GEOLOGICA CARPATHICA, APRIL 2008, 59, 2, 117—132
www.geologicacarpathica.sk
Trace element chemistry of low-temperature pyrites –
an indicator of past changes in fluid chemistry and fluid
migration paths (Eger Graben, Czech Republic)
JIŘÍ ZACHARIÁŠ
1
, JIŘÍ ADAMOVIČ
2
and ANNA LANGROVÁ
2
1
Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 43 Praha 2,
Czech Republic; zachar@natur.cuni.cz
2
Institute of Geology, Academy of Sciences CR, Rozvojová 269, 165 02 Praha 6, Czech Republic
(Manuscript received March 13, 2007; accepted in revised form November 13, 2007)
Abstract: Trace element contents in six generations of low-temperature ( < ~ 5 0 °C) pyrite (Py-1 to Py-6) recognized in
silicified Cretaceous sandstones near Jeníkov in the central Eger (Ohře) Graben, Bohemian Massif, are used to decipher
late Cenozoic fluid circulation patterns in the graben. Py-1 and framboidal Py-2 are generally coeval with silica
cementation, while Py-3 to Py-6 postdate this process. Py-6 forms inclusions in barite crystals, clearly separated in time
from silica cementation. The average arsenic contents of 0.01, 0.15, 2.94, 3.91, 6.14 and 0.76 wt. % As for Py-1 through
Py-6 and anomalous average nickel and cobalt contents in the oldest Py-6 inclusions (3.47 and 8.86 wt. %, respectively)
indicate the presence of two contrasting fluid circulation patterns in the graben fill during the period of pyrite formation
(?Pliocene to Recent). Early fluids (Py-1 to Py-5) are interpreted as progressive deepening, fault-driven fluids originating
from acidic volcanics in the basement. The late fluids are shallow, topography-driven fluids in contact with the Tertiary
lignite beds. Earlier data from structural geology allow us to explain such change in fluid circulation by regional tectonic
stress rearrangement, which inhibited the activity on the Krušné hory Fault at about 400 ka.
Key words: Eger Graben, silicification, fluid chemistry, fluid circulation patterns, pyrite, arsenian pyrite, arsenic.
Introduction
Low-temperature formation of pyrite is associated with di-
agenetic processes in sediments, often combined with bac-
terial precipitation under anoxic conditions, and with
hydrothermal activity at shallow depths under the sur-
face. Pyrite is commonly a stoichiometric FeS
2
phase;
however, significant amounts of As, Ni, Co and minor Au
amounts were found in many natural samples. As-rich py-
rites typically appear to have formed at relatively low
temperatures and often exhibit habits that suggest rapid
precipitation (Abraitis et al. 2004). The extent of As solu-
bility and type of bonding in the pyrite structure (iso-
morphous As
+ 1
S
—1
substitution vs. nanoinclusions of
arsenopyrite) was recently discussed by Reich & Becker
(2006). Klemm (1965) demonstrated experimentally ex-
tensive solid solutions between pyrite (FeS
2
), cattierite
(CoS
2
), and vaesite (NiS
2
) at high temperatures ( ~ 500 °C
and higher), and limited ones at low temperatures. In
contrast, low-temperature (< ~150 °C) pyrites are com-
monly rich in Co, or Ni, or both (bravoite). The formation
temperatures of natural mineral assemblages are, howev-
er, still poorly constrained.
In the Eger (Ohře) Graben of the Bohemian Massif,
low-temperature pyrite formation is associated with fluid
advection along major faults in Cenozoic times. Flows of
low-saline epithermal fluids were driven by topography,
being heated on deep circulation paths, or by the heat
gradient along subvolcanic bodies. These fluids were
streamed into adjacent aquifers of coarse detrital sedi-
ments, thereby causing pervasive silicification and sul-
phidic mineralization in zones several kilometers broad.
The lateral extent of the epithermal mineralization is fur-
ther enhanced by the presence of sealing horizons above
the aquifers.
In this paper we describe textural and chemical
variations in six successive pyrite generations, all
associated with low-temperature ( ~ 30—60 °C) shallow
fossil subsurface geothermal discharge, in the silicified
Lower Turonian quartzose sandstones along the Krušné
hory Fault in northwestern Bohemia. The relationship
between mineral chemistry and present and past fluid
chemistry is also discussed. The studied samples come
from the quarry of Jeníkov west of Teplice in the Eger
Graben, ca. 4 km from its northern boundary fault.
Geological setting
Late Alpine epithermal mineralization is associated
with the northern boundary fault of the graben – the
Krušné hory Fault (Fig. 1). The oldest tectonic activity
on the Krušné hory Fault dates back to the Oligocene-
Miocene, but the largest movements are very young,
extending beyond the Pliocene/Pleistocene boundary
(Malkovský 1980; Coubal & Adamovič 2000). In the
Jeníkov area, the total vertical displacement magnitude
on the Krušné hory Fault is ~ 700 m (Malkovský 1980).
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ZACHARIÁŠ, ADAMOVIČ and LANGROVÁ
Fig. 1. A – A location map of the study area (frame) within the Eger Graben. B – A schematic geological map of the Jeníkov area.
119
TRACE ELEMENT CHEMISTRY OF LOW-TEMPERATURE PYRITES (EGER GRABEN)
Newly formed mineral phases are typically hosted by the
Teplice rhyolite ignimbrite body and the overlying
Cretaceous
and
Neogene
detrital
sediments.
Three
mineralization stages were recognized by Čadek et al.
(1963): 1. silicification stage, 2. sulphidic stage with
pyrite, marcasite, sphalerite and U oxides, and the
youngest 3. barite-fluorite stage also characterized by Fe
and U oxides, galena, sphalerite, pyrite etc. As for the
character of fluids and ages of mineralization, the individ-
ual stages have been poorly constrained up to now. Barite
and fluorite of stage 3, however, are clearly separated from
stages 1—2 in time, and are compatible with the chemistry
of the present thermal springs in the area.
