05 and 0.45 ppm, respectively, for helium and hydrogen. Spatial distribution maps of soil He and H2 were constructed by using sequential Gaussian simulation (sGs) algorithm, provided by the sgsim program (Deutsch and Journel 1998; Cardellini et al. 2003). Soil helium enrichments were depicted as ΔHe (ΔHe = [He]soil atmosphere–[He]air). The atmospheric He concentration ([He]air) used was 5.24 ppm (Gluekauf 1946). The sGs procedure allows us to interpolate the measured variable at not-sampled sites. The simulation is conditional and sequential, i.e., the variable is simulated at each unsampled location by random sampling of a Gaussian conditional cumulative distribution function (Cardellini et al. 2003). To check whether the soil He and H2 data came from mixed polymodal distributions (Figs. 3, 5), the probability-plot technique (Tennant and White 1959; Sinclair 1974) was applied to the entire data sets. This technique is based on the recognition of inflection points along a curve defined by plotting cumulative percentile of the data on a log-normal scale. A bimodal distribution consisting of two log-normal populations plots as an S-type curve, where the inflection point shows the presence of two different modes: normal I and normal II. The two distinct populations are known as background (commonly derived from atmospheric He and H2) and peak (soil degassing), respectively.

4 Results

Table 1 shows a statistical summary of the soil He and H2 data measured in the five areas. Soil He values ranged from typical atmospheric values up to 35.1 ppm (measured in Abeque). The other mining areas showed values up to 17.4 ppm in Garehagua, 11.7 ppm in Berolo, 7.2 ppm in Guayafanta and 6.1 ppm in Atidama. The median values were similar or slightly higher than the atmospheric one: 5.42, 5.83, 5.67, 5.64 and 5.25 ppm for Abeque, Garehagua, Berolo, Guayafanta and Atidama, respectively (Table 1). Soil H2 concentrations measured in the five mining licenses ranged from typical atmospheric values (~0.5 ppm) up to 24.4 ppm in Garehagua, 8.6 ppm in Abeque, 8.3 ppm in Guayafanta, 7.2 ppm in Atidama and 4.3 ppm in Berolo. The median values measured for H2 were 0.43, 0.75, 1.23, 0.54 and 1.43 for Abeque, Garehagua, Berolo, Guayafanta and Atidama, respectively (Table 1).

Table 1 Statistical summary of the analytical results of He and H2 soil gas concentration measured at mining licenses for geothermal exploration in the Canary Islands
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4.1 Data Analysis and Spatial Distribution

4.1.1 Garehagua

The possible presence of mixed polymodal distributions in the soil helium data caused by the existence of different geochemical populations was examined by means of the probability-plot technique (Fig. 3a). Soil helium data showed two log-normal geochemical populations with a mean value of 5.79 ppm, for the background population (98.76 % of the total data) and a 9.87 ppm, for the peak population (0.7 % of the total data). The rest of the data is considered as intermediate values between background and peak populations. Peak values exceeded 1.70 times the background ones. The spatial distribution ΔHe (Fig. 4a) showed that highest values roughly defined a N–S trend (>1.6 ppm). The highest relative enrichment (>5.0 ppm) was measured in the central part of the study area. Other important values were measured in the north of the study area in good spatial correlation with the vertical surface projection of the bubbling gas spot already mentioned above (Pérez et al. 1996; Marrero 2010).

Fig. 4
figure 4

Spatial distributions of soil ΔHe at Garehagua (a), Berolo (b), Guayafanta (c), Abeque (d) and Atidama (e) mining licenses, constructed by the sGs algorithm

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The possible presence of mixed polymodal distributions in the soil H2 data was also examined by means of the probability-plot technique (Fig. 5a). Two geochemical populations were distinguished whose mean and percentage were 0.64 ppm (80.41 %) and 12.06 ppm (1.17 %) for the background and peak populations, respectively. The peak value was 18.84 times the background population value. The spatial distribution maps of soil H2 depicted in Fig. 6a showed multiple isolated anomalies, with a main one located in the central area, following a volcanic alignment of eruption centers, with highest values higher than 10 ppm.

