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Geochimica et Cosmochimica Acta 102 (2013) 143–161
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Variability of sulphur isotope ratios in pyrite and
dissolved sulphate in granitoid fractures down to 1 km depth
– Evidence for widespread activity of sulphur reducing bacteria
Henrik Drake a,⇑, Mats E. Åström a, Eva-Lena Tullborg b, Martin Whitehouse c,
Anthony E. Fallick d
a
Linnus University, School of Natural Sciences, SE-39182 Kalmar, Sweden
Department of Earth Sciences, University of Gothenburg, Box 460, SE-40530 Göteborg, Sweden
c
Laboratory for Isotope Geology, Swedish Museum of Natural History, P.O. Box 50 007, SE-10405 Stockholm, Sweden
d
Scottish Universities Environmental Research Centre, East Kilbride, G75 0QF Scotland, UK
b
Received 27 June 2012; accepted in revised form 18 October 2012; Available online 29 October 2012
Abstract
Euhedral pyrite crystals in 46 open bedrock (granitoid) fractures at depths down to nearly 1 km were analysed for sulphur
isotope ratios (d34S) by the in situ secondary ion mass spectrometry (SIMS) technique and by conventional bulk-grain analysis, and were compared with groundwater data. Twenty nine of the fractures sampled for pyrite had corresponding data for
groundwater, including chemistry and isotopic ratios of sulphate, which provided a unique opportunity to compare the sulphur-isotopic ratios of pyrite and dissolved sulphate both at site and fracture-specific scales. Assessment of pyrite age and
formation conditions were based on the geological evolution of the area (Laxemar, SE Sweden), and on data on co-genetic
calcite as follows: (1) the isotopic ratios of the calcite crystals (d18O, d13C, 87Sr/86Sr) were compared with previously defined
isotopic features of fracture mineral assemblages precipitated during various geological periods, and (2) the d18O of the calcites were compared with the d18O of groundwater in fractures corresponding to those where the calcite/pyrite assemblages
were sampled. Taken together, the data show that all the sampled fractures carried pyrite/calcite that are low-temperature and
precipitated from the current groundwater or similar pre-existing groundwater, except at depths of 300 to 600 m where
water with a glacial component dominates and the crystals are from pre-modern fluids. An age of <10 Ma are anticipated
for the pre-modern fluids. The d34Spyr showed huge variations across individual crystals (such as 32 to +73&) and extreme
minimum (50&) and maximum (+91&) values. For this kind of extreme S-isotopic variation at earth-surface conditions
there is no other explanation than activity of sulphur reducing bacteria coupled with sulphate-limited conditions. Indeed,
the most common subgrain feature was an increase in d34Spyr values from interior to rim of the crystal, which we interpret
are related to successively higher d34S values of the dissolved source SO42 caused by ongoing bacterial sulphate reduction
in fractures with low-flow or stagnant waters. The measured groundwater had d34SSO4 values of +9& to +37&, with the highest values associated with low sulphate concentrations. These values are overall, and especially in the sulphate-poor waters
down to 400 m, somewhat higher than the anticipated initial values, and can thus, like for the 34S-enriched pyrites, be
explained by a Rayleigh distillation process driven by microbial sulphate reduction. An intriguing feature was that the
d34SSO4 values of the groundwater were in no case reaching up to the values required to produce biogenic pyrite with d34S
values of +40& to +91&. To explain this feature, we suggest that groundwater in low-flow fractures with near-stagnant water
(carrying sulphate and pyrite with high d34S) is masked by high-flow parts of the fracture system carrying groundwater that
often contains sulphate in abundance and considerably less fractionated with respect to 34S and 32S. In order to gain detailed
⇑ Corresponding author. Tel.: +46 480447300; fax: +46 480447305.
E-mail address: [email protected] (H. Drake).
0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.gca.2012.10.036
144
H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
knowledge of chemical processes and patterns in groundwater in fractured rock, fracture-mineral investigations are a powerful tool, as we have shown here for the sulphur system.
Ó 2012 Elsevier Ltd. All rights reserved.
1. INTRODUCTION
Under low-temperature conditions in water-conductive
bedrock fractures, the sulphur system, including both dissolved and solid species, may be profoundly affected by sulphate-reducing bacteria (SRB) (e.g. Fredrickson et al., 1995;
Pedersen et al., 1997; Pedersen, 2000). This is supported by
the identification of SRB in deep boreholes and underground
openings in both crystalline and sedimentary rocks at a variety of sites. Such sites include depths of 600–1060 m in sandstone near Berlin, Germany (Sass and Cypionka, 2004),
approximately 550 m in supracrustal rocks of the Toyoha
mine, Japan (Nakagawa et al., 2002), 2800 m in Taylorsville
Basin hydrocarbon reservoir, Virginia, USA (Onstott et al.,
1999), 2800 m in a South African gold mine (Chivian et al.,
2008), 330 m in crystalline bedrock at Olkiluoto, Finland
(Pedersen, 2008), more than 500 m in crystalline bedrock in
Laxemar/Äspö, southern Sweden (Pedersen et al., 1997;
Hallbeck and Pedersen, 2009; Rosdahl et al., 2010), and
approximately 1000 m in crystalline bedrock in Forsmark,
central Sweden (Hallbeck and Pedersen, 2008). Additionally,
nutrient supply and sulphate concentrations can vary considerably from one site to another (e.g. Smellie et al., 1985,
1987; Blomqvist, 1999; Gascoyne, 2004; Pitkänen et al.,
2007; Laaksoharju et al., 2008; Kukkonen, 2011), which
may affect SRB activity. However, despite the wide occurrence of SRB, it often remains unclear to what extent the bacteria were active before disturbance (drilling and sampling)
and during pristine conditions in the past.
Dissolved sulphide in bedrock fracture fluids is a strong
indicator of ongoing SRB activity and sulphate reduction
(Rye et al., 1981). Often there is, however, no correlation
between number of SRB cells and sulphide concentrations,
and the groundwater sampling itself seems to strongly affect
sulphide concentrations (Pitkänen et al., 2009; Penttinen
et al., 2010; Tullborg et al., 2010, 2011). At the Swedish
sites listed above, sulphide concentrations in boreholes
immediately after drilling and pumping were low (mainly
<0.005 mg/L) but increased (up to several mg/L) during
the following years when stationary borehole equipment
was installed, followed yet again by decreasing values with
time during pumping (Tullborg et al., 2010, 2011). This may
be due to a number of possible reasons: (1) the borehole
section provides, due to the stationary equipment, potential
for local microbial-induced anaerobic corrosion as well as
nutrients for sulphate-reducing bacteria, (2) initial sulphide
values are too low as a result of disturbances caused by the
drilling and pumping (Rosdahl et al., 2010), and (3) in-mixing of shallow groundwater to deeper levels may increase
the content of nutrients, and thus SRB activity, in the borehole sections sampled. It thus remains unclear to what extent and to which depth SRB are active during
undisturbed conditions, and how the changes caused by
drilling may have influenced their activity.
