Organic Matter Associated with Mineralized Reduction ... - Springer Link

Organic Matter Associated with Mineralized Reduction Spots in Red Beds B.A. Hofmann l

Abstract Reduction spots in marine and continental red beds were investigated for the presence of organic matter. The samples are from a wide variety of localities and from host rocks ranging in age from Precambrian to Cretaceous. It was found that organic carbon is present in a minor number of samples only, where it is a major component. Organic carbon in reduction spot cores is always associated with uranium minerals. Rock-Eval pyrolysis shows that this organic matter is depleted in H and enriched in 0 relative to possible precursor materials. Pyrolysis-GC-MS indicates that this insoluble, high-molecular weight organic material primarily consists of aromatic structures, similar to other occurrences of organic matter associated with uranium mineralization. The stable isotopic composition of the organic matter is highly variable (-23.1 to -49.8%0 relative to PDB) but consistent with a derivation from oil, gas, or bacterial biomass. In reduction spots devoid of organic carbon, stable isotopes (C, 0) of carbonates are not significantly different in cores and in host rocks and therefore do not provide evidence for the former presence of organic carbon. Several possibilities for the origin of organic carbon in reduction spots are discussed but no conclusive evidence can be presented yet. Mineralogically, reduction spots with organic matter are generally similar to those without. A significant difference is the abundance of roscoelite in organic-poor reduction spots, while this mineral is absent in organic-rich ones.

1 Introduction Reduction spots are small-scale reduction phenomena commonly found in continental and marine red beds and in some altered crystalline rocks (Hofmann 1990). Reduction spots often have a dark center which has been ascribed to the presence of ore minerals and of organic matter. While ore minerals have been documented from many occurrences of reduction spots (e.g., Fesser 1971; Harrison 1975; Parnell 1985; Hofmann 1986, 1990, 1991a), considerable confusion exists in the literature regarding the occurrence of organic matter in the dark reduction spot cores. Several authors assumed the presence of organic

I

Natural History Museum, Department of Earth Sciences, Bernastr. 15, CH-3005 Bern, Switzerland

J. Parnell et al. (eds.), Bitumens in Ore Deposits © Springer-Verlag Berlin Heidelberg 1993

Organic Matter Associated with Mineralized Reduction Spots in Red Beds

363

Table 1. Occurrence and mineralogy of reduction spots containing organic matter Kiowa Co, Oklahoma (Sample: KIO) Host rock: Hennessey shale, Permian (clay- and siltstone) Minerals: coffinite, Ni-Co-arsenides, clausthalite Reference: Curiale et al. (1983); this chapter Dingwall, Scotland (Sample: DW) Host rock: Old Red sandstone, Devonian (calcite-cemented sandstone) Minerals: uraninite, brannerite, xenotime Reference: Parnell (1985); Parnell and Eakin (1987) Knowle Borehole, West Midlands U.K. (Sample: KNO) Host rock: Mercia Mudstone, Triassic Minerals: coffinite, V-oxides, Ni-Co-arsenides, Cu-sulfides References: Harrison et al. (1983) Gipsy Lane Brickpit, Leicester, U.K. (Sample: LEC) Host rock: Mercia Mudstone, Triassic Minerals: coffinite, Cu-sulfides, Ni-Co-arsenides Reference: Faithfull and Hubbard (1988) Weiach Borehole, Switzerland, depth 1076m (Sample: WEI) Host rock: coarse fluviatile sandstone, Permian Minerals: uraninite Reference: Hofmann (1990) Other occurrences, not investigated in this chapter: Nahe, Germany Host rock: Permian red beds Reference: Eichhoff and Reineck (1952) Northern Texas Host rock: Permian red beds overlying Panhandle gas field Minerals: Ni-Co-arsenides, pyrite, uraninite, xenotime Reference: Pierce et al. (1964)

matter on the basis of the dark color of cores only (Miller 1910; Mempel 1960; Prest et al. 1969; Wu 1971; Manning 1975; Durrance et al. 1978; Turner 1980), while few actual analyses for organic matter are available. The scope of this chapter is to present new measurements on the amount of organic carbon and to investigate the nature of organic matter where it is present in reduction spots. Occurrences with and without organic matter are compared and reasons are sought for its presence or absence. Because the nature of the reductant that caused the formation of reduction spots is unknown, the origin of organic matter is of particular interest. Only small amounts of organic matter were available for analysis from most organic-rich cores, therefore not all analytical techniques employed could be applied to all samples.