Teplice rhyolite ignimbrite
The basement of the central part of the Eger Graben is
formed by rhyolite ignimbrite of Carboniferous age,
pertaining to the Altenberg-Teplice Caldera, extensively
exposed in the adjacent part of the Krušné hory
Mountains (e.g. Breiter 1997; Ulrych et al. 2006). A
WSW—ENE-trending segment of the basement between
Teplice and Jeníkov (Fig. 1), called the Teplice-Lahoš
Ridge, functioned as a paleohigh in the pre-Late
Cretaceous relief. Later, it was buried beneath Cretaceous
marine sediments and beneath the Tertiary sedimentary
and volcanic fill of the Eger Graben. Faulted boundaries
of the Teplice-Lahoš Ridge suggest its tectonic uplift
after the deposition of the Eger Graben fill. Rhyolite
ignimbrite in its core became re-exposed to the surface
by headward erosion of the Bílina River tributaries at
approximately 500 ka (Čadek et al. 1964; Čadek &
Malkovský 1968). Feldspar crystals in the rhyolite are
sericitized and kaolinized. Dense joints are lined with
bleached zones and covered with young mineral phases,
notably hematite and quartz.
Upper Cretaceous sediments
In the central part of the Eger Graben, Cretaceous
sediments have a stratigraphic range of Cenomanian to
Coniacian, and a preserved thickness of max. 80 m. Most
of the sedimentary package is formed by mud-dominated
lithologies characterized by low permeabilities, not
permitting effective fluid circulation.
Cenomanian sediments of continental/estuarine origin
(Čech & Váně 1988) were deposited in topographic lows
of the pre-Cenomanian relief only. They are arranged
into upwards-fining cycles of conglomerate – arkosic
sandstone – micaceous mudstone, and reach thicknesses
of max. 20 m (Hrob area NW of Jeníkov).
Lower Turonian marine sandstones transgressively over-
lie the Teplice rhyolite. They reach a thickness of 7—25 m,
wedging out towards the axis of the Teplice-Lahoš Ridge.
The sandstone is massive or cross-bedded, composed of
quartz with an admixture of feldspar and rhyolite frag-
ments ( < 3 %). Rhyolite boulders up to several meters in
size are common immediately above the basal transgres-
sive surface, together with coalified plant debris. Rhyolite
granules to pebbles can be found near the top of the sand-
stone body, and locally concentrate into a bed of matrix-
supported conglomerate. The amount of glauconite in-
creases upwards in the topmost 2 meters. In the area
between the Krušné hory Fault and the Teplice-Lahoš
Ridge, Lower Turonian sandstones are pervasively
cemented by secondary quartz. The amount of cement
generally increases towards the top of the sandstone body.
Silicified sandstones range from very light grey to dark
grey in colour, depending on the proportion of sulphides
(mostly pyrite) in the cement (Fig. 2a—c). A prominent
peak in the content of sulphides occurs in the topmost
0.5 m (Fig. 3). Higher up, the silicified sandstones are
unconformably overlain by Middle Turonian greyish
brown mudstones and marlstones (Čech & Váně 1988) of
very low primary permeability.
Structural controls on quartz cementation and sulphidic
mineralization
Quartz cement is widely distributed in permeable
detrital sediments of Cretaceous and Neogene age over a
large area in the central Eger Graben, along the Krušné
hory Fault. The highest intensity of quartz cementation
is visible in areas where the sediments are directly
underlain by the Teplice rhyolite and overlain by a low-
permeability sealant rock. This suggests an advective
fluid flow mediated by the dense network of fractures in
the underlying rhyolite. Feldspar kaolinization along the
flow paths could have served as a potential source of
silica, as has already been demonstrated by Čadek &
Malkovský (1968).
Brittle tectonic features in the silicified sediments of
the central Eger Graben are more or less coeval with
silicification but most of them clearly post-date the main
silicification episode, as suggested by frequent striated
fault planes in the cemented rock. This excludes a very
young (Holocene) age for the main silicification episode.
However, the silicification process must be younger than
~18 Ma, which is the age of the youngest preserved
silicified strata (Burdigalian) along the Krušné hory
Fault (Váně 1961). In the early stages, the advective/
convective fluid flow could have been a thermally-
driven process, connected with the Serravallian to
Tortonian (13.4 to 9 Ma) volcanic activity in this part of
the graben (Cajz 2000). However, no direct evidence has
been reported.
The precipitation of fluorite and barite is clearly
younger than the silicification process. Its Late Pliocene
to Pleistocene age, coeval with the largest movements on
the Krušné hory Fault and fault-scarp formation, is
suggested by various lines of geological evidence
(Čadek et al. 1963; Fengl 1995). These include the
compatible chemistry of the present thermal waters in the
area, the concentration of barite and fluorite occurrences
in zones of modern groundwater issues, and their
presence on joints dilated during the formation of the
Krušné hory Fault scarp. In the Teplice area, timing of
the barite mineralization can be constrained by the
120
ZACHARIÁŠ, ADAMOVIČ and LANGROVÁ
exhumation of the Teplice-Lahoš Ridge ( ~ 500 ka;
Čadek et al. 1964) but the youngest “radiobarites” are
Holocene in age, as indicated by the disequilibrium be-
tween the activity of
226
Ra and the parent uranium in
barite (Ulrych et al. 2007).
The REE patterns and uniform
ε
Nd
(t) data close to zero
for the fluorites from the Krušné hory Mountains show
interaction of the mineralizing fluids with granites
(Höhndorf et al. 1994). Fluid inclusion studies of
sandstone-hosted vein fluorite indicated homogenization
temperatures between 50 and 160 °C and transport by
fluids of Na—HCO
3
—Cl type (Höhndorf et al. 1994).
