1. Introduction

Innovative, cutting-edge techniques to improve geochemical exploration of geothermal systems are currently under development. These techniques are based on the measurement of diffuse gas emissions (e.g., Hg, CO₂, H₂S, Rn, and He) in soils to find anomalies that can be used as geothermal resources exploration tools (Owens, 2013). The study of diffuse gas emissions in soils has helped to define the limits of geothermal systems, delineate preferential flow zones of greater permeability as fault zones, and better understand water-rock interaction processes (Murray, 1997; Fridriksson, 2009; Jennejohn, 2009; Owens, 2013; Hernández et al., 2015). These techniques involve placing detectors in the ground, and the resulting measurements represent geochemical signals from the deeper layers. These signals provide evidence for changes that occur in geological systems, such as volcanic activity, the presence of faults, fracture systems, and the trajectories of vertical flows of hot reservoirs (Hernández et al., 2015).

According to Fridriksson (2009), CO₂ measurements (concentration and flux) in released steam allow for the quantification of steam and heat flow. This gas is used as a magma degassing tracer due to its low solubility in silicate melts at moderate pressure, and CO₂ anomalies usually coincide with high-vertical-permeability zones (Mazot et al., 2011; Hernández et al., 2015; Peiffer et al., 2018; Jácome-Paz et al., 2019). According to Padrón et al. (2003), high surface concentrations of ²²²Rn and ²²⁰Rn (Thoron) allow for the identification of high-permeability zones and the prediction of flow rates in the same region; thoron, in particular, is associated with high velocity flows due to its short half-life of 5.56 s (Durridge, 2019).

On the other hand, the study of surface vapour mercury anomalies provides information about the existence of hydrothermal circulation patterns. Together with geological data, surface vapour mercury anomalies allow for the limits of the reservoir to be defined and provide a general image of the geometry of the geothermal field (Varekamp and Buseck, 1983). Gaseous elemental mercury (GEM) is associated with active volcanic structures; therefore, surface degassing studies of GEM are often used as tools for the exploration of geothermal resources and for the identification of hydrothermal systems (Bagnato et al., 2009, 2018; Cabassi et al., 2017). The vaporization and emission of Hg under the surface in high-temperature geothermal systems occurs via diffusion through permeable rock regions (Murray, 1997). Hg gas diffusion can be measured upon reaching the surface, while the portion of Hg immobilized by clay and organic material in the soil can be detected by analyzing the material present in the soil surface (Van Kooten, 1987). According to the information presented above, the application of this type of study can provide useful information for the determination of high-permeability areas and types of fluid transport.

The Araró-Simirao geothermal system is located in the southeastern part of the Cuitzeo Lake depression, northeast of the state of Michoacan in the central part of Mexico (Fig. 1A). This geothermal system has been studied since the 1970s (Gutiérrez-Negrín et al., 1989; Viggiano-Guerra and Gutiérrez-Negrín, 2005). According the findings of previous studies, it is a convective system dominated by liquid with sodium chloride waters and high boron content; further, it appears to be a fracture zone reservoir fed by a deep, narrow conduit related to the faults in the area (Viggiano-Guerra and Gutiérrez-Negrín, 2005). The soil type that dominate in the geothermal system area is gleysol, and in the southern part, bordering this system is vertisol (INEGI, 2009). The objective of the present study was to determine the relationships between diffuse gas flow in soils (CO₂, Rn, and GEM) in the areas of the greatest permeability (regional faults) to delimit the vertical ascent areas of fluids in the Araró-Simirao geothermal system. This study provides information regarding the diffuse emissions of gases in soils, which has not been previously reported in this geothermal area.

 

Fig. 1. Localization map of the Araró-Simirao geothermal system. A. Current geotectonic configuration of the Mexican territory, in gray the TransMexican Volcanic Belt (TMVB). B. Location of the Araró-Simirao area in the central part of the TMVB. C. Geological map of the Araró-Simirao area, and the geothermal field. (Modified from: Garduño-Monroy et al., 2009 and Gómez-Tuena et al., 2005. The geological map modified from DETENAL, 1978).

2. Geologic setting

The Araró geothermal zone is located in a region referred to as the Cuitzeo Lake depression, which is part of a large tectonic graben (with an E-W orientation) that extends from Chapala Lake to Tepetongo (Viggiano-Guerra and Gutiérrez-Negrín, 2003) and is contained within the central sector of the TransMexican Volcanic Belt (TMVB; Israde-Alcántara and Garduño-Monroy, 1999; Ferrari et al., 2012; Gómez-Tuena et al., 2018; Fig. 1B). Three deformation phases have been identified in the Cuitzeo basin: (1) a phase in the mid-late Miocene characterized by left-lateral strike-slip faults with E-W and NE-SW orientations, followed by (2) a distensive tectonic phase with a NW-SE orientation that likely started in the Miocene, reactivating the structures of the previous phase, and (3) a phase responsible for the formation of several lake basins in areas such as the Chapala and Cuitzeo lakes and the Cuitzeo graben (Garduño-Monroy et al., 1997).

