Barite–pyrite mineralization of the Wiesbaden thermalspring system, Germany: a 500-kyr record of geochemicalevolution

T. WAGNER1, T. KIRNBAUER2, A. J . BOYCE3 AND A. E. FALLICK3

1Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec, Canada; 2Technische Fachhochschule

Georg Agricola zu Bochum, Bochum, Germany; 3Scottish Universities Environmental Research Centre (SUERC), East

Kilbride, Glasgow, UK

ABSTRACT

Barite–(pyrite) mineralizations from the thermal springs of Wiesbaden, Rhenish Massif, Germany, have been stud-

ied to place constraints on the geochemical evolution of the hydrothermal system in space and time. The thermal

springs, characterized by high total dissolved solids (TDS) contents and predominance of NaCl, ascend from aqui-

fers at 3–4 km depth and discharge at a temperature of 65–70�C. The barite–(pyrite) mineralization is found in

upflow and discharge zones of the present-day thermal springs as well as at elevations up to 50 m above the cur-

rent water table. Hence, this mineralization style constitutes a continuous record of the hydrothermal activity,

linking the past evolution with the present state of this geothermal system. The sulphur isotope signatures of the

mineralization indicate a continuous decrease of the d34S of sulphate from +16.9& in the oldest barite to

+10.1& in the present-day thermal water. The d34S values of barite closely resemble various recently active ther-

mal springs along the southern margin of the Rhenish Massif and contrast strongly with different regional ground

and mineral waters. The mineralogical and isotopic signatures, combined with calculations based on uplift rates

and the regional geological history, indicate a minimum activity of the thermal spring system at Wiesbaden of

about 500 000 years. This timeframe is considerably larger than conservative models, which estimate the duration

of thermal spring systems in continental intraplate settings to last for several 10 000 years. The calculated equilib-

rium sulphur isotope temperatures of coexisting barite and pyrite range between 65 and 80�C, close to the dis-

charge temperature of the springs, which would indicate apparent equilibrium precipitation. Kinetic modelling of

the re-equilibration of the sulphate–sulphide pair during water ascent shows that this process would require

220 Myr. Therefore, we conclude that pyrite is formed from precursor Fe monosulphide phases, which rapidly

precipitate in the near-surface environment, preserving the isotope fractionation between dissolved sulphate and

sulphide established in the deep aquifer. Equilibrium modelling of water–mineral reactions shows slight supersatu-

ration of barite at the discharge temperature. Pyrite is already strongly supersaturated at the temperatures estima-

ted for the aquifer (110�C) and processes in the near-surface environment are most probably related to contact

of the thermal water with atmospheric oxygen, resulting in formation of oxidized intermediate sulphur species

and precipitation of Fe monosulphide phases, which subsequently recrystallize to pyrite.

Key words: age, mineralization, precipitation processes, sulphur isotopes, thermal springs, Wiesbaden

Received 27 June 2003, 25 August 2004; accepted 16 September 2004

Corresponding author: Thomas Wagner, Institut fur Geowissenschaften, Universitat Tubingen, Wilhelmstr. 56,

D-72074 Tubingen, Germany.

Email: th.wagner@uni-tuebingen.de. Tel: 0049-7071-2973080, Fax: 0049-7071-293060.

Geofluids (2005) 5, 124–139

INTRODUCTION

The thermal springs of Wiesbaden, Germany, have been

famous since Roman times, when Gaius Plinius Secundus

(23–79 AD) described the upwelling thermal water and

carbonatic sinter in his famous Natural History. Fe-oxide

precipitates from the springs have been used for colouring

hair in antique times, and were traded as far as Rome. The

thermal springs must have been known even during pre-

historic times, because about 60 stone and numerous bone

Geofluids (2005) 5, 124–139

� 2005 Blackwell Publishing Ltd

artefacts have been excavated in one of the ancient spring

basins (Floss 1991). On the basis of these archaeological

observations, it has been concluded that the springs were a

sacrificial site of pre-historic man. The thermal springs of

Wiesbaden are well studied by conventional hydrogeologi-

cal and hydrochemical methods (for compilation see

Kirnbauer 1997), but not much is known about their gen-

etic evolution and the sources of the water and its various

solutes.

Several recent studies have focused on the hydrogeologi-

cal and tectonic processes involved in thermal water sys-

tems in continental intraplate settings, notably the Rhenish

Massif in Germany. This research includes detailed hydro-

chemical and isotopic analyses, modelling of water–rock

interaction and modelling of crustal-scale transport proces-

ses (e.g. Hofmann & Baumann 1986; Griesshaber et al.

1992; May et al. 1996; Franko & Franko 2000; Herch

2000; Lopez-Chicano et al. 2001). Complementary to the

hydrogeological and hydrochemical data, which character-

ize the very recent state of a thermal water system, geo-

chemical investigation of precipitates from thermal waters

provide an exciting window into the evolutionary history.

This approach is obviously fruitful if fossil precipitates of

different ages are preserved, as is the case at Wiesbaden.

The present study centres on the sulphur isotope geo-

chemistry of (sub)-recent and fossil barite–(sulphide) min-

eralization of the Wiesbaden thermal water system, which

enable a reconstruction of the isotopic evolution during

the past 500 000 years. In addition, our investigation also

sheds light on the processes involved in water–rock interac-

tion and deposition of the barite–(sulphide) mineralization,

by applying alteration geochemistry and geochemical mod-

elling.

GEOLOGY, HYDROGEOLOGY,HYDROCHEMISTRY

Geology

Wiesbaden is situated at the southern margin of the

fold-thrust-belt of the Rhenish Massif (Rheinisches Schie-

fergebirge), which forms a major part of the Rhenohercy-

nian Zone, the northern margin of the Central European

Variscan orogen. The Rhenish Massif is composed of

Palaeozoic (Ordovician to Upper Carboniferous) shelf sedi-

ments and volcanic rocks, several km in thickness, which

were deposited in a passive continental margin setting. The

sequence underwent extensive deformation and very low

grade metamorphism during the Variscan orogeny in the

Upper Carboniferous, 325–305 Ma (Ahrendt et al. 1978).

Only the southernmost tectono-stratigraphic unit (Nor-

thern Phyllite Zone including Vordertaunus Unit) has

been metamorphosed under greenschist facies conditions

(Dallmeyer et al. 1995). This tectono-stratigraphic unit is

separated from the Mid-German Crystalline Rise, a part of

the internal zone of the Variscan orogen, by a first order

thrust system (Taunus Border Fault), which can be traced

by reflection seismics dipping to beneath the Moho

(Murawski 1975).

The youngest tectonic evolution of the Rhenish Massif is

characterized by strong Neogene and Quaternary uplift,

which abruptly accelerated about 800 000 years ago in the

Late Pleistocene and continues today (Meyer & Stets

1998). Present-day tectonic movements are observed by

continuing seismic activity related to the regional stress

field in central Europe, resulting from the dominantly

NW–SE oriented collision of the African and Eurasian lith-

ospheric plates (Ahorner 1975). Seismicity is related to

large-scale block movements that do not show a direct

relationship to the uplift (Ahorner 1983). An active rift

zone, which is part of the central European rift system,

transects the Rhenish Massif along the middle Rhine valley

(Fig. 1). This rift zone connects to the Upper Rhine Gra-

ben in the south and the Lower Rhine Trough in the

north (Ahorner et al. 1983).

