The role of organic matter during copper enrichment in
Kupferschiefer from the Sangerhausen basin, Germany
Yu-Zhuang Sun
a,*
,1, Wilhem PuÈttmann
baHebei Institute of Architectural Science and Technology, 056038 Handan, Hebei, PR China
bJohann Wolfgang Goethe-Universitaet Frankfurt, FB 17 Geowissenschaften-Umweltanalytik, Georg-Voigt-Str. 14, D-60054 Frankfurt a. M., Germany
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 Kup-ferschiefer 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 dierent 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;
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 tem-perature (less than about 200oC) hydrothermal and
chemical sedimentary ore deposits. These mechanisms are listed as follows:
1. modifying chemical environments, 2. serving as a reducing or oxidizing agent, 3. catalyzing reactions,
4. immobilization of elements, 5. mobilization of elements.
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 1 1 7 - 0
www.elsevier.nl/locate/orggeochem
* Corresponding author.
E-mail address:[email protected] (Y.-Z. Sun).
1 Present address: Basin Reservoir Research Center,
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 spe-cies in solvent extracts and kerogen composition was determined in a narrow-sampled pro®le consisting of 13 samples of Kupferschiefer and the overlying lime-stones.
2. Sample material
A 58 cm pro®le of Kupferschiefer (T1) and Zechstein carbonates (Ca1) were sampled in 1991 at the Nie-derroÈblingen 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 exten-sion of each sub-section is included in Fig. 1.
In ``Feine Lette'', the rock consists of black shale with a thickness of approximately 2 cm. ``Grobe Lette'' repre-sents 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 aected with visibleRote
FaÈule. Rote FaÈule is a red-coloured zone, which is in
contact (and sometimes even crosscuts) the Kupferschie-fer 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 (Ryd-zewski, 1978; Rentzsch, 1991).
3. Analytical methods
Finely ground (<0.2 mm) rock samples were Soxhlet-extracted for 24 h with dichloromethane. Removal of elemental sulfur was achieved by the addition of copper foil to the ¯ask during extraction. The extracts were separated into three fractions by column chromato-graphy over pre-washed silica gel (70±230 mesh, 501 cm). The alkanes were eluted withn-hexane, the aromatic hydrocarbons with dichloromethane and the polar com-pounds (heterocomcom-pounds) with methanol (40 ml of each solvent ).
GC analyses of saturated and aromatic hydrocarbon fractions were carried out on a CARLO ERBA 5160 gas chromatograph equipped with a 25 m fused silica col-umn (0.25 mm I.D.) coated with SE-54. The oven tem-perature was programmed from 80 to 300
C at 4
C minÿ1
followed by an isothermal period of 15 min. Hydrogen was used as carrier gas. The compounds were quanti®ed by adding squalane as standard prior to analysis. In the second step, the areas of standard and compound peaks in GC traces were quanti®ed using the Mchrom pro-gram. The compound contents were calculated by com-paring with the standard concentration.
Identi®cation of individual compounds was per-formed by GC±MS analysis using a VARIAN 3700 gas chromatograph coupled directly to a FINNIGAN MAT 8200 mass spectrometer using a 25 m0.25 mm (i.d.) fused silica column coated with SE 54. Helium was used as the carrier gas. Mass spectra were recorded in the cyclic scan mode (1.1 s total cycle time). Conditions for mass spectrometry were: electron energy 70 eV, emission current 1 mA and scan range 50±700 Da. Data were processed using an INCOS data system. Peak identi®ca-tion was based on comparison with standard spectra in the NBS library and, when necessary, con®rmed by gas chromatographic coelution with authentic compounds.
Fig. 1. Lithological pro®le of the studied Kupferschiefer sec-tion from the Sangerhausen basin.
The organic carbon content (Corg) was measured
using a Leco CR-12 carbon determinator. Carbonates were removed from the samples by prior treatment with concentrated hydrochloric acid. The Hydrogen Index (HI) was determined by Rock-Eval analysis using a DELSI Rock-Eval Instrument Version RE II. Copper contents were determined by instrumental neutron acti-vation analysis (INAA) at ACTLABS, Toronto, Canada.