Fig. 5
figure 5

Cumulative probability plots of soil H2 measured at Garehagua (a), Berolo (b), Guayafanta (c), Abeque (d) and Atidama (e) mining licenses. Solid black lines in the probability plots indicate different log-normal geochemical population in the original data. Solid gray lines indicate the separated background and peak log-normal populations

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Fig. 6
figure 6

Spatial distributions of soil H2 at Garehagua (a), Berolo (b), Guayafanta (c), Abeque (d) and Atidama (e) mining licenses, constructed by the sGs algorithm

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4.1.2 Berolo

Background and peak populations of soil He showed mean values of 5.48 ppm (88.54 % of the total data) and 8.41 ppm (2.92 % of the total data), respectively (Fig. 3b). The rest of the data formed intermediate values. Peak values exceeded by 1.53 times the background population in Berolo. Multiple isolated anomalies were detected at the spatial distribution map of soil ΔHe (Fig. 4b). The most important ones were measured in the northern part of the area (>5.0 ppm) and are composed by ~35 sampling sites that correspond spatially with one of the eruptive centers of the 1704–1705 volcanic eruption (Fasnia eruptive center).

The probability-plot technique (Fig. 5b) depicted two populations in the soil H2 data, characterized by a mean and percentage of 0.76 ppm (37.54 %) and 2.47 ppm (26.03 %). In this case, peak values exceeded by 3.25 times the background population. The highest soil H2 enrichments were measured at the southwestern part of the study area (>4 ppm) and partially overlap with a soil helium enrichment (Fig. 6b).

4.1.3 Guayafanta

One single log-normal was identified in the probability-plot analysis of the soil He data analyzed (Fig. 3c), with a mean value of 5.66 ppm. Guayafanta area (Fig. 4c) showed just a single main soil ΔHe anomaly (>1.5 ppm) at southern part, in the border limit of Arafo lava flow emitted during the 1704–1705 volcanic eruption.

The mean and percentage of the background and peak population of soil H2 were 0.53 ppm (97.63 %) and 1.72 ppm (1.04 %) (Fig. 5c). Peak values of soil H2 exceeded by 3.24 times the background population. The spatial distribution of soil H2 (Fig. 6c) did not show any area with a significant enrichment.

4.1.4 Abeque

The probability-plot analysis for soil He data measured in Abeque showed two log-normal geochemical populations (Fig. 3d): background, with a mean of 5.39 ppm (78.80 % of the total data) and peak, with a mean of 8.47 ppm (2.37 % of the total data), exceeding 1.57 times the background value. Abeque area showed the highest soil ΔHe value observed in this work and was measured in the west part of the area (>5.0 ppm), but other multiple relatives enrichments were detected as isolated anomalies (Fig. 4d).

In the case of soil H2 data, Abeque showed two overlapping log-normal geochemical populations, with mean and percentage of 0.52 ppm (91.40 %) and 4.28 ppm (1.49 %) (Fig. 5d). In this case, peak values of soil H2 exceeded by 8.23 times the background population. The only slight enrichments at Abeque were measured at the western area (Fig. 6d).

4.1.5 Atidama

In the case of soil helium data, two overlapping log-normal geochemical populations were also observed (Fig. 3e): background (95.18 % of the data) with a mean value of 5.22 ppm and a peak (1.97 % of the data) with a mean value of 6.03 ppm, and intermediate values between both populations. In this case, peak values exceeded 1.15 times the background population. Atidama area had the lower soil ΔHe values of the five areas studied in this work (Fig. 4e). Some slights enrichments were observed at southern part of the study area, with a highest value up to 0.8 ppm, close to the location of the exploratory drilling (Fig. 4e).

Soil H2 measured at Atidama showed two overlapping log-normal geochemical populations with mean and percentage of 1.37 ppm (88.52 %) and 5.20 ppm (0.57 %) (Fig. 5e). In this case, peak values of soil H2 exceeded by 3.79 times the background population. Atidama showed important values (~7 ppm) along a NW–SE trend at the northern part of the study area (Fig. 6e) in good spatial correlation with the structural alignment of the Plio-Quaternary volcanic rift (Anguita et al. 1991; Guillou et al. 2004).

