Detrital origin of hydrothermal Witwatersrand goldÐa review
Hartwig E. Frimmel
Department of Geological Sciences, University of Cape Town, Rondebosch, 7700, South Africa
ABSTRACT
The Witwatersrand `basin' is the largest known gold province in
the world. The gold deposits have been worked for moren than
100 years but there is still controversy about the ore forming
process. Detailed petrographic studies often reveal that the gold
is late in the paragenetic sequence, which has led many
researchers to propose a hydrothermal origin for the gold.
However, observations, such as the occurrence of rounded, disclike gold particles next to irregularly shaped or idiomorphic
secondary gold particles in the same sample, suggest an initial
detrital gold source within the Witwatersrand strata. Mineral
chemical and isotopic data, together with SEM cathodoluminescence
Introduction
The fluvial, low-grade metamorphosed
quartz pebble conglomerates (reefs) of the
late Archaean Witwatersrand Supergroup in South Africa (Fig. 1) host the
world's largest known gold `reservoir'.
The origin of the gold, which is commonly associated with pyrite, uraninite
or hydrocarbon, seems to remain as
controversial as seven decades ago.
Since Graton (1930) first challenged
the sedimentary model originally proposed by Mellor (1916), by advocating
a magmatic-hydrothermal model, a hydrothermal origin of the gold has been
postulated almost at regular intervals,
only to be rejected each time by arguments in favour of a detrital origin. A
decade ago a metamorphic-hydrothermal model was proposed (Phillips et al.,
1987; Phillips and Myers, 1989) which
prompted a vigorous debate leading to
a number of papers again refuting this
hydrothermal origin (e.g. Reimer and
Mossman, 1990; Robb and Meyer, 1991).
Recently, Barnicoat et al. (1997) reexamined the epigenetic-hydrothermal
model and have established a fourstage paragenetic sequence based on
detailed petrographic observations.
The authors argue that because gold
occurs late in their paragenetic sequence, a `compelling' argument for a
hydrothermal origin of all the gold, as
well as all the pyrite, uraninite and
hydrocarbon in the Witwatersrand `basin' can be made, without attributing
the required hydrothermal activity to a
Correspondence: Fax: + 27-21-650 3783;
E-mail: [email protected]
imaging and fluid inclusion studies, provide evidence for smallscale variations in the fluid chemistry ± a requirement for the
short-range mobilization of the gold. The existing data and
observations on the Witwatersrand rocks support a model of
hydrothermally altered, metamorphosed placer deposits, with at
least two subsequent gold mobilization events: hydrothermal
infiltration in early Transvaal time (2.6±2.5 Ga) and during the
2.020 Ga Vredefort impact event.
Terra Nova, 9, 192±197, 1997
specific geological event. It should be
noted that the term `basin', though often
used, is misleading as the 3.09±2.71 Gyr
old Witwatersrand Supergroup (Fig. 1)
represents an erosional remnant of tectonically stacked sediments originally
deposited in a variety of tectonic settings (Coward et al., 1995). Elevated
gold concentrations are confined only
to fluvial deposits. These deposits, accumulated during compressional phases,
are found predominantly in the Central
Rand Group, and are related to uplift
in a hinterland believed to have resulted
from the collision of the Kaapvaal and
Zimbabwe cratons (Limpopo orogeny).
Phillips and Law (1994) have proposed a link between the emplacement
of the Bushveld Igneous Complex at
2.05±2.06 Ga and metamorphic/hydrothermal alteration in the Witwatersrand `basin'. Recent studies on the postdepositional alteration history of the
Witwatersrand strata (Robb and Meyer,
1995; Frimmel, 1997) have revealed a
series of at least five hydrothermal/
metamorphic phases/events over the
time span of 2.7±2.0 Ga. They range
from pre-Transvaal (2.640±2.709 Ga)
thrust-related metamorphism along the
northern basin margin (Coetzee et al.,
1996); to early Transvaal (2.55±2.58 Ga)
Fig. 1 Simplified surface and subsurface geological map of the Witwatersrand `basin'
and the adjacent Bushveld Igneous Complex. Stratigraphic column shows gold-bearing
conglomerates (italics) referred to in the text, and post-depositional alteration events
(Frimmel, 1997; Robb and Meyer, 1995): I, thrust-related metamorphism along northern
basin margin; II, hydrothermal infiltration in Basal Reef, Welkom goldfield; III, burial
metamorphism; IV, Bushveld-related thermal metamorphism; V, Vredefort impactrelated hydrothermal infiltration; Asterisks indicate gold mobilizing events dated so far;
K, Klerksdorp; J, Johannesburg; W, Welkom.
