A geochemical study of solid bitumen in an Eocene epithermal deposit; Owen Lake, British Columbia, Canada

CHEMICAL GEOLOGY ISOTOPE" GEOSCIENCE

ELSEVIER

Chemical Geology 115 (1994) 249-262

A geochemical study of solid bitumen in an Eocene epithermal deposit; Owen Lake, British Columbia, Canada M. M a s t a l e r z a, M.L. T h o m s o n u, A. Stankiewicz c, R . M . Bustin a, A.J. Sinclair" "Department of Geological Sciences, The University of British Columbia, Vancouver, B.C. V6T 1Z4, Canada bNational Research Council, Ottawa, Ont. K1A OR6, Canada CSouthern Illinois University at Carbondale, Carbondale, IL 62901-4324, USA (Received November 10, 1992; revision accepted December 22, 1993 )

Abstract

The Owen Lake deposit (British Columbia, Canada) is an epithermal polymetallic deposit related to Eocene volcanic activity and igneous intrusions. The deposit contains both solid and liquid hydrocarbons generated during early and late stages of ore deposition. GC-MS, Py-GC, Rock-Eval ®, electron microprobe as well as carbon isotope, fluid inclusion and optical techniques have been applied and the results integrated into a deposit model incorporating fluid flow, fluid source and thermal structure of the volcanic terrane. Most solid bitumen examined is of the albertite variety and is related to the latest stage of mineral deposition. Gas chromatography of saturated hydrocarbons yielded n-alkanes from C15 to C33, with a maximum at r/-C23. Steranes and diasteranes are of low abundance relative to n-alkanes, terpanes and hopanes. Regular steranes dominate over rearranged steranes. Hopanes dominate over tricyclic terpanes and gammacerane is a significant component. Aromatic compounds are very low in abundance and are represented mainly by triaromatic steroids. These results together with carbon isotope values suggest a mixed marine-terrestrial organic source of the bitumen with saline or hyper-saline conditions prevailing during deposition or very early diagenesis of the source rock. Based on the geological framework of the deposit, two possible sources of the solid bitumen together with the fluid path are suggested. 1. Introduction

The generation of hydrocarbons through hydrothermal processes has been widely discussed and documented (e.g., Kawka and Simoneit, 1987; Didyk and Simoneit, 1989; Clifton et al., 1990; Kvenvolden and Simoneit, 1990; Peter el al., 1991 ). It is a product of maturation of organic matter due to hydrothermal fluid convection generally related to magmatic or volcanic activity. Unlike sedimentary basins, where oil IRA]

generation usually occurs within temperature range of 50-120°C (e.g., Hunt, 1979; Tissot and Welte, 1984) and cracking to natural gas takes place between 150 ° and 250°C (e.g., Kartsev et al., 1971; Vassoevich et al., 1974), hydrothermal systems can generate oil even at temperature as high as 400°C (Simoneit, 1990). Furthermore, hydrothermal processes were found to generate petroleum-like products almost "instantaneously" (Simoneit and Lonsdale, 1 9 8 2 ) f r o m organic matter of any maturation level (e.g., Simoneit, 1988). As a result, immature biomarkers as well as compounds derived from highly

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M. Mastalerz et al. I Chemical Geology 115 (1994) 249-262