There are three regional systems of normal faults associated with these deformation phases that affect the central part of the TMVB: (1) the NNW-SSE system formed by the extension of the Basin and Range, which is dated to the post-Laramide period (<45 Ma), (2) the NESW system, which is contemporary to the TMVB, and (3) the E-W Morelia-Acambay fault system (MAFS) of the Pleistocene (Garduño-Monroy et al., 2009). The first fault system (NNW-SSE) could be contemporary to the E-W extensional faults and could affect the rock of the Miocene and the Oligocene (Garduño-Monroy, 2019). The MAFS consists of a series of normal E-W and NE-SW faults that cut through the central part of the TMVB and are associated with tectonic lacustrine depressions, such as Chapala, Zacapu, and Cuitzeo (Garduño-Monroy et al., 2009). The tectonic basins are seismically active with large periods of recurrence related to the activity of the MAFS (Mendoza-Ponce et al., 2018). The E-W faults are approximately 7 to 9 Ma old and appeared during the Miocene with evidence of reactivation in paleoseismological records from the Pleistocene and in recent estimates of shallow earthquake hypocenters (<20 km; Mendoza-Ponce et al., 2018). In Cuitzeo, the system has hanging blocks in the northern and southern edges of the lacustrine basin, which form a graben in the western part and a half-graben in the eastern part of the basin (Fig. 1B)

2.1. Lithology and structures

The base-to-roof volcanic stratigraphy of the zone is given by andesites, basaltic andesites, vitreous rhyolites, vitreous fluid rhyolites, rhyolitic tuffs and basalts, and cinder cones. These units are covered by slope deposits and alluvium (Viggiano-Guerra and Gutiérrez-Negrín, 2003). Outcrops in the area can be grouped into three lithological units:(1) basaltic andesites of the Early Pleistocene with a slight degree of fracturing that correspond with the escarpments of the Huingo fault (Casarrubias¹ et al., 1990), (2) rhyolitic tuffs aged between 1.2 and 9 Ma (Casarrubias¹ et al., 1990; Viggiano-Guerra and Gutiérrez-Negrín, 2003), and (3) alluvial deposits (Viggiano-Guerra and Gutiérrez-Negrín, 2005; Fig. 1C).

In the studied area, there are two normal faults (Fig. 2) with a general E-W tendency: the Huingo fault in the north and the Araró-Simirao fault in the south, both of which are apparently active (Viggiano-Guerra and Gutiérrez-Negrín, 2003). These structures form a staggered system toward the NW and are slightly inclined toward the NE. In addition to these principal faults, surface lineaments have been observed (Ávalos, 2016; Vigil, 2017) to preferentially follow an E-W pattern, followed by a NNW-SSE and a NNE-SSW patterns (Fig. 1C).

 

Fig. 2. Location map of diffuse gas sampling points. A) Sampling tracks for GEM (August) and diffuse CO2 Flux (November), both in 2018. B) Sampling points for CO2 Flux, 222Rn and 220Rn, May 2019. C) GEM sampling tracks, November 2018.

2.2. Hydrology and hydrothermalism at the Araró-Simirao geothermal system

The Araró-Simirao geothermal system is part of the Cuitzeo Lake hydrographic basin. The aquifer in this region is heterogeneous and anisotropic with semiconfined conditions due to the presence of clay sediments in the area near the lake (CONAGUA, 2018). Where the flow direction is S-N toward the lake. The aquifer also consists of andesitic breccias, basalts, basaltic breccias, and ashes (CONAGUA, 2018). The shallow aquifer in the Araró area consists of clastic fragments and lacustrine sediments with variable thicknesses ranging between 100 and 200 m. The water table can be located at depths ranging from 0 to 25 m (Viggiano-Guerra and Gutiérrez-Negrín, 2003). In the Nicolás Simirao area, the water table is shallow (approximately 1 m deep) and mixed with thermal water. In this area, a clay layer constituted of lacustrine sediments can be inferred to act as an impermeable layer (Viggiano-Guerra and Gutiérrez-Negrín, 2003; CONAGUA, 2018); however, this layer breaks in areas where it is intercepted by faults and fractures, allowing thermalism to manifest and producing superficial deposits of sinter, calcite, and NaCl (Viggiano-Guerra and Gutiérrez-Negrín, 2003).

