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Influence of organic material on mineralization processes in the Permian Kupferschiefer formation, Poland
Advanea ia Orgtnk Geochemimry19ff7 Org. Geochem. Vol. 13, Nos I-3, pp. 357-363, 1988 Printed in Great Britain. All rights reserved
0146-6380/88 $3.00 + 0.00 Copyright © 1988 Pergamon Press plc
Influence of organic material on mineralization processes in the Permian Kupferschiefer Formation, Poland W. POTTM^~ ~, H. W. H^G,~,A~ ~, C. M~Atz~ and S. SPECZIK2 ILehrstuhl ffir Geologie, Geochemie und Lagerstatten des Erd61s und der Kohle, RWTH Aachen, Lochnerstr. 4-20, 51 Aachen, F.R.G. 2Faculty of Geology, University of Warsaw, 02-089 Warsaw, Zwirki i Wigury 93, Poland Abstract--The Permian Kupferschiefer horizon in Southwest Poland acted as a geochemical trap by accumulating metals from an ascending oxidizing brine. The hydrogen-rich organic material in the Kupferschiefer supplied sufficient reduction equivalents for the precipitation of base and precious metals from these brines by redox reactions. This is indicated by regular changes in the molecular composition of the extractable organic material in a set of samples collected from a 1.4 m thick horizon in a Polish mine. The degree of oxidation is shown to change drastically from the bottom to the top of the horizon. In the bottom section, saturated hydrocarbons are diminished and heteroaromatic systems containing oxygen and sulphur are enriched significantly; alkylated aromatic hydrocarbon abundances are relatively low. The ratio of phenanthrene/sum of methyiphenanthrenes varies with the degree of oxidation. Spectral fluorescence measurements reveal an increase in fluorescence intensities of the extracts with increasing oxidation. Moreover, the green shift of fluorescence maxima is related to oxidation effects. Parallel to the intensity of oxidation, as observed from changes in the extractable organic matter, the content of copper and silver changes within the horizon. The results confirm that organic matter in sediments, under appropriate geological conditions, can play a significant role in ore formation processes. Key words: aromatic hydrocarbons, dibenzofuran, biphenyl, dibenzothiophene, mineralization processes, redox reactions, Kupferschiefer, fluorescence
INTRODUCTION
The role of organic matter in ore formation processes has been discussed in numerous reports (Dunham, 1961; Kranz, 1968; Saxby, 1976; Giordano, 1985; Eugster, 1985; Disnar et al., 1986). Several mechanisms for the contribution of organic matter in metal accumulatio 9 are suggested. A well established mechanism for a primary accumulation of transition metals in sediments is the formation of nickel and vandium porphyrins by replacement of magnesium in chlorophyll (Treibs, 1934, 1935). Evidence to support the hypothesis that ore bodies could result from the destabilization of metal-organic complexes (Giordano, 1985) has yet to be presented. Ferguson and Bubela (1974) have shown that green algae can remove large quantities of copper, lead and zinc from aqueous solutions by both physical adsorption on particulate organic matter and by the formation o f metal complexes. However, an enrichment of metals sufficient for the formation of an ore body by such primary concentration processes is considered to be unlikely (Krauskopf, 1971). Nevertheless, a copper concentration of up to 10% in a peat has been ascribed to chelation effects (Fraser, 1961). In addition, some specific microorganisms were shown to concentrate elements from sea water with a concentration factor up to 106 (Trudinger and Bubela, 1967). Connected to this are more recent findings, which demonstrated that the growth of methanogenic bacteria is dependant on the nickel, cobalt and
molybdenum concentrations (Sch6nheit et al., 1979). The explanation for this result is given by the identification of nickel porphyrins as growth factors of methanogenic bacteria (Pfaltz et al., 1982). Until now it remains unclear whether this type of metal accumulation could contribute to a significant metal enrichment in sediments. Besides primary accumulation mechanisms of metals in sediments, some secondary processes involving the contribution of organic material to metal precipitation must also be considered. Studies on extracts of samples from the Saint-Privat mine (Lod~ve Basin, France) revealed a bacterial degradation of the bitumen (Connan and Orgeval, 1976). The observed alteration is suggested to be a result of the activity of both hydrocarbon-oxidizing and sulphate reducing bacteria which produced hydrogen sulphide necessary for metal sulphide precipitation. Organic geochemical investigations on Mississippi Valley.Type ore deposits have shown that mineralization resulted in a striking increase in the concentrations of n-alkanes in the range C,-C~9 and in changes of the aromatics/(aromatics + saturates) ratio (Gize and Barnes, 1987). In one case (Gays River deposit) changes in the organic distribution are interpreted as the addition of an epigenetic organic fraction to the indigenous organic matter in the host reef. In contrast, the investigation of the organic material in the Pine-Point lead-zinc deposit, Canada, revealed indigenous material which was derived from the local rocks (Mac,queen and Powell, 1983). In their study, bitumens associated
