archaean oil migration in the wit waters rand basin of south africa
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Journal of the Geological Society, London, Vol. 159, 2002, pp. 189201. Printed in Great Britain.
Archaean oil migration in the Witwatersrand Basin of South Africa
G. L. ENGLAND1,2, B. RASMUSSEN1, B. KRAPEZ{ 1 & D. I. GROVES1
1Centre for Global Metallogeny, Department of Geology and Geophysics, University of Western Australia,
35 Stirling Highway, Crawley, WA 6009, Australia2Present Address: Department of Geology and Geophysics, Grant Institute, University of Edinburgh,
Edinburgh, EH9 3JW, UK (e-mail: [email protected])
Abstract: The Late Archaean Witwatersrand Supergroup of South Africa hosts the largest known
gold-uranium-pyrite ore deposits. Oil preserved in fluid inclusions in quartz grains in siliciclastic
sedimentary rocks of that supergroup implies that hydrocarbon generation and migration occurred during
the Archaean, and may have been involved in mineralization processes. Through reference to Phanerozoic
analogues, oil-bearing fluid inclusions entrapped in healed microfractures in detrital quartz grains and in
early syntaxial quartz-overgrowths imply, that the onset of oil migration coincided with early to
intermediate stages of burial, while intra-granular porosity was still preserved. Multiple generations of oil
migration are indicated by: (i) oil inclusions within early diagenetic cements at different levels in the
stratigraphic succession; (ii) more than one type of oil in entrapment sites; (iii) oil entrapment in multiple
stages of the quartz paragenetic sequence. Oil generation and migration are considered to have occurred
throughout, and for some considerable time after, development of the Witwatersrand Basin, consistent
with progressive burial and kerogen maturation in more than one tectonic regime. Oil-bearing fluid
inclusions within detrital sandstone fragments suggest that oil migration also occurred in a sedimentary
succession on the Kaapvaal Craton prior to 2.9 Ga. Oil in the Witwatersrand Supergroup was most likely
derived from multiple source areas, with the principal source probably being shales within the lower
Witwatersrand Supergroup. The hydrocarbon migration history of the basin has important implications
for understanding the textural relationship between gold, bituminized oil and uraninite in the giant
gold-uranium-pyrite ore deposits.
Keywords: Witwatersrand, gold, uraninite, hydrocarbons, fluid inclusions.
Conglomerate- and sandstone-hosted gold-uranium-pyrite ore
deposits of the Witwatersrand in South Africa have providednearly 40% of world gold production over the whole span ofrecorded history (Pretorius 1991), although previous estimates
have suggested a proportion as high as 55% (Pretorius 1976).In all its statistics, whether tonnes of ore mined, tonnes of gold,
uranium and even pyrite produced, depth and areal extentof mining, or number of mines, the Witwatersrand ranksunreservedly as giant and of unparalleled economic signifi-
cance. The ore-deposits, their host sedimentary units and thefour depositional basins to those successions (Dominion,
Witwatersrand, Ventersdorp, Transvaal) are also of greatgeological interest. Of particular interest here is that three of
the successions (Witwatersrand, Ventersdorp, Transvaal) pre-serve evidence for the migration and trapping of oil during theLate Archaean.
Notwithstanding the long and continuing debate on theorigin of the gold, uranium and pyrite mineralization, the
origin of bituminous nodules and seams within the ore deposits(or reefs), and particularly within the Late ArchaeanWitwatersrand Supergroup, has also long been a source of
controversy (Pretorius 1991; Gray et al. 1998). Early investi-gators (e.g., Young 1917) recognized bitumen (referred to then
as carbon) as having a strong spatial relationship with gold,uraninite and pyrite. There was, however no detailed research
on the origin of bitumen until the 1950s and 1960s (e.g.,Davidson & Bowie 1951; Liebenberg 1955; Ramdohr 1958;
Snyman 1965). The two principal hypotheses on the origin ofthe bitumen are that it is either: (i) the fossil remains of in situalgae which colonized sediment surfaces (Snyman 1965;
Hallbauer 1975; Zumberge et al. 1981; Ebert et al. 1990);
or (ii) the residual product of migrating liquid hydrocarbons(Liebenberg 1955; Schidlowski 1981; Parnell 1996; Buick
et al. 1998; Gray et al. 1998). While the hypothesis of a
syngenetic algal residue was prominent during the 1970s and1980s, more-recent organic-geochemical, stable-isotopic and
petrographic studies (Gray et al. 1998; Robb et al. 1999;Spangenberg & Frimmel 2001) support the hypothesis that
bitumen originated from migrating hydrocarbons. Bituminousnodules are interpreted to have formed by the polymerizationand crosslinking of liquid hydrocarbons around irradiating
detrital heavy-mineral grains (principally uraninite) in the hostsedimentary rock (Liebenberg 1955; Schidlowski 1981).
