Saline waters in basement rocks of the Kaapvaal Craton, South Africa

Saline waters in basement rocks of the Kaapvaal Craton, South Africa

Chemical Geology 289 (2011) 163–170 Contents lists available at SciVerse ScienceDirect Chemical Geology j o u r n a l h o m e p a g e : w w w. e l s...

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Chemical Geology 289 (2011) 163–170

Contents lists available at SciVerse ScienceDirect

Chemical Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c h e m g e o

Research paper

Saline waters in basement rocks of the Kaapvaal Craton, South Africa Amitai Katz a,⁎, Abraham Starinsky a, Giles M. Marion b a b

Institute of Earth Sciences, The Hebrew University of Jerusalem, Edmond Safra campus, Givat-Ram, Jerusalem 91904, Israel Desert Research Institute (DRI), 2215 Raggio Parkway, Reno, NV 89512, USA

a r t i c l e

i n f o

Article history: Received 23 February 2011 Received in revised form 22 July 2011 Accepted 2 August 2011 Available online 16 August 2011 Editor: J.D. Blum Keywords: Cryogenic brines Carboniferous–Permian glaciations Seawater freezing Water–rock interaction South African brines Crystalline shield brines

a b s t r a c t The chemical composition and geochemical evolution of saline waters found in deep (1.9–3.5 km) gold mines in the Kaapvaal Craton, South Africa, are reported and discussed. Twenty eight samples, representing the most saline waters available were studied. The salinity of the samples ranges between 2.5 and 99 g·TDS·L− 1 with chloride being the dominating anion. Magnesium is practically absent from most samples. The Kaapvaal waters were sampled in the Carletonville region (Driefontein, Kloof, Mponeng and Tau Tona mines) and at Evander. The samples, defined as fracture waters, were retrieved from the Witwatersrand Basin Proterozoic quartzite rocks and from metamorphosed basalts and basaltic andesites. Based on water salinity, Na/Cl–Br/Cl ratios and Ca–Mg–SO4 relationship, we conclude that the chemical composition of the waters stems from ancient seawater that experienced three major processes by the following order: (1) concentration of seawater by freezing; (2) density-driven infiltration into the underlying crystalline basement, accompanied by water–rock interaction (WRI); and (3) dilution of the brine by freshwater. Because the last glaciation in South Africa occurred in Carboniferous–Permian (Gastaldo et al., 1996) we attribute this age to the Kaapvaal brines, rather than to a much earlier period proposed previously. After their cryogenic concentration in a marginal trough around the Gondwanan ice sheet, the brines infiltrated and migrated into the underlying crystalline rocks through fissures and shear zones, displacing less dense resident freshwater. Therein, the brines were further modified via water–rock interaction, which included chloritization and albitization of Ca-plagioclase. Chloritization was responsible for the almost total removal of Mg and SO4 from solution and for the significant Ca enrichment observed. Albitization, which affected only a part of the samples, caused a loss of Na and an additional, equivalent gain in Ca. In view of similar findings, limited so far to crystalline shields in the Northern Hemisphere, our interpretation of the South African data may have a much farther-reaching meaning. We argue that similar saline waters should be expected at depth in all rocks, crystalline and sedimentary alike, in regions that were covered by major glaciers, such as eastern South America, large parts of Australia, India and Antarctica during the Carboniferous– Permian. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Saline subsurface waters have been known for a long time from deep wells, mines and boreholes in ancient crystalline rocks in the Northern Hemisphere (e.g. Frape et al., 1984; Frape and Fritz, 1987; Herut et al., 1990; Möller et al., 1997; Bottomley et al., 1999; Starinsky and Katz, 2003; Greene et al., 2008; Bucher and Stober, 2010). In some respects, they resemble marine evaporitic basinal brines found in sedimentary environments. They are Ca-chloridic in composition (Ca2+ N (SO42− + HCO3−), expressed in equivalent units), displaying lower Mg/Cl, SO4/Cl, K/Cl and Na/Cl ratios and higher Ca/Cl and Br/Cl ratios, than the corresponding ratios in modern seawater.