The surface manifestations of the hydrothermal system consist of sodium chloride hot springs with high boron content, escaping gases, high electrical conductivity (4,600 μS/cm), and a pH between 6 and 8. The temperatures of these manifestations vary from 30 °C to the local boiling point. Geothermometers show temperatures at depths above 200 °C (Viggiano-Guerra and Gutiérrez-Negrín, 2003). The main gases of some springs are CO₂ and, in lower concentrations, H₂S, H₂, and noble gases (He, Ne, and Ar). These characteristics of the water suggest that they come from the mixture of deep geothermal fluids with shallow freshwater aquifers (Viggiano-Guerra and Gutiérrez-Negrín, 2003). This results in rapid discharge with little heat loss where self-sealing processes have occurred, causing the system to deepen (Viggiano-Guerra and Gutiérrez- Negrín, 2003).

3. Methodology

Three sampling campaigns were carried out in the study area during the rainy (August and November) and dry (May) seasons of 2018 and 2019, respectively. In these campaigns, diffuse gas sampling was carried out to measure CO₂, GEM, and ²²²Rn in the soil, and soil temperatures were measured as well (Fig. 2). To obtain CO₂ flux data, two sampling campaigns were carried out. The first campaign was carried out in November 2018 (the end of the rainy season) from the northern part of the study area near the Huingo and Los Hervideros balnearies to the 15D Atlacomulco-Guadalajara Highway and from the highway to the Araró-Simirao fault trace in the southern part of the study area. Eighty-six CO₂ flux and soil temperature measurements were taken throughout the geothermal system area (Fig. 2A). The other campaign was carried out in May 2019 (the dry season); in this campaign, measurements were taken in specific zones based on the anomalous data obtained from the first sampling campaign as well as data from the zone of interest regarding diffuse gases, such as GEM. Thirty-eight CO₂ flux and soil temperature measurements were taken in this latter campaign (Fig. 2B). Two sampling campaigns were carried out to obtain the GEM concentration data: one in August 2018 and the other in November of the same year. The August route was taken southeast of the town of Araró and south of Highway 15D, which is 8 km in length and occupies approximately 4.2 km² in area; 8,385 GEM measurements were taken on this route in total (Fig. 2A). In November, the August tour was repeated with an additional surface in the immediate area of the Huingo and Los Hervideros balnearies. A total area of 6.6 km² (length=21.3 km) was covered, resulting in 18,576 GEM measurements (Fig. 2C). To obtain ²²²Rn concentration data, a sampling campaign was carried out during the dry season (May) of 2019; however, for technical reasons, only six sampling points were obtained. The point distribution was 10 m between each point. The diffuse gas analyses of CO₂, GEM, and ²²²Rn were carried out using different methods.

3.1. CO₂ measurement method

A West System portable fluxmeter with an A type accumulation chamber was used to measure CO₂. This portable fluxmeter measures CO₂ concentrations through a non-dispersive infrared spectrometer (NDIR) with a measuring range of 0-20,000 ppm. Readings from this instrument are accurate to 3%. The cumulative chamber is placed on the floor, and concentration variations (ppm) are measured over 90-240 s (West System, 2019). The sampling points were georeferenced using a GPS device, and soil temperatures were recorded using a thermocouple inserted 10 cm into the ground. Subsequently, the data were corrected by soil temperature and pressure and converted from ppm/s to mol m^-2d^-1 and later to g m^-2d^-1, according to the equation:

 

where:

F: CO₂ flux in mol m^-2d^-1

Flux: measurement taken in the field (ppm/s)

P : barometric pressure in mbar

V : the cumulative chamber volume in m³

R : Gas constant 0.08314510 barLK^-1mol^-1

T : soil temperature in °K

A : cumulative chamber area in m²

 

3.2. GEM (gaseous elemental mercury) measurement method

The GEM concentrations were measured using the Lumex Portable Mercury Vapor Analyzer RA-915+® in monitoring mode (data logger). The route was followed on foot with a GPS device to control the monitoring position. The Lumex Portable Mercury Analyzer RA-915+® is a highsensitivity quantitative analytical device designed for real-time atmospheric Hg measurement. This instrument is a portable multifunctional atomic absorption spectrometer that applies a Zeeman correction for background absorption, which eliminates the effects of interfering impurities. The Hg analysis system pumps air (10 dm³/min) through an inlet that filters air particles to the detection cell. A zero correction is applied every 20 minutes during the sampling process to restore the baseline by changing the air flow through an Hg absorption filter. The detection limits of this instrument are 0.5-30,000 ng/m³, and the instrument is accurate to 20% (OhioLumex, 2017).