mature organic matter are found in hydrocarbons from hydrothermal systems. The environments in which hydrothermal oil and oil products have been documented include active deep-sea rift zones such as the Guaymas Basin, Gulf of California (Simoneit and Lonsdale, 1982; Simoneit, 1984; Peter et al., 1990), Escanabe Trough, northeast Pacific (Kvenvolden et al., 1986) and the northern Juan de Fuca Ridge, northeast Pacific (Leitch, 1991; Simoneit et al., 1992) as well as active continental geothermal systems such as the Cerro Prieto, Baja California, Mexico, (Barker, 1991), Yellowstone National Park, Wyoming, U.S.A. (Clifton et al., 1990) and the Waiotapu geothermal region in North Island, New Zealand (Czochanska et al., 1986 ). Similar to hydrocarbons generated from organic matter in sedimentary basins, geothermal characteristics of hydrothermal oils depend substantially on temperature. In high-temperature hydrothermal systems, such as the Guaymas Basin and Cape Verde Rise (North Atlantic), yield of hydrocarbons is higher than in low-temperature systems (e.g., the Atlantis II Deep, Red Sea; Simoneit et al., 1987). Polycyclic aromatic hydrocarbons (PAH) originated from pyrolytic-type reactions during a thermal event occur in high quantities in high-temperature systems (Simoneit and Lonsdale, 1982; Simoneit, 1983, 1984) whereas they are sporadic or not detectable in low-temperature systems (Simoneit et al., 1987). The Eocene Owen Lake deposit (central British Columbia, Canada) described in this study is an epithermal polymetallic deposit related to volcanic activity and igneous intrusions, containing both solid bitumen and petroleum within veins coeval with the early and late stages of ore deposition. Thus, this deposit, provides an opportunity to conduct a biomarker study on the solid bitumen and integrate these data into the deposit model of fluid flow, fluid source and thermal structure of a volcanic terrane. It also provides the opportunity to add to the understanding of the nature of hydrothermal maturation of organic matter. In our previous study on the Owen Lake deposit, we presented thermal history of bitumen-bearing veins based on opti-

cal properties and carbon isotope data of bitumen as well as fluid inclusions (Thomson et al., 1992). The purpose of this paper is to discuss the source of solid bitumen and fluid flow on the basis of organic geochemistry data and incorporate these data into the deposit model.

2. Geological setting The Owen Lake deposit is a complex epithermal vein system (Lang, 1929; Church, 1970; Hood, 1991; Leitch et al., 1991; Thomson et al., 1992), located centrally in the Intermontane Belt of British Columbia (Fig. 1 ). It is hosted by Upper Cretaceous (84.6 _+0.8 Ma, U-Pb) Tip-Top Hill Formation volcanic rocks of the Stikine Terrane (Leitch et al., 1991). Mineralization is Eocene in age, as determined by the relationship

N~IA ~1. .... :::--- "

4:2

"

v

".

"-) v , , , , ""

v

v

v

v

v v

"Ovven'

,-

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'2K'°?: :;

o

i.o

Fig. 1. a. Map of Canada showing location of British Columbia. b. Generalized geological map of British Columbia, showing location of the Owen Lake deposit. c. Geologic map of the Owen take district [after Lang (1929) and Leitch et al. (1990) ].

M. Mastalerz et aLI Chemical Geology 115 (1994) 249-262

of altered, and therefore older, dykes (51.3 +_1.8 Ma, K-Ar, whole rock) and unaltered (younger) dykes (51.9 + 1.8 Ma, K-Ar, whole rock; Leitch et al., 1992). None of the dykes appear to have local extrusive equivalents, although, 15 km to the east, Eocene volcanic flows make up much of the exposed rock (Mathews and Rouse, 1963 ). At nearby Mt. Nadina a microdiorite stock (Fig. 1 ), dated at 52.9_+2.9 Ma (K-Ar; Carter, 1981 ) intrudes a non-marine, organic-bearing argillite-greywacke unit (Lang, 1929). The sedimentary unit is limited to the margins of Nadina and Tsa-Lit Mountains and is highly indurated, which was interpreted by Lang (1929) to be the result of contact metamorphism. Lang describes a conglomerate unit within the greywacke-argillite as consisting of angular fragments of argillite with a few rounded pebbles of tuff and limestone. The source for the limestone is unknown. Lang (1929) interpreted this unit to disconformably overlie the Tip-Top Hill Formation volcanic rock and gives an age no older

