Hydrogeochemical and isotope geochemical study of northwestern ...

Arab J Geosci (2016) 9:169 DOI 10.1007/s12517-015-2252-2

ORIGINAL PAPER

Hydrogeochemical and isotope geochemical study of northwestern Algerian thermal waters Mohamed Belhai 1 & Yasuhiro Fujimitsu 2 & Fatima Zohra Bouchareb-Haouchine 3 & Tatsuto Iwanaga 4 & Masami Noto 4 & Jun Nishijima 2

Received: 6 April 2015 / Accepted: 7 October 2015 # Saudi Society for Geosciences 2016

Abstract Northwestern Algeria is characterized by a large number of thermal waters and volcanic eruptions and belongs to the Alpine-Magrebide belt. The geothermal reservoirs that feed these reservoirs are mainly hosted by a fractured Jurassic limestone and dolomite sequence. Seven samples were collected from thermal springs of near-neutral pH (6.2 to 7.56) with discharge temperatures between 42.9 and 66.1 °C. Hydrogeochemical analyses of the thermal waters reveal four types (Na+-Ca2+-Cl−, Na+-Ca2+-Cl−-HCO3−, Na+-Ca2+-Cl−SO42−, and Na+-HCO3−-Cl−) and show high total dissolved solids up to 4002 mg/L. Stable isotopic results (δ18 O and δ D) indicate that the thermal waters are of meteoric origin deeply infiltrated and heated by advective heat anomalies and raised up to the surface through deep-seated faults acting as hydrothermal conduits. The estimated reservoir temperatures using silica geothermometers and fluid-mineral equilibria overlap b et w e en 6 6 a n d 12 5 ° C , w h ile N a/ K a n d K /M g geothermometers give much higher and lower results, respectively, and are mainly influenced by mixing with cooler Mg groundwaters as indicated by the Na-K-Mg plot in the immature water field and in silica and chloride mixing models. Thermal waters deeply circulated and heated at a depth of

* Mohamed Belhai [email protected] 1

Department of Earth Resources Engineering, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan

2

Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan

3

Laboratoire de Géo-Environnement FSTGAT/USTHB, BP 32, El-Alia 16000, Algiers, Algeria

4

Department of Environment, Kyuden Sangyo Company, Fukuoka 813-0043, Japan

2 km were supplied by the higher geothermal gradients, which can reach 42.8 °C km−1 due to the complex geological setting. Keywords Northwestern Algeria . Thermal waters . Stable isotopes . Fluid-mineral equilibria . Hydrogeochemistry . Geothermometry . Deep circulation

Introduction Algeria belongs to the northern part of Africa, bordered by the Mediterranean Sea, and is situated between Morocco and Tunisia (Fig. 1a). Tectonically and seismically, the northern part of Algeria is located in the Alpine-Magrebide orogenic belt, which resulted from an active collision belt between the African and European plates. In northwestern Algeria, this vigorous tectonic activity has generated recent Mio-Plio-Quaternary volcanism and led to the emergence of several thermal manifestations exclusively related to the presence of a regionally active NE–SW trending active fault system of Miocene age (Thomas 1985; Yelles 2004). Previous studies conducted in the northwestern part of Algeria have shown that the hydrogeochemistry and location of thermal waters are strongly influenced by the regional geology (Issaadi 1992; Fekraoui 2007; Saibi 2009; Bahri et al. 2010; Bouchareb-Haouchine 2012). The main geothermal reservoir of the study area is Liassic limestone and dolomite sequence up to 500 m in thickness (Barkaoui 2014). The study area is located in the northwestern part of Algeria, in the socalled Occidental Tell, which extends for over 300 km from the Ouarsenis Mountains and Chlef basin in the east toward the Tlemcen Mountains in the west (Fig. 1b). The Mleta and Habra basins form a NE–SW sink that separates the coastal mountains and the Mediterranean Sea in the north from the

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Fig. 1 a Study area in northwestern Algeria. b Location of hot spring sampling points

Tellian thrust zones of Tessala and Beni-Chougrane Mountains further to the south. The thermal activity of the Occidental Tell consists of the BHammam^ thermal springs, travertine deposition, and sulfate precipitations around some of the thermal emergences and some hydrothermal alteration zones. The distribution of the hot springs is roughly parallel to the NE–SW fault system and very close to the Plio-Quaternary volcanism. Hammam Bouhdjar (BH1 and BH2), Hammam Rabbi (HB), Hammam Boughrara (BGR), and Hammam Bouhnifia (BHF1, BHF2, and BHF3) thermal springs are the main manifestations (Fig. 1b). The outlet temperatures of the thermal waters range between 42.9 and 67.1 °C, with an artesian flow rate of 6 to 9 L/s. The majority of the thermal waters are mainly used for balneology and medical treatments provided by thermal establishments. The aims of this study are as follows: (1) constructing a convincing geothermal conceptual model for the northwestern Algerian thermal manifestations based on hydrogeochemistry of collected samples and to highlight the water mineralization heterogeneity that results from the complex geological setting. However, different correlations were applied to show the interaction of hot waters with the host lithology based on dissolution and/or precipitation of minerals that play a role in key source control on the hot spring’s water chemistry. (2) Estimating the maximum reservoir temperatures using cationic and silica geothermometers, as well as silica-enthalpy and chloride-enthalpy diagrams to assess the rate of mixing with cold waters and/or the effects of a conductive cooling process during rising, and to confirm the evaluated reservoir temperatures estimated by chemical geothermometers, the fluidmineral equilibria were also used in this study, in terms of

the saturation state of some common geothermal minerals with their scaling in each hot spring. (3) Resolving the origin of the thermal waters in order to ascertain the relationship between Plio-Quaternary volcanism and these geothermal waters using isotopic data of δ18 O and δ D from five hot springs and meteoric waters.

