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Organic Geochemistry 31 (2000) 1143±1161
www.elsevier.nl/locate/orggeochem

The role of organic matter during copper enrichment in
Kupferschiefer from the Sangerhausen basin, Germany
Yu-Zhuang Sun a,*,1, Wilhem PuÈttmann b
a
Hebei Institute of Architectural Science and Technology, 056038 Handan, Hebei, PR China
Johann Wolfgang Goethe-Universitaet Frankfurt, FB 17 Geowissenschaften-Umweltanalytik, Georg-Voigt-Str. 14,
D-60054 Frankfurt a. M., Germany

b

Received 7 April 1998; accepted 7 August 2000
(returned to author for revision 10 November 1998)

Abstract
In a previous study (Sun, Y.-Z., PuÈttmann, W., 1997, Metal accumulation during and after deposition of the Kupferschiefer from NiederroÈblingen, Sangerhausen basin, Germany. Applied Geochemistry 12, 577±592) Cu enrichments
of up to 20% in the Kupferschiefer have been reported from the Sangerhausen basin, Germany. Petrological and
geochemical analyses have shown that Cu was precipitated by two di€erent processes: pyrite replacement (PR) and
thermochemical sulfate reduction (TSR). In the present study, the composition of the organic matter has been studied

in a pro®le from the Sangerhausen basin in order to estimate the amount of Cu precipitated by PR and by TSR.
Analyses of the soluble organic matter, by GC and GC/MS, and of kerogen by Rock-Eval pyrolysis, indicated that PR
is responsible for Cu precipitation with enrichments of less than 8%. Organic matter is not involved in PR but is
required as hydrogen donor in the thermochemical formation of H2S through sulfate reduction. In a sample from the
bottom section of the Kupferschiefer, with 19.88% Cu, the degree of hydrogen depletion in the organic matter allows
one to assess the amount of Cu precipitated by TSR to be approximately 12% (60% of the total Cu). Saturated and
aromatic hydrocarbons, kerogen, and possibly methane served as hydrogen donors for TSR. The results argue for
stepwise precipitation of Cu sul®des during diagenesis of the shale. First, Cu is precipitated during pyrite replacement,
and when the reduced S stored in the sediment is used up, additional H2S is generated by TSR. # 2000 Elsevier Science
Ltd. All rights reserved.
Keywords: Kupferschiefer; Thermochemical sulfate reduction; Copper enrichment

1. Introduction
The role of organic matter in concentrating metals in
sediments has been studied in a variety of sediment-hosted
ore deposits. The results have led to various hypotheses
concerning associations between ores and organic matter
(Saxby, 1976; MacQueen and Powell, 1983; Giordano,
1985; Gize and Barnes, 1987; PuÈttmann et al., 1988;
* Corresponding author.

E-mail address: sun_yz@hotmail.com (Y.-Z. Sun).
1
Present address: Basin Reservoir Research Center, University of Petroleum, Beijing, Shuiku Road, Changping, Beijing, 102200, PR China.

PuÈttmann and Gossel, 1990; Jowett, 1992; Sun et al.,
1995; Sun and PuÈttmann, 1996; Lin et al., 1997). Saxby
(1976) and Barton (1982) have outlined some possible
mechanisms by which organic matter in sediments and
living organisms may in¯uence the genesis of low temperature (less than about 200oC) hydrothermal and
chemical sedimentary ore deposits. These mechanisms
are listed as follows:
1.
2.
3.
4.
5.

modifying chemical environments,
serving as a reducing or oxidizing agent,
catalyzing reactions,

immobilization of elements,
mobilization of elements.

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00117-0

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Y.-Z. Sun, W. PuÈttman / Organic Geochemistry 31 (2000) 1143±1161

In general, only the in¯uence of bulk organic matter
on trace metal enrichment processes has been studied.
The particular role of individual hydrocarbon groups
and mass balance is poorly understood. In order to
clarify the role of organic matter in copper enrichment
in Kupferschiefer and Zechstein carbonates from the
Sangerhausen basin, the variation of hydrocarbon species in solvent extracts and kerogen composition was
determined in a narrow-sampled pro®le consisting of
13 samples of Kupferschiefer and the overlying limestones.

