Organic geochemistry of the Dongsheng sedimentary uranium ore deposits, China

Applied Geochemistry Applied Geochemistry 22 (2007) 1949–1969 www.elsevier.com/locate/apgeochem

Organic geochemistry of the Dongsheng sedimentary uranium ore deposits, China Jincai Tuo a

a,*

, Wanyun Ma

a,b

, Mingfeng Zhang

a,b

, Xianbin Wang

a

Key Laboratory of Gas Geochemistry, Chinese Academy of Sciences, No. 382 Donggang West Road, Lanzhou, 730000 Gansu, China b Graduate University of the Chinese Academy of Sciences, Beijing 100039, China Received 20 December 2006; accepted 9 March 2007 Editorial handling by B.R.T. Simoneit Available online 13 May 2007

Abstract Organic matter (OM) associated with the Dongsheng sedimentary U ore hosting sandstone/siltstone was characterized by Rock-Eval, gas chromatography–mass spectrometry and stable C isotope analysis and compared to other OM in the sandstone/siltstone interbedded organic matter-rich strata. The OM in all of the analyzed samples is Type III with Ro less than 0.6%, indicating that the OM associated with these U ore deposits can be classified as a poor hydrocarbon source potential for oil and gas. n-Alkanes in the organic-rich strata are characterized by a higher relative abundance of high-molecularweight (HMW) homologues and are dominated by C25, C27 or C29 with distinct odd-to-even C number predominances from C23 to C29. In contrast, in the sandstone/siltstone samples, the n-alkanes have a higher relative abundance of medium-molecular-weight homologues and are dominated by C22 with no or only slight odd-to-even C number predominances from C23 to C29. Methyl alkanoates in the sandstone/siltstone extracts range from C14 to C30, maximizing at C16, with a strong even C number predominance, but in the organic-rich layers the HMW homologues are higher, maximizing at C24, C26 or C28, also with an even predominance above C22. n-Alkanes in the sandstone/siltstone sequence are significantly depleted in 13C relative to n-alkanes in most of the organic-rich strata. Diasterenes, bb-hopanes and hopenes are present in nearly all the organic-rich sediments but in the sandstone/siltstone samples they occur as the geologically mature isomers. All the results indicate that the OM in the Dongsheng U ore body is derived from different kinds of source materials. The organic compounds in the organic-rich strata are mainly terrestrial, whereas, in the sand/siltstones, they are derived mainly from aquatic biota. Similar distribution patterns and consistent d13C variations between n-alkanes and methyl alkanoates in corresponding samples suggest they are derived from the same precursors. The OM in the organic-rich strata does not appear to have a direct role in the precipitation of the U ore in the sandstone, but an indirect role cannot be excluded. The OM in the U hosting sandstone shows a relatively low hydrogen index, presumably due to oxidation or radiolytic damage.  2007 Elsevier Ltd. All rights reserved.

1. Introduction Organic matter (OM) has long been known to be associated with many types of mineral deposits, * Corresponding author. Tel.: +86 931 4960854; fax: +86 931 8278667. E-mail address: [email protected] (J. Tuo).

especially certain types of U deposits (Spirakis, 1996). OM is a very sensitive marker of paleoenvironment and diagenetic history as well as alteration processes. Thus, it may provide useful information that is not easily obtained through other means (Landais, 1996). The roles of organic constituents in the mobilization, reduction and concentration

0883-2927/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2007.03.060

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J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

of U have been studied in a number U deposits as well as experimentally (Rouzaud et al., 1981; Nakashima et al., 1984; Landais and Dereppe, 1985; Leventhal and Daws, 1986; Leventhal et al., 1987; Parnell and Eakin, 1987; Disnar and Sureau, 1990; Oh et al., 1990; Landais et al., 1990; Landais, 1996; Min et al., 2000). Characterization of OM in U deposits has been undertaken using various analytical techniques including infrared, 13C nuclear magnetic resonance, UV fluorescence spectroscopic methods (Landais et al., 1984; Landais and Dereppe, 1985; Turner-Peterson et al., 1993), gas chromatography (GC) and pyrolysis GC–mass spectrometry (MS) (Zumberge et al., 1978; Dahl et al., 1988), elemental analysis (Gize, 1993), and GC, pyrolysis-GC and pyrolysis-GC–MS (Landais, 1996; Min et al., 2000). In the present paper, RockEval analyses, GC–MS techniques and C isotopic compositions of individual compounds are used to characterize the origin, the maturity and the diagenetic character of OM associated with the Dongsheng sedimentary U ore deposits in China. Possible relationships between the ore and OM are also discussed. 2. Geologic setting The Ordos basin is one of the most important oil and gas producing basins in China. Most of the oil and gas fields are located in the central and southern part of the basin (Fig. 1). Identified source rocks for the petroleum and gas accumulations include

Middle-Upper Proterozoic to Lower Paleozoic marine sediments, Upper Paleozoic Coals and Mesozoic continental deposits. Most of the oil and gas production is from Jurassic, Triassic and Upper Paleozoic reservoirs. The Dongsheng sedimentary U ore deposits are restricted to the northeastern part of Ordos basin. These economically important sedimentary U ore deposits are hosted by the lower member of Jurassic Zhiluo Formation. It can be divided into upper and lower sub-members. The lower sub-member comprises a braided stream system and braided delta regime. The upper submember of the Zhiluo Formation comprises a meandering river system and meandering river delta regime (Jiao et al., 2005). Generally, the ore host rocks occur at a depth of 100–200 m with thickness ranging from 30 to 40 m. The U ore deposit host rocks are mainly sandstones and siltstones, but these are usually interbedded with lesser volumes of barren organic matter-rich mudstones, carbonaceous mudstones and coals. The coexistence of U-rich sandstones and barren but organic-rich strata is typical in the Dongsheng sedimentary U ore deposits. High U contents have been reported in many organic sediments such as peat bogs (Kochenov et al., 1965; Lopatkina, 1967; Schmidt-Collerus, 1979; Granier et al., 1979; Idiz et al., 1986; Zielinski and Meier, 1988), sapropels (Kolodny and Kaplan, 1973; Halbach et al., 1980; Johnson et al., 1987), coals (Breger et al., 1955; Ergun et al., 1960; Breger, 1974; Ilger et al., 1987), black shales (Beers, 1945;

Fig. 1. Map showing the locations of the Ordos basin, the Dongsheng sedimentary U ore deposits and sampling area.