4. Results
4.1. Variation of copper contents and organic bulk parameters
The variation of Cu contents along the pro®le of Kupferschiefer and Zechstein carbonates of the Sanger-hausen basin is shown in Fig. 2 and Table 1. The Cu content does not exceed values of 0.8% in the top part of the pro®le (samples 5±13). A signi®cant enrichment of Cu is observed in the bottom part, maximizing in sample 2 with 19.88%. The distribution pro®le of Cu largely parallels the organic carbon content (Corg) also
shown in Fig. 2 and Table 1. However, the increase in the concentration of Cu in sample 2, relative to the adjacent samples, is not re¯ected in a similar increase of the Corgcontent, which only increases slightly relative to
the amounts observed in the vicinity of sample 2. The distribution pro®le of the extract yield (in ppm) shown in Fig. 2 is similar to the Corg pro®le, but sample 2
represents a signi®cant exception. Here, the extract yield is low despite the highest Corg content within the total
pro®le. A similar phenomenon is observed in the Corg
-normalized extract yields (mg Extr/g Corg). The lowest
value of the pro®le (19 mg Extr/g Corg) is measured in
sample 2 with the highest Corgcontent. In other parts of
the Kupferschiefer pro®le, the extract yields are almost constant (around a value of 50 mg Extr/g Corg) and
increase in samples from the Zechstein carbonates to values of up to 157 mg Extr/g Corg. The increasing
extract yield in Zechstein carbonates might be due to the increasing amount of alga-derived organic material. However the extract yield in sample 2 cannot be explained by facies variations.
4.2. Variation of hydrocarbon species
Hydrocarbon group separation was carried out in order to detect similarities and dierences in the varia-tion of individual hydrocarbon groups along the pro®le. The percentage of the saturated hydrocarbon fractions in the total extracts is shown in Fig. 2 (Col. 5). Low percentages of saturated hydrocarbons are observed in the bottom part of the pro®le with values below 16% (Table 1). The lowest amount (10%) is measured in sample 2. In the top part of the pro®le (samples 5±13), the values vary around 30% and show no dependency on the lithology. The highest value of 38.1% is observed at the topmost sample of the pro®le.
The percentages of the aromatic hydrocarbon frac-tions (Col. 6, Fig. 2) largely show an opposite trend to the saturated hydrocarbon factions. The highest amount of aromatic hydrocarbons occurs in sample 2 with a value of 40%. Towards the top of the pro®le the relative amount of aromatic hydrocarbons tends to decrease par-ticularly at the top of the Zechstein carbonates. An inverse relationship of saturated and aromatic hydrocarbon per-centages is only disturbed in the case of samples 2 and 3.
Here, the percentage of aromatic hydrocarbons is lower than that one would expect from the amount observed in samples below and above in the pro®le.
The proportion of heterocomponents in solvent extracts shows a largely inverse trend to the aromatic hydrocarbons in the Kupferschiefer section (Col. 7, Fig. 2). The lower amounts of aromatic hydrocarbons in samples 3 and 4 correlate with the maximum amounts of hetero-components. The section covered by these samples is located above those with the highest Cu content in the pro®le. In addition, in sample 3 the highest extract yield is observed.
4.3. Composition of saturated hydrocarbon fractions
The composition of the saturated hydrocarbon frac-tions was further analyzed by GC and GC±MS. Fig. 3 shows two GC traces for comparison. The gas chroma-togram of saturated hydrocarbons from the top sample of the pro®le (no. 13) shows a distribution ofn-alkanes in the range from n-C12 to n-C38. The highest
con-centration is observed at n-C17andn-C18. In a sample
from the center of the pro®le (no. 8) the relative con-centrations of the long-chain alkanes are lower in com-parison to those at the top of the pro®le. The maximum concentrations are shifted ton-C15andn-C16(Table 2).
In the sample near the bottom of the pro®le (no. 2), a further decrease of long-chain alkanes is observed, resulting in maximum concentrations atn-C14andn-C15
(Fig. 3). From the top to the bottom of the pro®le, the changes in the distribution of saturated hydrocarbons
are characterized by decreasing relative concentrations of long-chainn-alkanes, and by a shift of the dominant homologues towards shorter chain length. In addition, Fig. 3 and Table 1 show that from sample 13 through sample 8 to sample 2 the copper content increases from 91 ppm over 1306 ppm to 198,777 ppm. The qualitative shift in the distribution ofn-alkanes, thus, shows a clear relationship with the variation of the Cu content.
4.4. Composition of aromatic hydrocarbon fractions
Fig. 4 shows the GC traces of the aromatic hydro-carbon fractions of samples 2 and 10. The aromatic hydrocarbons identi®ed by GC/MS are listed in Tables 3 and 4.