Geological and tectonic setting The northern part of Algeria displays complex geological features belonging to the North African margin and is a part of the Alpine-Magrebide belt that extends from Gibraltar to SicilyCalabria (Domzig et al. 2006; Auboin and Durand-Delga 1971). The study area lies in the external zones (Fig. 2), so-called Tellian zones (Wildi 1983), which are mainly characterized by Miocene folds and Bnappes^ thrusted over the Atlasic foreland toward the south. In the north, the coastal shale massives of Cap Carbon and Cap Falcon consist of a granitic and metamorphic basement of Paleozoic age, overlain by Mesozoic clay and an evaporitic sequence. To the south, the Habra, Mleta, and Tafna basins of Miocene and Plio-Pleistocene age were essentially formed by sandstones, clay, and conglomerate developed during the early Miocene extensive phase. This extensional tectonic activity in the Mleta basin is characterized by a flat area of salt pan (Sebkha of Oran) mainly composed of a Messinian evaporitic sequence formed during the upper Miocene with the closure of Mediterranean Sea (Aifa et al. 2003). Since the Late Cretaceous, this salt pan has


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Fig. 2 Geological map of the northwestern Algeria (Fenet 1975)

followed its own geodynamic evolution that was affected by the NE–SW deep-seated fault. The Miocene post-thrust basins are mainly affected by NE– SW reverse faults and NE–SW en echelon folds developed during the late Miocene to the late quaternary compressive phase of deformation as a result of the closure of the Mediterranean Sea. This shortening between the Iberian and Eurasian plates is approximately 3 to 6 mm/year (Belabbes et al. 2008; Nocquet and Calais 2004; Serpelloni et al. 2007). These Miocene post-thrust basins are located between the Sebaa Shioukh thrust sheet unit and the coastal massive in the north. The Sebaa Shioukh unit is mostly composed of marine carbonates and dolomites of Cretaceous age that outcrop between the Tlemcen Mountains and the Mleta Miocene basin. In this area, the Triassic evaporitic basement rocks are bordering deep NE–SW faults and gave rise to limited extrusion of mainly gypsum and anhydrite deposits during the late Triassic age (Fig. 3b). Further to the south, the Saida Mountains consist of thick Jurassic dolomites and carbonates as a result of the superposition of several extensional and compressive phases of Alpine age since the late Cretaceous, and they extend to the Tlemcen Mountains with an orientation of ENE–WSW to E– W forming horst structures. In the northernmost part of the region, between the Ain Timouchent and Beni Saf areas, a post-collisional magmatic activity of calc-alkaline and alkali-basaltic type took place (Fig. 3a), which is mainly related to the regional tectonic process occurring along a NE-trending belt extending from the Alboran zone to the Tellian sector in Algeria and the middle Atlas and the Anti-Atlas in Morocco. This activity has a geochemical signature similar to an intra-oceanic island basalt (Chalouan 2008).

For this Neogene orogenic magma of northwestern Algeria, the age of the calc-alkaline type ranges from 7.5 to 11.5 Ma, while the age of the alkali-basaltic type ranges only from 0.8 to 3.9 Ma (Coulon et al. 2000, Louni-Hacini et al. 1995). These eruptions are present in the Sebaa Shioukh unit and Ain Timouchent area and near the Bouhdjar geothermal field (BH1 and BH2).

Methodology Seven thermal springs were sampled between February 5 and 10, 2013 in four geothermal fields (Bouhdjar, Boughrara, Bouhnifia, and Hammam Rabbi). Temperatures, pH, and electrical conductivities were measured in situ. All analyses for major ions Na+, K+, Ca2+, Mg2+ and SiO2 concentrations were measured by atomic absorption spectrophotometry, while SO42− concentrations were determined by spectrophotometer and HCO3 was determined using standard titration techniques. Cl− was analyzed by the AgNO3 titration method in the National Agency of Hydraulic Resources (ANRH-Algiers). The ionic balance provides an indication of the accuracy and reliability of the analysis and is given by X Balanceð%Þ ¼ X

Cationsðmeq=LÞ −

X

Anionsðmeq=LÞ 100 X Cationsðmeq=LÞ þ Anionsðmeq=LÞ

In this study, charge balance errors in all analyses were less than 5 %; analyses beyond this value were excluded from data set. The isotopes δ 18O and δ D were analyzed by mass spectrophotometry in the Department of Environment of Kyuden Sangyo Co., Inc. (Japan). The isotopic values are reported

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Fig. 3 Geological cross sections through profiles (AB and CD see Fig. 2)

using standard notation relative to NIST/IAEA reference material V-SMOW. The analytical precision is ±0.1 and ±1‰ for δ18 O and δ D, respectively.

Results and discussion The collected water samples are characterized by significant heterogeneity in their chemical-physical features (Table 1). The water temperatures ranged from 42.9 to 67.1 °C. Thermal waters show near-neutral pH values ranging from 6.2 to 7.53. Higher total dissolved solid (TDS) values from collected samples (up to 4002 mg/L) reflect longer circulation and residence times. According to the piper and ternary diagram (Fig. 4a, b), northwestern Algeria’s geothermal waters show four types mainly influenced by deep carbonate reservoir rocks and the interaction with saline salt flat water and/or the

dissolution of an evaporitic sequence near the upflow area of each hot spring. The alkali-chloride (Na+-Ca2+-Cl−) type is found at sites BH1 and BH2 in the Bouhdjar geothermal field. The diluted alkali-chloride-bicarbonate (Na+-Ca2+-Cl−-HCO3−) type is from the Bouhnifia geothermal fields, the alkali-sulfatecalcium (Na+-Ca2+-Cl−-SO42−) type is from the Hammam Rabbi geothermal field, and the alkaline carbonated (Na+HCO3−-Cl−) type is from the Boughrara geothermal field. In sedimentary environments such as the Tellian zones of northwestern Algeria, the sources of ions in thermal waters are more difficult to interpret because the mineralogical and chemical proprieties of the hosting reservoirs are inherently more heterogeneous, and physical and hydrological conditions are likely to be different and specific to each field. In view of this complexity, we attempt below to discuss the key control factors and processes influencing the evolution of northwestern Algerian thermal water.