The basement of the city of Wiesbaden is formed by

metavolcanic and metasedimentary rocks of Ordovician

to Silurian age (Fig. 2), both weakly overprinted by

Den Haag

Bremen

Münster

AachenKöln

MainzWiesbaden

Metz100 km

Fig. 1. Regional map of NW Germany and the surrounding areas, showing

the contours of the Rhenish Massif (dark grey) and the Upper Rhine Gra-

ben (light grey) as well as the young tectonics. Dashed-dotted lines repre-

sent fault zones related to the Lower Rhine Trough; solid lines are other

young faults. Redrawn and modified after Illies & Fuchs (1983).

Barite–pyrite mineralization of the Wiesbaden thermal spring system 125

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

Carboniferous deformation at 325 Ma. The Palaeozoic

rocks of the inner part of the city belong to two tectonic

slices of the Vordertaunus Unit. The north-western slice is

dominated by Ordovician to Silurian rhyolitic to rhyoda-

citic metavolcanics (so-called sericite gneiss) with thin

intercalations of metasediments (phyllites) (Sommermann

et al. 1992). In contrast, the south-eastern slice comprises

phyllites, which can be regarded as an equivalent of a

Lower Ordovician phyllite unit (Reitz et al. 1995). The

Taunus Border Fault is located about 700 m to the south-

east of the upwelling zone of the thermal springs of Wiesb-

aden (Kirnbauer 1997). In the southern part of the city,

the Variscan basement is discordantly overlain by Neogene

sediments belonging to the northern part of the Upper

Rhine Graben and the Mainz Basin. Palynological investi-

gations show that these clastic sediments (sands, gravels,

partly clays) correspond to marine Lower Miocene beds of

the Mainz Basin. Thin Quaternary deposits (fluvial sedi-

ments of Pleistocene and Holocene age) cover parts of the

Palaeozoic and Tertiary rocks.

Hydrogeology and hydrochemistry

Discharges of thermal and mineral waters are known from

roughly 400 localities in the Rhenish Massif. Based on

chemistry, temperature and isotopic evidence (He, C),

regionally, several types of mineral waters with two distinct

end members can be distinguished (Griesshaber et al.

1992; May 1994; May et al. 1996; Griesshaber 2000).

These end members are:

(1) Ca–Mg–CO2–waters, characterized by very low total

dissolved solids (TDS) contents, low discharge temper-

atures (mean of calculated aquifer temperatures:

29.8�C), high concentrations of CO2 and high total

carbon content. High contents of mantle-derived

helium (partly exceeding 70%) and a considerable con-

tribution of mantle-derived carbon are characteristic

for this type. These waters are common throughout

the Rhenish Massif, particularly in the Eifel volcanic

fields. The distribution pattern of these springs mat-

ches well with the location of the mantle plume below

the Eifel (Raikes & Bonjer 1983; Ritter et al. 2001)

and the recent uplift of this area (Meyer & Stets

1998).

(2) Waters of the NaCl-type are characterized by very high

TDS contents and high discharge temperatures, up to

75�C (mean of calculated aquifer temperatures:

93.4�C). The low concentrations of CO2 and a signifi-

cant contribution of organic-derived, sedimentary car-

bon correlate with an enrichment in radiogenic

helium. Waters of this type occur in the Upper Rhine

Graben, the Saar-Nahe Basin, along the southern mar-

gin of the Rhenish Massif and in the Lower Rhine

Trough. Based on temperature, compositional charac-

teristics and the hydrodynamics, Wiesbaden is a typical

and prominent member of this sodium chloride type.

Ascent of the thermal and mineral waters in the Rhenish

Massif is facilitated by joints and fissures in the upper crust,

controlled by the recent extensional regime in Central

Europe. The location, direction and opening dynamics of

the hydraulically active joints are effectively controlled by

block movements along major crustal fracture zones, e.g.

the Upper Rhine Graben. The temperature of the ascending

thermal and mineral waters depends on (1) the elevation of

the discharge sites, because thermal waters preferably

discharge at orographically low altitudes (May 1994), and

(2) on the circulation depth along the faults. As a conse-

quence, NaCl-type thermal springs are essentially bound to

the margins of the Rhenish Massif (e.g. Aachen, Wiesbaden)

and to deep valleys like the Rhine and the Lahn valley

(e.g. Bad Ems). The most important active thermal spring

system of the Rhenish Massif is developed along its southern

margin, related to the first order Taunus Border Fault.

Prominent historic spas use these waters, e.g. Wiesbaden,

Bad Soden, Bad Homburg and Bad Nauheim. In addition,

several important fossil spring locations, based on their char-

acteristic mineral assemblage, are located along this fault

(Kirnbauer 1998).

Wiesbaden is the most important locality of the large

active spring system with about 40 individual springs

known from the inner part of the city; today 27 are cap-

tured in wells or spring chambers. The total discharge per

100m

Taunus Border Fault

WI-31

WI-34

WI-106WI-107

WISA-6

WISA-10

WI-102

WI-101WI-100WI-103WICO-1

WICO-2

WI-35

Kochbrunnen

Schützenhofquelle

Adlerquelle

Faulbrunnen

Holocene sedimentsPleistocene sedimentsTertiary sedimentsOrdovician-Silurian metavolcanicsLower Ordovician phyllites

Sample localities

Thermal and mineral waters

Temp.(°C) Discharge (l/min)

>100 <100>60

20–60<20

Fig. 2. Geological sketch map of the city of Wiesbaden, showing the loca-

tion of thermal springs and sampling sites.

126 T. WAGNER et al.

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

year is estimated to be 1.3 · 106 m3. The general position

of the springs is controlled by the Taunus Border Fault. In

addition to the tectonic control, the local morphological

gradient defines the actual location of the Wiesbaden

springs (Kirnbauer 1997). The recent extensional stress

field in Central Europe enables NNW–SSE and ENE–WSW

trending joints to function as hydraulically efficient chan-

nels for the ascending waters. The most important thermal

springs of Wiesbaden, referred to as primary springs, are

characterized by high water quantities (>100 l min)1),

discharge temperatures of 65–70�C, high TDS

(>8000 mg l)1) and artesian pressure. The most prominent

spring is the Kochbrunnen, today a well, capturing a ther-

mal spring at 43 m depth with a constant wellhead tem-

perature of about 67�C and a discharge of 642 m3 day)1.

Three main discharge areas can be distinguished: (1) the

Kochbrunnen group in the NE (including Salmquelle,

Spiegelquelle, Romerquelle, Pariser-Hof-Quelle), (2) the

Adlerquelle group at a distance of about 200 m SW from

the Kochbrunnen area, and (3) the Schutzenhofquelle

group, 200 m SW from the Adlerquelle group (Fig. 2). All

three groups discharge at an elevation of about 119 m

above sea level. At a distance of 700 m SW from the

Kochbrunnen area, the Faulbrunnen, a low-temperature

mineral aquifer is intersected by a well. The majority of the

Wiesbaden springs and wells are situated south of the

Kochbrunnen and Adlerquelle groups. They are character-

ized by low discharge rates, temperatures below 60�C, low

TDS and low pressure. These waters represent secondary

springs, formed by mixing of thermal water flowing out

from the metavolcanics with cooler ground water from the

overlying Quaternary shallow aquifer.