The aromatic hydrocarbon distribution diers sig-ni®cantly within the pro®le. In the upper part of the pro®le (sample 10) phenanthrene (Ph=30), methylphe-nanthrenes (Mph=36-39) and dimethylphemethylphe-nanthrenes (DMPh=46-55) predominate. Additionally, trimethyl-phenanthrenes (60±63) are present in low amounts.
In sample 2, naphthalene and alkylated naphthalenes predominate within the aromatic compounds. These include: naphthalene (Na=1), methylnaphthalenes (MNa= 2, 3), dimethylnaphthalenes (DMNa=5-12), trimethylnaphthalenes (TrMNa=16-23) and a tetra-methyl naphthalene (TeMNa=25). The relative inten-sities of phenanthrene and its methyl derivatives are lower than those of naphthalenes. The predominant compound in the aromatic hydrocarbon fraction of sample 2 is biphenyl (4=Bp), which was a compound of
Table 1
Copper contents and organic geochemical parameters of a Kupferschiefer pro®le from the Sangerhausen basin
Sample no. Corga
c Aro=aromatic hydrocarbon fraction. d Alk=saturated hydrocarbon fraction.
e Het=heterocompounds (polar compounds).
f Asph=asphaltenes.
g HI=hydrogen index.
minor abundance in sample 10. Biphenyl is accom-panied by relatively low concentrations of methylbiphe-nyls (13, 14).
In sample 1 the predominant compounds belong to the series of naphthalenes, biphenyls and phenanthrenes dis-cussed above. Some compounds of minor concentration in samples 10 and 2 increase in relative concentration. These are dibenzothiophene (29) and methyldibenzo-thiophenes (34, 35).
4.5. Quantitation of all identi®ed hydrocarbons and HI values
In Fig. 5 the Cu contents and summarized con-centrations of identi®ed alkanes (id-Alk) and polycyclic aromatic hydrocarbons (id-PAH) are plotted along the lithological pro®le in order to investigate the possible role of saturated and aromatic hydrocarbons in metal accumulation. The parameter id-Alk represents the
absolute concentration of all identi®ed n-alkanes, pris-tane and phypris-tane. The parameter id-PAH includes all aromatic hydrocarbons marked in Fig. 4. The plots show an inverse relation between Cu contents and con-centrations of identi®ed alkanes. The inverse correlation indicates that alkane depletion in the bottom section of the pro®le is associated with Cu-sul®de formation: alkanes possibly served as hydrogen donors in the pro-cess of H2S formation. The amount of id-PAH varies
almost inversely to the amount of id-Alk. The lowest amount of aromatic hydrocarbons is observed in the top part of the pro®le and the highest amount is measured in sample 2 with the highest Cu content. Thus, saturated hydrocarbons are depleted, whereas aromatic hydro-carbons may also be depleted, but not so much as satu-rated hydrocarbons. The distribution of HI values also tends to decrease from sample 3 to the top of the pro®le similar to the amount of id-PAH and shows no rela-tionship with the amount of saturated hydrocarbons.
Table 2
Quantities of saturated hydrocarbons of Kupferschiefer samples from the Sangerhausen basin
Only in the mineralized bottom part of the pro®le (samples 1 and 2) are decreasing HI values associated with increasing amounts of aromatic hydrocarbons. A possible explanation for this phenomenon is given later in this chapter.
4.6. Quantitation of individual aromatic hydrocarbons
In order to describe dierences in the PAH composi-tion of the samples, individual constituents were quan-ti®ed (Fig. 6) and several PAH subgroups were developed according to structural characteristics. These subgroups are:
Na-PAH: sum of naphthalene and alkylated naphthalenes (16 compounds) Ph-PAH: sum of phenanthrenes and alkylated
phenanthrenes (18 compounds)
Bi-PAH: sum of biphenyl and alkylated biphenyls (3 compounds)
S-PAH: sum of all S-containing aromatic hydrocarbons (10 compounds) O-PAH: sum of all O-containing aromatic
hydrocarbons (1 compound) id-PAH: sum of all individual aromatic
hydrocarbons (48 compounds)
Fig. 6 and Table 4 show the variation of the quan-tities of these subgroups of PAH along the pro®le of Kupferschiefer and Zechstein carbonates. The results indicate that the absolute amount of subgroups varies largely parallel with the Cu content (except S-PAH). Sample 2 with the highest Cu content (Fig. 5) also has the highest amounts of PAH subgroups except for the S-PAH (Fig. 6). Na-S-PAH and Bi-S-PAH show the highest similarity with the distribution pro®le of Cu. Ph-PAH, S-PAH and O-PAH do not follow the concentration pro®le of Cu preferentially in the bottommost sample. Despite the low Cu concentration in sample 1, these aromatic compounds are present in high concentrations
comparable to those in sample 2 of each aromatic hydrocarbon subgroup.