Arab J Geosci (2016) 9:169 Table 1

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Chemical composition (mg/L) of collected hot springs of northwestern Algeria

Sample Station ID Temperature pH ID (°C)

Conductivity (μS/cm)

TDS (mg/L)

SO42− Cl− (mg/L) (mg/L)

HCO3− (mg/L)

K+ Na+ (mg/L) (mg/L)

Mg2+ (mg/L)

Ca2+ (mg/L)

SiO2 (mg/L) 65.7

BH1

Bouhjar

66

6.45 5720

3693

1650

60

656

46

950

35

310

BH2

Bouhjar

55.3

6.55 5880

4003

1890

30

622

60

1010

46

351

63.7

HB

Rabbi

45.9

6.81 2670

1742

398

486

320

13

300

32

234

43.9

BGR

Bouhghrara 42.9

7.53 730

492

69

21

296

7

103

13

28

26.0

BHF1

Bouhnifia

64.5

6.38 2380

1553

409

80

610

25

264

28

177

46.4

BHF2

Bouhnifia

66.1

6.2

2460

1523

397

97

586

25

255

24

201

45.4

BHF3

Bouhnifia

52.8

6.59 2350

2142

391

98

650

27

318

31

153

45.9

Na+ and Cl− contents of the collected thermal waters from northwestern Algeria are shown in the scatter diagram in (Fig. 5a). Thermal waters belonging to the Tellian zones show close correlation with the halite dissolution line. There is a positive correlation between Na+ and Cl− contents, while Cl−-rich waters are characterized by a Na+/Cl− ratio ∼1 (Fig. 5a). These thermal waters have a saturation index (SI) for halite (SI halite from −6.77 to −4.49; Table 3) that suggests they contain extensively leached soluble salts; however, they have not yet reached equilibrium with halite. According to Fig. 5b, c, most of the collected samples have a higher HCO3−/SO42− ratio and higher Ca2+ + Mg2+, indicating a carbonate and evaporitic source for the thermal waters. In Fig. 5b, higher Mg2+ contents of up to 46 mg/L (Table 1) indicate a near-surface reaction that leaches Mg2+ from the local Jurassic dolomite and alkali-basaltic rocks of the Tellian zones.

Additionally, Ca2+, Mg2+, and HCO3− can be derived from alteration of carbonates and silicates, as explained by the following reactions: CaMg ðCO3 Þ2 þ 2H2 CO3 Ca2þ þ Mg2þ þ 4 HCO3 − ð1Þ

Dolomite

Ca2þ ; Mg2þ ; Naþ ; Kþ silicates þ 2H2 CO3 − < ¼ > Ca2þ þ Mg2þ þ Naþ þ Kþ þ H4 SiO4 þ HCO3 − þ clay minerals

ð2Þ

Therefore, thermal waters are close to equilibrium with dolomite (SI dolomite of −0.16 to 0.65; Table 3). However, a scatter diagram of Ca2+-SO42− shows a weaker correlation (R2 =0.4) with the gypsum/anhydrite dissolution line. A typical Na+-Ca2+-Cl−-SO42− water of HB in Fig. 5c that is plotted

Fig. 4 a Piper diagram. b Ternary plot (anions species), showing the wide heterogeneity in chemistry of the collected thermal waters from northwestern Algeria

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Fig. 5 a Na+ vs. Cl−, b Ca2+ + Mg2+ vs. HCO3−/SO42− c Ca2+ vs. SO42−, e Ca2+ vs. HCO3−, f Cl− vs. HCO3− scatter diagrams of northwestern Algerian thermal waters, showing (as red lines) dissolution of halite, gypsum and calcite. d Na+/Cl− vs. HCO3−/ (Ca2+ + SO42−) plot, summarizing the key source process controlling the chemistry of collected thermal waters (data are given in meq/L)

a

b

c

d

e

f

along this line reveals dissolution of sulfate minerals likely hosted by a Triassic evaporitic sequence (rich in halite and gypsum/anhydrite) raised up along the horsts and grabbens of the Saida Mountains. Tellian zones of northwestern Algeria are mainly carbonates formed during the Jurassic and include Miocene marl and limestone. In fact, most of the thermal waters (BHF1, BHF2, and BHF3 in Fig. 5e) have a Ca2+/HCO3− ratio of ∼1, which is typical for waters that undergo calcite dissolution as a major mineralization process. Thermal water samples are consistently close to equilibrium with calcite (−0.36 < SI < 0.13). However, a Ca 2+ / HCO3− ratio >1 involves a source of Ca2+ other than

carbonates, which is explained by dissolution of sulfates and leaching of Ca2+ from Triassic Na+-Ca2+-Cl−-SO42− waters (e.g., HB) or a Miocene evaporitic source water of the Na+-Ca2+-Cl− type (e.g., BH1 and BH2). A fair correlation (R2 =0.7) exists between Cl− and HCO3− as seen in the scatter diagrams in Fig. 5f. In the Na+-Cl−HCO3− waters of BHF1, BHF2, and BHF3, the Cl−/HCO3− ratio is ∼1, indicating a probable dilution with shallow saline groundwater of the Miocene basin. Higher Cl−/HCO3− ratios >1 in BH1 and BH2 are exclusively linked to dissolution of halite from a salt pan evaporitic sequence or dilution with shallower groundwater rich in Cl− during ascension of deeper carbonated water.