2 Sample Provenance and Analytical Procedures Sample Provenance. Known occurrences of organic-rich reduction spots including the investigated samples are listed in Table 1. Host rocks range in age

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from Precambrian to Early Cretaceous. Most samples are from continental red beds, mostly clay-, silt- and sandstones. Samples from Oman are from radiolarian cherts and samples from Pennsylvania are from marine clastic red beds. Sample Preparation. Samples of mineralized and organic-rich reduction spots were obtained for analysis by coarsely crushing material containing one or several cores followed by hand-picking of the core material with tweezers and grinding in an agate mortar (small samples) or in an alumina oxide disc mill (larger samples). Analytical results are thus representative of one complete core or of a composite of several complete cores. Analytical Procedures. The mineralogy of mineralized cores of reduction spots was determined by reflected light microscopy and qualitative (EDAX) analysis on the SEM. Total and organic carbon were determined with a Leco CR-12 carbon analyzer by combustion of samples in O2 at 1370°C and IR detection of CO2 (Jackson et al. 1987). Prior to the determination of organic carbon, samples were treated with 6N HCI at 60°C to remove all carbonates. RockEval pyrolysis is described in Tissot and Welte (1978). Samples were heated in He from 250 to 550°C at 25°C/min. Volatile hydrocarbons released up to 300°C are designated Sl (mgHC/g sample). Pyrolysis products released above 300°C are S2 (mgHC/g). Organic CO2 released from 300 to 390°C is called S3 (mgC02/g). S2 and S3 are used for the calculation of the Hydrogen Index (HI = mgHC/g Corg) and of the Oxygen Index (01 = mgC02/g Corg). Tmax is the temperature of maximum pyrolysis yield and increases with increasing maturity of the organic matter. Insoluble organic matter from reduction haloes was investigated by pyrolysis-gas chromatography and pyrolysis-gas chromatography-mass spectrometry. Stepwise pyrolysis at 450,600, 750, 900°C and in some cases at 1050°C (Leventhal 1976) was performed with a Chemical Data Systems Pyroprobe pyrolysis device using a pyrolysis time of 10 s in helium carrier gas. Pyrolysis products were directly introduced in a gas chromatograph with a fused silica capillary column (15 m X 0.5 mm, coated with Apiezon L) and a flame ionization detector. After pyrolysis, column temperature was held at room temperature for 2 min, at 80°C for another 2 min, then the column temperature was programmed from 80 to 260°C at 5°C/min. The same PY-GC conditions were used for PY-GC-MS except that the temperature was programmed at 4°C/min. Eluents from the capillary column were analyzed with a Finegan 800 ion trap detector type mass analyzer. The scan rate was 2 s/decade for the mass range range 40-400 AMU. Stable carbon isotopes of organic matter were measured on decarbonated (6N-HCI-treated) samples after combustion (Pratt and Threlkeld 1984). CO2 was evolved from carbonates using the phosphoric acid method.


365

Fig. lA-D. Photomicrographs showing organic matter in reduction spots. A Organic matter with variable reflectance and birefringence. Dingwall , Scotland, UK. 8 Matrix consisting of an intimate mixture of organic matter and coffinite with some mica flakes. Gray spots consist of organic matter free of mineral inclusions. White areas are copper sulfides. Gipsy Lane Brickpit , Leicester, UK. C Highly anisotropic organic matter associated with uraninite. Weiach well , J076m, northern Switzerland. D Coffinite surrounded by haloes of higher reflectance in low-reflectivity organic matter. Kiowa Co. , Oklahoma, USA

3 Results 3.1 Description of Organic Matter and Associated Mineralization in Reduction Spots Organic matter in reduction spots is always closely associated with authigenic mineral phases, most commonly with uranium phases such as uraninite, coffinite and brannerite (Fig. 1). As a result of the irradiation from uranium and its daughter elements, the organic matter usually is highly mature, displaying high reflectivity and birefringence . Extremely intimate, submicroscopic intergrowths of organic matter and coffinite were observed in the Leicester and Knowle localities, England (Fig. 18). With the exception of samples from Kiowa Co, Oklahoma, only highly radiation-influenced high-reflectivity organic matter was present in the cores. In the Kiowa occurrence, high-reflectivity haloes are found only around small coffinite inclusions while the rest of the organic-rich cores show no optically visible radiation damage.