The sulphide anions are likely to react with dissolved
Fe2+, which often occurs abundantly in the deep groundwaters at these Swedish sites (Laaksoharju et al., 2009), and
consequently are precipitated as metastable iron sulphide
(FeS) that is ultimately transformed to pyrite. In this process, fractionation of sulphur isotopes is small or insignificant, both when FeS precipitates from H2S or HS
(Böttcher et al., 1998) and FeS transforms to FeS2 (Wilkin
and Barnes, 1996). Fractionation is, however, commonly
large (Goldhaber and Kaplan, 1975; Wortmann et al.,
2001; Sim et al., 2011), although occasionally as low as
2& (e.g. Detmers et al., 2001), during microbial reduction
because SRB favour 32S over 34S. Therefore, measurements
of S isotope ratios in pyrite (d34Spyr) provide information
on past microbial sulphate reduction, which is utilized in
this study as an indirect method to assess SRB activity during pristine (undisturbed) conditions in deep-lying bedrock
fractures.
Identification of low-temperature pyrite in bedrock fractures is, however, challenging because there is no protocol
for differentiating crystals precipitated under low-temperature conditions (microbial activity possible) from other
crystals formed at higher temperatures (abiotic conditions).
This problem can, however, to a large extent be overcome
where co-genetic minerals exist and can be dated by absolute or relative techniques. An important mineral in this
context is calcite, which typically is abundant in bedrock
fractures. Additionally, the large biogenic kinetic sulphurisotope fractionation can, obviously, in itself be used as
an indicator because abiotic thermochemical reduction typically feature maximum kinetic fractionation of 10–20&
(Kiyosu and Krouse, 1990) and has a temperature limit of
>100–140 °C (Machel et al., 1995; Machel, 2001). Pyrite is
frequent in fractures at many Proterozoic-Archean crystalline rock sites (Bottomley, 1987; Drake et al., 2006; Gehör
et al., 2007). However, most pyrite studied has generally
been of old hydrothermal origin, and only occasionally of
low-temperature origin (Paleozoic and later) but formed
from fluids out of isotopic equilibrium with the currentday groundwaters as shown by isotopes in calcite (e.g. Bottomley, 1987; Drake and Tullborg, 2009; Sandström and
Tullborg, 2009). Only very detailed studies of a single fracture plane at 200 m depth in Sweden have revealed and included pyrite that has a biogenic origin and was
precipitated from fluids corresponding to present groundwater (Pedersen et al., 1997; Tullborg et al., 1999).
This study focuses on pyrite precipitated in granitoid
fractures down to almost 1 km below the surface in the
Laxemar area, southern Sweden, where previous fracture
mineralogical investigations have been carried out (Drake
and Tullborg, 2009; Drake et al., 2009b, 2012). At this site,
extensive fracture mapping has shown that pyrite is readily
precipitated in open fractures indicative of SRB and/or
hydrothermal activity; in this study we have processed this
H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
mapping data in order to visualise the distribution of pyrite
in the fracture network. In many cases calcite is co-genetic
(partly intergrown) with pyrite, which means that approximate dating can be made by relating the isotopic composition of calcite (carbon, oxygen and strontium) to existing
hydrological, hydrochemical and fracture-mineralogical
knowledge/data of this region. U-series dating of the calcites is, unfortunately, not possible due to low U concentrations and small volumes of the calcites. Additionally, the
existing hydrochemical and hydrological data mean that a
unique comparison can be made between sulphur compounds in current-day fracture groundwater (decades to
several hundreds of thousands of years in age) and in pyrite
crystals (age up to 10 Ma) on the fracture walls.
The aim was to increase the understanding of sulphur
behaviour under low-temperature conditions in granitoid
fractures down to 1 km depth. Focus is on both pyrite frequency (mapping study) and isotopic composition (in situ
secondary ion mass spectrometry – SIMS – and bulk analysis), revealing phenomena unaffected by drilling activities.
In water-conducting fractures, the isotopic composition
also of dissolved sulphate was determined and compared
with the pyrite, in order to assess site-scale and, with some
caution, fracture-specific fractionation dynamics. A question of particular relevance is whether microbial sulphide
reduction occurs in the bedrock fractures, considering the
inherent difficulties (as described above) in the determination of dissolved sulphide. This question was approached
by a hypothesis: If SRB have been active in the fractures
of the upper 1 km of bedrock during pre-drilling conditions, then d34S of pyrite crystals shows a large variability
and evolution towards higher values if closed-system conditions prevailed and low but constant values if open-system
conditions prevailed (cf. McKibben and Eldridge, 1994;
Canfield, 2001a).
2. SETTING
2.1. Bedrock geology and fracture mineralogy
The bedrock consists of overall well preserved Paleoproterozoic 1.8 Ga granitoids (Wahlgren et al., 2006, 2008) of
the Transscandinavian Igneous Belt (Gaàl and Gorbatschev, 1987; Fig. 1). In the Paleozoic, marine transgression
resulted in a sedimentary cover (Koistinen et al., 2001)
which, according to apatite fission track analyses (Zeck
et al., 1988; Larson et al., 1999; Cederbom, 2001), reached
a thickness of several km. Subsequent uplift caused erosion
leading to a near complete removal of the sedimentary cover during Neogene erosional episodes in the Miocene and
Plio-Pleistocene (Japsen et al., 2002).
Fracture zones of regional character (surface trace
lengths >1000 m, with moderate to steep dips and thicknesses of about 10–150 m), minor deformation zones (trace
lengths <1000 m, thicknesses <10 m) and a variety of open
or sealed single fractures with four main orientation sets
(one sub-horizontal set and three sub-vertical striking N–S,
ENE and WNW) occur in the area (La Pointe et al., 2008;
Wahlgren et al., 2008). Frequency of open sub-fractures varies widely among the fracture zones but is generally 10–20/m
145
(at least in the core zone; transition zones are less fractured);
zones of crushed rock also occur. Within the fractures there
are up to six types of fracture mineral assemblages (Table 1)
characterised by different mineralogical composition as a result mainly of successively lower formation temperature
(Drake and Tullborg, 2009; Drake et al., 2009b). The first
two assemblages are Paleo-Proterozoic mylonite and cataclasites restricted to fracture zones featuring multiple reactivation phases (Wahlgren et al., 2008; Drake et al., 2009b; Viola
et al., 2009). Assemblage 3 mainly formed during brittle fracture reactivation in relation to nearby granite intrusions at
1.45 Ga, based on 40Ar/39Ar dating of mica in veins and altered wall rock (Drake et al., 2009b). This assemblage is
found in sealed fractures and has inorganic isotope signatures of d13Ccalcite (6& to 3& VPDB) and d34S in anhedral–subhedral pyrite (3& to +3& VCDT – Canyon
Diablo Troilite, bulk analysis) and fluid inclusion homogenisation temperatures of c. 200–360 °C (Drake and Tullborg,
2009). Fracture reactivation related to the 0.9–1.1 Ga Sveconorwegian orogeny (e.g. Bingen et al., 2005) lead to formation of fracture mineral assemblage 4 (Söderbäck, 2008),
supported by 40Ar/39Ar adularia dating of 0.99 Ga (Drake
et al., 2009b). During the Caledonian orogeny fracture minerals of assemblage 5 were formed mainly from c. 80–145 °C
brine fluids (Drake and Tullborg, 2009) at c. 440–400 Ma
(40Ar/39Ar adularia dating; Drake et al., 2009b). Scattered
high d34S values of bulk pyrite of assemblage 5 (up to
60& VCDT) indicate closed-system microbial activity in
the bedrock fractures, down to some hundreds of meters
depth (current elevation), but thermochemical sulphate
reduction may also have been ongoing during the most elevated temperatures of this period (Drake and Tullborg,
2009). Throughout the Quaternary, when the bedrock has
been exposed or covered by only a thin overburden of unconsolidated material, episodes of marine transgressions, melting glaciers and meteoric recharge have allowed water of
different composition to repeatedly intrude the bedrock fractures to great depths, (Laaksoharju et al., 2009). This has
triggered mineral formation, including low-temperature calcite of both marine and meteoric origin (Drake and Tullborg,
2009; Drake et al., 2012). We therefore argue that only after
the sedimentary cover of the area was significantly reduced
and eventually disappeared completely in Neogene (at
10 Ma), a dynamic groundwater system similar to that in
Quaternary could be established leading to mixing of water
types and associated fracture-mineral precipitation. Euhedral pyrite of assemblage 6 coatings is the focus of this study.
