Fluid-induced mineral composition adjustments during exhumation ...

Contrib Mineral Petrol (2013) 166:1489–1503 DOI 10.1007/s00410-013-0939-5

ORIGINAL PAPER

Fluid-induced mineral composition adjustments during exhumation: the case of Alpine stilbite Kurt Bucher • Tobias B. Weisenberger

Received: 23 March 2013 / Accepted: 5 September 2013 / Published online: 26 September 2013 Ó Springer-Verlag Berlin Heidelberg 2013

Abstract Stilbite is locally present as a very late mineral on fractures and fissures of granitic basement in the Central Swiss Alps. Stilbite samples from the Gotthard rail base tunnel provide evidence that they originally formed as a K-absent variety at depth. However, all stilbite samples from surface outcrops above the tunnel display significant potassium concentrations. Interestingly, water from fractures in the tunnel (at 50 °C) is oversaturated with respect to stilbite and essentially potassium-free whereas waters from high-Alpine brooks above the tunnel (and at other high-Alpine areas) have unusually high K/Na ratios. The data suggest that stilbite that may actively form on fissures at tunnel level as a K-absent variety by precipitation from water. Older stilbite that originally formed as coatings on fracture walls was gradually exhumed and uplifted and finally reached the today’s erosion surface about 2,000 m above the tunnel. However, the stilbite reaches the erosion surface as a K-rich variety as a result of interaction of the original low-K stilbite with surface water and near-surface groundwater. This leads to the conclusion that minerals once formed at depth may significantly change their composition once they reach the ground water zone on their way to the erosion surface. In the case of the stilbite, if surface outcrops would have been the only source of samples and data, the K-rich composition could have been

Communicated by C. Ballhaus. K. Bucher (&) T. B. Weisenberger Institute of Earth and Environmental Sciences, Albert-LudwigsUniversita¨t, Albertstrasse 23b, 79104 Freiburg, Germany e-mail: [email protected] T. B. Weisenberger Department of Geosciences, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland

mistaken for the composition of the mineral when it formed, which is not the case. Late-stage compositional readjustments may be difficult to discern in samples from surface outcrops. The provided data show that original mineral compositions may be adjusted by late-stage water– rock interaction in a highly selective way. Keywords Stilbite Petrology Fluid–rock interaction Zeolites Alpine minerals

Introduction Typical methods used in petrology for deducing pressure and temperature conditions during petrogenesis are based on the assumption of preserved and locked equilibria that can be deciphered by equilibrium thermodynamics. A key condition for the success of thermobarometric methods (including ‘‘pseudosections’’) is that minerals once formed at elevated temperature and pressure perfectly preserves their original composition. Several processes may modify the distribution of components within minerals but also between minerals as rocks gets exhumed. Intra- and intercrystalline diffusion tends to homogenize compositional gradients in minerals. The homogenization process slows at low temperature, and thus, chemically zoned minerals are very common. However, low-T modifications of mineral compositions are mostly performed by fluid–rock interaction processes that, for example, transforms K-feldspar to albite by a process called albitization (K-feldspar ? Na? = Albite? K?). There is ample evidence that minerals that have been formed at some stage of the metamorphic evolution of an area have been chemically modified by later processes (Bucher-Nurminen 1988). The observed compositional

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variation of biotite in the contact aureole of the Bergell intrusion may serve as a classic example. The equilibrium composition of contact metamorphic phlogopite is characterized by high XMg and fixed Tschermak component with high Al in the tetrahedral layer. Later infiltration of the marbles by hydrothermal fluids dramatically decreased XMg in these phlogopites but left the tetrahedral layer untouched. New phlogopite precipitated from the same fluid have low XMg and low Al in the tetrahedral layer. The Bergell phlogopites show that low-grade hydrothermal overprint may selectively change parts of a mineral but leaving others untouched. In this paper, we report on a further delicate chemical modification observed on Alpine fissure minerals during cooling and exhumation. Alpine fissure minerals precipitate in open cavities from an aqueous fluid. Mineral growth often ceased before the open cavity has been completely filled in resulting in perfectly euhedral crystals coating the fissure walls. The type and composition of the minerals formed this way reflects the pressure and temperature at the time of formation as well as the compositional details of the fluid they have precipitated from. Alpine fissures formed when the rocks passed from the ductile to the brittle regime during cooling, uplift and exhumation. The most typical mineral fissure contains euhedral quartzes (rock crystals, smoky quartz and morion quartz) of very variable sizes. Typically, Alpine fissure quartzes are rich in fluid inclusions. Fluid inclusion studies showed that the hottest and earliest mineral fissures formed at about 400 °C and quartz-filled fissures continued to form to temperatures of less than 200 °C during exhumation (Mullis et al. 1994). The very latest minerals that precipitated from hydrothermal fluids include various zeolites, apophyllite and hematite (Weisenberger and Bucher 2010; Weisenberger et al. 2012). Here, we report on compositional changes of fissure zeolite in response to subtle changes in fluid composition caused by cooling and exhumation.

Zeolite minerals Zeolites are tectosilicates characterized by an open threedimensional framework of (Si, Al)O4 tetrahedra. The tetrahedra form a network of open channels containing molecular water and charge-balancing cations of alkali and alkali-earth metals. Their distinctive crystal structures result in the ability to hydrate–dehydrate reversibly and to exchange cations with aqueous solutions. Zeolites have a distinctly high cation exchange capacity (Pabalan and Bertetti 2001). Zeolites typically form from very-low-grade alteration of primary minerals and glass of volcanic rocks by reaction with an aqueous fluid at high pH conditions (Bish and Ming 2001). The chemical composition of

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individual zeolite species can vary widely often related to the geological setting and to the host rock. The zeolites of interest in this study are stellerite Ca4Al8 Si28O7228H2O and stilbite, NaCa4Al9Si27O7228H2O. The two species mix continuously and form a solid-solution series (Fridriksson et al. 2001) that we call stilbite in this paper for simplicity. Most stilbites contain only small amounts of potassium. Sodium is normally much higher than potassium (Na/(Na ? K) = 0.70–0.98) in stilbite from different geologic settings (Passaglia and Sheppard 2001). Exceptional stilbites, however, occur in crystalline basement rocks, which may contain a significant amount of K (Passaglia et al. 1978; Weisenberger and Bucher 2010). Interestingly, only stilbite samples from surface outcrops show potassium enrichment. Subsurface stilbite samples, e.g., from tunnels contain very little potassium. In this study on cation exchange in a natural system, we characterize the exchange mechanism using both zeolite and fluid composition data from both the site of formation of zeolite at depth and from samples from surface outcrops.

