Major-ion geochemistry and mineralogy of the Salt Lake (Tuz Gölü) basin, Turkey

CHEMICAL GEOLOGY IA'C LUDIN~;

ISOTOPE GEOSCIENCE

ELSEVIER

Chemical Geology 127 (1996) 313-329

Major-ion geochemistry and mineralogy of the Salt Lake (Tuz GiSlii) basin, Turkey M. Zeki Camur *, Halim Mutlu General Directorate of Mineral Research and Exploration (MTA), 06520 Ankara, Turkey

Received 13 March 1995; accepted 11 August 1995

Abstract In the Salt Lake basin of Turkey, chemical composition of inflow surface waters defines a continuous trend from Ca-HCO3-rich spring waters to Na-SO4-Cl-rich brines. Considerable compositional variation exists among the surface waters. Water-rock interaction governs compositional variations in springs, streams and rivers, and is enhanced by evaporation and precipitation of calcite and protodolomite. Solute concentrations of the streams and the rivers are partially controlled by the mineralogy of the playa deposits. The concentration increase from inflow surface waters to a Na-Cl-type lake surface brine is not accessible to direct observation. The principal cause of the evolution from SO4-rich brine to Cl-rich brine in the lake is interpreted as the recycling of solutes through the differential dissolution of efflorescent crusts. In the lake, major-ion concentrations generally exhibit an evaporation-dependent evolution trend that is further modified by precipitating halite, gypsum, aragonite and calcite. Sediments of the lake are predominantly composed of gypsum, dolomite, huntite, magnesite, and subordinately of polyhalite minerals. Magnesite and huntite are mostly early diagenetic minerals that have been formed by the transformation of dolomite in the presence of pore fluids with a high M g / C a ratio.

1. Introduction

The Salt Lake (Tuz Gtilii) basin is located in central Anatolia, Turkey, and covers ~ 16,000 km 2 (Fig. 1). The basin is a playa lake complex surrounded by six major highlands. The Kar~ehir Massif to the east is composed of metamorphic rocks that are represented by gneiss-granite complex, schistmarble alternations and marbles from bottom to top, respectively. Sandstones and shales compose the Pa~ada~ Group to the north. The Karacadafg volcanics to the northwest include basaltic and andesitic lavas and associated pyroclastics. Samsam ophiolitic

* Corresponding author.

belt to the west consists mainly of serpentinites. Metamorphic rocks of the Bozda~ Group to the southwest include calc schist-quartz schist-mica schist, marble, calc schist-quartz schist-mica schist, dolomitic limestone and serpentinite-radiolaritelimestone-glaucophane schist sequences from bottom to top, respectively. Rhyolitic tufts and ignimbrites are the main members of the Aksaray volcanics to the south-southeast. The flat area between these units and the lake is covered by gypsum-bearing marl and limestone of Neogene age closer to the outer edge of the basin and by Quaternary alluvium deposits (gravel, sand, silt, clay and lacustrine evaporites) closer to the lake (Fig. 1). One N W - S E trending major fault zone extends parallel to the Klr~ehir Massif along the eastern edge of the lake.

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Sediment and water sample location

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EXPLANATIONS Water sample location

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Fig. 1. l)rainage and simplified geological map of the Salt Lake basin. Modified from 1:500.000 scaled geological map of Turkey. Sample numhers 1-4 are fixers, 5-12 are streams and 13 19 are springs. A and B are the locations of the cross-section given in Fig. 6.

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M.Z Camur. H. Mutlu / Chemical Geology 127 (1996) 313-329 The zone is characterized by both normal and rightlateral faults ( U y g u n , 1981; Haldun, 1995). The lake is 1600 k m 2 in area and is fed by three m a j o r rivers; Pe~enek6zti, U l u l r m a k and insuyu, with m e a n annual discharge rates o f 37 • 106, 41 • 10 6 and 10- 106 m 3, respectively. Several e p h e m e r a l streams and one m a n - m a d e agricultural discharge canal with a m e a n annual discharge rate o f 8 7 . 106 m 3 also feed the lake (Fig. 1). Inflow o f the discharge canal has diluted the lake w a t e r to the extent that the lake does not c o m p l e t e l y dry out since its o p e n i n g in 1974. Total annual surface f l o w to the lake, as m e a s u r e d b e t w e e n 1975 and 1978, is ~ 75 • 106 m 3 and varies m o d e r a t e l y by the a m o u n t o f released water both f r o m the discharge canal and from the irrigation dam on the U l m r m a k R i v e r (DSI, 1978). M e a n annual precipitation averages 353 _+ 36 m m and the potential m o n t h l y evaporation ranges f r o m 1 175 to 1390 m m ( M T A , 1982). On the basis o f an o b s e r v e d shallow sub-water sill barrier, the lake was d i v i d e d into two zones by Erol (1969): a main zone to the west and a deep z o n e to the east (Fig. 1). The main zone water level averages 70 c m in spring but

315

dries out in s u m m e r or early fall. The m i n i m u m and the m a x i m u m water levels h a v e been o b s e r v e d in the S e p t e m b e r / O c t o b e r and M a r c h / A p r i l periods, respectively. The deep zone, on the other hand, maintains its water content throughout the year and the water level reaches > 1 m in spring. As suggested by Irion (1970) and U y g u n and Sen (1978), these two zones exhibit different c h e m i c a l and m i n e r a l o g i cal characteristics as well. In this study, both inflow surface waters (springs, streams and rivers) o f the basin and the lake surface brine were investigated and a g e o c h e m i c a l m o d e l is p r o p o s e d on the basis o f h y r o g e o c h e m i c a l and mineralogical data a c c u m u lated o v e r a 4 - y e a r period. 1.1. Analytical methods The water, salt crust and sediment samples were collected by the General Directorate o f Mineral Research and E x p l o r a t i o n staff ( M T A , 1982). The sample locations are shown in Fig. 1. The water was s a m p l e d as three sets and stored in 500-ml polyethylene containers. The first set o f samples was filtered

Table l Mean ion concentrations (rag/l) of inflow surface waters in the Salt Lake basin No

Ca

Mg

AIk.