The S-PAH contribute between 1 and 7.5% to the total identi®ed PAH (Table 4). The highest absolute S-PAH content is measured in the bottommost sample of the pro®le with a value of 11.5 ppm (Fig. 6 and Table 4). A regular decrease is observed towards the top of the pro-®le to the topmost sample with a value of 0.04 ppm. High absolute O-PAH contents are observed in samples 1 and 2 (Fig. 6).
4.7. Relative concentration shifts of individual PAH and PAH subgroups
Calculations of the relative intensities of individual PAH and PAH subgroups are summarized in Table 4. The results show that the percentages of the Na-PAH/id-PAH vary between 41.2 and 71.8%. The lowest percentage is found in the bottommost sample, and the highest percen-tage is observed in sample 13 (Zechstein carbonates). A regular variation within the pro®le is not detectable. The observed enrichment of Cu in sample 2 shows no particular in¯uence on this parameter. The proportions of alNa/Na-PAH vary between 83.5 and 98.8% along the pro®le. Fig. 7 shows a negative correlation with Cu content.
Within the pro®le, phenanthrenes contribute between 14.3 and 35.1% to the total identi®ed PAH (Table 4). The variation of alPh/Ph-PAH ratios along the pro®le is plotted in Fig. 7. The lowest values are determined in the Cu-bearing zone of the pro®le and the highest values are reached above this zone, at the top of the pro®le in the Zechstein carbonates. The pro®le of alPh/Ph-PAH ratios partly parallels the pro®le obtained from the alNa/Na-PAH ratios (Fig. 7).
The third important group of aromatic compounds is Bi-PAH (biphenyl (Bi) and methylbiphenyls (alBi)), even though only three compounds are present in this group. The highest value of Bi-PAH/id-PAH (21.7%) is in sample 2. Moving upwards, a regular decrease is observed (Table 4). The lowest ratio of Bi-PAH/id-PAH is measured in the topmost sample with a value of 4.3%. The value of alBi/Bi-PAH reaches 24.4% in sample 2, and an increase is observed towards the top. The dis-tribution pro®le is similar to those obtained from alNa/ Na-PAH and alPh/Ph-PAH ratios (Fig. 7).
The percentages of most O-PAH/id-PAH values are lower than 1%, and the highest value of 1.8% is observed in the bottommost sample (Table 4).
5. Discussion
5.1. Evidence for thermochemical sulfate reduction The ®rst evidence for TSR in the Kupferschiefer pro®le from Sangerhausen, was provided, based on microscopical
Table 3
List of aromatic compounds identi®ed in samples 2 and 10 (peak numbers refer to Fig. 4)
Peak No. Compound Name Ab.
1 Naphthalene N
Table 4
Quantities of individual aromatic hydrocarbons in Kupferschiefer samples from the Sangerhausen basin. Abbreviations of most compounds are listed in Table 3
Sample
13 8.64 41.82 37.27 16.36 48.64 68.18 77.27 20.91 20.00 10.00 0.45 35.00 42.27 43.18
12 7.94 22.00 32.20 42.40 21.32 34.92 39.91 15.65 13.61 10.20 7.03 28.57 31.97 29.93
11 10.17 20.90 36.14 49.70 31.63 56.19 61.84 21.46 25.41 13.55 11.58 40.94 52.92 51.23
10 6.90 6.90 17.24 29.31 15.52 36.21 31.03 12.07 10.34 5.17 8.62 22.41 29.31 27.59
9 16.46 67.29 29.71 81.96 46.89 75.16 100.93 30.78 35.06 19.69 11.10 43.66 62.63 61.56
8 8.70 42.75 56.52 76.81 44.20 81.16 84.06 28.26 31.16 18.12 11.59 47.83 55.80 54.35
7 38.52 50.00 59.92 69.26 39.69 59.92 72.96 28.40 23.54 11.09 3.50 33.85 36.58 37.55