The chemical composition and temperature of the

important springs have been monitored for over 150 years;

representative hydrochemical data of the major springs are

listed in Table 1. A notable trace component is As, most

of it in the reduced arsenite form, in concentrations of

over 100 lg l)1 (Schwenzer et al. 2001). The 87Sr/86Sr

isotope ratio of the Kochbrunnen water is 0.7154 (Hof-

mann & Baumann 1986). Based on the chemical data, we

have carried out estimates of the aquifer temperatures

applying a variety of different solute geothermometers.

The results of these calculations indicate that the tempera-

tures in the aquifer are close to about 110�C (Table 1).

MINERALOGY OF THERMAL WATERMINERALIZATION

On the basis of their mineral assemblage and textural fea-

tures, four different mineralization types genetically related

to the upwelling thermal water can be distinguished. These

are (i) carbonatic sinter, (ii) oxidic sinter, (iii) silica-rich

precipitates with minor barite, and (iv) barite–pyrite fissure

vein fillings (Kirnbauer 1997).

Large amounts of carbonatic sinter (assemblage 1) have

been formed within and surrounding the ancient natural

basins of the thermal springs of Wiesbaden. The occur-

rence of this mineralization type is widespread but vertic-

ally restricted to a narrow zone reaching from the surface

to a maximum depth of 5 m. The carbonatic sinter, which

is composed of calcite, aragonite and minor amounts of Fe

hydroxides, displays a characteristic banded texture. Geo-

chemical analyses of different samples of carbonatic sinter

indicate relatively high concentrations of As (1–2 wt.%

As2O3), most likely totally adsorbed onto the Fe hydrox-

ides (Schwenzer et al. 2001). Oxidic sinter (assemblage 2),

mainly composed of amorphous and cryptocrystalline Fe

oxides and hydroxides, occurs as part of both recent and

fossil thermal water precipitates. Similarly to the carbonatic

sinter, the Fe-rich oxidic sinter contains elevated concen-

trations of As. The environmental relevance of the As

enrichment in soils and anthropogenic debris of wide areas

of the inner part of the city has been recently discussed

(Rosenberg et al. 1999). Silica-rich precipitates (assem-

blage 3), which occur as colloform silica (chalcedony, hya-

lite), are restricted to the Tertiary sediments. They have

preferentially formed along the bedding planes between

permeable sandstones and the underlying less permeable

siltstones and metavolcanics. Large portions of the sand-

stones have been pervasively impregnated with massive sil-

ica and, more locally, jasper. Barite concretions and crystals

containing sand inclusions were formed in incompletely

silicified sand-rich sediments. The silica-rich precipitates are

rarely associated with idiomorphic tabular barite crystals.

More commonly, barite forms separate veinlet fillings

crosscutting the silicifications (Fig. 3A). It is important to

note that silica-rich precipitates have been exclusively found

above the recent water table, i.e. at elevations between 120

and 160 m above sea level.

The barite–pyrite mineralization (assemblage 4) generally

occurs as fillings of mm- to cm-sized fractures, veinlets and

fissures within altered metavolcanics (metarhyolite to meta-

rhyodacite) and the lowermost portions of the overlying

Tertiary sediments. Rarely, pyrite impregnates hydraulically

fractured and brecciated quartz veins within the metavolca-

nics (Fig. 3B). Most of the barite–pyrite veins are subvertical

and strike NNW–SSE, but both the s1 cleavage planes of the

hostrocks and subhorizontal joints are also weakly mineral-

ized. Locally, altered metavolcanics are exposed as stock-

work-type mineralization, with pervasive impregnation of

fractures and veinlets with pyrite. Open fissures are partially

filled by pyrite and well-developed tabular barite crystals, up

to 4–5 cm in size. The vertical distribution of the barite–pyr-

ite mineralization in the Kochbrunnen area reaches from

112.5 m (i.e. 6–7 m below the recent water table) down to

a minimum depth of 73–77 m above sea level, as evidenced

from drillcore data. Based on reconstruction of the palaeo-

morphology and the palaeohydrology, the relative age of the

Barite–pyrite mineralization of the Wiesbaden thermal spring system 127

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

mineralizations decreases generally with decreasing eleva-

tion. The maximum elevation of the mineralization decrea-

ses systematically towards the S and SE, following the slope

of the water table of the thermal water. Supergene weather-

ing of the barite–pyrite mineralization resulted in the forma-

tion of gypsum (CaSO4Æ2H2O), szomolnokite (FeSO4ÆH2O) and melanterite (FeSO4Æ7H2O).

Representative samples of (sub)-recent barite–pyrite veins

and fossil barite–(pyrite) mineralization were investigated

in detail (Table 2), using reflected-light microscopy and

X-ray powder diffraction analysis. Pyrite is the dominant Fe

sulphide phase, whereas marcasite is only present in small

amounts (<1 vol.%) in very few samples. Pyrite–barite

mineralization hosted in the metavolcanics forms a com-

plex network of numerous veins and fracture fillings. Most

of the veinlets are entirely filled by pyrite, whereas pyrite

coating the walls of individual larger veins is commonly

overgrown by tabular barite crystals (Fig. 4A). The pyrite

Table 1 Representative chemical analyses of the

thermal and mineral springs of Wiesbaden,

measured by the ESWE laboratory Wiesbaden

(Pilz & Schneider 1998), and results of aquifer

temperature estimations applying different solute

geothermometers.

Kochbrunnen

(03/1998)

Salmquelle

(03/1998)

Adlerquelle

(03/1998)

Schutzenhofquelle

(03/1998)

Faulbrunnen

(03/1998)

Temperature (�C) 66.1 64.4 63.0 49.3 17.5

pH 5.97 5.96 5.95 6.01 6.36

Eh (mV) )80 )94 )101 )79 )38

Li+ 2.9 2.9 3.0 3.8 2.6

Na+ 2524 2532 2570 1936 1346

K+ 88.0 90.7 91.2 84.2 60.3

NHþ4 5.8 5.7 5.8 4.4 2.1

Mg2+ 44.7 45.3 45.0 31.8 27.2

Ca2+ 344 345 344 294 229

Sr2+ 12.9 13.1 13.3 12.4 9.1

Ba2+ 0.72 0.72 0.70 0.27 0.15

Mn2+ 0.57 0.57 0.57 0.29 0.21

Fe2+ 2.92 2.80 2.91 1.08 0.90

Al3+ 0.005 0.004 0.004 0.004 0.012

F) 0.56 0.55 0.55 0.74 0.66

Cl) 4380 4370 4410 3380 2385

Br) 3.9 3.9 4.0 3.4 2.4

I) 0.05 0.05 0.05 0.03 0.02

SO2�4 68.9 69.0 71.6 116.0 100.0

H2S 0.004 0.01 <0.004 <0.004 <0.004

HPO2�4 0.06 0.03 0.03 0.02 0.07

HAsO2�4 0.19 0.19 0.17 0.14 0.10

HCO�3 557 555 552 369 337

H2SiO3 79.8 80.2 79.8 68.0 72.5

HBO2 4.5 4.5 4.7 5.7 4.2

CO2 454 489 475 308 246

Geothermometry (�C)

Na-Mg-K [1] 125 126 126 135 133

Na-K [2] 108 109 109 123 125

Na-K [3] 160 162 161 174 175

Na-K [4] 141 142 142 155 157

Na-K [5] 146 148 147 160 162

Na-K [6] 94 96 95 109 112

K-Mg [3] 102 102 103 106 98

Na-Mg [3] 32 32 32 27 14

Na-Li [7] 84 84 85 116 115

Na-Li [5] 94 94 95 126 125

Chalcedony [2] 83 83 83 75 78

Quartz [6] 112 112 112 105 108

Chalcedony [6] 81 81 81 72 76

Quartz [8] 115 115 115 106 110

Quartz [5] 112 112 112 104 107

Mean 106 107 106 113 113

SD 31 32 31 37 39

SE 8 8 8 10 10

Element concentrations are tabulated in mg l)1. The following calibrations were used for geothermo-metric calculations: [1] Nieva & Nieva (1987); [2] Arnorsson et al. (1983); [3] Giggenbach (1988); [4]Fournier (1979); [5] Verma & Santoyo (1997); [6] Truesdell (1976); [7] Fouillac & Michard (1981); [8]Fournier & Potter (1982).