3.3. ²²²Rn measurement method

The DURRIDGE RAD 7 Electronic Radon Detector 2684™ was used to measure ²²²Rn gas. The RAD 7 uses a solid-state alpha radiation detector made out of a semiconductor material (silicon) that converts alpha radiation directly into an electrical signal. This instrument has a soil probe that is placed approximately 50 cm deep into the ground. The measurement mode was set to Sniff in the present study, as the concentrations of ²²²Rn gas in the soil were high, requiring little time to adjust the accuracy. The measurement method used in the present study was the protocol for thoron (²²⁰Rn; Durridge, 2019), in which the equipment takes measurements every five minutes; however, for Rn measurements, the first two readings were ignored to reach equilibrium and obtain an accuracy of ±10%. Thus, readings were taken every five minutes (per quadruplicate) at each sampling point to obtain the concentrations of ²²²Rn and ²²⁰Rn. Subsequently, data management was performed using CAPTURE software (DURRIDGE) to determine the concentrations at each sampling point, corrected by temperature and humidity. Soil temperature was measured at each sampling site using a thermocouple placed 50 cm deep.

3.4. Statistical data management

Common descriptive statistics (mean, median, and standard deviation) were obtained for the data concerning CO₂ flux, GEM concentrations, and soil temperature. Subsequently, statistical data processing was carried out according to the methodology followed by Sinclair (1974) and Lepeltier (1969) for a set of geochemical data. In this methodology, histograms are used to determine whether the data follows a normal distribution. If the data does not follow a normal distribution, a log-normal transformation is performed for the statistical analysis. From this transformation, a cumulative probability diagram is constructed to observe the data behavior, determine whether there are different populations, and establish the background values of the area as well as any possible anomalous values. After the statistical treatment of the data, the background and anomalous values from the first sampling of CO₂ fluxes, soil temperatures, and GEM concentrations were used to construct distribution maps. The statistical management of the data was carried out using Minitab v.19 software. Surfer v.16 software was used with the simple Kriging interpolation method to construct the distribution maps of CO₂ fluxes and soil temperatures. The minimum curvature interpolation method was used to construct the distribution map of GEM concentrations; this method sought to soften the interpolated surface according to a prespecified parameter.

4. Results

4.1. Statistical distribution of diffuse CO₂ fluxes, GEM concentrations, and soil temperatures

The CO₂ flux, GEM concentration, and soil temperature data were analyzed using the methodology described by Sinclair (1974) and Lepeltier (1969). According to the histograms (Figs. 3A, 4A, and 4B), the dataset shows a distribution different from a normal one. Cumulative probability graphs (Figs. 3A, 4A, and 4B) were constructed using the log-normally (Ln) transformed data to determine the possible existence of several populations within the range of the obtained data and also to determine the background values and possible anomalous values within the study area. All data concerning the Ln transformed CO₂ fluxes (Ln CO₂) followed the same trend on the normal distribution line (Fig. 3B). However, three data points did not follow a normal distribution, and these data points had the highest values; these points were categorized as anomalous values of the population because they represented areas in which there was greater degassing compared to rest of the studied area. The average value (50%) of the CO₂ fluxes that followed the same trend, which made up 95.25% of the total data (Fig. 3B), was used to establish the background value (15.39 g m^-2d^-1). The background value was used as the minimum limit for the flux distribution maps, while the maximum CO₂ flux value of this population (44.12 g m^-2d^-1) was used to determine the normal degassing values (biogenic and/or geogenic) for the study area.

 

Fig. 3. Histograms and cumulative probability curve the CO2 flux for samples gathered in November 2018. A) The first histogram shows the non-normal distribution behavior of the CO2 fluxes (g/m2/d) data set, and the second histogram shows the normal distribution behavior of the CO2 lognormal transformed data set. B) The cumulative probability curve shows the normal distribution behavior of LnCO2 data set, the background population average value (15.39 g/m2/d) and the threshold flux value (44.12 g/m2/d) in the study area according to data statistical treatment (Sinclair, 1974; Lepeltier, 1969).

 

Fig. 4. Histograms and cumulative probability curve of the GEM (gaseous elemental mercury) data obtained in August and November 2018. A) The histograms shows the non-normal distribution behavior of the GEM data and normal distribution behavior of the trans-formed data set (Ln GEM) for August sampling campaign. The cumulative probability curve shows the background average (3.45 ng/m3), threshold (7.24 ng/m3) and extremes (>210.69 ng/m3) values for the study zone. B) The histograms shows the non-normal distribution behavior of the GEM data and normal distribution behavior of the transformed data set (Ln GEM) for November sampling campaign. The cumulative probability curve shows the background average (3.78 ng/m3), threshold (12.31 ng/m3), anomalies zones (69.24 ng/m3) and extremes (>667.50 ng/m3) values for the study zone.

 

The same calculations were carried out for the GEM concentration dataset (Figs. 4A and 4B) and for the soil temperature data obtained at each CO₂ flux sampling point, which exhibited log-normal behavior in the data transformation process. As with the data concerning CO₂ fluxes, the statistical management of the data concerning GEM concentrations and temperatures was used to determine the background, normal values, and anomalies of the area.