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The key source processes controlling the chemistry of thermal water in northwestern Algeria are summarized in Fig. 5d. The data set confirms that the Na+ and Cl− contents of BH1, BH2, BHF1, BHF2, and BHF3 are mainly controlled by dissolution of saline Messinian formations rich in halite (Fekraoui 2007; Bouchareb-Haouchine 2012). Therefore, the Na+/Cl− ratios of most samples are buffered at ∼1. The thermal waters of the Tellian zones are shifted mostly to the right toward calcite dissolution (HCO3−/(Ca2+ + SO42−) ratio >10), which reflects the carbonated nature of the reservoir. However, a higher Na+/Cl− ratio in BGR is due to excess Na+ that is likely explained by the percolation of Na+ from altered sandstone and clays of the Tafna Miocene basin. Furthermore, this Na+ increase could be supplied by cation exchange processes with clays accompanied by Ca2+ deficiency (Davisson and Criss 1996). Table 2 Estimated temperatures (°C) of northwestern Algerian thermal waters using different cationic and silica geothermometers

Chemical geothermometry application Reservoir temperatures are a significant factor in the evaluation of the formation mechanism and utilization potential of geothermal systems. Solute geothermometers are valuable tools to estimate reservoir temperatures for these systems. Chemical geothermometers are based on fluid/mineral equilibria and provide the last temperatures of water-rock interaction attained in the reservoir. In this study, various silica and cationic geothermometers were used to infer the temperatures of northwestern Algerian geothermal systems, as shown in Table 2 and Fig. 6a, b: Na-K-Ca (Fournier and Truesdell 1973) (a), CCG (Nieva and Nieva 1987) (b), Na/K (Fournier 1979) (c), Na/K (Truesdell 1976) (d), Na/K (Tonani 1980) (e), Na/K (Giggenbach 1988) (f), K/Mg (Giggenbach 1988) (g), silica (Fournier and Potter 1982) (h), silica (Giggenbach 1992)

Geothermometers/ID sample

BH1

BH2

HB

BGR

BHF1

BHF2

BHF3

Na-K-Ca (Fournier and Truesdell 1973)a R Mg correction CCG (Nieva and Nieva 1987)b Na/K (Fournier 1979)c Na/K (Truesdell 1976)d Na/K (Tonani 1980)e Na/K (Giggenbach 1988)f K/Mg (Giggenbach 1988)g Silica (Fournier and Potter 1982)h Silica (Giggenbach 1992)i Chalcedony (Fournier 1992)j Silica no steam loss (Fournier 1977)k

102 15 −1051 150 162 121 148 181 23 115 93 86 126

109 17 −927 164 176 138 167 194 22 114 91 84 124

93 18 −1150 143 155 113 139 173 34 96 71 65 106

116 40 −284 174 186 150 180 203 36 74 46 42 82

124 20 −730 199 213 182 217 228 28 99 74 68 108

124 16 −765 202 215 186 221 231 27 98 73 67 107

122 23 −686 191 203 171 204 220 27 98 73 68 108

Cristobalite (α) (Fournier 1977)l Quartz (Verma 2000)m Silica max steam loss (Fournier 1977)n

65 111 114

63 109 113

46 90 98

24 66 78

48 93 100

47 92 99

48 92 99

Na-K-Ca (Fournier and Truesdell 1973): T=[1112/log (Na/K)+β*log (Ca/Na)+2.24]−273.15, β=1/3 as t 15 at BH1, BH2, HB indicates

water reached the surface rapidly, while a Na+/K+ R>5 [i.e., R=(Mg2+ ×100)/(Mg2+ +Ca2+ +K+)]. However, the Mg2+ correction is negative and cannot be applied. These results indicate that Mg2+ arises from cooler environments, and the Na-K-Ca contents were not greatly affected by mixing with cold groundwaters. The temperatures inferred by K/Mg (Giggenbach 1988) (g), which range from 22 to 36 °C, were much lower than


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use of immature waters to evaluate geothermal reservoir temperatures is not robust and confers only weak reliability of cationic geothermometers (Giggenbach 1988; Tarcan 2005). Geothermometers based on dissolved silica contents and the solubility of different silica species were applied to assess reservoir temperature. Quartz is generally applied in lowenthalpy situations that are more appropriate to northwestern Algerian thermal waters. Accordingly, silica (Fournier and Potter 1982) (h), quartz (Verma 2000) (m), and silica max steam loss (Fournier 1977) (n) provide a very close range of temperature (BH1 and BH2 from 109 to 126 °C; BHF1, BHF2, and BHF3 from 92 to 108 °C; HB from 90 to 106 °C; BGR from 74 to 82 °C), while silica (Giggenbach 1992) (i) and chalcedony (Fournier 1992) (j) provide a lower temperature than those yielded by quartz solubility, with a difference ranging from 20 to 30 °C. Temperatures estimated using cristobalite (α) (Fournier 1977) (l) are close to those measured at the surface. The use of silica (Fournier and Potter 1982) (h), quartz (Verma 2000) (m), silica max steam loss (Fournier 1977) (o), silica (Giggenbach 1992) (i), and chalcedony (Fournier 1992) (j) may be accepted as more reasonable than the use of Na/K and Mg-based geothermometers to estimate reservoir temperatures for the immature water type (Gokgos and Tarcan 2006; Shakeri et al. 2008; El-Fiky 2009; Matlu and Kilic 2009; Joseph et al. 2011). The use of Na-K-Ca (Fournier and Truesdell 1973) (a) yields a good result and temperatures very close to equilibrium with silica geothermometers in NaCl and Ca-SO4 waters of BH1, BH2, and HB, while the slightly higher temperatures observed are due to the calcite dissolution in carbonated Na-HCO3 waters of BHF1, BHF2, BHF3, and BGR.

