SEG 45 Final_qx4 - Society of Economic Geologists

SEG NEWS
20 SEG NEWSLETTER No 63 • OCTOBER 2005
Editor’s note: Part I of the report on the gold forum, covering Carlin-type deposits, was written by Greg Arehart and appeared in the
July Newsletter. This second part, on the Witwatersrand, was contributed by Laurence Robb.
A large audience of several hundred
attended the much-anticipated debate
on the origin of the Witwatersrand gold
and uranium deposits in South
Africa…testimony to the fascination
that this mother of all gold fields still
holds for the economic geology community
at large. Ironically, it was a
hydrothermal quartz vein in the West
Rand Group near Roodepoort from
which the very first gold was extracted
in the Wits Basin….this was by the
Streuben brothers a few years prior to
the discovery of conglomerate-hosted
gold, on the Ridge of White Waters, by
George Harrison and George Walker in
1886.
Right from the very beginnings of
mining, it was clear that an understanding
of the occurrence of gold, and
its origin, was no simple matter. As
long ago as 1917, Robert Burns Young,
in his book The Banket of South African
Gold Fields, described the authigenic
character of gold within what was
clearly a fossilized, heavy mineral-bearing
gravel placer. And so it is, still in
the year 2005, that geologists continue
to argue about the nature of the gold
and its origin, either as detrital particles
sourced from a hinterland that must
itself have been fabulously rich in primary
gold deposits, or hydrothermal
precipitates from a fluid—it, too, either
very voluminous or extraordinarily
enriched in the precious metal.
The Reno debate started off with a
talk by Hartwig Frimmel, who presented
the case for the modified placer point of
view. This notion is based on the existence
in the basin of flat, rounded and
“peened” grains that are interpreted as
detrital gold toroids. This gold was initially
concentrated by hydrodynamic
processes during sedimentation and
then substantially dissolved and remobilized
by later fluids flowing through
the sediments. It was shown that there
must have been at least 2 episodes of
fluid flow, resulting in the re-precipitation
of gold and pyrite as authigenic
particles within the conglomerates. The
argument in favor of a purely
SEG Forum on Gold Deposits: Part II
May 14, 2005 • Reno, Nevada
Laurence J. Robb (SEG 1985 F) Greg B. Arehart (SEG 1988 F)
Report on the Witwatersrand Debate
hydrothermal introduction of gold was
presented by Neil Phillips, whose model
invokes early circulation of diagenetic/meteoric
fluids with hydrocarbon
generation, followed by a pre-Platberg
(the Platberg volcanics form part of the
flood basaltic succession that immediately
overlies the Witwatersrand sediments
at 2709 Ma) introduction of
metamorphogenic fluid that sulfidized
the “black sand” heavy mineral suite
that had accumulated during sedimentation
and also precipitated gold. The
essential differences between the 2 viewpoints
are that the modified placer
model requires initial accumulation of
detrital gold grains, with later modification,
whereas the hydrothermal model
does not recognize the presence of allogenic
gold and mineralization is essentially
by precipitation from a fluid
phase.
What followed these two presentations
was a combination of audience
questions and comments, interspersed
with brief presentations by each of five
panel members, themselves experts on
the Witwatersrand Basin. David Groves
initiated the discussion by questioning
the discrepancy between the very large
scale alteration halos around putative
fluid conduits and the mineralization
that occurs at a more limited scale and
is restricted to the thin conglomerate
horizons. He also posed the question as
to what exactly was the hydrothermal
model, pointing out that its proponents
did not seem to agree on the timing of
the principal ore fluid circulation event.
John Chesley also spoke in favor of a
detrital origin of gold by summarizing
the recent Re-Os isotope dating and
characterization of gold and pyrite in
the basin. This data, although itself
controversial—as pointed out by
Jonathan Law—is interpreted to indicate
an unusual mantle source for the
gold and its introduction as detrital particles
at a time that preceded the end of
sedimentation in the basin. Proponents
of the hydrothermal view, such as Andy
Barnicoat, re-emphasized the ubiquity
of alteration assemblages and pointed
to evidence showing that the latter are
intimately associated with both gold
and uraninite, often in fractures within
the conglomerate packages. Vic Wall
provided evidence for a fluid circulation
and mineralizing event in post-
Transvaal times (i.e., later than ca.
2550 Ma) and drew interesting analogies
between the Au-U-C-PGM association
in the Witwatersrand and a similar
association in Proterozoic unconformity-related
uranium deposits.
Audience participation was lively
and the four hours allocated to the
debate passed by in a flash. No vote
was taken at the end of proceedings
and it is difficult to say which way the
body of opinion swayed. Is there more
consensus now about the origin of this
most enigmatic of deposits than there
was in the past? The answer is probably
yes, in that more is known about the
processes involved and also because
proponents of the two models do agree
on a wide range of issues. A fundamental
division, nevertheless, still remains—
namely, that of whether a fertile hinterland
existed from which pyrite, gold,
and uraninite detritus was sourced. The
morphology of grains is not sufficient,
in the view of some, to provide a conclusive
answer to this question.
Hundreds of zircons have now been
individually dated and provide
unequivocal indications of the age and
location of the source of clastic sediment
in the depository. The eventual
answers to the Witwatersrand debate
may also have to come from well-constrained
and more accurate ages of key
minerals such as pyrite, uraninite, and
gold. The writer for one is fervent in the
hope that we will have such answers
well before the last bucket of ore is
extracted from this phenomenal set of
deposits. 1