128 T. WAGNER et al.

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

is composed of numerous isometric hypidiomorphic to idi-

omorphic crystals (Fig. 4B), 20–500 lm in size, which dis-

play a significant growth zonation (Fig. 4C) and are

partially rimmed by fine-grained anhedral pyrite. Portions

of the isometric pyrite are crosscut by younger fractures,

which are filled by fine-grained pyrite and anhedral barite.

Rarely, idiomorphic quartz crystals are present as inclusions

within pyrite. Portions of quartz veins within the metavol-

canics have been subjected to strong hydraulic fracturing.

Angular fragments of the vein quartz, ranging from 5 lm

to 7 cm in size, were cemented by anhedral isometric to

elongate pyrite grains (Fig. 4D). The central portions of

wider fissure veins are partially filled with tabular barite

crystals, ranging from 0.5 to 4–5 cm in size. Several of the

barite crystals contain numerous elongated pyrite inclu-

sions, 5–10 lm by 50–200 lm in size, which are oriented

perpendicular to the growth direction of the crystals. The

pyrite mineralization within the Tertiary sediments is com-

monly present as massive pyrite layers, which are composed

of isometric grains. Pyrite encloses idiomorphic quartz

crystals, which are crosscut by microfractures filled with

fine-grained anhedral pyrite. The sediment close to the

massive pyrite layers is impregnated with very fine-grained

pyrite, present as clusters composed of anhedral grains,

infilling along grain boundaries and microfractures, and

pyrite displaying an atoll texture (Fig. 4B). No growth

zonation comparable to the fissure vein pyrite could be

observed.

Systematic electron-microprobe and LA-ICP-MS analysis

has shown that pyrite carries elevated concentrations of As

and Tl, whereas the contents of other minor and trace ele-

ments are significantly lower (Schwenzer et al. 2000). The

following maximum concentrations have been analysed: As

(5.4 wt.%), Tl (2.0 wt.%), Sb (0.9 wt.%), Pb (0.3 wt.%), Cu

(0.8 wt.%), Mn (100 ppm), Zn (70 ppm), Ag (50 ppm).

No systematic compositional difference among the various

textural types of pyrite could be detected.

SULPHUR ISOTOPE DATA

Experimental procedures

Mineral separates of coarse-grained pyrite and barite sam-

ples were prepared by careful hand-picking under a binocu-

lar microscope, followed by cleaning in doubly distilled

water. These samples were analysed by conventional com-

bustion procedures using cuprous oxide (Robinson &

Kusakabe 1975; Coleman & Moore 1978). Fine-grained

pyrite fracture fillings and impregnations were extracted by

an on-line in situ laser combustion from standard polished

blocks, using a SPECTRON LASERS 902Q CW Nd:YAG

laser (1 W power), operating in TEM00 mode (Kelley &

Fallick 1990; Fallick et al. 1992). The SO2 gas released by

either method was purified in a vacuum line, using cryo-

genic separation techniques. Determination of the sulphur

isotope composition of the purified SO2 gas (d66 SO2) was

carried out by a VG SIRA II gas mass spectrometer.

Reproducibility of the conventional results, and mass spec-

trometer calibration, was monitored through replicate

measurements of international standards NBS-123

(+17.1&), IAEA-S-3 ()31&) and SUERC’s internal

laboratory standard CP-1 ()4.6&). The analytical preci-

sion for both techniques was around ±0.2&. All sulphur

isotope compositions were calculated relative to Vienna

Canon Diablo Troilite (V-CDT), and are reported in

standard notation. The laser extraction method results in a

sulphur isotope fractionation between the host mineral and

the SO2 gas produced via combustion, which is mineral-

specific and can be corrected applying experimentally calib-

rated analytical factors (d34Spyrite ¼ d34Sgas + 0.8&; see

Wagner et al. 2002, for full discussion).

Analytical results

Table 3 lists all sulphur isotope data. Pyrite displays a con-

siderable variation of the d34S between )42.9& and

Fig. 3. Representative samples of barite–(sulphide) mineralization from the

thermal water district of Wiesbaden. (A) Idiomorphic tabular barite crystals

with minor pyrite, originating from a vertical fissure in silicified Tertiary

sandstone. Width of field: 13 cm. (B) Hydraulic quartz breccia, which has

been cemented by pyrite. The boundaries of quartz clasts are traced by

black lines. Width of field: 8 cm.

Barite–pyrite mineralization of the Wiesbaden thermal spring system 129

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

)33.9&, whereas the sulphur isotope composition of bar-

ite is more homogeneous varying between +11.6& and

+16.9& (Fig. 5). Detailed laser analyses of different tex-

tural types of pyrite reveals limited variations within indi-

vidual samples, which are usually smaller than 3–4&

(Table 3). It is important to note that the d34S values of

the different barite samples show a statistically significant

(>95%) correlation with the elevation of these samples

(Fig. 6), i.e. barites from barite–sulphide veinlets close to

and below the present water table have generally less posit-

ive d34S values (+11.6& to +14.7&) than fossil barite

mineralizations exposed above the zone of the recently

upwelling thermal water (+15.0& to +16.9&). This, com-

bined with the present-day sulphur isotope composition of

dissolved sulphate in the Kochbrunnen spring

(d34S ¼ +10.1&; Nielsen & Rambow 1969) is consistent

with a decrease of the d34S value of sulphate with time.

The isotopic fractionation between texturally coexisting

pyrite and barite is large with Dpy-ba values between 48.6&

and 52.9&; the resulting sulphur isotope temperatures

applying equilibrium isotopic fractionation factors (Ohmo-

to & Goldhaber 1997) range between 65 and 80�C, relat-

ively close to the present-day temperature of the thermal

springs (about 65�C). Alternatively, isotopic temperatures

for equilibrium fractionation between sulphate and differ-

ent dissolved sulphur species assuming d34S of pyrite to be

equivalent to d34S of the respective sulphur species have

been calculated as well (Table 4). The resulting tempera-

tures (76–116�C) are significantly higher and approach the

estimated aquifer temperatures.

