S .Afr.J.Geol., 1991,94(1),86-95
86
Chronological framework for the Witwatersrand Basin and environs: towards a
time-constrained depositional model
L.J. Robb
Department of Geology, University of the Witwatersrand, P.O. Wits 2050, Republic of South Africa
D.W. Davis and S.L. Kamo
Jack Satterly Geochronology Laboratory, Royal Ontario Museum, Toronto, Canada
Accepted 15 February 1991
Recent U-Pb zircon age determinations now provide accurate constraints for the complex sequence of events
responsible for the formation of the Late Archaean Witwatersrand Triad. These dates are summarized and, together
with existing whole rock and mineral ages, provide information on the timing of plutonic, volcanic and metarnorphichydrothermal events, as well as loosely constraining the ages of sediment deposition. At least three stages of granitoid
plutonism can now be recognized and these include pre-Dominion events (> 3 100 Ma), Dominion events
(3 100 - 3 010 Ma) and Witwatersrandian events (2980 - 2 720 Ma). The Dominion and Ventersdorp lavas represent
two major volcanic events associated with the Witwatersrand Triad and have been dated at 3 074 Ma and 2 714 Ma
respectively (Armstrong et ai., 1991). These, and other constraints, together with detrital zircon and monazite ages
from sedimentary units, indicate that the Dominion sediments were deposited between 3 096 - 3 074 Ma, while the
West Rand and Central Rand Groups were deposited between 2 980 - 2 914 Ma and 2840 - 2714 Ma respectively.
Several events both during and subsequent to the period of Witwatersrand sediment deposition resulted in re-setting of
the Rb-Sr and Pb-Pb whole rock isotope systems and can be attributed to metamorphic and/or hydrothermal processes.
Post-depositional events at c. 2 500 Ma, c. 2 300 Ma and c. 2 000 Ma apparently record significant metamorphic and
hydrothermal overprints, but these cannot in most cases yet be satisfactorily linked to other major events on the
Kaapvaal Craton. Four principal tectonic stages are now recognized as being pertinent to the secular evolution of the
Witwatersrand Triad. These stages are believed to develop within a Wilson Cycle (Stanistreet & McCarthy, 1991) and
include (i) an extensional rift phase between 3 100 - 3 010 Ma during which time the Dominion Group was
deposited; (ii) a compressional stage that saw the transformation from an epicontinental cover sequence (lower West
Rand Group) to a foreland basin setting (upper West Rand and lower Central Rand Groups) instigated by the
encroachment of the Zimbabwe Craton, between 2 980 - 2 900 Ma; (iii) a collisional event between
2 840 - 2 720 Ma when the Zimbabwe and Kaapvaal Cratons collided resulting in deposition of the upper Central
Rand Group; and (iv) an impactogenal rift phase at c. 2 710 Ma when the Klipriviersberg lavas and Platberg sediments
were deposited. Felsic plutonism formed an integral part of at least the fIrst three stages and is now considered as an
important component of the processes active along the basin edge.
Onlangse U-Pb sirkoonouderdomsbepalings verskaf nou akkurate afbakenings vir die ingewikkelde opeenvolging van
gebeure wat verantwoordelik is vir vorming van die Laat-ArgeYese Witwatersranddrietal. Hierdie datums word
opgesom en, tesame met bestaande heelgesteente- en mineraalouderdomme, verskaf hulle inligting aangaande die
tydsbepalings van plutoniese, vulkaniese, en metamorf-hidrotermale gebeurtenisse, en baken ook die ouderdomme van
sedimentafsetting min of meer af. Ten minste drie stadiums van granitiese plutonisme kan nou herken word en hulle
sluit die Pre-Dominiumgebeure (>3 100 Ma), Dominiumgebeure (3 100-3010 Ma), en die Witwatersrandiumgebeure
(2 980-2 720 Ma) in. Die Dominium- en Ventersdorplawas verteenwoordig twee belangrike vulkaniese insidente wat
met die Witwatersranddrietal geassosieer word en teen 3 074 Ma en 2 714 Ma respektiewelik, gedateer word
(Armstrong et al., 1991). Hierdie, en ander afbakenings,tesame met ouderdomme van detritale sirkoon en monasiet uit
sedimentere eenhede, dui aan dat die Dominiumsedimente in 3 096 - 3 074 Ma afgeset is, en die Wes--Rand en
Sentrale Rand Groepe in 2980 - 2 914 Ma en 2840 - 2714 Ma respektiewelik, afgeset is. Verskeie gebeure, beide
gedurende die peri ode van Witwatersrandafsetting en daarna, het tot die herinstelling van die Rb-Sr en Pb-Pb
heelgesteente-isotoopsisteme gelei en kan aan die metamorfe en/of hidrotermale prosesse toegeskryf word. Naafsettingdgebeure teen ca. 2 500 Ma, c. 2 300 Ma en c. 2 000 Ma lys skynbaar betekenisvolle metamorfe en
hidrotermale oordrukke, maar die kan in die meeste gevalle nog nie bevredigend aan ander belangrike gebeure op die
Kaapvaalkraton gekoppel word nie. Vier belangrike tektoniese stadiums word nou as behorend tot die sekulere
evolusie van die Witwatersranddrietal beskou. Daar word aangeneem dar hierdie stadiums binne 'n Wilsonsiklus
ontwikkel en sluit: (i) 'n uitgebreide sinkingstadium tussen 3 100 - 3 010 Ma, waartydens die Dominium Groep
afgeset is; (ii) 'n kompressiestadium wat die transformasie van 'n epikontinentale bedekkingsopeenvolging (onderste
Wes-Rand Groep) tot 'n voorlandkomomgewing (boonste Wes-Rand en onderste Sentrale Rand Groepe), veroorsaak
deur die voortskrydings van die Zimbabwe-kraton tussen 2 980 - 2 900 Ma; (iii) 'n botsingsgebeure tussen
2 840 - 2 720 Ma waartydens die Zimbabwe- en Kaapvaalkratons gebots het en dit tot die afsetting van die boonste
Sentrale Rand Groep aanleiding gegee het; en (iv) 'n botsingsveroorsaakte sinkingsstadium teen ca. 2 710 Ma
waartydens die Klipriviersberglawas en Platbergsedimente afgeset is, in. Felsiese plutonisme het 'n integrale deel van
ten minste die eerste drie stadiums gevorm en word nou as 'n belangrike komponent van die prosesse wat langs die
komrand aktief was, geag.