Fig. 7 The Na-K-Mg1/2 triangular plot for thermal waters collected in the northwestern Algeria combining fast-responding K/Mg geothermometer with non-equilibrating Na/K geothermometer (Giggenbach 1988)

temperatures measured at the surface, and they reveal the fast equilibration of Mg after mixing with groundwaters (Giggenbach 1988; Pasvanoğlu and Chandrasekharam 2011). Thus, conductive cooling and mixing with cooler Mg-rich waters has a much greater influence on the temperature decrease of geothermal waters as it flows upward to the spring vents (Han et al. 2010; Pasvanoğlu 2013). The ternary diagram of Na/1000-K/100-Mg1/2 (Fig. 7) suggested by Giggenbach (1988) is used to estimate the reservoir temperatures and to select the waters most suitable for geothermometry, by recognizing the fluid maturity of waters that have attained equilibrium with the host lithology and to demonstrate the mixing effect. All collected thermal waters from northwestern Algeria fall in the immature water field, rather close to the Mg1/2 corner. Therefore, this plot may result from the mixing of fully equilibrated or partly equilibrated thermal waters with Mg-rich cooler groundwaters that come from dissolution of the widespread Jurassic dolomitic sequence of the Tellian zones and leaching of Mg into the waters. However, the Table 3

Fluid-mineral equilibria The water-rock interaction in the northwestern Algerian thermal manifestations was studied by using the PHREEQC software (Parkhurst and Appelo 1999). The results of spring discharge temperatures are reported in Table 3. Furthermore, collected samples were assessed in

Results of saturation indices with respect to hydrothermal minerals at the discharge temperature of the collected northwestern thermal waters

Sample ID

SI anhydrite

SI aragonite

SI calcite

SI chalcedony

SI chrysotile

SI dolomite

SI gypsum

SI quartz

SI SiO2(a)

SI halite

SI CO2(g)

BH1 (66.1 °C)

1.58

0.39

0.5

0.18

−4.7

0.56

−1.66

0.5

−0.54

−4.59

−0.43

BH2 (55.3 °C) HB (45.9 °C) BGR (42.9 °C) BHF1 (64.5 °C) BHF2 (66.1 °C) BHF3 (52.8 °C)

−1.98 −0.86 −2.77 −1.5 −1.35 −1.59

0.38 0.11 0.07 0.15 0.02 0.17

0.5 0.24 0.2 0.27 0.14 0.3

0.27 0.19 −0.01 0.04 0.01 0.14

−4.81 −5.11 −2.06 −5.64 −6.81 −5.43

0.65 0.09 0.57 0.23 −0.16 0.42

−1.95 −0.76 −2.65 −1.56 −1.42 −1.55

0.61 0.56 0.37 0.36 0.33 0.49

−0.48 −0.58 −0.79 −0.68 −0.7 −0.61

−4.49 −5.62 −6.77 −5.69 −5.72 −5.6

−0.64 −1.22 −1.95 −0.36 −0.19 −0.63

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terms of their saturation states of some common hydrothermal minerals present in the reservoir. The results are estimated as an approach to the (1) determination of scaling potential and (2) reservoir temperatures. As indicated in Table 3, the collected thermal waters are oversaturated with respect to aragonite, calcite, dolomite, quartz, and chalcedony and undersaturated with respect to halite, gypsum, anhydrite, amorphous silica, and CO2(g). Chalcedony, in particular, is close to equilibrium in BGR, BHF1, and BHF2. The presence of travertine and some silica deposits around BH1 and BH2 (Hammam Bouhdjar) and

b 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0

2.0

Anhydrite

BGR BH1 BH2 HB BHF1 BHF2 BHF3

Precipitation

Dissolution

Aragonite


1.5 1.0

SI

SI

a

0.5

Precipitation

0.0

Dissolution

-0.5 -1.0 0

50

100

150

0

200

50

Temperature (°C)

c


SI

SI

1.0 0.5

Precipitation 0.0

Dissolution

-0.5 0

50

100 150 Temperature (°C)

0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4

200

200

Chalcedony Precipitation

Dissolution

0

50 100 150 Temperature (°C)


200

f BGR BH1 BH2 HB BHF1 BHF2 BHF3

Chrysotile Precipitation

SI

SI

10 8 6 4 2 0 -2 -4 -6 -8 -10 -12 -14

150

d

2.0 1.5

e

100

Temperature (°C)

Calcite

Dissolution

0

50

0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0 -2.2

CO2 (g) BGR BH1 BH2 HB BHF1 BHF2 BHF3

Precipitation

Dissolution

0

100 150 200 Temperature (°C)

g

50


200

h 1.5

Dolomite

1.5


1.0 Precipitation 0.5

1.0

-0.5

Dissolution

-1.0

Quartz

0.5

SI

0.0

SI

Fig. 8 Changes in the saturation states of the selected minerals vs. temperatures in thermal waters of northwestern Algeria. a Anhydrite, b aragonite, c calcite, d chalcedony, e chrysotile, f CO2(g), g dolomite, h quartz. SI> 0 precipitation, SI=equilibrium, SI>0 dissolution

BHF1, BHF2, and BHF3 (Hammam Bouhnifia) are likely due to precipitation of silica and calcite in the shallower part of the upflow zone. Chemical analyses on Hammam Bouhdjar (in weight %) performed by Issaadi (1992) yielded the following results: 3.58 % SiO2, 0.11 % Al2O3, 0.6 % Fe2O3, 0.05 % MnO, 42.9 CaO, 6 % MgO, 0.08 Na2O, and 0.1 % TiO2. These data confirm the mineral oversaturation found in this study and commonly predicted precipitation of calcite and aragonite at temperatures above 40–50 °C (Fig. 8b, c), quartz below 75–125 °C, chalcedony below 50–100 °C (Fig. 8d, h), and dolomite above 25–60 °C.

