Silica Geothermometer Was Applied to Estimate Reservoir Temperature [11], Which Ranges from 96°C for Padagi Kaur and 144°C

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INVESTIGATION OF ORIGIN, SUBSURFACE PROCESSES AND RESERVOIR TEMPERATURE OF GEOTHERMAL SPRINGS AROUND KOH-I-SULTAN VOLCANO, CHAGAI, PAKISTAN

Manzoor Ahmad Muhammad Rafiq Naveed Iqbal Muhammad Rafique Muhammad Fazil .

Isotope Application Division Directorate of Technology Pakistan Institute of Nuclear Science and Technology P.O. Nilore, Islamabad, Pakistan July, 2009

Abstract

There are many geothermal sites in Pakistan having low to high temperature (boiling water). In Chagai area, seven springs with maximum surface temperature of 32.2°C located in the vicinity of Miri Crater of Koh-i-Sultan Volcano were investigated using isotope and chemical techniques. Two springs of Padagi Kaur are MgSO4 type, while all the other springs at Batal

Kaur, Miri Kaur and Chigin Dik are Na–Cl type. Alteration of water to SO4 type takes place by absorption of magmatic H2S and the acidic solution is further responsible to dissolve rock salt and carbonate minerals. EC increases from Padagi springs (4940 and 8170 µS/cm) to Chigin Dik springs (45600µS/cm). Chagai thermal manifestations receive recharge from meteoric waters in the vicinity of Padagi Kaur (east side of Miri Crater), which is heated by the hot magma chamber of Koh-i-Sultan most probably through deep circulation. Movement of the thermal water is from Miri Crater towards Chigin Dik area. Residence time is more than 60 years. The thermal waters do not have any contribution of shallow young groundwater and they have high 18O–shift (6 to 8‰) due to rock–water interaction. Reservoir temperatures estimated by different chemical geothermometers like Na–K, Na–K–Ca, Na–K–Mg1/2

(triangular plot) are quite high (200–300°C), while the silica and ∆(SO4–H2O) geothermometers give relatively low temperature ranges (107–144°C and 112–206°C respectively).

Key words: Koh-i-Sultan, geothermal waters, origin, subsurface history, reservoir temperatures, isotope geothermometers; chemical geothermometers.

CONTENTS

1. INTRODUCTION ------1 2. DESCRIPTION OF THE AREA AND GEOTHERMAL MANIFESTATIONS ------2 3. GEOLOGY ------3

3.1. Stratigraphy ------3

3.2. Geological Structure ------6

3.3. Heat source ------7 4. MATERIALS AND METHODS ------8 5. RESULTS AND DISCUSSION ------9

5.1. Chemical facies ------9

5.2. Origin and evolution ------10

5.3. Residence Time ------15

5.4. Reservoir temperature ------16

5.4.1. Chemical geothermometry ------16

5.4.2. Isotope Geothermometer ------17

5.5. Conceptual Model ------18 6. CONCLUSIONS ------20

ACKNOWLEDGMENTS ------21

REFERENCES ------22

INVESTIGATION OF ORIGIN, SUBSURFACE PROCESSES AND RESERVOIR TEMPERATURE OF GEOTHERMAL SPRINGS AROUND KOH-I-SULTAN VOLCANO, CHAGAI, PAKISTAN

1. INTRODUCTION

Most of the high enthalpy geothermal resources of the world are within seismic belts associated with zones of crustal weakness such as plate margins and centers of volcanic activity. A global seismic belt passes through Pakistan and the country has a long geological history of geotectonic events: Permocarboniferous volcanism (Panjal traps in Kashmir) as a result of rifting of Iran-Afghanistan microplates (Sillitoe and Khan, 1977), Late Jurassic to Early Cretaceous rifting of the Indo-Pakistan Plate (Powell, 1979), widespread volcanism during Late Cretaceous (Deccan traps) attributed to the appearance of a "hot spot" in the region, emergence of a chain of volcanic islands along the margins of the Indo-Pakistan Plate (Kazmi, 1979; Dejong and Subhani; 1979), collision of Indian Ocean plate Eurasia (Late Cretaceous–Paleocene) and the consequent Himalayan upheaval (Powell, 1979; Khan et al., 1987), and Neogene–Quaternary volcanism in the Chagai District (Koh-i- Sultan Volcano) (Arthurton et al., 1979). This geotectonic framework suggests that Pakistan should not be lacking in commercially exploitable sources of geothermal energy. This view is further strengthened by the fairly extensive development of alteration zones and fumaroles in many regions of Pakistan, presence of a fairly large number of hot springs in different parts of the country, and indications of Quaternary volcanism. These manifestations of geothermal energy are found within the following three geotectonic or geothermal environments (Bender and Raza, 1995): Geopressurized systems related to basin subsidence, Seismotectonic or suture-related systems, Systems related to Neogene–Quaternary volcanism.