⁎ Corresponding author. Tel.: + 972 2 658 4620; fax: + 972 2 566 2581. E-mail addresses: [email protected] (A. Katz), [email protected] (A. Starinsky), [email protected] (G.M. Marion). 0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.08.002

The Kaapvaal waters under study were encountered and sampled in deep (1.9–3.5 km) gold mines (Driefontein, Kloof, Mponeng and Tau Tona) at Carletonville, and at Evander (Fig. 1). They were retrieved from the Witwatersrand Basin Proterozoic quartzite rocks and from metamorphosed basalts and basaltic andesites, where they were defined as fracture waters (Onstott et al., 2006). The sampling depth in Evander (1.47–1.95 kmbls) is shallower than in the other sites and the waters discharge from conglomeratic reefs. In the Carletonville region the sampling depth ranged between 2.7 and 3.4 kmbls. On the basis of stable isotope (δ2D, δ18O) data, Onstott et al. (2006) proposed a hydrothermal origin of the water. Moreover, they stated that the trends in the two isotopes and chloride indicate mixing of the original hydrothermal brine with paleo-meteoric waters. The attribution of salinity to a marine invasion later than the 2.0 Ga hydrothermal episode was ruled out by the authors on geological grounds. Evaluation of the origin of the Ca-chloride brines in crystalline basement rocks started before a few decades. Interpretation of the

A. Katz et al. / Chemical Geology 289 (2011) 163–170

Fig. 1. Location map (KL = Kloof; TT = Tau Tona; DR = Driefontein; MP = Mponeng; EV = Evander).

findings was, and remained in disagreement from the beginning, including the brine source(s), the origin of the solutes and the concentration mechanism. Until the nineties of last century, researchers favored the idea that the brine evolved from freshwater contained in the rocks. Accordingly, the salts in the brine were derived from the surrounding crystalline rocks by WRI with the original freshwater. The possibility that seawater constituted the original fluid was, to that point, rejected altogether (e.g. papers in the GAC Special Paper #33 edited by Frape and Fritz (1987), Lodemann et al. (1997), and Möller et al. (1997) and many others). Accumulating evidence led students of brines in crystalline rocks, as of around 1990, to propose ancient seawater as the appropriate parent solution that gave birth to the brine following its concentration on the surface. The characteristic composition of these fluids was attributed to WRI between the concentrated seawater and crystalline rocks, after infiltration into deep terrain. This major shift from the early, “all internal” concept opened the way to deeper and wider testing of “external” sources (seawater) and concentration modes (evaporation or freezing) but left unresolved, for the time, the debate about the kind of the external concentration mechanism that took place. The main difficulties arising from attribution of crystalline shield brines solely to “internal” fluid sources and to subsequent concentration by WRI within a closed system are the following: 1. The “internal” concentration of freshwater to brine is attributed to removal of H2O into (secondary) hydrated minerals such as chlorite, clay and zeolites (e.g. Bucher and Stober, 2010). These are generally of quite low abundance in the relevant rocks, and tend to occur as thin coatings on fissure faces. The concentration of “freshwater” (a few hundred mg·TDS·L− 1) to brine (200–300 thousand mg·TDS·L − 1) involves an unconceivable leap of three orders of magnitude. 2. The second unanswered question stemming from the “all internal” concept is the consistent fit of the deep brines, irrespective of age or geographical location, to a marine cryogenic evolution path as displayed in a Na–Br–Cl coordinate system (Starinsky and Katz, 2003), and their Ca–Mg–SO4 balance that is in concert with a seawater source. 3. According to the “internal” concept, each aquifer rock type dictates the specific composition of its resident brine. Actually though, saline waters in crystalline shields display, basically, similar compositions. Moreover, recent finding of similar brines in the Antarctic submarine sediment column (Frank et al., 2010) proves this to be true even in a completely different mineralogical environment. Hence, one is left with two processes for concentration of the parent water to a hypersaline fluid, evaporation or freezing of sea-