Geological setting The studied stilbite samples have been collected in the Central Swiss Alps, an orogenic belt that formed during Late Cretaceous and Tertiary time. (e.g., Frisch 1979; Schmid et al. 2004; Tru¨mpy 1960). The late collision stage of the orogeny caused a complicated tectonic structure and a late Barrovian-style regional metamorphism in the Central Swiss Alps. Collision was followed by uplift and erosion beginning in the late Tertiary. It continuously exhumes deeper structural elements and brings them to the erosion surface (Engi et al. 2004; Weisenberger et al. 2012). Alpine zeolites occur predominantly in the fracture porosity of the Aar and Gotthard massifs (e.g., Tru¨mpy 1980; Weisenberger and Bucher 2010). These so-called external massifs represent parautochthonous units of the European plate (Pfiffner 1986), forming a 115-km-long and 23–40-km-wide SW–NE trending outcrop of pre-Alpine basement (Fig. 1). The massifs represent erosional windows and expose typical continental crust, predominantly granites and gneisses. This pre-Alpine basement has been overprinted by the Tertiary Alpine metamorphism. Alpine peak metamorphic grade in the Aar- and Gotthard Massif region increases from lower greenschist facies in the north to lower amphibolite facies in the south. Maximum temperatures in the northern Aar massif reach about 300 °C. This temperature has been derived from data on illite crystallinity, vitrinite reflection, and fluid inclusion measurements by Breitschmid (1982). It is just above the temperature window for zeolite stability (Weisenberger et al. 2012).


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(a)

(b)

Fig. 1 Geological map (simplified from Tru¨mpy 1980) and sample locations

During post-orogenic uplift and erosion, the Central Swiss Alps underwent extensive late brittle deformation that created a complex multistage network of fractures and faults including extensional structures. The produced fracture porosity is normally occupied by an aqueous fluid that is in thermal equilibrium with the rock matrix but may be at hydrostatic pressure. If the connectivity of the fracture network is high, it also will permit fluid advection. Migrating fluid may not be in chemical equilibrium with the rock matrix and may react with the rocks exposed along the fissure walls to form secondary vein or fissure minerals (Bucher-Nurminen 1981; Bucher et al. 2012). In particular, characteristic sequences of fissure minerals precipitate from fluids with decreasing temperature. Zeolites and apophyllite are the youngest silicate mineral formed in Alpine fissures of the massifs. Ca-dominated zeolites precipitated from fluid with decreasing temperature in the order (old to young = hot to cold): scolecite, laumontite, heulandite, chabazite and stilbite (Weisenberger and Bucher 2010). The necessary components for zeolite formation are derived from dissolving primary minerals of granite and gneiss. The nature of these minerals depends, among other factors, on the metamorphic history of the host rock.

Zeolites of the Aar massif derived from the dissolution of epidote, secondary calcite and albite that were originally formed during Alpine greenschist metamorphism from primary granite and gneiss assemblages. Zeolite in Alpine amphibolite facies rocks of the Gotthard massif and the Lepontine Alps formed during fluid-induced decomposition and albitization of plagioclase. The released anorthite component has been used for zeolite formation (Weisenberger and Bucher 2011). Zeolite fissures occur in areas of H2O-dominated fluids. This is consistent with equilibrium calculations that predict a low CO2 tolerance of zeolite assemblages, particularly at low temperature (Weisenberger and Bucher 2010). The formation of zeolite is limited to the late exhumation period of the Central Swiss Alps. Weisenberger et al. (2012) showed that the abundant late fissure minerals laumontite and apophyllite predate stilbite. Stilbite formed very late at low temperature in the shallow crust. Laumontite formed at a maximum depth of 6,600 m and apophyllite at a depth 2,550 m below surface at a minimum temperature of 70 °C. Thus, late stilbite formed at even lower temperature. Stilbite may precipitate on fissures at conditions presently encountered at the sampling site in the rail base tunnel (up to 45 °C; Bucher et al. 2012).

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Mineral and water samples

Methods

We collected and analyzed zeolite samples from surface and subsurface localities (Table 1) in the Central Swiss Alps. Subsurface samples derive from two major Alpine tunnels: the Gotthard road tunnel, which was built in the 1970s and the Gotthard rail base tunnel (NEAT), which is currently under construction (Figs. 1, 2). Excellent mineral samples from the Amsteg section of the Gotthard tunnel, were made available by the authorized mineral inspector Peter Amacher. The Amsteg section of the NEAT tunnel was drilled in the period 2003–2006. Samples from the Gotthard Road tunnel have been provided by the Natural History Museum Bern. Surface samples originate from several high-Alpine outcrops in the eastern Aar massif. In addition, two surface samples from Gibelsbach in the Western Aar massif are analyzed (Table 1; Fig. 1). The NEAT tunnel stilbite specimens are samples from several fissures between tunnel meter 14,375 and 17,295. Central Aar granite, Southern Aar granite and the southern granite gneisses are host rocks of the stilbite-bearing fissures. The rock column above the sampling locations ranges from 1,580 to 2,130 m. Stilbite occurs as one of the latest minerals in Alpine fissures (Weisenberger and Bucher 2010). Crystals typically occur in aggregates of variable size and shape but rarely as isolated crystals. Observed crystal morphologies include sheaf-type, bow-tie, globular, radially fibrous and columnar. The {010} plane is the prominent plane, which is the contact plane for crystal aggregates. Stilbite, in general, is transparent to translucent, colorless to white. Globular stilbite from surface outcrops in the Rien valley hosted in quartz veins occurs as light reddish to brownish aggregates. For detail description of stilbite and zeolite occurrences in the Central Swiss Alps see Weisenberger and Bucher (2010). About 120 water samples from the Amsteg section of the NEAT tunnel have been collected and analyzed. All analytical data are documented in Bucher et al. (2012). The water enters the tunnel along water-conducting structures that were opened by the excavation, mostly fractures, fissures, gash veins and brittle fault zones (Fig. 2). Water data and water–rock interaction models for the stilbite sampling localities have been presented by Bucher and Stober (2010) and Bucher et al. (2012). Surface water samples from Alpine brooks above the NEAT tunnel axis have been collected and analyzed by Seelig (2009). The surface water samples from the mountains above the tunnel are supplemented by [80 water analyses from high-Alpine catchments of the Zermatt area (Zhou 2010).

Quantitative analysis of stilbite mineral composition was performed at the Institute of Geosciences, University of Freiburg, using a CAMECA SX 100 electron microprobe equipped with five WD spectrometers and one EDS detector with an internal PAP-correction program (Pouchou and Pichior 1991). Major and minor elements for zeolites were determined at 15 kV accelerating voltage and 10 nA beam current with a defocused electron beam of 20 lm in diameter and counting times up to 20 s. Na and K were measured first, to minimize the effect of Na and K loss during determination. Since the zeolite loses water when heated, the stilbite crystals were mounted in epoxy resin to minimize loss of water. Natural and synthetic standards were used for calibration. The charge balance of zeolite formulas is a reliable measure for the quality of the analysis. It correlates with the extent of thermal decomposition of zeolite during microprobe analysis. A useful test is based on the charge balance between the nonframework cations and the amount of tetrahedral Al (Passaglia 1970). Analyses are considered acceptable if the sum E% = ([100*((Al)-(Na ? K) ? 2(Mg ? Ca ? Sr ? Ba)/ (Na ? K) ? 2(Mg ? Ca ? Sr ? Ba))]) of the charge of the extraframework cations (Ca2?, Sr2?, Na?, and K?) is within 10 % of the framework charge. Quantitative analysis of water samples have been prepared as described in Zhou (2010), Seelig and Bucher (2010), and Bucher et al. (2012).