The geothermal springs in the Chagai district, Balochistan emerge from the vicinity of Koh-i-Sultan Volcano, which is a system related to Neogene–Quaternary volcanism. It is the least developed area in the country. The thermal springs were studied by Geological Survey of Pakistan (GSP) in terms of geological setting and general chemical characteristics of groundwater compositions (Todaka et al., 1988). A study on nature and evolution of geothermal water such as the origin, age, water/rock interaction or mixing between geothermal fluids and shallow groundwater and assessment of reservoir temperature was conducted by Pakistan Institute of

Nuclear Science and Technology under IAEA/RCA Project RAS/8/092 “Investigating the Environment and Water Resources in Geothermal Areas”, which is reported.

A set of water samples was collected from Chagai area in June 2003 and analyzed for various isotopes and chemical concentrations. The isotopes 2H, 18O & 3H of water, 13C of dissolved inorganic carbon (DIC), 18O & 34S of sulphates were used in the study. Such applications of isotopic and geochemical techniques for geothermal investigations were discussed by many scientists (Giggenbach et al., 1983; Giggenbach, 1992; Truesdell and Hulston, 1980; Krouse, 1980). Various geochemical and isotope geothermometers applicable under different conditions established by (Fournier and Truesdell, 1973; Fournier, 1977; Fournier and Potter, 1979; Arnorsson, 1983; Tonani, 1980; Fouillac and Michard, 1981; Giggenbach et al., 1983; Giggenbach and Goguel, 1989; Kharaka and Mariner, 1989) were applied to estimate reservoir temperatures.

2. DESCRIPTION OF THE AREA AND GEOTHERMAL MANIFESTATIONS

Topography of Chagai area comprises a vast sandy plain interrupted by low hills and isolated hillocks locally called ‘dik’. Miri Koh is the highest in the area with an altitude of 2332 m. The surrounding plain has an altitude ranging between 750 m and 900 m. The gradients are very gentle. Radial pattern of drainage is dominant as all the streams radiate away from the top of the volcanic cones (Todaka et al., 1988). The area can be classified as arid region since it receives scanty rainfall (about 160 mm/annum). Summers are very hot and winters are very cold. There are several hot springs in this area.

The Koh-i-Sultan volcano in Chagai area is shown in and Fig. 1 (Arthurton et al., 1982) and the thermal springs at Batal Kaur, Miri Kaur and Padagi Kaur located in the close vicinity of Miri Crater are given in Fig. 2 (Bender and Raza, 1995) and Plates 1-4. Five springs seep from the beds of rainwater-courses, which consist of lava and agglomerate of the Koh-i-Sultan Volcanic Group. The water temperature ranges from 25.6 to 34.8°C, which is lower than the ambient temperature in summer season. The acidic alteration zone is distributed to southwest of the Miri peak. The rocks along the Miri Kaur are strongly silicified. Hydrogen Sulphide gas is also noted in places, and the sulphur deposits, which are formed by Hydrogen Sulphide gas are

scattered in the alteration zone. Two spring waters in the alteration zone studied by GSP (Todaka et al., 1988) were strongly acidic with pH 2–3 but most of these springs were completely dry during the samplings of this investigation except one having pH 1.8. The other springs in the alteration zone show neutral pH i.e. 6.1–7.9.

The Chigin Dik Springs (one at Onyx Marble Quarry and one at Chigin Dik called Burbur spring) are located at about 10 km SE of Mashki Chah (Fig. 1). Their altitudes are 830 m and 1003 m respectively. One spring originates from a more or less 100 m deep hole drilled by Pakistan Industrial Development Corporation for iron ore exploration around Mashki Chah. It discharge water intermittently like a 2–3 m high fountain and sometimes its water table goes down up to 2 m depth. Its EC is 34500 µS/cm. The surface rocks are recent deposits. However, the hole may penetrate the Chagai Intrusion or the Sinjrani Volcanic Group. The other spring is located in the nearby plain, which keeps on shifting the position after clogging the path due to deposition of mineral salts (mostly CaCO3). It is also highly saline having EC values 44800 and 45600 µS/cm in the first and second sampling.

3. GEOLOGY

3.1. Stratigraphy

The Chagai volcano-intrusive complex occurs in a region constituted by three principal structural and tectonic zones. These are marked by three major physiographic units of the region, namely: Chagai Hills, Ras Koh Range and Mirjawa Range. The stratigraphic classification of the investigated area is as following (Arthurton et al., 1979; Hunting Survey Corporation Ltd. Hunting, 1960).

Sinjrani Volcanic Group is the oldest unit about 1000 m thick. Massive lavas dominate the volcanic pile through tuff and agglomerates, and locally thin-bedded limestone is also present, particularly in the upper part of the sequence. These volcanic rocks in general are well cemented and hard. The group has been assumed to be Cretaceous on the basis of its stratigraphic position.