Although most recent studies agree that the formation ofbituminous nodules in Witwatersrand (and Ventersdorp andTransvaal-Black Reef) ore deposits involved migrating hydro-
carbons, the timing of oil migration and the mechanism bywhich oil entered reef systems remain unclear. Whereas some
investigators consider that hydrocarbon migration occurredduring early burial and was focused into horizons that retained
primary porosity (Buick et al. 1998; England et al. 2001),others have suggested that the major conduit for oil migrationwas fracture-dominant secondary porosity that post-dated
occlusion of primary porosity by burial quartz cementationand pressure solution (Robb et al. 1997; Gray et al. 1998;
Parnell 1999). Some authors have suggested also that hydro-carbon generation and migration occurred during deposition
of the Transvaal Supergroup (Robb et al. 1997; Drennan et al.1999), some 180270 million years after deposition of theWitwatersrand Supergroup.
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Liquid hydrocarbon inclusions have been identified recentlywithin authigenic quartz cements in mineralized conglomeratesfrom the Witwatersrand (Dutkiewicz et al . 1998). Similar
oil-bearing fluid inclusions in quartz, carbonate and feldsparcements from Phanerozoic reservoir rocks (Burruss 1981; Lisk
& Eadington 1994; Parnell et al. 1998) are used commonly toconstrain the timing of hydrocarbon migration relative tocement paragenesis. By analogy, this paper focuses on the
petrographic and stratigraphic distribution of oil-bearing fluidinclusions in the Witwatersrand Supergroup, the Ventersdorp
Contact Reef at the base of the Ventersdorp Supergroup,and the Black Reef at the base of the Transvaal Supergroup.
The study examines: (i) the timing and mechanisms for oilmigration in the Witwatersrand Basin, in relation to bothquartz cementation history and basin evolution and (ii) the
relationship between oil migration and the formation ofbitumen nodules and gold mineralization. The results indicate
that processes of oil generation and migration, and theirtiming relative to burial history, have not changed since theArchaean.
Geological setting
The Witwatersrand Supergroup is the structural remnant of
what was originally a more extensive succession deposited
within the central portions of the Kaapvaal Craton of SouthAfrica (Fig. 1). The Supergroup is an approximately 75 kmthick succession of mudrock, sandstone and minor conglom-
erate that was deposited some time between 309 and 271 Ga(Armstrong et al. 1991). The original Witwatersrand Basin isconsidered to have been similar in geotectonic setting to
modern retroarc (foreland) basins (Burke et al. 1986), such asthose east of the American Cordillera (e.g., Rocky Mountains
and Andean Foreland Basins). According to Winter (1987),the Witwatersrand Supergroup can be divided into: (i) a lowermarine-influenced deltaic stage (West Rand Group) and (ii) an
upper fluviodeltaic stage (Central Rand Group). The olderDominion Group is considered to record a back-arc basin that
predated the Witwatersrand Basin by at least 100 million
years. The Dominion Reef, a siliciclastic succession at the baseof the Dominion Group, is a uraninite-pyrite ore-deposit withlow gold content.
Compressive deformation associated with the LimpopoOrogeny is considered to have produced synsedimentarythrust- and wrench-faulting of the West Rand and Central
Rand successions, with subsequent uplift, sediment recyclingand stacking of unconformities (Coward et al. 1995). Each
unconformity surface is overlain by transgressive quartz-pebble conglomerate lags and pyritic cross-bedded sandstones,which, in selected stratigraphic locations, are the host to gold
and uranium ore bodies (i.e., reefs).In addition to burial and deformation related to
episodic synsedimentary subsidence and uplift, several post-Witwatersrand, Archaean and Proterozoic events have modified
the Witwatersrand Supergroup. These include (after Cowardet al. 1995; Martin et al. 1998): (i) stacked episodes of flood-basalt volcanism, uplift, erosion and half-graben deposition of
the Ventersdorp Supergroup; (ii) folding and thrusting prior todeposition of the Transvaal Supergroup; (iii) passive-margin
thermal subsidence and flexural reactivation during depositionof the Chuniespoort Group (lower Transvaal Supergroup); (iv)rift-basin deposition of the Duitschland Formation and Preto-
ria Group (middle Transvaal Supergroup); (v) emplacement ofthe Bushveld Igneous Complex associated with lithospheric
extension and high heat-flow, coeval with deposition of theRooiberg Group (upper Transvaal Supergroup); (vi) strike-slipdeformation associated with uplift of the Vredefort Dome.