Precipitation

0.0 -0.5

Dissolution

-1.5 -1.0

-2.0 0

50

100

150

Temperature (°C)

200

0

50


200



Page 11 of 17 169

Undersaturation of waters in halite, gypsum, and anhydrite at temperatures less than 160–100 °C (Fig. 8a) suggests the dissolution of these minerals in the thermal waters that is further attested to by the absence of saline deposits in the spring vent.

At Hammam Boughrara (BGR), chalcedony and aragonite intersect with each other at equilibrium line SI=0, and the assessed temperature of 42 °C is close to that of the spring discharge temperature, while the quartz and chrysotile Mg3Si2O5(OH)4 curves intersect at 66 °C, which is the

b

a

6

10 BGR Anhydrite Calcite Aragonite Chalcedony Dolomite Chrysotile Quartz CO2(g)

6 4

Discharge temperature (42°C)

SI

2 0

4

(67°C) Anhydrite Aragonite Calcite Chalcedony Chrysotile Dolomite Quartz CO2(g)

0 -2 -4 -6

-2

-8

-4

m Quartz

(66°C)

h Silica

-12 0

50

100

150

200

0

50

d

6 4

BH2

SI

-4 (109°C)

-6

m

-8

Quartz (Verma, 2000)

Anhydrite Aragonite Calcite Chalcedony Chrysotile Dolomite CO2(g) Quartz

(64°C)

-4 -6 -8

k Silica

(105°C) ~ no steam loss (Fournier, 1977)

-10

-10

-12

-12 0

50

100

150

0

200

50

e

f (67°C)

0 -2 -4 (95°C) ~ h Silica (Fournier and Potter, 1982)

-6 -8

Anhydrite Aragonite Calcite Chalcedony Chrysotile Dolomite Quartz CO2(g)

150

200

6

BHF3

4

Discharge temperature

2

(52°C)


0 -2 SI

BHF2

100

Temperature (°C)

Temperature (°C)

SI


-2 SI

0

(113 to 118°C)

-4 -6

-10

-8

-12

-10

-14

-12 0

50

100

150

200

0

50

100

150

200

Temperature (°C)

Temperature (°C)

g HB

4

Discharge temperature (49°C)

2 0

SI

200

0

-2

6

150

4 BHF1 2

(52°C)

2

2

100

Temperature (°C)

Temperature (°C)

c

(116°C) ~ (Fournier and Potter, 1982)

-10

(Verma, 2000)

-6 -8

Discharge temperature

BH1

2

SI

8

-2 -4 -6

k Silica

(106°C) ~ no steam loss (Fournier, 1977)


-8 -10 0

50

100

150

200

Temperature (°C)

Fig. 9 Mineral equilibrium diagrams for collected thermal waters of northwestern Algeria. (Discharge temperature in red arrows corresponds to chalcedony-aragonite or calcite intersections at SI=0;

reservoir temperatures shown in black arrows corresponds to quartzchrysotile or CO2(g) at SI=0)


Page 12 of 17

Fig. 10 Silica-enthalpy mixing model (Truesdell and Fournier 1977). Dashed line in red from a′, b′, c′, d′, and e′ indicate estimated reservoir temperatures. Psa′, Psb′, and Psc′ represent original silica content in the reservoir, see Table 4

BGR BH1 BH2 BHF1 BHF1 BHF2 BHF2 BHF3 BHF3 HB HB BH1

500

ml

419 KJ/kg

no s

300

tea

xim

SiO2 (mg/L)

um s

400

oss

tea m

los s

BH2

ma

169

200

100

SiO2 at a’

a

a’

SiO2 at PSa’

b c

b’ PSa’ c’ d’ PSb’ PSc’ e’

d e

a’: Estimated reservoir temperature for BH1~149°C. PSa’: Silica content for parent fluid of BH1~134 mg/L. b’: Estimated reservoir temperature for BH2 and HB~141°C. PSb’: Silica content for parent fluid of BH2 and HB ~106 mg/L. c’:Estimated reservoir temperature for BHF3~137°C. PSc’: Silica content for parent fluid of BHF3~95 mg/L. d’:Estimated reservoir temperature for BHF1 and BHF2~144°C. e’:Estimated reservoir temperature for BGR~130°C.

