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Petrography of the Shuitian bitumen deposit, Hunan Province, China
Journal of Southeast Asian Earth Sciences, Vol. 9, No. 3, pp. 221-227, 1994 Elsevier Science Ltd Printed in Great Britain
Pergamon
Petrography of the Shuitian bitumen deposit, Hunan Province, China GENG ANSONG a n d JOHN PARNELL School of Geosciences, The Queen's University of Belfast, Belfast BT7 INN, U.K.
(Received 30 September 1992; accepted for publication 7 June 1993) A~traet--Solid bitumen vein deposits were mined until recently in Shuitian, Hunan Province, China. The
Shuitian bitumen veins occur in LowerCambrian black siltstones beneath the pre-Cretaceousunconformity.The H/C ratio of the bitumen is very low, suggesting that the bitumen is very mature and can be classified as impsonite. The carbon in both the bitumen and the host rock is completelyaromatic. The minerals associated with the bitumen are quartz, calcite, pyrite and barite. Two kinds of vanadium-bearing mineral inclusions, montroseite and roscoelite,occur within the bitumen. It can be deduced that the vein must have formed within the interval from Caledonian Orogeny to Cretaceous sedimentation, although there is not enough evidenceto indicate the exact timing of the vein formation.
INTRODUCTION BITUMENvein deposits have been exploited in many parts of the world. Indeed, they are still being mined in Utah, U.S.A. (Monson and Parnell 1992), northwest China (Parnell et al. 1992) and Argentina. Almost all of the deposits are simple fracture-fillings, in relatively young rocks (Mesozoic and Cenozoic), of low-maturity bitumen which can be readily manipulated by dissolution in organic solvents. They find use particularly in sealants, blacking agents for paints and artistic materials, and as chemical feedstock. They can be easily processed because of their high purity, containing only very sparse sand grains/mudrock fragments and small traces of calcite infilling contraction cracks. We report here a bitumen deposit, from Shuitian in southeast China, in much older (Cambrian) rocks, which has consequently suffered a more complex geological history than those instances above. It is more mature, insoluble, in part mixed with other minerals, and therefore less versatile than bitumen in the younger deposits. The bitumen was extracted for use as a fuel, but mining of the deposit was suspended in 1987 due to dangerous working conditions. Our studies sought particularly to determine the paragenesis of the bitumen with the other minerals at Shuitian and hence the reason for the complexity of the deposit. REGIONAL SETTING The geology of eastern Guizhou Province/western Hunan Province is dominated by Cambrian rocks and underlying Precambrian basement, and an unconformable Cretaceous cover. The older rocks are disposed in a series of N N E - S S W folds, while the Cretaceous is mostly fiat-lying (Fig. 1). The Shuitian bitumen deposits occurs in Lower Cambrian black siltstones near to the pre-Cretaceous unconformity. The deposit occurs on the west side of a broad anticline (Fig. 1). The region has experienced three
orogenic events since these rocks were deposited (Caledonian, Indosinian, Yanshanian), of which only the effects of the Caledonian Orogeny are definitely recorded in the rocks of the Shuitian district (see below). Bitumen occurs widely in the Cambrian rocks of the region, including occurrences in the large mercury deposit at Tongren in Guizhou Province (Fig. 1) and in lead-zinc deposits of Hunan Province (Jia et al. 1990). Shuitian bitumen deposit
The deposit consists of three veins, of which two were accessed by the adits aligned at 217 ° and 221 ° above and below the road respectively (Fig. 2). These trends follow the orientation of the predominant joint set in the host rocks, and also the strike of these rocks. We are uncertain of the location of the third vein. The adits at 298 ° are probably solely for access, although some loose blocks in the spoil heap (Fig. 3) show bitumen adhering to two sets of vein surfaces at 85 ° . Unpublished records of the Survey Team for Stone Coal Resources in South China suggest that all three veins had a similar orientation. The veins are near-vertical (minimum dip 85°), and therefore at an angle of about 15° to the bedding, equivalent to the dip of the bedding. No bitumen occurs parallel to bedding, as has been observed in some other bitumen deposits in fissile host rocks (e.g. Meyerhoff 1949). Indeed bitumen and other mineralization appears to be limited to the vein structures, as none is exposed in the siltstone at the roadside, although these exposures exhibit well-developed joints parallel to the veins. The veins are known to extend to at least 1300 m length and have been penetrated to a depth of 180 m. The vein width within the mine varies from 0 to 3 m. The petrographic evidence (see below) suggests that they are related to zones of disturbance, involving comminution of the wallrock, but the surface exposure and exposures in the 217 ° adit show no evidence of shearing. However, a normal fault displacement of 5.15 m is recorded across the veins (Survey Team for Stone Coal Resources in South China, unpublished data).
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GENO ANEOblO and JOHNPARNELL quartz extracted by acid dissolution were submitted for fluid inclusion analysis using a Linkam TH600 heating-freezing stage.