357 O.O. 1311-3X
358
W. POTT~O~N'~et al.
with sulphide mineralization are shown to have lower atomic H/C ratios than pre-existing unchanged bitumens, which could permit to an interaction between organic material and ore-bearing fluids. An attempt to explain the mineralization of the Permian Kupferschiefer in Poland is given by Jowett (1986). According to his "convection cell" model, Rotliegend brines carried metals leached from the volcanic detritus and migrated through the red beds into the overlying Kupferschiefer and Zechstein limestone. The author presumed that these brines overturned along the Zechstein-Rotliegende contact and sank down again into the Rotliegende, completing the convection cell. In accordance with this model, the present study provides evidence for the contribution of organic material to metal precipitation from upwards migrating metal-bearing solutions. EXPERIMENTAL
Finely ground shale samples were Soxhletextracted in pre-extracted timbles using dichloromethane as the solvent (24 h). The extracted bitumens were analysed as total extracts by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). For hydrocarbon group separation the extracts were separated by column chromatography over activated (1 h, 110°C) silica gel (70-230 mesh, 50 × 1 era). The elution of saturated hydrocarbons was achieved using n-hexane. Aromatic hydrocarbons were eluted with dichloromethane and polar heterocompounds with methanol. 40 ml of each solvent were used. GC analyses were carried out using a Carlo Erba 5160 gas chromatograph equipped with a 25m × 0.25 mm i.d. fused silica column coated with SE 54 silicone (0.25/zm film thickness) as the stationary phase. Hydrogen was applied as carrier gas. The GC oven was temperature programmed from 80 to 300°C at 4°C per min and maintained for 15 rain at final temperature. Quantification of individual compounds was carried out by the injection of a standard mixture (external standard procedure). GC-MS analyses were performed on a Varian 3700 GC coupled to a Finnigan MAT 8200 mass spectrometer using the same type of capillary column as used for GC analyses. Helium was applied as carrier gas. Mass spectra were recorded in the cyclic scan mode (1.1 s). An INCOS data system was used for data processing. The organic carbon content (Co¢) was determined using a LECO-WR-I 2 carbon determinator. Carbonates were removed from the samples by the prior treatment with concentrated hydrochloric acid. Spectral fluorescence measurements of the liptinites and the extracts were carried out at room temperature using a photometric reflecting light microscope equipped with a grating monochromator in order to dissociate the emitted fluorescence light. The liptinites were measured on polished sections and the extracts
on glass slides covered with Kieselgnhr G. The method has been previously described in detail by Hagemann and Hollerbach (1981, 1986). SAMPLE DESCRIPTION
Figure 1 shows the profile of one Kupferschiefer section from Konrad mine (North-Sudetic Syncline, Southwest Poland). The total profile has been separated into seven samples, each representing about 20 cm of the section. The depth of the sample containing the Rote Fiule material (K~) is at 440.3 m; the top-most sample K~ is at 441.5 m. The low burial depth of the Kupferschiefer horizon investigated is due to strong tectonical disturbances in the Konrad mine area (Speczik et al., 1986). In this profile the Rote F~ule horizon impregnates the dolomitic limestone underlying the marly shale (Kupferschiefer). The whole marly shale section is represented by five samples (K2-I~). Sample K7 was collected from the carbonates of Lower Werra overlying the Kupferschiefer horizon. The total organic carbon (TOC) content ranges from 1.2 to 1.8% in the Kupferschiefer samples. The over- and underlying limestones show lower values (Table 1). The rank of the kerogen was determined on vitrinite particles by reflectance measurements to a mean value of 0.86%. However, this value may not characterize the maturation exactly as oxidation may have affected the vitrinite reflectance value. RESULTS AND DISCUSSION
Gas chromatography
Extracts from seven samples of a Kupferschiefer profile including the over- and underlying limestones were analysed by gas chromatography. The GC traces are shown in Fig. I. Major differences in the composition of the volatile bitumen of the investigated samples can be observed. The bitumen obtained from the top of the Rote Fiule horizon (K~) is characterized by high intensities of only a few distinct compounds, which were determined (C~-MS) to be naphthalene (A), biphenyl (B), dibenzofuran (C), dibenzothiophene (D) and phenanthrcne (E). The geochemical significance of these compounds has been discussed in detail elsewhere (P~ttmann et al., 1988). With increasing distance from the Rote Fiule horizon, the intensities of these compounds in the G-C traces are reduced relative to a series of saturated hydrocarbons. In the upper part of the Kupferschiefer profile, as well as in the overlying limestone, the saturated hydrocarbons dominate within the volatile bitumen (Fig. 1). Confirmation is given by the hydrocarbon group separation which indicates that the amount of saturated hydrocarbons increase from the bottom to the top of the Kupferschiefer horizon (Table 1). The simpficity of the bitumen composition in the lower part of the
Influence of organic material on mineralization processes
359
B: blphenyL C:dlbenzofumn
[
[
,I,
D: dibenzothlophene E: p/~nanthrene
/ !" ' ' " ' i
7: , _ ' _ , - ' . . - ' : /
441.5 =..- . . . . .
•
"
E
3 :-:----':--~'-"---"--"----'
44(3.3 !,--
= DE
\\llil
\ \1 Illi L L I .I U~J.tlLk*,t,. ,,
[~
ooo,,i:
/ i[ 1" ."u
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,E
Fig. 1. Geological profile of the section investigated and gas chromatograms of the extracts from samples KI-K7 (marly series = Kupferschiefer).
section is thought to be caused by secondary alteration reactions which affected the organic material during late diagenesis or after diagencsis (Piittmann et aL, 1988).
The significant intensities of the unsubstituted aromatic compounds A-E is accompanied by relatively low intensities of their alkylated homologs. In general, the generation of bituminous constituents
Table 1. Geochemical data obtained from samples K~-K7 Sample No.
C ° (e/o~
Extr) (ppm)
Sat/ (%)
Aro.' (%)
Het." (%)
Asph/ (%)
KI K2 K3 K4 K5 K~ K7
0.46 1.50 1.80 1.20 1.40 1.40 0.70
86 547 441 309 324 801 242
2.1 2.2 5.3 6.0 9.6 10.5 2. I
45.8 41.7 30.5 32.8 37.9 31.4 16.7
47.7 52.9 61.5 58.5 47.9 35.6 64.5
4.4 3.2 2.7 2.7 4.6 22.5 16.7
Ph s I~ MePh 1i.41 8.58 2.78 2.65 1,91 2.10 1.93
Cu (%)
A$ (ppm)
0.04 0.20 4.19 2.73 1.56 1.58 0.66
2 6 66 59 54 50 II
Abbrevladons: "organic carbon content; bextract yield; ~saturated hydrocarbons; ~aromatic hydrocarbom; 'heterocompounds; fmlphldl~M~; Sphenanthrene/sum of methylpheunthrenes.