Humai Formation is assigned Late Cretaceous age on the basis of fossil content. It consists of limestone, shale, sandstone, conglomerate, tuff, and subordinate lava.

Jazzak Formation is believed to be mainly Paleocene and thickness of the formation is more than 2400 m containing mainly interbedded shale, sandstone, volcanic rock, and subordinate limestone.

Koh-i-Sultan Volcanic Group consists at least three volcanic cones with central craters with many smaller, satellite cones or plugs in addition. Age-wise, the group is divided into bassal agglomerate, top agglomerate, coarse pyroclastic deposit, tuff, lava flow, and ash flow. The group is unlikely to be older than Pleistocene. Subrecent deposits are semi-consolidated to loosely consolidated sand, gravel and pebble deposits. Recent deposits are poorly sorted alluvial sediments consisting of sand and silt, which occur in dunes and playas. Pebbles, cobbles and boulders occur in dry washes.

Miri Crater of Koh-i-Sultan Geothermal Spring at Batal Kaur

Chigin Dik Spring-1 Chigin Dik Spring-2

Fig. 1. Location and geological sketch map of Chagai area (Arthurton et al., 1982)

Fig. 2. Geological sketch of the Koh-i-Sultan volcanogenic geothermal zone (Bender and Raza, 1995)

Chagai Intrusions consist extensive batholiths and stocks in the Chagai Hills. The age range of these intrusions is doubtful. On the basis of fission track method, the major part of Chagai intrusions was emplaced during Oligocene and Miocene.

The area is located in the Chagai and Sultan Arcs. Initially the Chagai Arc was formed by the Cretaceous volcanic products (Sinjrani Volcanic Group) erupted on the southern margin of the Afghan block and later was covered by the sedimentary rocks (the Humai and Jazzak Formations etc.) and intruded by plutonic rocks (the Chagai Intrusions).

3.2. Geological Structure

This area is located in the Chagai and Sultan Arcs as shown in Fig. 3 (Ahmad and Dasht-e-Kain, 1986). Initially the Chagai Arc was formed by the cretaceous volcanic products (the Sinjrani Volcanic Group) erupted on the southern margin block and later was covered by the sedimentary rocks (the Humai and Jazzak Formations etc.) and intruded by the plutonic rocks (the Chagai Intrusions). During the Quaternary age, the Sultan Arc extended southwest from. Koh-i-Sultan in Pakistan through Koh-i-Taftan to Koh-i-Bazman in Iran (Fig. 4). It intersects the Chagai Arc obliquely.

The accurate pattern of major deformation axis is shown in Fig. 1. Uplifted areas of the Chagai Arc and the Ras Koh geanticline parallel the axis of the Mirjawa- Dalbandin trough. These tectonic movements appear to have recurred in Quaternary age after the present structure is assumed to be established in Neogene time (Arthurton et al., 1979). This area belongs to the uplifted north of the trough.

No fault is traceable in this area because the Quaternary rocks are widespread. However, two fault systems of NE-SW and NW-SE dominate in the Sinjrani Volcanic Group, which is distributed in the Chagai Hills (Bender and Raza, 1995). Besides, many small NE-SW lineaments and subordinate NNW-SSE lineaments from, the aerial photographs are observed in this area. It is assumed that the NE-SW and NW-SE faults run in the Sinjrani Volcanic Group, which is covered by the Koh-i-Sultan Volcanic Group.

Fig.3. Map showing Chagai and Sultan Arcs and Quaternary volcanic centers (Todaka et al., 1988)

Fig.4. Map showing Quaternary volcanic from Koh-i-Sultan in Pakistan through Koh-i-Taftan and Koh-i-Bazman in Iran (Ahmad and Dasht-e-Kain, 1986)

3.3. Heat source

The Koh-i-Sultan Volcano of Pleistocene age is the youngest volcano in Pakistan. It is fairly large spreading over approximately 500 km2. It is assumed that the hot magma chamber is the heat source. Existence of high temperature magma underneath Koh-i-Sultan is inferred by the emission of hydrogen sulphide and sulphur dioxide gasses (Abid, 1975).

4. MATERIALS AND METHODS

Water samples from hot springs were collected for chemical analysis and determination of isotopes: 2H, 18O & 3H (water), 18O & 34S (sulphate), and 13C (TIC). Physico-chemical parameters such as temperature, pH and electrical conductivity (EC) were determined in the field. A total of seven sample aliquots were collected for each hot spring site for chemical and isotopic analyses. Where filtration was required, a 0.45 µm pore diameter filter was used. The volume of the sample and treatment in the field for various analyses are given in Table I.