2. Sample material
A 58 cm pro®le of Kupferschiefer (T1) and Zechstein
carbonates (Ca1) were sampled in 1991 at the NiederroÈblingen mine in the Sangerhausen basin. The pro®le consists of 9 samples of Kupferschiefer (32 cm in
total) and 4 samples of Zechstein carbonates (26 cm in
total). The lithological composition of the pro®le is
shown in Fig. 1. According to traditions of the former
mine workers, the Kupferschiefer pro®le is devided into
6 sub-sections taking into account the colour, hardness
and fabric of particular layers. These sub-sections are
termed ``Feine Lette'' (FL), ``Grobe Lette'' (GL),
``Kammschale'' (KS), ``Schieferkopf'' (SK), ``Schwarze
Berge'' (SB) and Zechstein carbonate (Ca1). The extension of each sub-section is included in Fig. 1.

Fig. 1. Lithological pro®le of the studied Kupferschiefer section from the Sangerhausen basin.

In ``Feine Lette'', the rock consists of black shale with a
thickness of approximately 2 cm. ``Grobe Lette'' represents black bituminous marly shale, and ``Kammschale'' is
a dark grey marly shale. ``Schieferkopf'' consists of
laminated marl and ``Schwarze Berge'' of grey marl.
Dolostone-limestones occur in the Zechstein carbonates

overlying the Kupferschiefer. From ``Grobe Lette'' to
``Schwarze Berge'' changes in colour are due to
decreasing contents of organic matter.
The studied pro®le was not a€ected with visible Rote
FaÈule. Rote FaÈule is a red-coloured zone, which is in
contact (and sometimes even crosscuts) the Kupferschiefer horizon. The red colour is caused by the presence of
hematite. The occurrence of Rote FaÈule indicates the
strongly oxidizing character of ¯uids which circulated
through the Kupferschiefer after its deposition (Rydzewski, 1978; Rentzsch, 1991).

3. Analytical methods
Finely ground (0.3%) and of
pyrobitumen (Rr>0.7%), the sul®des could not be
formed by BSR under a temperature of about 130 C
(Rr=0.9%) in the Sangerhausen basin.

Y.-Z. Sun, W. PuÈttman / Organic Geochemistry 31 (2000) 1143±1161

1153

Fig. 6. Variation of the contents of PAH groups along the Kupferschiefer pro®le from the Sangerhausen basin.

Other evidence is the occurrence of saddle dolomite and
calcite spars. According to Machel (1987, 1989), the
occurrence of TSR in rocks will increase the carbonate
alkalinity and give rise to the formation of saddle (sparry)
dolomite and calcite spars. Abundant saddle (sparry)
dolomite and calcite spars can be seen in Kupferschiefer
from the Sangerhausen basin (Sun, 1996). It further con®rms that sul®des are formed by TSR in this area.
Further evidence is the shape of the sul®des. Framboidal pyrite, generated during sedimentation by BSR,
is missing in the highly mineralized samples. It can be
seen microscopically that most other sul®des are present

as ®lling of fractures and pores (Sun and PuÈttmann,
1997). This sul®de ®lling was formed after the formation
of pyrobitumen (or migrabitumen), because pyrobitumen is replaced by these sul®des (Sun and PuÈttmann,
1997). To provide additional evidence for TSR the
soluble hydrocarbons have been investigated in detail in
the present study.
5.2. Saturated hydrocarbons

The relative amounts of saturated hydrocarbons have
shown a decrease in the bottom of the pro®le. GC analyses

Fig. 7. Plots of Cu contents and the ratios of alkylated PAH/PAH subgroups in Kupferschiefer pro®le from the Sangerhausen basin.