4.2. CO₂ fluxes and soil temperatures

The CO₂ fluxes obtained in the first November campaign ranged from 4.38 to 94.61 g m^-2d^-1 (Table 1). The average CO₂ flux value of the background data population was 15.39 g m^-2d^-1 (Fig. 3B), which was relatively close to the average value of the total dataset 19.61 g m^-2d^-1 and comparable to other background values in hydrothermal systems associated with high degassing, magmatic conditions, and nonmagmatic conditions at depth (Harvey et al., 2017). The range of CO₂ fluxes in the study area was similar to that of the reported values of normal soil respiration (several tens of g m^-2d^-1; Chiodini et al., 2008) as well as that of the degassing values of volcanic soils both with and without vegetation (approx. 103-104 g m^-2d^-1; Chiodini et al., 2008). The statistical analysis showed that the average CO₂ flux value of the background population was 15.39 g m^-2d^-1 and that the threshold value (biogenic and/or geogenic) of the study area was 44.12 g m^-2d^-1. Only values above the threshold value were plotted in the flux distribution map (Fig. 5), and values that exceeded 44.12 g m^-2d^-1 were taken to be anomalous degassing zones. According to figure 5, the highest fluxes (94.61 g m^-2d^-1) were located near the Araró-Simirao fault (mud pool, south of the study area); this was the only area with a higher-than-normal degassing value. This finding suggests that the study area has homogeneous degassing; this can be explained by the type of soil (gleysol) in the area (which forms an impermeable layer), by the lack of highpermeability areas reaching the surface, and/or by groundwater serving as a CO₂  carrier solubilized in the aquifer.

Fig. 5. CO2 flux (upper panel) and soil temperature (lower panel) distribution maps for the samplings carried out in November 2018 and May 2019. The CO2 flux distribution map for November 2018, shows an anomaly zone (A) with fluxes greater than the threshold value (44.12 g m-2d-1) for the study area; in the map only fluxes above threshold value were plotted. The CO2 flux anomaly zone sampled in May 2019 denominated mud pool, with maximum fluxes of 1,421.80 g m-2d-1. The soil temperature distribution map for November 2018, shows two anomalous zones (A and B) with temperatures greater than threshold temperature of the study area (17.4 °C). The anomaly A map (mud pool zone) show the maximum soil temperature (71.8 °C) observed in sampling of May 2019. The anomaly B (the thermal active spring area) shows an increase in soil temperature with increasing fracturing, observed in sampling of May 2019.

 

In the second sampling campaign (May), a narrower mesh (approximately 10 m between points) was applied to the anomalous area (mud pool) detected during the November sampling campaign (Zone A). Fluxes of 0.80 to 1,421.80 g m^-2d^-1 (Table 1) were obtained, with an average of 65.70 g m^-2d^-1; these values were higher than those found in the first sampling campaign. This was probably due to the dry season, which corresponds to less humidity in the soil and a deeper aquifer water table, as CO₂ is a gas that dissolves easily in groundwater and can be carried away from the principal upstream areas (Marrero et al., 2008; Clark, 2015). This results in a decrease in soil degassing during the rainy season. The CO2 distribution map of this area (Fig. 5, Zone A) displays two anomalous points very close to each other (<50 m). The highest fluxes (1,421.8 g m^-2d^-1) were found on the Araró-Simirao fault trace, where it intersects with one of the NNE-SSW lineaments. These values were similar to those reported in other fault-dominated geothermal systems with numerous lineaments (e.g., 1,467.5 g m^-2d^-1 in Brady’s system in the Basin and Range Province, Nevada, USA; Jolie et al., 2015). This indicates a point of high permeability, where CO₂ can ascend from a deep area to the surface. It should be noted that these points were very restricted and that the surrounding fluxes had normal values.

The soil temperature in the November sampling ranged between 15.8 and 46.2 °C (Table 1). The annual mean atmospheric temperature (17.4 °C) was taken as the threshold value of the zone. As shown in figure 5, one anomaly was located in the area with the largest CO₂ degassing (mud pool) and the highest soil temperature values (46.2 °C) in the southern part of the study area (Zone A). Another anomalous value was found in the central area at Los Hervideros (temperature=45.5 °C) and Huingo (temperature=36.3 °C) in Zone B. The temperature increase in this zone was likely due to its proximity to superficial thermal manifestations, while the first anomaly could have been associated with a point of greater permeability (normal fault) and high CO₂  degassing.