Table I. Volume and treatment of samples for chemical analysis and isotopes

Analysis Volume Treatment

Anions (Cl, HCO3, SO4) 1L Unfiltered, unpreserved

Cations (Na, K, Ca, Mg, Li) 250 mL Filtered, acidified with HNO3 Silica 100 mL Filtered, diluted with deionized water (1:1) Stable isotopes: (18O, 2H) 500 mL Unfiltered, unpreserved 13C (DIC) 100 mL Filtered and poisoned 18O, 34S Sulphate) 1L Filtered Tritium 1L Unfiltered, unpreserved

Chemical analyses were carried out using: atomic absorption spectrophotometry for Na, K, Ca and Mg; ICP for Li; UV–visible spectrophotometry for Si and SO4; titrimetry for HCO3; and ion selective electrode for Cl (APHA, 1985). 18 The δ O value of the water was measured by mass spectrometer using the CO2 equilibration method (Epstein and Mayeda, 1953). For analysis of δ18O of dissolved sulphates, the sulphates were precipitated as BaSO4 in the field (Giggenbach and Goguel, 1989). The co–precipitated carbonate was removed by reacting with hydrochloric acid, and BaSO4 was obtained in powder form. The BaSO4 was reacted with graphite in a vacuum system to convert its total oxygen into CO2 (Nehring et al., 1977), which was analyzed on the mass spectrometer for δ34S. δ18O values of water and dissolved sulphate were analyzed relative to V–SMOW with a standard error of ±0.1‰. The tritium content of the samples was determined by liquid scintillation counting after electrolytic enrichment of the water samples with a standard error of ±1 TU (Hussain and Asghar, 1982).

5. RESULTS AND DISCUSSION

5.1. Chemical Facies

Chemical data is given in Table II. Chemical facies of thermal springs and groundwater are shown in Fig. 5 and Fig. 6. In Chagai area the thermal waters of

Padagi have relatively low EC (4940 to 8170 µS/cm) and they are MgSO4 type. The other three fields have very high EC values varying from 25300 to 45600µS/cm and they are NaCl type. EC of three water samples taken from wells in the area varies from 3150 to 4720µS/cm. Two of these in Padagi area belong to MgSO4 while the third one (Mashki Chah near Chigin Dik) is NaCl type.

Na + K Legend 0 1 Thermal Open well

0.2 0.8

N 0.4 0.6 a

g +

M K

0.6 0.4

0.8 0.2

1 0 Mg Ca 0 0.2 0.4 0.6 0.8 1 Ca Fig. 5. Triangular plot of cations of Chagai Area

0 Cl 1 LEGEND Batal, Miri, Ziarat Springs Padagi Springs Open well

0.2 0.8

0.4 0.6 4 C O l S

0.6 0.4

0.8 0.2

1 0

SO HCO3 4 00.20.40.60.81

HCO3 Fig. 6. Triangular plot of anions of Chagai area

5.2. Origin and Evolution

Isotopic data (Table II) can well differentiate between the three possible types of origin of thermal water i.e. magmatic, oceanic and meteoric. Ranges of δ18O and δ2H of all the sampled geothermal manifestations of Chagai are –3.7 to +4.3‰ and -33.4 to –8.8‰ respectively. These data do not show the presence of any significant amount of magmatic water which generally has δ18O: +6 to +9‰ and δ2H: –40 to -80‰ (Pearson and Rightmire, 1980; Giggenbach, 1992). Normally the oceans have δ18O & δ2H about 0‰ (VSMOW). Depleted values of δ2H (–33.4 to –8.8‰) rules out contribution of oceanic water. These values are similar to the meteoric water of this area indicating possibility of recharge from meteoric water. Highly enriched values of δ18O can be explained by δ18O–δ2H plot (Fig. 7). δ18O of thermal springs except one spring of Padagi Kaur follow the trend of 18O–shift due to exchange between water and rocks at higher temperatures. Especially, the 18O–shift for Batal Kaur, Miri Kaur and Chigin Dik springs is very high (6 to 8‰), maybe due to rock– water interaction at high temperature (Clark and Fritz, 1997), which is very likely as the carbonate rocks are present in the area coupled with low water–rock ratio (Todaka et al., 1988). In the same plot, the thermal springs of Padagi indicate evaporation trend, which originates from the meteoric water index of Quetta area (Hussain et al., 1995). It indicates that the origin of Padagi area is meteoric water, which undergoes extensive evaporation. There is no correlation between δ18O and Cl of Batal Kaur, Miri Kaur and Chigin Dik springs plotted in Fig. 8 but Considering the Padagi springs separately (insert in Fig. 8), Cl has positive correlation with δ18O, which confirms the evaporation process. It seems that origin of all the geothermal springs is meteoric water recharging from the same area, most probably in the vicinity of Padagi Kaur. During deep circulation, it undergoes high 18O–shift as a result of rock–water interaction at high temperature and there is no significant mixing or evaporation in the thermal waters.