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Table 2. Organic carbon content in reduction spots, haloes and host rocks (mean of two to ten repeat analyses) Host rock (wt.%) Weitenau, Germany (Permian) Zuzgen, Switzerland (Permian) Weierfeld, Switzerland (Permian) Exmouth, Devon, England (Permo-Triassic) North Berwick, Scotland (Carboniferous) Bedrock, Colorado (Triassic) Lyons, Colorado (Carboniferous) Basalt, Colorado (Triassic) Climax, Colorado (Carboniferous) Moab 1, Utah (Triassic) Moab 2, Utah (Triassic) Grand Canyon, Arizona (Proterozoic) Shartlesville, Pennsylvania (Ordovician) Oman (Uppermost Jurassic) Kl02 (Permian) DW4 (Devonian) Wei1076 (Permian) KNO (Triassic) LEC (Triassic) Mean

Haloe (wt.%)

Core (wt. %)

Presence of roscoelite

0.02 0.01 0.03 0.02

0.01 n.a. 0.02 n.a.

0.01 0.D2 0.02 0.01

+ + + +

0.02

n.a.

50%. Organic carbon is macroscopically and microscopically conspicuous in these samples (Fig. 1). Host rocks of reduction spots containing COTg-rich cores are not enriched in reduced carbon compared to host rocks of Corg-poor cores. Reduced carbon contents of red beds found in this study are lower than most reported values. Dyck and McCorkell (1983) report 0.2 to 0.5% for red beds from Nova Scotia; Curiale et al. (1983) found 0.04 to 0.08% in red beds from Kiowa Co, Oklahoma. The average in this study is 0.015 ± 0.005% (n = 18). Because repeated blanks were run and the LECO equipment was carefully calibrated, we are confident of these low values. Replicate analyses averaged a reproducibility of ±40% for samples with 0.02% C. In bleached haloes, significantly elevated values of reduced carbon are present in two samples with Corg-rich cores (KI02, KNO). The measured concentrations of reduced carbon are low but significantly above the detection limit and most likely represent background values of reduced carbon in red beds that are due to the presence of inert types of reduced carbon such as detrital graphite which has been repeatedly identified by reflected light microscopy and possibly to charcoal and soot (Wolbach and Anders 1989). Other possible sources of trace amounts of C are incompletely removed carbonates, CO 2 in fluid inclusions and adsorbed CO 2 . 3.3 Isotopic Composition of Organic Matter in Cores Organic carbon from the Kiowa samples yielded stable carbon isotopic compositions of -30.2 and -30.0%0. These values, obtained on slightly weathered material, are similar to values reported by Curiale et al. (1983) and Curiale (1985) of -31.0 ± 0.6 (n = 14). The organic matter from a reduction spot from Dingwall, Scotland, is very light (-49.9) and significantly different from vein hydrocarbon from the same locality (-33.3). Samples from northern Switzerland and Leicester (UK) yielded values of -23.2 and -29.0%0, respectively. 3.4 Rock-Eval Pyrolysis Results of Rock-Eval pyrolysis of organic matter in reduction spots are presented in Table 3. Fig. 2 shows a plot of hydrogen index versus oxygen index of these samples together with other samples of organic matter from U-rich ores. Kiowa. Unaltered Kiowa samples (KI03) are composed of organic matter with relatively high hydrogen index (HI, 287 to 325), low oxygen index (01, 20 to 44) and Tmax ranges from 437 to 442°C. Altered samples (KIOl, 2) from smaller cores are very different with lower HI (10 to 82) and higher 01 (70 to 120). Tmax of these samples is very variable and ranges from 384 to 471 °C. Total carbon ranges from 10.5 to 59.7%. The hydrogen index of Kiowa