These events are linked to several phases of metamorphism and
alteration, with peak metamorphism reaching lower greenschisttemperatures of 350 50 C (Phillips & Law 1994).
There is extensive debate as to whether major ore com-
ponents (gold, uraninite, pyrite) in reefs were: (i) introducedas detrital heavy minerals and later remobilized during meta-morphism or hydrothermal alteration (Minter 1978; Frimmel
1997; Robb et al. 1997) or (ii) introduced by hydrothermalfluids during metamorphism (Phillips & Myers 1989; Barnicoat
et al. 1997; Phillips & Law 2000). The second hypothesisrequires more than one hydrothermal event because gold-pyrite uraninite mineralization is recorded from the basal
stratigraphic succession of the Ventersdorp Supergroup(Ventersdorp Contact Reef), which post-dates folding, faulting
and mineralization of the Witwatersrand Supergroup (Krapez
1985), and from the basal Black Reef of the TransvaalSupergroup, which similarly post-dates the Ventersdorp
Supergroup.
Fig. 1. Subsurface geological map and
stratigraphic column of the
Witwatersrand Basin, including the
localities of the Welkom (WGF),
Klerksdorp (KGF) and Carletonville
Goldfields (CGF): modified after
Frimmel (1997).
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Methods
Sampling
Samples were collected of mineralized and non-mineralized sedimen-
tary rocks from underground mine workings and diamond drill core
on the Welkom, Klerksdorp and Carletonville Goldfields (Fig. 1).Sampled intervals include: (i) the Steyn and Leader Reefs from
Freegold One (President Steyn Mine) on the Welkom Goldfield; (ii)
sub-economic reefs (e.g., A and B Reefs) and uneconomic conglomer-
ates, sandstones and mudrocks in the Freegold Mining Lease on the
Welkom Goldfield; (iii) the Vaal and C Reefs, as well as uneconomic
conglomerates, sandstones and mudrocks from Vaal Reef Numbers 8
and 9 Shaft on the Klerksdorp Goldfield; (iv) the Ventersdorp Contactand Dennys Reefs from Vaal Reef Number 10 Shaft on the
Klerksdorp Goldfield; (v) the Inner Basin Reef (upper West Rand
Group) from the Afrikander Lease on the Klerksdorp Goldfield; (vi)
the Dominion Reef from the Dominion Lease on the Klerksdorp
Goldfield; (vii) the Carbon Leader and Black Reef from Western Deep
Levels on the Carletonville Goldfield; (viii) the Ventersdorp ContactReef from Elandsrand Mine on the Carletonville Goldfield; (ix) the
Black Reef, from diamond drill core, in the Potchefstroom Gap Areabetween the Klerksdorp and Carletonville Goldfields.
UV-epifluorescent microscopy
Oil-bearing fluid inclusions were identified in polished thin-sections
from many of the samples using conventional transmitted light (TL)
and ultra-violet (UV) epifluorescence microscopy. The process in-
volved the attachment of a vertical UV illuminator to a conventional
TL microscope, allowing observation under long-wave UV verticalillumination (Burruss 1981). Liquid hydrocarbons, if present within
fluid inclusions, will fluoresce under ultra-violet excitement. The
various fluorescent colours and intensities relate to differences in
organic chemical composition and are controlled essentially by the type
and concentration of aromatic molecules (and to a lesser degree, N-,
S- and O-bearing compounds), relative to aliphatic compound
concentrations (Stasiuk & Snowden 1997).Various researchers that discuss oil fluorescence (Hagemann &
Hollerbach 1986; McLimans 1987; Bodnar 1990; Lisk & Eadington
1994) often relate variations in fluorescence colours to differences in oil
gravity (API number), which may directly relate to oil maturation. Oil
at the red end of the fluorescent spectrum is considered to be produced
from source rocks at the onset of oil generation, representing low
maturity heavy oils. The blue and white fluorescent colours at theother end of the spectrum represent light oil or condensate expelled
from source rocks at higher levels of maturity, corresponding with
peak to late generation (Lisk & Eadington 1994). This, however does
not take into account other complexities, which may alter hydro-
carbon composition and thus affect UV fluorescence (George et al.