6 37.89 63.53 65.81 90.31 47.29 67.24 90.31 24.79 33.90 15.95 14.53 41.88 55.27 56.70
5 32.83 50.00 68.33 71.17 39.33 55.00 78.17 22.00 15.17 4.50 6.00 38.83 43.67 43.83
4 42.91 41.90 93.16 99.24 36.84 58.99 71.01 22.03 27.72 15.57 7.09 38.10 43.29 45.06
3 84.62 73.95 81.06 143.68 72.35 90.88 43.92 15.40 3.64 3.22 2.20 15.40 35.71 40.28
2 120.11 82.69 96.93 238.25 12.84 80.50 109.77 25.03 24.14 17.11 17.69 19.22 40.55 33.27
1 47.92 31.50 51.53 99.73 27.53 49.46 63.63 25.81 10.29 14.26 24.37 25.27 56.05 41.79
Sample
13 30.00 12.73 15.00 10.91 6.82 10.45 10.00 9.55 17.73 11.82 1.82 1.82 1.82 24.09
12 25.40 28.34 20.18 25.40 14.51 41.04 21.54 18.14 39.68 19.27 3.40 9.75 1.81 34.24
11 46.86 24.85 34.73 23.88 18.35 44.05 22.31 18.92 34.73 23.44 5.08 5.37 3.39 33.04
10 24.14 15.52 13.79 13.79 17.24 48.28 27.59 20.69 41.38 20.69 3.45 5.17 1.72 36.21
9 52.11 22.19 36.51 18.11 20.76 42.95 24.34 14.67 45.10 15.03 3.94 3.94 3.22 39.37
8 48.55 31.88 32.61 29.71 21.01 38.41 25.36 14.49 36.23 16.67 3.62 3.62 2.90 41.30
7 34.05 14.20 24.12 12.26 16.34 50.39 31.52 20.04 47.67 21.79 3.31 3.50 2.92 31.71
6 51.85 16.24 49.00 13.39 19.66 45.30 23.65 13.96 37.61 16.24 6.27 8.83 6.27 29.06
5 34.00 20.83 22.50 17.50 21.83 56.50 32.50 17.33 42.17 29.17 5.17 4.33 2.83 38.17
4 37.47 18.48 28.86 17.22 28.61 52.53 30.38 18.35 32.03 21.90 2.15 1.90 1.39 23.54
3 26.73 11.62 19.55 8.77 25.64 41.55 16.84 12.78 21.83 10.66 3.47 3.55 3.05 18.36
2 24.20 11.75 14.37 6.77 38.57 90.68 31.99 21.20 51.66 24.33 4.53 3.96 2.81 28.16
1 31.50 6.86 25.99 16.34 40.52 91.43 37.18 24.82 58.75 29.06 11.82 9.84 8.30 36.82
Sample
13 10.45 8.64 7.73 6.82 5.00 1.82 7.27 6.36 689 4.04 2.90 0.79 0.04 0.00 0.17
12 9.98 7.94 7.03 7.26 4.99 5.44 7.48 5.67 696 9.97 4.71 3.08 0.25 0.09 1.01
11 12.71 10.45 12.99 6.49 3.39 3.39 6.78 4.24 883 6.86 3.72 1.66 0.19 0.08 0.72
10 10.34 6.90 6.90 5.17 3.45 5.17 6.90 5.17 589 4.22 1.81 1.48 0.14 0.05 0.36
9 6.44 5.37 4.65 6.44 4.29 5.73 10.02 6.08 1074 31.19 18.26 6.14 0.75 0.42 3.88
8 9.42 8.70 8.70 5.07 2.17 4.35 7.97 4.35 1038 16.63 9.67 3.22 0.40 0.16 2.03
7 16.15 13.23 6.23 4.67 2.33 3.11 5.25 3.50 933 52.76 29.81 13.74 1.09 0.18 5.88
6 10.26 6.84 5.98 3.70 3.13 3.42 4.84 3.70 1085 43.67 26.31 8.19 0.98 0.51 5.44
5 9.50 8.83 8.67 2.00 1.33 0.67 1.50 0.83 947 63.19 35.14 15.69 1.78 0.36 7.53
4 5.70 5.19 6.58 2.66 1.52 1.01 1.90 1.14 963 83.74 49.92 16.58 2.64 0.56 12.17
3 6.85 4.74 3.98 1.95 1.78 1.02 951 138.31 83.56 19.81 3.89 0.29 27.21
2 8.43 7.73 6.90 7.34 4.85 1.47 0.70 1311 227.33 113.97 46.17 7.97 2.77 49.33
1 15.07 13.81 13.36 10.56 9.12 2.26 5.51 2.44 1070 147.63 60.84 41.92 11.46 2.7 16.70
Sample
investigations by Sun and PuÈttmann (1997). Evidence for TSR includes values for vitrinite re¯ectance, the occurrence of pyrobitumen, saddle dolomite and calcite spars (Machel et al., 1995; Sun et al., 1995). The average vitrinite re¯ectance (Rr) of the samples from the
San-gerhausen pro®le is 0.83%. The highest value is observed in the bottom samples with a value of 0.95% (Sun and PuÈttmann, 1997). Based on these data one can assume that the maximum temperature reached in the Kupferschiefer was at least 130
C based on the tem-perature/vitrinite re¯ectance relationship calculated by Quigley et al. (1987) for other basins. TSR can occur at minimum temperatures of about 100±140
C (Krouse et al., 1988; Machel et al., 1995).