The anomalous areas (found previously) were sampled in the May campaign. The soil temperatures ranged between 25.1 and 71.5 °C (Table 1), and the highest temperature once again corresponded with the highest CO₂ flux area (mud pool; Fig. 5). This corroborates the anomaly found in the first sampling campaign, albeit at a higher temperature. As shown in figure 5, this area showed a tendency for increased soil temperatures toward the north with respect to the Araró-Simirao fault trace. High soil temperatures were observed (maximum=57.8 °C) in the anomaly at the center of the study area (Zone A, May campaign), corroborating what was found in the first sampling campaign. Figure 5 shows an increase in temperature which was associated with thermal springs, faults and the NNW-SSE lineaments. A similar pattern was also observed toward the western sector in an area where there was evidence of ancient thermal manifestations (sinter), which were probably associated with a NNE-SSW lineament (Zone B, May campaign). CO₂ flux measurements were also taken in this area, but the range of CO₂ flux values fell within the range of background values for the study area. With regard to CO₂ flux, the soil temperature was higher in the second field season sampling campaign; this was probably due to the dry weather conditions, in which differences in humidity and the water table position of the aquifer play important roles as flow and temperature medium moderators.

4.3. GEM concentrations

During the August campaign, the range of GEM concentrations fluctuated between 0.5 and 365.51 ng/m³ (Table 1). Statistical management of the data concerning GEM concentrations was carried out in the same way as for the data concerning CO₂ fluxes and soil temperatures (Fig. 4). The background population average of the GEM concentrations in the study area was 3.45 ng/m³; GEM concentration values were considered anomalous if they were above 7.24 ng/m³ (threshold value) and deemed extreme if they exceeded 210.69 ng/m³ (Fig. 4A). In this way, two GEM anomalies were detected (Fig. 6). One anomaly with a maximum value of 184.2 ng/m³ (Fig. 6, Zone B) was detected in the old thermal manifestations of the La Salud balneary near the town of San Nicolás Simirao and a NNW-SSE lineament. Another (higher) anomaly with a GEM concentration of up to 363.56 ng/m³ (Fig. 6, Zone A) was detected near the Araró-Simirao fault trace (extreme value), which coincided with the area with the highest CO₂ flux (mud pool) and soil temperature. These GEM values were similar to those found in other areas with both volcanic and geothermal activity (Varekamp and Buseck, 1986) oscillating between 35 and 1,031 ng/m³.

Fig. 6. GEM (gaseous elemental mercury) content distribution maps for the samplings carried out in August and November 2018. GEM sampling carried out in August (upper part) shows two anomalies, the mud pool zone (A) and east part zone next to San Nicolás Simirao town (B), with GEM values above the threshold value (7.24 ng/m3) for the study area. GEM content distribution map for sampling carried out in November (lower part) shows three anomalies, the mud pool zone (A) and east part zone next to Simirao town (B), corroborating the first sampling, and other zone in the thermal actives springs area (C), with GEM values above the threshold value (12.31 ng/m3). The GEM values reached a maximum of 2,914. 46 ng/m3 in the mud pool area.

The GEM values obtained in the November campaign ranged between 0.49 and 2,914.46 ng/m³ (Table 1); the average background value was 3.78 ng/m³, and the anomalies were considered to be above 12.31 ng/m³. Two anomalous populations were identified: one with a maximum value of 69.24 ng/m³, and another with a maximum value of 667.50 ng/m³. Extreme values were considered to be above the maximum value of the second anomalous population (Fig. 4B). Three anomalous zones were found, confirming the two anomalous zones that were previously detected (Fig. 6). The first anomalous zone had a low GEM concentration of 115.1 ng/m³ (Fig. 6, Zone C) and was located in the north-central part of the study area, near the active thermal manifestation (Los Hervideros Balneary) and over a NW-SE lineament. The second anomalous zone had a maximum GEM concentration of 200.56 ng/m³ (Fig. 6, Zone B), was located in the central part of the study area, and corresponded to one of the anomalies found in the first sampling campaign (Fig. 6, Zone B). The third anomaly presented with the highest GEM concentration of 2,914 ng/m³ (Fig. 6, Zone A) and coincided with the area of the mud pool found in the first sampling campaign. The GEM concentration measured in this last anomalous zone was above the GEM values reported in areas associated with volcanism, such the Colima Volcano (western Mexico) and Hawaii (1,422 and 1,031 ng/m³, respectively; Voltattorni et al., 2010; Varekamp and Buseck, 1986). According to Varekamp and Buseck (1986) and Murray (1997), these anomalies occur when geothermal fluids ascend from a deep reservoir to shallow levels; thus, the GEM anomalies found in the study area may have been related to the temperatures of ascending geothermal fluids acting as carriers, in the fault or lineaments zones which can act as migration routes for gases (Phuong et al., 2012). In addition, the largest GEM anomaly found in this study suggests a vertical gas migration zone extending from a deep reservoir to the surface.