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Y.-Z. Sun, W. PuÈttman / Organic Geochemistry 31 (2000) 1143±1161

have shown that long chain n-alkanes are preferentially
depleted in the mineralized section. From samples of an
uniform sediment in a narrow maturity range, one
might expect a close correlation between the amount of
hydrocarbons generated during diagenesis and the
organic carbon content.
The plot of id-Alk vs. Corg (Fig. 8a) shows a good
correlation as far as samples 5±13 from the center and
top part of the pro®le are concerned. In samples 1±4 the
amount of id-Alk is much lower than one would expect
from the Corg content.

A possible explanation for this depletion can be derived
from the plot of id-Alk vs. Cu. Id-Alk contents decrease
with an increase of the Cu contents (Fig. 8b). This indicates that the decrease of alkanes is related to Cu-sul®de
formation. Most likely, the alkanes served as a hydrogen
donor for thermochemical sulfate reduction (TSR) to
H2S as required for Cu sul®de precipitation.
5.3. Aromatic hydrocarbons
Previous investigations of Radke et al. (1982) on
Carboniferous coals from the Ruhr have shown that the
yields of soluble organic matter and aromatic hydrocarbons is maturity dependent. The production of soluble organic compounds, and also of aromatic
hydrocarbons, maximizes at a rank characterized by a
vitrinite re¯ectance of approximately 0.9% Rr. Investigations of aromatic subfractions have shown that together with increasing molecular weight of the aromatic
hydrocarbons, the maximum production is shifted
towards slightly higher vitrinite re¯ectance. 1- and 2ring aromatics are preferentially generated at a vitrinite
re¯ectance of 0.88% Rr whereas 3- and 4-ring aromatics
show their maximum generation at 0.93% Rr. The
vitrinite re¯ectance determined in samples from the
Sangerhausen pro®le is approximately 0.83% Rr (Sun
and PuÈttmann, 1997), indicating that the maximum of
hydrocarbon generation was not reached. The thickness

of the pro®le from which the samples originate is 58 cm.
This very low variance of depth allows one to conclude
that the maximum temperature e€ect was similar for all
samples under investigation. This is re¯ected by the
relatively low variation of vitrinite re¯ectance along the
pro®le (Sun and PuÈttmann, 1997). Therefore, variations
of the absolute amounts of aromatic hydrocarbons
cannot be accounted for by variations in the temperature e€ect but only by the content of organic matter.
In Fig. 9 the amounts of all identi®ed PAH and various PAH subgroups have been plotted vs. the Corg
content in order to verify whether a linear correlation
exists. Results show that for most samples a linear correlation exists between the concentration of id-PAH and
Corg (Fig. 9a). Only samples 1 and 2 from the bottom of
the pro®le deviate from the correlation. Both samples
are enriched in id-PAH. Similar results are obtained if
only the concentration of naphthalenes (naphthalene
and alkylated naphthalenes) is plotted vs. the Corg content (Fig. 9b). Here, the correlation is even better than
for all aromatic hydrocarbons and a minor deviation is
only observed for sample 2 which has accumulated the
highest amount of copper (19.88%). This sample is also

enriched in biphenyls (Bi-PAH) and phenanthrenes (PhPAH) (Fig. 9c,d). The most signi®cant enrichment of
aromatic hydrocarbons in the mineralized bottom section of the pro®le is detected for O-PAH (Fig. 9f) and SPAH (Fig. 9e). The amount of both groups of aromatic
compounds is more than double than the amount
expected from the Corg content.
The enrichment of aromatic compounds in the bottom part of the section can be attributed to two di€erent
e€ects. Either the compounds have been generated from
inherent organic matter under the in¯uence of the
mineralization process or the compounds originate from
the underlying strata and have been adsorbed by Kupferschiefer from ascending solutions.
In order to clarify this question the absolute amounts
of PAH, of PAH subgroups and PAH ratios are plotted

Fig. 8. Correlation of (a) id-Alk vs. Corg and (b) id-Alk vs. Cu contents in Kupferschiefer samples from the Sangerhausen basin.