2001). Complexities may include: (i) variation in source rock type,although this had a less-significant effect with Archaean oils, which
could have been derived from only bacterial-algal Type I or Type II
kerogens (Mossman & Tompson-Rizer 1993); (ii) oil fractionation dueto water flushing and biodegradation during migration (Bodnar 1990);
(ii) fractionation of oil during trapping (George et al. 2001); (iv)
thermal alteration of oil during migration (Killops & Killops 1993).
With little detailed information on the organic chemistry ofArchaean oil, and to what extent chemical, thermal or biological
interaction processes may have been involved during oil migration, it is
difficult to interpret the causes for the variations in fluorescence
evident from samples examined during this study. Whereas some
studies of Phanerozoic oil suggest that samples containing more than
one fluorescent colour reflect multiple oil migration events or di fferent
sources (McLimans 1987; Eadington et al. 1991), others recommendcaution because single oil charges can show different colour
populations (George et al. 2001).
SEMThe fluid-inclusion history derived from samples of sandstones and
conglomerates of the Witwatersrand Supergroup is complex. The
complexity arises not only from fluid inclusions trapped during
post-depositional activity, but also from fluid inclusions in detrital
quartz grains. In some cases, to assist in defining the paragenetictiming of oil-bearing fluid-inclusion entrapment, selected polished
thin-sections were examined also by cathodoluminescence scanning-
electron microscopy (CL-SEM), which provides a means of identify-
ing: (i) healed microfractures (evident as fluid inclusion trails under TL
microscopy) and (ii) secondary quartz cements, which are optically
indistinguishable from detrital quartz grains in conventional optical
microscopy. CL-SEM imagery of Phanerozoic sandstones is often usedto distinguish detrital quartz grains from diagenetic quartz over-
growths and fracture fill (Hogg et al. 1992; Sullivan et al. 1997;
Milliken & Labach 2000). Detrital quartz from an igneous source is
usually substantially brighter in luminescence than quartz of an
authigenic origin (i.e., overgrowth and fracture fill).
Oil-bearing fluid inclusions: results and discussion
Inclusion description
Of the 62 polished thin-sections examined under UV illumi-nation during this study, 41 have fluid inclusions that contain
liquid hydrocarbons. The oil-bearing fluid inclusions rangefrom 3 to 15 m in diameter. They are hosted in either healed
microfractures within detrital quartz grains or are primaryinclusions within syntaxial quartz overgrowths (Figs 2 and 3).Although some re-equilibration of fluid inclusions could have
been expected due to increasing temperature and burial during
basin subsidence and subsequent metamorphism (McLimans1987), in most cases the liquid hydrocarbons within the fluidinclusions are well preserved. They appear typically as three
Fig. 2. Schematic diagram showing entrapment sites of fluid
inclusions within sandstones and conglomerates.
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phases (clear liquid-oil-gas bubble), although four phases(clear liquid-oil-clear liquid-gas) and oil-only inclusions are
present. The oil portion of the inclusions is represented by
either a clear, light or dark brown liquid rim, which typicallysurrounds the mobile gas phase and ranges from 5 to 20% of
the total volume of each fluid inclusion (Fig. 3a and b).
Although some oil-bearing fluid inclusions exhibit texturalevidence of auto-decrepitation and necking-down, the majority
show rounded or negative crystal shapes. The oil-bearing fluid
inclusion morphologies include spherical, oval, ellipsoidal,lath-like and irregular shapes, although there is no obvious
relationship between inclusion morphology and quartz cement
Fig. 3. Photomicrographs showing oil-bearing fluid inclusions hosted in healed microfractures from various sandstones and conglomerates of the
Witwatersrand Supergroup. (a, b) Detrital quartz grain surrounded by a matrix of sericite and brannerite (opaque) (a,TL). The quartz grain
contains two large fluid inclusions (marked by arrow), Vaal Reef, Klerksdorp Goldfield. A higher magnification, TL-UV composite
photomicrographs (b) demonstrates that the two fluid inclusions fluoresce yellow-orange under UV illumination. The dark liquid rim (marked by
arrows in b) surrounding the gas bubble represents the oil portion of the inclusion. (c, d) Trail of oil-bearing fluid inclusions, fluorescing white,
green and blue (c; TL; d, UV), Leader Reef, Welkom Goldfield. (e, f) Detrital quartz grain with multiple trails of oil-bearing fluid inclusions,
displaying a variety of florescent colours including orange, red, yellow, green, and blue (e; TL; f, UV), Steyn Reef, Welkom Goldfield.