Compared with the barite mineralization from Wiesba-

den (Fig. 7), the d34S values of dissolved sulphate in nine

recently active mineral and thermal springs along the

Taunus Border Fault including Wiesbaden are relatively

similar and range between +9.8& and +17.2& (Nielsen &

Rambow 1969). In contrast, the d34S values of sulphate

from six thermal springs at the northern margin of the

Rhenish Massif (Aachen region) are significantly lower and

Table 2 Description of representative samples of

barite–sulphide mineralization from the thermal

springs of Wiesbaden.Sample Location

Elevation

a.s.l. (m) Mineralization style

WICO-1 Parking garage Coulinstrasse,

drillcore B7S, 7.5–12.4 m

109.9–114.8 Pyrite (±marcasite) in fissures and

cavities of brecciated post-Variscan

vein quartz

WICO-2 Parking garage Coulinstrasse,

drillcore

120 Breccia, cemented with fine-grained

pyrite and quartz

WISA-6 Saalgasse 10–14, excavation 122 NNW–SSE striking veinlet in altered

metarhyolite/metarhyodacite, filled

with barite crystals enclosing

abundant pyrite crystals

WISA-10 Saalgasse 10–14, excavation 120 Hydrothermal breccia of post-Variscan

vein quartz, cemented

with pyrite crystals

WI-31 Geisbergstrasse 17–19,

excavation

140–142 Veinlets in Tertiary conglomerate,

filled with grey-brownish tabular barite

crystals displaying growth zonation

WI-34 Cellar Geisbergweg 136 Veinlets in Tertiary conglomerate,

filled with grey tabular barite crystals

displaying growth zonation

WI-35 Paulinenschlosschen,

excavation

140 Veinlet in Tertiary sandstone,

filled with grey-yellowish tabular barite

crystals displaying growth zonation

WI-100 Schutzenhof 120 Veinlet in Tertiary sandstone,

filled with grey tabular barite crystals

WI-101 Coulinstrasse, shelter below

the old graveyard

126.5–129.5 Veinlets in Tertiary conglomeratic

sandstone, filled with grey tabular

barite crystals

WI-102 Coulinstrasse, shelter near

Romertor, drillcore

126.5–143 Veinlet in Tertiary conglomeratic sandstone,

filled with grey tabular barite crystals

WI-103 Schutzenhof 120–130 Post-Variscan vein quartz, coated

with clear tabular barite crystals

WI-106 Taunusstrasse,

drillcore BK-107

115 Fissures and veinlets in altered

metarhyolite/metarhyodacite, filled with

pyrite and tabular barite crystals

WI-107 Taunusstrasse,

drillcore BK-107

103.5 Veinlet in metarhyolite/metarhyodacite,

filled with pyrite and clear

tabular barite crystals

130 T. WAGNER et al.

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

show a much narrower range between +6.3& and +8.5&

(Herch 2000). The sulphur isotope composition of dis-

solved sulphate in a variety of ground and mineral waters

originating from aquifers of the Tertiary Mainz Basin and

the western margin of the Upper Rhine Graben is highly

variable, with d34S values between )13.8& and +29.4&

(Nielsen & Rambow 1969; Heyl et al. 1970). This isotopic

signature contrasts strongly with both groups of deep-

sourced thermal springs discharging at the margins of the

Rhenish Massif.

ALTERATION GEOCHEMISTRY

Samples and methods

A total number of 16 samples of the metarhyolite/meta-

rhyodacite have been analysed for major and trace element

composition. This sample suite includes altered samples

(n ¼ 2) from drillcores, originating from the strongly min-

eralized zone below the present table of the thermal water,

as well as least altered samples (n ¼ 14) from different

surface exposures. Any visible mineralization was removed

by careful hand-picking of the crushed sample material.

Concentrations of selected major and trace elements have

been determined by wavelength-dispersive X-ray fluores-

cence analysis using a Phillips PW 1480 instrument. Pre-

paration of glass tablets was performed by the fusion of

600 mg dried (120�C) rock powder with 3600 mg SPEC-

TROMELT A12 on a NUTECH station. The mineral

phases present have been determined by X-ray powder dif-

fraction analysis. Based on these data, calculation of the

normative mineralogy has been performed using the MO-

DAN program package (Pactunc 1998, 2001).

Results

Mean compositions of least altered and altered metarhyo-

lite/metarhyodacite samples are given in Table 5. Com-

pared with the least altered metarhyolite/metarhyodacite,

altered wallrocks from the direct contact to barite–sulphide

fissure veins show a significant increase in their K2O/Na2O

ratios. The least altered metavolcanics display K2O/Na2O

ratios in the range of 1.5–5.3 (mean ¼ 3.0), whereas the

Fig. 4. Photomicrographs in reflected light showing various representative textures of barite–(sulphide) mineralization from the Wiesbaden thermal springs.

(A) Veinlet in coarse-grained pyrite (py), which has been filled by tabular barite (ba) crystals. Sample WI-107. Width of field: 1.64 mm. (B) Irregular impreg-

nation of pyrite (py) within silicified Tertiary sediment. Sample WICO-2. Width of field: 1.06 mm. (C) Zoned idiomorphic pyrite (py) crystals grown in frac-

tures of quartz (qz). Sample WISA-10. Width of field: 1.39 mm. (D) Hydraulic breccia, composed of clasts of vein quartz (qz), which are cemented with

pyrite (py). Sample WISA-10. Width of field: 1.39 mm.

Barite–pyrite mineralization of the Wiesbaden thermal spring system 131

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

altered samples have K2O/Na2O ratios of 7.3 and 9.1.

Identification of geochemically immobile and mobile com-

ponents and calculation of mass balance has been carried

out using the isocon method (Grant 1986; Baumgartner

& Olsen 1995). The isocon diagram shows that a relatively

large number of elements display a relatively immobile

behaviour, notably Al, Fe, Mg, Zr, Y and Nb (Fig. 8). The

altered rocks have been essentially depleted in Na2O

()62%), CaO ()51%), TiO2 ()37%) and MnO ()35%),

whereas Zn (+168%), Ba (+62%), K2O (+21%), Rb (+23%),

and SiO2 (+18%) have been enriched during water–rock

interaction (Fig. 9). The enrichment in K2O coupled with

the strong depletion in Na2O and CaO can be related to

the sericitization of plagioclase. This interpretation is also

supported by the XRD data and the normative mineralogy

of the altered samples, which show a much lower abun-

dance of plagioclase in the altered samples compared with

the least altered metarhyolite/metarhyodacite. The enrich-

ment in Ba is most probably due to very fine-grained frac-

ture fillings of barite, which have been observed in thin

sections of the altered rocks. The high concentration levels

of Zn found in the altered rocks correlate with the elevated

concentrations of this element (105–258 ppb) compared

with other metals in the thermal springs (Kochbrunnen,

Salmquelle, Adlerquelle).

DISCUSSION

Precipitation processes

Based on the hydrochemical data, alteration geochemistry

and the sulphur isotope composition of the barite–sulphide

assemblages, the processes involved in the precipitation of

mineralization related to the upwelling of the thermal

water can be reconstructed. In order to obtain quantitative

information on these processes, the speciation of solutes

in the different thermal springs as well as the saturation

Table 3 Sulphur isotope data of barite–sulphide mineralization from the

Wiesbaden thermal springs.

Sample Mineral Textural type d34SV-CDT (&)

WICO-1-1* Pyrite Network of fine veinlets )36.3

WICO-1-2* Pyrite Network of fine veinlets )37.8

WICO-1-3* Pyrite Coarse veinlet )33.9

WICO-2-1* Pyrite Cluster of idiomorphic crystals )41.2

WICO-2-2* Pyrite Fine-grained impregnation )41.3

WISA-6-1* Pyrite Idiomorphic crystal )39.9

WISA-6-2* Pyrite Idiomorphic crystal, core )42.9

WISA-6-3* Pyrite Idiomorphic crystal, rim )39.9

WISA-6-A Barite Coarse-grained, close to pyrite +11.9

WISA-6-B Barite Coarse-grained +13.0

WISA-10-1* Pyrite Cement of quartz breccia )39.8

WISA-10-2* Pyrite Cement of quartz breccia )39.8

WI-31-A Barite Idiomorphic crystals +16.9

WI-34-A Barite Idiomorphic crystals +15.3

WI 35-A Barite Idiomorphic crystals +15.5

WI-100-A Barite Idiomorphic crystals +11.6

WI-100-B Barite Idiomorphic crystals +11.7

WI-101-A Barite Idiomorphic crystals +15.6

WI-102-A Barite Idiomorphic crystals +16.1

WI-103-A Barite Idiomorphic crystals +15.0

WI-106-A Barite Idiomorphic crystals +14.7

WI-106-B Pyrite Coarse veinlet )35.9

WI-107-A Barite Idiomorphic crystals +13.4

WI-107-B Pyrite Coarse veinlet )35.2

*All analysed by in situ laser system. All other data are conventionally pro-duced (see text).