In the Jeníkov Quarry and its wider vicinity, no lateral
variations in the intensity of silicification of Lower
Fig. 2. a – A close-up view of silicified sandstones (quartzites). A subvertical fracture with sulphide impregnation along its walls is
marked by an arrow. The strike of sulphidic impregnations, parallel to the original sandstone bedding, is highlighted by a dashed line.
b—c – Examples of sulphidic impregnations parallel to sandstone bedding. d—j – Photomicrographs in transmitted light: d – Clasts of
detrital quartz (Qd) rimmed by quartz overgrowths (Q3) with subordinate pyrite mineralization (Py-2, not labelled on the photo). Sand-
stone pores are partly filled with microcrystalline silica (Q4) with disseminated sulphides (Py-2, -3, -4). Pore spaces that were left free after
Q4 and Py-2 to Py-4 precipitation were finally lined by crystals of late quartz (Q5). e – Early pyrite (Py-1/Py-2) trapped in quartz over-
growths (Q3). Pyrite precipitation partly postdates the onset of silicification (dustline formation). f – A quartz clast with two dustlines (in-
dicated by arrows). Pyrite (Py-2) rims the outermost (intra-Qtz-3) dustline. Microcrystalline silica (Q4) and pyrites (Py-2 trough Py-4) fill
the pores in sandstone. g – A detail of a quartz clast where Py-2 formation postdates quartz overgrowth formation (Q3). Numerous glob-
ules of framboidal Py-2, rimmed by laths of Py-2 intergrown with microcrystalline silica (Q4). h – Silica-sulphidic filling of an early frac-
ture. Central part of the photo is occupied by an angular fragment of silicified (Q3) sandstone with pores completely filled with massive
pyrite. i – A pore that was left free after Py-2 and Qtz-4 formation was later filled with kaolinite. j – Successive pyrite overgrowths
(Py-1 Py-3 Py-4; see Fig. 5c for details) on a quartz clast (Qd) with a Qtz-3 (Q3) rim. Black arrows indicate the dust line.
121
TRACE ELEMENT CHEMISTRY OF LOW-TEMPERATURE PYRITES (EGER GRABEN)
Turonian sandstone have been observed. In contrast, dis-
seminated sulphidic mineralization is visible in its
southeastern sector only, that is above the apical part of
the pre-Cretaceous rhyolite paleohigh. Sulphides in the
Lower Turonian sandstones occur mostly disseminated
(those that coprecipitated with the silica cement), less
frequently fracture-related (Fig. 2a) and even enclosed in
barite crystals lining the joint planes.
The study of successive generations of sulphides,
spanning the period between the older silicification
process and the young baritization, should therefore
provide clues to the Pliocene, Pleistocene and Holocene
fluid flow parameters in the late evolution of a rift basin.
Methods
Polished thin sections were prepared from about 20
samples of silicified sandstone from the Jeníkov Quarry.
Sulphide textures and paragenetic relationships were
studied by transmitted and reflected light optical
microscopy and by the back-scattered electron mode
(BSE) of electron scanning microscopy.
In order to study the intensity of pyritization in the
quarry, 13 whole-rock samples each about 0.5—1 kg in
weight were taken in a vertical section across the quartz-
ites up to the sealing mudstone horizon (Fig. 3). After ho-
mogenization, the samples were analysed by XRF
(Institute of Chemical Technology in Prague, J. Maixner
and S. Randáková).
The chemistry of all pyrite generations was studied by
the microprobe WD system (Cameca SX 100, Institute of
Geology, AS CR, Prague). Elements measured: S (Ka), Fe
(Ka), Co (Ka), Ni (Ka), Zn (Ka), As (La) and Tl (Mb).
Spectrometers used: LTAP (As), LPET (Tl), PET (S), and LIF
(Fe, Co, Ni). Standards used: marcasite (Fe, S), metallic Co
(Co), metallic Ni (Ni), GaAs (As), ZnS (Zn) and TlBr (Tl). A
total of 147 analyses were performed using the WD system.
The ED system (CamScan S4 microscope, Link ISIS
300 ED system, operator R. Procházka, Faculty of Sci-
ence, Charles University in Prague, Prague) was used for a
systematic study of Py-6 in barite from a wafer prepared
for the study of fluid inclusions. Later, a thin section was
prepared from the rest of the barite crystal, and a similar
analytical profile was studied using the WD system. In to-
tal, 132 analyses were performed using the ED system.
Results
Schematic paragenetic relationships between individual
primary minerals from the silicified Cretaceous sandstones
at the Jeníkov Quarry are summarized in Fig. 4. In general
two mineralization stages can be distinguished. The older,
stage 1 comprises pervasive silica precipitation in the pores
of sandstones, accompanied – and partly followed – by
the precipitation of sulphides and minor uraninite. The
minerals of stage 2, among which barite is the most
frequent, and sulphides, uraninite and fluorite are less
common, was typically precipitated on the walls of
subvertical fractures (mostly NE-SW). Stage 2 is separated
from stage 1 by a time gap during which most of the brittle
deformation took place. In places, fractures whose
formation coincided with late stage 1 are filled with
microcrystalline quartz matrix with sulphidic and uraninite
impregnations and with angular (breccia-like) fragments of
Fig. 3. Major and trace element contents in a section across Lower Turonian sandstones (quartzites). Analytical data correlate well with
the visually remarkable sulphidic enrichment in the topmost 0.5 m of the quartzite. Sulphide (Fe, S) enrichment coincides with As, Zn,
U and Ba enrichment. Sampling sites are shown on the inserted photo. The course of the sharp boundary between sulphide-enriched
and sulphide-poor quartzites is highlighted by a white-dashed line. dQcg – dark coarse-grained quartzite.
122
ZACHARIÁŠ, ADAMOVIČ and LANGROVÁ
silicified sandstone (Fig. 2h). Five generations of quartz
were identified as products of sandstone silicification. Early
silica formation has the character of euhedral quartz
overgrowths (Qtz-1, Qtz-3) on detrital grains (Fig. 2e—g),
rarely interspersed with scepter-like (Qtz-2) quartz crystal
growth.
Microcrystalline
quartz
cement
(Qtz-4)
and
megaquartz crystals (Qtz-5) fill the remaining pores and
voids in the rock (Fig. 2d,g—i). Six generations of pyrite
(Py) were recognized, referred to as Py-1 (oldest) to Py-6
(youngest), and compared with the silicification products.
Typically, the early ones (Py-1, -2) crystallized more or less
together with the silica cement (Qtz-4) in the pores of
Turonian sandstones (Fig. 2g), while the late ones (Py-3, -4,
-5) fill the remaining pores and voids in the silicified
sandstones, or joints and fractures in the rock (Py-5, -6;
Fig. 5g). The main stage of pyrite formation (Py-2 and Py-3)
generally
postdates
early
silica
formation
(quartz
overgrowths; Qtz-3; Fig. 2g). In some samples, however, the
onset of pyrite precipitation is contemporaneous with the
formation of late zones of quartz overgrowths (Qtz-3;
Fig. 2e—f), that is it slightly precedes microcrystalline silica
cement (Qtz-4). The timing of rare Py-1 (Fig. 5a—c) relative
to the neoformed silica phases is disputable.