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

Swanson, 1960; Leventhal, 1981; Coveney and Martin, 1983; Andersson et al., 1983; Dahl et al., 1988) and bitumens (Capus, 1979; Cortial, 1985; Landais and Connan, 1986). The strong association of OM with U deposits in sedimentary rocks indicates that reduction is an import chemical process in their genesis (Spirakis, 1996). Therefore, the origin and characteristics of the ore associated OM is important for understanding the types of U deposits and their precipitation mechanisms. For these reasons, samples from both ore hosting sandstones and interbedded barren but organic-rich mudstones, carbonaceous mudstones and coals were collected in the Dongsheng sedimentary U ore deposit region. A total of 17 samples of conventional core were collected from 7 wells. The well locations are shown in Fig. 1. Six samples were also collected from the corresponding host sandstones and interbedded organic-rich layers in the nearby Shenshan coal mine and Shenshangou outcrop. Outcrop samples were collected about 30–50 cm into the outcrop depending on the weathering conditions and the coal mine samples were collected from a fresh exposure in the Shenshan coal mine. During the sample collection procedures, cray response was measured for all the sampling points with a c-ray scintillometer to ascertain the ore and barren strata. 3. Experimental methods For the samples collected from outcrops, the exterior OM was removed by washing the bulk sample with chloroform three times. The exterior layers were removed mechanically and the remaining samples were retained for later use. Rock-Eval analyses were performed using the standard technique. Organic C concentrations were determined after removing carbonates from crushed samples with HCl and measuring the amount of residual C by LECO analyses. All the samples were powdered to less than 120 mesh and Soxhlet extracted with chloroform for 72 h. The extracts (chloroform asphalt ‘‘A’’) were concentrated and deasphaltened by addition of excess hexane. Saturated and aromatic hydrocarbons and non-hydrocarbons (resins) were separated from the deasphaltened samples by column chromatography on a column of neutral alumina over silica gel (approx. 4 g of each). Saturated fractions were eluted with hexane (150 mL), aromatic fractions with methylene chloride (150 mL), and non-hydrocarbon fractions with methanol (30 mL). For com-

1951

parative purposes, deuterated tetracosane (C24D50) and p-terphenyl were used as internal reference standards for saturated and aromatic hydrocarbons, respectively. For each analysis the same weight of sample was dissolved in the same volume of solvent and the same amount of internal standard added. The same volume of each solution was injected into the GC–MS and therefore the concentrations of individual compounds from sample to sample can be compared relative to the C24D50 or p-terphenyl peak area or height in the chromatograms (Tuo and Philp, 2003). The GC–MS analyses of saturated and aromatic hydrocarbon fractions were performed on a Hewlett–Packard 6890N gas chromatograph interfaced with a Hewlett–Packard 5973N mass spectrometer. The gas chromatograph was equipped with a DB5 MS fused silica capillary column (30 m · 0.25 mm) and He used as carrier gas with a flow rate of 1 mL/min. The mass spectrometer was operated with an electron impact energy of 70 eV and ion source temperature of 230 C. The GC oven temperature was isothermal for 1 min at 80 C and then programmed from 80 to 280 C at 3 C/min and isothermal for 20 min at 280 C. The GC–MS data were acquired and processed with a Hewlett–Packard Chemstation data system. The C isotope analyses of individual compounds were performed on a Delta Plus XP gas chromatography–combustion-isotope ratio mass spectrometer. The gas chromatography was performed using a Thermo Finnigan GC COMBUSTION III system equipped with a DB-5 fused silica capillary column (30 m · 0.25 mm) and He carrier gas with a flow rate of 1 mL/min. The GC oven temperature was isothermal for 1 min at 80 C and then programmed from 80 to 280 C at 3 C/min and isothermal for 20 min at 280 C. Isotopic values were calculated by integrating the m/z 44, 45 and 46 ion currents of the peaks produced by combustion (830 C) of the chromatographically separated compounds and those of CO2 standard spikes admitted at regular intervals. The reproducibility and accuracy of the analysis were evaluated routinely using laboratory standards of known d13C values (C16–C32 n-alkanes). Typically, one injection of laboratory standard was performed for every eight sample analyses. The isotope values are given with respect to the PDB standard. Based on the data, some of the analysis were duplicated or triplicated and the results presented as an average value.

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J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

4. Results 4.1. Hydrocarbon source potential and the types of OM The Rock-Eval analyses results and other basic geochemical data for the samples under study are listed in Tables 1 and 2. Total organic C (TOC) concentrations show significant differences. For example, the measured TOC concentrations range from 53.3% to 67.6% for coals, from 6.2% to 31.2% for carbonaceous mudstones and from 0.2% to 2.7% for mudstones. The TOC concentrations are much lower in all the analyzed sandstone and siltstone samples, ranging from 0.10% to 1.95%. The RockEval Hydrogen Index (HI) values for most of the analyzed samples are less than 150 mg/g with the

highest value of 304 mg/g. The Rock-Eval Tmax for most of the samples are less than 430 C indicating the organic matter in all the analyzed samples is immature to ‘‘early oil window’’ maturity. Observations on organic matter types in general (Powell, 1978, 1988) suggest that in order for a source rock to be effective, 10–20% of its OM must be Type I, or 20–30% must be Type II (Powell and Boreham, 1994). The bulk atomic H/C ratios would therefore fall in the range 0.8–0.9 (Powell, 1988) or the HI from Rock-Eval analysis above 220–300 (Powell, 1988; Hunt, 1991; Noble et al., 1991; Powell and Boreham, 1994). However, in the HI vs Tmax diagram (Fig. 2), all of the analyzed samples in this study fall in the Type III OM region, and the maturity of the OM for nearly all the samples cluster in the <0.6% Ro region. These observations indicate

Table 1 Rock-Eval analytical data of the samples studied Sample no.

Well no.