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history. The inclusions show a wide range of fluorescent
colours including red, light brown, orange, yellow, green, whiteand blue (Fig. 3b, d and f). Yellow is the most prominentcolour recorded, and irregularly shaped inclusions are most
common. Oil-bearing fluid inclusions with an irregular mor-phology are generally larger than other morphological types.
Petrographic distribution
Fluid inclusions in samples from the WitwatersrandSupergroup, the Ventersdorp Contact Reef and the Black Reef(Fig. 2) are categorized as: (i) pre-depositional (mostly
aqueous) fluid inclusions hosted in detrital grains (Type 1); (ii)secondary inclusions hosted in point-contact fractures that
developed during physical compaction (Type 2); (iii) primaryinclusions hosted in quartz cements (Type 3); (iv) secondary
inclusions hosted in deformation-related fractures (Type 4); (v)
primary inclusions hosted in quartz veins (Type 5). In manycases, it can be difficult to distinguish between the various
types. Resolution of some entrapment sites was achieved onlyby CL-SEM examination of polished thin-sections. No corre-
lation was detected between entrapment site of the oil-bearingfluid inclusion and UV fluorescent colour.
Type 1 fluid inclusions are identified as those hosted in detrital
quartz grains and pebbles, and in lithic fragments (e.g.,rounded sandstone fragments), and that were entrapped prior
to sedimentary deposition. This type, which has obviousprovenance relevance, includes fluid inclusions that originatedin source hinterlands (Shepherd 1977; Hallbauer 1983) or in
previously deposited Witwatersrand sediments that were
recycled during intraformational uplift. Some inherited inclu-sions can be recognized easily within detrital quartz grains,because they are associated with microfractures and quartz-healing patterns that are different to those in other surround-
ing framework grains. However, inherited fluid inclusions inquartz pebbles and grains show no evidence of liquid hydro-
carbons. The only Type 1 fluid inclusions that contain oil arethose hosted in rounded pebbles of sandstone.
Type 2 inclusions are secondary fluid inclusions hosted
by healed microfractures, within detrital quartz grains(Dutkiewicz et al. 1998; Figs 2 and 4). Microfracturing, as a
burial process, is considered to initiate during early stages ofdiagenesis (
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pyrobitumen. This may explain the presence of bitumen ininclusions hosted in quartz veins, as recorded by Drennan et al.(1999).
Stratigraphic distributionOil-bearing fluid inclusions are recorded here from samples at
stratigraphic intervals throughout the Central Rand Group, as
Fig. 4. Photomicrographs and SEM image showing the petrographic setting of fluorescent fluid inclusions from Dennys Reef, Klerksdorp
Goldfield. (a) TL photomicrograph shows detrital quartz grains with intra-granular pores filled with quartz and late-phase bitumen (opaque).
(b) CL SEM image of (a) reveals non-luminescent quartz filling physical compaction-related microfractures and overgrowing detrital grains
(indicated by arrows). (c, d) Combined TL (c) and UV (d) photomicrographs are a close up of (a) and (b) (see inserts), showing that many of
the fluid inclusions associated with microfractures contain oil. The oil-bearing fluid inclusions in (d) fluoresce yellow and light blue.
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Fig. 5. Photomicrographs and SEM images showing the petrographic setting of fluorescent fluid inclusions from the Steyn Reef, Welkom
Goldfield. (a) TL photomicrograph showing detrital quartz grains with intra-granular pores filled with chlorite, sericite and quartz. ( b) CL SEM
image of (a), reveals non-luminescent quartz filling fine physical compaction-related microfractures and overgrowing detrital grains. TL
photomicrograph (c) and matching CL-SEM image (d) (close up of a and b) reveal a trail of fluid inclusions at the boundary between the quartz
overgrowth and the detrital quartz grain (indicated by arrows). Other fluid inclusion trails are confined to healed microfractures. ( e, f) Fluidinclusions hosted at the overgrowth-detrital grain boundary and those confined within healed microfractures (see insert in c) show evidence of
oil, indicated by green and blue fluorescence under UV illumination (f).