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hydrothermal infiltration; syn-Transvaal
diagenesis and burial metamorphism
(2.2±2.3 Ga); and thermal metamorphism at the time of the emplacement of
the 2.055±2.060 Ga Bushveld Igneous
Complex, and hydrothermal infiltration associated with the 2.020 Ga Vredefort impact event. Gold was mobilized during at least two of these events,
at early Transvaal times and at the
Vredefort impact event.
In the view of the contrasting models
recently proposed for the genesis of the
Witwatersrand gold, it seems appropriate to review the most important observations and data relevant to this ongoing debate on the origin of the Witwatersrand gold.
Gold, pyrite and uraninite
grain morphology
In a detailed scanning electron microscopic (SEM) case study, Minter et al.
(1993) documented the three-dimensional shape and morphology of over
5000 gold grains (38±473 mm in size)
carefully released by sample digestion
in hydrofluoric acid from the foresets of
a cross-bedded conglomerate sample of
the Basal Reef in the Welkom goldfield.
Two distinct morphological types of
gold particles were found next to each
other: (i) rounded gold particles with
spheroidal, disc-like and torroidal forms,
and with a rough indented micro relief
caused by surrounding rounded quartz
and heavy minerals, such as pyrite and
zircon; and (ii) irregularly shaped gold
particles with smooth surfaces, locally
forming euhedral overgrowths which
are clearly secondary. Many rounded
gold grains display double-sided overturned edges interpreted to be the result
of mechanical deformation (peening)
during deflation by saltating sand grains
(W.E.L. Minter, unpublished experimental data). In-situ disc-like particles
with overturned edges can also be found
in polished thin-sections (Fig. 2a).
Similarly, pyrite ± by far the most
common sulphide phase in the Witwatersrand strata ± occurs in different
textural forms, often within the same
sample (Hallbauer, 1986): (i) wellrounded, compact pyrite; (ii) wellrounded but larger and `porous' pyrite
(concretionary pyrite); and (iii) idiomorphic, recrystallized pyrite, commonly
as overgrowths over the rounded pyrite
types. The recrystallized pyrite is usually the richest in Au (Phillips and Myers,
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Fig. 2 Photomicrographs. (a) in situ oblate detrital gold particle with overturned edges
on both sides (arrow) between rounded pyrite (Py) and detrital quartz (Qz) clasts (as
revealed by SEM-cathodoluminescence imaging) in the Basal Reef, Welkom goldfield;
scale bar = 1 mm; (b) SEM cathodoluminescence image of the Ventersdorp Contact
Reef showing rounded, and in placed corroded pyrite (non-luminescent) set between
detrital quartz clasts with highly variable cathodoluminescence intensities. Note intense
fracturing of the quartz clasts with the secondary porosity filled with weakly luminescent
secondary quartz; scale bar = 1 mm; (c) SEM cathodoluminescence image of
hydrothermal quartz from an auriferous quartz vein in the Ventersdorp Contact Reef,
showing a complex growth zonation in the core, a partly corroded mantle (arrow) and
further growth zones forming the rim; scale bar = 0.1 mm.
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H.E. Frimmel
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1989) and is generally believed to be
related to some hydrothermal/metamorphic fluid. In the hydrothermal
model of Phillips and Myers (1989),
the rounded pyrite grains would represent pseudomorphous replacements
after detrital Fe-oxide particles. In contrast, in the placer model, the rounded
compact pyrite is considered detrital
and the porous pyrite of diagenetic
origin (Hallbauer, 1986).
As with gold and pyrite, uraninite
occurs in the form of rounded particles
and as subhedral grains associated
with hydrocarbon (Hallbauer, 1986).
The latter type is generally accepted as
being hydrothermal/metamorphic. The
rounded uraninite has been interpreted
as detrital in origin and suggestive of an
oxygen-deficient late Archaean atmosphere by those favouring a placer
model (e.g. Schidlowski, 1981; Hallbauer, 1986). In contrast, Barnicoat et
al. (1997) interpret all the uraninite to
be of hydrothermal origin.
The rounded varieties of gold, pyrite
and uraninite typically occur along the
footwall contacts of conglomerate
beds, together with other heavy mineral
constituents (Feather and Koen, 1975).