Introduction
Even as recently as 1986 a review of the age limits of the
Witwatersrand Supergroup (Allsopp & Welke, 1986) stated
that the age of the sequence had not been reliably
detennined' but that available evidence suggested broad
constraints between circa 2 800 and 2 300 Ma. Some years
earlier, however, Van Niekerk & Burger (1978) had
obtained a U-Pb age on composite zircon fractions from
acid volcanics of the Platberg Group (Ventersdorp
Supergroup) of 2643 ± 80 Ma. This age, which is
"rj
~.
c::
...,
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en
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a
ae;
'<
2000
0
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s:I>
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A SUMMARY OF AGES FROM THE
WITWATERSRAND TRIAD AND ENVIRONS
SOURCES OF DATA (and isotope system) .I
Robb at 01., in prep
(U - Pb zircon)
8. Hart II ai, 198'
(U - Pb wholl rock)
(Pb-Pb"
,,)
z.
Robb 01 aI., 1990
(U - Pb zircon)
9. Allsopp, 1961
(Rb-Sr wholl rock)
I
~I,
. ,I'
WITS1SHALES
2100
en
3 Barlon,E.S.11 ai, 1989 (U·Pb zircon)
q>
0
a
4. Orlnnon "01,1990
(U-Pb zircon)
10.
Von Niekerk a Burger, 1969 (Pb - Pb sulphide leach)
II. Harding 0101.,1974
(Rb-Sr whol. rock)
2200
5. Armllrong II aI., 1991
So
{t>
~
(U- Pb zircon)
6. Barlan,E.S.llal., 1986 Wg~~ "'::'''~kl
2300
7 AnholulSor
~
a
Burger, 1982 (U· Pb zircon)
s:I>
12 Von Nilkork 8 Burger,I964 (Pb-Pbwllolerock)
13 Rundl. 8 Snelling, 1977 (U-Pb minerai, wholerock)
'4. GlullI 0101.,1986
(Pb- Pb oulphide)
15.AIIIOpp 0101.,1986 (U-Pb wholo rack)
~
en
2400
""1
'6. Loyer OIol.,I9B8 (K-Ar wholo rock)
""1
§
17. Siblyo, 1988 (U - Pb zircon)
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RUTILE
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2600
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-+-
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IKlIPRIVIERSBERG GP.)
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(MAKWASSIE Q P.FM)
0
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_ _ 2879
(TK9Z)
2900
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,
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_ 2113I
RU11L!
WITWATERSRAND
-+--
(rAl)
2'14t8~
=
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DOMINION
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(SYF[RfONTEIN FM)
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....- - - - PLUTONIC EVENTS -----·.-il!.---VOLCANIC EVENTS
..
DETRITUS ----J'~I --METAMORPHIC-HYDROTHERMAL EVENTS
--1
S.-Afr.Tydskr.Geol.,1991,94(1)
88
significantly older than previous age determinations for the
Ventersdorp sequence (e.g. 2 300 ± 100 Ma; Van Niekerk
Burger, 1964), was largely ignored because of the
suggestion that it was biased by the presence of older
inherited zircons.
At a meeting celebrating the centenary of the discovery of
the Witwatersrand goldfields, Geocongress '86, a paper
presented by the late Dr H.J. Welke unequivocally defined
the Witwatersrand Supergroup as an Archaean succession
by presenting a precise V-Pb zircon age for the Makwassie
Quartz Porphyry of 2 699 ± 16 Ma (Armstrong et al.,
1986). The Klipriviersberg mafic volcanics at the base of the
Ventersdorp succession have now also been dated and yield
a precise age of 2714 ± 8 Ma (Armstrong et al., 1991).
The Makwassie age has also beep refined and is now quoted
as 2 709 ± 4 Ma (Armstrong et al., 1991). The same study
has also provided a well-constrained V-Pb zircon age for
acid porphyries of the Dominion Group, which immediately
underlie the Witwatersrand Supergroup, of 3 074 ± 6 Ma
(Armstrong et al., 1991). The studies by Armstrong and
coworkers utilizing the ion-microprobe at the Australian
National University have, for the first time therefore, placed
accurate constraints on the deposition of the Witwatersrand
sequence. The constraints of 3 074 - 2 714 Ma indicate that
a period of 360 Ma in the late Archaean was available for
the accumulation of, first the West Rand Group and then the
Central Rand Group, into the Witwatersrand Basin.