Lithology

Depth (m)

TOC (%)

Tmax (C)

S1 (mg/g)

S2 (mg/g)

S3 (mg/g)

T0401

ZKA7-0

135

20.06

428

1.58

57.28

5.86

285

29

T0402 T0403

ZKA39-14 ZKA39-14

143 180

0.10 8.49

434 432

0.04 0.22

9.12

0.13 2.94

107

130 34

T0404

ZKA95-11

154

7.64

430

0.14

10.44

4.9

136

64

T0405 T0406

ZKA95-11 ZKA139-35

181 175

0.22 14.63

437 427

0.02 0.36

0.01 30.1

0.16 6.28

4 205

72 42

T0407

ZKA139-35

176

15.48

423

0.6

31.24

6.4

201

41

T0408

ZKA139-35

205

6.15

434

0.14

5.02

1.96

81

31

T0409 T0410 T0411 T0412 T0413 T0414

ZKA183-79 ZKA183-79 ZKA183-87 ZKA183-87 ZKA183-87 ZKA341-60

165 170 124 172 175 191

1.72 0.87 0.14 1.32 61.25 12.15

426 434 356 429 431 429

0.05 0.06 0.05 0.03 0.34 0.88

2.89 0.6

168 68

1.95 21.14 37

0.67 0.36 0.31 0.6 16.48 3.38

147 34 304

38 41 221 375 26 27

T0415 T0416

ZKA341-60 ZKA341-60

246 249

2.65 31.24

424 432

0.03 0.68

4.65 31.18

1.01 8.48

175 99

38 27

T0417 T0418

ZKA341-60 Shenshan coal mine Shenshan coal mine Shenshan coal mine Shenshangou Shenshangou Shenshangou

Carbonaceous mudstone Siltstone Carbonaceous mudstone Carbonaceous mudstone Siltstone Carbonaceous mudstone Carbonaceous mudstone Carbonaceous mudstone Mudstone Mudstone Siltstone Mudstone Coal Carbonaceous mudstone Mudstone Carbonaceous mudstone Siltstone Coal

234 Mine

0.22 64.99

418 429

0.1 0.5

8.43 30.86

4.56 27.68

111 47

60 42

Coal

Mine

67.57

429

0.86

47.56

28.16

70

41

Coal

Mine

62.58

430

0.56

37.68

25.76

60

41

Coal Sandstone Sandstone

Outcrop Outcrop Outcrop

53.27 1.91 0.05

423 467 348

2.34 0.01 0.03

56.2 0.39

39.68 5.8 0.39

105 20

74 303 780

T0419 T0420 T0421 T0422 T0423

HI (mg/g, TOC)

OI (mg/g, TOC)

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

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Table 2 Basic geochemical parameters for the samples studied Sample no.

Depth (m)

TOC (%)

Bit.‘‘A’’ (%)a

‘‘A’’/TOC (%)

HC/‘‘A’’ (%)

HC/TOC (%)

T0401 T0402 T0403 T0404 T0405 T0406 T0407 T0408 T0409 T0410 T0411 T0412 T0413 T0414 T0415 T0416 T0417 T0418 T0419 T0420 T0421 T0422 T0423

135 143 180 154 181 175 176 205 165 170 124 172 175 191 246 249 234 Mine Mine Mine Outcrop Outcrop Outcrop

20.06 0.10 8.49 7.64 0.22 14.63 15.48 6.15 1.72 0.87 0.14 1.32 61.25 12.15 2.65 31.24 0.22 64.99 67.57 62.58 53.27 1.91 0.05

1.43508 0.00117 0.25818 0.33627 0.00354 0.78117 1.35172 0.21296 0.01783 0.05347 0.00430 0.07837 0.66756 0.60758 0.17540 1.10881 0.10412 0.55455 1.24070 1.05613 2.91292 0.00124 0.00111

7.1539 1.1699 3.0410 4.4014 1.6105 5.3395 8.7321 3.4628 1.0366 6.1464 3.0691 5.9374 1.0899 5.0007 6.6190 3.5493 1.3736 0.8533 1.8362 1.6876 5.4682 0.0650 2.2289

16.72 38.83 22.34 20.11 37.90 22.46 16.85 17.17 53.89 5.68 15.67 8.94 21.69 28.30 13.45 22.74 47.16 19.50 15.04 17.69 6.31 49.79 41.07

1.20 0.45 0.68 0.89 0.61 1.20 1.47 0.59 0.56 0.35 0.48 0.53 0.24 1.42 0.89 0.81 0.65 0.17 0.28 0.30 0.34 0.03 0.92

Bitumen ‘‘A’’ composition (%)a Asph.

Sat.

Arom.

NSO

38.10 58.51 49.10 48.76 42.81 31.05 39.34 51.47 11.15 81.94 79.75 83.13 45.39 57.41 68.81 51.85 18.31 41.58 54.97 51.13 50.28 46.47 56.70

9.12 21.81 8.82 10.03 23.35 9.92 7.66 4.60 32.54 2.32 9.02 3.51 3.78 13.94 4.18 6.22 4.37 5.26 4.94 4.79 2.55 38.17 26.79

7.60 17.02 13.52 10.08 14.55 12.53 9.19 12.57 21.35 3.35 6.66 5.43 17.91 14.36 9.26 16.52 42.81 14.25 10.10 12.89 3.76 11.62 14.29

45.18 2.66 28.56 31.13 19.29 46.49 43.82 31.36 34.96 12.38 4.58 7.93 32.91 14.29 17.75 25.42 34.50 38.92 29.99 31.18 43.41 3.73 2.23

a

Bit.‘‘A’’, chloroform soluble extract; Sat., saturated hydrocarbons; Arom., aromatic hydrocarbons; NSO, N, S, O compounds (‘‘resins’’); Asph., asphaltenes.

Fig. 2. Rock-Eval characteristics of the organic matter from the Dongsheng sedimentary U ore deposits. HI: hydrogen index.

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J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

that the OM associated with the Dongsheng sedimentary U ore deposits can be classified as having poor hydrocarbon source potential for oil/gas. Total extractable OM (bitumen ‘‘A’’) concentrations also show significant differences in various samples (Table 2). The measured extractable OM (bitumen ‘‘A’’) concentrations range from 0.56% to 2.91% for coals, from 0.21% to 1.44% for carbonaceous mudstones, from 0.004% to 0.18% for mudstones and from 0.001% to 0.10% for sandstones and siltstones. In the bulk composition of the bitumen ‘‘A’’, saturated and aromatic hydrocarbon fractions comprised less than 50 wt% of the total extractable OM, whereas the resin (non-hydrocarbon) and asphaltene fractions predominate, making up more than 50 wt% of the total extracts in nearly all the samples analyzed. This is also a typical characteristic indicating that the OM in the samples analyzed is in a relatively immature to early mature thermal evolutionary stage (Tissot and Welte, 1984). 4.2. Distribution of aliphatic hydrocarbons The concentrations and distribution patterns of the n-alkanes in all of the samples are summarized graphically in Fig. 3. Calculated aliphatic hydrocarbon parameters are listed in Table 3. Normal alkanes (n-alkanes) in the extracts range from C15 to C33. The concentrations and distribution patterns of the n-alkanes show significant differences in different samples. In most of the OM-rich samples (mudstones, carbonaceous mudstones and coals), n-alkanes are characterized by a higher relative abundance of HMW homologues and are dominated by C25, C27 or C29 in most cases. Straight chain n-alkanes from C23 to C29 show distinct odd-to-even C number predominances (CPI range: 1.23–4.82, OEP range: 1.42–5.50) (Table 3) indicating that the OM in all of the organic-rich samples is relatively immature to early mature. Pristane is generally much more abundant than phytane (Pr/Ph range: 1.40–22.20) and nC17 (Pr/nC17 range: 0.92– 13.6) in most of the samples which is typical for non-marine shales and coals (Koopmans et al., 1999). In contrast, in the sandstones and siltstones, the n-alkanes are characterized by a higher relative abundance of medium-molecular-weight (MMW) homologues and are dominated by C22. Straight chain n-alkanes from C23 to C29 show no, or very low, odd-to-even C number predominances (CPI range: 1.23–2.04, OEP range: 0.97–2.32) (Table 3). Most analyzed sandstone/siltstone samples have rel-