than late Cretaceous, based on doubtful identification of Aspidiophyllum platanifofium. G.E. Rouse (pers. commun., 1992) after re-examining sample 64-29 described by Lang (1929, p.72a) and assigning the vague impression of fossil material as Sequoia, considers this unit to be correlatable to the Eocene fauna found 15 km to the east. Mineralization in the Owen Lake deposit consists of Ag-, Cu-, Pb- and Zn-sulphides in a gangue dominated by quartz. Carbonate and barite are restricted to veins ranging from 1 to 50 cm wide. The largest vein system (No. 3) consists of several en 6chelon veins along a strike length of 1.5 km, with a depth of at least 200 m and widths varying from 0.1 to 2.0 m. The veins are interpreted to have resulted from extensional faulting related to magmatic intrusions (Thomson and Sinclair, 1991 ). Four mineral assemblages characterize the paragenesis within the No. 3 vein (Fig. 2; Hood, 1991 ). Stages I-III are the "ore-forming" depositional stages; Stage IV is clearly younger, cross-

STAGE1

LEGEND

STAGE2

BARITE

STAGE3 STAGE4

BITUMEN

HEMA'ffTE ~

CUBIC

PYRITE

Oo

o

~

EUHEC~N.. Q ~

~

OLUFORM SPHALERITE

~

~ssr~

- ~

~ >, L9 Z

=o

251

13,N~DED

CARBONATE

BP.~CC.~'I~D SULPHIDES OJAgTZ WIIH ~ T E & NIIJMEN

Fig. 2. Cartoon representing stages of mineralization in an idealized vein, Owen Lake deposit (after Hood, 1991 ).

252

M. Mastalerzet al. / ChemicalGeology 115 (1994) 249-262

cutting all earlier stages and devoid of sulphides and sulphosalts. Stage I consists of early euhedral quartz crystals with hematite followed by tapered blades of barite with internal, discontinuous trails of bitumen and minor intergranular bitumen; pyrite and calcite are rare. Stage II consists of colloform sphalerite, pyrite and banded siderite-rhodochrosite. Stage III consists of sphalerite and pyrite similar to Stage II, followed by galena, chalcopyrite, and sulphosalts in a matrix of dominantly carbonate and quartz. Stage IV is characterized by vuggy patches of euhedral quartz and coarse blades of barite separated by intergranular bitumen and calcite (Fig. 2 ).

raphy ( P y - G C ) was carded out as above, except with an initial temperature of 20°C held for 1 min, then programmed to 300°C at 5 ° m i n - ~and held at 300°C for 15 min. The samples were held for 20 s at the pyrolysis temperature of 610 ° C. Rock-Eval ® pyrolysis was carded out using conventional techniques, described originally by Espitali6 et al. (1977), whereas Fourier transform infrared spectroscopy (FTIR) data were obtained using a Nicolet ® 710 micro-FTIR at resolution of 4 c m - ~. A gold plate was used as a background and each spectrum is a composite of 128 scans. Bands were assigned according to Painter et al. (1981) and Wang and Griffith (1985).

3. Experimental 4. Results Solid bitumen samples were mounted in Transoptic ® and polished using standard coal preparation technique as described in Bustin et al. (1985). Reflectance was determined under oil immersion at a standard wavelength of 546 nm, using a Leitz ® MPV II microscope (white light and fluorescent modes). The chemical composition was determined using a Camrca ® SX50 electron microprobe, using the light-element routine described in Bustin et al. ( 1993 ). For gas chromatography-mass spectrometry ( G C - M S ) analysis, the samples were crushed to 0.07 m m and extracted ( 4 × ) with dichloromethane (65 ml) for 10 min in a sonicator. After evaporation and concentration, the extracts were fractionated by open-column chromatography, using activated silica gel. The column was eluted with hexane, dichloromethane and hexane (9: 1 ), dichloromethane as well as dichloromethane and methanol ( 1 : 1 ), giving the saturated, aromatic and two polar fractions, respectively. The saturated and aromatic fractions were analyzed by GC-MS with a 25-m Hewlett Packard-/® column (0.2-mm i.d., film thickness 0.33/zm), initially held at 100°C for 10 min, then programmed from 100 ° to 300°C at 3 ° m i n - ~, then held at 300°C for 18 min, using a Hewlett Packard ® 5890A GC coupled to an Hewlett Packard ® 5970B MDS operated in the selected ion monitoring mode. Pyrolysis-gas chromatog-