The present review will summarize the available age
determinations for the Witwatersrand Basin and environs
from both the published literature and recently obtained data
(Robb et al., in prep.). This summary is presented in
Figure 1 where the data are sub-divided into categories
defining plutonic events, volcanic events, age of detritus and
metamorphic-hydrothermal events. These age data are then
placed into a scenario depicting the tectonic framework of
this portion of the Kaapvaal Craton to produce a timeconstrained depositional model for the Witwatersrand Triad.
Ages for the Witwatersrand Triad and environs
Plutonic events
For the purposes of this review plutonic (and volcanic)
events are considered to have been defined by V-Pb zircon
ages, rather than by whole rock Rb-Sr or Pb-Pb data (see
later). It should be borne in mind that this may not always
be correct as problems with inheritance and/or authigenic
overgrowths of zircon can res ult in erroneous interpretation.
All ages presented in this review should be quoted with
reference to the original source of the data and not with
respect to this publication.
At least three stages of granitoid plutonism are now
recognized in the region around the edge of the
Witwatersrand Basin:
VC.R
(SOUTH ROODEPOORT)
DE PAN a DOORNFONTEIN
OUTLIERS
,
j
RAAIPAN GROUP
~ELSIC VOLCANICS
~
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o
TA2
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Goldfields
(JJ}
Exposed granite outcrop and name
"""" Exposed greenstone remnant
Sample locality:represented by borehole
intersection or mine sample (name of
mine or borehole number)
81
Figure 2 Simplified map showing the outline of the Witwatersrand Basin and the localities of certain geological units
mentioned in the text. Localities of samples from which detrital zircons were extracted are also shown.
S.Afr.J .Geol., 1991 ,94(1)
89
Table 1 Suggested 'events' in the Provenance areas
of the Dominion and Witwatersrand sequences based
on age populations of detrital zircon and monazite
grains from various conglomerate horizons
Age Population (Ma)
No. of Grains
Events (% of provenance area)
3305
3207
Pre-Dominion (30%)
3 188 ::t 3
4
3 148 ::t 4
3
5
311O::t5
(> 3 100 Ma)
3 093 ::t 5
4
3 076 ::t 3
7
Dominion (58%)
3 056 ::t 6
7
(3 100 - 3 010 Ma)
3036 ::t 6
5
3 010 ::t 6
4
2953 ::t 5
2
Randian (12%)
2936
1
(2 980 - 2 720 Ma)
2901 ::t 7
2
Notes:
(i) Data represent 207Pb /206Pb ages for individual detrital zircon and
monazite grains, from Robb et a/., 1990. 80% of these ages are less than
5% discordant.
(ii) Age population deduced by calculating the mean of those ages which
cluster noticeably around a particular value.
(iii) 'Error' represents the range of ages on either side of the mean value.
(iv) The 3 207 Ma population is reflected as a major population (22 grains
out of 39 with a mean age of 3 203 Ma) of detrital zircon ages obtained
from two conglomerate samples from the Central Rand and Carletonville
areas (Barton et al., 1989). In certain areas, therefore, preserved remnants
of the pre-Dominion basement appear to have been more abundant than
crust formed during Dominion and later events.
1. Pre-Dominion Basement
The oldest granitoid from the hinterland of the
Witwatersrand Basin has been dated at 3 250 Ma and is
represented by tonalitic gneisses and migmatites sampled to
the west of the Coles berg Magnetic Anomaly Trend
(CMAT; Figure 2) in the Kimberley region (Drennan et al.,
1990). Several other granites have now also been shown to
pre-date the Dominion Group and these include two separate
intrusions in the Hartbeesfontein area at 3 l74 Ma and
3 120 Ma,
tonalites and granodiorites from
the
Johannesburg dome at 3 l70 Ma and 3 132 Ma, the
Westerdam dome at 3 096 Ma and the borehole
intersections of granitic rocks east of the Welkom goldfield
(GHI and NG 1) at 3 109 Ma and 3 122 Ma respectively
(Figures 1 and 2). These pre-Dominion granites (i.e.
> 3 074 Ma) are also clearly reflected in the detrital zircon
record within the Witwatersrand conglomerates themselves
(Robb et al., 1990; Barton et al. 1989).
J
2. Dominion Granites
A later generation of granite intrusions - designated
Dominion granites on the basis of the detrital zircon record
(Robb et al., 1990) - were emplaced at around, or slightly
younger than, the age of Dominion volcanism at 3 074 Ma
(Figure 1). These include the Outer Granite Gneiss (OGG)
in the core of the Vredefort structure at 3 080 Ma and the
Ottosdal granite at 3 032 Ma (Figure 2). The emplacement
of these granites predated deposition of the West Rand
Group (Figure 1) and formed part of a major magmatic
episode which included granite plutonism and the
outpouring of up to 3 000 m of bimodal Dominion
volcanics. Abundant evidence for this felsic magmatic event
is also preserved in the record of zircon detritus within
Witwatersrand conglomerates themselves (Table 1; Robb et
al., 1990).