4.4. ²²²Rn concentrations

The ²²²Rn concentrations found in this study ranged between 1,060 and 124,100 Bq/m³ (Table 1), and the highest concentrations were found nearest to the mud pool (Fig. 7A); moving away from this area, the measured concentrations decreased. This concentration range was greater than those reported in geothermal and volcanic areas, such as the Tetitlan Valley in Nayarit, Mexico (average ²²²Rn concentration =2,702 Bq/m³; Voltattorni et al., 2010) and the San Miguel Volcano in El Salvador (average ²²²Rn concentration=30,821 Bq/m³; Cartagena et al., 2004). These measurements suggest that high levels of ²²²Rn degassing occurred over a high-permeability area due to the crossing of the two aforementioned geological structures as well as a deep origin (Jolie et al., 2015; Giammanco et al., 2007; Voltattorni et al., 2010; Israde-Alcántara and Garduño-Monroy, 1999; Viggiano-Guerra and Gutiérrez Negrín, 2003).

Fig. 7. 222Rn, 220Rn concentration and soil temperature maps in the mud pool zone of the geothermal system, for sampling campaign of May 2019. A) 222Rn concentrations with values between 1,060 and 124,100 Bq/m3. B) 220Rn concentrations with values between 0 and 7,511 Bq/m3. C) 222Rn / 220Rn ratios and D) soil temperatures in the sampling points of the Rn measurements.

 

On the other hand, ²²⁰Rn concentrations ranged from 0 to 7,511 Bq/m³ (Table 1 and Fig. 7B), indicating high concentrations of this isotope at the points that had lower levels of ²²²Rn degassing. ²²⁰Rn anomalies can be induced by geothermal effects in areas with average gas flow rates and can be related to surface structures or gas origins that are close to the surface. These anomalies are also associated with lacustrine and alluvial deposits, which favor the accumulation of this gas due to its low permeability (Jolie et al., 2015). In the case of the sampled area, being near an area with high permeability, the high concentrations are likely due to the geothermal effect.

The ²²²Rn/²²⁰Rn ratios ranged from 1 to 508.9 (Fig. 7C). These ratios have been used as indicators to determine areas of high permeability (e.g., faults and fractures), to distinguish between deep (high ratios) and shallow (low ratios) origins in active faults, and to determine the extent of deep faults that connect the deep zone to the surface (Jolie et al., 2015; Phuong et al., 2012). In the case of the sampled area, the ²²²Rn/²²⁰Rn ratios were high at points near the mud pool and decreased as they moved away from it (Fig. 7C). These high ratios suggest that the mud pool is a zone of direct, deep degassing of the geothermal reservoir.

The soil temperatures measured at these points ranged between 32.70 and 58.11 °C (Table 1 and Fig. 7D); however, the point with the highest temperature was the point at which the highest concentration of ²²⁰Rn was found, assuming that the gas can be transported from a deep source with a high migration rate, which is produced by geothermal effects (e.g., convective processes) in a high-permeability zone.

5. Discussion

5.1 Relationships between diffuse gas emission concentrations in soils and faults zones.

The statistical treatment of the data revealed anomalies directly associated with the Araró-Simirao fault (Fig. 1). The largest anomalies in CO₂ flux, GEM concentration, ²²²Rn concentration, and soil temperature were observed in the Araró-Simirao fault trace with the crossing of the NNE-SSW lineament, that was related to the zone of the greatest permeability (in the mud pool). Additionally, it was inferred that two of the anomalies in GEM concentration were located in a N30°W secondary fracture zone with argillic alteration and silicification (Ávalos, 2016). This zone was located in the eastern part of the study area, and fracturing was indicated by the lineaments of both ancient and current thermal manifestations. The soil temperature anomalies were spatially similar to the anomalies in GEM concentration, suggesting similar fracturing. Another anomaly was identified toward the central and western part of the study area in a sector of ancient thermal manifestations, which were evidenced by surface sinter deposits.

On the other hand, the CO₂ anomaly was consistent with the highest ²²²Rn concentrations, GEM concentrations, and soil temperatures. This finding suggests the presence of a gas channels in this area, which would allow CO₂ upward migration to act as a carrier of trace species (in this case, ²²²Rn and GEM) to the surface (Etiope and Martinelli, 2002; Cartagena et al., 2004; Voltattorni et al., 2010). CO₂ can be considered a carrier gas due to its high concentrations in geothermal systems (Durrance and Gregory, 1987; Mörner and Etiope, 2002). Based on the obtained ²²²Rn/²²⁰Rn ratios, these gases can be considered to originate in a deep reservoir and reach the surface through high-permeability migration channels associated with faults and/or fractures (Etiope and Martinelli, 2002; Voltattorni et al., 2010; Phuong et al., 2012; Jolie et al., 2015; Bagnato et al., 2018). These migration channels may have occurred in the mud pool area, resulting in a direct ascent from the geothermal reservoir to the surface. The other zones of anomalous GEM concentrations and temperatures are considered to have been caused by existing thermal springs associated with sites of structural weakness, such as the NNW-SSE and NNE-SSW lineaments. These lineaments may correspond to reactivated faults due to the activity of the primary E-W faults, as has been observed to the east of the Graben de Cuitzeo (e.g., Mendoza-Ponce et al., 2018); alternatively, they may correspond to R’ or tension (T) fracture-fault systems due to their orientation with respect to the main E-W faults. Thermal discharges in the Araró-Simirao geothermal system are related to zones of high permeability, which are controlled by the main E-W and NE-SW faults and by the NWSE system. Similarly, thermal discharges observed in the Los Azufres geothermal field and the Tizitzio-La Escalera geothermal system, south of Araró-Simirao, are controlled by E-W extensional faults within the TMVB (González-Partida et al., 2000; Jácome-Paz et al., 2019) and reactivated fault systems related to Riedel deformation models (e.g., R’, P’, N, and T faults with NW-SE and NE-SW orientations; Jácome-Paz et al., 2020).