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Results Composition of stilbite Ca is the dominant extraframework cation in all Alpine stilbite samples (Fig. 3). The Na content in subsurface and surface samples varies with the Si/Al ratio (Fig. 3). The average value for Na in surface stilbite is slightly lower than the average value for subsurface samples (0.45 atoms per formula unit; a.p.f.u.). In general, Na increases from the core to the rim in a single stilbite crystal. The K content of surface and subsurface samples is remarkably different (Tables 2, 3; Fig. 3). Subsurface samples contain only traces of K (average value 0.04 a.p.f.u.). Na and Ca are the absolutely dominant extraframework cations (Table 2; Fig. 3). In contrast, surface samples contain K as additional major extraframework cation (Table 3; Fig. 3). The K content of surface samples range between 0.28 and 0.93 a.p.f.u., with an average value of 0.59 a.p.f.u.


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Table 1 Stilbite assemblages in Alpine fissures analyzed in this study Sample no

Mineral assemblage

Description

Locality

35370

Stb

Light brownish Stb crystal, up to 3 mm, grown on top of fine Qtz crystals, fissure breccia 2,830 m after north portal

Gotthard road tunnel

35508 35843

Kfs-Chl-Stb Sco-Stb

Light pinkish Stb, southern granite gneisses (3,240 m) Stb on top of Sco

Gotthard road tunnel Gotthard road tunnel

36728

Qtz-Cc-Ms-Ttn-Stb

Stb crystals up to 5 mm growing on Cc, Fibbia granite gneisses

Gotthard road tunnel

A4

Qtz-Kfs-Stb

Stb covers Qtz and Kfs as dense mats, up to 3 mm long white euhedral needles

Gotthard NEAT

A6

Qtz-Stb

Stb forming flat-topped crystals and fan-like crystal aggregates, grown on Qtz

Gotthard NEAT

TW01.2

Qtz-Chl-Lmt-Stb

Stb associated with Lmt and Chl as early phases in a fissure

Gotthard NEAT

TW01.3

Lmt-Stb

Lmt associated with Stb as early phase in a fissure

Gotthard NEAT

Lmt on Qtz associated with Stb as early phase in a fissure

Gotthard NEAT Val Strem/Gr

Tunnel sample

TW02 Qtz-Lmt-Stb Surface samples A3

Stb

Light brownish Stb crystals

B11

Sco-Hul-Stb

Euhedral crystals of Sco, Hul and Stb

Schattig Wichel/Ur

B8

Qtz-Stb

Light brownish Stb, forming radial groups of 1 cm in diameter, hosted in quartz lenses in paragneisses

Riental/Ur

DT

Hul-Stb

Light brownish Stb crystals, forming 5 cm long fanlike bow ties, which associated with early-formed Hul crystals on top of a highly porous matrix

Drumtobel/Gr

Fi1

Qtz-Hul-Stb

Euhedral Stb crystals, up to 1 cm in size associated with Hul and Qtz

Gibelsbach/Vs

Fi2

Flt-Stb

Stb on green fluorite

Gibelsbach/Vs

R1

Qtz-Stb

Light brownish Stb, forming radial groups of 1 cm in diameter, hosted in quartz lenses in paragneisses

Riental/Ur

TW20

Sco-Hul-Stb

Euhedral crystals of Sco, Hul and Stb

Schattig Wichel/Ur

TW34

Hul-Stb

Light brownish Stb crystals, forming 5 mm long bow ties, which associated with early-formed Hul crystals, on a highly porous matrix

Val Strem/Gr

Also molar Na/(Na ? K) in Alpine stilbite is distinctly different in surface and subsurface samples (Fig. 3). The mole fraction of Na (Na/(Na ? K)) in subsurface samples varies between 0.78 and 1.00 with an average of 0.90. In contrast, surface samples yield Na/(Na ? K) between 0.00 and 0.51 with an average ratio of 0.31. The composition range of two populations does not overlap. The increased K content in surface stilbite is paralleled by a decrease in Ca and Na (Fig. 3). In addition, an increase in the sum of extraframework cations occurs in surface samples (Fig. 4). Subsurface stilbite yields a range between 4.03 and 5.01 a.p.f.u., with an average sum of 4.54 a.p.f.u. (Fig. 4; Table 2), whereas surface stilbite shows slightly higher extraframework cations occupancy between 4.32 and 5.47 a.p.f.u., with an average extraframework occupancy of 4.82 a.p.f.u. (Fig. 4; Table 3). Composition of water Na is the dominant cation in most NEAT Gotthard tunnel waters near the zeolite sampling localities (Table 4;

Figs. 2, 4). The associated anions are predominantly CO32- and HCO3- in addition to SO42- and Cl-. The waters are essentially solutions containing dissolved natrite (Na2CO3), thenardite (Na2SO4) and halite (NaCl). Consequently, pH is remarkably high and may reach 10.5 in the zones where zeolites are found on the fissures (Bucher et al. 2012). The average molar Na/Ca ratio is about 6. Potassium is very low in all waters. The molar Na/K ratio varies from 30 to more than 100 with an average of 62 (Table 4; Fig. 5). The temperature of the waters at the zeolite sampling points varies from 40 to 45 °C. All these waters are strongly supersaturated with respect to zeolite minerals (PHREEQC; LLNL data base). The highest saturation index (SI) values have been computed for stilbite (Bucher et al. 2012), which implies that stilbite is the stable zeolite phase under the present day conditions (Table 4). These findings are consistent with the mineral stability diagram (Fig. 6) showing the log activity of dissolved Ca2? (referenced to a hypothetical one molal solution at infinite dilution) normalized to pH versus the log activity of dissolved SiO2aq (at the same standard state). Most tunnel waters are

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Fig. 2 Simplified cross section along the Amsteg section of the Gotthard tunnel

Fig. 3 Composition of stilbite from localities shown on Figs. 1 and 2. Endmember stellerite and stilbite are indicated with green dots

consistent with the presence of stilbite. Note that quartz saturation varies widely with pH at the conditions of the diagram (5). Thus, the SI of the waters (Table 4) with respect to quartz SIQtz varies in the range of -0.3 to 0.3. All waters are very close to equilibrium with quartz. The total amount of dissolved solids (TDS) ranges from 120 to 500 mg/L for the Na-rich high-pH waters. Very few tunnel waters contain major amounts of Ca. The associated anion in high-Ca waters is SO4. CaSO4 waters are related to some distinct fissure systems where fissure anhydrite dissolves (Bucher et al. 2012). These waters are unrelated to the zeolite-bearing fissures. Some selected water analyses from zeolite fissures can be found in Table 4 together with the SI for stilbite.