3. Randian Granites
The Randian suite of granites are defined as these emplaced
synchronously with Witwatersrand deposition and these
include the Schweizer-Reneke granite (borehole TKB2)
dated at 2 879 Ma, granitic gneisses intersected in borehole
TA2 at 2 923 Ma, the Gaborone granite complex along the
Botswana border at 2 830 Ma and a granite intersected in
borehole 1633 northwest of the Welkom goldfield, dated at
2 726 Ma. Evidence for these granites also exists in the
detrital zircon record, particularly from the upper Central
Rand Group (Figure 1). No granites coeval with the age of
Ventersdorp volcanism have yet been detected in the
environs of the Witwatersrand Basin, although such granites
may well exist elsewhere in the Kaapvaal craton.
Volcanic events
As previously mentioned, the time of Witwatersrand
deposition is now accurately constrained between the age of
the terminal stages of Dominion volcanism at 3 074 Ma and
that of early Ventersdorp volcanism at 2714 Ma
(Annstrong et al., 1991). Both of these major volcanic
events probably occurred over a relatively short time-span.
A figure of about 6 Ma is suggested for the Dominion event
because the OGG at Vredefort, which unconformably
underlies the Dominion sediments, has yielded a whole rock
- Pb-Pb age of 3 080 ::t 20 Ma (Hart et al., 1981). A figure
of somewhat in excess of 5 Ma is suggested for the
Ventersdorp event, where the Makwassie porphyries
towards the top of the sequence have been dated at
2 709 Ma while the unit at the base gives 2 714 Ma.
A tentative age determination is now also available for
felsic volcanics from the Kraaipan Group north of
Schweizer-Reneke. A poor-quality, heterogeneous zircon
population has made dating this unit difficult, and the best
age currently available is 3 066 ::t 40 Ma (Robb et al., in
prep.). This age is very similar to that of the Dominion
volcanics and points to another component of the major
magmatic event at circa 3 070 Ma in the Witwatersrand
hinterland. A tentative age is also now available for the
Crown Lava, or Jeppestown Amygdaloid, near the top of the
West Rand Group. This age of 2 914 ::t 8 Ma (Armstrong
et at., 1991) is consistent with the constraints imposed by
the detrital zircon record, but, because of younger
overgrowths and inconsistent '2IJ7 Pb / '2IJ6Pb ratios, should be
regarded as a tentative age of crystallization of the Crown
Lava.
S.-Afr.Tydskr.Geol.,1991,94(1)
90
Detritus and sediment deposition
Two recent studies have provided 207Pb / 206 Pb ages for a
total of some 85 single zircon and monazite grains from a
variety of conglomeratic units extending from the base of
the Dominion Group up to the Ventersdorp Contact Reef
(Barton et al., 1989; Robb et al., 1990). These data
collectively indicate that the ages of individual detrital
zircon grains become consistently younger as one progresses
upwards in the stratigraphic record, and imply that detritus
was derived from unroofing and erosion of successively
younger granites as the basin evolved. These data can also
be used to broadly constrain the time of sediment input
because the youngest detrital zircon dated in any particular
horizon provides the maximum age of deposition of that
horizon. Using these constraints, together with the ages of
other relevant igneous units, the following brackets on the
age of sedimentation for the major sedimentary units can be
provided:
1. Dominion Group sediments (i.e. Renosterhoek Quartzite
Formation); 3 096 - 3 074 Ma. (The maximum age
constraint of 3 096 Ma is obtained from the U-Pb zircon
age of the Westerdam dome, which unconformably underlies
Dominion sediments along its southern flank. The age of
3 096 M a is probably a more reliable maximum constraint
on Dominion sedimentation than that provided by the whole
rock age for the OGG at Vredefort. See earlier).
2. West Rand Group; 2980 - 2 914 Ma. (The minimum
age constraint for the West Rand Group (2 914 Ma) is
tentatively provided by the U-Pb zircon age for the Crown
Lava of Armstrong et al., 1991, which occurs near the top of
the Jeppestown Sub-group. The maximum age constraint is
provided by the youngest zircon in the Orange Grove
Quartzite Formation (Barton et al., 1989)).
3. Central Rand Group; 2840 - 2714 Ma (Figure 1). The
maximum age for the Central Rand Group is loosely
constrained at present by the age of the youngest zircon
grain dated from the Ventersdorp Contact Reef; Figure 1).
These constraints indicate that deposition of the
Witwatersrand sediments occurred episodically, but
continuously, over the entire 360 Ma interval between
3 074 Ma and 2 714 Ma. These age constraints provide a
chronological framework for the complex evolutionary
sequence of events that resulted, inter alia in the deposition
of the auriferous Witwatersrand sediments. These sediments
clearly reflect the depositional responses to a series of ongoing tectonic and magmatic events in the hinterland (Robb
et al., 1990; Stanistreet & McCarthy, 1991).