B.A. Hofmann

368 Table 3. Rock·Eval data for organic matter in reduction spots

%U DWI DW4 KIOl KIOla KIOlb KIOlc KI02 KI03a KI03b KI03c KNO

Vein hydrocarbon Spheroid core Altered core Altered core Altered core Altered core Altered core Fresh core Fresh core Fresh core Spheroid core

1.1 0.2

0.5 5-10

C org

Tmax

HI

OJ

72.7 46.1 29.1 37.5 59.7 23.5 10.5 73.9 73.9 60.0 7.56

444 453 384 468 471 398 435 437 437 442 454

556 52 28 10 11 36 82 325 308 287 51

34 95 71 70 120 81 80 22 20 44 61

% U: Uranium content. Values determined on different but analogue samples. Source: DW4, Parnell (1985); KIOl, Hofmann (1991b); KI03, mean of five analyses by Curiale et al. (1983) and Hofmann (1991b); KNO, estimated from autoradiograph. Tmax: Temperature of maximum pyrolysis yield, °C. HI: Hydrogen Index, HC/Corg (mg/g). 01: Oxygen Index CO 2 /Co ,g (mg/g).

400

II 300

c

KIO Oklahoma

0

KNO Midlands

•

c

0

::I:

OW Dingwall

Corg in U deposits

200

00

100

c 0

00

o

0 0

20

0

.. r,

40

0

c

tJ

c

60

0

80

III

• 100

c 120

140

01 Fig. 2. Plot of oxygen index versus hydrogen index (Rock Eval data) for organic matter from reduction spots compared with organic matter from uranium-rich deposits (data from Leventhal et al. 1986, 1987, and Hofmann, unpubli. data from the Morrison Formation in SW Colorado and of Canadian thucholites). For comparison, evolution paths of type I, II, and III kerogens are given


369

samples unaffected by surface alteration is unusually high compared with other types of uranium-rich organic matter (Fig. 2). Because all samples were pretreated with HCI, the 01 are not due to the release of CO2 from carbonates.

Dingwall. Vein hydrocarbon (DW1) and the centers of reduction spots (DW4) from Dingwall show very different Rock-Eval characteristics. DW1 is rich in C (72.7%), has a high HI (556) and low 01 (34). DW4 is lower in C 46.1% (due to dilution by mineral constituents), has a low HI (52) and a high 01 (95) and higher Tmax. Knowle. Centers of reduction spots from the Knowle borehole are relatively low in Corg (7.6%) but show characteristics similar to the Dingwall sample DW4. The Rock-Eval characteristics of samples DW4 and KNO are similar to other types of organic matter spatially related with uranium ore (Leventhal et al. 1986, 1987). The Kiowa samples stand out relative to the others in this group because of their relatively high HI and low Tmax. This may be related to the relatively high Corg/U ratio in these samples or, alternatively, to a relatively young age resulting in less severe radiation damage. 3.5 Pyrolysis-Gas Chromatography-Mass Spectrometry Pyrolysis products identified in the different steps by Py-GC-MS are listed in Table 4. Organic matter from reduction spots always yielded pyrolysis products dominated by aromatic compounds. The samples from different localities will be discussed individually.

Kiowa Samples. The unaltered Kiowa samples (KI03) yielded mostly aromatic pyrolysis products with molecular weights up to 260. The most abundant compounds identified are benzene, toluene, xylenes, benzene-C3 to Cs , indene, naphthalene, naphtalene-C 1 to C6 , phenanthrene/anthracene, and pyrene. Surprisingly, no organic sulfur compunds were found, although the organic matter contains 0.5% S, similar to the value of 0.6% given by Curiale et al. (1983). This sulfur must be present as organic S because no sulfide minerals were present in these samples. The altered samples (KI01, 2) yielded less pyrolysis products, mostly unsubstituted aromatic compounds such as benzene, toluene, xylenes and naphthalene. Small amounts of S02, thiophene, and benzthiophene were identified in 900 °C steps. Knowle Borehole (KNO). This sample yielded mostly low-molecular aromatic molecules, traces of phenanthrene/anthracene and some alkenes. In the 6000 step, minor amounts of n-alkanes were observed. Leicester (LEe). Very similar pyrolysis products as in the Knowle borehole sample. S02, thiophene and benzthiophene were present in small amounts. An ion with m/z 300 was tentatively identified as AS4 that probably was derived from the decomposition of admixed arsenide minerals.