2.2. Hydrochemistry and microbiology
Within the fracture network (fracture zones and single
open fractures) there is generally relatively low flow (transmissivity is in rare cases up to 1 105 m2/s; Rhén and
Hartley, 2009), reducing conditions (decreasing Eh with
depth and redox front generally at 15–20 m; Auqué et al.,
2006; Drake et al., 2009a; Laaksoharju et al., 2009), pH
of 8 ± 0.25 and low to intermediate temperature (5–20 °C;
Laaksoharju et al., 2009). The groundwater composition
varies in general systematically with depth, although considerable variability exists due to local transmissivity of
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H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
Fig. 1. (a) Geological map of the Laxemar area with cored boreholes sampled for pyrite and groundwater chemistry indicated. Minor fracture
zones are not shown. (b) Map of Sweden with the Laxemar area indicated.
Table 1
Sequence of fracture filling assemblages (1 = oldest, 6 = youngest). Modified from Drake et al. (2009b).
Main related event and/or age
1. Mylonite; quartz, epidote, muscovite, chlorite, albite ± K-feldspar, calcite
2. Cataclasite; epidote, quartz, chlorite, K-feldspar, albite, hematite ± illite
3a. Calcite, quartz, epidote, chlorite, pyrite, prehnite, fluorite, muscovite,
laumontite, adularia, hematite
4. Calcite, adularia, laumontite, chlorite, quartz, illite, hematite
Cambrian sandstone (near surface)
5. Calcite, adularia, chlorite, hematite, fluorite, quartz, pyrite, barite, gypsum,
clay minerals, apophyllite, harmotome, REE-carbonate, ±galena, chalcopyrite,
laumontite, sphalerite, analcime
6. Calcite, pyrite, clay minerals, goethite (near surface), minor barite
fractures, topography, and distance to the present shoreline
(SKB, 2006). The oldest (in the order of hundreds of thousands of years) water type (saline) resides in the deepest part
of the bedrock at <600 m (the waters without marine
component in Fig. 3a). When the latest continental ice sheet
(Weichselian) melted and retreated at 14 ka, glacial meltwater was injected into the bedrock fractures to depths of at
least several hundred meters due to high head pressure
(SKB, 2006). These waters currently dominate at 250–
600 m depth and are mixed with brackish water intruded
during subsequent transgressions of Littorina Sea water
(salinity maximum approximately 6500 mg/L Cl at 6.5–
5.0 ka; Westman et al., 1999) in the eastern part of the study
area and in valleys further inland. Water with a marine
component is generally distinguished by enhanced Mg/Cl,
K/Cl and SO4/Cl ratios and high d18O values compared
with non marine waters although these marine signatures
are to variable degree affected by mixing with other water
types and partly changed by water–rock interaction (Laaksoharju et al., 2009). In the parts not covered by the Littorina Sea, meteoric water circulation introduced freshwater
Waning stages of the Svecokarelian orogeny (>1750 Ma)
Probably 1750–1,620 Ma
Intrusion of Götemar and Uthammar granites related to
the 1.47–1.44 Ga Danapolonian orogeny
Sveconorwegian orogeny (1.1–0.9 Ga)
Early Cambrian extension
Caledonian orogeny at 440–400 Ma
Late Paleozoic to possibly Quaternary
on top of the glacial, brackish and saline waters, and driven
by the land uplift, partly flushed out older water types
(Laaksoharju et al., 2009). Modern meteoric recharge water
currently dominates the upper 100–200 m of the bedrock
(SKB, 2006). Sulphide concentrations are, as presented
above, variable in the groundwater and under the reducing
conditions prevailing (except above the redox front) the
common presence of dissolved Fe(II) facilitates the
precipitation of Fe(II)-monosulphides (subsequent transformation to pyrite), as indicated by modelled FeS saturation (Gimeno et al., 2009). The source of the groundwater
has no significant control on total number of cells and
concentration of adenosine-tri-phosphate (ATP, indicating
ongoing microbial activity) (Hallbeck and Pedersen, 2008,
2009). Sulphate-reducing bacteria have been identified
down to 920 m (deepest water investigated), but show
wide variations in population levels (generally 3 101 to
3 103 cells mL1), in similarity with Fe- and Mnreducing bacteria and acetogens, although these observations do not constrain whether the microbes are actively
metabolizing.
H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
3. METHODS
3.1. Sampling and material
The distribution of open fractures and proportion of
pyrite-bearing open fractures within the bedrock were
determined based on existing data (SKB’s database SICADA) of 33,000 open fractures identified in approximately
25 km of drill cores from a total of 50 boreholes of various
lengths and dips (Fig. 1). For details on the mapping method, see e.g. Ehrenborg and Dahlin (2005). Absolute volumes of the mapped fracture minerals were not defined.
Forty-six pyrite-bearing open fractures at depths between 1.9 and 860 m.a.s.l. (boreholes drilled from
ground surface at 5–25 m.a.s.l) were sampled from cores
drilled with the triple-tube technique (e.g. Ask et al.,
2005), which is highly superior to the conventional singlebarrel technique, because during triple-tube drilling the
core remains stationary within the inner tube where it is
also kept isolated from flushing cooling water; only the outer tube rotates with the drill rod. Fragile minerals on the
fracture surfaces are therefore not flushed away and preserved in good condition. These selected fractures were chosen for two reasons. First, they carry co-genetic calcite that
expresses stable isotopic composition (d13C and d18O) and
Sr isotopic signatures (analysed only in part of the samples)
indicative of low-temperature formation conditions in postPaleozoic age (Fig. 2). Second, based on information and
experience from routine drill-core fracture mapping, in-hole
flow measurements and groundwater-oriented projects
(Ehrenborg and Dahlin, 2005; Rouhiainen et al., 2005;
Wikström et al., 2008; Smellie and Tullborg, 2009) we chose
an approximately equal number of (1) single open fractures
located outside fracture zones and having a low water flow
147
(<3 109 m2/s). This kind of low flow was not detected by
the flow log, which is a down-hole equipment used for section-wise flow rate measurements from the whole borehole
(5 m long sections, stepwise moved 0.5 m; Rouhiainen
et al., 2005), (2) low-flow fractures within fracture zones
(<3 109 m2/s), and (3) high-flow fractures with dominantly large aperture within fracture zones (up to
8 105 m2/s). In addition, barite was sampled from
three fractures.