Fre

qu

ency

δ34SV-CDT (‰)

5

4

3

2

1

0–40 –30 –20 –10 0 10 20

Barite

Pyrite

Fig. 5. Histogram showing the sulphur isotope composition of pyrite and

barite from thermal water mineralization.

Elevation of presentwater table

Ele

vati

on

(m

)

δ34SV-CDT (‰)

150

140

130

120

110

10010 12 14 16 18

y = 3.7x+76.9

R2 = 0.77

Fig. 6. Correlation between sample elevation and the sulphur isotope com-

position of barite. The two samples from drillholes below the water table

(WI-106, WI-107) have not been included into the regression.

Table 4 Sulphur isotope thermometry, using d34S values from texturally

coexisting pyrite and barite.

Sample d34S (py) d34S (ba) T (py) T (H2S) T (HS)) T (S2))

WISA-6 )39.9 +11.9 68.4 79.4 81.3 101.8

WISA-6 )39.9 +13.0 64.8 75.8 77.6 97.7

WI-106 )35.9 +14.7 73.0 84.3 86.1 107.2

WI-107 )35.2 +13.4 80.1 91.6 93.5 115.5

The temperatures (�C) have been calculated for equilibrium fractionation

between sulphate and pyrite, H2S, HS) and S2) using the fractionation fac-tors of Ohmoto & Goldhaber (1997).

132 T. WAGNER et al.

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

indices (SI) for important mineral phases have been calcu-

lated as a function of temperature, using the PHREEQC soft-

ware package (Parkhurst & Appelo 1999), version 2.8. We

have verified the results by comparing them with calcula-

tions of equilibria using the SOLVEQ speciation code (Reed

& Spycher 1984; Pang & Reed 1998). Despite difference

in absolute SI for a few minerals, notably the feldspars, the

results of both calculations are essentially consistent. The

following discussion is mainly based on the results of

the calculations using PHREEQC, because this is the most

widely used speciation code in hydrogeochemistry, and the

results obtained can be easily compared to other similar

studies.

The results for all undiluted springs (Kochbrunnen, Sal-

mquelle, Adlerquelle) are essentially identical. Barite is

undersaturated at higher temperatures; the SI increases

with decreasing temperature until around 50–70�C barite

δ34SV-CDT (‰)

Fre

qu

ency

A

Wiesbadenmineralization

5

4

32

10

–10 0 10 20 30

–10 0 10 20 30

–10 0 10 20 30

–10 0 10 20 30

Barite

Dissolvedsulphate

B

5

4

3

2

1

0

Taunussprings

C

5

43

2

1

0

Aachensprings

D

5

4

3

2

1

0

Tertiarywaters

Fig. 7. Sulphur isotope composition of (A) barite from the Wiesbaden min-

eralization compared with (B) dissolved sulphate of recently active mineral

and thermal springs along the Taunus Border Fault including Wiesbaden,

(C) dissolved sulphate of thermal waters of the northern margin of the

Rhenish Massif (Aachen region), and (D) dissolved sulphate of ground and

mineral waters from aquifers of the Tertiary Mainz Basin and the western

margin of the Upper Rhine Graben. Data from Nielsen & Rambow (1969),

Heyl et al. (1970), Herch (2000) and this study.

Table 5 Mean compositions and standard errors of hydrothermally altered

and least altered metarhyolite/metarhyodacite samples from Wiesbaden.

Least altered (n ¼ 14) SE Altered (n ¼ 2) SE

Wt.%

SiO2 72.87 0.52 75.03 3.19

TiO2 0.40 0.03 0.22 0.02

Al2O3 13.61 0.24 11.89 1.15

Fe2O3* 3.09 0.19 2.83 1.00

MnO 0.03 0.01 0.02 0.01

MgO 0.44 0.04 0.34 0.09

CaO 0.27 0.03 0.12 0.01

Na2O 2.13 0.18 0.72 0.01

K2O 5.53 0.16 5.87 0.61

P2O5 0.08 0.01 0.06 0.01

LOI 1.65 0.10 2.01 0.40

Total 100.10 99.11

ppm

Zn 63 4 148 14

Ga 18 1 17 3

Rb 240 11 259 43

Sr 38 2 28 0

Y 53 2 49 1

Zr 254 13 199 18

Nb 15 1 15 1

Ba 969 43 1374 484

*Fe2O3 ¼ Fetot.

Alt

ered

ro

cks

(wt.

%)

100

10

1.0

0.1

0.01

0.001

Least altered rocks (wt.%)0.001 0.01 0.1 1.0 10 100

SiO2

Al2O3K2O

Fe2O3

Na2OMgO

TiO2

CaOP2O5

Ba

Zr

MnO

RbZn

Y

SrNb

Fig. 8. Isocon diagram constructed for mean compositions of least altered

and altered metarhyolite/metarhyodacite from the upwelling zone of the

thermal waters. Due to the logarithmic scaling, the slope of the isocon line

is unity.

Barite–pyrite mineralization of the Wiesbaden thermal spring system 133

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

becomes saturated (Fig. 10A). This explains satisfactorily

why barite is found down to about 40–45 m below the

present-day water table of the thermal springs, where water

temperatures are close to the discharge temperature of

63–67�C. The observed crystal morphology of barite

(well-developed tabular crystals) is consistent with deposi-

tion from a saturated to slightly supersaturated hydro-

thermal solution. Different precipitation experiments in

the temperature range 45–150�C have shown that barite

deposited from solutions with low supersaturation shows

rectangular and polyhedral crystal shapes, whereas at high

supersaturation dendritic and skeletal crystals are formed

(Benton et al. 1993; Christy & Putnis 1993; Shikazono

1994).

The calculated SI for pyrite is around 6.8 at a tempera-

ture of 110�C (mean of the aquifer temperature estimates)

and increases with decreasing temperature to 7.9–8.1 at

65–70�C. The thermal water of the springs is therefore

strongly supersaturated with respect to pyrite, which

should result in pyrite precipitation during ascent of the

water. In contrast to the results of the calculations, pyrite

mineralization is restricted to a relatively limited zone

below the table of the thermal water and decreases with

depth. This discrepancy can be explained by the complex

reaction kinetics involved in the formation of pyrite from

aqueous solutions. A large number of experiments have

shown that in low-temperature environments pyrite essen-

tially forms via Fe monosulphide precursor phases such as

amorphous FeS, mackinawite (Fe9S8) and greigite (Fe3S4).

The results of our calculations show that the SI for mack-

inawite is around 1.0 at 110�C and decreases with decreas-

ing temperature to reach saturation (SI ¼ 0.0) around

35–45�C (Fig. 10A). This would indicate that pyrite

becomes more stable over mackinawite with decreasing

temperature. Considering the faster precipitation kinetics of

mackinawite compared to pyrite (Schoonen & Barnes

1991a,b), the stability relationships would predict that the

mackinawite precipitated in the discharge zone of the ther-

mal springs should recrystallize to pyrite upon cooling.