Steam loss before mixing

no steam loss before mixing

M

0 0

200

400

600

800

1000

1200

1400

1600

ENTHALPY (KJ/kg)

reservoir temperature estimated by quartz (Verma 2000) (m). At BH1, CO2(g) and quartz curves intersect with the equilibrium line at SI=0 and infer a reservoir temperature of 116 °C that is close to that estimated by silica (Fournier and Potter 1982) (h), while the chrysotile and quartz curves are connected at equilibrium at 125 °C as estimated by silica no steam loss (Fournier 1977) (k). At BH2, quartz and chrysotile intersect at the equilibrium line at 109 °C, which is estimated by quartz (Verma 2000) (m). In most of the collected springs, the intersection of chalcedony with aragonite and/or calcite commonly yields a temperature close to the discharge temperatures. This is likely the most suitable thermal condition for a sedimentary environment where the deep-seated water reaches equilibrium with chalcedony at the surface. BHF1 and BHF2 attain equilibrium at 105 and 95 °C, respectively, where dolomite and anhydrite and CO2(g) intersect close to SI=0 at 162 °C in BHF1 (Fig. 9e) and BHF3 reaches equilibrium between 113 and 118 °C close to Na-K-Ca (Fournier and Truesdell 1973) (a). A probable interpretation is that the thermal waters of

Table 4 Results of estimated temperatures (in °C) and original silica content (mg/L) of the reservoir fluid obtained from SiO2-enthalpy model

Bouhnifia (BHF1, BHF2, and BHF3) interacted with the dolomitic sequence of the Jurassic layers and the raised Triassic CaSO4 minerals at 2 km, and they therefore mixed with shallower carbonated waters during the process of rising. HB attains equilibrium at 106 °C according to the silica no steam loss geothermometer (Fournier 1977) (k). However, a range of 66 to 125 °C zone of overlap between silica geothermometers and fluid-mineral equilibria is mostly representative of a geothermal reservoir for the collected northwestern Algeria thermal springs.

Mixing models Silica enthalpy An enthalpy-silica mixing model suggested by Fournier and Truesdell (1974) and Truesdell and Fournier (1977) was applied to infer the temperature of the hot water component in

Sample ID

Silica concentration in the discharge spring (mg/L)

Silica concentration in the reservoir fluid (mg/L) inferred from the enthalpySiO2 model

Fraction of hot water in the mixed discharge (Xs)

Estimated reservoir temperature determined from the enthalpy-SiO2 model (°C)

BHF1 BHF2 BHF3 BH1 BH2 HB BGR

46.38 45.38 45.88 65.72 63.74 43.90 26.04

103.12 103.12 95.31 134.37 106.25 106.25 70.31

– – 0.15 0.14 0.15 0.15 –

144 144 137 149 141 141 130

M meteoric water sample/T 15 °C, SiO2 10 mg/L


Page 13 of 17 169 Conductive heating

Fig. 11 Chloride-enthalpy mixing model for northwestern Algerian collected thermal waters showing reservoir temperatures (PGL1 and PGL2) with mixing and conductive cooling trends

To parent fluid

Steam gain

3000

Steam loss

Dilution or mixing

Steam point 2775 KJ/kg

Conductive cooling

BGR

BGR BH1 BH2 BHF1 BHF1 BHF2 BHF2 BHF3 BHF3 HB HB

2500

BH1

Enthalpy (KJ/kg)

BH2

2000 Ad

iab

ati

cc

1500

oo

lin

g

1000 Parent Geothermal Liquid (PGL2) 123°C~ Na-k-Ca

500

Conductive cooling meteoric water box

Parent Geothermal Liquid (PGL1) 108°C~ quartz (Verma, 2000)

g

in Mix

Conductive cooling

BHC

0 0

500

1000

1500

2000

2500

Chloride (mg/L) BHC: cold groundwater near BH1 and BH2

mixed water and to estimate reservoir temperatures of northwestern Algerian thermal waters. The application is dependent upon three main basic assumptions: No silica deposition or dissolution occurred after mixing, quartz controls the solubility of silica in the thermal water, and no heat loss took place after mixing. Enthalpy values used to estimate northwestern Algerian reservoir temperatures were derived from measured spring discharge temperatures and the steam table (Keenan et al. 1969), and meteoric water was used as the reference cold water. Relating the cold sample water to thermal waters, within separation and steam escape (100 °C, 419 kJ/kg), yields points a, b, c, d, and e in Fig. 10. Thus, the reservoir temperature is assessed by connecting horizontal lines to the quartz solubility curve corresponding to the maximum steam line in a′ for BH1, b′ for BH2 and HB, and c′ for BHF3, while a vertical line from a′, b′, and c′ intersected with the quartz solubility no steam loss curve at Psa′, Psb′, and Psc′, indicating the original silica Table 5 Isotopic results of δ D (‰) and δ18 O (‰) relative to SMOW, with altitude (m above sea level) of collected northwestern Algerian thermal waters Geothermal field (Hammam)

Sample ID

δD (‰)

δ18 O (‰)

Altitude (m ASL)

Hammam Bouhnifia

BHF1

−47

−7.1

263

Hammam Rabbi

HB

−56

−8.1

637

Hammam Bouhdjar

BH1

−48

−7.6

161

Hammam Bouhdjar

BH2

−47

−7.2

161

Local meteoric water

M

−10

−2.6

–

Hammam Boughrara

BGR

−48

−7.2

263

concentration in the reservoir (see Table 4). The fraction of the hot water component in the spring discharge lost as steam before mixing (Xs) is given by Xs = 1 − (SiO2 at (PSa′, PSb′, PSc′)/SiO2 at (a′, b′, c′), and it ranged between 0.14 and 0.15. The intersection of cold meteoric water with the no steam loss curve yields estimated temperatures d′ (BHF1 and BHF2) and e′ (BGR), which indicates the composition of the parent geothermal water prior to mixing. Results obtained by the silica-enthalpy model were slightly higher than those estimated by different silica geothermometers and ranged between 130 and 149 °C. These results denote that this overestimated temperature is likely due to silica precipitation caused by mixing with cold water and cooling. Chloride enthalpy The enthalpy-chloride mixing model of Fournier (1979) is very useful for characterizing the parent geothermal liquid for northwestern Algerian thermal waters and delineating their upflow and cooling process in order to determine the hydrologic complexities of the different hydrothermal systems and to estimate the reservoir temperatures (Mutlu 1998; Guo et al. 2009; Guo and Wang 2012). Two principal trends can be inferred from (Fig. 11): (1) conductive cooling, where the fluid loses heat to the surrounding host rock and enthalpy increases but the chloride content remains constant. This process is detected in both low and high Cl− springs (BHF1, BHF2, BHF3, HB, and BH2). (2) Mixing with cold groundwater that is less enriched in Cl−, which produces a decrease in the enthalpy and Cl− content. This process occurred clearly at BH1, which is mixed with BHC (cold groundwater near Hammam Bouhdjar). Chlorides are often used to estimate