For 29 of the 46 sampled pyrite coatings, groundwater
data existed for the corresponding fracture section. This
groundwater data has been collected in other previous projects, mainly after drilling of each borehole during 2004–
2008 or during monitoring in 2008–2009, and are presented
in Appendix 1. In addition, for 11 pyrite coatings, comparison was made with groundwater data from other nearby
borehole sections. Groundwater was sampled in 3–10 m
long borehole sections (a few longer sections also exist)
which were isolated using inflatable packers to prevent inflow from other parts of the borehole. These water samples
are mainly from fracture zones and thus generally represent
water from several water-flowing sub-fractures with various
flow, i.e. both low-flow and high-flow sub-fractures. The
first round of groundwater samples were collected shortly
after drilling utilizing pumping, on-line measurements and
water sampling for chemical analyses in isolated borehole
sections (e.g. Bergelin et al., 2006; Kalinowski, 2010). Subsequent monitoring was performed in borehole sections
with permanently installed packers (Regander et al.,
2009). Quality assurance of the chemical data has been assured by systematic evaluation based on an integrated geological, hydrogeological and hydrochemical approach
(Smellie and Tullborg, 2009). Most groundwater variables
retrieved from the database, and presented in this study
Fig. 2. (a) d18O vs. d13C and (b) 87Sr/86Sr vs. d18O in calcite co-genetic with pyrite, shown along with hydrothermal to low-temperature
calcites included in earlier studies in the area, as well as the range for hypothetical calcite precipitated from the present groundwater at the site,
using the equation 1000lna = 2.78(106 T2) 2.89 (O’Neil et al., 1969) to calculate the fractionation factor (a) between oxygen in water and
calcite at borehole water-temperatures. Formation temperature and age of hydrothermal Proterozoic calcite (200–370 °C and 0.99–1.45 Ga,
respectively) and Paleozoic calcite (80–145 °C and 400–448 Ma) were determined from fluid inclusion analyses (calcite and co-genetic quartz;
Drake and Tullborg, 2009) and 40Ar/39Ar dating (adularia and mica; Drake et al., 2009b). Calcite younger than the Paleozoic is interpreted to
be formed from temperatures lower than 50 °C (cf. Drake et al., 2012).
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H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
Fig. 3. Selected groundwater variables vs. depth (a-e) and dissolved SO42 vs. its S-isotopic composition (f). Waters with distinguishable
marine component (i.e. high Mg/Cl ± high d18O ± high K/Cl) are indicated as white symbols in a and b.
(Appendix 1), showed only small variations between initial
sampling and the subsequent monitoring. For clarity, therefore, a single value is presented for each section (primarily
the data for the initial sample, or the last time-series measurement if data from initial sampling was lacking).
3.2. Analyses of the solid phase
3.2.1. SEM
Each fracture surface was examined with stereo microscope and SEM (Hitachi S-3400N SEM, equipped with an
H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
integrated EDS system), for identification of crystal habit,
paragenetic minerals, as well as to identify any older pyrite,
mainly by appearance; anhedral–subhedral, coarse-grained
crystals and/or paragenesis; e.g. presence of co-genetic epidote, prehnite, Fe–Mg chlorite and massive calcite of older
assemblages (Table 1). The acceleration voltage was 20 kV,
and working distance 9.9 mm. The crystals were then handpicked under the stereo microscope, for both conventional
analysis and in situ SIMS analysis. Crystals for SIMS analysis
were mounted in epoxy, gold-coated (30 nm) and before the
analysis investigated with SEM (back-scattered electrons;
BSE and cathodoluminescence; CL) in order to identify
any zonation or overgrowth and any cracks or impurities.
3.2.2. Sulphur isotope analysis
d34S analyses of bulk samples of pyrite (5 mg) and additional barite (10 mg) were performed at the Scottish Universities Environmental Research Centre (SUERC), East
Kilbride, UK. SO2 was liberated from sulphides following
the method of Robinson and Kusakabe (1975), whereby
samples were combusted at 1070 °C for 25 min in the presence of excess Cu2O. For barite (two samples), the technique of Coleman and Moore (1978) was used whereby
silica is added to the Cu2O, and was combusted at
1120 °C to completion, typically 30 min. A high-tempera-
149
ture furnace containing pure Cu ensured that any SO3 in
the combustion products was reduced to SO2. SO2 was separated from excess oxygen and the other combustion products by standard vacuum line techniques, and the purified
gas was analysed for 34S/32S on a dual-inlet Micromass
SIRAII multiple-collector mass spectrometer. Results are
reported relative to VCDT, accuracy and precision was
±0.2& or better based on replicate analyses of sample
duplicates, an inter-laboratory standard (CP-1, CuFeS2,
d34S = 4.60&) and certified standard reference materials
(IAEA-S-1, Ag2S, d34S = 0.3&, Ding et al. 2001; IAEAS-2, Ag2S, d34S = +22.62&, Mann et al. 2009; IAEA-S-3,
Ag2S, d34S = 32.49&, Mann et al. 2009; NBS 123, ZnS,
d34S = +17.44, Zhang and Ding, 1989) measured routinely
with every batch of about ten samples.
SIMS-analyses of pyrite (and one barite) were carried
out at the Nordsim facility, Swedish Museum of Natural
History, Stockholm. One to three crystals (mainly two)
were analysed from each fracture coating and one to seven
analyses (mainly three) were made for each crystal, in transects. Microscale sulphur isotope ratio measurements were
carried out using a Cameca IMS1280 ion microprobe. Analytical settings are described briefly below, and closely follow those described by Kamber and Whitehouse (2007)
and Whitehouse (in press). Sulphur was sputtered using a
Fig. 4. (a) Variation in frequency of open fractures, calculated for 10 m vertical depth intervals (5 m intervals in the upper 70 m and 150 m
interval at the largest depth, represented by only a few cores). (b) Frequency of open fractures carrying pyrite for each 10 m depth interval
from cored boreholes.