Several different reaction paths and mechanisms have been

proposed for the transformation of the precursor phases to

pyrite, which all require an oxidant to produce pyrite from

the precursor phases (e.g. Schoonen & Barnes 1991b;

Wachtershauser 1993; Wilkin & Barnes 1996; Benning &

Barnes 1998). The most recent experimental study demon-

strates that oxidation of precursor Fe monosulphide phases

or reduced aqueous sulphur species is necessary to promote

pyrite formation (Benning et al. 2000). Contact of the

thermal water with atmospheric oxygen and subsequent for-

mation of oxidized intermediate sulphur species is therefore

considered to be the dominant mechanism responsible for

the formation of pyrite in the fissure veins at Wiesbaden.

Water–rock interaction

The calculated SI for different rock-forming minerals

(Fig. 10B) present as constituents of the fresh and altered

+168Enrichment

Depletion

Rel

ativ

e co

nce

ntr

atio

n c

han

ge

(%)

+100

+50

0

–50

–100Si Ti Al Fe Mn Mg Ca Na K P LOI Zn Ga Rb Sr Y Zr Nb Ba

Fig. 9. Relative concentration change (%) of a suite of geochemically rele-

vant elements during hydrothermal alteration of metarhyolite/metarhyoda-

cite.

A

Pyrite

Sat

ura

tio

n in

dex

Sat

ura

tio

n in

dex

10

8

6

4

2

0

–2

–4

–6

–8

–10

10

8

6

4

2

0

–2

–4

–6

–8

–10

T(°C)

T(°C)

2000 50 100 150

Hematite

BariteGoethite

Mackinawite

Chalcedony

B

-10 50 100 150 200

Muscovite

Kaolinite

Quartz

Anorthite

Albite K-feldspar

Kochbrunnen

Kochbrunnen

Fig. 10. Saturation indices (SI) of different minerals for thermal water from

the Kochbrunnen calculated as a function of temperature, using PHREEQC

(Parkhurst & Appelo 1999). The SI have been calculated for minerals of (A)

the mineralization, and (B) the altered metarhyolite/metarhyodacite.

134 T. WAGNER et al.

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

metarhyolite/metarhyodacite explain the alteration reac-

tions and the patterns of element enrichment/depletion.

Water–rock interaction has strongly affected the plagioclase

of the metavolcanics, as shown by the lower normative

abundance of plagioclase and the depletion of Ca and Na

in the altered rocks. The thermal water of the springs is

undersaturated with both albite and anorthite, which

results in dissolution of plagioclase in the metavolcanics

during reaction with the water. In contrast, the SI of

K-feldspar is slightly positive around 50–70�C and, accord-

ingly, K-feldspar should remain stable during water–rock

interaction. The thermodynamic data of K-feldspar in the

PHREEQC software package are consistent with the thermo-

dynamic data of adularia included in the WATEQ4F database

(Ball & Nordstrom 1991); our calculated SI are therefore

relative to adularia. The temperature-dependent variations

in SI of albite and K-feldspar are relatively small (about

1 log unit) in the temperature range 25–75�C. Considering

the comparatively low concentrations of Al in the thermal

waters and the associated analytical errors, the calculated SI

for K-feldspar and albite have to be interpreted with some

caution (Pang & Reed 1998). We note, however, that

mineralogical and XRD studies have shown that the altered

metarhyolite/metarhyodacite samples still contain most of

their original K-feldspar content, but no detectable plagio-

clase, which would be consistent with the results of the

water–rock interaction calculations. Muscovite is strongly

supersaturated at the discharge temperature, consistent with

the observation that fine-grained muscovite replaces plagio-

clase in the altered rocks. Saturation with quartz is already

reached at a temperature of about 110�C, whereas satura-

tion with chalcedony is only reached at significantly lower

temperatures. This explains why weak to strong silicification

of altered and mineralized rocks is so widespread both

vertically and laterally, whereas formation of chalcedony

does only occur locally in the thermal water system.

Sulphur isotope fractionation

The sulphur isotope signatures and the isotopic fraction-

ation between barite and texturally coexisting pyrite of the

mineralization at Wiesbaden contain important information

about the precipitation process and the genetic evolution

of the thermal spring system. If equilibrium fractionation

factors (Ohmoto & Goldhaber 1997) are applied, the

Dpy-ba (48.6–52.9&) would correspond to precipitation

temperatures in the range between 65 and 80�C, which is

very close to the discharge temperature of the springs. The

interpretation of the observed isotopic fractionation

between barite and pyrite as true equilibrium fractionation

established during contemporaneous precipitation of both

mineral phases is not consistent with the reaction kinetics

of the sulphate–sulphide pair. Applying the model of

kinetic sulphur isotope fractionation (Ohmoto & Lasaga

1982), the time required for establishment of isotopic

equilibrium from an initial fractionation factor of 0& (i.e.

instantaneous sulphate reduction or sulphide oxidation)

would be 1.25 Ma at 110�C (Fig. 11). This calculation

assumes that both species were initially equilibrated in the

aquifer supplying the thermal waters, for which calculations

based on solute geothermometry have resulted in a mean

temperature estimate of 110�C. The time required for re-

establishment of isotopic equilibrium between coexisting

dissolved sulphate and sulphide at a temperature of 65�C(the actual temperature of the thermal water) would

require a significantly larger time of 220 Ma.

Our calculations and available data on the regional geo-

thermal gradient, hydrodynamics and the measured water

flow rates of the springs indicate that ascent of the thermal

water from deep-sourced aquifers at a depth of about

3–4 km (May 1994; May et al. 1996) to the surface

requires a time of several months to about a year. The time

available for re-equilibration of sulphur isotopes between

dissolved sulphate and sulphide is therefore dramatically

shorter than the time required for equilibration, based on

the kinetic model. Therefore, the observed fractionation

has to be considered as an apparent equilibrium fraction-

ation. If isotopic temperatures for equilibrium fractionation

between sulphate and dissolved sulphur species such as H2S

(aq), HS) and S2) are calculated assuming d34S of pyrite to

be equivalent to d34S of the respective sulphur species, they

are close to the estimated aquifer temperatures

(76–116�C). We therefore propose a model of rapid instan-

taneous iron sulphide precipitation in a surface-near envi-

ronment, which essentially preserves the sulphur isotope

fractionation between dissolved sulphate and sulphide

established in the deep-sourced aquifer. It is clear from the

pH =4–7

Tim

e (y

ear)

1016

1014

1012

1010

108

106

104

102

T(°C)200150100500

pH =9

pH =2

T2

T1

Fig. 11. Time required for establishment of 95% equilibrium between dis-

solved sulphate and sulphide calculated for the thermal water composition

of the Kochbrunnen, Wiesbaden, applying the model of Ohmoto & Lasaga

(1982). The diagram shows that 95% equilibration at 110�C (T1, the calcu-

lated aquifer temperature) requires 1.25 Ma, whereas at 65�C (T2, the act-

ual temperature of the water) this process takes about 220 Ma.