360
W. P O r t r e s s et al.
in shales is mainly controlled by diagenetic and catagenetic effects. Thereby, alkylated aromatics are usually abundant constituents of the aromatic fractions (Ishiwatari and Fukushima, 1979; Radke et al., 1982a). It is believed that the distribution of methylphenanthrene isomers is temperature controlled due to steric-strain driven shifts of methyl groups from ~,- to ~-positions of phenanthrene (Radke et al., 1986). Consequently, the relative concentration of phenanthrene and its methyl derivatives could be applied as a maturation parameter (Radke et al., 1982b). Moreover, methylbiphenyls and dimethylbiphenyls were shown to be often apparent in the GC traces of aromatic fractions of crude oils and coal extracts (White and Lee, 1980; Rowland et al., 1984, 1986; Cumbers et al., 1987). The relative intensities of methylbiphenyls appears to depend on the thermal maturity of crude oils (Alexander et al., 1986). In addition, alkylnaphthalenes are found to be dominating constituents of the aromatic fractions in both coal and shale extracts and petroleums (Radke et al., 1982b; Alexander et al., 1985). In contrast to the studies mentioned above, the composition of the aromatic compounds obtained from samples K] and K2 (Fig. 1) is dominated by non-substituted aromatic systems. Alkylated derivatives occur only in trace amounts. This unusual composition can be explained by the oxidative degradation of the aliphatic sidechains of aromatic systems to carboxylic acids followed by decarboxylation reactions (Hayatsu et al., 1978; Pfittmann et al., 1988). In order to examine the validity of this assumption, quantitative aspects have been taken into consideration. The quantification of the five outstanding alteration indicators (A-E) in the investigated samples revealed a significant decrease of these compounds from the Rote F~ule horizon upwards to the top of the Kupferschiefer (Fig. 2). Only naphthalene shows a partly irregular trend which can be due to evaporation losses during the removal of the solvent. In sample K~ the concentration of each compound amounts to more than 100/~g]g Cots. For comparison, compounds A-E in sample K7 contribute less than 5% of the value measured in Kin. This difference cannot be explained NaphthaLene
BlphenyL
solely by the degradation of the diagenetically/ produced alkylated aromatic hydrocarbons to their non-substituted counterparts. The changing lithology from sample Kmto sample Ks may also affect the TOC-normalized concentration values of these compounds. In addition, the destruction of asphaitenes and kerogen must be considered as a further source for compounds A-E. Based on oxidation experiments on coals, a degradation pathway of phenanthrene-type structures to biphenyl diacid (Anderson and Johns, 1986), a potential precursor of bipbenyl, is suggested. This can explain the formation of such high amounts of biphenyl (120 #g/g Coo) in K~. The investigation of bitumens obtained from a set of coals of a different rank range revealed that the biphenyl occurs only in higher rank (>0.93% R,) coals (Schaefer and Pfittmann, 1987). Consequently, the presence of biphenyl in the Kupferschiefer of lower rank (0.86% Rr) may be caused by secondary oxidation reactions and kerogen degradation. Moreover, desalkylation of methylbiphenyls under oxidizing conditions should be envisaged as a further source of biphenyl. The relatively high intensity of dibenzofuran in the lower part of the section (Kt-I~) supports this interpretation. The presence of dibenzofuran in crude oils is reported by Williams et al. (1986). Alkylated derivatives of dibenzofuran were shown to be important constituents of coal extracts (Radke et al., 1982b). Moreover, when a bituminous coal was treated with Na,Cr207 solution, dibenzofuran was the dominant compound within the extractable bitumen (Hayatsu et ai., 1978). The enhanced formation ofdibenzofuran in samples K2-K4 can again be explained by the oxidative degradation of aliphatic side-chains of alkylated dibenzofurans. This type of reaction may also be responsible for the abundance of dibenzothiophene (D) in these samples. Consequently, the bitumen composition of the lower part of the Kupferschiefer is basically similar to the one obtained from in situ oxidation experiments on bituminous coals using metal oxide solutions mentioned above. The oxidation of subbituminous coals under the same conditions yield a bitumen composition different from
• catagenetically
)lbenzofuron
i
o
t00
2000
1O0
2ooo
I 100
'
! 200
Dibenzothlophene
•
I
0
I
I00
I
;
200
Phenonthmne
L
0
Fglg CorQ Fig~ 2. Quantification of five selected aromatic compounds in samples Kt-K~.
loo
20o
Influence of organic material on mineralization processes
sample (K~) and decreases at first rapidly and then, in the upper part, more slowly towards the top of the Kupferschiefer horizon. In contrast, the copper concentrations are low in the bottom part (Kin and K2), increase in K3, and then decrease towards the top of the Kupferschiefer. The silver concentrations follow a similar trend. An explanation for this striking difference regarding the development of the metal content and the Ph/~MePh ratio is provided by the genetic model discussed for the Kupferschiefer in Poland (Speczik et al., 1986; Jowett, 1986). In agreement with these models, it can be envisaged that the upwards migrating oxidizing and metal-bearing solutions came in contact with the organic matter adsorbed or coating on mineral surfaces in the Kupferschiefer horizon. The hydrogen equivalents of the bitumen were used up in the contact area by the reduction of the oxidizing strength of the metal solutions without being able to precipitate the metals, When reaching areas deeper inside the Kupferschiefer horizon, sufficient hydrogen was available for the precipitation of copper mainly as the sulphide. The further decrease of the copper content towards the top may be caused by the restricted permeability of the Marly series due to the increasing carbonate content. Results described here support that the Kupferschiefer horizon can be regarded as a geochemical trap for mineralized solutions.