atively lower Pr/Ph ratios (Pr/Ph range: 0.45–13.75, mostly <1.0) suggesting dysoxic depositional environments during their formation (Tuo et al., 2003). The relative abundance of pristane and phytane show a larger variation (Pr/nC17 range: 0.89– 1.95, Ph/nC18 range: 0.14–0.88) indicating that the OM in sandstones/siltstones is formed in variable and complex environments. In most of the organic-rich samples, the concentrations of n-alkanes are more abundant than in U ore host sandstones/siltstones; and comparatively, the n-alkanes are also more abundant in most of the core samples (T0401–T0417) than in the mine and outcrop samples (T0418–T0423). 4.3. Distribution of methyl alkanoates An homologous series of methyl alkanoates was detected in the aromatic hydrocarbon fractions of most of the analyzed samples. The concentrations of the methyl alkanoates also show significant differences among various samples. They occur in relatively minor concentrations in most of the sandstone/siltstone samples and range from 5 to <1 lg/g TOC (Fig. 4). The concentrations of methyl esters are relatively high in most of the organic-rich sediments and range from 1.5 to as high as 500 lg/g TOC. Drastic differences can be seen in the distribution patterns of methyl esters between the sandstone/siltstone and organic-rich layers. In the sandstone/siltstone extracts they range from C14 to C30, maximizing at C16, with an even C number predominance and low concentrations >C20 (Fig. 4). In contrast, the organic-rich sediments are characterized by a higher relative abundance of HMW homologues, maximizing at C24, C26 or C28, and even C number predominance >C22. In a few samples (T0403, T0404 and T0410), the methyl esters occur with a bimodal distribution, maximizing at C14 or C16 and C26, also with an even C number predominance. 4.4. Distributions of sterenes and triterpanes Steroid hydrocarbon distributions of the different lithologies are also distinct. Most of the organic-rich sediments contained a series of diaster-13(17)-enes, ranging from C27 to C30 with C29 (20S and 20R) as the dominant peaks (Fig. 5a). This distribution is a typical characteristic of immaturity and predominance of OM originating from higher plants (Pancost and Boot, 2004). Regular C27–C29 steranes can

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

300

T0401 CM

27

27 29

50

25 23

31

0

27 29

80

23

25

29 31

20

23

25 27 29

19

25

T0408 CM

120

27

23 21

22

T0411 SS

25

T0413 CO

27

200

23

27

90

25

T0414 CM

29

22

T0417 SS

24 27

18

0.3

23 19 21

0

0.3

T0419 CO

29

31

25 23

18

0.7

23

19

20

25 23

31

25 23 27 31

25 27

T042 1 CO

29 19

23 31

0 22 T0423 SS

25

25 27

19

31 0.0

T0418 CO

27 29 31

22

29

29

10

0.0 T0422 SS

27

T0415 MS

31

25

T0420 CO

29

0.0

31

0.0

18

27

29

18

0.0

0.8

25 27

0

0.8

29

31

T041 2 MS

19 21

31

0

27

23

0

23

25

29

19 21

27 29

19 21

T0416 CM

27 25

23 25

29

0.00

31

T0409 MS 19 21

60

0 23 19 21

21

0

19

0

29

29

31

110

19

31

22

25 27

T0406 CM

0

0 T0410 MS

31

23

31

0

29

19 21

27

T0405 SS

19

19 21

27

23

0

19

2

25

T0407 CM

25

T0403 CM

31

0

23

0.15

29

0

19 21

14

27

31

T0404 CM

70

25

20

29

0

80

T0402 SS

21

21

50

22

3

25

23

1955

27 31

0

Fig. 3. Distributions and concentrations (lg/g TOC) of n-alkanes from the Dongsheng sedimentary U ore deposits. Peak numbers refer to chain length. CM, carbonaceous mudstone; SS, siltstone or sandstone; MS, mudstone; CO, coal.

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J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

Table 3 The aliphatic hydrocarbon parameters for the samples studied Sample no.

Depth (m)

Cmax

Pr/Ph

Pr/nC17

Ph/nC18

T0401 T0402 T0403 T0404 T0405 T0406 T0407 T0408 T0409 T0410 T0411 T0412 T0413 T0414 T0415 T0416 T0417 T0418 T0419 T0420 T0421 T0422 T0423

135 143 180 154 181 175 176 205 165 170 124 172 175 191 246 249 234 Mine Mine Mine Outcrop Outcrop Outcrop

C25 C22 C25 C27 C27 C27 C25 C25 C27 C22 C22 C27 C25 C27 C29 C25 C22 C25 C25 C25 C25 C22 C22

10.17 13.75 22.20 2.96 0.49 2.63 2.95 6.79 1.78 1.05 0.45 2.18 2.13 0.95 4.50 7.98 1.02 2.75 1.40 1.09 2.87 0.56 2.64

2.18 2.50 13.60 2.76 0.89 1.20 2.91 3.85 1.26 1.08 0.98 1.16 2.24 0.84 2.88 3.48 1.00 2.38 1.86 0.92 1.28 1.78 1.51