Throughout the Witwatersrand `basin'
there is little systematic relationship
between gold, pyrite, uraninite and
their respective textural varieties. However, within a given reef, a strong correlation can exist between gold and
pyrite, gold and uraninite, or gold and
hydrocarbon. The ratio between rounded
and subhedral to euhedral, secondary
particles of these minerals strongly varies between reefs and goldfields. Gold
particles of unequivocal detrital morphology are the exception rather than
the rule. Apparently the chances of
survival of delicate morphological features of placer gold particles, such as
over-turned rims, during post depositional alteration are slim. This is not
surprising, considering the recrystallization of the host rocks at metamorphic
temperatures and pressures in excess of
300 8C and 2.5 kbar, respectively (Frimmel, 1994, 1997). In the gold-bearing
conglomerate beds where hydrothermal alteration is pronounced (Barnicoat et al., 1997), such as the Ventersdorp Contact Reef (VCR) above the
Witwatersrand Supergroup, detrital
gold is rare to absent due to secondary
mobilization and recrystallization. The
same applies to pyrite and uraninite.
The dominance of the respective sec194
ondary, unequivocally hydrothermal
textural types over rounded ones, is
well correlated with the local extent of
fluid±rock interaction.
Gold, pyrite and uraninite
grain chemistry
Individual gold particles in the Witwatersrand `basin', irrespective of their
morphology and textural framework,
are homogenous with respect to the
distribution of Ag and Hg. This is consistent with diffusion models for these
elements through gold (Frimmel et al.,
1993; Frimmel and Gartz, 1997). To
date, the only exception is gold in late
hydrothermal quartz veins that crosscut, and thus post-date, pseudotachylites which have been dated at 2.02 Ga ±
the time of the Vredefort impact event
(Trieloff et al., 1994; Spray et al., 1995;
Kamo et al., 1996; Moser, 1997).
Other studies on the chemistry of
gold particles (Reid et al., 1988; Frimmel and Gartz, 1997) have documented
a considerable intrasample variation in
Ag and Hg concentrations for individual gold particles. On a thin-section
scale, the Ag and Hg concentrations
between gold grains can vary by up to
3 and 6 wt%, respectively. If the gold
were of hydrothermal origin, a uniform
composition of the gold particles along
a postulated fluid channelway (i.e. a
single ore horizon) would be expected
(cf. Barnicoat et al., 1997). We have
studied the reef that experienced the
most intense fluid±rock interaction,
the VCR. The relatively high fluid:rock
ratio there may be explained by a basinwide relatively impermeable cover in
the form of a blanket of metabasic
rocks of the Ventersdorp Supergroup
(Fig. 1). At a mine-scale, the gold chemistry in the VCR is relatively uniform,
though not homogenous. Between neighbouring mines, however, there are major differences in the gold chemistry.
For example, the Ag contents of gold
particles in the VCR at the Driefontein
mine range between 9.0 and 10.5 wt%,
whereas those in the same reef at the
West Driefontein mine range between
17.1 and 19.0 wt% (Frimmel and Gartz,
1997). Unless the gold is related to two
distinct mineralizing fluids, it is difficult
to reconcile a hydrothermal origin of the
gold with these mineral-chemical data.
Phillips and Myers (1989) postulated
that a single Au-S-bearing metamorphic
fluid was responsible for the gold
mineralization in the reefs. In that model, heavy mineral sands, consisting of
Fe- and Ti-oxides, were sulphidized by
the gold-mineralizing fluid. Although
there are examples of local sulphidation
of various detrital clasts (Ramdohr,
1958; Myers et al., 1993), a basin-wide
sulphidation process is not likely and
previous arguments against it have already been summarized by Reimer and
Mossman (1990). Preliminary SHRIMP
analyses across individual pyrite grains
indicate large variations in the d34S values
between adjacent pyrite grains and also
within single grains (Eldrige et al., 1993;
Armstrong et al., 1995). These variations are not consistent with a hydrothermal replacement model. Furthermore, compact rounded pyrite and
arsenopyrite often show oscillatory zonation, defined by their As concentrations. The growth zones are frequently
truncated at grain boundaries (McLean
and Fleet, 1990), clearly demonstrating
mechanical abrasion which could have
occurred only prior to deposition.
The Black Reef, a conglomerate bed
at the base of the Transvaal Supergroup (Fig. 1), is comparable to the
gold-, pyrite-, and uraninite-bearing
metaconglomerates within the Witwatersrand Supergroup except for its lower metamorphic grade. Variable trace
element and U±Pb isotope compositions of the different morphological
types of pyrite in that reef support a
detrital origin for the compact rounded
pyrite and a syn-sedimentary formation of the concretionary pyrite (Barton
and Hallbauer, 1996). By analogy, the
same genesis is inferred here for these
two morphological pyrite types in the
Witwatersrand sediments.
