Archaean Oil Migration in the Wit Waters Rand Basin of South Africa

8/2/2019 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).

190 G. L. ENGLAND ET AL.


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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.

ARCHAEAN OIL MIGRATION IN SOUTH AFRICA 191


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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.