4.1. General characteristics of hydrocarbons

The bitumen at Owen Lake commonly occurs as interstitial to euhedral, 1-4-mm-long blades of barite and patchy siderite which line and fill open spaces within brecciated and altered host rock. Less commonly bitumen occurs as < 1-mmwide black seams within altered porphyritic andesite. Megascopically all bitumen samples are dark brown or glossy black. In reflected light microscopy, two generations of bitumen are recognized. The first type exhibits smooth, non-granular and homogeneous surfaces with local shrinkage cracks. It is isotropic and shows weak to no fluorescence. Its reflectance (in oil) ranges from 0.30% to 0.35% (Fig. 3A), which corresponds to a vitrinite reflectance of 0.6% (H. Jacob et al., 1981 ) and, thus, to a mature stage with respect to oil generation (Tissot and Welte, 1984). Optical properties and insolubility in carbon disulphide (solubility <2%) indicate the bitumen is of the albertite variety (M. Jacob, 1989 ). This bitumen type is related to Stage-IV mineral deposition as documented in Hood ( 1991 ). All the bitumen of this stage is almost identical with respect to optical properties, chemical composition and carbon isotope signature (Thomson et al., 1992) and therefore the four samples selected for geochemical studies are

M. Mastalerz et al. I Chemical Geology 115 (1994) 249-262 100 80--

50

A

60--

a" ii

40--

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B o~ ella

253

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s

25

THhc/

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20--

T~-

r7 0.2

0.3

I

I O0

150

1

0.4

0.5

0.9

1.0

1.1

Fig. 3. Histogramof bitumenreflectance:(A) Stage-IVmineralization (modifiedfrom Thomsonet al., 1992); and (B) Stage-Imineralization. representative of the whole population. The second type of bitumen exhibits a granular surface and has a reflectance of 1.05% (Fig. 3B) which corresponds to a vitrinite reflectance of also 1.05% (H. Jacob et al., 1981 ). This type of bitumen is the epi-impsonite variety and is associated with Stage-I mineralization. Stage-I bitumen is very rare and very dispersed and therefore of necessity all geochemical studies were restricted to the bitumen representing Stage IV. Brine ( < 10 eq wt% NaC1) aqueous and hydrocarbon fluid inclusions occur within both Stage-I and -IV barite. The hydrocarbon inclusions are amber in colour and range in size from 1 to 30 mm. Vapour bubbles, making up 10-20 vol% of the inclusions are common. Hydrocarbon inclusions fluoresce yellow under ultraviolet and blue excitation, confirming the presence of petroleum (Burruss, 1981; Peter et al., 1990). Details of fluid inclusion studies on the Owen Lake deposit can be found in Thomson and Sinclair ( 1991 ) and Thomson et al. (1992). A summary of the fluid inclusion data is presented in Fig. 4. Fluid inclusion data suggest that the temperature and pressure conditions of Stage I are elevated compared to those of Stage IV, which corroborates the thermal maturation data of the associated solid bitumen.

T 250

Temperature ( o C )

1.2

Reflectance (%)

? 200

Fig. 4. B - T - V d i a g r a m for h y d r o c a r b o n fluid inclusions and 10 eq wt% NaC1 solution. Homogenization temperatures (TH) o f hydrocarbons (hc) and aqueous (aq) fluid inclusion are related by way o f hydrocarbon and aqueous isochore lines to indicate m a x i m u m pressure and temperature (P-T) conditions for Stage-I and -IV systems. Table 1 Chemical and Rock-Eval ® pyrolysis data on the bitumen of Stage-IV mineralization Sample H/C O/C

S/C

T~.~ Sl

$2

HI

328.9 327.4 327.4

690 6 690 5 692 0

(°c) (mgg-') (mgg-')

88-5-5 1.59 0.008 0.003 441 265 1.75 0.007 0.004 443 1-90-9 1.61 0.008 0.003 444

9,0 9.2 9.4

Ol

T~=temperature in Rock-Eval® pyrolysis at which the maximum amount of hydrocarbons is evolved; S~ = milligrams of hydrocarbons that can be thermally distilled from I g of rock; $2---milligrams of hydrocarbons generated by pyrolytic degradation of the kerogen in 1 g of the rock; HI = hydrogen index; Ol = oxygen index.