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Surface waters in high-Alpine brooks, after having passed through rocky talus or run in bare rocky beds above the treeline have low TDS ranging from below 20 to about 100 mg/ L (Table 5). All surface waters from the Aar massif and also the Zermatt waters from granitic and gneissic catchments are clearly Ca dominated. The molar Na/Ca ration varies from 0.01 to 0.7 with an average value of 0.1 corresponding to 10 times more Ca than Na (Seelig 2009; Zhou 2010). The dominant anion is always HCO3-. Carbonate equilibria buffer pH to the range 7.5 and 8.2. A very significant feature of high-Alpine surface waters is their unusually low Na/K molar ratio (Fig. 5). The average molar Na/K ratio of more than 100 Zermatt water samples is 0.82. Na/K is generally low irrespective of the dominant catchment rock (Zhou 2010). Similar data have been collected from waters in the Aar massif (Seelig 2009). The average molar Na/K of surface water from Go¨scheneralp immediately west of the NEAT tunnel (1.28) and from the Etzli valley and the Val Strem area directly above the tunnel is 1.26 (Table 5; Fig. 5). It is evident from the data that high-Alpine surface waters contain about equal molar amounts of potassium and sodium (K/ Na * 1.0). This is in very sharp contrast to the tunnel waters with an average Na/K ratio of 62.

Discussion Cation exchange in stilbite Subsurface and surface stilbite show specific chemical variation with respect to framework and extraframework cations. Specific chemical characteristics of surface and subsurface stilbite are inherited during primary mineralization. During exhumation and cooling subsurface stilbite

58.12

62.55

16.48

8.48

0.57

0.10

88.22

SiO2

Al2O3

CaO

Na2O

K2O

Totalsa 80.60

0.07

0.70

7.73

14.72

57.26

TW02 4

84.33

0.05

0.51

8.36

15.78

59.55

TW02 5

72

2.0

0.76

0.90

3.99

0.49

0.05

72

0.1

0.76

Ca

Na

K

O

E%b

Si/(Si/Al)

0.32

0.08

35370 2

58.24

13.62

7.90

0.04

0.05

79.87

Sample no Analysis no

SiO2

Al2O3

CaO

Na2O

K2O

Totalsa 79.87

0.11

0.64

7.58

14.37

57.12

35370 4

0.01

0.94

0.77

-3.7

72

0.04

0.65

3.98

8.35

27.54

80.29

0.03

0.27

7.74

14.14

58.11

35370 5

0.01

0.94

0.76

-2.2

72

0.03

0.46

4.12

8.56

27.39

0.78

0.55

0.01

K/(K ? Na ? Ca)

E%b

Na/(Na ? K)

72

-6.0

O

Si/(Si/Al)

0.04

0.03

4.09

Ca

K

7.75

Al

Na

28.12

Si

0.01

0.86

0.77

-2.1

72

0.05

0.30

3.94

8.11

27.84

0.01

0.90

0.77

-4.1

72

0.07

0.60

3.94

8.21

27.70

0.00

0.93

0.78

-2.8

72

0.02

0.25

3.99

8.01

27.93

Anhydrous formula unit composition

79.67

7.58

14.18

57.39

35370 3

0.01

0.90

0.01

Na/(Na ? K)

K/(K ? Na ? Ca)

0.04

0.38

3.94

8.50

8.53

Al

27.55

27.46

Si


81.63

0.07

0.41

7.75

15.21

TW02 2

TW02 1


0.01

0.86

0.78

-3.0

72

0.02

0.14

4.06

8.07

27.86

85.82

0.04

0.16

8.42

15.22

61.90

35370 6

0.01

0.91

0.77

-5.0

72

0.04

0.39

3.99

8.08

27.81

80.47

0.06

0.42

7.75

14.25

57.80

TW02 7

0.01

0.83

0.78

-0.5

72

0.03

0.14

3.96

8.05

27.93

87.92

0.05

0.16

8.42

15.57

63.65

0.01

0.82

0.78

-4.8

72

0.04

0.17

4.06

7.93

27.94

79.94

0.06

0.18

7.85

13.93

57.83

0.02

0.78

0.77

-4.5

72

0.08

0.28

4.07

8.13

27.76

87.00

0.14

0.32

8.54

15.50

62.42

0.01

0.82

0.78

-4.1

72

0.03

0.13

4.08

7.98

27.92

83.93

0.05

0.15

8.27

14.71

60.69

35370 10

0.01

0.95

0.76

2.3

72

0.03

0.63

3.95

8.76

27.29

79.87

0.05

0.67

7.59

15.31

56.23

TW01,2 2

35370 9

0.01

0.92

0.77

-0.9

72

0.03

0.36

3.94

8.21

27.76

79.76

0.05

0.39

7.58

14.38

57.31

TW01,2 1

35370 8

0.01

0.95

0.77

-0.5

72

0.03

0.62

3.85

8.33

27.66

80.88

0.05

0.67

7.52

14.78

57.83

TW02 11

35370 7

0.01

0.92

0.77

-0.5

72

0.04

0.49

3.92

8.34

27.65

80.06

0.07

0.52

7.57

14.65

57.23

TW02 9

Table 2 Mineral composition of stilbite subsurface (tunnel) samples

0.00

0.60

0.78

-1.6

72

0.02

0.03

3.98

7.89

28.08

79.66

0.03

0.03

7.69

13.84

58.05

35370 11

0.01

0.93

0.76

3.8

72

0.03

0.38

3.97

8.66

27.42

80.69

0.05

0.41

7.72

15.33

57.18

TW01,2 3

0.01

0.78

0.78

-3.3

72

0.04

0.14

4.04

7.97

27.94

87.32

0.07

0.16

8.52

15.30

63.17

35370 12

0.01

0.97

0.77

-3.5

72

0.03

0.82

3.82

8.20

27.73

80.14

0.05

0.88

7.39

14.40

57.41

TW01,2 5

0.02

0.86

0.76

-4.4

72

0.09

0.57

4.07

8.45

27.45

87.89

0.16

0.66

8.60

16.23

62.15

35843 1

0.00

0.98

0.76

-1.8

72

0.01

0.76

4.04

8.70

27.26

89.82

0.02

0.91

8.72

17.09

63.07

TW01,2 4

0.01

0.78

0.78

-7.0

72

0.06

0.20

4.18

8.00

27.84

87.54

0.10

0.23

8.82

15.37

63.01

35843 5

0.01

0.96

0.76

-1.3

72

0.03

0.83

3.92

8.60

27.37

81.36

0.05

0.90

7.67

15.31

57.41

TW01,3 1

0.01

0.90

0.77

-3.0

72

0.03

0.31

4.10

8.29

27.63

86.10

0.06

0.35

8.52

15.64

61.46

35843 7

0.01

0.90

0.77

-1.9

72

0.05

0.46

4.02

8.40

27.56

79.30

0.08

0.49

7.69

14.59

56.44

TW01,3 2

0.01

0.90

0.77

-2.9

72

0.03

0.26

4.05

8.14

27.80

86.77

0.05

0.30

8.48

15.51

62.42

35843 8

0.01

0.43

0.78

-1.8

72

0.02

0.02

4.00

7.90

28.06

81.20

0.04

0.02

7.87

14.12

59.13

35370 1

0.01

0.83

0.77

-2.5

72

0.04

0.20

4.10

8.23

27.69

86.09

0.07

0.23

8.52

15.54

61.62

35843 10

0.01

0.55

0.78

-6.0

72

0.03

0.04

4.09

7.75

28.12

79.87

0.05

0.04

7.90

13.62

58.24

35370 2

0.01

0.92

0.77

-8.2

72

0.04

0.52

4.08

8.05

27.73

80.35

0.07

0.56

7.90

14.16

57.47

35843 11

0.01

0.43

0.78

-1.8

72

0.02

0.02

4.00

7.90

28.06

81.20

0.04

0.02

7.87

14.12

59.13

35370 1

Contrib Mineral Petrol (2013) 166:1489–1503 1495

123

123

SiO2 Al2O3 CaO Na2O K2O Totalsa

b

a

Totals include traces of Ba, Sr, Mg, Ti, Mn and Fe estimate of charge balance

Si Al Ca Na K O E%b Si/(Si/Al) Na/(Na ? K) K/(K ? Na ? Ca)