0.19 0.14 0.53 0.80 0.59 0.48 0.87 0.33 0.62 0.73 0.49 0.49 0.57 1.02 0.49 0.35 0.88 0.72 1.15 0.56 0.29 0.84 0.42

a

þ C 21 =C 22

C21 + C22/C28 + C29

0.19 0.93 0.44 8.68 0.23 0.53 0.27 0.33 0.21 0.82 0.31 0.65 0.21 0.35 0.40 1.28 0.24 0.55 0.52 3.34 0.85 26.92 0.18 0.36 0.56 1.58 0.26 0.47 0.20 0.26 0.12 1.41 1.17 8.98 0.41 1.41 0.48 1.55 0.41 1.57 0.20 0.47 0.46 6.05 0.36 4.92    ð1Þiþ1 1 C 25 þ C 27 þ C 29 þ C 31 þ C 33 C 25 þ C 27 þ C 29 þ C 31 þ C 33 C i2 þ 6C i þ C iþ2 CPI ¼ þ : : OEP ¼ 2 C 24 þ C 26 þ C 28 þ C 30 þ C 32 C 26 þ C 28 þ C 30 þ C 32 þ C 34 4C i1 þ 4C iþ1

be detected in some of the sandstone/siltstones and their distributions are comparable as shown by the mass chromatogram of m/z 217 in Fig. 5b. The sterane distributions are characterized by the predominance of biological configurations which show the following order of abundance: C27 P C29 > C28, indicating a typical mixture of contributions from aquatic and terrigenous OM. The biological steranes (20R) dominate over the 20S-isomers, with 14b and 17b-steranes at relatively lower concentrations. In general, the steroidal hydrocarbon assemblage in the sandstone/siltstones is characteristic of lipids which are contributed by a mixture of major aquatic and minor terrigenous OM and are at an early to medium stage of diagenesis (McEvoy and Maxwell, 1983; Matsumoto et al., 1987), coinciding with the interpretation of the n-alkane and methyl alkanoate distributions and in agreement with the Tmax (<435 C) from Rock-Eval pyrolysis. Hopane-type triterpanes are present in nearly all the OM-rich sediments (Fig. 6a). The immature, 22R-b, b-hopanes (C27–C31, no C28) are major (Fig. 6a) and hopenes are more dominant than the

CPIa

OEPa

3.15 1.38 2.75 3.55 2.04 3.87 4.82 2.07 3.36 1.95 1.42 3.02 1.76 3.82 3.13 3.55 1.41 1.44 1.23 1.25 3.11 1.23 1.24

3.48 1.10 3.14 3.71 2.32 4.57 5.50 2.91 4.02 1.00 1.05 3.42 1.84 4.46 4.00 4.02 0.97 1.71 1.52 1.42 3.91 1.04 1.02

a,b- or b,a-hopanes in most of the samples. Thus, the triterpene distribution also reflects immaturity in agreement with the sterene, n-alkane and methyl alkanoate distributions. While in the sandstone/siltstones, the terpane distribution patterns are marked by a predominance of the mature ab-hopanes over tri- and tetracyclic terpanes (Fig. 6b). No sterenes, 22R-b,b-hopanes and hopenes are detectable in the sandstone/siltstones, which indicates the OM is more mature than that in organic-rich strata. 4.5. Carbon isotopic compositions of n-alkanes The d13C values measured for the n-alkanes are listed in Table 4 and summarized graphically in Fig. 7a–g. Some typical m/z 44 chromatograms of GC–IRMS analyses are also given (Fig. 7h–j) to show the possible effects which could influence the accuracy of the d13C values. For most of the alkanes, the standard deviations were below 0.5&. Some greater standard deviations and variable d13C values were probably due to co-eluting peaks, UCM or small peaks (Fig. 7h–j). They were included in order

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969 12

24 26

T0401 CM

1.6

16 18

28 22 14

1.6

5

26 28 24 30

T0404 CM 16

22

28

32

26 28 24 30

T0408 CM

22

28 T0412 MS

16

80

26 30

0

28 16 T0410 26 MS 30 24 22 32

2.5

26

16

22

28 30 32

2.5

16 T0411 SS

18

22 26 30 0.0

2628 T0414 CM

50

24 30

T0415 MS

28 26 30 24

22 32

1618

32 22

0

0

T0416 CM

26

T0407 CM

0

16 18

32

24 22

30 32

24

28 30 2426 32

0

1618

500

28

T0405 SS

14

32

0

26 28 24

0.0

0

16

32

16

18

0.0

T0403 CM

16 2224 28 30

0.0

14

14

22

30 32

0

1.5

T0402 SS

1957

T0418 CO

16

26

26 T0419 CO 28

28

28 24 0

38

3032

1618 22

26

0.12

T0420 CO

2224

0.0 16

22

24

3032

24

3032

16 T0423 SS

T0422 SS

18 24 28

0.00

22

0 3.8

18

28 0

3032

0.0

24 28

Fig. 4. Distributions and concentrations (lg/g TOC) of methyl alkanoates from the Dongsheng sedimentary U ore deposits. Peak numbers refer to chain length of fatty acid constituents. CM, carbonaceous mudstone; SS, siltstone or sandstone; MS, mudstone; CO, coal.

to provide an unbiased view of the results. The major feature is a significant difference in C isotopic compositions in the n-alkanes in different samples. The isotopic values of the n-alkanes in the carbonaceous mudstone from well ZKA39-14 vary from 30.6& to 27.2&, whereas the values range from 33.7& to 30.1& in the siltstone sample from the same well (Fig. 7a). Comparatively, in the same well, the n-alkanes in the siltstone are about 2–5& depleted in 13C relative to the same corresponding

chain length n-alkanes in the carbonaceous mudstone. In well ZKA95-11 (Fig. 7b), the short chain n-alkanes (C23) in the mudstone are about 2–6& depleted in 13C relative to the same compound in the siltstone. In well ZKA139-35 (Fig. 7c), the d13C values of the n-alkanes in two carbonaceous mudstone samples showed no significant variation

1958

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

Fig. 5. Comparison showing the distribution of: (a) diaster-13(17)-enes and (b) steranes detected in a typical organic matter-rich sample (well ZKA95-11, carbonaceous mudstone, 154 m) and in a typical U-hosting sandstone sample (well ZKA183-79, sandstone, 156 m).

ranging from 29.7& to 31.9&. In well ZKA18379 (Fig. 7d), the HMW n-alkanes (>C25) in the two mudstone samples have fairly uniform d13C values of 27.5& to 31.2&, whereas the shorter chain nalkanes (
5. Discussion 5.1. Sources of organic matter The investigation of lipid biomarkers in sediments permits the determination of the relative contributions of terrigenous and aquatic OM sources from the analysis of n-alkanes and n-fatty acids (e.g., Silliman et al., 1996; Wilkes et al., 1999; Brincat et al., 2000; Fuhrmann et al., 2003). Generally, n-alkanes of terrestrial plants are characterized by strong odd predominances in the C25–C35 C-number range (Castillo et al., 1967; Rieley et al., 1991; Collister et al., 1994; Chikaraishi and Naraoka, 2003), whereas aquatic plants are characterized by an enrichment of C23 and C25 n-alkanes (Baas et al., 2000; Ficken et al., 2000). Relatively short-chain n-alkanes (C15, C17 and C19) are often attributed to algae and cyanobacteria (Han et al., 1968; Gelpi et al., 1970). The nfatty acids from higher plant wax show a distribution from C22 to C32 with a maximum at C24 or C26, whereas those from algae and bacteria range between

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

1959

Fig. 6. Comparison showing the distributions of terpanes detected in: (a) a typical organic matter-rich sample (T0412, well ZKA183-87, mudstone, 172 m), and (b) a typical U-hosting sandstone sample (well ZKA39-14, sandstone, 166 m).