Page 14 of 17

D (‰ vs SMOW)

0

a

Local Meteoric Water

BGR BH1 BH2 BHF1 HB

SMOW

-10 )

93

L 19 W al., GM i et k ns za

-20 -30

BGR

BGR BH1 BH2 BHF1 HB

BH1 BH2

-20

BHF1 BHF2

o

(R

b

-10

D (‰)

169

-40

BHF3

-30

HB

-40 -50

-50

-60

100

200

300

400

500

600

700

Altitude, m ASL

-60 -8

-6

-4

-2

0

18O (‰ vs SMOW)

c

d

BGR

BGR BH1 BH2 BHF1 HB

BH1

2000

BGR

BH2

Group 1

BHF1 BHF2

BHF2 BHF3

HB

HB

Cl (mg/L)

Cl (mg/L)

Group 1

1000

1500

1000

Group 2

Group 2

500

Local Meteoric Water

-7

-6

500

Local Meteoric Water Group 3

Group 3

0 -8

BHF1

BHF3

1500

-9

BGR BH1 BH2 BHF1 HB

BH1

2000

BH2

0 -5

-4

-3

-2

18O (‰ vs SMOW)

-60

-50

-40

-30

-20

-10

D (‰ vs SMOW)

Fig. 12 Isotopic composition of thermal waters and local meteoric water from northwestern Algeria (a); all compositions lie close to global meteoric world meteoric line (GMWL); inserted plots indicate

variations of δ D (‰) with altitudes at spring discharge (b). c, d The relationship between Cl− vs. δ D (‰) and Cl− vs. δ18O (‰)

the mixing ratio as they do not chemically react even at high temperatures (Han et al. 2010). The mixing ratio in Hammam Bouhdjar is calculated using this equation:

geothermal liquid (PGL2) of BHF1, BHF2, and BHF3 of Hammam Bouhnifia yields temperatures of approximately 123 °C, which is close to Na-K-Ca (Fournier and Truesdell 1973) (a) because Na, K, and Ca are less affected by mixing and dissolution of Messinian saline minerals rich in Na.

− − Cl − Cl 100ð%Þ R ¼ T− MIX ClT − Cl−C

Isotope geochemistry where R is the mixing ratio, expressed as the percentage of non-thermal groundwater, Cl−MIX is chloride content in the mixed thermal water BH1, which was approximately 1650 mg/L, [Cl−T] is the chloride content in the thermal water BH2, which was approximately 1890 mg/L, and [Cl−C] is the chloride content in cold groundwater BHC, which was approximately 1128 mg/L. The estimated mixing ratio in Hammam Bouhdjar is approximately 31 %. This high percentage reflects the greatest Cl− concentration in thermal water supplied by a salt pan (Sebkha of Oran) through dissolution of Na and Cl− from halite minerals and the presence of NE– SW trending fault zone structures that increase the mixing ratio. The estimated temperatures of the parent geothermal liquid (PGL1) of BH1 and BH2 are both approximately 108 °C, which are equal to those estimated by quartz (Verma 2000) (m) in the two hot springs. However, the parent

The isotopic signature of thermal waters can act as a tracer of the fluid origins and reservoir processes in geothermal systems (Craig et al. 1956; Craig 1963; White 1986). The isotopic data of δ18 O and δ D are reported in Table 5, and they range between −8.1 to −7.1‰ and −56 to −47‰, respectively (Fig. 12a). Collected northwestern Algerian thermal waters fall along the Global World Meteoric Line (GMWL; Fig. 12a), the equation for which is δ D=8.13 δ18 O+10.8 (Rozanski et al. 1993), indicating a meteoric origin for the thermal waters. The absence of an oxygen shift toward a positive value indicates that there is no isotopic exchange with the host rock and that the thermal waters belong to a low-enthalpy resource. Thus, waters infiltrated through a deep-seated fault network and became heated during deep advective flow. The recharge


Page 15 of 17 169

Fig. 13 Conceptual evolution model for the northwestern Algerian thermal waters

probably takes place at fractured Jurassic limestone. However, depletion of δ18 O toward a less negative value at BH1 compared to BH2 is likely due to mixing with shallower cold groundwater with lower Cl− content, mainly by carbonated aquifers of Miocene age of Hammam Bouhdjar, as indicated in group 1 (Fig. 12a, c). The downwards shifting of δ D of the collected samples relative to local meteoric water reflects the high altitude of the spring discharge and distance from seawater, i.e., HB 637 m ASL, δ D=−56‰ in (Fig. 12b). The light isotopic value of δ D in group 1 with the higher Cl− content water (BH1 and BH2) is due to a long residence time and a deeper circulation. Tritium values from Hammam Bouhdjar BH1 and BH2 vary between 3.1 tritium unit (TU) and 3.5 TU, while those of meteoric water of northern Algeria can reach 50 TU (Issaadi 1992). However, thermal waters with tritium content