57.22 61.85 57.01 16.00 16.31 15.73 8.15 8.61 8.36 0.92 0.59 0.59 0.08 0.15 0.15 82.44 87.54 81.92 formula unit composition 27.02 27.41 27.09 8.90 8.52 8.81 4.12 4.09 4.26 0.84 0.51 0.54 0.05 0.08 0.09 72 72 72 -2.8 -3.0 -4.1 0.75 0.76 0.75 0.95 0.86 0.86 0.01 0.02 0.02

56.20 15.96 8.09 0.88 0.06 81.26 Anhydrous 26.93 9.01 4.15 0.82 0.04 72 -1.6 0.75 0.96 0.01


36738 11

36738 9

36738 8

Si Al Ca Na K O E%b Si/(Si/Al) Na/(Na ? K) K/(K ? Na ? Ca) 36738 10

55.95 13.80 7.95 0.46 0.06 78.24

56.31 56.12 56.08 55.88 14.03 14.06 13.90 13.95 7.55 7.60 7.71 7.49 0.69 0.30 0.58 0.51 0.11 0.09 0.08 0.16 78.74 78.23 78.38 78.12 Anhydrous formula unit composition 27.71 27.74 27.73 27.72 8.14 8.19 8.10 8.16 3.98 4.03 4.08 3.98 0.66 0.29 0.56 0.49 0.07 0.06 0.05 0.10 72 72 72 72 -6.4 -2.5 -7.7 -5.1 0.77 0.77 0.77 0.77 0.91 0.84 0.92 0.83 0.01 0.01 0.01 0.02

SiO2 Al2O3 CaO Na2O K2O Totalsa

27.26 8.64 4.19 0.56 0.08 72 -4.1 0.76 0.88 0.02

57.78 15.55 8.29 0.61 0.13 82.39

36738 12

27.72 8.06 4.22 0.44 0.04 72 -9.8 0.77 0.92 0.01

35508 5

35508 3

35508 4

35508 2

35508 1


Table 2 continued

27.20 8.74 4.15 0.57 0.06 72 -2.2 0.76 0.90 0.01

59.81 16.31 8.52 0.65 0.11 85.42

27.30 8.62 4.12 0.61 0.04 72 -3.4 0.76 0.93 0.01

54.28 14.55 7.64 0.63 0.07 77.25

35508 8

27.60 8.43 4.02 0.23 0.02 72 1.4 0.77 0.91 0.01

56.70 14.70 7.70 0.25 0.04 79.43

A4 3

27.75 8.12 4.01 0.45 0.03 72 -4.5 0.77 0.95 0.01

56.19 13.95 7.58 0.47 0.04 78.34

35508 7

36738 13

27.83 8.08 3.99 0.33 0.06 72 -4.0 0.78 0.84 0.01

56.88 14.01 7.62 0.35 0.10 79.07

35508 6

27.40 8.65 4.02 0.36 0.01 72 2.6 0.76 0.96 0.00

56.64 15.16 7.75 0.39 0.02 80.02

A4 4

27.90 7.97 3.98 0.44 0.04 72 -5.4 0.78 0.91 0.01

57.19 13.87 7.61 0.46 0.07 79.23

35508 9

27.51 8.44 4.02 0.51 0.00 72 -1.6 0.77 1.00 0.00

58.15 15.14 7.93 0.56 0.00 81.94

A4 5

27.66 8.17 4.07 0.63 0.06 72 -7.8 0.77 0.91 0.01

55.24 13.84 7.59 0.65 0.10 77.46

35508 13

27.39 8.53 4.09 0.58 0.02 72 -3.0 0.76 0.97 0.00

56.30 14.88 7.84 0.61 0.03 79.81

A6 1

27.67 8.23 4.11 0.37 0.04 72 -4.7 0.77 0.89 0.01

57.05 14.39 7.91 0.39 0.07 79.82

36738 1

27.57 8.37 4.13 0.29 0.05 72 -2.8 0.77 0.86 0.01

62.29 16.04 8.71 0.34 0.08 87.46

A6 2

27.20 8.67 4.22 0.63 0.04 72 -5.1 0.76 0.94 0.01

60.11 16.26 8.71 0.72 0.07 85.94

36738 2

27.45 8.56 3.97 0.42 0.04 72 0.7 0.76 0.92 0.01

61.42 16.25 8.28 0.49 0.07 86.72

A6 3

27.22 8.68 4.24 0.54 0.02 72 -4.2 0.76 0.97 0.00

60.15 16.27 8.74 0.62 0.03 85.90

36738 3

27.35 8.63 4.03 0.49 0.02 72 -0.1 0.76 0.95 0.01

56.17 15.04 7.72 0.52 0.04 79.64

A6 4

27.20 8.67 4.21 0.71 0.05 72 -5.9 0.76 0.93 0.01

59.48 16.08 8.59 0.80 0.09 85.10

36738 4

27.43 8.54 4.00 0.64 0.04 72 -1.6 0.76 0.94 0.01

56.50 14.92 7.68 0.68 0.07 79.86

A6 5

27.34 8.60 4.12 0.52 0.05 72 -2.4 0.76 0.91 0.01

59.43 15.86 8.36 0.58 0.09 84.36

36738 5

27.13 8.85 4.02 0.69 0.03 72 0.4 0.75 0.96 0.01

61.80 17.11 8.55 0.81 0.05 88.54

A6 7

27.17 8.81 4.03 0.67 0.06 72 0.1 0.76 0.92 0.01

55.99 15.40 7.75 0.71 0.10 80.05

36738 6

26.98 9.06 4.01 0.75 0.07 72 2.1 0.75 0.92 0.01

61.34 17.47 8.50 0.88 0.12 88.43

A6 8

27.48 8.47 4.12 0.39 0.04 72 -2.4 0.76 0.90 0.01

56.66 14.81 7.92 0.42 0.07 79.89

36738 7

1496 Contrib Mineral Petrol (2013) 166:1489–1503

55.94

59.32

15.00

8.55

0.18

0.65

83.74

SiO2

Al2O3

CaO

Na2O

K2O

Totalsa

0.37 0.66

16.62

8.73

0.39

0.63

89.49

Al2O3

CaO

Na2O

K2O

Totalsa

80.86

0.76

0.16

7.56

15.19

57.06

R1 3

82.61

0.52

0.20

7.91

15.20

58.74

Fi2 4

0.13

0.15

0.76

1.9

72

0.58

0.10

3.92

8.70

27.34

88.61

0.60

0.35

8.64

16.53

62.45

Fi2 5

0.10

0.24

0.76

1.6

72

0.47

0.15

3.89

8.60

27.42

72

0.46 0.08

0.48

0.07

Na/(Na ? K)