We have observed the following textural relationships
between individual pyrite generations: i) Py-1 Py-3
Py-4 (Figs. 5a—c,f); ii) Py-2 Py-4 (Fig. 5d); iii) brittle-
fractures related pyrites (Py-5 Py-6; Fig. 5g) are young-
er than the early ones (Py-1 to Py-4). The temporal rela-
tionship between Py-1 and Py-2 is based on indirect
evidence only: on the presence of corrosion features in
Py-1 and their absence in Py-2 and younger generations.
Representative microprobe analyses of pyrites of the
different generations are given in Table 1.
Pyrite-1
Remnants of Py-1, exhibiting chemical corrosion fea-
tures, were found in the cores of some Py-3 and Py-4
grains. No individual Py-1 grains were found. Py-1 relics
are up to 80 mm in size, corrosion is weak (Fig. 5a,c) to
strong (Fig. 5b). Composite grains with pyrite relics
typically fill pores in silicified sandstones, or less
frequently imperfectly overgrow quartz clasts (Fig. 5c,f).
Py-1 is free of marked internal structures like As-rich or
As-low bands on BSE images. Some grains, however, exhibit
tiny micropores (Fig. 5a). Py-1 is almost stoichiometric
FeS
2
.
A
minor admixture of As (0.06 wt. %) was identified in
one analysis only. The arithmetical mean and standard
deviation of arsenic content is 0.01 ± 0.02 wt. % As (n = 6).
No other elements were detected.
Pyrite-2
Py-2 forms framboids ( ~ 3 to ~ 70 mm in size), mostly
with disordered (Fig. 6b), less frequently with ordered
(Fig. 6a;
cubic
close
packing)
internal
structures.
Individual pyrite crystallites form cubes ( ~ 0.5 to ~ 1 mm
in size), no octahedral ones were recorded. Framboids
occur either isolated or in clusters consisting of up to 5
framboids. In contrast to Py-1, Py-2 lacks any corrosion
features.
We further noticed a process of “welding” of individu-
al pyrite crystallites within the framboids, the result of
which are massive framboids with little pore space and
with relatively large grains (Fig. 6c). Framboids are also
frequently overgrown with pyrite (Py-4) cubes ( ~ 5 mm in
size; Fig. 6b and 6d), or laths (Fig. 6f).
The empirical formulae of Py-2 suggest a low anion
surplus (S + As + Tl: 2.044—2.014) and a cation deficit (Fe:
0.956—0.986). Of trace elements, only arsenic was detected
(0.11—0.60 wt. % As). The arithmetical mean and standard
deviation of arsenic contents are 0.15 ± 0.04 wt. % As (n = 6).
Framboid size was measured systematically in two
samples (Fig. 7). Sample JL-9B represents quartzite with
disseminated sulphides from the most sulphide-rich zone
of Lower Turonian sandstones just below (0—0.5 m) the
Fig. 4. Paragenetic relationships
between primary minerals in
silicified Cretaceous sandstones
of the Jeníkov Quarry near
Teplice, northern Bohemia.
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TRACE ELEMENT CHEMISTRY OF LOW-TEMPERATURE PYRITES (EGER GRABEN)
Fig. 5. Textures of the studied pyrites; crosses indicate sites of microprobe analyses; numbers refer to As wt. %; Sph = sphalerite:
a – corroded Py-1 relics enclosed by a Py-3 grain (BSE image); b – intensively corroded Py-1 grain enclosed in Py-3 and overgrown by
Py-4; c – Py-1 relics growing on a quartz grain with Qtz-2 overgrowths (BSE); d – Py-4 aggregate enclosing framboidal pyrite (Py-2;
BSE); e – euhedral Py-4 grain with several As-rich oscillation growth zones (BSE); f – complex grain with Py-4 overgrowth and fracture
fillings (BSE); g – fracture in silicified sandstone with coatings of Py-5 and a younger barite filling (transmitted light); h—i – As-rich Py-5
grains (BSE); j – a barite crystal (section parallel to 010 ??) with a large unzoned core and compositionally zoned rim. Line A—B—C
marks the position of the microanalytical profile where Py-6 chemistry was studied (transmitted light); k – a part of barite rim showing
composite Py-6 needles (transmitted light); l – a detailed view of a Py-6 needle composed of tens of individual Py-6 crystals (transmitted
light); m – Py-6 crystals from the core of barite crystal (BSE); n – a detail of a Py-6 lath/needle from the outermost growth zone
exhibiting complex fan-like textures (marcasite?; BSE).
124
ZACHARIÁŠ, ADAMOVIČ and LANGROVÁ
sealing horizon of Middle Turonian claystones. Sample
JL-15 comes from a fracture with sulphides in otherwise
sulphide-free quartzite. This sample is located about
150 m to the NW away from the most sulphide-rich zone
and stratigraphically about 4—5 m below the Lower/
Middle Turonian boundary. Both data show lognormal
distribution of similar standard deviations, but of
significantly different median values ( ~ 12
µm and
~30.5
µm for JL-9B and JL-15, respectively). Data from
other, less systematically studied samples, are similar to
those from JL-9B.
Pyrite-3
Py-3 forms crystal-faced overgrowth zones on Py-1
(Fig. 5a—c,f). It is homogeneous on BSE (0.17—39 wt. %
Table 1: Representative microprobe (WDX) analyses of the studied pyrites and numbers of atoms per formula unit calculated to 3
positions (n.d. – not detected).
As), except for 1 or 2 thin ( ~ 1—2
µm) As-rich growth
bands (4.11 wt. % As). The arithmetical mean and standard
deviation of arsenic contents are 0.31 ± 0.20 wt. % As
(n = 12). No other trace elements were detected.
Pyrite-4
Py-4 forms euhedral overgrowths on the Py-3 to Py-1
surfaces (Fig. 5b—d) and/or individual grains (cubes max.
50
µm in size; Fig. 6b—d,e), disseminated in (intergrown
with?) fine-grained silica cement (Qtz-4).