C12 and C20 with a maximum at C16 (e. g., Simoneit, 1978; Xie et al., 2003). Therefore, the significant differences in the distributions of n-alkanes and methyl alkanoates between the different kinds of sediments may imply that the OM in the U ore host sandstones/siltstones and interbedded organic-rich layers is derived from drastically different kinds of source materials. The OM in the organic-rich sediments may be predominately derived from terrestrial higher plants, whereas in the sandstones/siltstones, it is mainly from aquatic biota. The lower OM contents with no or low odd-to-even C number

predominances and higher relative abundance of MMW n-alkanes (Cmax at 22) in the sandstones/ siltstones also imply that, if the OM was from terrestrial higher plants, it has probably been reworked by microorganisms. It has been reported that microbial reworking could be responsible for the transformation of pronounced odd-to-even C number preferences of HMW n-alkanes to short chain n-alkanes with no C number preference (Grimalt et al., 1988). However, no evidence from this study has indicated that the organic matter in the sandstones/siltstones has been strongly altered by microorganisms.

1960

Table 4 Carbon isotopic compositions of n-alkanes in the samples studied (d13C&, PDB) C16

T0401 T0402 T0403 T0404 T0405 T0406 T0407 T0409 T0410 T0411 T0412 T0414 T0415 T0416 T0421 T0422 T0423

28.6 ± 0.4 30.6 ± 0.4 30.7 ± 0.7 31.9 ± 0.7 29.3 ± 0.3 30.1 ± 0.3 30.2 ± 0.5 30.6 ± 0.2 30.1 ± 0.4 28.2 ± 0.1 30.3 ± 0.4 30.6 ± 0.5 30.7 ± 0.8 31.7 ± 0.8 32.3 ± 0.6 29.4 ± 0.5 30.4 ± 0.7 31.7 ± 0.7 31.2 ± 0.9 30.9 ± 0.4 29.2 ± 0.4 30.6 ± 0.1 31.9 ± 0.6 29.8 ± 0.4 32.8 ± 0.6 31.9 ± 0.3 29.7 ± 0.3 29.7 ± 0.5 31.8 ± 0.6 31.9 ± 0.4 29.2 ± 0.4 29.5 ± 0.1 29.8 ± 0.5 28.3 ± 0.3 29.3 ± 0.4 30.0 ± 0.2 29.3 ± 0.3 30.8 ± 0.6 30.2 ± 0.6 30.8 ± 0.8 30.5 ± 0.5 32.3 ± 0.7 32.3 ± 0.6 31.6 ± 0.5 28.9 ± 0.3 28.6 ± 0.2 29.2 ± 0.1 28.0 ± 0.1 28.5 32.7 ± 1.2 32.7 ± 0.7 33.4 ± 0.6 32.4 ± 1.0 32.3 ± 1.2 34.4 ± 0.7 27.4 26.7 ± 0.1 27.5 ± 0.5 26.5 ± 0.5 26.9 ± 0.5 27.0 ± 0.1 26.9 ± 0.2 28.2 ± 0.8 28.8 ± 0.3 27.4 ± 0.7 26.5 ± 0.2 30.1 ± 0.5 27.4 ± 0.8 29.0 ± 0.2 32.1 ± 0.9 33.2 ± 1.1 33.4 ± 0.7 31.4 ± 0.4 31.8 ± 1.0

135 143 180 154 181 175 176 165 170 124 172 191 246 249 Outcrop Outcrop Outcrop

C17

C18

C19

C20

C21

C22

C23

C24

C25

C26

C27

30.1 ± 0.3 30.6 ± 0.1 28.2 ± 0.2 31.4 ± 0.2 31.6 ± 0.3 31.5 ± 0.1 30.2 ± 0.2 29.6 ± 0.2 30.5 ± 0.2 31.4 ± 0.2 29.4 ± 0.2 33.4 ± 0.3 27.0 ± 0.1 27.5 ± 0.2 33.1 33.5 ± 0.2 32.3 ± 0.2

28.5 ± 0.1 33.2 ± 0.1 27.2 ± 0.1 32.2 ± 0.1 30.6 ± 0.1 30.7 ± 0.2 30.2 ± 0.1 28.5 ± 0.1 29.8 ± 0.1 31.2 29.2 ± 0.1 33.9 ± 0.1 26.0 ± 0.1 27.0 ± 0.2 27.4 ± 0.1 32.9 ± 0.2 32.4 ± 0.3

30.1 ± 0.7 30.3 ± 0.1 28.7 ± 0.1 31.8 ± 0.6 30.6 ± 0.6 31.1 31.2 ± 0.5 28.4 ± 0.1 29.4 ± 0.4 30.8 ± 0.7 27.8 ± 0.1 34.7 ± 1.2 26.6 ± 0.1 27.6 29.3 ± 0.7 33.6 ± 0.6 31.3 ± 1.1

28.6 ± 0.1 33.7 ± 0.1 27.6 ± 0.1 32.1 ± 0.1 28.3 ± 0.1 30.5 ± 0.1 30.9 ± 0.1 28.0 ± 0.1 27.5 ± 0.1 31.0 ± 0.1 27.5 ± 0.1 33.0 ± 0.1 25.9 ± 0.1 26.9 ± 0.1 28.1 ± 0.1 31.7 ± 0.2 30.3 ± 0.2

30.2 ± 0.5 32.7 ± 1.1 27.7 ± 0.5 34.9 ± 0.9 28.5 ± 0.3 32.8 ± 0.9 31.6 ± 0.8 28.0 ± 0.8 28.1 ± 0.7 31.2 ± 0.5 26.9 ± 0.6 32.7 ± 0.2 24.4 ± 0.3 27.9 ± 0.3 31.5 ± 0.2 34.6 ± 0.8 32.6 ± 1.2

29.6 ± 0.1 32.8 ± 0.2 28.0 ± 0.1 35.4 ± 0.2 28.6 ± 0.2 31.1 ± 0.2 31.7 ± 0.2 27.7 ± 0.2 27.6 ± 0.2 30.5 ± 0.1 26.9 ± 0.1 33.3 ± 0.2 26.1 ± 0.3 27.2 ± 0.2 29.1 ± 0.1