Characterization The bitumen contains 51.5 wt% carbon and 1.50 wt% hydrogen, yielding an atomic hydrogen/carbon ratio of 0.35. This is characteristic of a thermally mature bitumen, classifiable as impsonite of some workers (e.g. Jacob 1989). Consequently, the bitumen is insoluble, as verified by a test of solubility in both n-pentane and chloroform. The sulphur content of the bitumen, as determined by microprobe, is 0.32-0.51%. The carbon content in the host rock is 1.84%, which is viable as a source of hydrocarbons, and therefore we assume that it was the source of the bitumen in the Shuitian deposit. Characterization by carbon-13 nuclear magnetic remanence spectroscopy (Fig. 4) shows that the carbon in both the bitumen and the host rock is completely aromatic. The aromaticity is calculated as the proportion of the total peak area which is occupied by the aromatic peak in the N M R spectrum. The very slight aliphatic signal from the Shuitian bitumen in Fig. 4 occupies less than 1% of the total area, so that the aromaticity is 1.00. The high aromaticity is consistent with the low H/C ratio. N M R aromaticities corresponding to H/C ratios for bitumens are plotted by Curiale (1986), although the plot does not extend to an aromaticity of 1.0. By contrast, a younger bitumen from Cretaceous rocks at Urho, Xinjiang Province, analysed for comparison, contains predominantly aliphatic carbon (Fig. 4). Despite the completely aromatic character of the Shuitian bitumen, an X-ray diffraction study showed that it does not have a graphitic structure. The bitumen reflectance measured in oil is 3.11 + 0.24%. Ten determinations of maximumminimum bireflectance gave a mean value of 1.61. Both
Fig. I. Regional geological map of eastern Guizhou and western Hunan Provinces, south China, showing locationof Shuitian bitumen mine and bitumen-bearing Tongren mercury mine.
BITUMEN PETROGRAPHY
Methodology Numerous specimens were collected from the spoil heaps at the mine, then cut and polished to observe petrographic relationships. Smaller specimens were prepared for scanning electron microscopy/electron microprobe analysis. Microprobe analyses were made using a JOEL 733 instrument in backscattering mode, operated at 25 kV and with a 1-micron beam diameter. Samples of bitumen and wallrock were characterized by combustion in a Perkin-Elmer 6400 elemental analyser; and carbon-13 nuclear magnetic remanence spectroscopy at the University of Durham. Samples of
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~217 °
main mine buildings s ' " 298
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Fenghuang spoil
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///i i \ ~ ~ \ \ \ ~ \ Fig. 2. Map of mine site at Shuitian.
Fig. 3. View, looking north, of spoil heaps in foreground, Shuitian mine. Note black colour of spoil due to high organic content. The spoil heaps are now used to grow red chilli peppers. Fig. 5. Hand specimen of bitumen within quartz veinrock. Field width 0.5 cm. Fig. 6. Hand specimen of veinlet showing broken wallrock cut by two phases of calcite and quartz veinlet, and bitumen masses in quartz/calcite veinrock. Field width 1/~m.
Petrography of the Shuitian bitumen deposit, Hunan Province, China
Figs 3, 5 and 6.
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GENG ANSONG a n d JOHN PARNELL
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Fig. 7. Backscattered electron micrograph of veinrock of euhedral quartz crystals, containing masses of bitumen (black). Field width 600 microns. Fig. 8. Hand specimen of bitumen between rhombs of calcite but also cut by calcite veinlet. Field width 0.5 cm. Fig. 9. Backscattered electron micrograph of veinrock composed of quartz (dark grey), calcite (light grey), bitumen (black), barite (bright), excepting masses of pyrite (P) in lower field. Field width 2000 microns. Fig. 12. Backscattered electron micrograph of bitumen (black) containing calcite veinlet and small inclusions of the vanadium oxide montroseite (arrowed). Field width 500 microns.
Petrography of the Shuitian bitumen deposit, Hunan Province, China
Shultlen souroe rock
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Urho bitumen Fa : 0 . 2 4
21o
' 2~o
1~o '
1~o'
do
'
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'
8 ppm
aromatic
allphatlc
Fig. 4. Carbon- 13 nuclear magnetic resonance spectra for bitumen and source rock at Shuitian, showing complete aromaticity (Fa = 1.0), and for comparison bitumen from Urho, Xinjiang Province, whose carbon is predominantly aliphatic, sbs = side band spinning peak.
reflectance and bireflectance values are also characteristic of highly mature bitumen.
Description Parts of the vein, the best quality material mined, are pure bitumen. However, much of the vein consists of mixtures of bitumen, quartz, calcite and fragments o f black siltstone wallrock, with traces of pyrite and barite. Quartz is the predominant mineral phase in the veins, and also occurs as thin veinlets (up to 0.5 cm wide) containing bitumen and comminuted wallrock, within
the wallrock and parallel to the main veins. The contacts between veins and waUrock are very sharp. Petrographic observations on a large number of specimens from the spoil heaps suggest the following sequence of events. (i) The precipitation of quartz in fractures, growing a s subbedral crystals across the fractures to prop them open. (ii) Precipitation of calcite infilling the space remaining in the earlier quartz veins and also forming some new veins. The calcite veinrock was porous where the calcite consisted of coalesced rhombs. (iii) Precipitation of further quartz, coeval with bitumen (Fig. 5) but prolonged for long enough to allow veining and displacement of the bitumen by the quartz. The earlier calcite veins were partially brecciated and collapsed with attached wallrock into this second stage of quartz veining, and the wallrock fragments were veined with quartz (Fig. 6). The quartz in places consists of a framework of euhedral crystals (Fig. 7). The bitumen of this phase infilled the pores of the rhomboid calcite (Fig. 8) and also formed large pure vein-fillings which were the object of the mining activity. A stockpile of mined bitumen showed it to be pure, with no mineral admixture. (iv) Remaining pore space in the second stage of quartz veining with bitumen was filled with calcite and limited barite and pyrite (Fig. 9). Calcite also veined through the wallrock fragments and the bitumen, causing some displacement of them. This is the simplest interpretation of the observations. The two phases of calcite could not be distinguished by their cathodoluminescence (which was uniformly mid to bright orange), but there are undoubtedly two phases as a cross-cutting relationship can be seen (Fig. 6). The paragenetic relationships are summarized in Fig. 10. The quartz contains only very sparse fluid inclusions, which are simple two-phase (fluid, vapour). Homogenization temperatures are summarized in Fig. 11. They span the range 170-240°C, with irregular distribution at three distinct temperatures, although each measurement was made from a different crystal.