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well as from samples of the Inner Basin Reef (upper West
Rand Group), the Ventersdorp Contact and Black Reefs(Fig. 6). There is no obvious correlation between stratigraphicposition and UV fluorescent colour of oil-bearing fluid inclu-
sions. The oldest stratigraphic interval investigated (DominionReef) shows no evidence of oil-bearing fluid inclusions (see
also Feather & Glatthaar 1987). The implication is that theDominion Reef most likely received no or only a minimal oilcharge.
Samples of the Inner Basin Reef (at the base of the
Jeppestown Subgroup) contain fluid inclusions with liquidhydrocarbons in Type 1 and 2 sites. Inherited oil-bearing fluidinclusions (Type 1) identified in those samples are hosted in
a well-cemented and partially recrystallized, rounded pebbleof sandstone. Although the pebble preserves several sets of
microfractures and evidence for several phases of quartzrecrystallization, oil-inclusions are confined to early point-contact fractures within detrital quartz grains enclosed by
Fig. 6. Stratigraphic distribution of oil-bearing fluid inclusions within the Witwatersrand Supergroup, in the Welkom, Klerksdorp, and
Carletonville Goldfields: stratigraphic section modified from SouthAfrican Committee for Stratigraphy (1981). Arrows indicate sections of the
stratigraphic succession examined during the study.
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authigenic quartz cements. Irrespective of whether the
pebble were derived intraformationally from Witwatersrandsediments or from pre-Witwatersrand source rocks, its oil-bearing fluid inclusions indicate that liquid hydrocarbons
migrated through sedimentary rocks before deposition of theJeppestown Subgroup.
Samples that contain oil-bearing fluid inclusions come fromthe Central Rand Group in the Klerksdorp, Carletonville and
Welkom Goldfields. The oil-filled inclusions are recordedmostly in samples from conglomerate units, particularly thosethat contain bituminous nodules. Oil entrapment is most
prevalent in Type 2 fluid inclusions, although Types 1, 3 and 4fluid inclusions are preserved also. In several cases, oil inclu-
sions appear to have been entrapped at various stages in thequartz paragenetic sequence. Reefs that contain abundantoil-bearing fluid inclusions, as well as bituminous nodules, are
the Vaal, C and Dennys Reefs of the Klerksdorp Goldfield,the Steyn, Leader, A and B Reefs of the Welkom Goldfield,
and the Carbon Leader Reef of the Carletonville Goldfield. Insome polished thin sections, up to 30% of the total fluid
inclusions contain liquid hydrocarbons.Non-mineralized sandstones and conglomerate lags distal
to the mineralized conglomerates also contain oil-bearing
fluid inclusions, but they are comparatively less abundant.The implication is that the reef horizons were the principal,
but not sole, pathways for early oil migration. Furthermore,their larger average grain sizes and higher porosities may wellhave made reef horizons more susceptible to point-contact
fracturing during physical compaction (Zhang et al. 1990),thereby providing more sites for oil entrapment during early
burial.Samples examined from the Ventersdorp Contact Reef
contain two populations of oil-bearing fluid inclusions. The
first population comprises small (
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deposition of the source rocks, thus falling well within the
time frame of hydrocarbon systems in Phanerozoic forelandbasins (Erlich & Barrett 1992; Osadetz et al. 1992). Such an
interpretation is in conflict with the suggestion by Robb et al.(1997) that oil generation and migration from Witwatersrandshales occurred at c. 2300 Ma. It is conceivable that by that
time, which was at least 400 million years after Witwatersranddeposition, most of the source rocks were outside the oil
window (Fig. 7) and probably near the limits of gas generation(c. 230 C, Tissot & Welte 1984), particularly given the post-
Witwatersrand tectonic history of the Kaapvaal Craton.
Furthermore, the quoted age of c. 2300 Ma, which is derivedfrom UPb dating of uraniferous bituminous nodules in
Witwatersrand conglomerates (Robb et al. 1994), may recorduranium remobilization (Schidlowski 1981) rather than oil
migration and the formation age of bituminous nodules.