4.2. Rock-Eval ® pyrolysis and GC-MS

Rock-Eval ® pyrolysis data show that the temperature at which the maximum amount of hydrocarbons are evolved from the bitumen (/'max) is between 441 ° and 444°C (Table 1 ). This temperature indicates a mature stage with respect to oil generation (e.g., Tissot and WeRe, 1984), and generally agrees with reflectance results. The very high hydrogen index values (690-692) are comparable to oil generated from kerogen Type I (Table 1 ). Gas chromatography of the saturated hydrocarbons from the bitumen yields unimodal distribution of n-alkanes ranging from C15 to C33 , with a maximum at n-C23 (Fig. 5A). n-Alkanes above C26 are minor. No carbon number pre-

254

M. Mastalerz et al. / Chemical Geology 115 (1994) 249-262

A Ion 99

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74

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78

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C28(20R)

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<

500 Ion 218 I

66

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l

70 Time, (min.)

I

74

I

I

78

Fig. 5. Partial mass chromatograms of the saturated hydrocarbon fraction ( m / z = 99 ) (A); and the steranes (m/z = 217) (B) and m / z = 2 1 8 (C), of the bitumen extract, sample 1-90-9.

dominance occurs. Isoprenoids are of low abundance with phytane dominating over pristane ( P r / P h = 0 . 8 9 ) . G C - M S analysis of the saturated fraction indicates that sterane and diasterane contents are low in relation to n-alkanes, tricyclic terpanes and hopanes (Figs. 5 and 6). Regular steranes of which the C29 compounds are most prominent are more abundant relative to the rearranged steranes (Fig. 5B and C). The ratio of 20S/20R in C27-C29 steranes is close to 1

(0.98), which suggests a level of maturation equivalent to the peak of oil generation (Mackenzie et al., 1980). The m/z 191 mass chromatogram shows a predominance of hopanes over tricyclic terpanes of which C21 as the dominant homologue (Fig. 6A and B). A C24 tetracyclic terpane and the gammacerane are also significant components (Fig. 6A and B ). Abundant otfl hopane-type triterpanes and minor pot hopanes [ f o r C29 and C3o o t p / ( o t p . J t - p o t ) =0.91 ] suggest

M. Mastalerz et aL / Chemical Geology 115 (1994) 249-262 ==-

40000-

255

Ion 191

A

O t¢-1

< 10000

I

I

70

I

75

I

80

16000-

I

85

90

B

Ion 191

C21

t-"

C23 C24 I

¢-1

<

C 2O

C24 tetr.

4000

u

45

i

50

i

55

i

60

Time (min.)

Fig. 6. Partial masschromatogramsof the saturatedhydrocarbonfraction (m/z = 191), showing:(A) the pentacyclichopanes (G=gammacerane);and (B) the tricyclicterpanes,sample 1-90-9. an advanced catagenetic zone (e.g., Seifert and Moldowan, 1980). GC-MS analysis of the aromatic fraction of all bitumen samples detected only triaromatic steroid hydrocarbons (Fig. 7A and B). Triaromatic steroids do not show evidence of biodegradation; a C2~ homologue known to be susceptible to biodegradation (Wardroper et al., 1984) is relatively abundant (Fig. 7B ). Minimal amount of the aromatic fraction for all the bitumen samples made it impossible to determine whether triaromatic steroids were the only aromatic compounds present and, consequently, whether PAH were present or not. Because PAH originate from high-temperature, pyrolytic-type reactions (e.g., Geissman et al., 1967; Blumer, 1975; Hunt, 1979) and none of our data indicate such a hightemperature regime, the presence of these compounds in the bitumen is rather unlikely. It is noteworthy that PAH compounds have not been found in the low-temperature hydrothermal sys-