In addition to constraining the ages of sedimentation, the
detrital zircon-monazite data also reflect the span of ages in
the sediment provenance area. Table 1 presents a variety of
age populations deduced subjectively on the basis of
obvious clustering in the ages of detrital zircon or monazite
from
within
the
Dominion
and
Witwatersrand
conglomerates. The pattern of detrital grain ages exhibits a
normal distribution (Robb et al., 1990) with the geometric
mean occurring at 3 073 Ma. This age accords with that of
Dominion volcanism (Armstrong et al., 1991) and,
consequently, all plutonic and volcanic events that fall
within a few tens of millions of years on either side of this
age are designated 'Dominion' (Table 1). All granites (and
volcanic rocks) older than 3 100 Ma are designated 'PreDominion', while younger igneous events that coincide with
the time span during which the Witwatersrand sediments
were deposited are classified as Witwatersrandian events. If
these detrital grains constitute a representative sampling of
the provenance area, it would appear that some 30% of the
source rocks are pre-Dominion in age, 58% formed during
the Dominion event and a smaller, yet significant,
proportion (12%) of the source area was introduced into the
crust during Witwatersrand deposition.
Metamorphic-hydrothermal events
Metamorphic and/or hydrothermal events in the rock record
are generally difficult to date isotopically because of a lack
of suitable minerals and a poor understanding of the
complexities of element redistribution and isotope re-setting
during sub-solidus events. It is, however, becoming
increasingly apparent, particularly in old rocks that have
been subjected to complex prehistories, that Rb-Sr whole
rock ages are often younger than U-Pb zircon ages from the
same suite. In Table 2 a comparison is made between
recently published U-Pb zircon ages from several different
rock types in the Kaapvaal Craton and corresponding Rb-Sr
(and Pb-Pb) whole rock ages. It is apparent that very little
correspondence exists between ages obtained from the U-Pb
zircon and the two whole rock isotope systems, even given
the fact that errors are usually considerably larger for the
Rb-Sr isochron technique. Rb-Sr ages are generally younger
than U-Pb zircon ages, but occasionally may be older, as
indicated by the Kaap Valley tonalite pluton, for example
(Table 2). Only very seldom do the U-Pb zircon and Rb-Sr
Table 2 Selected comparisons of Rb-Sr whole rock
and U-Pb zircon ages in rocks of the Kaapvaal Craton
U-Pb zircon
(Ma)
Rb-Sr whole rock
(Ma)
Kaap Valley tonalite pluton
3 227 ± 11
3491 ± 1662
1
Theespruit trondhjemite pluton 3 443 + 4/_3
3432 ± 1352
1
Doornhoek trondhjemite pluton 3 448 + 4/_3
3 176 ± 3022
1
Nelspruit batholith
3 106 + 2/_3
3 149 ± 1252
1
Mpuluzi batholith
3 107 + 4/_3
3028 ± 142
Bosmanskop syenite pluton
3 107 ± 21
2848 ± 31 2
1
Mpageni granite pluton
c. 2 740
2496 ± 1763
5
Johannesburg dome - tonalite 3 170 ± 34
2264 ± 944
6
Westerdam dome - granodiorite 3096 ± 32
2759 ± 674
Schweizer-Reneke dome adamellite
2879 ± 26
2 767 ± 12if
Gaborone granite complex
2830 ± 108
c. 2 6007
Dominion Group - volcanics
3 074 ± 69 2 725 ± 75(Pb-Pb/o
Ventersdorp Group - volcanics 2 714 ± 89
c. 2 15011
Sources of data:
1. Kamo et ai. (1990) 2. Barton et al. (1983) 3. De Gasparis
(1967) 4. Barton et al. (1986) 5. Anhaeusser & Burger (1982)
6. Robb et ai. in prep. 7. Harding et ai. (1974) 8. Sibiya (1988)
9. Armstrong et ai. (1991, in press) 10. Armstrong (1987)
11. Crampton (1974)
S.AfrJ.Geol.,1991,94(1)
whole rock ages agree. This trend is particularly evident
among rocks associated with the Witwatersrand triad and
environs because of the long and complex record of
superimposed events that pertain to this region.
Accordingly, for the purposes of the present compilation, all
whole rock Rb-Sr ages have been placed, together with ages
that clearly are re-set, into a category termed 'metamorphichydrothermal events' (Figure 1). In many cases this
categorization can be shown to be justified, but in others this
interpretation may be contentious as there is no way of
detennining whether the data have meaning or are simply
spurious. The following ages can justifiably be regarded as
representing later metamorphic and/or hydrothermal events
in the environs of the Witwatersrand Basin (refer to
Figure 1 for sources of data):
1. 2 940 Ma old metamorphic overgrowths on zircons
within 3 250 Ma old tonalitic gneisses from borehole
CB 1 on the CMA T (Figure 2) in the Kimberley region.
2. 2 883 Ma old rutile from a hydrothermal vein that cuts
across compositionally banded tonalitic gneisses in
borehole T A2 to the south-west of the SchweizerReneke. This hydrothermal event is probably related to
the emplacement of the nearby Schweizer-Reneke dome
(Figure 2) at 2 879 Ma.
3. The Schweizer-Reneke dome itself appears to have been
re-set between approximately 2 780 Ma (Pb-Pb) and
2 767 Ma (Rb-Sr).
4. The Dominion lavas may also have been metamorphosed
and/or hydrothermally altered subsequent to their
deposition, as suggested by ages of c. 2 950 Ma (pb-Pb
pyrite) and c. 2 800 Ma (Pb-Pb).
5. In the Vredefort dome the OGG, which undeilies
> 3 074 Ma old Dominion sediments in the core of the
structure, appears to have been affected by an event at c.