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B.A. Hofmann

Table 4. Pyrolysis products from stepwise pyrolysis of organic matter in reduction spheroids Figures correspond to the numbers of identified isomers Sample KI03 KIOI KNO LEC OWl OW4 WEIl076 fresh altered Temperature, DC 600 900 600 900 600 900 600 900 750 600 900 600 900 No." 1 Alkanes 2 Alkenes 3 Benzene 4 Toluene 5 Xylenes 6 Benzene-C3 7 Benzene-C4 8 Benzene-CS 9 Indene 10 Methylindene 11 Naphthalene 12 Indene-C2 13 Methylnaphthalene 14 Acenaphthalene 15 Pyrene 16 Naphthalene-C2 17 Methylbiphenyl 18 Naphthalene-C3 19 Naphthalene-C4 20 Naphthalene-C5 21 Naphthalene-C6 22 Phen/anthracene 23 Phen/an-Cl 24 Phen/an-C2 25 Pyrene-Cl 26 Pyrene-C2 27 Phenol 28 Thiophene 29 Benzthiophene

1 2 2 5 1

2 1 4

1 1 1 3 1 3 2 1 2 1 2 2

1 1

1 I

1 1 1 2 2 2

1 1

1 1 1 2 4 1

2 4

1 1 2 3

1 2 1

2 1

1 2 1

1 2 I

1 2 1

2

2

2

2

3

1 4

I

2 1

1 5

I

2 2 3

25 >25 1 1 2 1 1 2 1 2

1 1 1 1

1 1 1

1

1 1 1 2 2

1

1

4 2 1 1 1

1 2 2 1 1

2

I

1

1

1

1

"Number of corresponding peaks in Fig. 3.

Weiach 1076. This sample yielded only low-molecular weight aromatic compounds with naphthalene as the heaviest molecule. Phenol is present in significant amounts. Kai246. Very similar to Weiach 1076, with very low molecular weight compounds only (no naphthalene). Phenol is again present. Small amounts of S02 and CS2. Dingwall Samples (DW). The organic matter from the cores of reduction spots (DW4) showed low molecular weight aromatic compunds with phenol as a major component. Associated vein hydrocarbon from (DWl) yielded mostly nalkanes and n-alkenes as pyrolysis products. Aromatic compounds (benzene, toluene, substituted benzenes) were present in very small amounts only.


371

Table S. Stable Isotopes of organic matter and carbonates Organic matter KI01 KI02 DW1 DW4 Wei1076 LEC

«5\3C (PDB) -30.2 -30.0 -33.3 -49.9 -23.2 -29.0

% organic carbon 29.1 10.5 72.7 46.1 >50 22.0

Carbonates DW4red DW4halo KI02red KI02core KNO red KNOhalo EX1 red EX2red EX2core EX4core WFred WFcore KAI230red KAI217 core ZUZ227 core RIN91Ocore

«5\3C (PDB) -1.36 -1.29 -6.63 -6.07 -0.97 -0.99 -2.06 -3.08 -2.30 -2.83 -5.66 -5.14 -5.68 -5.02 -5.32 -4.76

«5180 (SMOW) +23.47 +22.66 +29.19 +27.30 +31.59 +31.79 +27.73 +27.47 +28.67 +26.27 +24.93 +25.25 +23.92 +24.87 +18.81 +24.97

Exmouth, South Devon, UK Permian red beds Weierfeld well, Switzerland Permian red beds Kaisten well, Switzerland Zuzgen well, Switzerland Riniken well, Switzerland Permian red beds

3.6 Stable Isotopes in Carbonates

Data on the stable isotopic composition of carbonates in host rocks and core material are given in Table 5. There is no systematic difference in t5 180 and t5 13C of carbonates in cores (or near-core haloes) and the red host rocks. Even in samples with Corg-rich cores, no indication of lighter carbon isotopes in the core carbonates have been found, indicating that oxidation of organic carbon is not reflected in the isotopic composition of carbonate near the cores of reduction spots. This is consistent with an increase of carbonate solubility due to carbon dioxide evolution rather than precipitation of carbonates in a carbonate-buffered system and therefore does not exclude the possibility of oxidation of organic carbon.

