Lessons from the past
Exploring the Black Reef
Edited by Leon Louw
The Black Reef is one of the less-known reefs of the Witwatersrand Basin.
Following is a summary of the geology of the Black Reef as it appears in “The
Great Mineral Fields” from Episodes the Journal of International Geoscience
(edited by Richard Viljoen).
“
The Black Reef Formation lies at the base
of the much younger 2.65 Ga. Transvaal
Supergroup (Eriksson et al., 2006). It is
a pyritic quartz pebble conglomerate in a
fluvial setting with a black, carbonaceous shale
from whence its name. Conglomerate filled
channels occur near the base of the succession
and in places contain well developed coarse
buckshot pyrite and carbonaceous material,
especially in the vicinity of underlying gold-
bearing Witwatersrand reefs.
“At the Randfontein Estates Gold Mine
in the West Rand Goldfields, the Black
Reef uncomformably overlies Ventersdorp
and Witwatersrand Supergroup strata.
A clear special association of gold-rich
Black Reef conglomerates and underlying
Witwatersrand conglomerates exists. Black
Reef conglomerates were deposited in linear
channels trending roughly north-south, which
cut into less resistant sediments in close
proximity to underlying outcrops of Main,
Bird and Kimberley Reefs. This strongly
suggests that the source of the gold was from
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these underlying Witwatersrand reefs.
“Recent microprobe analyses of pyrite
from the Black Reef have revealed a trace
element signature which differs from that of
the underlying Witwatersrand reefs (Fuchs
et al., 2016). These workers suggest that while
the depositional environment was similar
to that of the underlying Witwatersrand
clastic sediments, hydrothermal alteration by
circulating aqueous and hydrocarbon fluids
(oils) deposited large volumes of native gold,
uranium minerals and pyrobitumen. Their
work indicates that while the pyrite was
not the result of reworking the underlying
Witwatersrand strata, the close spatial
association suggests strongly that gold and
uranium were hydrothermally recycled from
the underlying Witwatersrand reefs. It is
thus likely that the Witwatersrand reefs were
the proto-ore of the Black Reef and the later
hydrothermal activity remobilised the gold and
changed its character and appearance.
“Black Reef conglomerates were exploited
at the Government Gold Mining Areas,
Geduld Propriety and Modderfontein Mines
in the East Rand Goldfields and to a lesser
extent from the West Rand and Klerksdorp
Goldfields. In excess of 30 million tonnes
of ore was extracted from Black Reef
conglomerates (Robb & Robb, 1998).
Reference list
Eriksson, P., Altermann, W. and Hartzer,
F., 2006. The Transvaal Supergroup and its
precursors. In: The Geology of South Africa.
Johannesburg: Pretoria: Geol. Soc. S. Afr; Coun.
Geosci., pp. 237-260.
Fuchs, S., Williams-Jones, A. &
Przybylowicz, W., 2016. The origin of the gold
and uranium ores of the Black Reef Formation,
Transvaal Supergroup, South Africa., Ore Geol.
Rev. v.72, pp. 149-164.
Robb, L.J. and Robb, V.M., 1998. Gold
in the Witwatersrand Basin in The Mineral
Resources of South Africa (M.G.C Wilson and
C.R. Anhaeusser, eds.): Handbook, Council for
Geoscience no. 16, p. 294-349.
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