water, each of which takes place under a characteristic and very different temperature range, resulting in a specific mineral series. During evaporation, the early crystallization sequence is given by aragonite → gypsum → halite (Braitsch, 1971 and many later publications). Upon seawater freezing, the first mineral to crystallize is ikaite (CaCO3·6H2O), then mirabilite (Na2SO4·10H2O) at − 6 to − 8 °C, followed by hydrohalite (NaCl·2H2O) and gypsum (CaSO4·2H2O) at −22 °C (Ringer, 1906; Gitterman, 1937; Nelson and Thompson, 1954; Marion et al., 1999). Further cooling does not lead to crystallization of additional minerals until, at − 34 to −36 °C, sylvite (KCl) and MgCl2·12H2O precipitate. The evaporative and cryogenic paths for modern seawater are readily resolvable on a Na/Cl–Br/Cl diagram constructed from seawater evaporation and freezing experiments (Fig. 2). Obviously, such diagrams are sensitive to composition of the seawater under regard, hence to ocean age. During our preliminary search for saline waters in crystalline rocks of the Southern Hemisphere we encountered and analyzed two samples from the Tau Tona gold mine in South Africa (Table 1). Both samples showed features characteristic of marine cryogenic brines in the Northern Hemisphere studied by us earlier (Herut et al., 1990; Bottomley et al., 1999; Starinsky and Katz, 2003), and provided the needed motivation for a more thorough study of the same area. Application of the aforementioned Na/Cl–Br/Cl relationship for identification of the concentration process responsible for the Kaapvaal waters (freezing or evaporation), required the detailed knowledge of the major ion composition of the seawater under regard. All the cryogenic saline waters found so far in the Northern Hemisphere were related to Pleistocene glaciations (Herut et al. 1990; Bottomley et al. 1999; Starinsky and Katz, 2003), to which a modern seawater composition could be attributed. Because no Pleistocene glaciation is known in South Africa, this assumption is obviously invalid for the Kaapvaal Craton. The latest ice age that affected the area under study took place in Carboniferous–Permian times (Fig. 3). Therefore, the chemical composition selected by us corresponds to that of a Carboniferous–Permian ocean. Fig. 4 represents the relevant Na/Cl–Br/Cl system. The blue evaporation line represents a calculated linear regression on fluid inclusion data from halite crystals (Horita et al., 2002). The freezing is represented by two curves, calculated by means of the FREZCHEM program (Marion and Kargel, 2008), where the green and red colors represent, respectively, equilibrium and fractional crystallization under freezing conditions. Advances in hydrogeology and in the study of long range subsurface water and brine migration have shown that, given adequate hydraulic

Gitterman (1937) Nelson and Thompson (1954) Raab evaporation data (1996) Linear regression (on Raab's data)

0.9 0.8

Na/Cl (equivalents)

164

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Br/Clx100 (equivalents) Fig. 2. The Na/Cl–Br/Cl relationship during evaporation and freezing of modern seawater in the laboratory. Red and green lines describe seawater freezing. Evaporation data from Raab, 1996.

A. Katz et al. / Chemical Geology 289 (2011) 163–170

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Table 1 Chemical composition of the Kaapvaal waters. Sources: (1) Onstott et al. (2006); (2) this study; (3) Horita et al. (2002) and Lowenstein et al. (2005). Name

Source

Depth

T°C

kmbls

Na

K

Mg

Ca

Cl

Br

SO4

Br/Cl

Na/Cl

Ca/Cl

Dilution

mol/L

mol/L

mol/L

mol/L

mol/L

mol/L

mol/L

× 100

m/m

m/m

Brcryo/Brf

Driefontein DR548 FW 090901 DR546 BH1 110198 DR546 BH1 110802 DR546 BH1 101501 DR546 BH1 103001 DR546 BH1 020199 DR546 BH1 120798

1 1 1 1 1 1 1

3.3 3.213 3.213 3.213 3.213 3.213 3.213

50 35 27 32 32 37.2 37

7.24E-01 2.78E-01 3.00E-01 3.80E-01 3.07E-01 2.59E-01 2.48E-01

8.23E-03 1.53E-03 1.96E-03 8.82E-04 1.10E-03 1.44E-03 1.32E-03

9.72E-02 7.40E-05 1.02E-05 7.82E-05 1.69E-05 4.57E-05 4.44E-05

4.48E-01 2.94E-01 2.18E-01 1.61E-01 1.87E-01 2.19E-01 1.69E-01

1.80E+00 8.89E-01 7.13E-01 6.98E-01 6.76E-01 6.37E-01 6.34E-01

5.48E-03 2.43E-03 1.52E-03 1.66E-03 1.66E-03 1.71E-03 1.33E-03

2.23E-05 5.59E-04 1.13E-03 9.91E-05 1.11E-03 9.64E-04 4.81E-04

0.304 0.273 0.213 0.238 0.246 0.268 0.210

0.402 0.313 0.421 0.544 0.454 0.407 0.391

0.249 0.331 0.306 0.231 0.277 0.344 0.267

2.9 5.8 7.1 7.3 7.5 8.1 8.0

Evander EV818 H6 121902 EV818 H5 102702 EV818 H5 102502 EV818 H6 111502 EV818 H6 102702 EV818 FW 030601 EV818 FW 051001 EV818 FW 062101