Y.-Z. Sun, W. PuÈttman / Organic Geochemistry 31 (2000) 1143±1161

Fig. 9. Correlation of id-PAH and PAH subgroups with Corg in Kupferschiefer samples from the Sangerhausen basin.

1155

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Y.-Z. Sun, W. PuÈttman / Organic Geochemistry 31 (2000) 1143±1161

Fig. 10. Correlation of id-PAH and Cu concentrations in Kupferschiefer samples from the Sangerhausen basin.

Y.-Z. Sun, W. PuÈttman / Organic Geochemistry 31 (2000) 1143±1161

vs. the Cu content. Fig. 10a shows that in those samples
originating from the center and the top of the pro®le, the
amount of PAH is largely independent of the Cu content,
but at elevated Cu contents, the amount of PAH increases
at the same rate. Fig. 10c,e,g, shows that the PAHsupgroups (naphthalenes, biphenyls and phenanthrenes)
follow similar trends. The compounds increase together
with increasing Cu concentrations. In Fig. 10b the absolute amount of S-PAH is plotted vs. the Cu content. An
enrichment of S-PAH is observed in samples with very
high copper contents. Only sample 1 deviates from the
linear relationship between both parameters. The
enrichment of S-PAH is higher than one would expect
from Cu contents. The results indicate that the process
of copper mineralization is linked with the formation of
S-PAH. The formation of Cu sul®des requires the
availability of H2S during both biological and thermochemical sulfate reduction. Therefore, the occurrence of
S-PAH does not allow one to distinguish between biological and thermochemical generation of H2S. A possible pathway for the formation of dibenzothiophenes
from aliphatic precursor reacting with H2S during sedimentation or early diagenesis has been proposed by
Rospondek et al. (1994) and is summarized in Fig. 11.
According to this hypothesis, H2S is trapped with organic
matter during sedimentation. This reaction sequence is
favored when reduced Fe for precipitation of H2S is not
available in the sediment. As long as reduced Fe is available, the formation of iron monosul®des will prevail as
the trapping mechanism for H2S (Berner, 1985).
Reduced sulfur can also react with aromatic hydrocarbons in immature sediment to form benzothiophenes
(White and Lee, 1980). A third possible pathway for the
formation of S-PAH might be the reaction of aromatic
units in kerogen with pyrite or other metal sul®des. This
pathway is also shown in Fig. 11.
The observed link between S-PAH concentrations and
Cu mineralization argues for a correlation of the processes leading to Cu mineralization with those leading to

Fig. 11. Proposed model for the formation of S-PAH in Kupferschiefer (modi®ed after Rospondek et al., 1994). Abbreviations: OSC=organic sulfur compounds, BSR=bacterial sulfate
reduction, PR=pyrite replacement and TSR=thermochemical
sulfate reduction.