1 2 3 4

K 2.97 16.86 15.69 13.61

Na 51.6 291.97 284.70 257.19

43.66 70.92 I 15.49 129.89

33.09 81.08 161.51 154.98

5.79 8.15 7.49 9.55

SO~ 62.25 249.12 686.3 540.95

C1 50.65 390.20 392.59 414.63

7.83 7.98 7.85 7.72

pH

5 6 7 8 9 10 11 12

18.37 3.51 8.92 8.89 3.01 6.86 3.40 9.99

2481.6 197.08 242.34 93.85 59.79 265.08 69.31 519.96

534.63 30.54 41.42 88.37 69.48 36.49 39.94 42.46

444.71 63.59 103.19 33.62 30.00 59.88 45.83 104.26

4.05 7.13 7.90 7.28 5.90 6.55 5.47 7.72

4367.9 100.04 270.91 73.03 53.12 364.93 66.04 495.30

2573.5 204.50 281.70 125.13 67.45 188.64 64.16 517.75

7.82 8.07 7.92 7.62 7.70 8.45 8.20 8.40

13

0.00

13.56

55.91

7.05

3.90

14.80

19.14

7.10

14 15 16 17 18 19

0.00 0.78 0.00 0.00 0.00 0.00

28.97 25.98 6.90 12.40 63.00 I 1.03

85.97 69.94 35.00 36.50 30.80 36.07

21.00 7.05 8.99 24.00 22.36 5.83

6.10 4.10 2.61 3.8 5.00 2.20

49.90 29.70 12.90 3.40 7.20 21.10

46.00 8.86 17.00 25.17 49.60 9.22

7.20 7.60 8.05 7.70 7.40 8.30

Alkalinity is expressed in meq/l. Sample locations are given in Fig. l. Sample numbers 1-4 are rivers, 5-12 are streams and 13-19 are springs. Mean temperature (°C) distribution from January to December is 8, 9, 8, 8, 18, 20, 21, 23, 19, 16, 11 and 6, respectively. Density is equal to one for all.

316

M.Z. Camur, H. Mutlu / Chemical Geolo~4y 127 (1996) 313 329

in the field, and used for CI, Na, K and SO 4 analyses. The second set was filtered and acidified in the field, and used for Mg and Ca analyses. The last untreated set was utilized for alkalinity, pH and conductivity measurements which were also carried out at the sample locations. The salt crust samples collected from a northern limb of the main zone were dissolved both in water and in acid for chemical analysis. The chemical and mineralogical analyses of the samples were done in the General Directorate laboratories using standard methods reported in USGS (1970, 1979, 1989). Na and K concentrations were determined with flame photometry. Gravimetric and titrimetric methods were used for S Q , Ca, Mg, C1 and alkalinity analyses. Mean spring, stream and river compositions were averaged from the concentrations of 172 analyses and are presented in Table 1. A total of 36 lake water and salt crust analyses were used for the determination of the mean chemical

composition of the lake on a monthly base and these concentrations are listed in Table 2 (a) and (b) for the main and the deep zones, respectively. In the northern limb of the main zone, sediment samples from a depth of 4.3 m were collected with a hand-driller after the lake had mostly dried out. One-meter sediment thickness of the deep zone was sampled with polyvinylchloride (PVC) pipes (7 cm in diameter). The mineralogical determinations of the samples were done using X-ray diffractometry (XRD). 1.2. Numerical methods

Saturation state of the waters in the basin was investigated through thermodynamic calculations. Saturation state prediction of an aqueous solution requires that the free-ion activity coefficients in the solution are known. Since individual ion properties cannot be independently measured, some theoretical

Table 2 Mean ion concentrations (rag/l) in: (a) the main zone; and (b) the deep zone. of the Salt Lake brine in a yearly cycle Month

K

Na

Ca

Mg

Alk.

SO 4

CI

pH

p (g/l)

7" (°C)

Jan. Feb. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

1610 800 800 944 1458 3358 6300 9400 9900 9950 10000

115625 107500 106300 101980 114717 106667 113000 89625 69750 105417 102083

617 870 1000 925 772 429 380 273 192 621 600

4445 2100 2200 2860 3963 10591 10932 20686 36667 6194 5095

3.85 2.70 2.10 2.84 3.23 9.20 8.67 14.43 13.28 4.46 4.23

9675 6100 6600 7371 8838 19097 20809 37018 67785 13646 10885

188812 161100 173100 167438 185454 172710 184104 196448 171159 176527 168018

7.42 7.50 7.10 7.34 7.30 7.33 7.45 7.15 6.95 7.33 7.55

1.15 1.15 1.15 1.15 1.18 1.21 1.25 1.30 1.15 1.15 I. 15

15 13 15 18 23 24 25 26 22 7 5

c2~ (.r

10

4

12

13

0.31

14

3

May Jun. Jul. Aug. Sep. Oct.

300 453 715 1200 1700 2200

28000 31500 60000 100000 100000 110000

365 478 718 920 524 561

1000 1327 2370 3720 5302 6955

3.80 2.41 2.69 4.51 4.10 2.95

2600 3485 6109 9594 12798 15110

40900 50875 91500 149075 185316 185011

I. 15 1.18 1.21 1.25 1.30 1.15

8 22 23 24 25 21

% o-

7

9

3

9

0.19

6

7

(a) Main zone:

0.8

(b) Deep cone: 8.15 8.50 7.80 7.40 7.20 7.30 0.9

8

Sample locations are given in Fig. 1. Alkalinity, is expressed in meq/I, c/~ Cr is the mean percent standard deviation and p is density.