1 1 1 1 1 1 1 1

1.89 1.89 1.89 1.89 1.89 1.95 1.95 1.95

46 42 40.5 46 47 45 45 45

1.05E-01 9.36E-02 9.80E-02 1.01E-01 9.36E-02 8.92E-02 8.26E-02 7.83E-02

3.94E-04 5.32E-04 5.47E-04 5.58E-04 4.17E-04 2.86E-04 3.86E-04 3.45E-04

4.69E-05 1.57E-04 2.04E-04 4.77E-05 4.94E-05 4.07E-04 3.52E-04 3.31E-04

4.04E-02 3.93E-02 3.90E-02 3.86E-02 3.70E-02 3.54E-02 3.59E-02 3.64E-02

1.77E-01 1.73E-01 1.71E-01 1.70E-01 1.61E-01 1.58E-01 1.52E-01 1.47E-01

4.25E-04 4.77E-04 4.03E-04 4.20E-04 4.05E-04 4.00E-04 4.09E-04 3.97E-04

2.91E-03 2.97E-03 2.62E-03 3.09E-03 2.89E-03 1.56E-04 2.19E-04 5.46E-04

0.240 0.276 0.236 0.247 0.252 0.253 0.269 0.270

0.593 0.541 0.573 0.594 0.581 0.565 0.543 0.533

0.228 0.227 0.228 0.227 0.230 0.224 0.236 0.248

28.8 30.0 29.8 30.1 31.9 32.5 34.0 35.2

Kloof KL443 KL441 KL739 KL441 KL441 KL441 KL441 KL441

1 1 1 1 1 1 1 1

3.4 3.082 3.1 3.3 3.082 3.3 3.082 3.3

59 52.4 61.1 N.A. 60 56 58.2 56

1.20E-01 1.38E-01 9.48E-02 1.02E-01 2.82E-02 3.97E-02 2.79E-02 2.14E-02

1.41E-03 9.26E-04 1.06E-03 9.49E-04 1.97E-04 1.76E-04 1.74E-04 2.22E-04

3.08E-04 1.96E-05 6.01E-06 1.74E-04 7.43E-06 1.65E-06 6.98E-06

8.18E-02 5.46E-02 6.99E-02 5.30E-02 1.28E-02 7.58E-03 9.18E-03 1.14E-02

2.90E-01 2.48E-01 2.31E-01 2.14E-01 5.79E-02 4.91E-02 4.77E-02 4.40E-02

5.75E-04 5.78E-04 5.89E-04 4.73E-04 1.49E-04 1.31E-04 1.23E-04 9.39E-05

7.29E-04 1.46E-03 1.08E-04 1.02E-03 1.48E-06 4.98E-05 2.81E-05 6.39E-05

0.198 0.233 0.255 0.221 0.257 0.267 0.258 0.213

0.414 0.556 0.410 0.477 0.487 0.809 0.585 0.486

0.282 0.220 0.303 0.248 0.221 0.154 0.192 0.259

17.5 20.5 22.2 23.7 89.0 105.4 108.0 115.7

Mponeng MP109 FW 101701 MP104 E65XC H1 MP104 E65XC H1

1 1 1

3.305 2.825 2.825

50 52 59

7.89E-02 5.06E-02 2.76E-02

1.17E-03 4.30E-04 4.04E-04

1.65E-05 1.85E-06 1.03E-06

5.98E-02 1.98E-02 2.37E-02

2.09E-01 8.49E-02 7.19E-02

5.22E-04 2.15E-04 1.75E-04

7.72E-04 1.86E-03 8.99E-04

0.250 0.253 0.243

0.378 0.596 0.384

0.286 0.233 0.330

24.5 60.5 71.2

Tau Tona G-4712 032006 G-4774 011007

2 2

3.55 3.55

55 55

1.42E-01 1.12E-01

1.01E-03 8.44E-04

3.78E-06 BDL

7.72E-02 5.56E-02

2.96E-01 2.26E-01

7.32E-04 6.97E-04

1.25E-03 6.45E-04

0.247 0.308

0.479 0.496

0.261 0.246

17.2 23.5

Permian seawater Modern seawater

3 3

n/a n/a

n/a n/a

4.54E-01 4.80E-01

1.00E-02 1.09E-02

5.50E-01 5.45E-01

1.50E-02 1.09E-02

5.60E-01 5.60E-01

8.50E-04 8.50E-04

2.10E-02 2.87E-02

1.52E-03 1.52E-03

8.11E-01 8.57E-01

0.027 0.019

n/a n/a

HWDN FW 050801 HWDN FW 020199 FW 062901 BH1 022801 HWDS H1 120198 HWDS H2 051702 HWDS H1 020199 HWDS H2 050201

Remark: Dilution factor (Brcryo/Brf) is the ratio between the Br concentration in the original cryogenic brine and in the analyzed sample.

head, permeability and time, brines may migrate along continent-scale distances (Garven, 1995). In crystalline rocks the permeability is connected to large-scale faults and fissures rather than to intra-granular flow and may therefore greatly vary from region to region. The objectives of this study are to evaluate the source of salinity in the Kaapvaal waters, the processes that led to their present composition, and their relation to saline waters in crystalline rocks elsewhere in Earth's crust.