4.2 Chemical Ratios of the Soil Gases

The relative contributions of the soil gases studied here (He and H2) with other nonreactive gases present in the soil environment (N2 and Ar) are displayed as ternary N2–Ar–He and N2–Ar–H2 diagrams (Figs. 7, 8). The chemical composition of soil gases in the five study areas plots along a typical atmospheric component partially polluted by endogenous He and H2. As depicted in Fig. 7, the N2–Ar–He ternary plot suggests a linear mixing trend of atmospheric air with endogenous gases. This possible endogenous addition of helium is evident in Abeque and moderate in Garehagua and Berolo. The results shown by the N2–Ar–H2 ternary plot (Fig. 8) seem to add some slight significance to Atidama, but the most obvious endogenous contribution is found in Garehagua. In both diagrams, the endogenous component in Abeque, Garehagua and Berolo is higher than in Guayafanta and Atidama.

Fig. 7
figure 7

Ternary N2–Ar–He diagrams of soil gases at Garehagua (a), Berolo (b), Guayafanta (c), Abeque (d) and Atidama (e) mining licenses. Red squares indicate the typical air concentration, and dashed line shows the mixing trend with deep-seated gas

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Fig. 8
figure 8

Ternary N2–Ar–H2 diagram of soil gases at Garehagua (a), Berolo (b), Guayafanta (c), Abeque (d) and Atidama (e) mining licenses. Red squares indicate the typical air concentration, and dashed line shows the mixing trend with deep-seated gas

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The molar ratio between an endogenous gas (helium or hydrogen) and a typical atmospheric component can be useful to discriminate the origin of the anomalous emission zones and to weigh up the input of the deep-seated gases. Therefore, assuming a pure atmospheric origin for Ne and Ar, we used three specific two-components molar ratios (He/Ar, He/Ne and H2/Ar) since their values would act as indicators of endogenous gases from volcano-hydrothermal activity, assuming that Ar and Ne are present in the hydrothermal fluids in relative contents close to those of air-saturated groundwater (Giggenbach 1991; Chiodini and Marini, 1998; Pedroni et al. 1999). Table 2 shows the percentage of He/Ar, He/Ne and H2/Ar ratios that exceed by three times that observed in the air. This percentage of He/Ar and He/Ne ratios was higher for Abeque and Garehagua, whereas that corresponding to H2/Ar ratio was greater for Atidama and Berolo. With the aim of sorting the endogenous contribution at each study area, a dimensionless parameter (value in Table 2) from 1 to 5 was assigned to each chemical ratio, being the lowest assigned to the highest chemical ratio values. The same procedure was also applied to the probability-plot parameters of soil ΔHe and H2 (peak values expressed as times background value, Table 2). The sum of the total assigned values gave us a way to sort geochemically the five different mining licenses to weigh up the relative potential of finding geothermal resources at each of the study area, thus reducing the uncertainty in the selection of the area with the highest success of future exploratory works.

Table 2 Geochemical and geographical characteristics of the mining licenses
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5 Discussion

Through a visual inspection of soil He and H2 spatial distributions (Figs. 4, 6) and Fig. 1, it can be stated that there are no spatial relations of soil He and H2 enrichments and the different lithotypes of Tenerife and Gran Canaria. Hernández et al. (2004) investigated the helium emission and U and Th contents in shallow rocks and soils of Las Cañadas caldera and surroundings, Tenerife Island. Their data indicate that helium was supplied mostly from a deep source, with a minor contribution from U- and Th-rich shallow rocks and soils. These results suggest that peak populations observed in the soil helium probability plots shown in the present work are caused mainly by degassing of deep-seated helium. Although the two different possible sources for helium (shallow degassing by radioactive decay of U- and Th-rich rocks and deep degassing from geothermal systems developed beneath the study areas) cannot be ruled out, those areas with higher values of peak population of soil helium (expressed as times the background population) would show a higher proportion of the deepest component. In the case of Tenerife island, where 4 mining licenses were studied, other evidence to support this assertion is the observed high levels of mantle degassing along the three volcanic rift zones of Tenerife, as pointed out by the constant levels of 3He/4He ratio (~7 RA) in groundwater of the island, even though there is no clear evidence of geothermal activity in the surface environment (Pérez et al. 1996). Additionally, the recent results reported by Padrón et al. (2013a, b) during the volcanic unrest that led a submarine volcanic eruption at the southern coast of El Hierro island (see location of El Hierro in Fig. 1) also support that, even when there were clear evidences of magmatic degassing, soil helium enrichments did not exceed the enrichment values shown in this work.