The important feature of the fluids emerging from thermal manifestations is their high salt contents, especially in Chigin Dik springs. With reference to EC, thermal springs can be divided in to three groups i.e. low at Padagi Kaur (4940–8170 µS/cm, high at Batal and Miri Kaurs (25300–26300 µS/cm) and very high at Chigin

Dik (34600–45600 µS/cm). The Cl and HCO3 of thermal water increase from Padagi Kaur towards Chigin Dik area. The Padagi springs have relatively low Cl (145–409 ppm) as compared to that of others having very high Cl (6560 to 13871 µS/cm). Two

springs at Chigin Dik have Cl values of 10326 ppm and 13871ppm. Normally the oceans have salinity from 33500 to 37600 ppm and chloride about 19300 ppm (Clark and Fritz, 1997). Such a high Cl indicates dissolution of marine origin salts/minerals. 13 With the increase in HCO3, δ C also increases from Padagi Kaur (-6.2 to –8.5‰) towards Chigin Dik springs –1.4 to +0.9‰, which is similar to that of marine 13 carbonate (Clark and Fritz, 1997). As carbonate is dissolved, δ CDIC is evolved to 13 18 more enriched values. Enrichment of CDIC is also synchronized with the O– enrichment due to rock–water interaction (Fig. 9).

Behaviour of sulphate content is opposite to that of Cl and HCO3. It is high in Padagi springs and other springs of Batal Kaur and Miri Kaur around the Miri Crater

(1558–4214 ppm), which reduces to (578–1289 ppm) at Chigin Dik springs. H2S was observed during sampling in these areas. As explained in D’Amore and Arnorson

(2000), H2S reacts with the oxygen dissolved in water or directly with the oxygen of the atmosphere.

Partial oxidation leads to the formation of native sulphur, but complete oxidation leads to the formation of sulphuric acid. Dissolution of SO2 also leads to the formation of sulphuric acid, which is a strong acid. Thermal waters in such a system may attain pH as low as 1. The two springs in the alteration zone studied by GSP (Todaka et al., 1988) having pH 2–3 reflect similar process of sulphate evolution. Dominance of magnesium might be due dissolution of magnesium mineral like magnesite. The acidic steam waters are strongly under saturated with common rock forming minerals. As a result, soil and rock are extensively dissolved and undergo drastic chemical changes and complex mineralogical transformation. The minerals present depend on the original rock and the extent to which leaching has progressed. In the sedimentary rocks of the area, main component of Humai Formation is limestone and Jazzak Formations contains subordinate limestone, whereas the subrecent deposits may also have carbonate minerals. Rock salt resulting from drying of seasonal lakes is also present in the area (Bender and Raza, 1995). Therefore, when the acidic sulphate type water flows horizontally towards down– slope areas, it reacts with carbonate minerals and rock salt. Consequently, the waters seeping beyond the alteration zone at lower altitude become NaCl type. It is also observed that the shallow groundwater samples from two open wells in the alteration zone is MgSO4 type, while the water sample of open well of (Mashki Chah) is NaCl type.

40 Batal Kaur Thermal Spring LMWL: 2H = 7.9 18O + 12.5 Miri Kaur Thermal Springs δ δ Padagi Kaur Thermal Springs 20 Chigin Dik Thermal Springs Evaporation tred Shallow w ells Quetta Rain at Padagi Kour 0 LMWL H (‰) 2

δ -20

O-18 Shift -40

-60 -8 -6 -4 -2 0 2 4 6 δ18O (‰) Fig. 7. δ 2H vs. δ 18O of geothermal springs

Fig. 8. Chloride vs. δ 18O of geothermal springs

2.0 Padagi Springs 0.0 Other Thermal Springs

) -2.0 C (‰

13 -4.0

-6.0

-8.0

-10.0 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 δ18O (‰) Fig. 9. δ 13C vs. δ 18O of Chagai thermal springs

Anion Index (A.I.) was calculated for the thermal springs (Todaka et al., 1988).

A.I. = 0.5 [{SO4 / (Cl + SO4)} + {(Cl + SO4) / (Cl + SO4 + HCO3)}] (1)

The higher values of A.I. indicate higher geothermal activity in a volcanic area. Direction of flow of water may be inferred from A.I. values. Water may flow from a locality with the highest A.I. towards localities with lower A.I. values. In this area anion indices of Padagi Kaur, Batal/Miri Kaurs and Chigin Dik springs are 1.7, 1.1 and 0.9 respectively, which confirm the origin and movement responsible of chemical evolution.