Fig. 3. Ancient ice ages in time (after J.C.C. Crowell in Crowley and North, 1991).

2. Methods The water samples are from 5 deep mines at Driefontein, Evander, Kloof, Mponeng and Tau Tona, South Africa (Fig. 1). The original collection by Onstott et al. (2006) holds 170 samples, many of which are freshwater and therefore unsuitable for the purpose of our study. Fortunately though, we were able to sort out a large number of analyses (26) representing the requested waters of sufficiently salinity (Table 1). This selection minimizes modification effects of the original ion ratios by freshwater dilution. The samples from Tau Tona represent freely discharging water, were collected in 1 L plastic bottles and brought to the geochemical laboratory at the Hebrew University, Jerusalem for analysis. The water was filtered through Millipore (0.47 μm), diluted with de-ionized water (18.3 M Ohm·cm − 1) where required and analyzed. Na, K, Mg, Ca, and SO4 (as S) were determined by ICP-AES spectroscopy (PerkinElmer model Optima-3000 Radial ICP-AES spectrometer) at an internal (1σ) precision of ±0.2–0.8%. Chloride and bromide were analyzed using a Lachat Instruments QE flow injection analyzer (FIA) with colorimetric detection, following Eaton's et al. (1995) procedures, at an internal (1σ) precision of ±1% or better. The equilibrium and fractional crystallization lines for freezing Permian seawater were calculated using the FREZCHEM thermodynamic program (Marion and Kargel, 2008). Alternative pathways for seawater equilibrium and fractional crystallization were quantitatively

166

A. Katz et al. / Chemical Geology 289 (2011) 163–170

0.9

EVANDER

0.8

0.7

0.7

0.6

0.6

Na/Cl (m)

0.8

0.5 0.4

0.5 0.4

0.3

0.3

0.2

0.2

0.1 0.1

0.2

0.3

0.4

0.5

0.6

0.1 0.1

0.7

KLOOF Albitization

Na/Cl (m)

0.9

0.2

0.3

Br/Clx100 (m)

0.9

0.9

DRIEFONTEIN

0.7

0.6 0.5 0.4

0.5 0.4 0.3

0.2

0.2

0.2

0.3

0.4

0.5

0.6

0.7

0.1 0.1

0.2

0.3

Br/Clx100 (m)

0.4

0.5

0.6

0.7

Br/Clx100 (m)

TAU TONA

0.9

0.7

0.6

0.3

0.1 0.1

0.6

MPONENG

0.8

Na/Cl (m)

Na/Cl (m)

0.7

0.5

Albitization

Albitization

0.8

0.4

Br/Clx100 (m)

LEGEND

0.8

Fluid inclusions in marine-evaporitic halite

0.7

Seawater freezing under fractional crystallization

Na/Cl (m)

Seawater freezing under equilibrium crystallization

Data points

0.6

Seawater

0.5 0.4 0.3 0.2 0.1 0.1

0.2

0.3

0.4

0.5

0.6

0.7

Br/Clx100 (m) Fig. 4. Na/Cl–Br/Cl ratios in the Kaapvaal samples (5 sites). The lines represent Carboniferous-Permian seawater. All data points fall on, or below the cryogenic line, where the lower points tend to disperse along a vertical trend, reflecting albitization. Notably, all samples in the Evander site fall on or very close to the freezing line, indicating that no albitization took place.

demonstrated by the FREZCHEM model (Marion et al., 1999). The seawater evaporation line was obtained by linear regression fit to fluid inclusion data in Permian evaporites (Horita et al., 2002). The concentrations of Na, K, Mg, Ca, Cl and SO4 in Permian seawater were taken from Lowenstein et al. (2005), who based their calculations on analyses of fluid inclusions in ancient marine halite crystals and provided compositions of seawater for the Tatarian, Artinskian–Kungurian and Asselian–Sakmarian Permian stages. The

composition of Permian seawater given here represents an average of these compositions. Recent seawater value was assumed by us for bromide concentration in the Permian sea (Table 1). 3. Results and discussion The samples detailed in Table 1 have a salinity above 2.5 g·TDS·L− 1, reaching a maximum of 99 g·TDS·L− 1 (sample DR548 FW 090901),

A. Katz et al. / Chemical Geology 289 (2011) 163–170 TT

MP

KL

EV

DR

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Cl (mol/L) Fig. 5. Chloridity distribution in the studied waters.