Time
Minerals
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Paragenetic sequence
quartz calcite bitumen barite pyrite
Fig. 10. Paragenesis of bitumen and other minerals in veins at Shuitian.
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GENG ANSONG and JorrN PARNELL
SHUITIAN
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HOMOGENIZATION TEMPERATURE (*C) Fig. 11. Histogram of fluid inclusion homogenization temperatures from quartz in bitumen-bearing veinrock, Shuitian.
Vanadium enrichment
The bitumen contains large numbers of mineral inclusions up to 10 x 2 microns in size (Fig. 12). They appear variably lath-shaped to equidimensional, which suggests a range of cross-sections through randomlyorientated elongate crystals. Microprobe analyses show that the inclusions are vanadium-bearing minerals. Most consist predominantly of vanadium, with a trace of iron (Table 1) and are probably oxides. Based on the weight percentage values of vanadium measured, stoichiometric calculation for the mass of the common vanadium mineral montroseite VOOH would yield values of 95%. Our experience is that mineral inclusions of this size rarely yield totals exceeding 90% because of beam penetration to the carbon beyond the inclusion. The substitution by iron in montroseite is normal (Weeks et al. 1953), as iron similarly forms phases of composition Table 1. Typical analysis (wt%) of vanadium minerals in bitumen samples from Shuitian, Hunan, China 1
2
3
4
V Fe Si AI K Ca Cu Mg Ni Zn O* H*
6.64 1.77 11.77 4.57 3.45 0.08 0.00 1.54 0.23 0.07 26.81 0.28
7.47 1.67 13.39 5.13 4.08 0.13 0.00 1.47 0.00 0.04 30.50 0.32
44.29 12.42 0.20 0.00 0.00 0.43 0.27 0.11 0.00 0.71 35.53 1.10
47.06 4.81 0.16 0.10 0.01 0.28 0.15 0.12 0.30 0.09 32.90 1.02
Total
57.23
64.20
95.06
87.00
1,2: roscoelite 3,4: montroseite *O,H calculated by stoichiometry
FeOOH, and indeed there are several published analyses of iron oxides which include traces of vanadium (Shieh and Duedall 1988, Jedwab et al. 1989, Kribek et al. 1992). The iron substitution in the Shuitian montroseite reaches (V0.sFe0.2)OOH. Some other very rare vanadium oxides are known, but we cannot confidently distinguish them from montroseite by microprobe. However, only montroseite regularly exhibits substitution by iron for vanadium, and we are confident that this is the identity of the oxide phase. In addition to the oxide phase, rare occurrences of a vanadium aluminosilicate mineral have been founded as inclusions, of similar size to the oxide. Analyses of the aluminosilicate show that it also contains potassium as a cation, and is referable to the vanadium mica roscoelite, which has a variable composition. Analyses listed in Table 1 yield a formula of Ko.66(V,AI,Mg,Fe)l.TIAISi3010(OH)2, comparable to the generalised roscoelite formula of K(V,AI)E,AISi3Ot0(OH):. Other analyses of roscoelite also show some deficiency in the potassium content (Parnell 1988). The vanadium/silicon atomic ratio of roscoelite ranges from 0.11 to 0.74 (Wells and Brannock 1946, Fisher et al. 1947): the roscoelite inclusions from Shuitian yield a ratio of 0.31. No vanadium minerals have been found in the black organic-rich wallrock incorporated into the veins. DISCUSSION Only limited constraints can be placed upon the timing of vein formation. The region has suffered three orogenic events: Caledonian (Silurian-Devonian), Indosinian (Triassic) and Yanshanian (Jurassic-Cretaceous). The Cambrian country rocks were folded during the Caledonian Orogeny. The veins definitely post-date this
Petrography of the Shuitian bitumen deposit, Hunan Province, China folding, as they are near-vertical while the country rocks at the mine dip at 14° (Fig. 2). There are no younger rocks in the vicinity of Shuitian to determine the effects of the younger orogenies in this district. Further east in Hunan Province, Carboniferous to Jurassic rocks are folded beneath the fiat-lying Cretaceous, but it is possible that the Precambrian and Cambrian rocks near Shuitian behaved as a rigid block which was relatively undisturbed. At some stage, the viscous liquid bitumen which entered the fractures was dehydrogenated and completely devolatilised to its present completely aromatic state. This could have occurred very early if the fracture-filling was a late stage event in the Caledonian orogenic activity, or may possibly have been due to heating during a subsequent orogeny. There has not been adequate burial/ heating since the Cretaceous to achieve this. Consequently vein formation must have occurred within the interval from the Caledonian Orogeny to Cretaceous sedimentation. The high fluid inclusion temperatures of up to 240°C are consistent with the high maturity of the organic material: they are sufficiently great to have realized all of the hydrocarbon potential of the source rock. Regional studies suggest that the Lower Cambrian would have been buried to a 2 or 3 km depth before Caledonian orogenic activity, and therefore may have entered the oil window, but not released the bulk of their hydrocarbon potential. The temperatures are most likely to have been achieved during the Caledonian Orogeny, as suggested above. They are higher than have been recorded from other mined bitumen vein deposits, in Utah (92-109°C; Monson and Parnell 1992) and Oklahoma (125-150°C; B. Monson pers. comm.). The higher temperatures also explain the presence of quantities of quartz much greater than occur in