the one obtained from bituminous coals (Hayatsu et al., 1978). This can be explained by the well established observation that the typical pattern of naphthalenes, dibenzofuranes and phenanthrenes is generated not before the rank range of bituminous coals has been reached (Radke et al., 1982b). Based on these findings, the oxidation of the Kupferschiefer in Poland (Konrad mine) occurred probably during late diagenesis or early catagenesis. This is in agreement with the suggestion that an epigenetic accumulation of metals such as copper and silver in the Kupferschiefer of Southwest Poland happened during Triassic rifting (Jowett, 1986). Results obtained from hydrocarbon group separation reveal that in the bottom section polar heterocompounds and asphaltenes are reduced while aromatic hydrocarbons are increased in comparison to the upper part of the section (Table 1). Most likely, primary insoluble aromatic material was released from the kerogen by oxidative degradation followed by decarboxylation and decarbonylation reactions yielding increasing amounts of non-substituted aromatics. Metal contents
The measurements of the metal contents in samples Kj to K? were undertaken in order to examine their variation throughout the investigated section. Metals like lead, zinc, nickel, cobalt and vanadium were present in ppm amounts and were largely homogeneously distributed throughout the Kupferschiefer horizon. In contrast, the copper content reached values of up to 4.19% in sample K3 decreasing to the top and to the bottom of the Kupferschiefer. The determination of the silver content provides a similar distribution profile showing a maximum value of 66ppm in sample K 3 (Table 1). Figure 3 shows the distribution of the copper and silver concentration over the whole profile in comparison to the ratio ofphenanthrene to sum of methylphenanthrenes. An inverse relationship between both parameters can be observed in the lower part of the section. The oxidation indicating parameter (Ph/~MePh) reaches its highest value in the bottom Ph/~MePh
Spectral fluorescence measurements
Evidence for the theory mentioned above also exists from spectral fluorescence measurements on both extracts and liptinites. Figure 4 shows that the maximum intensity of the fluorescence spectra obtained from the extracts (Kj-KT) is shifted to shorter wavelengths towards the Rote F~iule horizon. The fluorescence intensities increase in the same direction (Fig. 4). The most drastic shifts of both parameters are observed in the lower section of the horizon (K~-K3). In the upper part, the fluorescence intensity and the maximum wavelength remains largely constant (Table 2). This result corresponds with the measurement of the absolute quantities of the oxidation indicating compounds A-E, which is in Cu (%)
,
.
,
Ag (ppm)
,
.._
2 4 5o ,oo-Fig. 3. Variation of the phenanthrene/sum of methylphenanthrenes ratio (Ph/~MePh), the copper content and the silver content with increasing distance from the Rote Fiule. 0
5
1o
o
361
362
W. P O T n ~ 12
et al.
K1
.L._
qO
tI
"- K2
it / / i
t ff
s
•
/ /
0
/
Q
,'"
i
g
K3 _ _ L.
,
",
".
0
B
o
4o0
45o
5oo
550
eoo
65o
70o
Weveten0th (nm) Fig. 4. Relationship between fluorescence intensity (%) and the peak wavelength (nm) obtained by
fluorescence measurements of the extracts from K r K ~. agreement with earlier results obtained from studies on the fluorescence behaviour of crude oils and their fractions (Hagemann and Hollerbach, 1986). Additional fluorescence measurements on liptinites (aiginites and sporinites) show largely the same tendency but less intensive. The peak wavelength of the liptinites varies only from 580 to 530 nm, which indicates that the kerogen was less affected by the metal-bearing solutions than the bituminous material. In the Rote Fiule sample K,, fiptinites could not be identified. The best preserved liptinites were observed in the upper part of the section (K~-Ke). This confirms that the degradation of the kerogen induced by oxidizing solutions occurred mainly in the lower part of the section. SUMMARY AND CONCLUSIONS
The organic geochemical study of a narrow sampled
Kupferschiefer section (Konrad mine, Poland) revealed a significant alteration of the extractable bitumen in the lower part of the section. The bitumen is mainly composed of five unsubstituted aromatic compounds (naphthalene, biphenyl, dibenzofuran, dibenzothiophene and phenanthrene) with relatively low amounts of related alkyl derivatives and saturated hydrocarbons. Table 2. Fluorescenceparameters of samples K,-K7 Extract analysis Liptinite analysis Sample No. G.," (rim) FL-I? (%) , t ~ (nm) ~.-I. (%) Km 515 11.19 --K 2 585 8.90 530(AIg.)c 0.56 K~ 610 4.77 560(AI8.) 0.83 K4 630 1.99 580(Spo.)d 0.73 Ks 650 1.83 575 (8po.) 0.72 K6 615 2.04 555 (AIR.) 0.71
This particular composition departs greatly from well established alteration effects of organic material such as meteoric weathering, microbial degradation and water washing. An explanation for this composition can be given by the removal of aliphatic hydrocarbons as well as aliphatic side-chains o f aromatic systems under the influence of upwards migrating, metal-bearing and oxidizing solutions. By that, metals like copper and silver were introduced into the Kupfersehiefer and precipitated through the reduction equivalents provided by the hydrocarbons present in the shale. Consequently, an epigenetic mineralization of the Kupfersehiefer in the area of Southwest Poland is envisaged. This area is related to major tectonic rift zones. Results presented herein are in agreement with the proposal of Jowett (1986), in saying that the interaction between migrating solutions and the Kupfersehiefer barrier can be dated to the Upper Triassic age. Acknowledgements--Thanks go to the lnstitut f'fir Mineral-
ogle und LagerstLttenlehre (RWTH Aachen) for providing the measurements of the copper content and to Analytisches Labor Aachen for the silver measurements. Support by Professor Dr M. Wolf (RWTH Aachen) is gratefully acknowledged. The paper benefited from critical reviews by Professor Dr D. Leythaeuser (KFA, Jfi!ich, F.R.G.) and Dr J. E. Zumberge (Ruska Labs, Houston, U.S.A.). We thank the Deutsche Forschungsgemeinschaft for financial support.