The relationships between the tectonic structures (faults, fractures, and/or lineaments) that affect the hydrothermal system and the diffuse gas anomalies found in this study were used to develop a conceptual model of the hydrothermal system (Fig. 8). This model suggests that the area of the mud pool located in the Araró-Simirao fault is the vertical ascent zone of diffuse gases from the deep reservoir (upflow zone) and that the lineaments and/or fractures are areas of structural weakness, in which the GEM gases are associated with the temperatures of the escaping thermal manifestations (outflow zone). Also, the model showed that the low CO₂ content in these areas is most likely due to the high solubility of CO₂ in water, which allows it to be carried by the groundwater flow that runs in the S-N direction toward the Cuitzeo Lake. According to Viggiano-Guerra and Gutiérrez-Negrín (2003), this system resembles a stockwork reservoir, which is associated with fracture zones and located between two deepening faults, exhibiting high vertical permeability in fault zones and low permeability in other areas.

Fig.8. Conceptual model of the Araró-Simirao geothermal system. The figure shows the upflow zone where the diffuse gases (CO2, Hg and Rn) are transported from the depth to the surface through an area of greater permeability (Araró-Simirao fault trace), and the outflow zone in the area where the thermal springs and fractures and/or lineaments are located (The depth of the heat source is not to scale).

6. Conclusions

The Araró-Simirao geothermal system, located in the southeastern part of the Cuitzeo basin, presents surface manifestations such as thermal springs with temperatures reaching the boiling point (Los Hervideros and Huingo springs) as well as degassing zones through the soil and a mud pool. This area is affected by an E-W normal fault system that forms a halfgraben with dipping blocks northward. The studied area contains two main faults—the Araró-Simirao and Huingo normal faults to the south and north of the area, respectively—as well as associated NNW-SSE and NNE-SSW lineaments or fractures. Based on the diffuse gas emissions (CO₂, GEM, and ²²²Rn) in the soils of this geothermal system, the following conclusions were reached:

• The Araró-Simirao fault and the NNW-SSE and NNE-SSW lineaments are directly associated with the surface thermal manifestations and diffuse gas anomalies found in the hydrothermal system because these are areas of high permeability.

• The area called the mud pool (located at the intersection of the Araró-Simirao fault trace with a NNE-SSW fracture) with a high degassing rate, represents a vertical upflow zone with ascent of gases from a deep reservoir.

• The NNW-SSE and NNE-SSW lineaments serve as a secondary degassing zone (outflow zone).

• The concentrations of the diffuse gases (CO₂, GEM, and ²²²Rn) in the studied soils were similar or higher than those reported in other geothermal areas affected by volcanism or large tectonic faults.

• CO₂ may function as a carrier of Hg and ²²²Rn in the vertical ascent zone (upflow zone). In the outflow zone, CO₂ may be present dissolved in the groundwater. However, it is necessary to perform a more complete sampling of ²²²Rn to determine its correlation with CO₂.

• It is important to consider sampling temporality, as diffuse gas emissions in soils decrease during the rainy season due to soil saturation, thus reducing the diffusion of gases on the surface.

Acknowledgement
This work was funded by project P0₂ of the CeMIE-GEO, as well as by internal project- Geophysics Institute, UNAM. We thank the Geothermal Fluid Geochemistry Unit (GFGU) of the Geophysics Institute, Morelia Unit, UNAM for the scientific and analytic support, as well as Ph.D. C. Inguaggiato of the Ensenada Scientific and Higher Education Research Center (CICESE) and Ph.D. V. Yutsis of the Applied Geosciences Division, Potosin Institute of Scientific and Technological Research, (IPICYT) for scientific support, M. López Bastida and B. Ávila Ciprés for field assistance. The authors acknowledgement the anonymous reviewer for his valuable comments.

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