4 Discussion 4.1 Stable Isotf)pes in Carbonates

The absence of an isotopic signature of organic contributions to carbonate carbon in and near cores can be due to a lack of such a contribution, to a difference in time between carbonate precipitation and organic matter oxidation, to dissolution of carbonates during organic matter oxidation rather

372

B.A. Hofmann

than precipitation, and/or to the strong dilution by isotopically heavy inorganic carbon. A similar lack of isotopically light carbonates associated with V-Umineralization in the Henry Basin (Utah) is explained by dilution (Northrop et al. 1990). 4.2 Organic Geochemistry and the Origin of Organic Matter in Reduction Spots Analyses for reduced carbon clearly demonstrate that in most reduction spots organic carbon is not enriched. Samples from 18 localities showed no elevated contents of reduced carbon in the cores, while only six localities are known where reduction spots rich in organic carbon do occur (Table 1). Several reports of high organic carbon contents in reduction spots in the literature are erroneous. Care must be taken when the organic carbon content is judged from color only. The mineral roscoelite is dark green to brown (if oxidized) and occurs in the matrix of mineralized rocks, resulting in an appearance similar to that of organic-rich rocks. Because roscoe lite is considered as quite uncommon, its confusion with organic matter is understandable. The range of the isotopic composition of organic carbon in reduction spots (-23.15 to -49.9%0) indicates that organic matter of a wide range of sources was involved. Methane seems the only likely carbon source in the case of very light carbon. All samples are highly uraniferous, therefore the isotopic composition prior to a possible, radiation-induced positive shift (Leventhal and Threlkeld 1978; Forster 1986; Eakin 1989) was possibly even lighter by a few permil. Where the core organic matter is much lighter than associated vein hydrocarbon (OWl + 4), obviously the process of organic matter accumulation in the core selectively favored isotopically light compounds such as methane. Rock-Eval analyses of organic matter in reduction spots show that this organic matter has characteristics similar to organic substances found in uranium mineralization in a wide variety of geological settings: Typically, the hydrogen index is low and the oxygen index is high. The similarity with other uraniferous organic matter is shown in Fig. 2. For organic matter from reduction spots, oxygen indices are even higher than in most samples of organic matter from U-rich mineralizations. The properties of the samples from SW Oklahoma (KI01, 2) with low HI and high 01 most likely are influenced by recent oxidative degradation under near-surface conditions. This is confirmed by the presence of a small amount of base-extractable humic-type organic matter in these samples. Sample KI03 with a relatively low U content (0.5%) has a higher HI and lower 01 than samples OW4 and KNO, with U contents >1%. This is in accordance with the assumption that irradiation of organic matter lowers the hydrogen content due to release of low-molecular weight hydrocarbons and increases the oxygen content by radiation-induced oxidation (Leventhal et al. 1986). Pyrolysis-GC-MS also is indicative of a radiation-influenced nature of the organic matter. The prevalence of aromatic compounds among the pyrolysis products is typical of uranium-rich organic matter (Zumberge et al. 1978; Willingham et al. 1985; Leventhal et al. 1986, 1987). Because of this radiation damage, it is not possible to obtain molecular information about the precursor