M.Z Camur, H. Mutlu / Chemical Geology 127 (1996) 313-329

equations have been developed and calibrated on the basis of experimental data to calculate the free-ion activity coefficients in electrolyte solutions (for summary see Whitfield, 1979, 1991). Truesdel| and Jones (1974) combined the BrCnsted-Guggenheim equation with the mean-salt method and derived empirical equations for free-ion activity coefficients of relatively dilute water ions. The WATSPEC speciation computer program that was produced by Wigley (1977), adapted from Truesdell and Jones (1974) was used in this study for dilute inflow surface waters. Temperatures used in the calculations were measured from the surface waters on a monthly basis in the field. The specific ion-interaction formulation of Pitzer (1973, 1979, 1991) which has been suggested to be applicable to the concentrated solutions having ionic strengths in the range of brines (Gueddari et al., 1983; Brantley et al., 1984; Harvie et al., 1984; Monnin and Schott, 1984; Nordstrom and Munoz, 1986; Weare, 1987) was used for the lake surface brine. Pitzer (1973, 1979, 1987, 1991) has used the statistical-mechanical approach to express the nonideality observed in concentrated electrolyte solutions. The formulation is based on a virial equation of state that accounts for specific ion interactions. The development and basis of the Pitzer equations are given in detail by Pitzer (1979, 1987, 1991), Harvie et al. (1984) and Weare (1987). The virial coefficients in the equations were calculated using the ion-interaction parameters of Harvie et al. (1984) and Weare (1987). All the ionic species listed in the data set were considered. The thermodynamic data set used in the calculations is from Harvie et al. (1984) and Johnson et al. (1991). The integrals in the electrostatic unsymmetrical mixing equations were solved using Chebyshev polynominal approximations (see Pitzer, 1987). A computer program code PITDI based on these equations and the data set was written and used to determine the saturation state of the lake at 25°C (Camur, 1995). pH-values of hypersaline brines obtained by direct emission of an electrode can be erroneous as Pasztor and Snove (1983) pointed out. Nevertheless, in our calculations pH is assumed to be representative of a purely reversible (equilibrium) property of the system. Therefore, the results of carbonate equilibria should be viewed cautiously. In the interpretations of the carbonate equi-

317

libria, the effects of higher pH-values should be considered (Sonnenfeld, 1984).

2. Geochemistry of the inflow surface waters

2.1. Spatial cariation Major solute input to the lake is provided by the perennial rivers; Pe~enekiSzii (#1), Ulmrmak (#2) and insuyu (#4), and the discharge canal (#3) whose mean concentrations are listed in Table 1 (Fig. 1). Source-rock-dependent solute concentration differences among the rivers were qualitatively investigated. The Pe~enek~zii River which extends parallel to the long axis of the lake in the N W - S E direction at east along the fault zone, together with its tributaries traverse the metamorphic, ultramafic and granitic rocks of the K~r~ehir Massif (Fig. 1). The Pe~enekiSzii River with mean total dissolved solids (TDS) of 630 mg/1 is the most dilute among the rivers and its water chemistry is dominated by HCO 3 ions (Fig. 2a). Waters of the canal and the insuyu River, on the other hand, are the most concentrated with mean TDS of ~ 2600 m g / l . The canal at the southwest traverses the Bozda~g Group (II) which consists predominantly of marble-dominated metamorphics and limestones and subordinately of ophiolitic m~lange before flowing over gypsum-bearing marls and limestones of Neogene (VII) age which also host the Insuyu River to the west. This common path of the canal and the Insuyu River is shown by similar amounts of high Ca, Mg and SO 4 contents in comparison to those of the Ululrmak and Pe~enek~iz~i (Fig. 2a). The Ulmrmak River drainage system is located at the southeastern part of the basin. The fiver and its tributaries drain basaltic and andesitic lavas and associated pyroclastics of tile Aksaray volcanics (V) and traverse the alluvium deposits (VIII) before reaching the lake. The Ulmrmak River, with mean TDS of 1800 mg/1, has lower concentrations of Ca, Mg and SO 4 than the canal and the Insuyu River. Alkalinity is similar in the rivers and the canal. Spring waters of the Salt Lake basin are calciumbicarbonate rich (Fig. 2b; Table 1). The drainage systems of the streams are situated in flyschoid rocks

318

M.Z. Camur, H. Mutlu / Chemical Geology 127 (1996) 313-329

of the Pa~ada~ Group (IV) in the north, basaltic and andesitic volcanics of Karacada~ (VI) in the northwest, and the ophiolitic rocks and limestones of

Samsam (III) in the west. Several Neogene gypsum deposits also crop out in the basin and are situated along the paths of some streams. All streams in the

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Fig. 2. Mean major-ionchemistryof: (a) the rivers Pe~enek6zii(six-rayed star), Ulmrmak(circle) and Insuyu(triangle), and the discharge canal (square); and (b) the representativespring (diamond) and streams (star) in the Salt Lake basin.