tem from the Atlantis II Deep (Simoneit et al., 1987 ) and very minor aromatic fractions do occur in solid bitumens from other ancient hydrothermal systems (Nooner et al., 1973). Pyrograms (Py-GC) of the bitumen samples reveal a series of doublets consisting of a complete range of n-alkanes and their corresponding n-alkenes (Fig. 8 ), similar to pyrograms of kerogens and asphaltenes (Horsfield, 1984; Larter, 1984). The pyrograms also show an overwhelming predominance of the aliphatic over the aromatic compounds (Fig. 8), thus corroborating the GC-MS data.

4.3. Elemental composition and carbon isotopes of the bitumen The elemental chemical composition of the bitumen of Stage IV as well as its carbon isotope composition have been presented in Thomson et

M. Mastalerz et al. / Chemical Geology 115 (1994) 249-262

256

8000 -

A

6000 O~ 0 cf.Q

<

4000 -

2000 -

I

I

30

40

I

I

50

I

60

70

I

80

Time (rain.)

B

4000

Ion 231 O~ 0 e-

3000

"0 t-

C 26+27

C

28

2000

C 21 1000

.~

C 26

i

i

!

65

70

75

i

80

Time (rain.) Fig. 7. Chromatograms of." (A) the aromatic fraction of the bitumen; and (B) aromatic steroids of the bitumen ( m / z = 231 ), sample 1-90-9.

al. (1992) and, thus, are only summarized here. Carbon isotope analyses yield a very narrow range of ~'3C from - 2 9 . 0 7 to -28.34%o. Carbon content varies from 85.11% to 86.61%, sulphur content is < 1% (Thomson et al., 1992). Atomic H/C, O / C and S/C ratios for selected bitumen samples are presented in Table 1. A1-

though hydrogen was calculated by difference and, as a result, H / C ratios may be in fact somewhat lower than those presented, they are higher than most reservoir bitumens (Rogers et al., 1974). Sulphur content is extremely low with S/ C atomic ratios lower than for most reservoir bitumen.

M. Mastalerz et aL / Chemical Geology 115 (1994) 249-262

257

t-. t-

C-

35

e"

C9 Cll C13

(13

oe t~ "o

C1

e--

C17

.o

C22

t

C27

N et~

d 2

i

0

8

16

24

32

40

d

Time (min.) Fig. 8. Pyrogram of the bitumen, sample 1-90-9.

5. Discussion and conclusions

5.1. Source of hydrocarbons The carbon isotope ratios ( - 2 9 . 0 7 to -28.34%o) dearly indicate an organic source for the bitumen (Degens, 1966; Vinogradov and Kropotova, 1968; Schidlowski et al., 1983 ). The ~$] 3C-values are significantly lower than those reported from m o d e m marine sediments (e.g., Sackett and Thomsen, 1963), which was one of the reasons why Thomson et al. (1992) suggested that the terrestrial organic matter was the source of the bitumen. Such low carbon isotope values, however, are known from numerous ancient marine sediments (e.g., Buchardt et al., 1986), and nowadays it is commonly accepted that whole-oil carbon isotopic composition cannot be used to differentiate marine and non-mafine source (Yeh and Epstein, 1981; Sofer, 1984; Peters et al., 1986). An allochthonous, terrestrial input (terrestrial plant waxes) for the bitumen studied is suggested, however, by the presence of n-alkanes with carbon numbers of >25 (e.g., Kawka and Simoneit, 1987). A terrestrial source