some three times higher than that of modern seawater. Chloride is the dominant anion, displaying a wide concentration range between the different locations sampled (Fig. 5). Magnesium is practically absent in all samples, except in the most saline water from Driefontein. In this sample the Mg/Ca (molar) ratio is around 0.21. Considering that our comparison of evaporation vs. freezing processes relates to the same parental ocean, the following features, displayed in Fig. 4, are crucial to our study: a) all the data points, irrespective of site, fall on or below the cryogenic fractional crystallization line; b) most of the sites have at least one data point sitting exactly on the cryogenic fractional crystallization line; c) in one case (Evander) all the data points are situated on the cryogenic fractional crystallization line. In other words, none of the data points fall on the evaporation line or even in the void space between that and the cryogenic fractional crystallization line. Thus, Fig. 4 rules out evaporation of seawater as a concentration process for the Kaapvaal Craton saline waters, leaving this task for cryogenesis. Because the latest major glaciation in South Africa occurred during Carboniferous– Permian time (Fig. 3), the age of the ocean from which the salts were derived must be the same. The preservation of brine that originated during an earlier freezing of ocean water, albeit possible, is obviously of lower probability. Ca-chloride brines, similar in composition to the South African waters have been reported earlier from North American and North European crystalline shields. Starinsky and Katz (2003) proposed a conceptual scheme, reproduced here in Fig. 6, for the evolution of a large-scale marine-cryogenic basin, some 500–1000 km across, to account for Northern Hemisphere brines. Accordingly, following the onset of an ice age and loading of a growing continental ice sheet, the underlying crust is down warped and a forebulge is formed between the glacio-tectonic depression and the coast, with a peripheral marginal trough between the ice edge and the forebulge (Andrews, 1975). Seawater infiltrates into this trough and sea ice crystallizes on its surface. Driven by its elevated density, the resultant brine sinks to the bottom and infiltrates into the underlying terrain via fissures and shear zones, 1 and migrates inland, towards the center of the depression. Loss of brine from the trough is compensated by fresh supply of seawater through the forebulge. Brine production continues towards the glacial maximum, saturating the basement porosity with cryogenic fluid. Upon subsequent warm-up and ice sheet decay, the crust rebounds and melt water flushing of the shallower brine outwards from the center of the glacial basin takes place. Similar waters were found in Neogene, subglacial, glacimarine, and marine sediments in the McMurdo Sound, Antarctica, surrounded by an existing ice sheet. These were shown to be of marine cryogenic 1 Ice loading enhances the formation of fissures and shear zones in underlying crystalline crust that experiences glacio-static depression (Mörner, 1991; Talbot, 1999; Stewart et al., 2000). This may greatly increase the mobility of migrating cryogenic brines.

167

origin (Frank et al. 2010). Earlier findings of Ca-chloride brines in Antarctica were reported by Meyer et al. (1962), Tedrow et al. (1963), Torii and Ossaka (1965), Carlson et al. (1990) and by Marion (1997). The glacial period in South Africa during which the brines under study were formed (Fig. 3) stretched between 330 and 245 Ma, including 2 peaks at 275 and 300 Ma (Gastaldo et al., 1996), as compared with only 1–2 Ma for the Northern Hemisphere Pleistocene ice age. While the Canadian Shield fluids possess salinities similar to the original, undiluted brine (up to 300 g/L) already at a depth of 1.5 km, the South African waters are very dilute even at a depth of 3.5 km. The higher dilution in the South African waters may probably be related to the much longer exposure (≈250 Ma) of the crystalline rock aquifers to flushing processes and/or to differences in the glacio-tectonic activity and its duration between the Permian glacials in South Africa and Pleistocene freezing events in the Northern Hemisphere. Geochemical modification of the waters continued also after their cryogenic concentration. All but one of the samples listed in Table 1 (DR548 FW 090901) show an almost total loss of their original (seawater) magnesium. This is reflected in their Mg/Cl ratios, which are 2–3 orders of magnitude below the corresponding ratio in cryogenically concentrated seawater (≈0.18). We attribute the very efficient removal of Mg2+ to chloritization, which has been reported to occur in fractures in the gold mines by several researchers (e.g. Hayward et al., 2005), thereby explaining also the Ca-chloridic composition of all the samples that plot on the cryogenic evolution path. Loss of Mg2+ from seawater due to WRI has been demonstrated both for natural and experimental systems. Humphries and Thompson (1978) showed that Mg is generally taken up by basalt during hydrothermal alteration and incorporated in chlorite, releasing Ca2+ to solution at approximately 1:1 molar ratio. Seyfried and Bischoff (1979) reacted seawater with diabase and with basaltic glass at 150 °C at a W/R mass ratio of 10. Under these conditions the exchange of Mg2+ for Ca2+ was a matter of a few tens of days for the glass and a few hundred for the diabase. The chloritization process was complemented by another important WRI, as reflected by the dispersal of several data points along a generally vertical trend below the cryogenic lines (Fig. 4). Data point dispersal may be attributed to Na + and/or Br − ion depletion. However, bromide loss would result in a leftwards, horizontal shift rather than in the observed trend. Thus, the process responsible for Na + depletion was very likely plagioclase albitization (Eq. 1). CaAl2 Si2 O8 ðCaplagioclaseÞ