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H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
133
Cs+ primary beam with 20 kV incident energy (10 kV
primary, 10 kV secondary) and a primary beam current
of 1.5 nA, producing secondary ions from a slightly elliptical area of 10 lm (long axis). A normal incidence electron gun was used for charge compensation. Analyses
were performed in automated sequences, with each analysis
comprising a 70 s presputter to remove the gold coating
over a rastered 25 25 lm area, centring of the secondary
beam in the field aperture to correct for small variations in
surface relief, and data acquisition in 16 4 s integration cycles. The magnetic field was locked at the beginning of the
session using an NMR field sensor. Secondary ion signals
for 32S and 34S were detected simultaneously using two Faraday detectors with a common mass resolution of 4860 (M/
DM). Data were normalised for instrumental mass fractionation using matrix matched standards which were mounted
together with the sample mounts and analysed after every
sixth sample analysis. For pyrite, the Ruttan standard with
a conventionally determined d34SCDT value of 1.2& (Crowe
and Vaughan, 1996) was used and for barite, the Fig Tree
reference material (Reuschel et al., 2012), with a conventionally determined d34SCDT value of 4.5& (normalisation
values are presented in Appendix 2). Typical precision on
a single d34S value, after propagating the within run and
external uncertainties from the standard measurements
was ±0.3–0.4&. All results are reported with respect to
the V-CDT standard (Ding et al., 2001).
4. RESULTS
4.1. Groundwater
The concentrations of Cl and SO42 increased with
depth (Fig. 3a and b), whereas the concentrations of organic
carbon and bicarbonate decreased with depth (Fig. 3c and
d). The d34S of SO42 in fractures in the upper 400 m (mostly
>25&, Fig. 3e) was distinctly higher than in deeper fractures
(generally +9& to +20&) and than in modern ocean water
(21&; Rees et al., 1978) and Baltic Sea water (20 ± 1&;
SKB, 2006). Another feature of the d34S distribution is that
high values (25–37&) were strongly clustered to SO42
< 100 mg/L and the lowest values (around +10&) to
SO42 > 450 mg/L (Fig. 3f).
4.2. Pyrite
The frequency of open fractures decreased overall from
the surface (nearly 3/m) to depths of <700 m (approximately 1/m; Fig. 4a). The proportion of these fractures carrying pyrite also varied with depth: less than 5% in the
upper tens of meters above the redox front, 20 ± 4% (mean
and standard deviation) throughout most of the depth profile (30 to 675 m), and overall a successively dropping
frequency at depths below 675 m (Fig. 4b).
The sampled pyrite was euhedral, including individual
crystals and/or polycrystalline aggregates (Fig. 5). A few
fractures additionally contained minor amounts of anhedral to subhedral crystals. Cubic crystals dominated
(Fig. 5a–c) but pyritohedral and octahedral crystals were
also identified (Fig. 5d). Paragenetic minerals included
mainly calcite, but also clay minerals and occasionally barite. SEM-BSE investigation of polished epoxy-mounted
crystals revealed very few overgrowths or zonations in the
pyrite crystals (<15% of the samples, see e.g. Figs. 7 and
8 below). Pyrite crystals were generally 100–300 lm in size.
The pyrite crystals showed extreme variation in d34S: (1)
bulk grain analyses ranged over 96& (42 to +54&), (2)
subgrain (in situ) analyses ranged over 141& (50 to
Fig. 5. BSE–SEM images of pyrite crystals on fracture surfaces. (a) Cubic pyrite crystals (py) intergrown with calcite (cc). Sample KLX07A:
373 m. (b) Cubic pyrite crystals (py) intergrown with a smooth chlorite-clay mineral-dominated fracture coating (cm), KLX04: 555 m. (c)
Cubic pyrite crystals intergrown with euhedral calcite (cc) and clay minerals (cm), KLX07A: 356 m, (d) Aggregates of octahedral and
pyritohedral pyrite (py) along with calcite (cc) and clay minerals (cm), KLX10: 357 m.
H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
+91), (3) the youngest outermost parts of the crystals ranged over 133& (42 to +91&) and (4) single crystal ranged
over 105& (32 to +73&) (Fig. 6, Appendix 1). There
were three general trends in isotopic composition with mineral grains: (1) an increase in d34S from centre to rim (al-
151
most two thirds of the samples, Figs. 6b and 7), (2)
relatively constant d34S (almost one third of the samples,
Fig. 8), and (3) spikes or dips in isotopic composition across
the grains, although several of these had an overall trend of
increasing d34S with growth (Fig. 9). In the first group the
difference between d34Srim and d34Scentre was commonly
15–45&, but also higher. Variations in d34S range and pattern between different crystals on the same fracture surface
occurred and are interpreted to reflect different phases of
the precipitation event and/or analysis of different growth
layers in the polished crystals. The barite also showed
increasing values with growth (+17 to +27&), however to
a smaller extent than in pyrite from the same fracture surface (Appendix 1). The samples with relatively constant
d34S across the crystal showed consistent d34S in several
grains on the same fracture surface and were generally in
agreement with bulk analyses (Figs. 6b and 8). Crystals
with spikes and dips in d34S either showed a change from
increasing d34S with growth in the crystal interior to
decreasing d34S at the crystal edge (sample KLX09F:102,
KLX15A:630; moderate), or the opposite, a dip in d34S between centre and rim (KLX13A:450, Fig. 9). Generally,
most of these crystals showed fluctuating but overall
increasing d34S from centre to rim (KLX03:904,
KLX20A:275). The difference between d34Srim and d34Scentre
was generally uncorrelated with the depth of the fracture
(except that d34Srim d34Scentre > 50& was restricted to
<300 m) but was lower in fractures with a high water flow
– often close to 0& – than in fractures with low water flow –
mostly +10& to +50& (Fig. 6b). The d34Srim values were
on average higher in pyrite collected in the upper 300 m
than in pyrite sampled below that depth, whereas the few
negative d34Srim values were mainly from below 300 m
(Fig. 6c).
5. DISCUSSION
5.1. Groundwater
Fig. 6. d34S in pyrite vs. depth. (a) in situ (SIMS) analyses;
individual fracture coatings are connected by a line. (b) difference:
d34Srim d34Scentre (c) d34S of the outermost parts of the crystals. In
b and c, samples are divided based on the flow (lowflow = <3 109 m2/s; high-flow = 3 109 up to 8.0 105 m2/
s) of the pyrite fracture in the section sampled for groundwater (for
pyrite from the low-flow fractures, groundwater samples represent
mainly other fractures with higher flow rates; see text), and based on
whether the corresponding groundwater sample has a marine
component.
The water types that have infiltrated the bedrock,
including marine (Baltic Sea) water and fresh waters (meteoric and glacial melt water), all presumably had d34SSO4 values close to that of the marine water (+20&) considering
the near-coastal location of the site. This is supported by
the d34S values of the central parts of barite crystals
(+17& to +22&) and bulk barite crystals (+17& to
+23&). On one of the barite crystals the rim had a value
of +27&, which however most likely reflects precipitation
from an already modified source. In the deeper aquifers (below 400 m) the d34SSO4 may have attained lower values
due to addition of relatively 34S-depleted sulphate from dissolution of fracture-bound Paleozoic gypsum (d34S: +4&
to +13&; Drake and Tullborg, 2009), which indeed is indicated in the groundwater d34SSO4 values at these depths
(+10& to +20&, Fig. 3e).
d34SSO4 values of 25–37&, occurring predominantly in
the upper part of the bedrock (down to 400 m) and at
low SO42 concentrations (<100 mg/L), are therefore
attributed to bacterial sulphate reduction. The reason for
this conclusion is that the light S isotope (32S) is preferred
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H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
Fig. 7. (a) d34S increase from crystal centre to rim in selected pyrite crystals, shown in BSE–SEM images in (b–h, not all), where analysis
targets are indicated with circles and discerned overgrowths or zonations are indicated by stippled white lines.
over the heavy isotope (34S) during metabolism (e.g. Kaplan and Rittenberg, 1964), causing the precipitating metalsulphides (pyrite) to have lower d34S than the source sulphate. As a consequence of this phenomenon, in systems
where the reduction rate exceeds the supply rate of sulphate, the remaining shrinking sulphate pool will attain
progressively higher d34S values as the light isotope is favoured by the microorganisms and hence ends up in pyrite.