373

7 'Il

13

KI03 9 7

900

13

°c

n

t-

Z

III

Ii: Ii: :l

o Z o

KNO

6

13

9

900°C

13 7

100

10

200

300

400

500

600

SCAN NUMBER Fig. 3. Total ion chromatogram of the 900° pyrolysis steps for two typical samples of organic matter from reduction spots. Sample KNO shows much more extensive radiation damage than sample KI03. For compound identification see Table 4

materials of this organic matter. This situation is similar to the case of organic matter from the Witwatersrand (Zumberge et al. 1978), Blind River (Willingham et al. 1985), and Cluff Lake (Leventhal et al. 1987). It is not clear whether the generally high sulfur content of the organic matter is inherited from the precursor material or whether this also is a result of radiation damage (Eakin 1989). Again, samples with lower uranium content (KI03) yielded more pyrolysis products with higher molecular weights than very U-rich samples (KNO, DW4, WEIl076), but none of these samples yielded appreciable nalkanes (Table 4, Fig. 3). The results of PY-GC-MS of sample KI03 (Table 4, Fig. 3) are very similar to samples of Kolm from the Cambrian Alum Shale of Sweden (Leventhal et al. 1986). The unmineralized vein hydrocarbon sample

374

B.A. Hofmann

DWI (associated with mineralized DW4) showed mostly n-alkanes and is typical of oil-derived solid hydrocarbons. In the light of these results, no definitive answer can be given on the origin of organic matter in reduction spots. This organic matter might be directly related to the origin of the reduction spots or it might not be related, its deposition in the cores being due to radiation-induced polymerization of mobile organics after the completion of spot formation. Considering the large percentage of Corg in some cores (Oklahoma, Dingwall) showing accretionary growth structures (Curiale et al. 1983), it seems likely that at least in these cases organic matter is directly related to the origin of the cores. The isotopically very light sample (DW4, -49.9%0) is sulfur-rich like all organic carbon samples from the cores of reduction spots (based on microprobe analyses). Light carbon is difficult to explain as a derivative from sulfur-rich crude oil. An origin of this carbon from light hydrocarbon gases seems more likely. If that is true, the high sulfur content must be due to incorporation after the immobilization from a gas phase. Considering a possible microbiological origin of reduction spots during diagenesis (Hofmann 1990, 1991a), the organic matter could represent degraded bacterial biomass. This could explain the highly variable isotopic composition, due to the use of highly specific carbon sources with distinct isotopic compositions (Freeman et al. 1990), as well as the high sulfur content, as this element is retained in organic carbon during radiation-induced alteration (Eakin 1989). The presence or absence of organic matter in the cores of reduction spots could depend on whether the conditions during its formation were in favor of conservation of organic matter or not. Most authors favored an origin of reduction spots due to detrital organic debris (e.g., Van de Poll and Sutherland 1976; Durrance et al. 1978; Mykura and Hampton 1984) or to migrated hydrocarbons (Curiale et al. 1983; Parnell and Eakin 1987). The first model cannot explain the occurrence in altered crystalline basement rocks and in Precambrian and Early Paleozoic continental red beds. Assuming that some kind of migrated organic matter was involved in the genesis, this organic matter must have been inert enough to allow migration through red beds without reacting with hematite. Figure 4 shows a V-Ni plot of organic-rich and organic-poor cores. VlNi ratios in cores are highly variable, but the highest VlNi ratios are shown by organic-rich cores. VINi ratios in crude oils are lower than 16 (Tissot and Welte 1978, Fig. IV.1.20.). Figure 4 shows that organic-rich cores from Oklahoma have V INi ratios well below 16, while most organic-poor samples have V INi ratios > 16. For organic-poor cores, a derivation of the V and Ni contents from oil is therefore unlikely. 4.3 Geological Setting of Reduction Spots Rich in Organic Matter

In order to find reasons for the presence of organic-rich reduction spots, the geological setting of these occurrences has to be taken into account, especially the association of red beds containing reduction spots with organic-rich sedi-

Organic Matter Associated with Mineralized Reduction Spots in Red Beds 40000

~----------..---------......,

•

o

30000

E

~

375

organic rich no organics

20000

•

>

o

10000

o

1000

2000

• •

3000

4000

5000

NI ppm

Fig. 4. Plot of nickel versus vanadium in cores of reduction spots. Corg-rich cores are all from Kiowa Co., Oklahoma (data from Hofmann 1991b and from Curiale 1985). Crude oils have V/Ni ratios