1157

S-PAH formation. One possible explanation of this
interaction could be the transfer of reduced sulfur from
pyrite to other metal ions (pyrite replacement) at elevated temperatures. Alternatively, uptake of S-PAH in
Kupferschiefer from basinal formation waters has to be
considered assuming that the brines carried Cu and SPAH concomitantly (PuÈttmann and Goûel, 1990). The
elevated concentration of S-PAH in sample 1 from the
bottom of the pro®le argues for this interpretation,
because in this sample Cu is not enriched adequately.
Fig. 10d,f,h shows that copper mineralization has also
an in¯uence on the PAH distribution pattern. The amount
of alkylated naphthalenes in relation to all naphthalenes
decreases together with increasing amounts of Cu. A
similar e€ect is observed in the case of alkylated biphenyls
and phenanthrenes. The non-substituted aromatic
hydrocarbons tend to increase relative to the alkylated
counterparts. However, the e€ect is much weaker than
in Kupferschiefer from the Konrad mine in southwest
Poland. Here, alkylated PAH disappeared signi®cantly
in relation to the non-substituted aromatic hydrocarbons (PuÈttmann et al., 1989). The distribution shift
from alkylated to non-substituted PAH, again can be
explained by two di€erent mechanisms. Either the nonsubstituted aromatics have been added preferentially to
the bottom part of the shale from external sources or the
non-subsituted PAH were generated in-situ by oxidative
dealkylation. The intensity shift from alkylated to dealkylated aromatic hydrocarbons in the mineralized section of the pro®le argues for in situ transformation
reactions rather than the accumulation of aromatic
hydrocarbons from an external source. The pathway of
oxidative alkylation has been discussed previously in
detail (PuÈttmann et al., 1989).
5.4. Kerogen
The plot of HI vs. the absolute amount of identi®ed
alkanes in the solvent extracts is shown in Fig. 12a. Only
the bottommost sample of the pro®le deviates from the
overall positive correlation. This sample originates from
the redox boundary Weissliegendes-Kupferschiefer and
is characterized either by an additional amount of
alkanes or by a selective oxidation of the kerogen. Oxidation of the kerogen prior to oxidation of the soluble
fraction is unlikely because the soluble fraction is more
susceptible to oxidation reactions than the solid material. Most likely, the saturated hydrocarbons and the
solid material are both oxidized and donate hydrogen
for TSR, but the excess of saturated hydrocarbons at
the redox boundary is due to the presence of migrated
hydrocarbons. The positive correlation for all samples
of Kupferschiefer and Zechstein carbonates indicates
that the hydrogen depletion observed in the mineralized
section of the pro®le a€ected both the soluble hydrocarbons (id-Alk) and the kerogen to a similar degree.

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Y.-Z. Sun, W. PuÈttman / Organic Geochemistry 31 (2000) 1143±1161

Fig. 12. Correlation diagram (a) id-Alk vs. HI and (b) id-PAH vs. HI contents in Kupferschiefer samples from the Sangerhausen basin.

Fig. 12b shows the correlation of the absolute amount
of identi®ed PAH and the HI values. The parameter idPAH shows a positive correlation to HI values similar
to the correlation diagram of id-Alk vs. HI. Exceptions
from the correlation are represented by samples 1 and 2.
Here, the absolute amount of id-PAH is higher than one
would expect from the HI values. This deviation can be
explained by a decrease of the HI values under the
in¯uence of Cu mineralization in the bottom section or
by the addition of aromatic hydrocarbons from an
external source exclusively at the bottom of the pro®le.
5.5. Mass balance calculation
According to the chemical reaction proposed for
TSR (Orr, 1977; Powell and MacQueen, 1984; Machel
et al., 1995), H2S is generated at the expense of organic
matter:
2CH2 O ‡ SO2ÿ
! 2HCOÿ
4
3 ‡ H2 S
H2S can be used for the precipitation of dissolve Cu
according to the following reaction schemes:
H2 S ‡ 2Cu‡ ! Cu2 S ‡ 2H‡
H2 S ‡ Cu‡‡ ! CuS ‡ 2H‡
About 4.72 g of hydrogen originating from hydrocarbons is necessary for the precipitation of approximately 200 g of copper as 50% of each Cu2S and CuS
(1000 g ore rock from sample 2). Assuming that the
hydrogen is supplied from saturated hydrocarbons
(CH), the precipitation of 100 g of copper (as Cu2S)
requires 20.46 g of hydrocarbons (CH). For the precipitation of 100 g Cu as CuS 40.92 g of hydrocarbons
(CH) are required. That is, 61.38 g CH is required for
the precipitation of approximately 200 g of copper as