Dissolved sulphate of four samples was analyzed for δ34S (CDT, Canyon

34 Diablo Troilite). Plot of δ S vs. SO4 content (Fig. 10) gives negative linear correlation. The sample of Miri Kaur spring having highest sulphate (3008 ppm) has relatively depleted δ34S (+6.3‰), while the Chigin Dik sample with lowest sulphate (578 ppm) has enriched δ34S (+18.3‰). In a geothermal system, δ34S of sulphate with a magmatic origin ranges from 0 to 2‰ and sulphate resulting from the dissolution of evaporites can have δ34S from +10 (Permian) to +35‰ (Cambrian), whereas δ34S of modern oceanic sulphate is about +20‰ (Krouse, 1980). If the δ34S of sulphate had increased due to dissolution of evaporates, the SO4 concentration would also have increased. Therefore, dissolution of evaporites is not the source of sulphate. Giggenbach explained the sulphur origin of sulphate of Taupo volcanic

zone, New Zealand having δ34S value of +6.5‰ from magmatic sulphur dioxide (Giggenbach, 1977). The sulphate in the thermal springs originates from magmatic source, whose δ34S values are modified due the fact that both equilibrium and kinetic fractionation tend to enrich 34S in the oxidized sulphate and deplete in the reduced 34 species (Pearson and Rightmire, 1980). It seems that δ S of SO4 becomes more enriched (+6.3‰) in the oxidation process of magmatic H2S. The decrease in the SO4 concentration by reduction process is accompanied by enrichment of 34S in 34 sulphates. Therefore, SO4 of Chigin Dik springs is enriched in S.

18 The δ O(SO4) of hot springs varies from +13.2 to +16.3‰, which is plotted 34 against δ S(SO4) in Fig. 12 (Clark and Fritz 1997). Two data points fall in the magmatic/ hydrothermal zone and the third one having slightly higher δ34S is located near the boundary of this zone. Locations of these points confirm the source of sulphates as magmatic. Although, the forth point falls in the Cenozoic band showing dissolution of marine evaporitic sulphate, which is not true in this case. As discussed above, the decrease in SO4 concentration due to reduction is accompanied by the increase in δ34S.

S (‰) 10 34 δ

0 0 500 1000 1500 2000 2500 3000 3500

SO4 content (ppm)

34 Fig. 10. Enrichment of δ S(SO4 ) with the decrease in SO4 content of Chagai thermal springs

modern marine SO4 30 early Paleozoic 25 Tertiary 20 magmatic and hydrothermal SO4 Cenozoic

(‰) 15 4 Devonian to LowerTriassic 10 O of SO

18 5

0 terrestrial evaporites atmospheric SO4 -5

-10

-15 -10-50 5101520

18 δ O of SO4 (‰)

Fig. 11. Source of sulphates in Himalayan geothermal springs with the help of 34 18 δ SCDT(SO4) and δ OVSMOW(SO4) ranges in various origins of dissolved sulphates in groundwater (after Clark and Fritz, 1997)

5.3. Residence Time

Tritium content of the thermal waters of Chagai areas is about zero (less than detection limit i.e. 0.7 TU). In case of no tritium in the thermal water the exact input data of tritium is not needed for dating because, whatever the values may be, its would decay to the level of detection limit within 60 years. Hence the tritium data show that all these thermal waters were recharged before the start of nuclear weapon testing (1952) (Clark and Fritz, 1997). Tritium of the shallow open wells in Padagi Kaur area is also less than the detection limit showing residence time more than 60 years, whereas one well of Mashki Chah has considerable tritium (1.6 TU), which indicates significant contribution of young water in the recharge. Similarly, no tritium was found in Tatta Pani geothermal springs indicating residence time more than 60 years (Ahmad et al., 2000). Carbon-14 dating would give information about the age of thermal waters.

5.4. Reservoir Temperature

5.4.1. Chemical geothermometry

Various chemical geothermometers like Na–K, K–Mg, Na–K–Ca, Na-K-Ca-Mg and quartz geothermometers were used to estimate reservoir temperatures. The following equations were used and temperatures (°C) are given in(Table II and Table III.

TNa-K = {1217 / [1.483 + log (Na/K)]} – 273.15 (Fournier, 1979) (2)

TNa-K = {933/ [0.993+log(Na/K)]} – 273.15] (Arnorsson et al. 1983) (3)

TNa-K = {1178 / [1.470+log (Na/K)]} – 273.15 (Nieva and Nieva, 1987) (4)

TNa-K = {1390/ [1.70+ log (Na/K)]} – 273.15] (Giggenbach, 1988) (5)

0.5 TNa-K-Ca = {1647/[log(Na/K)+β log(Ca /Na)+2.24}–273.15] (Fournier & Truesdell, 1973) (6)

2 TNa-K-Ca-Mg ={16000/(3log(Na/K)+3log(Ca/Na )+18.3)} – 273.15 (7)

Tquartz = {1309 / [5.19 – log (SiO2)]} – 273.15 (Fournier, 1977) (8)