Barite–pyrite mineralization of the Wiesbaden thermal spring system 135

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

experimental data in the literature and the results of our

calculations that pyrite does not precipitate directly from

solution, but forms via Fe sulphide precursor phases such as

amorphous FeS, mackinawite or greigite. The isotopic effect

of the subsequent transformation of the precipitated Fe sul-

phide phase to pyrite is difficult to assess, because no experi-

mental or theoretical determinations of the equilibrium

sulphur isotope fractionation factor of any of these potential

pyrite precursor phases have been carried out so far.

However, the well-established equilibrium fractionation fac-

tor between pyrrhotite and pyrite (Ohmoto & Goldhaber

1997) can be used to estimate the isotopic effect involved.

For the temperature interval 50–65�C, the calculated

D34Spy-po is in the range of 2.6–2.9&, which would indicate

that no significant modification of the large isotopic frac-

tionation between sulphide and sulphate should occur dur-

ing recrystallization of the Fe sulphide precursor phases.

Alternatively, bacteriogenic sulphate reduction could also

explain the large measured isotopic fractionation between

barite and pyrite, but is not supported by textural or analyt-

ical data. Bacteriologic analysis of the thermal water, which

is carried out regularly for health purposes, has not indica-

ted that any sulphate-reducing organisms are present.

Age and evolution of the thermal springs

The mineralogical and isotopic data obtained from the dif-

ferent (sub)-recent and fossil mineralizations substantiate

the available geologic information on the age and evolu-

tion of the thermal water system of Wiesbaden. A mini-

mum age of the thermal springs of Wiesbaden is indicated

by the presence of the Palaeolithic artefacts, which are

15 000–25 000 years old (Floss 1991). This age estimate

is in good agreement with a 14C model age of 25 400

(with errors of +1500 and )1300 years) obtained for the

Kochbrunnen water (Lutkemeier 1975). Silica sinters preci-

pitated by thermal water generally show an increase in

structural order and particle density with increasing time;

silica sinters older than approximately 50 000 years are

recrystallized to microcrystalline quartz (Herdianita et al.

2000). Portions of the Si-rich sinter at Wiesbaden are

already altered to jasper, which supports a Pleistocene

minimum age of the Wiesbaden spring system. Further-

more, pebbles of silica concretions in the Middle Pleisto-

cene Mosbach Formation south of Wiesbaden, derived

from the Wiesbaden thermal water area, could prove an

age of about 500 000 years (Kirnbauer 1997).

The sulphur isotope composition of the barite from both

fossil and (sub)-recent mineralization indicates a continuous

evolution of the thermal water system; the d34S values of

barite decreasing systematically with time. The reasons for

this systematic shift of the isotopic composition are presently

not properly known. Potential processes responsible for this

isotopic trend could involve (i) change of the discharge

temperature with time, (ii) decrease of d34S of dissolved

sulphate in the deep-sourced aquifer, possibly due to influx

of isotopically distinct waters in the recharge area, and (iii)

different hydrological conditions in the past, which could

have enabled mixing with surface or shallow ground waters.

The tectonic and hydrologic situation around Wiesbaden is

rather complex, which makes it difficult to properly con-

strain the setting of the recharge area and the sources of dis-

solved components. The mineralogical observations, notably

the textures and trace element compositions of the barite–

pyrite mineralization, and the systematic isotopic evolution

support a long-term continuous activity of the thermal

spring system at Wiesbaden. The oldest preserved fossil

barite precipitates are found in an altitude of about 50 m

above the recent thermal water table (Kirnbauer 1997).

Based on an average Quaternary uplift rate of 11 cm per

1000 years for this area of the Rhenish Massif (Ploschenz

1994), these 50 m represent a time span of 455 000 years,

which is in good agreement with the Middle Pleistocene

age derived from the presence of silica concretions in sedi-

ments. Considering the fact that the age estimate of

500 000 years represents a minimum age, because mineral-

izations at higher elevations could have been already

eroded, the onset of thermal water activity would have

coincided with the strong acceleration of the uplift of the

Rhenish Massif in the Pleistocene. The estimated minimum

lifetime of 500 000 years of the thermal springs of

Wiesbaden is in contrast to May et al. (1996), who pro-

posed that individual spring systems are only active for a

few 10 000 years in this area.

CONCLUSIONS

(1) Sulphur isotope studies of (sub)-recent and fossil bar-

ite–(sulphide) mineralization from the thermal springs

of Wiesbaden, Rhenish Massif, demonstrate a continu-

ous evolution of the hydrothermal system with time,

with d34S of sulphate decreasing from +16.9& in the

oldest barite to +10.1&, the present-day value of the

thermal water. The d34S values of barite are very

similar to the present-day composition of dissolved

sulphate in recently active mineral and thermal springs

along the southern margin of the Rhenish Massif, but

contrast with the highly variable d34S values of ground

and mineral waters of the Tertiary Mainz Basin and

the Upper Rhine Graben.

(2) Mineralogical observations, calculations based on uplift

rates and the systematic isotopic evolution support a

continuous long-term activity of the thermal spring

system at Wiesbaden, which can be estimated to have

lasted for at least 500 000 years. The duration of

hydrothermal activity is therefore considerably longer

than estimates applying conservative models, which are

in the range of several 10 000 years.

136 T. WAGNER et al.

� 2005 Blackwell Publishing Ltd, Geofluids, 5, 124–139

(3) Isotope temperatures calculated from the d34S values

of coexisting barite and pyrite are in the range of

65–80�C, very close to the discharge temperature of

the springs. The reaction kinetics of the sulphate–

sulphide pair is too slow to permit isotopic re-

equilibration during ascent of the thermal water from

the deep aquifer; the time required for this process

being 220 Myr. We propose a model of rapid pyrite

precipitation in the near-surface environment, preser-

ving the isotope fractionation of the sulphate–sulphide

pair established in the aquifer.

(4) Geochemical modelling of water–mineral equilibria

indicates that barite is slightly supersaturated at the dis-

charge temperature of the springs. The thermal water is

strongly supersaturated with pyrite, even at the temper-

atures estimated for the deep aquifer. Considering the

kinetics of pyrite formation via monosulphide precursor

phases at low temperatures, we conclude that contact of

the thermal water with atmospheric oxygen and subse-

quent formation of oxidized intermediate sulphur

species is the dominant mechanism involved in pyrite

precipitation at Wiesbaden.

ACKNOWLEDGEMENTS

We thank the spa management of the municipality of

Wiesbaden (Mr Lipecki) for the permission to publish the

recent chemical analyses of important thermal and mineral

waters of Wiesbaden, which were measured in the ESWE

laboratory, Wiesbaden. Mr Boos and Mr Dorr (ESWE

Wiesbaden) are thanked for providing access to drillcores

and detailed drilling maps. G. Strecker (Hessisches Lande-

samt fur Umwelt und Geologie, Wiesbaden) has carried

out the XRF measurements of unaltered metavolcanics and

kindly provided the data. F. Geller-Grimm (Museum of

Wiesbaden) helped by providing several mineral samples.

T. Willert (Institut fur Geowissenschaftliche Gem-

einschaftsaufgaben, Hannover) provided actual data on the

regional geothermal gradient. P. Spaethe (University of

Wurzburg) is thanked for the preparation of polished sec-

tions carried out in excellent quality. The assistance of

K. Hradil during XRD analysis is greatly appreciated. We

thank Paul Gorman and Julie Dougans for technical assist-

ance with S isotope analyses. SUERC is funded by the

Natural Environment research Council (NERC) and the

Consortium of Scottish Universities. AJB is funded

through NERC support of the Isotope Community

Support Facility at SUERC.

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