M.Z Camur, H, Mutlu / Chemical Geology 127 (1996) 313-329

basin are ephemeral and sink into the alluvium before reaching the lake. TDS of the streams range from 670 to 2670 mg/1. Representative mean stream compositions are shown in Fig. 2b. The chemical composition of the streams defines a compositional spectrum between the springs and the rivers and between the rivers and the lake with water types dominated by Na, C1 and SO 4 ions that are partially contributed by the evaporative precipitates of playa flats. 2.2. Seasonal euolution

In order to establish whether major ions behaved conservatively on mixing and evaporation, the approach that has been pioneered by Garrels and Mackenzie (1967) and Eugster and Hardie (1978) was used; the best-fit trends of seasonal concentrations of the ions in the inflow surface waters and the lake surface brine were plotted against chloride concentration (Fig. 3). Na concentrations define a straight line between the springs and the lake, suggesting an evaporative concentration control. The trend of K is nearly parallel in slope to Na and exhibits a slight depletion between the springs and the rivers. Mg 6No

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Fig. 3. Seasonal variation of ion concentrations in the surface waters o f the Salt L a k e basin. The best-fit lines are b a s e d on spring (square), stream (diamond), river (star) a n d lake surface brine (filled square) compositions.

319

concentrations also follow linear and parallel trends to Na in the springs, but the slope is lower than those of Na and K between the rivers and the lake, indicating a gradual depletion. The trend of Ca concentrations and its slope in the river and the stream waters are nearly similar to that of Mg. In the spring waters and between the river and the lake waters, however, the Ca trend decreases whereas that of Mg increases. Overall the concentration trend of Ca indicates that it is removed from the water via precipitation. The trend of H C O 3 concentrations exhibits a reverse trend to that of Ca: HCO 3 decreases in the rivers and increases between the rivers and the lake. H C O 3 concentrations are also displaced from the evaporative evolution line of spring waters toward the decreasing side, thus suggesting that bicarbonate species are also removed from the water. In the rivers and the streams, SO 4 exhibits a rather flat trend with nearly constant concentration around 400 mg/1 (Fig. 3). When compared to the concentrations of springs, waters of the several streams and rivers contain, in general, less sulfate than what would be possible by simple evaporation. The trends of Ca and bicarbonate ions indicate that CaCO 3 solid phase is progressively removed from the surface waters at all stages of evolution upon evaporation. The lower slope of the Mg trend with respect to that of the conservative ion further indicates that Mg either coprecipitates with CaCO 3 a n d / o r joins the precipitation of another Mg-bearing mineral such as dolomite, hydromagnesite or magnesite. The slight depletion of K could be assigned to the sediment adsorption. Possible mechanisms for deficient SO 4 are: (a) selective enrichment of chloride against sulfate; (b) reduction; and (c) precipitation. Because sulfate-bearing mineral precipitation (e.g., gypsum, mirabilite, barite) is precluded by the results of the saturation calculations presented in the next paragraph, it may be deduced that sulfate reduction and partly selective enrichment of chloride could be responsible for low concentrations of SO 4 in the surface waters. Because inflow water compositions influence the composition of salts formed in the brine, the saturation state of the waters was investigated. Saturation index (SI) results for calcite, dolomite, magnesite, gypsum and halite (all of which have been observed in the sediment samples of the lake) are displayed in

M.Z. Camur, H. Mutlu/ Chemical Geology 127 (1996) 313 329

320

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Fig. 4. Saturation state best-tit lines of the sud'ace waters for halite (cross), gypsum (triangle), calcite (six-rayed star), dolomite (short-dashed line) and magnesite (long-dashed line) in the Salt Lake basin. Ionic strength is in molal scale.

Fig. 4 against ionic strength. The most dilute water composition in the basin is represented by springs, some of which are saturated only with respect to calcite. Some streams and rivers, on the other hand, are saturated with respect to not only calcite but also dolomite and magnesite. The supersaturation of these minerals may be explained by kinetic reasons (Morse, 1983), or because of their solid solution characteristics, which were not incorporated into the calculations. Gypsum and halite saturation points are reached only in the surface brines of the lake. These results support the previously described concentration trends of Na, K, Ca, Mg, CI and HCO 3 ions, and suggest that calcite, dolomite and magnesite are brought into the lake by the inflow surface waters.

2.3. Deuelopment of the lake surface brine Chemical composition of the inflow surface waters defines a continuous trend from Ca-HCO3-rich

spring waters to N a - S Q - C l - r i c h brines (Fig. 5). A considerable compositional variation was observed in the inflow surface waters. The variation, that is initiated by water-rock interactions along the paths of springs, streams and rivers, is enhanced by both evaporation and precipitation of calcite and sometimes protodolomite and magnesite. The compositional trends in the cation and anion triangles are characterized by increasing M g / C a and C I / H C O 3 ratios and Na, K and SO 4 contents (Fig. 5). The concentration increase from the inflow waters to the lake brine is not accessible to direct observation in the anion triangle. The final change in the lake brine consists of C1 enrichment over SO 4. The evolution from SO4-rich brine to Cl-rich brine in the lake may have been governed at one time by the gradual enrichment of C1 in the brine through fractional crystallization of other salts. But at present, it is governed by the recycling of solutes through fractional dissolution of effiorescent crusts. The other

M.Z. Camur, H. Mutlu / Chemical Geology 127 (1996) 313-329

321

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80

60 w - -

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[Col

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C fi ~ I 8 N S Fig. 5. Chemical trends of the surface waters: spring

Na+K

HC03'C@3

2@

dE] ~hlo~ne

,,om_q,,,"1 °" ,~ (open circle), river and

v BE (C)

80

C I +N[.., J

A h ~ 0 N S stream

(square)

and lake

(.filled circle) in

the Salt Lake basin.

