Abstract
Geothermal energy has a wide range of uses in our life. It is very important to characterize the temperature and the depth of geothermal reservoirs. The aim of this paper is the determination of type, origin source of water temperature, and depth of water circulation in the Ayub-Peighambar and Shafa (AP and SH) hot springs, located in NE Iran, using hydrogeochemistry and environmental isotopes (2H and 18O). AP hot spring has elevated temperature (36–40°C) and as such is very important for balneotherapy and geotourism industry purposes. The average values of δ18O and δ2H for this hot spring (−10 and −73, resp.) are analogous to that of geothermal and meteoric waters. This indicates that the heat source cannot be related to volcanic activities (with average δ18O value of about 5) and it is most probably associated with geothermal gradient with deep circulation of groundwater through faults. Based on Na-K geothermometers coupled with isotopic (18O and 2H) geochemistry the temperature of the AP geothermal reservoir was estimated to be in the range of 100–150°C with 3–5 and 4.2 kilometres’ depth, respectively. Chemically, the AP samples are CaSO4 facies with a chemically homogeneous source and steam heated waters type.
1. Introduction
Characteristics of springs (especially hot springs) in orogenic regions are due to the interaction between water, the lithosphere, and environmental conditions like lithology, pathways, residence time underground, and many other factors. The origin of elevated temperatures in hydrothermal reservoirs (hot/warm springs) can be due to the geothermal gradient with deep circulation of groundwater through faults, chemical reactions (e.g., in evaporates and oil field brines), heat produced through radioactive decay of long-lived radioactive material (isotopes), and mixing of meteoric water with magmatic water or volcanic vapour in volcanic area [1–11]. The hydrothermal phenomenon most often occurs where the released water and vapour are related to regional volcanic activities. The geothermal heat transfer to the ground surface occurs through aqueous and magmatic/volcanic systems. In an aqueous system, cold surface water can penetrate to depth through deep faults/fractures, where convective currents cause the boiling of water. Subsequently the high temperature groundwater flows upward creating a hydrothermal system on or near the ground surface as hot springs or thermal groundwater discharge within an aquifer [12]. In a geothermal field, an aqueous system can be seen as hot spring, geyser, and mudpot subsystems. Understanding the origin of hydrothermal fluids and the reason for the elevation of temperature is a very important step in revealing the temperature and pressure conditions of the underground reservoirs [13]. To determine the reason and origin of the temperature/heat in a geothermal field, several methods such as remote sensing, geological investigation, geophysical survey, thermal anomalies measurements, and hydrochemical and isotopic method can be applied [14]. Today oxygen and hydrogen isotopes are widely used in geothermal studies to determine the origin and history of geothermal fluids and often serve as natural tracers for the provenance of geothermal water. Most geothermal systems are recharged by meteoric waters and if recharging from shallower groundwater these systems have isotope compositions consistent with local meteoric precipitation [15]. The main objective of this paper is to determine the geothermal reservoir characteristics (type, origin, temperature, and depth of circulation) of Ayub-Peighambar and Shafa (AP and SH) hot springs, in the NE of Iran, using geothermometers and environmental 2H and 18O isotopes along with geological and thermal anomalies data of the study area.
2. Sampling Locations and Geology of the Study Area
The AP and SH hot springs, with a temperature range of 36 to 40°C, are located in the NE of Iran (Figure 1). Field parameter measurement locations and sampling points are illustrated in Figure 2(a). The geological map of the study area was generated using field observations, satellite images, and remote sensing techniques (Figure 2(a)). As illustrated on the geological map, various sedimentary lithological units including massive limestone (Tirgan Formation), marl as pencil marl (Sarcheshmeh Formation), marl-shale (Sanganeh Formation), and sandstone-shale with glauconite (Aitamir Formation) have outcropped in the study area. These formations make the Asiazow anticline with a 15-degree plunge to the west, thrusting and faulting (Figure 2(b)). By oxidation of pyrite, as clearly indicated in thin sections of both Sarcheshmeh and Aitamir Formations, dissolved sulfate was produced [17], which in turn can be reduced through microbial and thermal reduction process (Hill, 1987; [18]), which finally results in H2SO4 production in the groundwater [18, 19].
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Travertine around AP hot spring is evidence of a long discharge history at this spring (Figure 3). This indicates that the water of AP hot spring has previously moved upward through faults where the surface temperature and pressure conditions led to oversaturation of the water with respect to carbonate minerals (CaCO3) which in turn caused travertine precipitation. This is confirmed by the presence of calcite and aragonite (usually aragonite forming in deep conditions rather than calcite) structures in thin sections and in Scanning Electron Microscope (SEM) images (Figure 3). The X-Ray Fluorescence (XRF) analysis of travertine indicates that CaO contains 55% of travertine and Atomic Absorption Spectrophotometry (AAS) and Energy Dispersive Spectrometer (EDX) results show that calcium (with 27%) is the major cation in AP travertine (Figure 3). The existence of a large amount of voids in the travertine can be seen clearly in both hand specimens and thin sections (Figure 3), indicating that CO2 was escaping during formation of this thermogenic travertine. There is no evidence for volcanic activities or existence of intrusive igneous rocks, as confirmed by the field observations and reported by previously report [20].