Through the results obtained by the assigned values to the statistical–graphical parameters of soil ΔHe, the importance of the studied mining licenses is sorted as Garehagua > Abeque > Berolo > Atidama > Guayafanta; this result is very similar to that of soil H2: Garehagua > Abeque > Atidama > Berolo > Guayafanta. A visual inspection of the spatial distribution maps (Figs. 4, 6) seems to suggest a different order: Berolo > Abeque > Garehagua > Guayafanta > Atidama for the soil ΔHe data and Berolo > Garehagua > Atidama > Abeque > Guayafanta for soil H2. However, the visual information of the spatial distribution should be treated with caution, especially in Berolo.

Table 3 summarizes the parameters used to interpolate the unsampled locations by sGs for soil ΔHe and H2. The spatial interpolation of data to unsampled locations is strongly influenced by the spatial density of real data, because once a value is simulated in an unsampled location, sGs adds it to the data set and uses as an original data to estimate the variable at the next locations of the grid (Cardellini et al. 2003). Minimum values of spatial densities were used to construct the maps for Berolo and Abeque mining licenses (Table 3). Additionally, those sample locations separated by distances closer than the range value of the semi-variogram model used are spatially autocorrelated, whereas locations farther apart than the range are not. Soil H2 and ΔHe data in Berolo and Garehagua, respectively, showed the higher spatial autocorrelation. These results provide a critical view of the spatial distribution obtained by sGs in Berolo, because the relative low spatial density together with the high spatial autocorrelation likely yields an oversized anomaly distribution.

Table 3 Parameters used in the sGs interpolation to construct the spatial distribution maps of soil gas ΔHe and H2 in the mining licenses
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The number of sampling sites with He/Ar, He/Ne and H2/Ar ratios higher than 3 times the atmospheric value suggests a geothermal potential sorted as Abeque > Garehagua > Atidama > Berolo > Guayafanta (Table 2). Finally, the visual inspection of the ternary N2–Ar–He and N2–Ar–H2 diagrams (Figs. 7, 8) seems to support the relative order suggested by the statistical–graphical analysis, because the volcano-hydrothermal contribution in the soil helium follows the order Garehagua > Abeque > Berolo > Atidama > Guayafanta in the case of soil He and Garehagua > Abeque > Atidama > Berolo > Guayafanta in the case of soil H2. Combining the overall information shown in this work, we sorted the geothermal potential of the five mining licenses studied here by assigning values from 1 to 5 (where 1 means higher endogenous component; Table 2), resulting as Garehagua > Abeque > Berolo > Atidama > Guayafanta.

6 Conclusions

There is no current evidence of endogenous fluids manifestations (active or fossil) at the surface environment of the mining licenses studied here that might evidence the presence of active geothermal systems at depth, except the magmatic bubbling gas spot located inside a water gallery at Garehagua. However, the presence of relative anomalous concentrations of nonreactive and/or highly mobile gases in the soil gas atmosphere such as helium and hydrogen suggests the current existence of a significant input of deep-seated gases. The spatial distribution maps of the five study areas confirm in some cases the existence of a good spatial correlation between anomalous concentrations values of soil ΔHe and H2 and historic and/or recent volcanic alignments, which could act as preferential zones of vertical permeability allowing the migration of deep source gases.

Combining the overall information shown in this work, based on statistical–graphical analysis of the data, visual inspection of the spatial distribution and analysis of interesting chemical ratios in the soil gas, it is possible to obtain weighting tools to sort the five different study areas in terms of their relative potential of finding geothermal resources. In this context, Garehagua and Abeque (corresponding to the southern and western volcanic rifts of Tenerife, respectively) seemed to show the highest geothermal potential of the five mining licenses studied. Additional geochemical and geophysical studies would be necessary to reduce the uncertainty inherent to the selection of the area with the highest success in future exploration works.