C28

C29

C30

C31

Average

30.9 ± 0.3 32.9 ± 0.3 33.7 ± 0.9 32.8 ± 0.2 30.6 32.4 ± 0.4 32.0 27.3 ± 0.7 29.0 ± 0.1 28.6 ± 0.6 30.2 ± 0.2 28.7 32.1 28.4 ± 0.8 28.5 ± 0.3 29.1 ± 1.3 30.1 ± 0.1 29.9 31.2 31.5 ± 0.3 31.9 ± 0.2 31.0 28.7 ± 0.6 28.0 ± 0.1 28.6 ± 0.8 29.2 ± 0.2 28.8 28.8 ± 0.7 28.8 ± 0.3 28.5 ± 1.3 29.3 29.9 ± 0.4 31.4 27.1 ± 0.2 28.0 ± 0.1 27.8 ± 0.3 28.5 ± 0.3 28.2 33.6 ± 0.4 33.5 ± 0.5 36.0 ± 1.7 36.1 ± 0.1 33.6 26.8 ± 0.6 26.6 ± 0.3 25.8 ± 1.3 29.1 ± 0.2 26.7 29.6 ± 1.1 29.0 ± 0.1 27.6 ± 0.8 31.1 ± 0.1 28.0 26.8 ± 0.6 30.6 ± 0.2 28.3 ± 0.8 29.2 33.0 ± 0.9 31.4 ± 0.4 32.9 31.1 ± 0.3 31.6

Steranes are derived from sterols and reflect their composition in OM of sedimentary environments, and are thus important biomarkers (Seifert and Moldowan, 1981; Mackenzie et al., 1982; de Leeuw and Baas, 1986; Matsumoto et al., 1987; Rushdi et al., 2003). Steroids are common in higher plants but also in marine microbiota (plankton) and marine organisms (Volkman, 1986), typically comprised of campesterol (C28), sitosterol (C29) and stigmasterol (C29) (Pancost and Boot, 2004). Based on this, C29 steranes in ancient sediments have been attributed to both sources (e.g., Huang and Meinschein, 1979; de Leeuw and Baas, 1986). Therefore, the distribution patterns of diaster13(17)-enes in organic-rich samples may indicate higher plant inputs. Whereas, the distributions of the steranes in the U hosting sandstones imply that the organic matter is a mixture of major aquatic and minor terrigenous plants input, coinciding with the results from the n-alkane and methyl alkanoate distributions. Hopanes are ubiquitous biological markers in fossil fuels, and their precursors are widely found in bacteria and cyanobacteria (blue-green algae), but only C30 hopane occurs in some vegetation (Ourisson et al., 1979; Philp, 1985). Tricyclic terpanes have been recognized in many oils and source rocks (e.g., Aquino Neto et al., 1982, 1983; Chicarelli et al., 1988; Tuo et al., 1999) and may be derived from bacterial or algal membrane lipids (Ourisson et al., 1982; Simoneit et al., 1990). Thus, the hopanes and tricyclic terpanes found in these samples are probably derived from various microbiota. The C isotope compositions of the n-alkanes are controlled by two factors: (1) the isotope composition of the biosynthetic precursors and (2) fractionation and exchange during OM maturation and evaporation. An increased depletion of 12C in hydrocarbons from artificially matured source rocks and oils of increasing maturity has been shown by various authors (Chung et al., 1981; Lewan, 1983). Clayton and Bjorøy (1994) reported that individual n-alkanes of North Sea (UK) oils became isotopically heavier with increasing maturity but the magnitude of the change was less than 2&. Dzou and Hughes (1993) attributed changes in the isotope composition of fluvio-detaic oils and condensates to the phenomenon of evaporative fractionation but that also cannot significantly affect the isotope ratio of individual n-alkanes. This is confirmed by the results of Dzou and Hughes

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

Sample Depth no. (m)

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

1961

Fig. 7. Plots of C isotopic compositions of n-alkanes from the Dongsheng sedimentary U ore deposits (a–g). Examples of chromatograms of m/z 44 from GC–IRMS analysis showing the possible effects which could influence the accuracy of the d13C values of n-alkanes (h–j). CM, carbonaceous mudstone; SS, siltstone or sandstone; MS, mudstone; CO, coal.

(1993) showing differences of <0.5& between the d13C values of C12–C14 n-alkanes in a condensate compared those in the residual oil. Such small differences are within the precision of the technique (Bakel et al., 1994). In this study, the C isotopic compositions of the n-alkanes show significant differencs in the different samples. This large difference cannot be due only to maturity variation because it is larger than reported by other authors

as the maximum maturity variation for a single or similar source rock (Bjorøy et al., 1994; Hall et al., 1994). Therefore, this difference is likely due to source differences. This observation also supports the assumption that the organic matter in the carbonaceous mudstone is derived mainly from terrestrial higher plants, whereas in the sandstones/siltstones, it is primarily from aquatic microbiota.

1962

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

5.2. Source of n-alkanes and methyl alkanoates In this study, methyl alkanoates can be detected in aromatic hydrocarbon fractions in most of the analyzed samples (18 of total 23 samples). But methyl esters are concentrated enough in only four samples to allow C isotopic analysis of individual compounds (T0412, T0414, T0415 and T0416). In these samples, the distribution pat-

terns of the methyl alkanoates are remarkably similar to the distributions of the n-alkanes in the same samples (Fig. 8). In all other samples, the methyl alkanoates also have similar distributions to those of the HMW n-alkanes (compare Figs. 3 and 4). The surprising similarity in distributions between n-alkanes and methyl alkanoates seems to suggest they are derived from the same precursors.

Fig. 8. Comparison showing the distributions of n-alkanes (a–d) and methyl alkanoates (e–h) in corresponding samples.

28.0 32.9 27.0 28.4 28.9 34.0 28.1 31.6 26.3 33.8 30.3 28.7 28.1 31.8 26.5 27.8 28.8 32.4 26.8 28.8 33.8 27.9 28.2 ZKA183-87 ZKA341-60 ZKA341-60 ZKA341-60 T0412 T0414 T0415 T0416

172 191 246 249

Mudstone Carbonaceous mudstone Mudstone Carbonaceous mudstone

32.6 25.7

34.0

31.1 27.2 29.6

33.8 27.0 27.4

27.0 33.2 25.6 26.9

28.0 31.4 25.2 26.9

C30 C29 C28 C27 C26 C25 C24 C23 C22 C21 C20 Lithology Depth (m) Well no.