Highly variable ThO2/UO2 ratios
have been reported for the rounded
uraninite grains and used as evidence
for a granitic/pegmatitic source (Feather and Glatthaar, 1987). This variation,
together with relatively high Th contents, preclude a low-temperature hydrothermal origin as suggested by Barnicoat et al. (1997). A detrital origin is
further supported by U±Pb and Pb±Pb
isotope data for rounded uraninite,
pyrite, and sphalerite grains, and Os±
Ir isotope data for osmiridium grains,
which indicate derivation from sources
of variable age and chemistry which are
older than the Witwatersrand sediments (Rundle and Snelling, 1977; Saager, 1981; Hart and Kinloch, 1989;
Barton and Hallbauer, 1996).
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Small-scale variations in fluid
chemistry
Hydrothermal models for the origin of
the Witwatersrand gold involve the influx of auriferous fluid through favourable sites of precipitation. These sites
would have to correspond to distinct
sedimentary features in order to explain
the well-known sedimentological control on the distribution of the gold. The
correlation between mineralization and
sedimentary features (e.g. Minter, 1978;
Buck and Minter, 1985), such as unconformities, fluvial channels, crossbed foreset laminae and deflation surfaces has been, and is still being used to
guide mining and exploration. The original pore space of the siliciclastic protoliths was largely destroyed during
diagenesis due to cementation. Considering the metamorphic overprint at
greenschist facies conditions, the creation of a secondary porosity is required
for any significant late to post-diagenetic hydrothermal infiltration. Postdiagenetic fluid flow was focused along
discrete channelways created by a series
of deformation phases (Coward et al.,
1995). Pervasive fluid flow across stratigraphic boundaries was limited as the
fluid composition was largely stratigraphically controlled (Frimmel, 1996):
the presence of magnetite and haematite in iron-formation beds as opposed
to that of pyrite in the fluvial siliciclastic rocks indicate fluctuations in the
ambient oxygen fugacities. Evidence
for focused fluid flow is, however, widespread in the form of chemical and
mineralogical alteration along faults
and bedding-parallel shear zones, and
hydrothermal veins that intersect bedding planes at both very shallow and
steep angles (Phillips and Myers, 1989;
Gartz, 1996; Barnicoat et al., 1997;
Frimmel and Gartz, 1997).
Barnicoat et al. (1997) conclude that
there is no evidence for the rapid, smallscale variations in fluid and mineral
chemistry that would be required to
remobilize detrital gold over short distances. However, variations in the chemistry of gold, pyrite, and uraninite particles on a millimetre- to centimetre-scale,
together with the widespread textural
evidence for more than one generation
of these minerals in the same thin-section (Minter et al., 1993) indicate a lack of
chemical equilibrium on this small scale.
An explanation for such small-scale
chemical variations may be found in the
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behaviour of quartz, by far the dominant mineral in the Witwatersrand
strata. Quartz was both consumed
and produced, depending on the type
of metamorphic/metasomatic reaction.
The VCR may serve as an illustrative
example as it reflects three superimposed stages of metamorphic/metasomatic alteration with progressively increasing pH (Gartz, 1996; Frimmel and
Gartz, 1997). The dominant Al-silicate
in the footwall quartzite is pyrophyllite.
However, an alteration halo, extending
to about 10 m above and 5 m below the
reef, exists along the reef and is characterized by K-enrichment reflected by
the formation of muscovite. Within the
reef, this metasomatic zone is overprinted by a narrow reef-parallel channel along which the previously formed
muscovite appears chloritized.
It remains unclear whether the pyrophyllite was formed during post-depositional H+-metasomatism (Barnicoat et al., 1997) or whether it is
related to the predominance of kaolinite in the clay fraction of the original
sediments. The prograde dehydration
of kaolinite to pyrophyllite involves the
consumption of quartz. This may explain the common observation of pyrophyllite infilling pore space created by
the corrosion of quartz (Barnicoat et
al., 1997). In contrast, the sericitization
of pyrophyllite leads to the liberation of
silica and precipitation of quartz. The
subsequent chloritization, in turn,
caused the dissolution of quartz again.