The overgrowths typically consist of several thick As-
rich zones separated by thin ( ~ 1—2
µm) As-poor bands
(Fig. 5c—d). Numerous As-poor patches located close to
the base of Py-4 overgrowths most probably represent
crystallization seeds/centres. Individual pyrite grains/
125
TRACE ELEMENT CHEMISTRY OF LOW-TEMPERATURE PYRITES (EGER GRABEN)
zones exhibit similar textures. Py-4 rarely encloses/
overgrows small (up to 20
µm) grains of sphalerite
or galena.
Arsenic admixture is high (2.16—6.62 wt. % As),
but nickel and cobalt were below detection limits.
The arithmetical mean and standard deviation of
arsenic contents are 3.91 ± 1.64 wt. % As (n = 24).
Thallium was found in some grains only, ranging
from 0.01 to 0.12 wt. %.
In some samples, we have found BSE-homogeneous
pyrite cubes (isolated or overgrowing Py-2 framboids;
Fig. 6b—e) with high As contents (2.94 ± 1.46 wt. % As;
n = 8) similar to those in typical Py-4.
Pyrite-5
Py-5 forms massive coatings on post-silicification
fractures (where it is overgrown by barite with Py-6;
Fig. 5g) or individual grains up to 80
µm in size in
the pores of silicified sandstone in the vicinity of
these fractures.
Py-5 grains frequently exhibit a crystallization
core, consisting of one or more As-rich grains
(Fig. 5h—i) and a massive thick overgrowth zone.
Small variations in the As/S ratio can be found in
the overgrowths (Fig. 5h). In contrast to Py-3 and
Py-4, all growth zones (including grain cores) are
extremely As-rich (4.05—7.96 wt. %
As). The arith-
Fig. 6. Various framboid textures. Crosses indicate sites of microprobe analyses. Numbers refer to As wt. %. a – a framboid cluster with
cubic closely packed internal structure (BSE image); b – a framboid with disordered texture and Py-4f cube overgrowths; c – a “welded”
framboid approaching massive texture (BSE); d – a framboid overgrown with Py-4f and isolated Py-4f crystals (BSE); e – framboid
relics in the core of a Py-4 grain (BSE); f – a framboid globule overgrown with Py-4 laths (transmitted light).
Fig. 7. Frequency distribution of framboid size in two samples, both ex-
hibiting lognormal data distribution.
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ZACHARIÁŠ, ADAMOVIČ and LANGROVÁ
metical mean and standard deviation of arsenic contents
are 6.14 ± 1.10 wt. % As (n = 29). Thallium contents range
between 0.0 and 0.082 wt. %; arithmetical mean and
standard deviation correspond to 0.05 ± 0.02 (n = 17). No
nickel or cobalt admixtures were detected.
Pyrite-6
Py-6 forms numerous tiny (max. 10 mm; Fig. 5m—n)
crystal inclusions in barite crystals that precipitated in
late fractures in the quartzites. Rows (lines) of single Py-6
inclusions typically rim individual barite growth zones
(Fig. 5j). Only those from the latest, or several latest,
growth zones are commonly arranged in needles,
consisting of welded single crystals (Fig. 5k—l). In rare
cases, an amorphous carbon phase was identified associ-
ated with these needle-shaped Py-6 crystals.
Significant variations in the pyrite trace element
chemistry were recorded along the profile across several
barite growth zones (Fig. 8a): the oldest Py-6 inclusions
are variably enriched in Ni (3.47 ± 2.09 wt. %), Co
(8.86 ± 3.68 wt. %) with traces of As and Zn, while the
younger ones are only slightly enriched in arsenic
(0.76 wt. % As). The transition zone at the boundary
between Ni-Co-rich core and Ni-Co-free rim is marked by
a sudden increase in arsenic content (up to 4 wt. % As;
restricted to one growth zone only). The arithmetical
mean and standard deviation of arsenic contents are
Fig. 8. Chemistry of Py-6 inclusions from a barite crystal: a – systematic variations in Py-6 crystals along the studied profile A—B—C
(Fig. 5j); b – a correlation of Co-Ni and As-Ni contents from Py-6.
127
TRACE ELEMENT CHEMISTRY OF LOW-TEMPERATURE PYRITES (EGER GRABEN)
0.76 ± 0.71 wt. % As (n = 33). Py-6 is
the
most
thallium-rich
generation
0.19 ± 0.13 wt. %
Tl
(n = 15;
mean
± SD), although Tl was not detected in
all studied Py-6 grains.
Trace element chemistry of pyrites –
a summary
Individual pyrite generations/types
differ significantly in arsenic content
(average As contents: 0.01, 0.15, 2.94,
3.91, 6.14 and 0.76 wt. % As for Py-1
through Py-6, respectively; Fig. 9) that
gradually increases towards younger
pyrite types (except for Py-6). Arsenic
contents in most of the analysed Py-1
and Py-2 grains were below the
detection limits.
Arsenic contents correlate well with
sulphur
deficit,
suggesting
As
+ 1
S
—1
Fig. 9. A summary of arsenic admixture statistics (arithmetical mean and standard
deviation) for the studied pyrites.
Fig. 10. Correlation of the arsenic admixture in pyrites with: a –
sulphur deficiency – indicating As
+1
S
—1
substitution mechanism;
b – cation deficiency (mostly iron deficiency).
substitution mechanism (Fig. 10a). No correlation exists
between Ni or Co and As contents in Py-6 grains, but it
does exist between the Ni and Co contents themselves
(Fig. 8b) for pyrite inclusions in the studied barite crystal
(Fig. 5j). We have further noticed a tendency for slight
iron deficiency (Ni and Co were below detection limits in
most analysed grains) with gradually increasing arsenic
content (Fig. 10). This trend, however, is not statistically
well documented, because of the given standard deviation
for iron contents. True occupancy of pyrite structural sites
therefore remains to be proved by the single crystal X-ray
method.