3 000 Ma (Rb-Sr), while the Inlandsea Leucogranofels
(ILG) underwent prograde granulite facies metamorphism
and dehydration at 2 830 Ma (Hart et ai., 1981).
6. The latter event may also be reflected in the 3 096 Ma
old Westerdam dome which records a Pb-Pb whole rock
age of 2 810 Ma. The same rocks also yield a Rb-Sr
whole rock age of 2759 Ma.
7. 3 170 Ma old tonalites in the Johannesburg dome appear
to have been isotopically reset during two discrete
metamorphic and/or hydrothermal events, one at
3 030 Ma (Pb-Pb) and the other considerably later at
2 268 Ma (Rb-Sr).
8. Several periods of metamorphism and/or hydrothermal
actIvIty may have been recorded within the
Witwatersrand Basin itself. An early event appears to
have resulted in the authigenic growth of rutile in the
Promise Reefs at c. 2 580 Ma, while various generations
of authigenic pyrite might have formed at c. 2 500 Ma
and c. 2 260 Ma. Kerogen, which occurs very late in the
paragenetic sequence of Witwatersrand mineralization,
appears to have been isotopically homogenized at c.
2 320 Ma. Uraninites yield a Pb-Pb model age of c.
2 040 Ma, a figure which coincides with the
emplacement of the Bushveld Complex. Finally,
Witwatersrand shales passed through their K-Ar blocking
temperature approximately 1 945 Ma ago.
91
9. The 2 700 Ma Ventersdorp volcanics also record a
complex sub-solidus history and appear to have been reset at c. 2 300 Ma (Pb-Pb) and 2 154 Ma (Rb-Sr).
The following ages possibly also reflect younger events
that have reset the whole rock isotopic systems, but these
cannot be substantiated because corresponding U-Pb zircon
ages are not yet available:a. 3 060 Ma (Pb-Pb) and 3 014 Ma (Rb-Sr) ages in the
granodioritic phase of the Johannesburg dome.
b. 3 015 Ma (Pb-Pb) and 2 875 Ma (Rb-Sr) ages for the De
Pan tonalite gneiss outlier along the Rand Anticline
(Figure 2).
c. a 2 525 Ma (Rb-Sr) age for the Doornfontein granite
outlier, also along the Rand anticline (Figure 2).
The whole rock ages described above collectively point to
a long history of events in the Archaean and early
Proterozoic rocks of the Kaapvaal Craton in and around the
Witwatersrand Basin. Several of the events, such as the
apparent resetting of the Rb-Sr isotopic clock in the
Schweizer-Reneke, Westerdam and Vredefort granites,
occurred during the 360 Ma of Witwatersrand deposition
and can most logically be attributed to the continuing
magmatic or hydrothermal activity now recorded within the
Witwatersrand Basin hinterland (Figure 1). Many more
granite ages are still needed before the full extent of syndepositional magmatic aCtiVIty is documented. The
formation of authigenic minerals such as (inter alia) rutile,
pyrite and kerogen within the Witwatersrand Basin cannot at
this stage be related unequivocally to any other events on
the Kaapvaal Craton. In spite of the uncertainties in
interpreting the whole rock isotopic data, two ages, at c.
2 500 Ma and c. 2 300 Ma, do, however, occur repeatedly
(Figure 1) and may be significant. Again, many more
precise age dates are required before the early Proterozoic
chronostratigraphic sequence on the Kaapvaal Craton is
fully documented. One of the last events to have affected
rocks of the Witwatersrand triad occurred at c. 2 000 Ma
and could conceivably be related to intrusion of the
Bushveld Complex and/or the Vredefort catastrophism.
Towards a time-constrained depositional framework
Many attempts have been made in the recent past to provide
models for the tectonic framework of Witwatersrand
deposition (Bickle & Erikssen, 1982; Burke et ai., 1986;
Drennan et al., 1990; Myers et al., 1990; Robb et ai., 1990;
Stanistreet et al., 1986, 1991; Winter, 1987, etc.). Although
these models differ in detail most of them concur in
regarding the encroachment of the Zimbabwe and Kaapvaal
Cratons as being a principal causative mechanism in the late
Archaean crustal evolution of southern Africa. The recent
overview by Stanistreet & McCarthy (1991) has put forward
the notion that this period of crustal evolution developed
within an entire Wilson Cycle. The following section will
,attempt to place some broad time-constraints on the recent
ideas regarding the evolution of the Kaapvaal Craton and
the tectonic framework of Witwatersrand deposition.
Although the ideas proposed in Stanistreet & McCarthy
(1991) have been selected as a framework within which to
establish a chronological sequence, the model presented
S.-Afr.Tydskr.Geol.,1991,94(1)
92
A. EXTENSIONAL RIFT BASIN
c. 3100-3010 Ma
DOMINION VOLCANOSEDIMENTARY SEQUENCE
C. INDENTATION STAGE -
"SQUEEZE OUT BASIN"
c. 2840-2720 Ma
UPPER CEN1 RAL
RAND GROUP
>3100 WI OLD
GRANITE -GREENSTONE
BASEMENT
D. IMPACTOGENAL RIFT
c. 2710 Ma
B. FORELAND BASIN STAGE
LIMPOPO OCEAN
Figure 3 Tectonic and depositional framework for the Witwatersrand Triad as envisaged by Stanistreet & McCarthy (1991), with timeconstraints imposed by recent U-Pb zircon ages summarized in the text.
below differs in certain respects, particularly those
pertaining to the earlier events. Furthermore, in the sections
that follow, emphasis is placed on delineating the various
magmatic episodes that occurred during this period of
crustal evolution, as this is one factor that has been largely
overlooked in previous considerations of the Witwatersrand
Basin.
event. Irrespective of whether the tectonic setting described
above is correct or not, the period of time between
3 100 - 3 010 Ma
witnessed
the
emplacement of
voluminous volcanic and plutonic rocks which collectively
represent as much as 60% of' the Witwatersrand Basin
source area (Table 1). This scenario is schematically
depicted in Figure 3a.