3. Material and Methods
Water samples were collected in polyethylene 25 mL bottles, from AP and SH hot springs during 2013-2014. All samples were filtered using 0.45 µm membrane and the cation samples were acidified using concentrated HNO3 acid. Field parameters, T, pH, Eh, EC, and TDS, were measured during sampling. The geochemical analyses of main cations and anions and the isotopic analyses of 18O and 2H in water were performed at the geochemistry and G. G. Hatch Stable Isotope laboratories at the University of Ottawa. Geological formations of the study area were investigated using available maps, remote sensing with satellite images, and field observation. The sediment samples were collected from the AP hot spring frontal pool and their mineral contents were determined from thin sections and binocular microscope. All AAS, XRF, SEM, and EDX analyses on travertine samples were done at the Ferdowsi University of Mashhad. The geothermometry of water samples was determined using Aq.QA hydrochemical software [21] and cation geothermometers (Na-K) coupled with isotopic (18O and 2H) geochemistry.
4. Results and Discussion
The measured field parameters and major ion concentrations of groundwater samples are listed in Table 1. The origin and type of water and geothermal reservoir characteristics (temperature and depth) of AP and SH hot spring were investigated using geochemistry and geological studies, geothermometers, and environmental 2H and 18O isotopes.
4.1. The Origin and Type of Water in the AP and SH Hot Spring
Based on piper diagram AP and SH hot springs fall into the CaSO4 facies (Figure 4) and according to Cl-HCO3-SO4 diagram (Figure 5) we have distinguished mature waters (high content of Cl and is saline), peripheral and shallow waters (HCO3 is more than 50%), steam heated waters (SO4 is more than 50% and HCO3 > Cl), and magmatic and volcanic waters (HCO3≈ 0, high SO4 and sulfate and chloride type) [22]. Chemically, the AP and SH hot springs samples have the same chemical composition as a chemically homogeneous source and are of the steam heated water type. These samples have considerable concentrations of HCO3, Ca, and Mg with Ca-SO4 type, indicating deep circulation of meteoric water through the carbonate formation and available deep faults in study area. The increasing of water temperature is probably due to deep circulation and exothermic chemical reactions (probably within the Shourijeh Formation with anhydrite and gypsum). However, a partial melting zone, which emerged through convection currents and faults in the axis of Asiazow anticline, cannot be ruled out.
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4.2. Deep Water Circulation Evidence in Quartz of AP Sediment, Thermal Anomalies, and CO2 Gas
Since there is no volcanic activity in the area [20], the possibility of a volcanic source for the AP spring’s hydrothermal reservoir has been rejected. The existence of travertine around AP spring provides insight into the circulation of water through faults in the past and present. Cold meteoric waters penetrated and dissolved deep Tirgan limestone formations and then became highly enriched in bicarbonate (HCO3) and sulfate (SO4) (due to reduction of mineral sulfates by bacteria with carbonate mineral dissolution at depth) and precipitated/or not precipitated CaCO3 on the ground surface, depending on the pH and Eh of the spring water. This porous thermogenic travertine has had very high amount of CO2 which resulted from deep circulation, exothermic chemical reactions, and dissolution of limestone by sulfuric acid. The thermal anomaly map of the study area, prepared based on temperature of local springs’ water (Table 1), indicates that the AP and SH hot springs in the study area have the most thermal anomalies (Figure 6). These thermal anomalies are caused by tectonically tensional conditions, which can be confirmed by the presence of travertine as well as joints and faults in the area (e.g., [23]; Liu et al., 2003). The sedimentary petrography of AP hot spring indicates quartz and rim-burned biotite has been altered to chlorite, via chloritization, which is characteristic of igneous biotite. Although assembled minerals of green chlorite, opaque mineral (pyrite), amphibole, and glauconite can be seen in thin sections (Figure 7), igneous biotite is reported for the first time in this research and needs further investigation. Large amounts of quartz in pool sediments and also in thin section of this sediment confirmed deep circulation of water. The amount of CO2 that dissolved in AP hot spring is 9.25 mg/L with a little H2S, exempt of bubbles that escaped [24], indicating deep circulation of water through the deep faults [25] that were generated by sulfuric acid in phreatic zone.