In the bottom samples of the Sangerhausen pro®le, abundant amorphous organic matter with relatively high re¯ectivity (0.8±1.2% Rr) is present, ®lling pores
and fractures (Sun, 1996). According to the classi®ca-tion of Landis and CastanÄo (1995), this material should be termed pyrobitumen. By de®nition, migrabitumen is distinguished from pyrobitumen only by its re¯ectivity. Below a re¯ectivity of 0.7%Rrsolid bitumen is termed
migrabitumen and above 0.7%Rrpyrobitumen (Landis
and CastanÄo, 1995). TheRrvalues in the middle and top
part of the pro®le are lower than 0.7%. This bitumen belongs to migrabitumen. According to Machel (1989), BSR occurs during sedimentation and early diagenesis. These reactions take place bacterially at temperatures up to about 85
C (equivalent to vitrinite re¯ectance values of approximately 0.2±0.3%Rr). Considering the
formation time of migrabitumen (Rr>0.3%) and of
pyrobitumen (Rr>0.7%), the sul®des could not be
formed by BSR under a temperature of about 130C
(Rr=0.9%) in the Sangerhausen basin.
Table 4 (continued)
Sample no.
alNa/Na-PAH (%)
alPh/Ph-PAH (%)
alBi/Bi-PAH (%)
Na-PAH /id-PAH (%)
Ph-PAH /id-PAH (%)
S-PAH /id-PAH (%)
O-PAH /id-PAH (%)
Bi-PAH /id-PAH (%)
10 97.8 81.1 52.8 42.9 35.1 3.3 1.2 8.5
9 97.7 82.2 46.4 58.5 19.7 2.4 1.2 12.4
8 98.8 83.5 47.8 58.1 19.4 2.4 1.0 12.2
7 93.4 81.1 39.5 56.5 26 2.1 0.3 11.1
6 94.9 90.6 41.7 60.2 20.9 2.2 1.2 12.5
5 94.4 88.4 43.3 55.6 24.8 2.8 0.6 11.9
4 93.2 75.0 35.6 58.9 19.6 3.1 0.7 14.4
3 86.8 68.1 31.4 60.4 14.7 2.8 0.2 19.7
2 83.5 69.9 24.4 50.1 20.3 3.5 1.2 21.7
1 91.3 76.3 33.8 41.2 28.4 7.5 1.8 11.3
Fig. 5. Concentration pro®les of Cu, (id-Alk), PAH (id-PAH) and of HI values in Kupferschiefer samples 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. Fram-boidal 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 pyrobitu-men 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
have shown that long chainn-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 Corgcontent.
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 indi-cates 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 hydro-carbons is maturity dependent. The production of solu-ble organic compounds, and also of aromatic hydrocarbons, maximizes at a rank characterized by a vitrinite re¯ectance of approximately 0.9% Rr.
Investi-gations of aromatic subfractions have shown that toge-ther with increasing molecular weight of the aromatic hydrocarbons, the maximum production is shifted towards slightly higher vitrinite re¯ectance. 1- and 2-ring aromatics are preferentially generated at a vitrinite re¯ectance of 0.88%Rrwhereas 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 eect 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 tempera-ture eect but only by the content of organic matter.
In Fig. 9 the amounts of all identi®ed PAH and var-ious 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 cor-relation 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
con-tent (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 (Ph-PAH) (Fig. 9c,d). The most signi®cant enrichment of aromatic hydrocarbons in the mineralized bottom sec-tion of the pro®le is detected for O-PAH (Fig. 9f) and S-PAH (Fig. 9e). The amount of both groups of aromatic compounds is more than double than the amount expected from the Corgcontent.
The enrichment of aromatic compounds in the bot-tom part of the section can be attributed to two dierent eects. 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 Kup-ferschiefer 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. Corgand (b) id-Alk vs. Cu contents in Kupferschiefer samples from the Sangerhausen basin.