Estimated reservoir temperatures at geothermal sites of Chagai area are given in Table III. The chemical geothermometers like Na–K and Na–K–Ca indicate high reservoir temperature estimates mostly in the range of 200–300°C. The geothermal reservoir temperature is also estimated using the Na–K–Mg1/2 triangular diagram (D’Amore and Arnorson, 2000) (Fig. 11). This approach was adopted in order to take into account the possible uncertainties of the Na–K geothermometer. The Chagai samples are spread along an alignment starting from the Mg1/2 vertex and pointing towards the full equilibrium region. Spread may be due to continuous modification of chemical concentrations at different localities. These thermal waters fall in the partial equilibrium zone and extrapolations to the full equilibrium curve indicate temperature range of 200–300°C, while the two points nearest to the full equilibrium curve are at about 225°C line. Na–K–Ca–Mg gives relatively low temperature range (95–148°C). In this case the estimated temperature of these areas i.e. Padagi Kaur (87–95°C), Batal/Miri Kaurs (136–138°C) and Chigin Dik (145–148°C) increases along the movement of geothermal water.

Silica geothermometer was applied to estimate reservoir temperature (Fournier, 1977), which ranges from 107°C for Chigin Dik to 144°C for Batal Kaur. As discussed above, the movement of thermal water is from east of Miri Crater (Padagi Kaur) to Chigin Dik. Padagi Kaur is the alteration zone, where chemical changes start. It seems that after achieving maximum temperature, the thermal water appears in Batal/Miri Kaur springs and during traveling relatively longer distance towards Chigin Dik drastic changes take place in its chemistry due to which silica concentration decreases resulting in less estimated temperature.

1 0

0.8 0.2 100

S q 0.6 0.4 r 0 t 0 150 ( 0 M /1 g a ) N 0.4 200 0.6

250

0.2 300 0.8

300 0 1 350 250 200 150 100

1 0.8 0.6 0.4 0.2 0 K/100

Fig. 11. Evaluation of Na–K–Mg temperatures of thermal waters of Chagai area

5.4.2. Isotope geothermometer

Although several isotope exchange reactions can be used as indicators of subsurface equilibrium temperature, the exchange of 18O between dissolved sulphates and water is often the most useful because isotope equilibrium can be attained in a reasonable lapse of time depending on temperature and pH (Lloyd 18 1968, Clark and Fritz 1997). The ∆ O(SO4–H2O) geothermometer is based on the oxygen isotope exchange reaction:

16 18 16 18 16 SO4 + H2 O ⇐⇒ S O3 O + H2 O (9)

The reservoir temperatures were calculated using the relationship of Mizutani and Rafter (1969).

6 −2 1000 ln αSO4–H2O = 2.88 x 10 T − 4.1 (10) where 18 18 α = [1000 + δ O(SO4)] / [1000 + δ O(H2O)]

18 The results of ∆ O(SO4–H2O) geothermometer are graphically 18 18 displayed in the δ O(SO4) vs. δ O(H2O) diagram (Fig. 12), in which isotherms from 50 to 250°C are also shown. The reservoir temperature ranges from 112 to 207°C. Scatter of the data points can be attributed, at 18 least in part, O shift and reduction of SO4. The average reservoir temperature (165°C) is similar to that of Miri Kaur spring.

30 Padagi o 25 Miri Kaur 50 C Chigin Dik 100oC 20 (‰) 4 15

O of SO o 18 10 150 C

200oC 5 250oC 0 -4 -3 -2 -1 0 1 2 3 4 5 18 δ O of H2O (‰)

18 18 Fig. 12. δ O(SO4) vs. δ O(H2O) diagram for Chagai Area showing relevant 18 isotherms based on the ∆ O(SO4–H2O) geothermometer

5.5. Conceptual Model

Koh-i-Sultan eruption in Quaternary times produced a number of faults in the older rocks bringing about upheavals. It is presumed that the NE-SW and subordinate NW-SE faults run in the Sinjrani Volcanic Group, which is covered by the

Koh-i-Sultan Volcanic Group and quaternary deposits in this area. Based on the field observations, many fractures resulting in the high permeability are likely to occur in the Sinjrani Volcanic Group around the faults (Todaka et al., 1988). Fig. 13 shows the Conceptual Model of Chagai geothermal system. The geothermal fluid, recharged by the meteoric water, is stored along the faults and in the fractured lava. Hydrogen sulphide gas rising from the deep zones is continuously dissolved into the little amount of groundwater available in the alteration zone. As a result, the water becomes strongly acidic concentrated SO4 type water in the alteration zone. The highly acidic springs, sometimes, dry up due to evaporation. Some of the acidic water horizontally flow in the shallow zone and react with the country rocks. Consequently, the water seeping beyond the alteration zone at lower altitude becomes Cl–SO4 type. The thermal springs are concentrated around the Miri Crater. Most likely the recharge of water to the alteration zone takes place in the north-east of Miri Crater and geothermal fluid flows towards Chigin Dik through Padagi Kaur, Miri Kaur and Batal Kaur. Chemical changes and 18O-shift takes place due to rock-water interaction.