is also supported by the predominance of the C29 homologues of steranes (Mackenzie et al., 1984; Moldowan et al., 1985). Other geochemical characteristics such as the presence of tricyclic terpanes along with abundant hopanes and the sterane distribution (composed of C27-C29steranes) strongly suggest bacterial and algal source for the bitumen (Ourisson et al., 1982; Philp and Gilbert, 1985; Aquino Neto et al., 1989; Volkman et al., 1989; Peters and Moldowan, 1993). A saline or hyper-saline environment of deposition of the source rock is indicated by the presence of the gammacerane (Moldowan et al., 1985; ten Haven et al., 1985 ). A siliceous clastic environment rather than carbonate or evaporate environment is suggested as a hydrocarbon source environment based on very low sulphur content. Low sulphur content in hydrocarbons is believed to result from quick fixation of reduced sulphur as iron sulphides in iron-rich clastic environments (Evans et al., 1971 ). The bitumen reflectance values, Tma x and biomarker indicators (sterane and hopane ratios) of maturity of the bitumen indicate that it has reached a mature stage with respect to the

258

M. Mastalerz et al. / Chemical Geology 115 (1994) 249-262

"oil window" (Tissot and WeRe, 1984). Lack of detectable monoaromatic steroids and prominent triaromatic steroids also suggest advanced catagenetic zone (Mackenzie et al., 1981 ). The thermal maturity of the bitumen may indicate that: (1) the bitumen comes from a mature source and it was neither elevated (or elevated very little) to higher maturity level nor bio-degraded after cracking (post-oil bitumen; Curiale, 1985), or (2) the bitumen comes from an immature source and reached its present maturity after cracking (pre-oil bitumen). Strong concentrations of n-alkanes in both the extractable (Fig. 6A) and the pyrolyzable bitumen (Fig. 8 ) with no baseline "humps" in the chromatograms (Fig. 6A) suggest non-biodegraded hydrocarbons generated from a mature source rock (Curiale, 1985). Furthermore, bacterial destruction of steranes is generally associated with elimination of n-alkanes and acyclic isoprenoids (Seifert et al., 1984), which is not the case in this study. Thus, the possibility of the bitumen formation due to water washing (washing away light hydrocarbons; Evans et al., 1971 ) or biodegradation can be eliminated. Thus, this must be thermal cracking and/or gas deasphalting responsible for the bitumen formation. No carbon number predominance in chromatograms suggests influence of thermal stress on n-alkane distribution (Simoneit, 1984). Isoprenoid distribution also shows some thermal effect. In unaltered source rocks, phytane usually is much more abundant than pristane (Didyk et al., 1978; Simoneit, 1990). In most hydrothermal systems, either preferential thermal destruction of phytane or intensive generation of pristane occurs and resuits in increase in P r / P h ratio to > 1 (Simoneit et al., 1981 ). For the bitumen studied, phytane concentration is only slightly higher than pristane, thus, relative enrichment in pristane due to the thermal event might have taken place. Thermal cracking and gas deasphalting processes are closely interrelated and, as a result, it is usually difficult to separate their effects (Evans et al., 1971; Rogers et al., 1974). Consequently, reflectance and Tmax values of a bitumen cannot be used as a direct substitute for vitrinite reflectance, nevertheless these values do suggest max-

imum temperature responsible for the maturation of the bitumen of Stage IV within 80-120 ° C (Thomson et al., 1992 ). No biomarker study has been done on the hydrocarbon fluid inclusions in the barite, so the question of whether or not they originate from the same source as the bitumen remains open. The close association between the bitumen and hydrocarbon fluid inclusions may suggest that they come from the same source. The migration of fluid inclusions from the source rock probably occurred earlier than the migration of the liquid precursor of the bitumen, as suggested by the presence of hydrocarbon inclusions in barite clearly pre-dating the bitumen (Thomson et al., 1992). The production of methane and other light hydrocarbon gases is common in a hydrothermal environment (Ilchik et al., 1986; Simoneit et al., 1988 ). Differentiation in the composition of the hydrocarbon fraction, possibly due to the action of gases may have resulted in some hydrocarbons to remain in solution and some others being precipitated (Evans et al., 1971 ). Variable conditions during trapping also might have been responsible for "solidification" of some hydrocarbons only with other remaining liquid. There are two possible sources of the bitumen in the Owen Lake deposit: ( 1 ) the Cretaceous and possibly younger argillite-greywacke found on the margins of the Nadina and Tsa-Lit Mountains; and (2) the Eocene sediments associated with andesitic volcanic rocks to the southeast of the deposit. The Cretaceous argiUite-greywacke is the most probable source. It must be at least 30 Ma older than the ore deposit (Leitch et al., 1991 ), allowing time for burial and, therefore, maturation. Although the rock currently exposed is highly metamorphosed (hornfelsic facies) and such a degree of metamorphism would result in organic matter maturation well beyond the "'oil window", it is likely that outside the contact metamorphic aureole these rocks were of lower maturation, allowing hydrocarbon generation. This unit contains some limestone, which might sug-