4.3. Temperature and Depth Measurements of the Hydrothermal Reservoirs Using Geothermometers
The temperature of hydrothermal reservoirs can be determined using geothermometry [26]. Since geothermometers equilibrate with their surrounding environments under certain temperature and pressure, their equilibrium concentrations in water (from hot springs, soil, and rock) are applied to determine the thermodynamic equilibrium temperature and pressure conditions [26, 27]. This method is very useful in areas without drilled boreholes/wells and it is applicable for initial studies in exploration geothermal fields. To estimate the ranges of temperature in a geothermal reservoir, a large suite of geothermometry tools (minerals, elements, and chemical composition such as silica (SiO2), cations (Na, K, and Li), and gases (H2S, CO2, H2, CO2/H2, and H2S/H2)) can be used [15, 26, 28]. The equilibrium concentrations of Na and K in aqueous and solid feldspar phases (1) strongly depend on temperature. Therefore, the Na-K geothermometer can be applied for both [29–31] hot water (250 < T < 300°C) and very hot water (T > 300 °C) [15, 29].
Based on chemical composition of the water, the temperature of AP hot springs reservoirs were estimated using the Na-K geothermometer and (2). Also the temperatures were estimated using Aq.QA software by defining Na-K and SiO2 as geothermometers. The depth of geothermal reservoir of these springs was calculated using (3) [32] and all estimated temperature and depth are listed in Table 2.
Here, D is the depth of reservoir (km), is the calculated temperature by geothermometers, and refer to the average annual temperature (10.4°C which is calculated by Domarton and Amberege methods [34]). Also is the geothermal gradient (0.03°C per 1 meter).
4.4. Heat Source Determination Using 18O and 2H Isotopes
Based on δ18O and δ2H values of a geothermal water, the heat source of the water may be determined [14]. The δ18O and δ2H values of AP and SH hot springs are plotted within the range of geothermal and meteoric waters (Figure 8). The average δ18O values of the AP and SH hot springs (−10, Figure 8) indicate that the heat source cannot be related to volcanic activities (with δ18O value of about 5) and therefore the geothermal gradient is the primary heat source. Meteoric water, by penetrating to depth through faults/fractures, is heated and altered to a geothermal water, subsequently rising and discharge at the land surface. The deuterium content of these springs (average of −73, ) is within the range of granitic and metamorphic rocks (−40 to −90, Figure 8(b)), which is probably due to partial melting and alteration in Kopet-Dagh bedrock that resulted in increasing temperature in spring water (reference for partial melting and alteration). Although this can be confirmed with the existence of biotite in AP hot spring sediment, it needs more investigation.
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4.5. Calculating the Depth of Water Circulation Using 18O and 2H Isotopes
The depth of water circulation in the AP hot spring reservoir was determined using δ18O and δ2H isotopic compositions and (4) [35]. The maximum depth of water circulation in AP spring is approximately 5 kilometres (Table 2).where is the depth of water circulation (in km) and Δ is δ18O or δ2H isotopes difference (in ) between hot water and cold water spring in the study area.
The DIC and DOC concentrations in AP hot spring are 91 and 3 mgL−1, respectively. The high amount of DIC and its enriched δ13C value (−1.3 VDPB) indicate carbonate dissolution in the geothermally heated water flow path. As illustrated in Figure 9, the 13C-DIC isotope in AP spring is analogous to the range of 13C derived from carbonate dissolution [36]. Therefore, the Tirgan marine carbonate formation (with δ13C content about 0) is probably the main recharge source of the AP spring. Meteoric waters percolated through the Tirgan limestone via faults and discharged from the AP spring after passing through underground karst systems. The very low amount of DOC in the AP spring (3 mgL−1) with δ13C-DOC value of −21.4 VDPB probably originated from organics in the Sanganeh Formation.
5. Conclusions
According to geological, hydrogeochemical, and stable isotope studies the AP spring can be classified as a steam heated waters type. The main heat source for the high water temperature (38°C) in this spring is the geothermal gradient. Using 18O and 2H the origin of the water is meteoric which has penetrated via deep faults/dissolution conduits in the study area. Meteoric waters percolated through the Tirgan marine carbonate formation (with δ13C content about 0) via faults and discharged from the AP spring after passing through underground karst systems. This can be confirmed by high amount of DIC and its enriched δ13C value (−1.3 VDPB). The deep circulation of water in this area can be inferred via high concentration of HCO3, Ca, Mg; CaSO4 water type; thermogenic travertine and high amount of CO2 gas; and thermal anomaly as well. The region is tectonized and has many folds, such as Asiazow anticline which is the main recharge zone of the AP springs. According to Na-K geothermometry, modelling, and stable isotope analyses, the depth of the AP hot spring geothermal reservoir is estimated to be 4-5 kilometres with a temperature of about 150°C. This great depth combined with the water type suggests a hypogenic karst system signature that needs more investigation.
Competing Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The authors would like to thank Northern Khorasan Regional Water Company (NKRWC), Groundwater Research Center (GRC) of Ferdowsi University of Mashhad and Ottawa University geochemistry, and G. G. Hatch Stable Isotope laboratories for their help on sampling and sample analysis. They also thank Dr. Matt Herod for reviewing the manuscript and appreciate Mr. Asghari and Mr. Alizadeh’s assistance during field work. Financial support was partially provided by NKRWC (CN: KNW89098).