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 PAH-supgroups (naphthalenes, biphenyls and phenanthrenes) follow similar trends. The compounds increase together with increasing Cu concentrations. In Fig. 10b the abso-lute 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
thermo-chemical sulfate reduction. Therefore, the occurrence of S-PAH does not allow one to distinguish between bio-logical and thermochemical generation of H2S. A
possi-ble pathway for the formation of dibenzothiophenes from aliphatic precursor reacting with H2S during
sedi-mentation 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 avail-able, the formation of iron monosul®des will prevail as the trapping mechanism for H2S (Berner, 1985).
Reduced sulfur can also react with aromatic hydro-carbons 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 pro-cesses leading to Cu mineralization with those leading to
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 ele-vated temperatures. Alternatively, uptake of S-PAH in Kupferschiefer from basinal formation waters has to be considered assuming that the brines carried Cu and S-PAH 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 eect 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 eect 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 hydro-carbons (PuÈttmann et al., 1989). The distribution shift from alkylated to non-substituted PAH, again can be explained by two dierent mechanisms. Either the non-substituted 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 deal-kylated aromatic hydrocarbons in the mineralized sec-tion of the pro®le argues for in situ transformasec-tion 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. Oxi-dation of the kerogen prior to oxiOxi-dation of the soluble fraction is unlikely because the soluble fraction is more susceptible to oxidation reactions than the solid mate-rial. 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 aected both the soluble hydro-carbons (id-Alk) and the kerogen to a similar degree.
Fig. 12b shows the correlation of the absolute amount of identi®ed PAH and the HI values. The parameter id-PAH 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:
2CH2OSO24ÿ ! 2HCO
ÿ
3 H2S
H2S can be used for the precipitation of dissolve Cu
according to the following reaction schemes:
H2S 2Cu
! Cu2S2H
H2SCu
! CuS2H
About 4.72 g of hydrogen originating from hydro-carbons is necessary for the precipitation of approxi-mately 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 pre-cipitation 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 hydro-carbons 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 Corghas
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.
Com-paring to sample 5, the depletion of extractable organic matter in sample 2 amounts to 37 mg Extr/g Corgduring
ore formation.
When the Corgcontent 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 dierence is partly due to the fact that quantitation of individual saturated hydrocarbons was
restricted ton-alkanes, pristane and phytane. All bran-ched 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 hetero-components.
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 Corgis
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 Na-PAH content occurs in sample 2 with a value of about 35 ppm. Bi-PAH are enriched by about 35 ppm and Ph-PAH 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 con-tent 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 Bi-PAH 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 dicult to calculate the donation of hydrogen from aromatic hydrocarbons, because the hydrogen donation of Alk, as mentioned above, led to an increase in the rela-tive 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 alky-lated naphthalenes and 5 ppm alkyalky-lated phenanthrenes were consumed for sul®de formation.
According to PuÈttmann et al. (1989), alkylated phe-nanthrenes can be transformed into biphenyls by the in¯uence of oxidizing brines after the Kupferschiefer deposition. In the Sangerhausen pro®le, sample 2 was only very weakly in¯uenced with the oxidizing brines. But the Bi-PAH content is much higher than in the other samples. This may argue that the Bi-PAH might be derived from Ph-PAH. From the average content an additional amount of 35 ppm Bi-PAH in sample 2 can be expected according to the reaction scheme from methylphenanthrenes to biphenyl (PuÈttmann et al., 1989), and about 44 ppm methylphenanthrenes would be required for the formation of 35 ppm Bi-PAH. In sample 2, about 14% of trimethylphenanthrenes are depleted in comparison to the average content of the pro®le (Table 4). Assuming that 35 ppm Bi-PAH are derived from trimethylphenanthrenes, about 52 ppm of those are required for the formation of 35 ppm Bi-PAH in sample 2. The donation content of Na-PAH is about 35 ppm in sample 2 (Table 4, Fig. 7). According to the same reaction scheme (PuÈttmann et al., 1989), about 59 ppm trimethylphenanthrenes were needed for the for-mation of 35 ppm Na-PAH.
From the three PAH groups mentioned above (Na-PAH, Ph-PAH and Bi-PAH), approximately 100 ppm alkyl-PAH were depleted in sample 2 during metal accumulation. This amount is not of quantitative importance for TSR, but is of diagnostic value for the reconstruction of processes in¯uenced by TSR.