-3.7

K/(K ? Na ? Ca)

E%b 0.77

72

-3.7

O

0.37

0.31

4.00

8.39

27.52

0.76

0.33

0.35

4.06

Ca

K

8.51

Al

Na

27.40

Si

0.07

0.37

0.77

-0.5

72

0.31

0.18

3.98

8.41

27.57

0.07

0.47

0.76

-2.5

72

0.34

0.30

4.06

8.55

27.40


88.87

8.54

16.28

62.94

63.06

SiO2

Fi2 2

0.18

Fi2 1

0.08

K/(K ? Na ? Ca)

0.27

0.76

-0.5

72

0.93

0.34

3.76

8.80

27.19


0.77

0.30

E%b

Na/(Na ? K)

72

-9.3

O

Si/(Si/Al)

0.16

0.38

4.26

Ca

K

8.21

Al

Na

27.56

Si

Si/(Si/Al)

80.98

0.95

0.11

7.62

15.36

56.91

R1 2


80.46

1.50

0.36

7.23

15.37

R1 1

TW020 5


Table 3 Mineral composition of stilbite surface samples

0.07

0.26

0.77

0.8

72

0.28

0.10

3.94

8.36

27.65

84.06

0.48

0.11

7.99

15.40

60.02

Fi2 6

0.10

0.41

0.76

3.0

72

0.46

0.32

3.85

8.75

27.30

84.01

0.78

0.36

7.77

16.05

59.01

R1 4

0.08

0.44

0.76

-2.8

72

0.35

0.28

4.01

8.46

27.48

89.48

0.63

0.33

8.62

16.52

63.25

Fi2 7

0.12

0.25

0.75

2.3

72

0.57

0.19

3.96

8.88

27.17

81.49

0.93

0.20

7.73

15.77

56.83

R1 5

0.11

0.30

0.76

-3.2

72

0.51

0.22

4.01

8.50

27.43

90.26

0.92

0.26

8.68

16.72

63.60

Fi2 8

0.10

0.19

0.76

2.2

72

0.46

0.11

3.86

8.51

27.52

82.45

0.77

0.12

7.66

15.34

58.44

R1 6

0.14

0.51

0.74

0.0

72

0.77

0.81

3.83

9.37

26.63

88.54

1.36

0.94

8.07

17.94

60.11

TW34 1

0.13

0.42

0.76

1.3

72

0.62

0.45

3.80

8.81

27.22

84.65

1.05

0.50

7.70

16.23

59.14

R1 7

0.15

0.46

0.74

-3.9

72

0.79

0.68

4.00

9.20

26.71

89.06

1.41

0.79

8.46

17.70

60.57

TW34 2

0.17

0.28

0.75

-0.3

72

0.84

0.33

3.86

8.90

27.07

87.68

1.48

0.38

8.07

16.93

60.67

R1 8

0.13

0.47

0.75

-5.3

72

0.70

0.62

3.99

8.95

26.92

88.00

1.23

0.72

8.36

17.03

60.39

TW34 3

0.13

0.39

0.75

-0.1

72

0.66

0.41

3.89

8.85

27.14

86.15

1.14

0.47

8.02

16.57

59.91

R1 9

0.12

0.31

0.76

-4.0

72

0.61

0.28

4.04

8.67

27.23

87.06

1.07

0.32

8.40

16.39

60.68

TW34 5

0.16

0.16

0.76

0.1

72

0.75

0.15

3.88

8.69

27.31

86.37

1.31

0.17

8.02

16.33

60.51

R1 10

0.13

0.50

0.76

3.3

72

0.63

0.63

3.50

8.60

27.45

82.61

1.05

0.69

6.94

15.49

58.27

DT 8

0.13

0.49

0.75

1.2

72

0.64

0.61

3.75

8.90

27.13

88.76

1.14

0.71

7.95

17.17

61.67

R1 11

0.15

0.48

0.77

-0.9

72

0.72

0.66

3.56

8.45

27.53

86.83

1.26

0.76

7.41

15.99

61.36

DT 9

0.15

0.14

0.75

-1.2

72

0.75

0.12

4.02

8.83

27.14

87.51

1.32

0.14

8.41

16.78

60.79

R1 12

0.13

0.50

0.76

-0.3

72

0.62

0.62

3.66

8.53

27.45

87.79

1.09

0.72

7.70

16.32

61.89

DT 11

0.12

0.28

0.77

-0.7

72

0.57

0.22

3.79

8.32

27.65

81.42

0.93

0.24

7.41

14.80

57.94

R1 13

0.10

0.42

0.74

4.3

72

0.49

0.37

4.06

9.47

26.63

81.00

0.80

0.39

7.84

16.64

55.15

A3 1

0.11

0.39

0.75

-0.2

72

0.56

0.36

4.13

9.17

26.82

87.22

0.98

0.41

8.59

17.36

59.83

Fi1 1

0.12

0.36

0.74

4.7

72

0.59

0.33

3.98

9.39

26.70

80.77

0.96

0.35

7.67

16.46

55.19

A3 2

0.10

0.34

0.74

-1.0

72

0.51

0.27

4.29

9.39

26.58

84.66

0.87

0.30

8.65

17.21

57.40

Fi1 2

Contrib Mineral Petrol (2013) 166:1489–1503 1497

123

123

55.19

55.15

16.64

7.84

0.39

0.80

81.00

SiO2

Al2O3

CaO

Na2O

K2O

Totalsa

0.10

K/(K ? Na ? Ca)

b

0.12

0.36

0.74

4.7

72

0.59

0.33

3.98

9.39

26.70

0.11

0.31

0.74

7.3

72

0.50

0.22

3.98

9.53

26.62

estimate of charge balance

Totals include traces of Ba, Sr, Mg, Ti, Mn and Fe

0.74

0.42

4.3

E%b

Na/(Na ? K)

72

O

Si/(Si/Al)