This process is described by a Rayleigh distillation process
where the sulphate pool is successively consumed during
microbial reduction and thus progressively depleted in 32S
relative to 34S (Ohmoto and Rye, 1979). Therefore, our
water data provide evidence of both microbial sulphate
reduction and a high ratio of reduction to supply rate of
sulphate in the upper 400 m.
At depths below 400 m, the d34SSO4 values are lower
(Fig. 3e). Therefore, at these depths, the d34SSO4 values pro-
vide no evidence of SRB activity. The explanation is neither
absence of SRB nor fast sulphate supply by advection, because SRB exist at these depths (Hallbeck and Pedersen,
2009) and water flow is overall lower here than in the upper
few hundred meters (Rhén et al., 2008). We therefore suggest that high sulphate concentrations, which are characteristic for these depths (Fig. 3) and originate from infiltration
of marine waters and breakdown of remnants of Paleozoic
gypsum, mask any SRB activity and sulphate reduction.
Geochemical modelling has indicated the potential dissolution of this gypsum (Gimeno et al., 2009).
5.2. Calcite age indications
Calcite readily incorporates Sr in comparison with Rb
and therefore retains the 87Sr/86Sr ratio of the groundwater
it precipitated from (McNutt et al., 1990). The 87Sr/86Sr
H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
153
Fig. 8. (a) Analytical transects of pyrite crystals with relatively constant d34S from centre to outermost parts. (b–f) BSE–SEM images of
crystals and analytical targets (indicated with circles). Only one crystal is shown for each sample even if two crystals (xx) are shown in a
(crystals shown in the BSE–SEM images are: b, KLX12A:538.3-xx2; c, KLX11D:96-xx2; d, KLX04:643-xx2; e, KLX08:679-xx1; KLX08:822xx2). Crystals b and c feature visible growth zonations.
ratios of groundwater have gradually increased from low
values in the Proterozoic and Paleozoic, shown by older
calcite (Fig. 2), due to interaction with wall rock minerals
that have been successively enriched in 87Sr due to decay
of 87Rb in Rb-rich minerals. The 87Sr/86Sr ratios of calcite
can therefore be used as a rough indicator of timing of
precipitation, as shown earlier for this area (Drake and
Tullborg, 2009; Drake et al., 2009b, 2012), especially since
87
Sr/86Sr ratios of inflowing water have been indicated to
be quickly homogenised through ion-exchange reactions
to values similar to those found in the wall rock (Peterman
and Wallin, 1999; Drake and Tullborg, 2009). Because the
calcites co-genetic with pyrite have 87Sr/86Sr ratios that
overlap with those of the present-day groundwater and of
the wall rock, precipitation of the calcites are relatively
recent.
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H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
Fig. 9. Spikes and/or dips in the d34S pattern during growth (a). BSE–SEM images of crystals and analytical targets indicated with circles (b–
f). In crystals KLX03:409 and KLX20A:275 (not shown in detail), analytical targets were not in straight transects but more randomly
distributed.
d18O of calcite has also varied over time, with lower values in the Proterozoic and Paleozoic (Fig. 2, when temperatures were higher than at present, Drake and Tullborg,
2009) than in the calcites of this study. As shown in
Fig. 2, all calcites co-genetic with the pyrites have d18O
within the range of hypothetical calcite precipitates from
the present-day groundwater, if using the equation
1000 lna = 2.78 (106 T2) 2.89 (O’Neil et al., 1969) to calculate the fractionation factor (a) between oxygen in water
and calcite at groundwater-temperatures of 0–20 °C (Drake
et al., 2012).
The d13C values of the calcites also largely indicate lowtemperature formation, especially the extremely low d13C
values (down to 70& VPDB), for which anaerobic bacterial oxidation of methane, possibly coupled with SRB activity, is the only possible explanation (cf. Drake et al., 2012).
The moderately to highly depleted d13C values of the majority of the calcites are typically associated with HCO3 originating from oxidation of organic matter, which leaves a 13Cdepleted reaction product that ultimately ends up in the calcite. The latter signature overlaps with Paleozoic calcite, but
generally differs from the Proterozoic calcite (Fig. 2).
The absence of two-phased fluid inclusions in the calcite,
typically indicative of fluid entrapment at fairly low temperatures (Roedder, 1984), also suggests low-temperature formation (<50 °C; Milodowski et al., 2005; Drake and
Tullborg, 2009).
All sampled calcites, and thus the pyrites, are therefore
of a low-temperature type, as opposed to Paleozoic and
Proterozoic calcites. However, during the long time span
of exposed bedrock (after the sedimentary rocks were removed by erosion at 10 Ma) throughout characterised by
low temperatures, the d18O composition (and salinity) of
the groundwater has changed several times during glaciations–interglaciations, featuring repeated intrusion of glacial meltwater, marine water and meteoric water
(Laaksoharju et al., 2009). Additionally, since the pyrites
may represent precipitation over a much larger time span
than covered by the ages of the current fracture groundwater, it is not likely that all sampled pyrites precipitated from
water of the same composition as presently residing in the
corresponding fractures. Indeed, if the equation by O’Neil
et al. (1969) is used for calcite–groundwater d18O comparisons on fracture scale, most fractures at 300 to 600 m, as
opposed to other depths, generally show no overlap, because the calcite has higher d18O values than expected from
the groundwater data (Fig. 10). This feature has a straightforward interpretation: current-day groundwater at 300
to 600 m carries a notable proportion of glacial water that
have d18O values as low as 21&, and the calcites are predating these waters and were precipitated from fluids
including a substantial proportion of marine water, meteoric water and/or saline water that have higher d18O values
than the glacial water (Laaksoharju et al., 2009). Conse-
H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
Fig. 10. d18O values measured in calcite coatings (single symbols)
compared with calculated d18O for calcite hypothetically precipitated from the present groundwater at 0–20 °C in corresponding
sections (horizontal line). The lower limit of the line range for
hypothetical calcite corresponds to the temperature of intruded
glacial meltwater and the upper limit is depth-dependent and is set
to 10 °C at ground surface and 20 °C at 1000 m (corresponding to
an increase with depth of 1 °C per 100 m) based on average watertemperatures measured in the boreholes. Sections with matching
(overlapping) measured-hypothetical d18O values are presented by
green colour and the others in red. (For interpretation of the
references to colour in this figure legend, the reader is referred to
the web version of this article.)
quently, in sections with glacial water the calcite seems to
have precipitated during previous Pleistocene interglacials,
whereas in sections with other waters it is either Holocene
or precipitated from ancient groundwater with a similar
composition as the present-day groundwater. Therefore,
d34S comparison between pyrite and dissolved sulphate on
the fracture level is tentative whereas on the site-scale is robust as all water types present during the Quaternary are
currently present in the fracture network.