As mentioned above, about 61,380 mg CH are required for the sul®de formation in 1000 g ore rock of sample 2. The extract is only depleted by about 5794 mg. The calculated hydrogen donation of alkanes and PAH is much lower than the donation of the total extract. Only about 618 mg of alkanes and 100 mg of PAH are represented in the calculation above, which is about 12% of the total extract.
This discrepancy is due to the fact that the identi®ed alkanes and aromatics represent only a minor part of all molecules of those fractions. Additionally, hetero-components can contribute to hydrogen donation for TSR. According to the maturity value of organic mat-ter, one presumes that methane could be another important hydrogen donator for TSR. Unfortunately, one cannot measure nor calculate it.
Table 5
Additional PAH contents in the dierent PAH groups of Kupferschiefer samples from the Sangerhausen basin
Sample no. Na-PAH (ppm) Bi-PAH (ppm) Ph-PAH (ppm) S-PAH (ppm) O-PAH (ppm)
2 35 35 20 5 2
5.6. Hydrogen donation of kerogen during TSR
Kerogen can be another important source of hydro-gen for TSR. The donation of hydrohydro-gen from kerohydro-gen can be determined by Rock-Eval analysis. The Hydro-gen Index (HI) is the amount of hydrocarbons Hydro- gener-ated from a rock sample during Rock-Eval analysis in relation to the organic carbon content.
Samples 2 and 3 belong to the same layer (Grobe Lette) and were deposited in the same environment. Both samples should have similar HI values. Compared with sample 3, the HI values are depleted at least by 100 mg HC/g Corgin sample 2 and even 200 mg HC/g Corg
in sample 1. Considering the Corgcontents of 15.66% in
sample 2 and of 11.08% in sample 1, at least 15,660 mg of HC were consumed in sample 2 for the formation of sul®des in the 1000 g ore rock. In sample 1 about 22,180 mg HC were depleted for sul®de formation and oxida-tion of the organic matter. Consequently, in sample 2 the depletion of hydrogen from kerogen was at least represented by 15,660 mg hydrocarbons considering that also sample 3 was aected by hydrogen depletion for sul®de formation.
The calculation shows that the soluble organic matter and the kerogen provided 5793 mg (Extr) plus 15,660 mg (kerogen) of hydrocarbons in sample 2. The total supply of hydrocarbons from organic matter was 22,453 mg but 61,380 mg was required if all metal sul®des had to be precipitated by TSR. Due to the observed lack of 38,927 mg hydrocarbons in sample 2 TSR was not the only prominent process of Cu precipitation in the pre-sent case.
Additional Cu precipitation might have been caused with processes such as:
1. Part of the Cu has been enriched by replacement of pyrite and pyrite precursors and did not require hydrogen depletion in the organic matter. 2. Part of the Cu enriched during sedimentation and
the hydrogen required for BSR was supplied from metabolizable detrital organic matter.
3. Part of the hydrocarbons migrated from other strata to the Kupferschiefer and was used for TSR. 4. Part of the hydrocarbons was donated by
meth-ane.
Process no. 1 is favored among these processes because of petrological evidence discussed in Sun and PuÈttmann (1997). Process no. 2 can be disregarded to be responsible for Cu accumulations in the bottom of the pro®le because of the absence of framboidal textures of Cu sul®des. Process no. 3 can be neglected because the present study of the composition of the organic matter provides evidence for in situ alteration of the organic matter but not for a signi®cant uptake of allochthonous hydrocarbons in the mineralized section of the pro®le. Process no. 4 is possible, but cannot be proved.
Based on these presumptions, about 12% Cu in sam-ple 2 was precipitated by thermochemical sulfate reduc-tion, and about 8% Cu was precipitated by replacement of pyrite and pyrite precursors.
6. Conclusions
Results from the analyses of soluble organic matter by GC and GC±MS and of kerogen by Rock-Eval pyr-olysis indicate that PR is responsible for Cu precipita-tion in this pro®le, as long as the Cu contents are enriched less than 8%. In a sample from the bottom section of 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%. Solvent extract, saturated and aro-matic hydrocarbons, kerogen, and possibly methane served as hydrogen donors for TSR in this process. The results argue for stepwise precipitation of Cu sul®des during diagenesis of the shale. At ®rst reduced sulfur present as pyrite and pyrite precursors is supplied for Cu precipitation by pyrite replacement. Only when the reduced S stored in the sediment is used up, is additional H2S generated by TSR.
Acknowledgements
The authors are grateful to Dr. L. Schwark, Dr. C. Jowett and an anonymous reviewer for their helpful comments to improve the manuscript.
Associate EditorÐL. Schwark
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