Oil migration
The presence of oil-bearing fluid inclusions provides directevidence that oil migration took place in the Witwatersrand
Supergroup. Their locations in compaction related micro-fractures (Type 2) and in early syntaxial quartz overgrowths
(Type 3) have Phanerozoic analogues (Lisk & Eadington 1994;Parnell et al. 1998), and imply the presence of oil in formation
fluids during early burial. The stratigraphic distribution ofoil-bearing fluid inclusions also indicates that hydrocarbongeneration and migration were most likely ongoing throughout
basin development, and is consistent with progressive sedi-
mentary compaction and kerogen maturation. Although oil-bearing fluid inclusions are most abundant in the ore horizons,their presence in non-mineralized conglomerates and sand-
stones indicates that oil migration was not confined to the reefsbut was basin-wide.
In the majority of samples examined from theWitwatersrand Supergroup, there is more than one population
of oil-inclusions. This is apparent not only from oil that wasentrapped at various stages of the quartz paragenetic sequence(Fig. 2), but possibly also from the variety of different coloured
inclusions that were entrapped in the same textural sites. InPhanerozoic basins, this multiplicity in oil-inclusions may
indicate multiple phases of oil migration or different sources ofoil (McLimans 1987; Eadington et al. 1991; Parnell et al. 1998).In the case where oil is entrapped in microfractures, inclusions
of various oil compositions are entrapped progressively as thefractures slowly heal. It is evident, from even the most oil-
saturated siliciclastic reservoirs in Phanerozoic successions,that water will remain the wetting phase, thereby enabling
quartz cementation and the trapping of oil inclusions tocontinue slowly (Lisk & Eadington 1994). As discussed above,the significance of the variation in fluorescence (or, more
precisely, oil composition) from Witwatersrand samples is stillunclear. Nevertheless, by analogy to several Phanerozoic
examples (McLimans 1987; Stasiuk & Snowdon 1997),Witwatersrand sandstones and conglomerates may havereceived multiple charges of oil of varying composition during
burial, as a consequence of changes in oil maturation or oilfractionation during migration.
As primary porosity diminishes due to quartz cementationand pressure solution during increasing depth and temperature
(Leder & Park 1986), oil migration or entrapment becomes
restricted to secondary porosity (e.g., fractures). With evidencefor several deformation events during the post-depositional
history of the Witwatersrand Supergroup (Coward et al. 1995;Frimmel 1997), the absolute timing of many of the late
fractures (Type 4) and veins (Type 5) is difficult to constrain.Only a few of the recognized late fractures (Type 4) containoil-bearing fluid inclusions. However, previous fluid inclusion
studies (see review in Klemd 1999) have shown thatlight hydrocarbons (e.g., CH4 and C2H6) and bitumen (e.g.,
Drennan et al. 1999; Gartz & Frimmel 1999) are present inprimary and secondary fluid inclusions entrapped in quartzveins. The hydrocarbon gases entrapped in late paragenetic
sites may reflect increasing maturation levels during laterperiods of basin evolution, with associated increases in
temperatures and burial depths (Fig. 7).
Summary and implications for the goldbitumenrelationship
Oil preserved in fluid inclusions within the WitwatersrandSupergroup indicates that there was hydrocarbon generation
and migration during the Archaean. Oil-bearing fluid inclu-sions are recorded in polished thin-sections of conglomerates
and, to a lesser degree, sandstones taken from samplesthroughout the Central Rand Group, as well as from repre-sentative samples of the upper West Rand Group, the
Ventersdorp Contact Reef at the base of the Ventersdorp
Supergroup, and the Black Reef at the base of the TransvaalSupergroup. The presence of those oil-bearing fluid inclusionsin healed microfractures, which developed in detrital quartz
Fig. 7. Comparison of the various burial depths required for the
onset of oil generation, dependent on the given geothermal gradient
applied (1; 15-16 C km-1; Jones 1988; Martini 1992; 2 & 4, Frimmel
et al. 1993; 3 Gibson et al., 1997) and a sequence thickness of
approximately 7 km. It appears, from the diagram, that oil was most
likely generated from lower West Rand Group mudrocks prior to
the end of deposition of the Central Rand Group. V, Ventersdorp
Supergroup; CR, Central Rand Group; WR, West Rand Group.