M.Z. Camur, H. Mutlu / Chemical Geology 127 (1996) 313-329

322

possible mechanism, removal of sulfate due to reduction under anoxic conditions, is not credible because only a trace amount of pyrite was observed in the sediments from a few localities. The mineralogy of the lake sediments, mainly sulfate and carbonate minerals, suggests that the fractional crystallization of salts (e.g., gypsum) other than Cl-bearing, was probably the initial process in the evolution. But after the lake brine had enriched in C1 up to the point of salt crust formation, the recycling of solutes by fractional dissolution of efflorescent crusts has taken over the fractional crystallization process in the evolution from SO4-rich brine to Cl-rich brine. Efflorescent crusts develop during the dry season in the lake. The soluble salts, consisting mainly of halite in the crust dissolve easily as indicated by the constancy of the Na/C1 ratio in the surface brine (Table 2). On the other hand, many other constituents remain in the crust and are lost to the interstitial brines.

ports the presence of anhydrite minerals in the northern edge of the main zone. Cubic halite minerals in the upper levels of the salt crust have dimensions of < 0.5 mm and are distinguished from the underlying halites by their clean white colors. The underlying halite minerals, on the other hand, exhibit a chevron pattern and include diagenetically developed unzoned halites in the lower levels as well (MTA, 1982). Discoidal gypsums crystals, located in the upper sections of the sediment underlying the salt crust, are present predominantly as individual monoclinic crystals and subordinately as groups of crystals known as "desert rose". The crystals compose nearly 50 v o l ~ of the sediments and some bear displacive growth signs. Gypsum crystals range from 0.3 to 4 cm in size and sometimes contain carbonitic inclusions along the crystallization interlaces. The characteristics (color, composition, etc.) of these inclusions were detected to be the same as those of the surrounding sediment (MTA, 1982; Ergun, 1988). Ergun (1988) reports that some discoidal gypsum crystals include anhydrite in varying amounts near their center. The mud surrounding gypsum crystals indicates a carbonate assemblage that is made of magnesite, huntite and dolomite. In dolomite analyses (221) and ( 111 ) surface reflections are absent (Ergun, 1988). These characteristics indicate that dolomite minerals in the mud are Ca-rich, hence they are named by (Ergun, 1988) as protodolomites (Graf, 1960: Curtis et al., 1963). Protodolomites in the mud are ~ 1-5 /,m in size and exhibit well-developed rhombohedral crystallographic forms (Ergun, 1988). In addition to these minerals, dispersed white anhydrite nodules, ranging from 0.5 to 4 cm in size, were also reported by Ergun (1988) from one location. These nodules are composed of tens to hundreds of small anhydrite crystals that are < 0.5 mm in length. The deep zone sediments, on the upper levels, are composed of a thin (few centimeters) layer contain-

3. Geochemistry and mineralogy of the lake 3.1. Mineralogy

The main zone evaporite mineralogy in the salt crust of the northern limb consists of halite, gypsum, aragonite and calcite. The mineralogy of the unconsolidated muddy sediment, below the salt crust ( 1 - 3 0 cm), starts with gypsum-, huntite- and magnesitebearing levels reaching ~ 25 cm in thickness and continues in the central parts with polyhalite occurrences. This level is ~ 35 cm thick and is underlain by gypsum-, huntite-, magnesite-, illite- and montmorillonite-bearing sediments. A cross-section from the northern part of the lake is shown in Fig. 6. Protodolomite, that was not detected in our sediment samples of the main zone, was reported by Irion (1970) in the central parts of the zone and by Ergun (1988) in the northern edge. Ergun (1988) also reA

B

I ~ _ _ .

~0

F..xpianatJon

l

HaJJte

~

Sand

~

Clay

~

C,yps~

I km Scale

Fig. 6. A northern limp cross-section o f the Salt Lake (compiled from M T A , 1982). The location is given in Fig. 1.

M.Z Camur, H. Mutlu / Chemical Geology 127 (1996)313-329

ing halite, gypsum and aragonite. The sedimentation continues with Mg-calcite and dolomite occurrences in the lower levels. The dolomite/Mg-calcite ratio is 1:1 and the MgCO 3 content in Mg-calcites ranges from 7 to 11 wt% as detected from XRD analyses. In addition, detrital minerals chlorite, montmorillonite, quartz, feldspar and mica were also observed in this zone of sediments.

323

6CI 54

(D~4-

E 0

3.2. Seasonal evolution

(_9 cn 2 . 0

The chemical data indicate that the lake surface water could be classified as a Na-Cl-type brine (Table 2). The spatial chemical variation in the lake water exists only between the surface brines of the main and the deep zones. Ion concentrations of the main zone surface brine are about twice as much as higher than those of the deep zone (Table 2). Ion concentrations decrease in the following order: CI, Na, SO4, Mg, K, Ca and HCO 3 in both zones and the mean percent of these ions (expressed as m g / l ) in the main zone is about 56.27, 32.77, 6.03, 3.06, 1.57, 0.19 and 0.12, respectively. Each zone, on the other hand, spatially exhibits small concentration differences. For example, mean percent standard deviation of the Na and C1 ions, that compose 90% of the total ions in the main zone, are only of 4% and 3%, respectively (Table 2). Also note that these deviations were calculated from the data that were collected over a 4-year period, which means that, in a given specific time, the deviations are even lower. Although the spatial compositional variation in the lake is small, seasonal variation is pronounced and enhanced by both evaporation and mineral precipitation. In order to evaluate annual chemical variation in each zone, ion concentrations have been plotted for both zones of the lake as a function of time (Fig. 7). The lines in the figure represent the polynominal best fit for each ion. Fig. 7 displays that the zones exhibit two different chemical evolution trends in a yearly cycle. In the main zone, K represents an evaporation-dependent concentration change in a closed lake; increases between April and October and decreases between November and March. The general trend cycle for SO4, Mg and H C O 3 ions is that the concentrations increase between April and September and then decrease between October and

1-

(a) Main Zone F

M

A

M

J

d

A

S

0

N

D

0

N

D

Month 6-

5-

C~4-

8 d,s-

Q cO

Cr~2 _ O

1-

(b) Deep Zone F

M

A

M

J

J

A

S

Month Fig. 7. Ion concentration c h a n g e in a yearly cycle for: (a) the main zone; and (b) the deep zone o f the Salt Lake, for N a (triangle), K (square), M g (cross), C1 (diamond), SO 4 (rice-cornered star) and H C O 3 (tilted cross). The capital letters on the X-axis are the first letters of months f r o m J a n u a r y to December.