other bitumen vein deposits. The quartz would have been mobilized from the country rocks and reprecipitated in fracture systems. The generation of hydrocarbons during/following orogenic activity is a natural consequence of the marked increase in geothermal gradient and/or crustal thickening. Oliver (1986) has emphasized the significance of continental collision in driving large-scale fluid flow, including hydrocarbons. The bitumen veins at Shuitian may represent hydrocarbon-rich fluids channelled along fracture pathways following an orogeny. There are many known examples of vanadiferous bitumens (McKinstry 1957, Hess 1922, Kribek et al. 1993), although the Shuitian deposit has not previously been recorded as vanadiferous. In immature bitumens and oils, the vanadium may be entirely organicallybound, but in more mature samples the vanadium occurs as mineral inclusions. The vanadium mineral may be a sulphide, oxide or aluminosilicate. As the Shuitian bitumen has a low sulphur content, we would not expect the vanadium to occur as a sulphide. The predominance of oxide inclusions over aluminosilicate inclusions suggests that aluminium and/or silicon were not available in significant quantities in the hydrocarbon-bearing fluid (note no clays were encountered in the gangue minerals of the vein). The lack of vanadium minerals in the organicrich wallrock fragments, in contrast to their abundance in
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the bitumen, suggests that an organic-rich environment alone was not the control on vanadium precipitation. It is likely that vanadium became progressively concentrated in the bitumen with maturation, as the metal tends to be related to the heavy asphaltene component (Reynolds et al. 1984) which becomes more abundant as lighter components are driven off by devolatilization. Initially the vanadium would be bonded through heteroatoms (O,S,N) (Reynolds et al. 1984) but ultimately, when the bitumen is effectively carbonized, the vanadium would precipitate out as an inorganic mineral. Acknowledgements--We are grateful for technical support from the
Queen's University Electron Microscope Unit and G. Alexander for cartographic services. The reflectance values were kindly supplied by B. Monson. The NMR analyses were undertaken at the University of Durham through an allocation granted by the S.E.R.C. The work was undertaken while Geng Ansong was in receipt of a Research Fellowship of the Royal Society. Professor Liu Dehan, Institute of Geochemistry, Chinese Academy of Sciences, provided help for the field work. We are grateful to a reviewer whose expert comments helped to clarify the manuscript.
REFERENCES Curiale, J. A. 1986. Origin of solid bitumens, with emphasis on biological marker results. Org. Geochem. 10, 559-580. Fischer, R. P., Hall, J. C. and Rominger, J. F. 1947. Vanadium deposits near Placerville, San Miguel County, Colorado. Colorado Scient. Soc. Proc. 15, 115-134. Hess, F. L. 1922. Uranium-beating asphaltite sediments of Utah. Engng Mining J. 114, 272-276 Jacob, H. 1989. Classification, structure, genesis and practical important of natural solid oil bitumen (migrabitumen). Int. Coal Geol. 11, 65-79. Jedwab, J., Blanc, G. and Bouleque, J. 1989. Vanadiferous minerals from the Nerews Deep, Red Sea. Terra Nova 1, 188-194. Jia, R., Liu, D. and Fu, J. 1990. Relationships between organic matter and metalliferous deposits in Lower Palaeozoic carbonate formations in China. Spec. Publ. Int. Assoc. Sediment. 11, 193-202. Kribek, B., Holubar, V., Parnell, J., Pouba, Z. and Hladikova, J. 1993. Thermal mesophase in vanadiferous bitumens from Upper Proterozoic lava flows, Mitov, Czechoslovakia. In: Bitumens in Ore Deposits (Edited by Parnell, J., Landais, P. and Kucha, H.). Springer, Heidelberg. McKinstry, H. 1957. El vanadio en el Peru (review). Econ. Geol. 52, 324-325. Meyerhoff, H. A. 1949. The occurrence and mining of solid bitumens in western Argentina. Trans. Amer. Inst. Min. Eng. 181,403-412. Monson, B. and Parnell, J. 1992. The origin of gilsonite vein deposits in the Uinta Basin, Utah. In: Hydrocarbon and Mineral Resources of the Uinta Basin, Utah and Colorado (Edited by Fouch T. D., Nuccio, V. F. and Chidsey T. C.), pp. 257-270. Utah Geological Association Guidebook 20. Oliver, J. 1986. Fluids expelled tectonically from orogenic belts: their role in hydrocarbon migration and other geologic phenomena. Geology 14, 99-102. Parnell, J. 1988. The mineralogy of red bed uranium-vanadium mineralization in the Permo-Triassic of Belfast. Irish. J. Earth Sci. 9, 119-124. Parnell, J., Geng, A., Fu, J. and Sheng, G. 1993. Geology of bitumen vein deposits of Ghost City, Junggar Basin, NW China. (submitted). Reynolds, J. G., Briggs, W. R., Fetzer, J. C., Gallegos, E. J., Fish, R. H., Komlenic, J. J. and Wines, B. K. 1984. Molecular characterization of vanadyl and nickel non-porphyrin compounds in heavy crude petroleums and residua. In: Characterization of Heavy Crude Oils and Petroleum Residues, pp. 153-157. Editions Techip, Paris. Shieh, C. S. and Duedall, I. W. 1988. Role of amorphous ferric oxyhydroxide in removal of anthropogenic vanadium from seawater. Marine Chem. 25, 121-139. Weeks, D. A., Cisney, E. A. and Sherwood, A. M. 1953. Montroseite, a new vanadium oxide from the Colorado Plateaus. Amer. Mineral. 35, 1235-1241. Wells, R. C. and Brannock, W. W. 1946. The composition of roseoelite. U.S. Geol. Surv. Bull. 1009-BV, 1-62.