Discussion
Relationships of pyrite types to silicification processes
Mutual textural relationships clearly indicate that Py-1
to Py-4 ( ± Py-5 ?) pyrite types formed as a consequence
of one complex mineralization process, accompanying
widespread silicification in the Jeníkov-Lahoš (Teplice)
area. Py-1 relics likely do not represent pre-silicification
pyrite, but postdate the onset of silicification. This can
be deduced from Py-1 overgrowths on quartz clasts with
neoformed silica (Qtz-3) overgrowth rims (Figs. 2j and
5c). Py-2 (framboids) precipitated largely in the pores of
sandstone, together with microcrystalline quartz cement
(Qtz-3). The same is true for Py-3 and Py-4. Py-5
postdates the main silicification stage (Qtz-2, -3) only.
The continuity in arsenic content from Py-4 to Py-5 and
the contrast in arsenic content between Py-5 and Py-6
allow us to suggest that Py-5, although texturally
postdating silicification (filling fractures in already
silicified sandstone; Fig. 5g), is still genetically related
to the silicification process. Py-1 to Py-5 therefore
represent a single more or less continuous evolution of
pyrite
crystallization.
Py-6,
having
the
form
of
128
ZACHARIÁŠ, ADAMOVIČ and LANGROVÁ
inclusions in late tabular barite crystals, is probably sep-
arated by a time gap from the previous types. The dura-
tion of this pyrite evolution gap is difficult to assess, but
the youngest growth zones of barite crystals are Ho-
locene in age (Ulrych et al. 2007).
The actual temperatures of the above mentioned
silicification are not known but the presence of only one-
phase liquid fluid inclusions (sparse at Jeníkov, but more
frequent at Salesiova výšina Hill, approximately 5 km from
Jeníkov) in quartz overgrowths (Qtz-3) and on dustlines
separating quartz clasts and overgrowths allow us to
suggest temperatures not exceeding approx. 50—60 °C.
Similar temperatures ( ~ 53 °C) were estimated by
Hladíková et al. (1979) for the formation of calcite vein-
lets (fracture fillings) from the Teplice area (suggesting
δ
18
O
water
: — 10‰ SMOW, similar to the present-day
groundwater).
The present-day temperature of mineralized thermal
waters at Teplice, a likely analogue for the fossil fluids,
is 32—49 °C (Čadek et al. 1968). When the present
chemistry of thermal waters (Čadek et al. 1968; analyses
1—3, 28 from table 24) is used, then the “fluid reservoir”
temperatures estimated from various empirical chemical
thermometers correspond to: ~ 88 °C (SiO
2
, quartz;
Fournier & Potter II 1982), ~ 57 °C (SiO
2
, chalcedony;
Table 6.1 in Kharaka & Mariner 1989), ~ 96 °C (Na—K—Ca;
Fournier & Truesdel 1973), and ~ 65 °C (Mg—Li; Kharaka
& Mariner 1989). Na-K as well as Na-Li thermometers
yielded unrealistically high temperatures (160—210 °C).
All these data unequivocally point to a low-
temperature ( ~ 50 °C) origin of the studied pyrites and
associated mineralizing fluids.
Solubility of arsenic, nickel and cobalt in pyrite
The solubility of arsenic solid solution in pyrite/
marcasite at low temperatures is difficult to study
experimentally. Contents of up to ~ 19 wt. % As were
recorded in some natural hydrothermal pyrites formed at
temperatures lower than 300 °C (Reich & Becker 2006
and
references
therein).
Recent
thermodynamic
calculations of Reich & Becker (2006) restricted the
solubility of arsenic in pyrite to about ~ 2.1, ~ 2.7 and
~6 wt. % As at ~25, ~ 100 and ~500 °C, respectively.
Concentrations exceeding such ranges correspond either
to metastable arsenic solid solution, or suggest a
presence of arsenopyrite nanoscale domains in the pyrite
structure.
Of the studied pyrites, average As contents (Fig. 9) in
Py-2, -3 and -6 are compatible with a stable arsenic solid
solution according to Reich & Becker (2006), while
those of Py-5 exceed this limit. Arsenic contents in Py-4
oscillate around this limit.
As the arsenic content in Py-6 does not exceed the the-
oretical limit for arsenic solubility in pyrite at 25 °C
(Reich & Becker 2006), we may suggest the same for
nickel and cobalt. Experimental or theoretical data for
solubility of cobalt and nickel in pyrite formed at
subambient temperatures are missing. Klemm (1965) sug-
gested solubilities of about 10 at. % of CoS
2
or NiS
2
in
FeS
2
at 400 °C.
Constraints on fluid saturation with respect to iron and
sulphur
Numerous experimental studies (Ohfuji & Rickard
2005 and references therein) demonstrated that major
prerequisites for framboid formation are 1. extremely
high supersaturation with respect to iron sulphides and
2. conditions when crystal nucleation rate significantly
exceeds the crystal growth rate. These favourable
conditions may result from increasing/higher fluid
temperature ( > 150 °C), from the presence of small
amounts of O
2
in the system, from the presence of S(0)
and polysulphides, or from increasing Eh.
Of these factors, Eh variations are the most likely
factor at Jeníkov. An increase in Eh can be indirectly
deduced from the presence of isolated cogenetic
inclusions of FeOOH—FeS
2
and/or siderite enclosed in
quartz overgrowths (Qtz-3) at Salesiova výšina Hill. Due
to much steeper decrease of pyrite solubilities along the
FeS
2
—FeOOH or FeS
2
—Fe
2+
phase boundary than along
other pyrite boundaries (Fig. 11a), a slight Eh increase
coupled with a subtle increase in pH is the most effective
way to produce fluids highly supersaturated with respect
to iron sulphides, thus initiating framboid formation (cf.
fig. 3 of Butler & Rickard 2000).
The presence of welded framboids (Fig. 6c), where
crystallites have grown at the expense of micropores,
reflects conditions when pyrite growth rate exceeded that
of nucleation. The same can be inferred from the absence
of framboid structures in later pyrite types (Py-3, -4, -5).
Relatively fast nucleation also accompanied later Py-4,
as shown by the chaotic BSE textures with numerous
nucleation centres (Fig. 5c—e), but not enough for
framboid formation.