March (Fig. 7a). Ca concentrations, on the other hand, exhibit an opposite trend to that of the general cycle; they decrease between April and September and increase between October and March. Na and CI concentrations are nearly constant all year around. The reverse trends of calcium and bicarbonate

324

M.Z. Camur, H. Mutlu / Chemical Geology 127 (1996) 313-329

concentrations indicate the presence of precipitating CaCO 3 mineral in the lake (Garrels and Mackenzie, 1967; Hardie and Eugster, 1970). Approximately constant concentrations of Na and C1 in the brine reveal that the crystallization point of halite is maintained throughout the year. Because the slopes of Mg and SO 4 concentrations are lower than that of K (which is assumed to be nearly conserved in the brine) between July and December, it can be deduced that Mg- and SO4-bearing mineral(s) (i.e. gypsum, dolomite) also precipitate in the main zone during this period. In the deep zone, all ion concentrations but those of Ca and HCO3 increase between May and October (the period for which data exist). Considering that this period covers a dry season in the region, it is an expected pattern. However, note that the slopes of C1, Na and SO 4 trends level off (less pronounced for SO 4) after July (Fig. 7b). Calcium and bicarbonate concentrations, on the other hand, increase and decrease, respectively, between May and June and decrease and increase, respectively, between July and October. The opposing trends of calcium and bicarbonate ions and the leveling off trends of C1, Na and SO 4 imply that the expected concentration increases due to evaporation are changed by the precipitating minerals halite, calcite (aragonite) and gypsum.

4

'

.~' ¢~

/¢{ ~5

2

0....

"W

r~ - - e ' "

9 . . :¢

0 /'

0~ 0

,

2 x •

4

x"

,"

--6

(a) -8

j

F

M

A

Main

M J j Month

Zone

A

s

o

N

~

8-

6 i 4 . 8 - - t~-

a. 2

us 0 O~ 0

A ....

t~--

1

.+.~

-2 4 ¸-

3.3. Saturation state 6

Genesis of the minerals, which are detected to be present in the lake sediments, was investigated through saturation state calculations using the lake surface brine. The saturation calculations were carried out for halite, gypsum, aragonite, calcite, huntite, magnesite, dolomite and polyhalite minerals at the temperature of 25°C. Because Mg-calcite solid solution properties are not covered by the model, saturation level of this mineral is not investigated. The results are shown in Fig. 8a and b for the main and the deep zones, respectively. In the main zone, halite and gypsum present an equilibrium saturation level throughout the year. Calcite and aragonite are slightly supersaturated in all months except in October when they are close to the equilibrium level. The results suggest that brine compositions of the main zone are supersaturated with

(b) - 8

F

M

A

M

d

V ' r te

Deep d

z 0 A

{-

(}

~{i

D

Month Fig. 8. S a t u r a t i o n state o f the lake s u r f a c e brine: (a) m the m a i n

zone: and (b) in the deep zone. of the Salt Lake at 25°C and 1 arm for halite (cross), gypsum (triangle), calcite (six-rayed star), aragonite (circle with cross), dolomite (square), magnesite (diamond), huntite (star) and polyhalite (tilted cross).

respect to huntite and magnesite and undersaturated with respect to polyhalite. The degree of polyhalite undersaturation decreases and gets close to the equilibrium value between September and November (Fig. 8a). The general log SI trends for the deep zone water compositions reveal an equilibrium saturation in the August-October period for halite and gypsum

M.Z Camur, H. Mutlu / Chemieal Geology 127 (1996) 313-329

325

Table 3 M a s s transfer models for the main zone brine e v a p o r a t i o n / d i l u t i o n processes Month

Evaporation

Dilution

Halite

Feb./Apr. Apr,/May May/Jun. Jun./Jul. Jul./Aug. Aug./Sep. Nov./Dec. Dec./Jan. Jan./Feb.

1.22 1.31 1.39 2.36 1.10 2.09 -

1.10 1.88 2.53

- 913.04 - 1982.66 - 1424.45 - 8638.32 - 282.92 -6609.89 195.60 3387.28 2825.48

Gypsum -

12.22 16.23 15.05 21.76 - 6.23 - 118.86 - 20.53 49.06 27.04

Calcite

CO 2

12.09 4.54 - 0.13 - 19.37 3.61 105.33 21.16 - 39.54 - 10.11

- 13.46 - 4.52 - 0.60 21.17 - 5.45 - 110.74 - 21.05 41.47 11.40

Values are in millimoles per k g H 2 0 .

(Fig.

8b). Calcite

slightly

above

and

aragonite

the equilibrium several

saturation value

year. Dolomite

shows

supersaturation

in the predictions.

orders

levels

throughout of magnitude

3.4. M a s s t r a n s f e r c a l c u l a t i o n s

are the of

In this section, related

to mass

evaporation

(mineral)

and dilution

transfer

processes

for the main

zone

]nc~._asingN~/Ca~ K

A

~

7 1M

Z ==

E

~

A

•

A' I E

I~

m

ARAGONnT. DOLOMITE

lot

0

?