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Gilles H. H., Diekert G. and Thauer R. K. (1982) 81. Zur Kenntnisdes Faktors F 430 aus methanogenen Bakterien: Struktur des porphinoiden Ligandensystems. Heir. Chim. Acta 65, 828-865. Piittmann W., Merz C. and Speczik S. (1988) The secondary oxidation of organic material and its influence on Kupferschiefer mineralization of South-West Poland. Submitted. Radke M., Welte D. H. and Willsch H. (1982a) Geochemical study on a well in the Western Canada Basin: relation of the aromatic distribution pattern to maturity of organic matter. Geochim. Cosmochim. Acta 46, 1-10. Radke M., Willsch H., Leythaeuser D. and Teichmiiller M. (1982b) Aromatic components of coal: relation of distribution pattern to rank. Geochim. Cosmochim. Acta 46, 1831-1848. Radke M., Welte D. H. and Willsch H. (1986) Maturity parameters based on aromatic hydrocarbons: Influence of organic matter type. In Advances in Organic Geochemistry 1985, (Edited by Leythaeuser D. and Rullk6tter J.). Org. Geochem. 10, 51--63. Pergamon Press, Oxford. Rowland S. J., Alexander R. and Kagl R. I. (1984) Analysis of trimethylnaphthalenes in petroleum by capillary gas chromatography. J. Chromatogr. 294, 407-412. Rowland S. J., Alexander R., Kagi R. I., Jones D. M. and Douglas A. G. (1986) Microbial degradation of aromatic components of crude oils: a comparison of laboratory and field observations. Org. Geochem. 9, 153--161. Saxby J. D. (1976) The significance of organic matter in ore genesis. In: Handbook of strata-bound and stratiform ore deposits, Vol. 2: Geochemical studies (Edited by Wolf K. H.), pp. 111-133. Elsevier, Amsterdam. Schaefer R. G. and Pfttmann W. (1987) Analysis of hydrocarbons in coals by means of a micro-thermodesorptioncapillarygas chromatography combination. J. Chromatogr. 395, 203-216. Sch6nheit P., Moll J. and Thauer R. K. (1979) Nickel, cobalt and molybdenum requirement for growth of Methanobacterium thermoautotrophicum. Arch. Microbiol. 123, 105--107. Speczik S., Skowronek C., Friedrich G., Diedel R., Schumacher C. and Schmidt F.-P. (1986) The environment of generation of some Zechstein base metal occurrences of Central Europe. Acta Geol. Polon. 39, 1-28. Treibs A. (1934) 0her das Vorkommen yon Chlorophyllderivaten in einem Olschiefer aus der oberen Trias. Ann. Chemie 509, 103-114. Treibs A. (1935) Chlorophyll-und H~minderivate in bitumin6sen Gesteinen, Erd61en, Kohlen, Phosphoriten. Ann. Chemie 517, 172-196. Trudinger P. A. and Bubela B. (1967) Microorganisms and the natural environment. Minera. Deposita 2, 147-157. White C. M. and Lee M. L. (1980) Identification and geochemical significance of some aromatic components of coal. Geochim, Cosmochim. Acta 44, 1825--1832. Williams J. A., Bjorey M., Dolcater D. L. and Winters J. C. (1986) Biodegradation in South Texas Eocene oils-Effects on aromatics and biomarkers. In Advances in Organic Geochemistry 1985 (Edited by Leythaeuser D. and Rullk6tter J.). Org. Geochem. I0, 451-461. Pergamon Press, Oxford.
0146-6380/88 $3.00 + 0.00 Copyright © 1988 Pergamon Press plc
Influence of organic material on mineralization processes in the Permian Kupferschiefer Formation, Poland W. POTTM^~ ~, H. W. H^G,~,A~ ~, C. M~Atz~ and S. SPECZIK2 ILehrstuhl ffir Geologie, Geochemie und Lagerstatten des Erd61s und der Kohle, RWTH Aachen, Lochnerstr. 4-20, 51 Aachen, F.R.G. 2Faculty of Geology, University of Warsaw, 02-089 Warsaw, Zwirki i Wigury 93, Poland Abstract--The Permian Kupferschiefer horizon in Southwest Poland acted as a geochemical trap by accumulating metals from an ascending oxidizing brine. The hydrogen-rich organic material in the Kupferschiefer supplied sufficient reduction equivalents for the precipitation of base and precious metals from these brines by redox reactions. This is indicated by regular changes in the molecular composition of the extractable organic material in a set of samples collected from a 1.4 m thick horizon in a Polish mine. The degree of oxidation is shown to change drastically from the bottom to the top of the horizon. In the bottom section, saturated hydrocarbons are diminished and heteroaromatic systems containing oxygen and sulphur are enriched significantly; alkylated aromatic hydrocarbon abundances are relatively low. The ratio of phenanthrene/sum of methyiphenanthrenes varies with the degree of oxidation. Spectral fluorescence measurements reveal an increase in fluorescence intensities of the extracts with increasing oxidation. Moreover, the green shift of fluorescence maxima is related to oxidation effects. Parallel to the intensity of oxidation, as observed from changes in the extractable organic matter, the content of copper and silver changes within the horizon. The results confirm that organic matter in sediments, under appropriate geological conditions, can play a significant role in ore formation processes. Key words: aromatic hydrocarbons, dibenzofuran, biphenyl, dibenzothiophene, mineralization processes, redox reactions, Kupferschiefer, fluorescence
INTRODUCTION
The role of organic matter in ore formation processes has been discussed in numerous reports (Dunham, 1961; Kranz, 1968; Saxby, 1976; Giordano, 1985; Eugster, 1985; Disnar et al., 1986). Several mechanisms for the contribution of organic matter in metal accumulatio 9 are suggested. A well established mechanism for a primary accumulation of transition metals in sediments is the formation of nickel and vandium porphyrins by replacement of magnesium in chlorophyll (Treibs, 1934, 1935). Evidence to support the hypothesis that ore bodies could result from the destabilization of metal-organic complexes (Giordano, 1985) has yet to be presented. Ferguson and Bubela (1974) have shown that green algae can remove large quantities of copper, lead and zinc from aqueous solutions by both physical adsorption on particulate organic matter and by the formation o f metal complexes. However, an enrichment of metals sufficient for the formation of an ore body by such primary concentration processes is considered to be unlikely (Krauskopf, 1971). Nevertheless, a copper concentration of up to 10% in a peat has been ascribed to chelation effects (Fraser, 1961). In addition, some specific microorganisms were shown to concentrate elements from sea water with a concentration factor up to 106 (Trudinger and Bubela, 1967). Connected to this are more recent findings, which demonstrated that the growth of methanogenic bacteria is dependant on the nickel, cobalt and
molybdenum concentrations (Sch6nheit et al., 1979). The explanation for this result is given by the identification of nickel porphyrins as growth factors of methanogenic bacteria (Pfaltz et al., 1982). Until now it remains unclear whether this type of metal accumulation could contribute to a significant metal enrichment in sediments. Besides primary accumulation mechanisms of metals in sediments, some secondary processes involving the contribution of organic material to metal precipitation must also be considered. Studies on extracts of samples from the Saint-Privat mine (Lod~ve Basin, France) revealed a bacterial degradation of the bitumen (Connan and Orgeval, 1976). The observed alteration is suggested to be a result of the activity of both hydrocarbon-oxidizing and sulphate reducing bacteria which produced hydrogen sulphide necessary for metal sulphide precipitation. Organic geochemical investigations on Mississippi Valley.Type ore deposits have shown that mineralization resulted in a striking increase in the concentrations of n-alkanes in the range C,-C~9 and in changes of the aromatics/(aromatics + saturates) ratio (Gize and Barnes, 1987). In one case (Gays River deposit) changes in the organic distribution are interpreted as the addition of an epigenetic organic fraction to the indigenous organic matter in the host reef. In contrast, the investigation of the organic material in the Pine-Point lead-zinc deposit, Canada, revealed indigenous material which was derived from the local rocks (Mac,queen and Powell, 1983). In their study, bitumens associated