Pergamon
Petrography of the Shuitian bitumen deposit, Hunan Province, China GENG ANSONG a n d JOHN PARNELL School of Geosciences, The Queen's University of Belfast, Belfast BT7 INN, U.K.
(Received 30 September 1992; accepted for publication 7 June 1993) A~traet--Solid bitumen vein deposits were mined until recently in Shuitian, Hunan Province, China. The
Shuitian bitumen veins occur in LowerCambrian black siltstones beneath the pre-Cretaceousunconformity.The H/C ratio of the bitumen is very low, suggesting that the bitumen is very mature and can be classified as impsonite. The carbon in both the bitumen and the host rock is completelyaromatic. The minerals associated with the bitumen are quartz, calcite, pyrite and barite. Two kinds of vanadium-bearing mineral inclusions, montroseite and roscoelite,occur within the bitumen. It can be deduced that the vein must have formed within the interval from Caledonian Orogeny to Cretaceous sedimentation, although there is not enough evidenceto indicate the exact timing of the vein formation.
INTRODUCTION BITUMENvein deposits have been exploited in many parts of the world. Indeed, they are still being mined in Utah, U.S.A. (Monson and Parnell 1992), northwest China (Parnell et al. 1992) and Argentina. Almost all of the deposits are simple fracture-fillings, in relatively young rocks (Mesozoic and Cenozoic), of low-maturity bitumen which can be readily manipulated by dissolution in organic solvents. They find use particularly in sealants, blacking agents for paints and artistic materials, and as chemical feedstock. They can be easily processed because of their high purity, containing only very sparse sand grains/mudrock fragments and small traces of calcite infilling contraction cracks. We report here a bitumen deposit, from Shuitian in southeast China, in much older (Cambrian) rocks, which has consequently suffered a more complex geological history than those instances above. It is more mature, insoluble, in part mixed with other minerals, and therefore less versatile than bitumen in the younger deposits. The bitumen was extracted for use as a fuel, but mining of the deposit was suspended in 1987 due to dangerous working conditions. Our studies sought particularly to determine the paragenesis of the bitumen with the other minerals at Shuitian and hence the reason for the complexity of the deposit. REGIONAL SETTING The geology of eastern Guizhou Province/western Hunan Province is dominated by Cambrian rocks and underlying Precambrian basement, and an unconformable Cretaceous cover. The older rocks are disposed in a series of N N E - S S W folds, while the Cretaceous is mostly fiat-lying (Fig. 1). The Shuitian bitumen deposits occurs in Lower Cambrian black siltstones near to the pre-Cretaceous unconformity. The deposit occurs on the west side of a broad anticline (Fig. 1). The region has experienced three
orogenic events since these rocks were deposited (Caledonian, Indosinian, Yanshanian), of which only the effects of the Caledonian Orogeny are definitely recorded in the rocks of the Shuitian district (see below). Bitumen occurs widely in the Cambrian rocks of the region, including occurrences in the large mercury deposit at Tongren in Guizhou Province (Fig. 1) and in lead-zinc deposits of Hunan Province (Jia et al. 1990). Shuitian bitumen deposit
The deposit consists of three veins, of which two were accessed by the adits aligned at 217 ° and 221 ° above and below the road respectively (Fig. 2). These trends follow the orientation of the predominant joint set in the host rocks, and also the strike of these rocks. We are uncertain of the location of the third vein. The adits at 298 ° are probably solely for access, although some loose blocks in the spoil heap (Fig. 3) show bitumen adhering to two sets of vein surfaces at 85 ° . Unpublished records of the Survey Team for Stone Coal Resources in South China suggest that all three veins had a similar orientation. The veins are near-vertical (minimum dip 85°), and therefore at an angle of about 15° to the bedding, equivalent to the dip of the bedding. No bitumen occurs parallel to bedding, as has been observed in some other bitumen deposits in fissile host rocks (e.g. Meyerhoff 1949). Indeed bitumen and other mineralization appears to be limited to the vein structures, as none is exposed in the siltstone at the roadside, although these exposures exhibit well-developed joints parallel to the veins. The veins are known to extend to at least 1300 m length and have been penetrated to a depth of 180 m. The vein width within the mine varies from 0 to 3 m. The petrographic evidence (see below) suggests that they are related to zones of disturbance, involving comminution of the wallrock, but the surface exposure and exposures in the 217 ° adit show no evidence of shearing. However, a normal fault displacement of 5.15 m is recorded across the veins (Survey Team for Stone Coal Resources in South China, unpublished data).
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GENO ANEOblO and JOHNPARNELL quartz extracted by acid dissolution were submitted for fluid inclusion analysis using a Linkam TH600 heating-freezing stage.
Characterization The bitumen contains 51.5 wt% carbon and 1.50 wt% hydrogen, yielding an atomic hydrogen/carbon ratio of 0.35. This is characteristic of a thermally mature bitumen, classifiable as impsonite of some workers (e.g. Jacob 1989). Consequently, the bitumen is insoluble, as verified by a test of solubility in both n-pentane and chloroform. The sulphur content of the bitumen, as determined by microprobe, is 0.32-0.51%. The carbon content in the host rock is 1.84%, which is viable as a source of hydrocarbons, and therefore we assume that it was the source of the bitumen in the Shuitian deposit. Characterization by carbon-13 nuclear magnetic remanence spectroscopy (Fig. 4) shows that the carbon in both the bitumen and the host rock is completely aromatic. The aromaticity is calculated as the proportion of the total peak area which is occupied by the aromatic peak in the N M R spectrum. The very slight aliphatic signal from the Shuitian bitumen in Fig. 4 occupies less than 1% of the total area, so that the aromaticity is 1.00. The high aromaticity is consistent with the low H/C ratio. N M R aromaticities corresponding to H/C ratios for bitumens are plotted by Curiale (1986), although the plot does not extend to an aromaticity of 1.0. By contrast, a younger bitumen from Cretaceous rocks at Urho, Xinjiang Province, analysed for comparison, contains predominantly aliphatic carbon (Fig. 4). Despite the completely aromatic character of the Shuitian bitumen, an X-ray diffraction study showed that it does not have a graphitic structure. The bitumen reflectance measured in oil is 3.11 + 0.24%. Ten determinations of maximumminimum bireflectance gave a mean value of 1.61. Both
Fig. I. Regional geological map of eastern Guizhou and western Hunan Provinces, south China, showing locationof Shuitian bitumen mine and bitumen-bearing Tongren mercury mine.