5.3. Origin of pyrite and its isotopic composition
The range in d34Spyr values (50& to +91&) is exceptional. A similarly large range for a single location has to
the best of our knowledge been reported only for pyrite in
sediments and veins in the Creede Caldera (30& to
+110&; McKibben and Eldridge, 1994), whereas slightly
smaller ranges have been reported for lower-Paleozoic sediment-hosted pyrite at Navan, Ireland (63& to +62&; Boy-
155
ce et al., 1994), and vein-hosted Paleozoic pyrite at SaintSalvy, France (33& to +74&; Munoz et al., 1994). For this
kind of extreme S-isotopic variation at earth-surface conditions there is no other explanation than SRB activity (Machel
et al., 1995; Canfield, 2001a; Seal, 2006). Thermochemical
sulphate reduction (TSR) may produce fractionation up to
10–20& under experimental conditions (Kiyosu and Krouse,
1990), but in natural settings, it normally produces sulphide
with similar or the same d34S values as the initial sulphate
(Machel et al., 1995). However, calcite co-genetic with the
sampled pyrites shows no indication of being formed at high
temperatures (see Section 5.2), which strongly point to a lowtemperature origin of all sampled pyrites.
The low pyrite d34S values (50& to 30&) would require larger fractionation than commonly identified for
SRB (Canfield, 2001a) in order to be accomplished from
an initial d34SSO4 pool of 15–20&, but is still within the
range of fractionations in natural settings and laboratory
(Wortmann et al., 2001; Sim et al., 2011). In situ disproportionation, which requires intermediate oxidative steps and in
settings such as shallow marine sediments can cause extreme
fractionation (Canfield and Thamdrup, 1994; Habicht and
Canfield, 1997; Canfield, 2001a; Ries et al., 2009) in the order of additional 4–5& under abiotic conditions (Fry et al.,
1988) and 18& in biotic processes (Kaplan and Rittenberg,
1964), are unlikely because all the sampled pyrites are located well below the redox front. Two possible mechanisms
are therefore envisaged for the ultralight pyrite sulphur: (1)
SRB have produced extreme fractionation during reduction
of sulphate of marine origin and/or (2) precipitation from
fluids carrying a sulphate pool generated mainly by dissolution of gypsum, a mineral that is found in the fractures and
has lower d34S values (+4& to +13&; Drake and Tullborg,
2009) than the marine sulphate.
The increase in d34Spyr from low values in crystal interiors to high values in the rim (Fig. 7) – the dominating subgrain feature – was a nearly ubiquitous feature in fractures
with low water flow and occurred occasionally in fractures
with high flow (Fig. 6b). This pattern suggests that in low
flow fractures, where the supply rate of chemical species
is relatively low, the rate of sulphate consumption by
SRB exceeds the rate of sulphate supply (by advection, diffusion and fracture-mineral dissolution) in a Rayleigh distillation process moving towards progressively heavier
values (also seen in the single barite sample analysed in detail). This feature is in line with microbial investigations,
which have identified SRB utilizing acetate and methane
at great depths and humic substances at shallower depths
(Hallbeck and Pedersen, 2009). SRB have been proposed
to be most active in the upper hundreds of meters where
dissolved organic carbon (DOC) contents are high
(Fig. 3c) (Laaksoharju et al., 2009). The possibility that
the d34S increase across the pyrite grains would be caused
by a series of different descending fluids with variable
d34SSO4, as reported for other systems elsewhere (McKibben and Eldridge, 1994), would be expected to produce
irregular intracrystal d34S patterns and/or an abundance
of overgrowths and zonation, which was not the case except
on odd occasions (Fig. 9). A Rayleigh fractionation model
is thus relevant to consider:
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H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
d34 Spyr ¼ ðd34 S0 þ 1000Þf ða1Þ 1000;
where f is the fraction remaining and a is the fractionation
factor and d34S0 is the initial sulphate (Ohmoto and Rye,
1979). This equation predicts, when using marine water
with d34SSO4 = 21& and a fractionation factor aSO4-H2S of
1.047 (Canfield, 2001a), that >90% of the SO42 pool was
consumed in fractures with the most extreme d34Spyr value
(91&) and 50–70% consumed in other fractures carrying
the most frequently occurring d34Spyr values of the latest
precipitate (12–39&). We accordingly argue that closedsystem conditions with respect to dissolved sulphate have
been frequent in low-flow fractures, and occasionally occurring also in high-flow fractures, down to 1 km depth
throughout the period when the pyrites were precipitated.
The few pyrite crystals that do not feature a uniform increase in d34Spyr from centre of crystal to rim (Fig. 9) point
to inflow, at one or several occasions, of water with a
SO42-pool that was less affected by closed-system SRB
activity, i.e. had a lower d34SSO4 value, than the pre-existing
water in the fracture (cf. McKibben and Eldridge, 1994).
Afterwards, such as in KLX13A:450-xx2, and
KLX03:904-xx1, the water inflow was followed by another
increase of d34Spyr indicating re-establishment of closed-system Rayleigh fractionation.
Unchanged d34Spyr values during growth appeared in
about one third of the samples, dominantly in SO42 rich,
high-flow fractures (Fig. 8). This makes sense, because in
high-flow fractures, in particular if the dissolved sulphate
concentrations are high, it is unlikely that SRB will cause
a measurable shift towards heavier S-sulphate. The absolute values of d34Spyr in these fractures, however, varied
considerably (Fig. 8), which is intriguing and as yet unexplained. Possible causes, which may be the focus of future
investigations, are spatially variable but in specific fractures
temporally stable d34Spyr values due to different microbial
populations with different intrinsic fractionation factors
(cf. Detmers et al., 2001) and differences in reduction rate
(Kaplan and Rittenberg, 1964), or substrate availability
(cf. Canfield, 2001b).
The superheavy pyrite-S, as found in this study on the
rim of the crystals, has earlier been described for ancient
marine settings (Ries et al., 2009; Ferrini et al., 2010; Fallick et al., 2012) and have, in environments with absence
of 34S-depleted pyrite, been explained by precipitation from
fluids pre-enriched in 34S during migration (Ferrini et al.,
2010). For our study area, a similar scenario is rejected
for several reasons. First, the large range in d34S values
within the collected set of pyrites (Fig. 6). Second, the ultralight-S for several pyrites. Third, the large spread in d34Spyr
within single pyrite crystals. Additionally, formation of
some of these pyrites from fluids descending from an organic-rich sedimentary cover (potential for fluids already enriched in 34S), as the Cambro–Silurian successions once
covering the area (Koistinen et al., 2001; Japsen et al.,
2002), is also rejected. The reason for this is that co-genetic
calcite differ from Paleozoic calcite (Fig. 2) formed beneath
these successions at >80 °C in different paragenesis (Drake
and Tullborg, 2009; Drake et al., 2009b). For specific fractures, the d34S values of bulk pyrite were overall lower than
Fig. 11. (a) Difference of d34Srim and d34Scentre in pyrite vs.