357 O.O. 1311-3X
358
W. POTT~O~N'~et al.
with sulphide mineralization are shown to have lower atomic H/C ratios than pre-existing unchanged bitumens, which could permit to an interaction between organic material and ore-bearing fluids. An attempt to explain the mineralization of the Permian Kupferschiefer in Poland is given by Jowett (1986). According to his "convection cell" model, Rotliegend brines carried metals leached from the volcanic detritus and migrated through the red beds into the overlying Kupferschiefer and Zechstein limestone. The author presumed that these brines overturned along the Zechstein-Rotliegende contact and sank down again into the Rotliegende, completing the convection cell. In accordance with this model, the present study provides evidence for the contribution of organic material to metal precipitation from upwards migrating metal-bearing solutions. EXPERIMENTAL
Finely ground shale samples were Soxhletextracted in pre-extracted timbles using dichloromethane as the solvent (24 h). The extracted bitumens were analysed as total extracts by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). For hydrocarbon group separation the extracts were separated by column chromatography over activated (1 h, 110°C) silica gel (70-230 mesh, 50 × 1 era). The elution of saturated hydrocarbons was achieved using n-hexane. Aromatic hydrocarbons were eluted with dichloromethane and polar heterocompounds with methanol. 40 ml of each solvent were used. GC analyses were carried out using a Carlo Erba 5160 gas chromatograph equipped with a 25m × 0.25 mm i.d. fused silica column coated with SE 54 silicone (0.25/zm film thickness) as the stationary phase. Hydrogen was applied as carrier gas. The GC oven was temperature programmed from 80 to 300°C at 4°C per min and maintained for 15 rain at final temperature. Quantification of individual compounds was carried out by the injection of a standard mixture (external standard procedure). GC-MS analyses were performed on a Varian 3700 GC coupled to a Finnigan MAT 8200 mass spectrometer using the same type of capillary column as used for GC analyses. Helium was applied as carrier gas. Mass spectra were recorded in the cyclic scan mode (1.1 s). An INCOS data system was used for data processing. The organic carbon content (Co¢) was determined using a LECO-WR-I 2 carbon determinator. Carbonates were removed from the samples by the prior treatment with concentrated hydrochloric acid. Spectral fluorescence measurements of the liptinites and the extracts were carried out at room temperature using a photometric reflecting light microscope equipped with a grating monochromator in order to dissociate the emitted fluorescence light. The liptinites were measured on polished sections and the extracts
on glass slides covered with Kieselgnhr G. The method has been previously described in detail by Hagemann and Hollerbach (1981, 1986). SAMPLE DESCRIPTION
Figure 1 shows the profile of one Kupferschiefer section from Konrad mine (North-Sudetic Syncline, Southwest Poland). The total profile has been separated into seven samples, each representing about 20 cm of the section. The depth of the sample containing the Rote Fiule material (K~) is at 440.3 m; the top-most sample K~ is at 441.5 m. The low burial depth of the Kupferschiefer horizon investigated is due to strong tectonical disturbances in the Konrad mine area (Speczik et al., 1986). In this profile the Rote F~ule horizon impregnates the dolomitic limestone underlying the marly shale (Kupferschiefer). The whole marly shale section is represented by five samples (K2-I~). Sample K7 was collected from the carbonates of Lower Werra overlying the Kupferschiefer horizon. The total organic carbon (TOC) content ranges from 1.2 to 1.8% in the Kupferschiefer samples. The over- and underlying limestones show lower values (Table 1). The rank of the kerogen was determined on vitrinite particles by reflectance measurements to a mean value of 0.86%. However, this value may not characterize the maturation exactly as oxidation may have affected the vitrinite reflectance value. RESULTS AND DISCUSSION
Gas chromatography
Extracts from seven samples of a Kupferschiefer profile including the over- and underlying limestones were analysed by gas chromatography. The GC traces are shown in Fig. I. Major differences in the composition of the volatile bitumen of the investigated samples can be observed. The bitumen obtained from the top of the Rote Fiule horizon (K~) is characterized by high intensities of only a few distinct compounds, which were determined (C~-MS) to be naphthalene (A), biphenyl (B), dibenzofuran (C), dibenzothiophene (D) and phenanthrcne (E). The geochemical significance of these compounds has been discussed in detail elsewhere (P~ttmann et al., 1988). With increasing distance from the Rote Fiule horizon, the intensities of these compounds in the G-C traces are reduced relative to a series of saturated hydrocarbons. In the upper part of the Kupferschiefer profile, as well as in the overlying limestone, the saturated hydrocarbons dominate within the volatile bitumen (Fig. 1). Confirmation is given by the hydrocarbon group separation which indicates that the amount of saturated hydrocarbons increase from the bottom to the top of the Kupferschiefer horizon (Table 1). The simpficity of the bitumen composition in the lower part of the
Influence of organic material on mineralization processes
359
B: blphenyL C:dlbenzofumn
[
[
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D: dibenzothlophene E: p/~nanthrene
/ !" ' ' " ' i
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E
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section is thought to be caused by secondary alteration reactions which affected the organic material during late diagenesis or after diagencsis (Piittmann et aL, 1988).
The significant intensities of the unsubstituted aromatic compounds A-E is accompanied by relatively low intensities of their alkylated homologs. In general, the generation of bituminous constituents
Table 1. Geochemical data obtained from samples K~-K7 Sample No.