Extensional Rift Basin Stage - Dominion Event -
Foreland Basin Stage - West Rand G roup and
Lower Central Rand Group - c. 2 980 - 2900 Ma
The lower West Rand Group sediments represent an
epicontinental cover sequence deposited on a tidallydominated platform deriving detritus from an upland region
somewhere to the north and probably north-west of the
present basin. Upper West Rand Group and lower Central
Rand Group sediments are considered to represent fluviodeltaic deposits, deriving detritus predominantly from the
north-west. Stanistreet & McCarthy (1991) regard early
West Rand Group deposition as the response to thermal
cooling and subsidence subsequent to the outpouring of the
voluminous bimodal Dominion volcanics. The detrital
zircon age data for the Orange Grove Quartzite Formation at
the base of the West Rand Group indicate, however, a
maximum age of deposition of 2 980 Ma (Barton et aI.,
1989). This implies that West Rand Group deposition did
not commence for almost 95 Ma after the Dominion lavas
had been extruded. Consequently, the present model prefers
to place the entire West Rand Group, together with the
lower Central Rand Group, into a single developmental
stage involving the transformation of a tidally-dominated
epicontinental sequence into a fluvio-deltaic-dominated
foreland basin. The initiation of a foreland basin may have
been a response to convergence initiated by the approach of
the Zimbabwe Craton from the north (Burke et al.,1986;
Stanistreet & McCarthy, 1991; Winter, 1987). This
convergent scenario is depicted in Figure 3b. This stage
also envisages emplacement of granites which formed
during subduction in the period between 2 980 - 2 900 Ma.
Evidence for the plutonic activity is poorly represented
among the present data base of source-area granitoids
(Figure 1), but is reflected in at least three age populations
from the detrital zircon age data (Table 1).
c. 3 100 - 3 010 Ma
The Dominion Group comprises a thin basal quartz arenite
and conglomerate sequence which is overlain by up to 3 km
of bimodal tholeiite-andesite and dacite-rhyolite volcanics.
The tectonic setting for the deposition of this sequence is
controversial and has been attributed to an Andean-type
volcanic arc (Burke et al., 1986), as well as to a failed rift
basin in an extensional setting (Bickle & Eriksson, 1982;
Stanistreet & McCarthy, 1991). The latter setting is
favoured here because detailed geochemical studies have
recently shown that the petrogenesis of Dominion volcanics
is consistent with eruption in a continental environment
characterized by rifting (Marsh et al., 1989). The Dominion
event is considered to have occurred between c.
3 100 - 3 010 Ma ago and apparently peaked with the
outpouring of the bimodal Dominion volcanics at 3 074 Ma
(Armstrong 1991). In addition to Dominion volcanism it is
now apparent that felsic volcanics associated with the
Kraaipan Group (Figure 2) are also c. 3 070 Ma old (Robb
et al., in prep) and possibly represent rift-related volcanism
that developed closer to the edge of the continental margin
at that time. The Dominion event was also accompanied by
plutonic magmatism which is recorded in at least five
distinct age populations among the detrital zircon age data
recorded in Table 1. One of these populations undoubtedly
reflects the Dominion volcanic event itself (i.e. 3 076 ±
3 Ma; 7 zircons; Table 1) whereas the others (3 093 ±
5 Ma, 3 056 ± 6 Ma, 3 036 ± 6 Ma and 3 010 ± 6 Ma;
Table 1) probably represent plutonic events associated with
the extensional rift basin stage. Dating of granites
themselves in the source area also record the presence of at
least two plutons (i.e. Westerdam, Ottosdal; Figure 1)
which formed during the period ascribed to the Dominion
S.Afr.J .Oeol., 1991 ,94(1)
It is pertinent to stress the possible significance of the
tentative U-Pb zircon age of 2914 ± 8 Ma recently
obtained by Armstrong et al., (1991) for the Crown lava, a
thin but persistent volcanic horizon that occurs towards the
top of the West Rand Group. This age is consistent with
detrital zircon age data which constrains West Rand Group
deposition (Figure 1). The event itself may reflect more
intensive volcanic activity elsewhere on the craton. The
Nsuze volcanics, for example, at the base of the Pongola
Supergroup in the southeastern Kaapvaal Craton, have been
dated at 2 940 ± 22 Ma (Hegner et ai., 1984) by U-Pb
zircon techniques. These two ages overlap within error and
may, therefore, reflect the same, or similar, events occurring
over widespread areas of the craton during the early
convergence stage of Witwatersrand Basin evolution.
Indentation Stage - Upper Central Rand Group c. 2 840 - 2 720 Ma.