0.37

0.49

4.06

Ca

K

9.47

Al

Na

26.63

Si

a

80.47

0.81

0.24

7.65

16.64

54.82

A3 3

80.98

0.90

0.15

7.69

15.65

56.36

A3 4

0.12

0.20

0.75

1.5

72

0.55

0.14

3.97

8.88

27.14


80.77

0.96

0.35

7.67

16.46

A3 2

A3 1


Table 3 continued

0.12

0.15

0.75

3.0

72

0.55

0.09

4.05

9.05

27.02

79.98

0.88

0.10

7.76

15.75

55.43

A3 5

0.12

0.47

0.75

-0.5

72

0.62

0.55

3.96

9.05

26.94

85.49

1.06

0.62

8.08

16.79

58.93

B8 1

0.14

0.39

0.75

4.3

72

0.66

0.41

3.75

8.95

27.14

87.33

1.16

0.48

7.83

17.01

60.81

B8 2

0.14

0.24

0.75

2.1

72

0.69

0.21

3.87

8.86

27.17

87.94

1.22

0.25

8.14

16.95

61.24

B8 3

0.13

0.25

0.75

1.0

72

0.64

0.21

3.93

8.85

27.15

82.10

1.06

0.23

7.72

15.79

57.13

B8 4

0.14

0.35

0.75

2.8

72

0.67

0.36

3.83

8.95

27.09

82.84

1.12

0.39

7.60

16.12

57.51

B8 5

0.15

0.44

0.75

0.5

72

0.76

0.60

3.72

8.85

27.15

85.07

1.29

0.67

7.55

16.35

59.12

B8 6

0.16

0.35

0.75

1.8

72

0.83

0.44

3.75

8.97

27.07

88.46

1.46

0.51

7.93

17.21

61.23

B8 7

0.12

0.01

0.76

2.7

72

0.54

0.01

3.96

8.73

27.33

82.61

0.90

0.01

7.84

15.73

58.04

B11 1

0.13

0.02

0.76

-1.2

72

0.60

0.01

4.03

8.61

27.33

84.86

1.02

0.02

8.20

15.92

59.52

B11 2

0.16

0.06

0.75

0.4

72

0.77

0.05

3.95

8.82

27.17

86.81

1.34

0.06

8.19

16.63

60.36

B11 3

0.16

0.03

0.75

2.0

72

0.78

0.03

3.93

8.84

27.20

85.41

1.34

0.03

8.03

16.42

59.55

B11 4

0.11

0.00

0.76

0.0

72

0.51

0.00

3.97

8.51

27.50

83.84

0.86

0.00

7.99

15.58

59.34

B11 5

1498 Contrib Mineral Petrol (2013) 166:1489–1503


1499

(a)

(b)

(c)

(d)

Fig. 4 Composition of stilbite minerals from localities shown in Figs. 1 and 2. a Extraframework K (apfu) versus molar Si/(Si ? Al). b Extraframework Na (apfu) versus molar Si/(Si ? Al).

c Extraframework Ca (apfu) versus molar Si/(Si ? Al). d molar (Na/(Na ? K) versus molar Si/(Si ? Al). Symbols given on Fig. 3

reacted with surface- or near-surface waters and created distinguishable chemical pattern in surface stilbite minerals. All tunnel stilbite samples (Figs. 3, 4; Table 2) show a compositional variation that can be described by the exchange reaction:

zeolites formed in a cooling environment in accord with the geological context. Stilbite collected at surface outcrops contains significant amounts of potassium (Fig. 3; Table 3). This is a fundamental difference to the tunnel samples. All other compositional variations in surface stilbite are similar to those of the tunnel samples. However, because of the presence of potassium in surface stilbites calcium and sodium is lower than in the tunnel samples, suggesting the extraframework cation exchange reactions:

Ca4 Al8 Si28 O72 28H2 O ðstelleriteÞ þ NaAl ¼ NaCa4 Al9 Si27 O72 28H2 O ðstilbiteÞ þ Si

ð1Þ

Reaction (1) describes the coupled substitution: Si4þ ) Al3þ þ Naþ

ð2Þ

The Si ¼) Al substitution increases the extraframework occupancy with h ¼) Na (e.g., Neuhoff and Ruhl 2006). Fridriksson et al. (2001) related this substitution with increased Na occupancy in the stilbite–stellerite solid solution to decreasing temperature during zeolite formation. In the tunnel samples, Na increases from the core to the rim in a single stilbite crystal suggesting that the

Ca2þ ) 2Kþ

ð3Þ

Naþ ) Kþ

ð4Þ

The total extraframework cation content is higher in surface samples compared to tunnel samples. This implies that reaction (3) is the dominant cation exchange reaction operating on surface samples.

123

1500


Table 4 Water samples from the southern part of the Amsteg section of the Gotthard rail base tunnel (Bucher et al. 2012) Sample Tunnel meter Overburden Rock unit

A025 13,974 1,895 cAg

A098 14,735 1,880 cAg

A027 14,794 1,920 Mig

A099 14,850 1,975 Mig

A101 14,972 2,080 Mig

A102 14,986 2,090 Mig

A114 16,416 1,760 sAg

A115 16,553 1,700 sAg

A116 16,569 1,690 sAg

A117 16,681 1,700 Grgn

A118 16,782 1,690 Grgn

Temp (°C)

43.3

45.2

43

42.6

43

43.4

41.9

42.6

42.7

42

41.9

pH

9.12

9.37

9.44

9.4

9.4

9.47

10.1

9.96

9.24

9.65

9.82

Ca2?

17.3

8.08

5.02

5.62

5.64

5.46

3.45

2.87

4.78

2.78

1.26

Mg2?

0.03

\0.01

\0.01

\0.01

\0.01

\0.01

\0.01

\0.01

\0.01

\0.01

0.03

Na?

119.8

30.6

26.6

26.1

25.8

25

33.7

31.8

49.9

39

41.8

K?

2.77

1.36

0.87

0.94

0.57

0.45

0.55

0.64

0.79

0.5

0.98

Al

0.025

0.141

0.094

0.093

0.086

0.098

0.063

0.046

0.056

0.048

0.05

HCO3-

88.7

29.8

19.2

21.6

22.3

23.4

6.5

8.1

43.5

21.2

24.8

CO32-

5.4

3.2

2.4

2.5

2.6

3.2

3.8

3.4

3.5

4.4

7.5

SO42-

171.5

51.8

39.3

39.8

37.7

35.5

22.2

18.4

46.1

20.7

19.5

Cl-

65.20

1.00

0.80

1.20

0.70

0.70

8.30

7.50

14.60

10.20

8.90

-

F

13.01

1.92

1.64

1.59

1.59

1.69

3.33

3.28

5.24

6.71

4.14

Br-

0.73

0.01

0.01

0.02

0.01

0.01

0.10

0.11

0.19

0.15

0.12

SiO2aq TDS

26.91 514

28.52 157

32.17 129

28.21 128

27.66 125

23.26 119

46.01 128

42.61 119

29.92 199

30.44 137

37.09 150

Na/K (molar)

74

38

52

47

77

94

104

84

107

132

72

SI stilbite

5.05

6.24

5.87

5.62

5.31

4.9

5.92

4.91

5.51

4.36

4.02

All concentrations in mg/L. TDS: Total dissolved solids includes small amounts of Sr, B, Li, Rb, As Rock units: cAg central Aar granite. mig migmatites (‘‘Schollen’’ unit), sAg southern Aar granite, grgn granite gneiss Overburden: meters above tunnel floor. Tunnel meter from North Portal

Fig. 5 Cation composition of high-Alpine surface waters and of tunnel waters from the Gotthard rail base tunnel (see text). Note that Mg is very low in surface waters and virtually absent from tunnel waters

The slightly lower Si/Al ratio for surface samples (Fig. 4) implies a further substitution that also affects the framework structure and increases the extraframework cation occupancy:

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Fig. 6 Mineral stability diagram for tunnel water data (Bucher et al. 2012). The diagram has been computed using the software Domino/ Theriak of de Capitani and Petrakakis (2010) and the thermodynamic data of Berman (1988) and Frey et al. (1991)


1501

Table 5 Surface waters from Val Strem, Etzli valley and mountains above the tunnel (Seelig and Bucher 2010) Sample Rock unit Locality

USOF12 Grgn Caschle

Temp (°C)

9.40

pH

USOF1 Grgn Val Strem 15.30

USOF2 Grgn Val Strem 12.80

USOF3 sAg ? grgn Puoza

USOF4 Mig ? sAg Alp Strem

USOF11 Mig Mittelplatten

USOF5 cAg Gulmenfed

USOF10 cAg Gulmenfed

USOF6 cAg Gwasmet

USOF9 cAg Wittenalp

18.50

12.40

7.80

7.90

10.30

8.50

7.90

7.45

7.26

7.33

7.24

7.05

6.93

6.61

6.75

6.66

6.61

Ca2?

18.73

5.98

6.70

3.06

2.85

7.81

3.89

4.35

5.59

4.34

Mg2?

0.76

0.45

0.40

0.12

0.17

0.27

0.13

0.22

0.20

0.25

Na?

1.56

1.21

1.19

0.46

0.46

0.97

0.36

0.39

0.30

0.30

?

K

Alkalinity (HCO3) SO42-

2.60

1.48

1.28

0.50

0.68

0.76

0.38

1.18

0.78

0.61

54.46

18.64

21.91

12.15

11.65

15.26

13.88

16.13

18.09

11.06

11.49

4.46

5.00

1.03

1.20

12.28

1.38

2.18

2.67

5.11

Cl-

0.15

0.00

0.11

0.00

0.00

0.00

0.00

0.00

0.11

0.00

NO3-

2.84

0.34

0.21

0.00

0.00

0.93

1.07

0.41

1.33

1.00

F-

0.06

0.06

0.06

0.02

0.02

0.03

0.02

0.02

0.04

0.02

SiO2,aq

7.38

7.20

7.29

2.52

3.26

3.29

2.51

1.69

2.29

1.87

100.10

39.82

44.16

19.85

20.29

41.60

23.61

26.57

31.40

24.56

1.02

1.39

1.57

1.58

1.14

2.17

1.62

0.55

0.67

0.84

TDS Na/K (molar)

All concentrations in mg/L. TDS: Total dissolved solids includes small amounts of Sr, B, Li Rock units: cAg central Aar granite. mig migmatites (‘‘Schollen’’ unit), sAg southern Aar granite, grgn granite gneiss

Si4þ ) Al3þ þ Kþ

ð5Þ

Cation exchange reactions basically affect extraframework cations and water molecules and therefore a change in framework cations are of very minor significance. Stilbite-forming reactions The exchange reactions formulated above chemically modified stilbite crystals coating granite and gneiss walls of water-conducting fractures. The primary reactions that formed the zeolites precipitated stellerite crystals from a hydrothermal solution that was strongly oversaturated with respect to stellerite (Bucher et al. 2012; Weisenberger and Bucher 2010). The prime zeolite of the zeolite stage of Alpine fissure mineralization is laumontite. Laumontite was later superseded by stilbite/stellerite (Weisenberger and Bucher 2010). A feasible reaction that formed stilbite/ stellerite uses albite and calcite of the rock matrix (in addition to quartz or SiO2aq in the fluid). Also zoisite/clinozoisite is a potential Ca source (Weisenberger and Bucher 2010). Reaction (6) produces the observed high-pH natrite fluid (see above). 8NaAlSi3 O8 þ 4CaCO3 þ 4SiO2 þ 28H2 O ¼ Ca4 Al8 Si28 O72 28H2 O þ 8Naþ þ 4CO2 3

ð6Þ

The early-formed stilbite/stellerite was probably pure stellerite with very small amounts of Na caused by reaction (1) and (2). With increasing progress of reaction (6), the

deep water on the fractures became progressively enriched in sodium and the molar Na/Ca ratio increased to 6.3 (Bucher et al. 2012). Reaction (6) progressed in an overall environment of uplift and cooling (Weisenberger et al. 2012). The consequences of the overall fluid development to pure Na fluids can be seen in zeolite compositions evolving from stellerite to stilbite. The observed stellerite cores and stilbite rims support this interpretation. The end product of the process is stilbite that coats fracture walls of the tunnel as the latest mineral in the series of fissure minerals. Exhumation brings stilbite to the erosion surface From the place, temperature and depth where the stilbite originally formed, it is a long-lasting and protracted process to reach the erosion surface. Uplift, erosion, cooling, and denudation are the processes that finally make the fissure minerals to appear at the earth surface where they can be collected at an outcrop and studied by the baffled geologist. The chemical composition of the sample material is typically assumed to reflect the circumstances and conditions of rock- and mineral-forming processes. It has taken a minimum of 2 million years for the stilbite crystals to reach the erosion surface and ‘‘see’’ the sunlight for a geologically very short moment of their total existence (Weisenberger et al. 2012). As the rocks slowly move upward during the late stages of Alpine orogeny (about 1 mm/a average) at some stage, the rocks get under the

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1502

influence of near-surface groundwater. Precipitation reacts with exposed rocks and modifies its composition as described above. The typical high-Alpine surface waters (Fig. 5) with an unusually low Na/K ratio finally infiltrate the fractured near-surface rocks. The Na–K exchange reaction (4) inevitably modifies the composition of the million years old stilbites and makes them distinctly K-enriched. The K-enrichment is, however, not a primary feature of the stilbites and relates to late near-surface processes involving surface waters.

Conclusions Our study demonstrates cation exchange in a natural system based on data for primary zeolite and fluids in equilibrium during crystallization and secondary cationexchanged zeolite, including fluid data for the fluid that caused cation exchange in the zeolite. The composition of subsurface and surface waters that are in contact with stilbite shows a potassium pattern analogous to the stilbites. We suggest that stilbite found at the present day erosion surface originally precipitated as a K-absent phase at depth. Exhumation brought the stilbite-bearing rocks in contact with K-rich surface water and cation exchange reactions modified the original stilbite composition introducing a significant amount of K. The following more general conclusions can be drawn from the presented observations. One should never trust a surface rock sample. It may be unsafe to a priory assume that the composition of rock and minerals reflects the original condition and processes during formation. Starting with the original mineral-forming process, there is a long protracted route to the end product found at an outcrop and ample opportunity to modify the material in many ways. Acknowledgments AlpTransit Gotthard AG is thanked for providing access to the tunnel and providing water samples. We would like to thank P. Amacher who provided high-quality mineral specimens from the Gotthard NEAT tunnel. We thank B. Hoffmann and P. Vollenweider from the Natural History Museum Bern for access to the NMBE mineral collection. Special thanks to the Friedrich Rinne foundation for the financial support. We thank three anonymous reviewers for their detailed and constructive comments and C. Ballhaus for his editorial efforts and the editorial handling of the paper.

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