Thus the available secondary porosity
becomes a function of the ambient pH,
and a function of the extent of sealing of
the micro-fractures by hydrothermal
quartz. The local fluid:rock ratio can
be expected to have been highly variable, depending on the competition
between the opening of pore space by
fracturing or quartz dissolution, and
the sealing of the pore space by secondary quartz and other precipitates. Even
in wider fractures, where silica was
mobilized to form distinct quartz veins,
small-scale variation in the hydrothermal fluid chemistry is evident from
complex zonation patterns, including
periods of partial quartz corrosion,
which are revealed by SEM cathodoluminescence imaging (Fig. 2c).
Gold-mobilizing events
At least two gold-mobilizing events can
be distinguished. The euhedral, irregu-
larly shaped gold which occurs together
with the rounded torroidal particles
in the Basal Reef (Minter et al., 1993),
is associated with secondary quartz
which, in turn, includes inter alia chlorite, sudoite, and minute hydrothermal
zircon and rutile needles (Frimmel et
al., 1993). Attempts to date the zircon
using the SHRIMP technique yielded
an approximate age of 2.5 Gyr (R.
Armstrong, unpubl. data). A Pb±Pb
age of 2.58 + 0.03 Gyr obtained on
rutile from the West Rand Group
(Robb et al., 1990) may date the same
hydrothermal infiltration event. Microthermometric and electron microprobe analysis of primary fluid inclusions attached to the hydrothermal
gold within quartz indicated a CaCl2rich, moderately saline aqueous fluid
with XCO2 & 0.2 and a slightly elevated
pH compared to the resident pore fluid,
the pH of which was buffered by pyrophyllite and muscovite (Frimmel et al.,
1993; Frimmel, 1997). In the view of the
fluid chemistry it appears likely that
this gold-mobilizing fluid was derived
from outside the Witwatersrand `basin'
as there is relatively little source material for Ca and CO2 available within the
`basin'. Together with the age obtained,
the fluid chemistry points towards interaction with the carbonate rocks of
the lower Transvaal Supergroup.
Extensive gold mobilization in the
VCR is also associated with chloritization. There, the chlorite is late in the
paragenetic sequence and post dates
pseudotachylites (Gartz, 1996). Ar±Ar
ages obtained on the pseudotachylites
(Trieloff et al., 1994; Spray et al., 1995)
overlap with a U±Pb age of 2.020 Ga
obtained on shock metamorphosed zircon dating the Vredefort impact event
(Kamo et al., 1996; Moser, 1997). By
analogy, the gold mobilization is considered to be a result of the Vredefort
impact which shattered the surrounding rocks, thus creating sufficient secondary porosity to allow for the circulation of fluids. Fluid inclusions in
auriferous hydrothermal quartz in the
VCR are CaCl2-rich, moderately saline
with small amounts of CO2 present
(Frimmel, 1997) Ð similar to the older
inclusions described from the Basal
Reef above.
In other areas, the locally pronounced association between gold and
hydrocarbon may point towards further
gold mobilization during the fluxing of
hydrocarbons following the matura195
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Detrital origin of gold
.
H.E. Frimmel
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tion of organic material which, according to Robb and Meyer (1995), occurred around 2.3 Ga. In summary,
the existing data suggest that not every
metamorphic/hydrothermal alteration
event caused the mobilization of the
gold, and that gold mobility was
strongly dependent on fluid chemistry,
probably more so than on the volume
of fluid that passed through.
Conclusion
There is little doubt that much of the
Witwatersrand gold in its present form
is the product of hydrothermal re-precipitation, but no observations prove
that the gold is necessarily of hydrothermal origin and allochthonous with
respect to its host rocks. On the contrary, there is a wealth of evidence for
the existence of detrital gold particles,
closely associated with other heavy
minerals, such as pyrite and uraninite,
in fluvial siliciclastic rocks of the Witwatersrand Supergroup. Evidence also
exists for small-scale fluctuations in
fluid chemistry which can explain the
local mobilization of detrital gold, pyrite, and uraninite. It appears therefore
that the gold grains with hydrothermal
characteristics were derived from the
mobilization of originally detrital, allochthonous gold particles. At the time of
burial and metamorphic/hydrothermal
alteration, the gold was, however, autochthonous within the sedimentary pile.
Acknowledgements
Funding by the Foundation for Research
Development is greatfully acknowledged. I
thank M. J. de Wit and N. J. Beukes for
helpful critical comments and K. Durocher
for proof-reading the manuscript. D. Gerneke from the Electron Microscope Unit at
the University of Cape Town assisted
greatly in aquiring the SEM-cathodoluminescence images.
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