groundwater SO42 concentrations. (b) d34S of bulk pyrite and
d34SSO4 of the groundwater from the same section vs. depth. (c)
d34Spyr of outermost part of pyrite crystal (SIMS) and d34SSO4 in
groundwater from the same section vs. depth. In (a), samples are
divided based on flow and the existence of marine water (see
caption for Fig. 6). In (b) and (c) each pyrite-groundwater pair is
represented by a line with pyrite symbol being green if, as expected
during SRB activity, d34Spyr is lower than d34SSO4. (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
for dissolved sulphate, as expected in a system of SRB
activity (Fig. 11b), whereas the outermost part of the pyrite
crystals, in contact with the water, was not always so
H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
(Fig. 11c). This pattern suggests that the pyrites are often
not precipitated from the kind of dissolved sulphate-pool
currently existing in the groundwater in the corresponding
fractures. Obviously, a possible explanation for this is that
this sulphate pool was different during previous geological
periods and that pyrite precipitation mainly occurred during such periods. We do, however, not find any evidence
to suggest that this would be the case. The reason is that
on site-scale, the present groundwater covers the range of
groundwater compositions that existed throughout the last
millions of years of glaciations–interglacials, and the range
of d34S values measured in the current groundwater is thus
regionally representative for the whole period during which
the pyrites potentially precipitated (Laaksoharju et al.,
2009). The fact that the d34SSO4 measured in the current
groundwaters (Fig. 3e) was not reaching up to the values required to produce the d34Spyr values of +40& to +91&
means that the latter were produced by another mechanism
than variability in groundwater d34SSO4 values over time.
We suggest the following mechanism. The groundwater
sampled represents a relatively large volume, and contains
water from up to 50–100 m away from the borehole, as
an effect of pumping during sampling (Kalinowski, 2010).
The sampled groundwater is therefore likely to represent
a mixture of waters from various sub-fractures in the
water-conducting structure, including both high and low
conducting parts of the fracture zone. The high conducting
parts, which obviously supply plenty of water, will generally
have unchanged or only marginally affected d34SSO4 values,
whereas the low conducting parts that contribute with only
limited water volumes will have generally heavy d34Spyr and
d34SSO4 values (Fig. 6) as a result of more stagnant waters
favouring closed-system conditions. Consequently, we propose that low-flow channels in these fracture zones and lowflow single fractures, both with closed-system SRB-created
superhigh d34Spyr (especially at shallow depth where dissolved concentrations of SO42 were low and DOC elevated, Fig. 6c, cf. Fig. 3), are in the groundwater
sampling masked (diluted) by water from high-flow parts
of the structures. This produces in the groundwater data
an underrepresentation of low-flow structures carrying sulphate preferentially enriched in heavy S and being the
source of pyrite with superhigh d34S values. Crystals and/
or samples with high d34Spyr (in situ) values of +30& to
+50& are in some cases corresponding to bulk analyses
with lower values. This indicates absence in the in situ analyses of the earliest part of the precipitation phase, as also
suggested for other systems elsewhere by (cf. Seal, 2006),
or multiple precipitation episodes (included in the bulk
samples but not present in the crystals analysed using
SIMS). Crystals from the same fracture surfaces sometimes
also show differing starting d34Spyr values reflecting that all
stages are not present in every crystal.
Nineteen percent of the 33,000 open fractures carry pyrite, which certainly to a large extent is low-temperature but
also partly (to an unknown extent) hydrothermal. In our
study, including 46 pyrite-bearing fractures, selected mainly
based on characteristics of co-genetic calcite, there is evidence that the pyrites were all precipitated during low-temperature conditions and mostly had d34S values indicative
157
of microbial sulphate reduction. Our pyrites were not randomly selected among the thousands of fractures carrying
pyrite, but were collected where co-genetic calcite indicates
low-temperature formation. Therefore we cannot directly
extrapolate our data to the large pyrite data set. However,
our pyrite samples are by no means unique, and therefore
there are reasons to suggest, as indicated by the patterns
in Fig. 4b, that SRB activity has been a ubiquitous phenomenon at least down to 675 m under current low-temperature conditions.
6. CONCLUSIONS
We have shown that in situ d34S analyses of fracture pyrite, co-precipitated with calcite, give detailed information
on the sulphur system and on the activity of sulphur reducing bacteria during natural conditions in the bedrock. Our
main findings are:
d34Spyr shows huge variations across individual crystals
(up to 32& to +73&) and extreme minimum
(50&) and maximum (+91&) values indicative of
SRB activity. These features, combined with the fact
that there are thousands of pyrite-bearing fractures
down to >675 m below the surface, provide evidence
that SRB activity has been a ubiquitous phenomenon
throughout the bedrock volume over the past 10 Ma.
Increase in d34Spyr values across individual crystals is a
common feature for pyrite residing in low-flow fractures,
revealing that sulphate has been limited (up to >90%
consumed) and SRB have caused isotopic fractionation
in a closed-system that has undergone Rayleigh
distillation.
Current-day groundwater with d34SSO4 values up to
+37& cannot explain the high d34Spyr values of +40&
to +91&. We propose that these pyrites reflect conditions developed in near stagnant parts of the fracture
system, e.g. low-flow channels with limited water volumes where prolonged closed condition SRB activity
have resulted in heavy d34Spyr and d34SSO4 and groundwater depleted in sulphate. Groundwater samples do
usually not reflect these environments and the d34SSO4
values are therefore easily diluted by more sulphate-rich
waters from high-flow parts of the structures, in which
open system often prevails as indicated by constant
d34Spyr during crystal growth.
The data and findings of this study are relevant for
understanding S2 production and pyrite formation under
in situ low-temperature conditions in bedrock fracture to
substantial depth. At the study site, it is obvious that the
supply of energy and oxidised sulphur species (sulphate)
have been enough for microorganisms to thrive and pyrite
consequently to be precipitated on the fracture walls. For
similar fracture settings elsewhere, there are no reasons to
generally suspect a lower supply of energy, whereas the sulphate concentrations may be lower. At the study site, sulphate is supplied by both marine transgressions and
fracture-gypsum dissolution, which may not be the case
everywhere. Therefore, in order to make global quantifica-
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H. Drake et al. / Geochimica et Cosmochimica Acta 102 (2013) 143–161
tions of the sulphur system in fractured bedrock, there is a
need for additional studies in low-sulphur environments,
which may be common within continents.
ACKNOWLEDGMENTS
The authors thank Nova – Centre for University Studies Research and Development, and the Swedish Nuclear Fuel and Waste
Management Co. (SKB) for providing access to the unique drill
core material and databases, such as the hydrochemical data, which
were collected by SKB during their 2004–2009 site investigation
program. The manuscript benefited from detailed and constructive
reviews by Dr. David Fike, Dr. Axel Schmitt, one anonymous reviewer and associate editor Dr. David Johnston. This is NordSIM
contribution number 329.
APPENDIX A. SUPPLEMENTARY DATA
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.gca.2012.10.036.
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Associate editor: David Johnston