The chemistry of the OM is sensitive to the conditions of deposition and early diagenesis. Furthermore, as a function of its origin (algal, planktonic or terrestrial), the OM will exhibit different characteristics (Landais, 1996). Fig. 10 shows the comparative distribution patterns of the organic compounds in the U-hosting sandstone and non-U OM-rich strata. Uranium hosting sandstone and non-U strata are drastically different in their organic compound distribution patterns. Differences in depth do not provide an explanation for the variations in the distributions of organic compounds. The changes seem to be related to lithologic trends rather than depth. The lithological differences reflect primarily the conditions of the depositional environment and alternately, the types of OM. Obviously, the differences in organic compound distributions between the U-hosting sandstone and non-U organic-rich strata indicate that no soluble OM migrated into the U-hosting sandstone from the interbeds. The indigenous OM in barren strata is thermally imma-

1963

Sample no.

5.3. Possible relationships between ore and OM

Table 5 Carbon isotopic compositions of methyl alkanoates in mudstone and carbonaceous mudstone samples (d13C&, PDB)

The d13C values for the methyl alkanoates are listed in Table 5. The d13C values of the n-alkanes and the methyl alkanoates in the corresponding samples are presented in Fig. 9. The results are quite interesting. For 3 samples (T0412, T0415 and T0416), the d13C values of the n-alkanes vary from 25.0& to 30.0& (Table 4, Fig. 9), whereas for sample T0414, they vary from 31.2& to 36.2& (average: 33.6&). By comparison, the d13C values of n-alkanes in sample T0414 are 5.1–6.7& depleted in 13C relative to n-alkanes in the other 3 samples. Similar to the n-alkanes, the d13C values of the methyl alkanoates of the same three samples (T0412, T0415 and T0416) also vary from 25& to 30&, but for sample T0414, they vary from 31.1& to 34.0& (average: 32.9&). The methyl alkanoates in sample T0414 are also 4.5–5.9& depleted in 13C relative to the methyl alkanoates in the other three samples (Table 5, Fig. 9). Whatever the reasons for the d13C variations of n-alkanes and methyl alkanoates in the different samples, both n-alkanes and methyl alkanoates show remarkable correlations in C isotopic compositions within the same samples. This observation suggests a product–precursor relationship between these two series of compounds, or at least, that they are derived from same C pool in biosynthesis (e.g., higher plant lipids).

Average

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

1964

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

T0412

Average:-28.2

T4140

T0412

T4140

Average:-32.9

13

C(‰, PDB)

Average:-33.6

Average:-28.0

T0415

T0416

Average:-26.7

Average:-28.0

n-Alkanes

T0415 Average:-27.0

T0416

Average:-28.4

Methyl alkanoates

Fig. 9. Comparison showing the plots of C isotopic compositions of n-alkanes and methyl alkanoates in corresponding samples.

ture, as discussed above, and therefore could not supply soluble OM to the uraniferous sandstone. The OM in the U-hosting sandstone may have imigrated from a source rock located in another part of the Ordos basin, a type with marine or mixed OM. If that is the case, the organic matter in the organicrich strata did not have an important or direct role in the precipitation of U ore in the sandstone. But an indirect role cannot be excluded, because OM alteration can create and maintain a reducing environment favorable for preconcentration of U and subsequent precipitation of U minerals (Min et al., 2000). The variability in the nature and maturity of the OM can also be attributed to radiolytic alteration in the zones of high U concentrations. But it is difficult to determine whether and to what extent such alteration occurred. Landais et al. (1990) have noted some discrepancies in structural and com-

pound distributions between barren and U-mineralized OM in the Saskatchewan U deposits, Canada. But direct effects of radiolysis cannot explain the differences observed between the barren and mineralized bitumens since variations of d13C are opposite to those which can be expected from radiolytic alteration and no significant variations of aromaticity factor and H/C atomic ratio are noticed (Leventhal and Threlkeld, 1978). Min et al. (2000) also found some differences in the pyrolysis-GC between U mineralized and unmineralized samples from paleokarst-hosted U deposits of South China. But the differences were attributed to different amounts of terrestrial OM input to the mineralized and unmineralized sample. In some cases, radiation damage has been shown to increase the aromatic C content and lower the amount of hydrocarbon pyrolysis products in the insoluble OM associated with U minerals (Leventhal and

J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

1965

Fig. 10. Comparison showing the distributions of the organic compounds between the organic-rich samples and the U-hosting sandstone samples in four typical wells. CM, carbonaceous mudstone; SS, siltstone or sandstone; MS, mudstone; CO, coal.

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J. Tuo et al. / Applied Geochemistry 22 (2007) 1949–1969

Daws, 1986). In this study, differences in the distributions of organic compounds and C isotopic compositions of n-alkanes between uraniferous sandstones/siltstones and interbedded organic-rich layers are observed. These differences cannot be attributed exclusively to different degrees of radiolytic alteration. In particular, the variations in d13C of n-alkanes between U-hosting sandstone and non-U organic-rich strata cannot be explained by radiolytic phenomena which would result in the preferential concentration of the 13C isotope in the more mature OM. Thus it is suggested that two different sources of OM are present in the Dongsheng U ore deposit. It should be noted that the OM in the U-hosting sandstone, which is a mixture of mainly aquatic and minor terrigenous detritus, has a relatively low HI index and falls in the Type III OM region in the HI vs Tmax diagram (Fig. 2). This lack of pyrolysis products (low HI) indicates a loss of organic H, presumably due to oxidation or radiolytic alteration of OM by the associated U minerals. This study can not ascertain radiolytic alteration of the OM due to the different original inputs and differences in preservation. 6. Conclusions Organic matter associated with the Dongsheng sedimentary U ore hosting sandstone/siltstone was characterized by Rock-Eval, gas chromatography– mass spectrometry and stable C isotope analysis and compared to other OM in the sandstone/siltstone interbedded organic matter-rich strata. The Organic matter in the Dongsheng U ore hosting sandstones/siltstones and interbedded barren organic-rich layers is very different in many respects. The results indicate that the OM in the sediments of the Dongsheng U ore body is derived from different sources. The organic compounds in the organic-rich sediments are derived mainly from terrestrial higher plants; whereas, in the U ore hosting sandstones/ siltstones, they are mainly from aquatic sources. Similar distribution patterns and consistent d13C signatures between n-alkanes and methyl alkanoates in the same samples suggest a common source between these two compound series. The OM in the organic-rich strata does not appear to be important in the precipitation of U ore in the sandstone, but an indirect role cannot be excluded. The OM in the U hosting sandstone has a relatively low HI index presumably due to oxidation or radiolytic damage.

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