C ° (e/o~
Extr) (ppm)
Sat/ (%)
Aro.' (%)
Het." (%)
Asph/ (%)
KI K2 K3 K4 K5 K~ K7
0.46 1.50 1.80 1.20 1.40 1.40 0.70
86 547 441 309 324 801 242
2.1 2.2 5.3 6.0 9.6 10.5 2. I
45.8 41.7 30.5 32.8 37.9 31.4 16.7
47.7 52.9 61.5 58.5 47.9 35.6 64.5
4.4 3.2 2.7 2.7 4.6 22.5 16.7
Ph s I~ MePh 1i.41 8.58 2.78 2.65 1,91 2.10 1.93
Cu (%)
A$ (ppm)
0.04 0.20 4.19 2.73 1.56 1.58 0.66
2 6 66 59 54 50 II
Abbrevladons: "organic carbon content; bextract yield; ~saturated hydrocarbons; ~aromatic hydrocarbom; 'heterocompounds; fmlphldl~M~; Sphenanthrene/sum of methylpheunthrenes.
360
W. P O r t r e s s et al.
in shales is mainly controlled by diagenetic and catagenetic effects. Thereby, alkylated aromatics are usually abundant constituents of the aromatic fractions (Ishiwatari and Fukushima, 1979; Radke et al., 1982a). It is believed that the distribution of methylphenanthrene isomers is temperature controlled due to steric-strain driven shifts of methyl groups from ~,- to ~-positions of phenanthrene (Radke et al., 1986). Consequently, the relative concentration of phenanthrene and its methyl derivatives could be applied as a maturation parameter (Radke et al., 1982b). Moreover, methylbiphenyls and dimethylbiphenyls were shown to be often apparent in the GC traces of aromatic fractions of crude oils and coal extracts (White and Lee, 1980; Rowland et al., 1984, 1986; Cumbers et al., 1987). The relative intensities of methylbiphenyls appears to depend on the thermal maturity of crude oils (Alexander et al., 1986). In addition, alkylnaphthalenes are found to be dominating constituents of the aromatic fractions in both coal and shale extracts and petroleums (Radke et al., 1982b; Alexander et al., 1985). In contrast to the studies mentioned above, the composition of the aromatic compounds obtained from samples K] and K2 (Fig. 1) is dominated by non-substituted aromatic systems. Alkylated derivatives occur only in trace amounts. This unusual composition can be explained by the oxidative degradation of the aliphatic sidechains of aromatic systems to carboxylic acids followed by decarboxylation reactions (Hayatsu et al., 1978; Pfittmann et al., 1988). In order to examine the validity of this assumption, quantitative aspects have been taken into consideration. The quantification of the five outstanding alteration indicators (A-E) in the investigated samples revealed a significant decrease of these compounds from the Rote F~ule horizon upwards to the top of the Kupferschiefer (Fig. 2). Only naphthalene shows a partly irregular trend which can be due to evaporation losses during the removal of the solvent. In sample K~ the concentration of each compound amounts to more than 100/~g]g Cots. For comparison, compounds A-E in sample K7 contribute less than 5% of the value measured in Kin. This difference cannot be explained NaphthaLene
BlphenyL
solely by the degradation of the diagenetically/ produced alkylated aromatic hydrocarbons to their non-substituted counterparts. The changing lithology from sample Kmto sample Ks may also affect the TOC-normalized concentration values of these compounds. In addition, the destruction of asphaitenes and kerogen must be considered as a further source for compounds A-E. Based on oxidation experiments on coals, a degradation pathway of phenanthrene-type structures to biphenyl diacid (Anderson and Johns, 1986), a potential precursor of bipbenyl, is suggested. This can explain the formation of such high amounts of biphenyl (120 #g/g Coo) in K~. The investigation of bitumens obtained from a set of coals of a different rank range revealed that the biphenyl occurs only in higher rank (>0.93% R,) coals (Schaefer and Pfittmann, 1987). Consequently, the presence of biphenyl in the Kupferschiefer of lower rank (0.86% Rr) may be caused by secondary oxidation reactions and kerogen degradation. Moreover, desalkylation of methylbiphenyls under oxidizing conditions should be envisaged as a further source of biphenyl. The relatively high intensity of dibenzofuran in the lower part of the section (Kt-I~) supports this interpretation. The presence of dibenzofuran in crude oils is reported by Williams et al. (1986). Alkylated derivatives of dibenzofuran were shown to be important constituents of coal extracts (Radke et al., 1982b). Moreover, when a bituminous coal was treated with Na,Cr207 solution, dibenzofuran was the dominant compound within the extractable bitumen (Hayatsu et ai., 1978). The enhanced formation ofdibenzofuran in samples K2-K4 can again be explained by the oxidative degradation of aliphatic side-chains of alkylated dibenzofurans. This type of reaction may also be responsible for the abundance of dibenzothiophene (D) in these samples. Consequently, the bitumen composition of the lower part of the Kupferschiefer is basically similar to the one obtained from in situ oxidation experiments on bituminous coals using metal oxide solutions mentioned above. The oxidation of subbituminous coals under the same conditions yield a bitumen composition different from
• catagenetically
)lbenzofuron
i
o
t00
2000
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I 100
'
! 200
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I
0
I
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I
;
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loo
20o
Influence of organic material on mineralization processes
sample (K~) and decreases at first rapidly and then, in the upper part, more slowly towards the top of the Kupferschiefer horizon. In contrast, the copper concentrations are low in the bottom part (Kin and K2), increase in K3, and then decrease towards the top of the Kupferschiefer. The silver concentrations follow a similar trend. An explanation for this striking difference regarding the development of the metal content and the Ph/~MePh ratio is provided by the genetic model discussed for the Kupferschiefer in Poland (Speczik et al., 1986; Jowett, 1986). In agreement with these models, it can be envisaged that the upwards migrating oxidizing and metal-bearing solutions came in contact with the organic matter adsorbed or coating on mineral surfaces in the Kupferschiefer horizon. The hydrogen equivalents of the bitumen were used up in the contact area by the reduction of the oxidizing strength of the metal solutions without being able to precipitate the metals, When reaching areas deeper inside the Kupferschiefer horizon, sufficient hydrogen was available for the precipitation of copper mainly as the sulphide. The further decrease of the copper content towards the top may be caused by the restricted permeability of the Marly series due to the increasing carbonate content. Results described here support that the Kupferschiefer horizon can be regarded as a geochemical trap for mineralized solutions.