Processes that led to a partial dissolution of early py-
rite (Py-1), notably the time gap between Py-1 dissolu-
tion and Py-2 formation, are more obscured. If we accept
that Py-1 postdates the early silicification phase, then
the formation and dissolution of Py-1 came only a short
time before the Py-2 (framboid) formation. With regard to
the kinetics of pyrite oxidation (Williamson & Rimstidt
1994), a short-lived and fast pyrite dissolution requires a
sudden high increase in the amount of dissolved oxygen
(e.g. by the factor of 10
5
—10
6
to increase the half-times of
pyrite dissolution by the factor of 300
× to 1000×; based
on the rates of Williamson & Rimstidt 1994). To allow
Py-2 formation in the next step, a return to reducing con-
ditions is necessary, followed by a less dramatic increase
in oxygen activity to promote pyrite supersaturation and
framboid formation (see above). Later, when Py-3 to Py-5
are formed, conditions in the stability field of pyrite can
be expected again. The observed mineral association
thus reflects oscillatory changes in the oxygen activity
of the mineralizing fluids, gradually diminishing with
time (Fig. 11b). With regard to the more or less neutral
pH of fossil and present thermal waters, dissolved oxy-
129
TRACE ELEMENT CHEMISTRY OF LOW-TEMPERATURE PYRITES (EGER GRABEN)
Fig. 11. Relationships between fluid chemistry and mineralogy: a – Eh-pH diagram for the system Fe-K-S-CO
2
-H
2
O-O
2
at 25 °C, 1 bar
pressure,
ΣFe(aq)=10
—4
mol/kg,
ΣK(aq)=10
—4
mol/kg,
ΣS(aq)=10
—2
mol/kg and P
CO
2
= 10
—2
bar (after Nordstrom & Munoz 1985 in Lang-
muir 1996) with dots indicating the chemistry of the present thermal waters from the Teplice area. Note that the thermal waters are far
from equilibrium with iron sulphides. The observed mineralogy (pyrite, siderite, uraninite, iron oxyhydroxides) is compatible with in-
creasing Eh (thick arrow) under pH similar to present-day thermal waters. b – schematic changes in Eh with time, necessary to produce
the observed variations in pyrite textures.
gen is the most likely agent for pyrite oxidation. Hydrox-
yl radicals (HO
•
), or H
2
O
2
produced by radiolysis of wa-
ter (Lefticariu et al. 2006) represent a less probable alter-
native to dissolved oxygen.
Framboids formed under euxinic conditions in modern
basins tend to be smaller and less variable in size than
those formed under dysoxic conditions (Wilkin et al.
1996). The dependence of mean framboid size on the
oxygen fugacity probably also exists at Jeníkov.
Framboids from the zone of most intense pyritization
immediately underlying the mudstone sealing horizon
are smaller than those from more distal places, where the
fluids could mix with other more oxidized groundwaters,
or where the role of exhaustion of reducing components
in the parent fluid was becoming important.
Chemistry of present-day thermal waters in the Teplice area
The thermal waters at Teplice have been monitored for
more than 100 years. Despite changes in instrumentation,
the
results
of
major
cation
chemistry
and
pH
measurements are comparable over this whole period:
pH: 6.6—6.9, Eh: + 0.137 to + 0.202 V and + 0.410V,
SiO
2
: 27—49 mg/l, Na: 160—240 mg/l, K: 10—18 mg/l,
Ca: 35—110 mg/l, total sulphur as SO
4
2—
: 83—136 mg/l,
HAsO
4
2—
: 0.01 mg/l (Čadek et al. 1968 and unpublished
recent analyses). The sampled waters, however, do not
represent a pure thermal water end-member, as mixing
with shallow groundwater occurs just beneath the dis-
charge zone.
Mineral saturation indices (SI) were calculated using
PhreeqC (Parkhurst & Appelo 1999) and thermodynamic
database minteq.v4 in order to evaluate the saturation of
present-day thermal waters with respect to the mineral of
interest: pyrite is strongly undersaturated (SI: over —90),
siderite is slightly undersaturated (SI: —4 to —10), barite
and quartz is more or less in equilibrium with the waters
(SI: —0.6 and 0.7, respectively), kaolinite and goethite
are slightly supersaturated (SI: 2 to 5). Fossil waters
responsible for sulphidic mineralization could have pH
values similar to those of present-day thermal waters, but
must have had a distinctly lower Eh.
The evolution of fluid trace element chemistry and
changes in fluid migration paths
The trace element chemistry of minerals can be used as
a paleoindicator of fluid chemistry, species activities,
and the degree of saturation. As stated earlier, all pyrite
types
precipitated
at
similar
temperatures,
with
differences not exceeding 20—30 °C. The influence of
temperature as a factor governing As, Ni, Co and Tl
contents in pyrites can therefore be ruled out. The As (Ni,
Co, Tl) admixture in pyrites likely reflects the contents/
saturation of these elements in fossil fluids. The effect of
variations in the trace element contents in the
130
ZACHARIÁŠ, ADAMOVIČ and LANGROVÁ
mineralizing fluids on the pyrite chemistry was con-
firmed and discussed in the study on uraninite, which co-
precipitated with Py-4 (Zachariáš et al., in prep.). Besides
element contents in the fluid phase, kinetics of pyrite
precipitation could also contribute to the formation of
As-rich and As-poor pyrite growth zones.
Arsenic contents in fluids at the site of fluid discharge
can result from two different interactions (besides the role
of temperature): i) from As-saturation/depletion due to
precipitation of various As-bearing compounds at, or close
to the discharge site (e.g. Cleverley et al. 2003); or ii) from
complex fluid-rock interactions governed by actual As-
minerals (usually arsenopyrite or As-pyrite) in a deep fluid
reservoir at temperatures appropriate for this reservoir (e.g.
Aiuppa et al. 2006). Fluid boiling in a deep reservoir, or
fluid evaporation at the site of surface discharge, would
also result in a gradual increase in arsenic content in the
remaining liquid phase; but, these processes are difficult
to demonstrate in our case. Because the As-bearing pyrite
is the only As-bearing mineral at Jeníkov, the process of
fluid saturation/depletion by As due to precipitation of
various As-bearing compounds can be neglected. The
gradual increase in As contents from Py-1 to Py-5
therefore most probably reflects a gradual increase in As
content in the parent fluid.