BITUMEN PETROGRAPHY
Methodology Numerous specimens were collected from the spoil heaps at the mine, then cut and polished to observe petrographic relationships. Smaller specimens were prepared for scanning electron microscopy/electron microprobe analysis. Microprobe analyses were made using a JOEL 733 instrument in backscattering mode, operated at 25 kV and with a 1-micron beam diameter. Samples of bitumen and wallrock were characterized by combustion in a Perkin-Elmer 6400 elemental analyser; and carbon-13 nuclear magnetic remanence spectroscopy at the University of Durham. Samples of
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~217 °
main mine buildings s ' " 298
Joints
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Fenghuang spoil
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///~l~ f~ ~\,, ",~"
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///i i \ ~ ~ \ \ \ ~ \ Fig. 2. Map of mine site at Shuitian.
Fig. 3. View, looking north, of spoil heaps in foreground, Shuitian mine. Note black colour of spoil due to high organic content. The spoil heaps are now used to grow red chilli peppers. Fig. 5. Hand specimen of bitumen within quartz veinrock. Field width 0.5 cm. Fig. 6. Hand specimen of veinlet showing broken wallrock cut by two phases of calcite and quartz veinlet, and bitumen masses in quartz/calcite veinrock. Field width 1/~m.
Petrography of the Shuitian bitumen deposit, Hunan Province, China
Figs 3, 5 and 6.
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GENG ANSONG a n d JOHN PARNELL
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Fig. 7. Backscattered electron micrograph of veinrock of euhedral quartz crystals, containing masses of bitumen (black). Field width 600 microns. Fig. 8. Hand specimen of bitumen between rhombs of calcite but also cut by calcite veinlet. Field width 0.5 cm. Fig. 9. Backscattered electron micrograph of veinrock composed of quartz (dark grey), calcite (light grey), bitumen (black), barite (bright), excepting masses of pyrite (P) in lower field. Field width 2000 microns. Fig. 12. Backscattered electron micrograph of bitumen (black) containing calcite veinlet and small inclusions of the vanadium oxide montroseite (arrowed). Field width 500 microns.
Petrography of the Shuitian bitumen deposit, Hunan Province, China
Shultlen souroe rock
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A
'turn.°
sbs
sbs
Urho bitumen Fa : 0 . 2 4
21o
' 2~o
1~o '
1~o'
do
'
4'o
'
8 ppm
aromatic
allphatlc
Fig. 4. Carbon- 13 nuclear magnetic resonance spectra for bitumen and source rock at Shuitian, showing complete aromaticity (Fa = 1.0), and for comparison bitumen from Urho, Xinjiang Province, whose carbon is predominantly aliphatic, sbs = side band spinning peak.
reflectance and bireflectance values are also characteristic of highly mature bitumen.
Description Parts of the vein, the best quality material mined, are pure bitumen. However, much of the vein consists of mixtures of bitumen, quartz, calcite and fragments o f black siltstone wallrock, with traces of pyrite and barite. Quartz is the predominant mineral phase in the veins, and also occurs as thin veinlets (up to 0.5 cm wide) containing bitumen and comminuted wallrock, within
the wallrock and parallel to the main veins. The contacts between veins and waUrock are very sharp. Petrographic observations on a large number of specimens from the spoil heaps suggest the following sequence of events. (i) The precipitation of quartz in fractures, growing a s subbedral crystals across the fractures to prop them open. (ii) Precipitation of calcite infilling the space remaining in the earlier quartz veins and also forming some new veins. The calcite veinrock was porous where the calcite consisted of coalesced rhombs. (iii) Precipitation of further quartz, coeval with bitumen (Fig. 5) but prolonged for long enough to allow veining and displacement of the bitumen by the quartz. The earlier calcite veins were partially brecciated and collapsed with attached wallrock into this second stage of quartz veining, and the wallrock fragments were veined with quartz (Fig. 6). The quartz in places consists of a framework of euhedral crystals (Fig. 7). The bitumen of this phase infilled the pores of the rhomboid calcite (Fig. 8) and also formed large pure vein-fillings which were the object of the mining activity. A stockpile of mined bitumen showed it to be pure, with no mineral admixture. (iv) Remaining pore space in the second stage of quartz veining with bitumen was filled with calcite and limited barite and pyrite (Fig. 9). Calcite also veined through the wallrock fragments and the bitumen, causing some displacement of them. This is the simplest interpretation of the observations. The two phases of calcite could not be distinguished by their cathodoluminescence (which was uniformly mid to bright orange), but there are undoubtedly two phases as a cross-cutting relationship can be seen (Fig. 6). The paragenetic relationships are summarized in Fig. 10. The quartz contains only very sparse fluid inclusions, which are simple two-phase (fluid, vapour). Homogenization temperatures are summarized in Fig. 11. They span the range 170-240°C, with irregular distribution at three distinct temperatures, although each measurement was made from a different crystal.