the one obtained from bituminous coals (Hayatsu et al., 1978). This can be explained by the well established observation that the typical pattern of naphthalenes, dibenzofuranes and phenanthrenes is generated not before the rank range of bituminous coals has been reached (Radke et al., 1982b). Based on these findings, the oxidation of the Kupferschiefer in Poland (Konrad mine) occurred probably during late diagenesis or early catagenesis. This is in agreement with the suggestion that an epigenetic accumulation of metals such as copper and silver in the Kupferschiefer of Southwest Poland happened during Triassic rifting (Jowett, 1986). Results obtained from hydrocarbon group separation reveal that in the bottom section polar heterocompounds and asphaltenes are reduced while aromatic hydrocarbons are increased in comparison to the upper part of the section (Table 1). Most likely, primary insoluble aromatic material was released from the kerogen by oxidative degradation followed by decarboxylation and decarbonylation reactions yielding increasing amounts of non-substituted aromatics. Metal contents
The measurements of the metal contents in samples Kj to K? were undertaken in order to examine their variation throughout the investigated section. Metals like lead, zinc, nickel, cobalt and vanadium were present in ppm amounts and were largely homogeneously distributed throughout the Kupferschiefer horizon. In contrast, the copper content reached values of up to 4.19% in sample K3 decreasing to the top and to the bottom of the Kupferschiefer. The determination of the silver content provides a similar distribution profile showing a maximum value of 66ppm in sample K 3 (Table 1). Figure 3 shows the distribution of the copper and silver concentration over the whole profile in comparison to the ratio ofphenanthrene to sum of methylphenanthrenes. An inverse relationship between both parameters can be observed in the lower part of the section. The oxidation indicating parameter (Ph/~MePh) reaches its highest value in the bottom Ph/~MePh
Spectral fluorescence measurements
Evidence for the theory mentioned above also exists from spectral fluorescence measurements on both extracts and liptinites. Figure 4 shows that the maximum intensity of the fluorescence spectra obtained from the extracts (Kj-KT) is shifted to shorter wavelengths towards the Rote F~iule horizon. The fluorescence intensities increase in the same direction (Fig. 4). The most drastic shifts of both parameters are observed in the lower section of the horizon (K~-K3). In the upper part, the fluorescence intensity and the maximum wavelength remains largely constant (Table 2). This result corresponds with the measurement of the absolute quantities of the oxidation indicating compounds A-E, which is in Cu (%)
,
.
,
Ag (ppm)
,
.._
2 4 5o ,oo-Fig. 3. Variation of the phenanthrene/sum of methylphenanthrenes ratio (Ph/~MePh), the copper content and the silver content with increasing distance from the Rote Fiule. 0
5
1o
o
361
362
W. P O T n ~ 12
et al.
K1
.L._
qO
tI
"- K2
it / / i
t ff
s
•
/ /
0
/
Q
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g
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,
",
".
0
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o
4o0
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550
eoo
65o
70o
Weveten0th (nm) Fig. 4. Relationship between fluorescence intensity (%) and the peak wavelength (nm) obtained by
fluorescence measurements of the extracts from K r K ~. agreement with earlier results obtained from studies on the fluorescence behaviour of crude oils and their fractions (Hagemann and Hollerbach, 1986). Additional fluorescence measurements on liptinites (aiginites and sporinites) show largely the same tendency but less intensive. The peak wavelength of the liptinites varies only from 580 to 530 nm, which indicates that the kerogen was less affected by the metal-bearing solutions than the bituminous material. In the Rote Fiule sample K,, fiptinites could not be identified. The best preserved liptinites were observed in the upper part of the section (K~-Ke). This confirms that the degradation of the kerogen induced by oxidizing solutions occurred mainly in the lower part of the section. SUMMARY AND CONCLUSIONS
The organic geochemical study of a narrow sampled
Kupferschiefer section (Konrad mine, Poland) revealed a significant alteration of the extractable bitumen in the lower part of the section. The bitumen is mainly composed of five unsubstituted aromatic compounds (naphthalene, biphenyl, dibenzofuran, dibenzothiophene and phenanthrene) with relatively low amounts of related alkyl derivatives and saturated hydrocarbons. Table 2. Fluorescenceparameters of samples K,-K7 Extract analysis Liptinite analysis Sample No. G.," (rim) FL-I? (%) , t ~ (nm) ~.-I. (%) Km 515 11.19 --K 2 585 8.90 530(AIg.)c 0.56 K~ 610 4.77 560(AI8.) 0.83 K4 630 1.99 580(Spo.)d 0.73 Ks 650 1.83 575 (8po.) 0.72 K6 615 2.04 555 (AIR.) 0.71
This particular composition departs greatly from well established alteration effects of organic material such as meteoric weathering, microbial degradation and water washing. An explanation for this composition can be given by the removal of aliphatic hydrocarbons as well as aliphatic side-chains o f aromatic systems under the influence of upwards migrating, metal-bearing and oxidizing solutions. By that, metals like copper and silver were introduced into the Kupfersehiefer and precipitated through the reduction equivalents provided by the hydrocarbons present in the shale. Consequently, an epigenetic mineralization of the Kupfersehiefer in the area of Southwest Poland is envisaged. This area is related to major tectonic rift zones. Results presented herein are in agreement with the proposal of Jowett (1986), in saying that the interaction between migrating solutions and the Kupfersehiefer barrier can be dated to the Upper Triassic age. Acknowledgements--Thanks go to the lnstitut f'fir Mineral-
ogle und LagerstLttenlehre (RWTH Aachen) for providing the measurements of the copper content and to Analytisches Labor Aachen for the silver measurements. Support by Professor Dr M. Wolf (RWTH Aachen) is gratefully acknowledged. The paper benefited from critical reviews by Professor Dr D. Leythaeuser (KFA, Jfi!ich, F.R.G.) and Dr J. E. Zumberge (Ruska Labs, Houston, U.S.A.). We thank the Deutsche Forschungsgemeinschaft for financial support.
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