Such a trend indicates fluid-rock interactions in a deep
crystalline reservoir at progressively increasing tempera-
tures and depths and/or progressively opening fluid mi-
gration paths. Temporal coincidence of pyrite formation
with large-scale movements along the Krušné hory Fault
suggests a fault-driven upward transport of fluids either
by the mechanism of solitary waves of Revil & Cathles
(2002) or some type of seismic pumping (e.g. Sibson et
al. 1975).
As has been stated earlier, Py-1 to Py-5 precipitation
was associated in space and in time with pervasive
regional silicification. Čadek & Malkovský (1968)
already demonstrated that the silica responsible for the
silicification comes from low-temperature hydrolysis of
alkali feldspars (and volcanic glass) of the Teplice
rhyolite
body
during
descending
groundwater
circulation. The volcanic rocks themselves, however, can
hardly be the source of all the observed variations in As,
Ni, Co contents in the studied pyrites because: i) trace
element contents in magmatic bodies on regional scale
are relatively homogeneous (average contents for acidic
volcanic rocks of the Altenberg—Teplice Caldera are:
10.2 ppm As (7.4—19.4 ppm), 9.3 ppm Ni (5—16 ppm) and
5 ppm Co (1—12 ppm); Ulrych et al. 2006); ii) groundwa-
ter flow tends further to homogenize the contents of trace
elements leached from host-rocks into the fluid phase.
A drop in the As content in Py-6, contrasting with the
increasing trend from Py-1 to Py-5, coupled with the
sudden appearance of Co and Ni in Py-6 from cores of
barite crystals, suggest a marked change in the fluid
circulation pattern.
Cobalt and nickel can be mobilized from mafic and ul-
tramafic rocks, from magmatic or hydrothermal Ni-Co-
sulphidic mineralizations, or from organic matter. Al-
though the first two sources can be found in the Krušné
hory Mts, they are missing in the Jeníkov area. On the
other hand, fluid circulation through the overlying Ter-
tiary lignite beds is a likely explanation for high Ni, Co
and also for moderate As contents in the fluids. Besides
organic sulphur compounds and disseminated sulphides
with variable As admixtures (0—8 at. % As; Weiss et al.
2001), the Tertiary lignite strata are rich in organic mat-
ter-bonded/adsorbed Ni and Co. Currently exploited lig-
nites from the near by Bílina Mine average 7.1 ppm As,
29.2 ppm Ni and 9.3 ppm Co; A Catalogue (2007).
Moreover, stable isotope and fluid inclusion data (Ul-
rych et al. 2007) suggest the Tertiary lignite strata as the
most likely source of sulphur for the barites (hosting our
Py-6) and disqualify crystalline basement sulphides as a
potential source.
All the above facts point to a shallow circulation of
Py-6 fluids through the Tertiary sediments or along their
contacts with the Teplice rhyolite, rather than a deep
fluid circulation in the Teplice rhyolite itself. Such
change in fluid circulation should be substantiated by
regional
stress
rearrangement
and
a
consequent
extinction of tectonic activity on the Krušné hory Fault.
Although no detailed tectonic measurements have been
performed on the Krušné hory Fault itself, kinematic
analyses of parallel faults (Coubal & Adamovič 2000)
point
to
a
post-Pliocene
ENE—WSW
compression
followed by an ENE—WSW extension effective until the
mid-Pleistocene (Mindel, ~ 0.4 Ma). In contrast, no fault-
ing younger than this age has been reported, and fluvial
terraces of Riss or Würm ages in the Eger Graben show
no abnormities in their vertical profiles. The data from
the Eger Graben are in agreement with the data on the
present tectonic stresses obtained from hydraulic fractur-
ing and borehole breakouts (World Stress Map Project,
Zoback 1992), indicating a uniform NW—SE compres-
sional stress field for most of the Alpine-Carpathian fore-
land. However, no tectonic deformations related to this
stress field have been observed in the Bohemian Massif.
This suggests a mid-Pleistocene stress reversal, which in-
hibited the Krušné hory Fault activity and marked the
end of the fault-scarp topographic up-building. Very
probably, this event can be correlated with the herein de-
scribed change in fluid flow pattern and the resulting
change in mineral chemistry.
Conclusions
Mineral textures and trace element chemistry of pyrites
can be used as a tool to decipher the history of fluid
chemistry and circulation patterns. The studied locality
in the Eger Graben in northwestern Bohemia reflects
near-surface mineralization processes (silicification and
associated sulphide formation) in a transition from deep,
fault-driven
fluid
flow
to
subsequent
shallow,
topography-driven groundwater flow. Six types of pyrite
(Py-1 to Py-6) were distinguished, Py-1 to Py-4 being
closely associated with silicification, Py-5 shortly post-
131
TRACE ELEMENT CHEMISTRY OF LOW-TEMPERATURE PYRITES (EGER GRABEN)
dating it and Py-6 separated by a significant time gap
from the silicification. The evolution of pyrite textures
from framboids (neglecting rare Py-1 relics) to individual
crystals (mostly cubes) suggests an oversaturation of ear-
ly fluids with respect to pyrite and its rapid crystalliza-
tion. The As contents in Py-3 and Py-4f correlate well
with the theoretically predicted As solubility in FeS
2
at
low temperatures (Reich & Becker 2006), while those in
Py-5 (up to 7.9 wt. % As) probably correspond to meta-
stable arsenic solid solution in pyrite.
Two different fluid circulation patterns, separated in
time, can be interpreted from the trace element chemistry
of pyrites: i) a progressively deepening circulation
pattern of low-temperature geothermal fluids derived
from the acidic volcanics in the Eger Graben basement
and driven by the Krušné hory Fault propagation, and ii)
shallow geothermal/groundwater circulation in contact
with the Tertiary lignite beds, driven by fault-scarp
topography. The former contributed to the chemistry of
Py-1 to Py-5, the latter to Py-6 and barite precipitation.
Acknowledgments: This research was supported by the
Grant Agency of the AS CR, Project A3013302 to JA and
JZ
and
by
the
Ministry
of
Education
Grant
MSM0021620855 (JZ) and the Research Plan AV0
Z30130516 of the Academy of Sciences CR (JA). The
authors wish to thank the Keramost a.s. Company for
providing access to the Jeníkov Quarry. We also thank B.
Kříbek and Z. Sawlowicz for their critical reviews and
helpful comments.
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