þ

2+

+ 2Na + 4SiO2 → 2NaAlSi3 O8 + Ca ðalbiteÞ

:

ð1Þ

In four of the sampled sites the waters on the cryogenic fractional crystallization line share a common location on the diagrams, namely around coordinates x = 0.25; y = 0.60. This allows concluding that the cryogenic concentration of brine from seawater was more or less uniform at the different sites. The evolution of the Kaapvaal waters from the onset of seawater freezing to their late dilution by freshwater is displayed in a Ca vs. Cl concentration diagram (Fig. 7). The black line (filled rectangular markers) represents the thermodynamically calculated relationship between Ca and Cl during freezing of Permian seawater. The red curve describes the (calculated) relationship, in the freezing brine, between the decreasing temperature (red, right hand ordinate and scale) and the increasing Ca concentration (abscissa). The average Br/Cl ratio of the analyzed waters is 0.248 (±9.44% 1σ), corresponding to a (calculated) freezing temperature between −23.3 and − 25.7 °C (Fig. 7). At this point (B on the black freezing line), the molal concentrations of Cl and Ca reach 5.4 and 0.15, respectively, and the brine becomes concentrated by a factor of 18.4 with respect to Permian seawater. The concentration of Cl at point B (5.4 m), which was achieved by the cryogenic process only,

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Fig. 6. The isostatic and hydrological evolution of a marine-cryogenic basin: a. Continent–ocean boundary before the onset of a glacial cycle. b. An ice sheet develops on the continent, depressing the crust underneath and forming a forebulge along the coast. Seawater infiltrates into the marginal trough between the ice edge and the forebulge and sea ice crystallizes on its surface. The resultant brine sinks to the bottom and infiltrates the underlying sediments and rocks via fissures and shear zones and by non-equilibrium melting of the ice sheet base. Then, it migrates inland, along the inclined ice–rock contact, towards the center of the depression. Loss of brine from the trench is compensated by fresh seawater flow through the forebulge. c. Upon glacial maximum the basement rocks below the ice sheet become saturated with brine. d. Increased melt water head developing during glacial decline, accompanied by postglacial lithospheric rebound, drive the brines outwards from the center of the glaciostatic depression to their present sites (from Starinsky and Katz, 2003).

marks also the brine's (upward) departure from the black freezing line and the onset of its WRI with the surrounding crystalline rocks. The main reflection of the WRI in the diagram is the increase in the Ca concentration in the brine, without a corresponding change in the Cl concentration. At the same time, though, the Mg concentration measured in the waters is negligibly low, indicating their active role in plagioclase chloritization. The Na analyses show that most, but

Fig. 7. A Ca vs. Cl diagrammatic model for the geochemical evolution of Permian seawater to deep saline waters in the Kaapvaal Craton, South Africa. The black line (filled rectangular markers) represents the calculated relationship between Ca and Cl during “net” freezing of Permian seawater. The red curve describes the (calculated) relationship, in the freezing brine, between the decreasing temperature (red, right hand ordinate and scale) and increasing Ca concentration (abscissa). The average Br/Cl ratio of the analyzed samples corresponds to a calculated freezing temperature between − 23.3 and − 25.7 °C. At point B on the freezing line, the molal concentrations of Cl and Ca reach 5.4 and 0.15, respectively, and the brine becomes concentrated by a factor of 18.4 with respect to Permian seawater. The concentration of Cl at point B marks also the onset of WRI. The main indication of the WRI is the increase in the Ca concentration in the brine, without a corresponding change in the Cl concentration. The Na analyses show that several waters participated also in albitization. Chloritization could have occurred before, or along with albitization.