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grains during physical compaction, and in early syntaxial
quartz overgrowths indicates that the onset of oil mi-gration coincided with early stages of sedimentary burial, whenintra-granular porosity was still preserved.
This investigation points strongly to the evolution ofmultiple generations of oil. Evidence includes: (i) that oil was
trapped in inclusions in the same type of early diageneticfabrics throughout the stratigraphic successions; (ii) that many
of the oil entrapment sites contain more than one type of oil,as indicated by the variation in UV-fluorescent colours (cf.McLimans 1987); (iii) that oil was typically entrapped at
multiple stages of the quartz paragenetic sequence that can beidentified within single polished thin-sections. It is likely that
oil preserved within sandstones and conglomerates of theCentral Rand Group and the Ventersdorp Contact Reef wasderived from multiple source areas. The lack of oil-bearing
fluid inclusions, and only the rare occurrence of residualhydrocarbon in the Dominion Reef, imply that the major oil
source-rocks were stratigraphically higher than the DominionGroup.
This study indicates that oil generation and migrationwere ongoing throughout and after development of theWitwatersrand Basin, consistent with progressive burial and
kerogen maturation. Liquid hydrocarbon identified in fluidinclusions from the Black Reef was probably derived from
a source other than the Witwatersrand Supergroup, suchas carbonaceous mudrocks within the same succession,carbonaceous mudrocks within the overlying Chuniespoort
Group, or carbonaceous shales within the underlyingWolkberg Group.
The results from this study are in conflict with the suggestionby Robb et al . (1997) that onset of oil generation and
migration in the Witwatersrand Supergroup occurred at
c. 2300 Ma, at some stage during deposition of the TransvaalSupergroup. Analogy with Phanerozoic successions implies
that at a similar stage of basin evolution (i.e. at least400 million years after deposition), source rocks in the
Witwatersrand Supergroup were unlikely to be still producingoil and may have already reached the limits of gas generationas the consequence of increasing depth of burial, increasing
temperature, and the impact of successive tectonic events.Early oil generation and migration can explain why only a
limited number of fractures that developed during late-stagedeformation contain oil, and why light hydrocarbons, such asmethane, are present in fluid inclusions that are hosted in late
authigenic quartz and secondary trails in late quartz veins(Drennan et al. 1999; Frimmel et al. 1999).
The presence of oil during diagenesis has important impli-cations for the origin of bituminous nodules within the
Witwatersrand Supergroup. If rounded uraninite grainsrepresent former detrital heavy minerals, as many petro-graphic studies have proposed (Ramdohr 1958; Minter 1978;
Schidlowski 1981), then oil migrating through primaryporosity during early stages of burial would almost certainly
have been radiogenically immobilized to form bituminousnodules. A similar mechanism for bituminous nodule forma-tion during diagenesis has been established from Phanerozoic
depositional basins (Rasmussen et al. 1989, 1993; Englandet al. 2001), where detrital grains of monazite, xenotime and
high-U zircon are enveloped in bitumen that is the residualproduct of immobilized hydrocarbons.
The well-documented occurrence of a significant proportionof Witwatersrand gold in or adjacent to bitumen seams ornodules (e.g., Pretorius 1991) implies that either detrital gold
was remobilized or hydrothermal gold introduced after initial
radiogenic immobilization of oil. The timing of this event,which controlled the present siting of most of the gold, was
probably late in basin history (i.e., post-Witwatersrand
deposition) when (i) primary porosity and permeability in theCentral Rand Group were limited, (ii) oil migration was at a
minimum and (iii) light hydrocarbons were the primary oilphase. The combination of these factors limited the nature of
auriferous fluids, if any, that could have deposited gold inzones of structurally induced permeability. Importantly, inter-
nal remobilization of gold could have been achieved in thepresence of water-poor, volatile-rich fluids (e.g., methane; seeEngland et al. 2001), potentially explaining the paucity of
synchronous quartz veins in the ore zones.
The authors acknowledge the support, assistance and co-operation of
Anglogold, and in particular Nick Fox and Keith Kenyon. We would
also like to thank the staff at the Centre for Microscopy and
Microanalysis, UWA for technical assistance. The paper has benefited
from comments by John Parnell, Andrew Gize and subject editor JoeMacquaker, and recommendations from Grant Young.
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Received 13 December 2000; revised typescript accepted 8 October 2001.
Scientific editing by Joe Macquaker.
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