Time
Minerals
225
Paragenetic sequence
quartz calcite bitumen barite pyrite
Fig. 10. Paragenesis of bitumen and other minerals in veins at Shuitian.
226
GENG ANSONG and JorrN PARNELL
SHUITIAN
0
I
I
I
I
160
170
180
190
I
200
210
220
230
240
HOMOGENIZATION TEMPERATURE (*C) Fig. 11. Histogram of fluid inclusion homogenization temperatures from quartz in bitumen-bearing veinrock, Shuitian.
Vanadium enrichment
The bitumen contains large numbers of mineral inclusions up to 10 x 2 microns in size (Fig. 12). They appear variably lath-shaped to equidimensional, which suggests a range of cross-sections through randomlyorientated elongate crystals. Microprobe analyses show that the inclusions are vanadium-bearing minerals. Most consist predominantly of vanadium, with a trace of iron (Table 1) and are probably oxides. Based on the weight percentage values of vanadium measured, stoichiometric calculation for the mass of the common vanadium mineral montroseite VOOH would yield values of 95%. Our experience is that mineral inclusions of this size rarely yield totals exceeding 90% because of beam penetration to the carbon beyond the inclusion. The substitution by iron in montroseite is normal (Weeks et al. 1953), as iron similarly forms phases of composition Table 1. Typical analysis (wt%) of vanadium minerals in bitumen samples from Shuitian, Hunan, China 1
2
3
4
V Fe Si AI K Ca Cu Mg Ni Zn O* H*
6.64 1.77 11.77 4.57 3.45 0.08 0.00 1.54 0.23 0.07 26.81 0.28
7.47 1.67 13.39 5.13 4.08 0.13 0.00 1.47 0.00 0.04 30.50 0.32
44.29 12.42 0.20 0.00 0.00 0.43 0.27 0.11 0.00 0.71 35.53 1.10
47.06 4.81 0.16 0.10 0.01 0.28 0.15 0.12 0.30 0.09 32.90 1.02
Total
57.23
64.20
95.06
87.00
1,2: roscoelite 3,4: montroseite *O,H calculated by stoichiometry
FeOOH, and indeed there are several published analyses of iron oxides which include traces of vanadium (Shieh and Duedall 1988, Jedwab et al. 1989, Kribek et al. 1992). The iron substitution in the Shuitian montroseite reaches (V0.sFe0.2)OOH. Some other very rare vanadium oxides are known, but we cannot confidently distinguish them from montroseite by microprobe. However, only montroseite regularly exhibits substitution by iron for vanadium, and we are confident that this is the identity of the oxide phase. In addition to the oxide phase, rare occurrences of a vanadium aluminosilicate mineral have been founded as inclusions, of similar size to the oxide. Analyses of the aluminosilicate show that it also contains potassium as a cation, and is referable to the vanadium mica roscoelite, which has a variable composition. Analyses listed in Table 1 yield a formula of Ko.66(V,AI,Mg,Fe)l.TIAISi3010(OH)2, comparable to the generalised roscoelite formula of K(V,AI)E,AISi3Ot0(OH):. Other analyses of roscoelite also show some deficiency in the potassium content (Parnell 1988). The vanadium/silicon atomic ratio of roscoelite ranges from 0.11 to 0.74 (Wells and Brannock 1946, Fisher et al. 1947): the roscoelite inclusions from Shuitian yield a ratio of 0.31. No vanadium minerals have been found in the black organic-rich wallrock incorporated into the veins. DISCUSSION Only limited constraints can be placed upon the timing of vein formation. The region has suffered three orogenic events: Caledonian (Silurian-Devonian), Indosinian (Triassic) and Yanshanian (Jurassic-Cretaceous). The Cambrian country rocks were folded during the Caledonian Orogeny. The veins definitely post-date this
Petrography of the Shuitian bitumen deposit, Hunan Province, China folding, as they are near-vertical while the country rocks at the mine dip at 14° (Fig. 2). There are no younger rocks in the vicinity of Shuitian to determine the effects of the younger orogenies in this district. Further east in Hunan Province, Carboniferous to Jurassic rocks are folded beneath the fiat-lying Cretaceous, but it is possible that the Precambrian and Cambrian rocks near Shuitian behaved as a rigid block which was relatively undisturbed. At some stage, the viscous liquid bitumen which entered the fractures was dehydrogenated and completely devolatilised to its present completely aromatic state. This could have occurred very early if the fracture-filling was a late stage event in the Caledonian orogenic activity, or may possibly have been due to heating during a subsequent orogeny. There has not been adequate burial/ heating since the Cretaceous to achieve this. Consequently vein formation must have occurred within the interval from the Caledonian Orogeny to Cretaceous sedimentation. The high fluid inclusion temperatures of up to 240°C are consistent with the high maturity of the organic material: they are sufficiently great to have realized all of the hydrocarbon potential of the source rock. Regional studies suggest that the Lower Cambrian would have been buried to a 2 or 3 km depth before Caledonian orogenic activity, and therefore may have entered the oil window, but not released the bulk of their hydrocarbon potential. The temperatures are most likely to have been achieved during the Caledonian Orogeny, as suggested above. They are higher than have been recorded from other mined bitumen vein deposits, in Utah (92-109°C; Monson and Parnell 1992) and Oklahoma (125-150°C; B. Monson pers. comm.). The higher temperatures also explain the presence of quantities of quartz much greater than occur in other bitumen vein