not all waters (e.g. the samples from Evander), participated also in albitization. Chloritization could have occurred before, or along with albitization. Obviously, the WRI requires temperatures greatly exceeding the sub-zero range of the cryogenic environment (exhibited by the red curve in Fig. 7). Such were readily available in the deep terrain invaded by the brine, as dictated by the local geothermal gradient. Hence, the dense, saline fluid started to infiltrate the underlying crystalline rocks, chloritizing plagioclase along the green line until all Mg 2+ was consumed, and an equivalent amount of Ca 2+ was released into the brine (point C). Additional Ca 2+ was liberated into the solution by plagioclase albitization along the orange line to point Dmax. The next stage in the water evolution was its significant dilution by meteoric water. The dilution factors corresponding to each sample, defined as the ratio between the Br concentration in the original (cryogenic) brine (BrCryo) and its concentration in the samples (Brf), are presented in Table 1. Two dilution ranges can be noted, with the Driefontein waters being the least diluted (x2.9-8.1) and all the rest falling within a dilution range of 17–116. The late timing of the dilution emerges from the chloride-calcium relationship (Fig. 7) The “All samples” linear regression line shown (R 2 = 0.986) runs through the origin, indicating that dilution was carried out by freshwater. Because most of the Ca was derived from the aquifer rocks, dilution must have occurred after water–rock interaction. Three dilution lines are shown in Fig. 7: An “All samples” line (Dav–E) which is practically crossing the origin; line C–E represents waters that participate in chloritization only (Evander site); line E–Dmax contains the sample with the highest degree of albitization (sample DR546 BH1 020199). The lengths of the green (chloritization) and orange (albitization) sections on the vertical water–rock interaction path (B–DMax) allow estimating the relative contribution of the two processes to the Ca 2+ mass balance in the brine. Thus, chloritization added 1.04 mol Ca 2+ to the existing 0.153 mol of the cryogenic brine at point B, while albitization added on top of that another 0.66 mol per kg H2O in the brine, thereby defining an albitization/chloritization ratio range of approximately 0–0.66 for the Kaapvaal fluids studied.

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of a few kilometers. This was achieved by chloritization, expressed in the almost total removal of Mg from solution, and was accompanied by a partial loss of Na due to albitization. Both losses were compensated by addition of Ca from the host rocks to the solution. 5. The last process that affected the brines was dilution by fresh groundwater, which probably continues to present time. 6. Similar saline waters should be expected at depth in all rocks, crystalline and sedimentary alike, in regions that were covered by major glaciers, such as eastern South America, large parts of Australia, India and Antarctica during the Carboniferous–Permian.

Acknowledgments

Fig. 8. The distribution of the late Carboniferous Glacials in Gondwana, showing glaciations on one large landmass. The arrows indicate directions of ice flow. ● = Permian pole position (Taken from Crowley and North, 1991, after Sullivan, 1974). + = area of study.

We thank Tullis Onstott for very constructive remarks to an earlier version of this paper and for the provision of chemical analyses and samples throughout the project. Fruitful discussions with Ze'ev Reches, who provided the samples from Tau Tona as well as important information on the geology of the area, were of great help. We greatly benefited from advice and conversations with Dick Holland on several geochemical issues related to the composition of the ancient seawater under regard. Special thanks are extended to Kurt Bucher and two anonymous reviewers for their critical reading and helpful evaluation of the manuscript. Ahuva Agranat contributed efficient and professional help in the laboratory. Line drawings were prepared by Carmel Gorni.

References Estimating the minimum amount of Ca-plagioclase required for the above reaction is straightforward as shown below: 1. The mass of Ca required to match the maximum Ca gain per 1 kg H2O is 1.04 + 0.66 = 1.70 mol. 2. The molecular weight of Ca-plagioclase (CaAl2·Si2O8) is 278.212 g. 3. The minimum mass of Ca-plagioclase needed to satisfy the two aforementioned water–rock reactions is 278.212 × 1.70 = 472.96 g per 1 kg H2O. Finally, our study suggests that the saline waters found in the Kaapvaal Craton were concentrated by freezing of seawater during the Carboniferous–Permian glaciations in South Africa. In view of similar findings and interpretation made on Northern Hemisphere subsurface brines in crystalline shields, it may have a much fartherreaching significance. One should expect similar saline waters to be found at depth in all rocks, crystalline and sedimentary alike, in regions that were covered by major glaciers, for example eastern South America, large parts of Australia, India and Antarctica (Fig. 8). 4. Conclusions The present study allows to draw the following conclusions: 1. The saline waters found in crystalline rocks in deep gold mines in the Kaapvaal Craton, South Africa represent ancient seawater that was concentrated by freezing. 2. The absence of post-Permian glacial deposits or structures in South Africa allows attributing the age of the cryogenic brines to the Carboniferous–Permian, or earlier ice ages. 3. The finding of saline, marine-cryogenic waters in the Southern Hemisphere, in addition to the recent finding in Antarctica, expands the spatial and temporal distributions of this phenomenon, which so far was restricted to Pleistocene glaciations in the Northern Hemisphere. 4. Modification of the original seawater beyond their concentration by freezing continued during infiltration and migration to a depth

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