Fig. 13. Conceptual Model of Chagai geothermal system

6. CONCLUSIONS

Two springs of Padagi Kaur are Mg–SO4 type, while all the other springs at

Batal Kaur, Miri Kaur and Chigin Dik are Na–Cl type. Alteration of water to SO4 type takes place by absorption of magmatic H2S and the acidic solution is further responsible to dissolve rock salt and carbonate minerals. EC increases from Padagi springs (4940 and 8170 µS/cm) to Chigin Dik springs (45600µS/cm)

Chagai thermal manifestations receive recharge from meteoric waters in the vicinity of Padagi Kaur (east side of Miri Crater), which is heated by the hot magma chamber of Koh-i-Sultan most probably through deep circulation. Horizontal movement of thermal water is from Miri Crater towards Chigin Dik area. Residence time is more than 60 years. The thermal waters do not have any contribution of shallow young groundwater but they have high 18O–shift (6 to 8‰) due to rock–water interaction.

Reservoir temperatures estimated by different chemical geothermometers like Na–K, Na–K–Ca, Na–K–Mg1/2 (triangular plot) are quite high (200–300°C), while the silica and ∆(SO4–H2O) geothermometers give relatively low temperature ranges (107–144°C and 112–206°C respectively).

ACKNOWLEDGMENTS

The present study was carried out under the IAEA/RCA Project RAS/8/092 “Investigating the Environment and Water Resources in Geothermal Areas”. The authors express their gratitude to Dr. Z. Pang, Technical Officer, Mr. P. Dias, RCA Coordinator, Mr. C.R. Aleta, Ex RCA Coordinator, Project Officer and the supporting staff at the Agency for their co–operation in technical and administrative matters. The authors would like to express their gratitude to PAEC authorities especially Director General PINSTECH, Director Technology and Head IAD for providing infrastructure and financial support for the research work. To carry out the fieldwork, facilities were provided by Geological Survey of Pakistan (GSP). Cooperation of Director General GSP, Director Planning and Information and Mr. Abdul Hafeez Malik is highly appreciated. The efforts of M/S M. Azam Tasneem, Zahid Latif, M. Islam Pasha, Mumtaz Khan, Imtiaz Ahmad, Bashir Ahmad, Abdul Razaq, Muhammad Zamir and Amjad Ali for various sample analyses are thankfully acknowledged.

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Table II. Chemical and isotopic data of geothermal water samples from Chagai Area

δ34S δ18O S. Temp EC HCO Cl SO Ca K Mg Na SiO δ18O δ2H δ13C Tritium Sample pH 3 4 2 (SO ) (SO ) ºC µS/cm ppm ppm ppm ppm ppm ppm ppm ppm ‰ ‰ ‰ 4 4 TU ‰ ‰ Spring-1 32.2 25300 6.12 1409 7446 1558 583 898 541 3360 111.7 +4.0 -36.1 0.9 0.3 Spring-2 26.4 26300 7.58 1682 6560 3008 668 716 1036 3510 110.0 +2.4 -31.5 6.1 13.8 0.7 Spring-3 -3.3 -25.4 -8.5 Spring-4 17.6 4940 7.64 738 145 1710 574 27 626 299 44.0 -3.6 -25.4 -6.2 11.3 13.2 1.6 Spring-5 13.0 8170 7.87 484 409 4214 547 32 2014 548 68.3 +0.5 -8.8 -8.0 0.7 Spring-6 26.2 34600 6.42 2834 10326 1289 739 1514 665 9580 56.2 +4.3 -22.2 -1.2 13.5 16.3 0.3 Spring-7 27.5 45600 6.56 2773 13871 578 685 1370 376 10250 63.5 +4.2 -34.3 -1.4 18.3 13.3 0.4 Well-1 17.4 4650 6.85 864 211 1861 632 28 598 244 -3.2 -25.4 0.4 Well-2 19.0 3150 7.77 164 112 1111 247 31 294 261 -3.4 -27.3 0.9 Well-3 4720 7.72 266 883 377 139 50 76 644 -37.0 -2.9 1.6

Table III: Reservoir temperatures (ºC) of thermal springs of Chagai by various chemical geothermometers

Na-K Na-K Na-K Na-K Na-K-Ca Na-K-Ca-Mg Sample (Eq. 2) (Eq. 3) (Eq. 4) (Eq. 5) (Eq.6) (Eq. 7) SiO2 (Eq. 8) Spring-1 319 323 304 325 212 344 144 Spring-2 287 281 272 297 232 330 143 Spring-4 209 186 196 225 447 214 96 Spring-5 174 145 162 192 424 216 117 Spring-6 260 247 246 272 203 331 107 Spring-7 243 227 230 257 203 314 113