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M. Mastalerz et al. / Chemical Geology 115 (1994) 249-262

gest a saline environment and explain the presence of gammacerane. The organic matter within the Eocene unit is brownish-black in colour and has a vitrinite reflectance (random) of 0.60% (Fig. 9). It is represented almost exclusively by homogeneous, structureless vitrinite, with low aliphatic H content as inferred from very low absorbance of aliphatic stretching modes (2750-3000-cm-' region) in FTIR spectra (Fig. 9). The present maturation of the Eocene rocks indicates the early stage of catagenesis (Tissot and Welte, 1984) and, thus, the organic matter is sufficiently mature to generate hydrocarbons. This organic matter, however, is exclusively of terrestrial origin and as such does not seem to be the source (or at least the only source) rock of the solid bitumen.

Top Hill Formation and their fluid path would be similar. The Owen Lake ores are restricted to adjacent extensional faults, suggesting that the faults acted as a focus for the fluid flow. It appears, therefore, that the organic-bearing fluids flowed down the topographic gradient as suggested by the model of Forster and Smith (1990). The fact that the bitumen of Stage-IV mineralization did not suffer any biomarker degradation as a result of subsequent flow of water suggests that once the bitumen was deposited, fluid influence was no longer important, which, in turn, suggests that the temperature decreased in the late stages of the hydrothermal event. The bitumen reflectance as well as the fluid inclusion data indicate that the thermal maturity of the bitumen in Stage-I mineralization is higher than that in Stage-IV mineralization. If Stage I represents the initial establishment of the hydrothermal cell then the temperature isograds were higher than in the final stage, which confirms the suggestion that the veins were deposited in a decreasing geothermal gradient. The role of organic matter in the genesis of ore in the Owen Lake deposit remains unclear. The organic matter in the source rock and that present in fluids might have interacted with the inorganic phase, provoking reduction and facilitating the precipitation of ore minerals (Disnar

5.2. Fluid flow path

Based on the present study, two units have to be taken into account as a possible source rock for hydrocarbons, however, flow path for the hydrothermal. Based on the present study two units have to be taken into account as a possible source rock for hydrocarbons, however, flow path for the hydrothermal fluids can be reconstructed. Both of these units occur structurally above the TipI O0 -

80

m e a n = 0.60

_

~0

(5

O_

60

I

sd = 0.06 n=25

--

03

3610

2830

2050

1270

490

Wavenurnber Q)

,o 4 0 - Q) 0_

@0

--

I 0.3

I

I 0.4

I

I 0.5

I

I 0.6

I

I 0.7

Reflectance (%)

Fig. 9. Histogramof vitrinitereflectanceand FTIR spectrumof Eocenesediments.

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and Sureau, 1990). Very low sulphur content in the solid bitumen does not favour this suggestion, however. It is more likely that its role in ore genesis was passive and limited to their common transport path.

Acknowledgments This work was funded by grants to R.M.B. and A.J.S. from the Natural Sciences and Engineering Research Council of Canada. The authors would like to thank M.A. Kruge, Southern Illinois University, for permission to use his laboratory facilities. Very thoughtful reviews by Simon George and two other reviewers immensely improved the paper.

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