•

.° I 1D

I

•

|

-

I

HUN/ITE

MAGN~FIT.

t 2D

H20

tJ

<

6 I

E

~-

I I

u,l

GYPSUM

I

I' I

<

E

~

? IP

~ c m , ar,~ng d e p t h Fig. 9. A schematic d i a g r a m o f mineral genesis in the Salt L a k e basin. E, A and O stand for endogenic, allogenic and authigenic origins, respectively. C, D, G, P a n d M are c a l c i t e / a r a g o n i t e , dolomite, g y p s u m , polyhalite a n d m a g n e s i t e / h u n t i t e paths, respectively. Dashed lines indicate local occurrences.

326

M.Z Camur, H. Mutlu / Chemical Geology 127 (1996) 313-329

surface brine was quantitatively evaluated using NETPATH computer program of Plummer et al. (1991). This program incorporates mass balance relations to mineral dissolution/precipitation reactions in the system under stated constraints and phase conditions (for details, see the above reference). Mass transfer calculations require two sets of data to be pre-determined. The constraints set is the ion concentrations that involve into the precipitation/dissolution reactions. For this set, ion concentrations reported in Table 2 (a) were used after speciation calculations, performed using the PITDI computer code of Camur (1995). K and Mg ions were excluded from the reactions because these elements are absent in the observed surface sediment minerals. The second data set is related to the phases that are likely to both dissolve and precipitate in the lake. For this set, minerals (halite, gypsum and calcite) detected in the surface sediments of the lake and CO~ were considered. The calculations were carried out for the brine composition of each month (the salt crust formation period, September-November, is excluded). The results are presented in Table 3. The limitations: (a) the calculations were carried out at 25°C; (b) the calculations do not include salt crust formation effects; (c) the calculations do not include solid solution series; and (d) the calculations were carried out for steady-state conditions should be taken into account while evaluating the amounts presented in Table 3. In Table 3, calculated positive mass transfers indicate that the phase entered the aqueous solution, indicating dissolution (of a mineral) or in-gassing (of a gas). Negative mass transfers indicate precipitation or out-gassing. For example, the second row in the table suggests that 1 1 of the May brine would have been evaporated form 1.31 1 of the April brine and in so doing precipitated 1982.66 mmol of halite, 16.23 mmol of gypsum, outgassed 4.52 mmol of CO 2 and dissolved 4.54 mmol of calcite per kilogram H20. The calculation results are in agreement with the observations that the main zone lake surface brine is evaporated between April and October and is diluted between November and February (Table 3). According to the results, during the evaporation period, > 97% of the total sedimentation consists of halite. The remaining ~ 3% is predominantly composed of gypsum and subordinately of calcite. Evaporation vs.

dilution annual mass budget calculations indicate a positive mass transfer (precipitation) for halite and gypsum and a negative mass transfer (dissolution) for calcite. The negative mass transfer for calcite supports the allogenic (brought into the lake by surface inflow waters) input since calcite mineral is observed in the surface sediments of the lake.

4. Discussion Mineral genesis model of the Salt Lake basin is outlined in Fig. 9. Halite, gypsum, calcite and aragonite are endogenic minerals (the minerals originating from processes occurring within the water column) in the lake as it is suggested by the sediment mineralogy, saturation calculations and the concentration trends. Since the 25°C temperature value is best approached between June and October by the lake, and since the decreasing temperature would shift the saturation points of the minerals further down to the undersaturation side in Fig. 8, it could be safe to deduce that the lake brine precipitates halite and gypsum between June and October in the main zone and between August and October in the deep zone. On the basis of the crystallographic similarities of gypsum minerals from Abu Dhabi and from the Salt Lake, Ergun (1988) adapted the model of Shearman (1963) and Kinsmann (1966) and suggested that gypsum, in general, has formed during early diagenesis from pore fluids rich in Ca and SO4 through capillary evaporation with displacive growth. Saturation calculations, however, indicate that the lake surface brine is saturated with respect to this phase, suggesting direct precipitation for gypsum in both zones of the lake. The endogenic character of gypsum is also supported by the evaporation experiments that were carried out by MTA (1978). In these experiments, the lake surface brine was poured into the Cr-Ni alloy tank (30 X 40 x 50 cm) and was subjected to an evaporation under sun energy during June-August. Morning, noon and evening mean temperatures were 21 °, 29 ° and 31°C, respectively, during the experiments. Halite and gypsum were determined as the experimental products in the XRD analyses of the precipitated solid materials. Because dolomite, huntite and magnesite solid solution series were not incorporated into the calcu-

M.Z. Camur, H. Mutlu/ Chemical Geology 127 (1996)313-329

lations, we cannot evaluate the supersaturation trends of these minerals against field data. Moreover, saturation calculations of carbonate minerals could be erroneous due to the assumption of pH as previously stated. Although the results of saturation calculations and M g / C a ratio do not preclude the primary precipitation of huntite, magnesite and dolomite (which is also detected to be allogenic), several lines of observations also favor authigenic (the minerals resulting from processes that occur within the sediments once deposited) origin for these minerals. The absence of calcite and aragonite in the main zone sediments below the salt crust indicates that either these minerals are non-equilibrium phases due to salt crust formation or they were altered in the sediments by the percolating brines. The results, in general, support the equilibrium precipitation of calcite and aragonite in the main zone, thus favoring the alteration hypothesis. The increasing M g / C a ratio in the brine, governed primarily by gypsum precipitation in the main zone, could cause dolomitization of endogenic as well as allogenic CaCO 3 in the sediments (path I C in Fig. 9) as also speculated by Irion and Mfiller (1968), MTA (1982), and Ergun (1988). Dolomitization of previously deposited aragonite by refluxing brines depleted in Ca from gypsum precipitation has been also observed in similar situations like Solar Lake, Israel (Aharon et al., 1977). Our observations about calcite and protodolomite minerals in the main zone sediments conflict with those of Ergun (1988). In the main zone sediments, we observed calcite (on the sediment surface) but found no protodolomite whereas Ergun (1988) detected protodolomite but found no calcite. Irion and Mtiller (1968), on the other hand, reported both calcite and protodolomite occurrences in the sediments from central parts of the main zone. These observations suggest that calcite and protodolomite distributions in the main zone sediments are not uniform. The absence of protodolomite minerals may indicate that they were all altered to magnesite and huntite by pore fluids with a high M g / C a ratio (path l D in Fig. 9). According to Irion and Miiller (1968), the further increase in the M g / C a ratio could produce magnesite and huntite as diagenetic minerals. Ergun (1988), on the other hand, suggests that dehydration of gypsum to anhydrite adds water to the environment