deposits. The quartz would have been mobilized from the country rocks and reprecipitated in fracture systems. The generation of hydrocarbons during/following orogenic activity is a natural consequence of the marked increase in geothermal gradient and/or crustal thickening. Oliver (1986) has emphasized the significance of continental collision in driving large-scale fluid flow, including hydrocarbons. The bitumen veins at Shuitian may represent hydrocarbon-rich fluids channelled along fracture pathways following an orogeny. There are many known examples of vanadiferous bitumens (McKinstry 1957, Hess 1922, Kribek et al. 1993), although the Shuitian deposit has not previously been recorded as vanadiferous. In immature bitumens and oils, the vanadium may be entirely organicallybound, but in more mature samples the vanadium occurs as mineral inclusions. The vanadium mineral may be a sulphide, oxide or aluminosilicate. As the Shuitian bitumen has a low sulphur content, we would not expect the vanadium to occur as a sulphide. The predominance of oxide inclusions over aluminosilicate inclusions suggests that aluminium and/or silicon were not available in significant quantities in the hydrocarbon-bearing fluid (note no clays were encountered in the gangue minerals of the vein). The lack of vanadium minerals in the organicrich wallrock fragments, in contrast to their abundance in
227
the bitumen, suggests that an organic-rich environment alone was not the control on vanadium precipitation. It is likely that vanadium became progressively concentrated in the bitumen with maturation, as the metal tends to be related to the heavy asphaltene component (Reynolds et al. 1984) which becomes more abundant as lighter components are driven off by devolatilization. Initially the vanadium would be bonded through heteroatoms (O,S,N) (Reynolds et al. 1984) but ultimately, when the bitumen is effectively carbonized, the vanadium would precipitate out as an inorganic mineral. Acknowledgements--We are grateful for technical support from the
Queen's University Electron Microscope Unit and G. Alexander for cartographic services. The reflectance values were kindly supplied by B. Monson. The NMR analyses were undertaken at the University of Durham through an allocation granted by the S.E.R.C. The work was undertaken while Geng Ansong was in receipt of a Research Fellowship of the Royal Society. Professor Liu Dehan, Institute of Geochemistry, Chinese Academy of Sciences, provided help for the field work. We are grateful to a reviewer whose expert comments helped to clarify the manuscript.
REFERENCES Curiale, J. A. 1986. Origin of solid bitumens, with emphasis on biological marker results. Org. Geochem. 10, 559-580. Fischer, R. P., Hall, J. C. and Rominger, J. F. 1947. Vanadium deposits near Placerville, San Miguel County, Colorado. Colorado Scient. Soc. Proc. 15, 115-134. Hess, F. L. 1922. Uranium-beating asphaltite sediments of Utah. Engng Mining J. 114, 272-276 Jacob, H. 1989. Classification, structure, genesis and practical important of natural solid oil bitumen (migrabitumen). Int. Coal Geol. 11, 65-79. Jedwab, J., Blanc, G. and Bouleque, J. 1989. Vanadiferous minerals from the Nerews Deep, Red Sea. Terra Nova 1, 188-194. Jia, R., Liu, D. and Fu, J. 1990. Relationships between organic matter and metalliferous deposits in Lower Palaeozoic carbonate formations in China. Spec. Publ. Int. Assoc. Sediment. 11, 193-202. Kribek, B., Holubar, V., Parnell, J., Pouba, Z. and Hladikova, J. 1993. Thermal mesophase in vanadiferous bitumens from Upper Proterozoic lava flows, Mitov, Czechoslovakia. In: Bitumens in Ore Deposits (Edited by Parnell, J., Landais, P. and Kucha, H.). Springer, Heidelberg. McKinstry, H. 1957. El vanadio en el Peru (review). Econ. Geol. 52, 324-325. Meyerhoff, H. A. 1949. The occurrence and mining of solid bitumens in western Argentina. Trans. Amer. Inst. Min. Eng. 181,403-412. Monson, B. and Parnell, J. 1992. The origin of gilsonite vein deposits in the Uinta Basin, Utah. In: Hydrocarbon and Mineral Resources of the Uinta Basin, Utah and Colorado (Edited by Fouch T. D., Nuccio, V. F. and Chidsey T. C.), pp. 257-270. Utah Geological Association Guidebook 20. Oliver, J. 1986. Fluids expelled tectonically from orogenic belts: their role in hydrocarbon migration and other geologic phenomena. Geology 14, 99-102. Parnell, J. 1988. The mineralogy of red bed uranium-vanadium mineralization in the Permo-Triassic of Belfast. Irish. J. Earth Sci. 9, 119-124. Parnell, J., Geng, A., Fu, J. and Sheng, G. 1993. Geology of bitumen vein deposits of Ghost City, Junggar Basin, NW China. (submitted). Reynolds, J. G., Briggs, W. R., Fetzer, J. C., Gallegos, E. J., Fish, R. H., Komlenic, J. J. and Wines, B. K. 1984. Molecular characterization of vanadyl and nickel non-porphyrin compounds in heavy crude petroleums and residua. In: Characterization of Heavy Crude Oils and Petroleum Residues, pp. 153-157. Editions Techip, Paris. Shieh, C. S. and Duedall, I. W. 1988. Role of amorphous ferric oxyhydroxide in removal of anthropogenic vanadium from seawater. Marine Chem. 25, 121-139. Weeks, D. A., Cisney, E. A. and Sherwood, A. M. 1953. Montroseite, a new vanadium oxide from the Colorado Plateaus. Amer. Mineral. 35, 1235-1241. Wells, R. C. and Brannock, W. W. 1946. The composition of roseoelite. U.S. Geol. Surv. Bull. 1009-BV, 1-62.