50% of each Cu2S and CuS (1000 g ore rock from
sample 2).
The Corg-related extract yields provide an appropriate
parameter for the calculation of the amount of hydrocarbons supplied for the precipitation of metal sul®des
in sample 2 although the extracts are not only composed
of saturated hydrocarbons.
The highest extract yield with 157 mg Extr/g Corg has
been determined in the topmost sample. This sample
represents Zechstein carbonates and the extract yield
might not be appropriate for comparison with extract
yields of Kupferschiefer without mineralization. An
appropriate reference value can be drawn from sample 5
with a low degree of base metal mineralization. The
extract yield of this sample is 56 mg Extr/g Corg. In
comparison, sample 2 with 19.88% of Cu mineralization
provides an extract yield of 19 mg Extr/g Corg. Comparing to sample 5, the depletion of extractable organic
matter in sample 2 amounts to 37 mg Extr/g Corg during
ore formation.
When the Corg content is considered, 1000 g ore rock
of sample 2 have consumed 5794 mg Extr (assumed as
CH) for Cu mineralization. According to the calculation
mentioned above, 61,380 mg CH is required for Cu
precipitation in 1000 g rock of sample 2. But the extract
yield is only depleted by 5794 mg. Assuming that all the
Cu was precipitated by TSR, either kerogen or migrated
bitumen from basement rocks had to contribute an
additional 55,586 mg CH for H2S formation.
The average content of alkanes from sample 5 with low
mineralization is 5.3 mg id-Alk/g Corg. The saturated
hydrocarbons in sample 2 amount to 1.3 mg/g Corg. If the
value obtained from sample 5 is regarded as standard, the
donation of alkanes in sample 2 was about 618 mg for the
formation of sul®des in 1000 g ore rock. This is only
about 11% of the depletion of total extract in sample 2.
The observed di€erence is partly due to the fact that
quantitation of individual saturated hydrocarbons was

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Y.-Z. Sun, W. PuÈttman / Organic Geochemistry 31 (2000) 1143±1161

restricted to n-alkanes, pristane and phytane. All branched and cyclic hydrocarbons were not included in the
calculation of id-Alk but contribute also to the total
extracts as well as aromatic hydrocarbons and heterocomponents.
In order to determine a possible contribution of PAH
to the hydrogen donation the correlation diagramms of
PAH and Corg (Fig. 9) can be used. For samples 3 to 13
a linear correlation of the amount of id-PAH and Corg is
observed (Fig. 9). An additional PAH content is
observed only in samples 1 and 2. The vertical distance
(x-intercept) of sample 2 from the regression line allows
one to calculate an additional amount of 100 ppm PAH
in this sample and of about 40 ppm additional PAH in
sample 1.
Figs. 9b and 6 show that the highest additional NaPAH content occurs in sample 2 with a value of about
35 ppm. Bi-PAH are enriched by about 35 ppm and PhPAH by about 20 ppm in sample 1 (Figs. 9c,d and 6).
Additional S-PAH of less than 10 ppm are available in
samples 1 and 2, but in sample 1 the amount is higher
than in sample 2 (Figs. 9e and 6). The additional content of O-PAH amounts to 2 ppm in both samples from
the bottom samples of the pro®le (Fig. 9f). According to
these correlations (Fig. 9), the estimation of additional
PAH contents is summarized in Table 5.
Exclusively, S-PAH is more enriched in sample 1 than
in sample 2. This phenomenon can be explained by the
addition of S-PAH concomitant with the metal bearing
solution from underlying strata (PuÈttmann and Goûel,
1990). High additional contents of Na-PAH and BiPAH are observed only in sample 2. This fact cannot be
explained by PAH migration from the underlying strata,
because in the bottommost sample additional Na-PAH
and Bi-PAH are not observed. Considering the high
trace metal and lower Ph-PAH contents in sample 2, the
additional Na-PAH and Bi-PAH must be caused by
trace metal enrichment. In this process some Ph-PAH
might have been transformed into Bi-PAH by hydrogen
donation for TSR.
It is dicult to calculate the donation of hydrogen from
aromatic hydrocarbons, because the hydrogen donation
of Alk, as mentioned above, led to an increase in the relative aromatic proportion in Extr yields. In this study,
some indirect methods have been used to calculate the
donation of PAH during trace metal accumulation.
Table 4 and Fig. 7 show that about 12% alkylated
naphthalenes and 11% alkylated phenanthrenes were

depleted in sample 2. That means, about 14 ppm alkylated naphthalenes and 5 ppm alkylated phenanthrenes
were consumed for sul®de formation.
According to PuÈttmann et al. (1989), alkyl