327

Table 4 Ion concentrations (mg/1) of the pore fluids in the northern limb of the main zone sediments of the Salt Lake (after Ergun, 1988) Sep.

Na

Ca

Mg

HCO 3

SO 4

Cl

1977 1983 1984

144000 97000 163000

211 172 195

31300 33700 27800

1816 2102 2337

35700 48400 46800

170600 192800 164300

Samples we~ collected ~om a depth of 50-60 cm.

which causes alteration of dolomite to magnesite and to huntite (path I G in Fig. 9) as experimentally demonstrated by Sayles and Fyfe (1973). This interpretation was supported by the observations of Ergun (1988) that: (a) a positive correlation exists between the anhydrite content of gypsum and the amount of magnesite present in the environment; (b) where magnesite is absent in the sediments, present gypsum includes no anhydrite; and (c) huntite content positively correlates with magnesite in the sediments. Because the presence of anhydrite minerals was locally observed in the sediments by Ergun (1988) and magnesite and huntite are present while anhydrite is absent in other localities, it is concluded that both mechanisms probably contribute to the formation of magnesite and huntite in the lake sediments. The high M g / C a ratio of pore fluids collected by Ergun (1988) supports this conclusion (Table 4). Direct precipitation as a third mechanism (path 1M in Fig. 9) could also be responsible for some magnesite and huntite minerals present in the sediments as suggested by the M g / C a ratio of the surface brines and by saturation calculations. The genesis of polyhalite found in the sediments of the main zone is presently a controversial matter. MTA (1982) suggests a primary origin for the polyhalite occurrences on the basis of the observation that the sediments exhibit a continuous layered morphology. Contrary to this deduction, the absence of gypsum minerals where polyhalite is present, led Irion and Mfiller (1968) to the interpretation that polyhalite has been diagenetically formed by the transformation of gypsum in the presence of solutions rich in Mg and K (path 2G in Fig. 9). Irion (1973), on the other hand, suggests a primary origin either directly from the lake surface brine or from the pore fluids on the basis of mineral stability studies (path I P in Fig. 9). Our thermodynamic

328

M.Z Camur. H. Mutlu / Chemical Geology 127 (1996) 313 329

calculations

presented

surface brine respect

might

earlier suggest that the lake have

to p o l y h a l i t e

prior

achieved to t h e

saturation opening

with

of the

discharge canal.

Acknowledgements We thank Director Nizamettin ~enttirk for providi n g a c c e s s to t h e u n p u b l i s h e d d a t a to M e v l t i t A y g i i n , F e v z i A y o k , H a s a n Ba~, T a y f u n B i l g i ~ , E r g f i n (~elik, Celal

Erkan,

Soner

Kayaklran,

Ali

Uygun

and

M u s t a f a Y a ~ a r f o r c o n t r i b u t i o n s to t h e c o l l e c t i o n o f t h e d a t a , a n d to t h e a n o n y m o u s comments

and suggestions.

reviewer for useful

This research

is f i n a n -

cially supported by the General Directorate of Mineral

Research

and

Exploration

(MTA)

of Turkey.

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Techniques of Water-resources Investigations of the U.S.G.S. U.S. Geol. Surv., Bk. 5, Ch. AI, 545 pp. Uygun, A., 1981. Tuz G~51il havzasmm jeolojisi, evaporit olu~umlari ve hidrokarbon olanaklari. Tilrk. Jeol. Kurumu, Bilim. Tek. Kurul. Bildir., 35: 66-71. Uygun, A. and Sen, E., 1978. Tuz G~51il havzasl ve dotal kaynaklark I. Tuz G~ilil suyunun jeokimyasl. Tilrk. Jeol. Kurumu Bill., 21: 113-120. Weare, J.H., i987. Models of mineral solubility in concentrated brines with application to field observations. In: I.S.E. Carmichael and H.P. Eugster (Editors), Thermodynamic Modelling of Geological Materials: Minerals, Fluids and Melts, Ch. 5. Mineral. Soc. Am., Rev. Mineral., 17: 143-176. Whitfield, M., 1979. Activity coefficients in natural waters. In: R.D. Pytkowitcz (Editor), Activity Coefficients in Electrolyte Solutions, Vol. 2, Ch. 3. CRC (Chem. Rubber Co.) Press, Boca Raton, Fla., pp. 154-299. Whitfield, M.. 1991. Activity coefficients in natural waters. In: R.D. Pytkowitcz (Editor), Activity Coefficients in Electrolyte Solutions, Vol. 2, Ch. 3. CRC (Chem. Rubber Co.) Press, Boca Raton, Fla., 2nd ed., pp. 154-299. Wigley, T.M.L.. 1977. WATSPEC: A computer program for determining the equilibrium speciation of aqueous solutions. Br. Geomorphol. Res. Group, Tech. Bull., 20: 3-39.