Central Rand Group sediments were laid down in a
shrinking basin where deposition was controlled by synsedimentary strike-slip faults (Stanistreet & McCarthy,
1991). Alluvial fan sedimentation, particularly towards the
upper Central Rand Group, was controlled by block faulting
developed under a compressive regime that was intensified
by collision of the Zimbabwe and Kaapvaal Cratons. The
overall sense of strike-slip motion associated with this stress
regime suggests that the Witwatersrand Basin was being
'squeezed-out' towards the south-east in response to the
tectonic escape of crust from continental interiors during
collision (Stanistreet & McCarthy, 1991). During this event
crustal thickening resulted in the continued generation of
felsic magma and emplacement of high-level granite plutons
adjacent to the edge of the Central Rand Group basin.
Evidence for this magmatism is not recorded in the present
detrital zircon age data (Table 1), but at least three granite
plutons (the Gaborone granite, the Schweizer-Reneke dome
intersected in borehole TKB2, and a granite north-west of
the Welkom goldfield intersected in borehole 1633;
Figure 2) yield ages that either immediately predate or
coincide with the period of upper Witwatersrand deposition.
It is pertinent to note that the borehole 1633 granite was
emplaced very late in the Witwatersrand depositional history
(i.e. 2 726 Ma; Figure 1) and this event could be related to
the stress regime that was responsible for overturning large
segments of the Central Rand Group along the western edge
of the Welkom goldfields in pre-Ventersdorp times. It is
also envisaged that the Kraaipan volcanic basin, which
formed close to the 'Nestern edge of the continental margin
during the Dominion event, was severely deformed by
compressive stress from the west (Stanistreet & McCarthy,
1991) set up during this stage (i.e. Amalia greenstone belt,
Figure 3c). The bulk of the Witwatersrand Basin itself
appears to have been largely beyond the influence of this
easterly-directed stress regime at this time.
Contrary to many previous ideas, therefore, it is clear that
granite magmatism persisted throughout the depositional
history of the Witwatersrand Basin, forming in response to
both the convergence stage of late West Rand Group
deposition as well as during the collisional stage that was
active during upper Central Rand Group times. The
93
Archaean granite basement did not merely represent a floor
upon which the sediments and volcanics of the
Witwatersrand triad were deposited, but continued evolving
in the source area during the long-lived depositional and
tectodic history of these sequences.
Impactogenal Rift - Ventersdorp Volcanism c. 2710 Ma
The compressional stress regime operating during the
indentation stage of Central Rand Group deposition
eventually culminated in the outpouring of the
Klipriviersberg Group flood basalts at 2 714 Ma (Armstrong
et al.,1991). This event was rapidly followed by relaxation
of the oblique-slip and reverse fault zones of the earlier
event, and the formation of grabens and half-grabens into
which the Platberg Group sediments were deposited (Myers
et al., 1990; Figure 3d). During this late extensional phase
it is noted that normal movement often occurred along the
same faults which acted as thrust faults during the earlier
compressional event, explaining why Platberg sediments are
commonly deposited within the area of the Witwatersrand
Basin itself (Myers et ai., 1990). No record of plutonic
activity associated with this extensional stage is known in
the immediate environs of the Witwatersrand Basin,
although :s; 2 700 Ma granites may have been intruded into
the crust elsewhere on the craton.
Conclusions
Many new inferences regarding the tectonic and
depositional framework of the Witwatersrand Basin have
been published over the last few years. Most of these
models share a fair degree of consensus and have the
collision between the Zimbabwe and Kaapvaal Cratons as
their common causative mechanism. While providing an
adequate plate tectonic-related conceptual framework for
Witwatersrand deposition, most of the models (i) do not
provide a well-constrained chronological framework for the
envisaged sequence of events and (ii) generally ignore the
magmatic aspects involved in these processes. The latter fact
is surprising considering that modern analogues of these
processes are characterized by intense plutonic, volcanic and
hydrothermal activity. Such considerations are all the more
important in the Witwatersrand context because of the
generally held consensus that a fertile source· area was
essential to the development of the initial placer
concentrations of gold and uranium. The depositional
framework presented above attempts to provide timeconstraints for a sequence of events that, although probably
still contentious in detail, incorporates most of the recent
ideas that have been presented regarding the secular tectonic
and depositional evolution of the Witwatersrand Basin.
These time-constraints are based largely on relatively new
U-Pb zircon dating by Armstrong et al. (1986; 1991),
Barton et al. (1989) and Robb et al. (1990; in prep.). In
addition, the above model, unlike its predecessors, attempts
to include the extremely important effects of volcanism and
granitoid plutonism, particularly in areas adjacent to the
edge of the basin, on the deposition of sediments. Although
the model will undoubtedly be refined and modified, the
magmatic aspects of the tectonic and depositional
94
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Acknowledgments
This paper represents a summary of many people's data,
some as yet unpublished. The authors are particularly
grateful to Richard Armstrong, Ian Stanistreet and Terence
McCarthy for glimpses at work that was still in press at the
time of writing. Three sets of reviewers and the volume
editor provided many helpful comments which improved the
manuscript. Any short-comings and biases in the
interpretation of all the data presented herein are solely the
responsibility of the authors. Lyn Whitfield and Pat King are
thanked for drafting and secretarial assitance.
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