Stewart R. Wallace — 1919–2009 - Society of Economic Geologists

JULY 2009 No 78 SEG NEWSLETTER 17
may be preserved (>50 g/t). In some
areas, PGE are associated with interstitial
base-metal sulfides that extend into
the footwall (Cawthorn, 1999). In some
parts of the eastern limb, cryptic layering
occurs on a scale of centimeters in a
wide-reef facies (Fig. 5C), as described in
the Winnaarshoek locality by Mitchell
and Scoon (2007). In the wide-reef
facies, two chromitite stringers constrain
the width of the mineralized reef
zone, as is typically the case, but they
are separated by a layer of feldspathic
orthopyroxenite that is relatively thick
(1.8 m on average) and includes a barren
middling. Moreover, the lithology
between the chromite stringers throughout
most of the western limb is a pegmatoidal
orthopyroxenite, an unusual
assemblage the origin of which has
been much debated (e.g., Cawthorn and
Boerst, 2006), whereas in the eastern
limb the principal layer of pegmatoid
occurs below the mineralized zone.
The average grade of the Merensky
reef is similar to the UG2, al though it is
far more variable. Typically, a much
larger number of PGM species occur
(Kinloch and Peyerl, 1990) and lateral
variation of the PGM contrasts with the
regularity of the primary layering. PGM
are spatially associated with base-metal
sulfides (1–2 %), constituting approximately
equal abundances of pyrrhotite,
pentlandite, and chalcopyrite; the relative
paucity of pyrrhotite is unusual
(Liebenberg, 1970). Base-metal sulfides
are an important by-product from the
Merensky and assist with downstream
smelting operations, whereas smelting
of sulfide-poor UG2 concentrates is
problematic.
The Platreef
The Platreef is restricted to a relatively
small area of the northern limb, and is
typically located a few tens of meters
above the base of the intrusion, where
the floor ranges from Archean granite
to sedimentary rocks of the Transvaal
Supergroup. The lowermost zones of the
Rustenburg Layered Suite are not developed
in this area. The Platreef is far
more irregular than the UG2 and
Merensky, is typically much thicker (up
to 100 m or more), and contains PGE
that are invariably associated with
base-metal sulfides throughout (Viljoen
and Schurmann, 1998). The Platreef is
also characterized by a much lower
Pt/Pd ratio than the Merensky and UG2
reefs, although the lower average grades
are offset by the width of the reef. The
Platreef may constitute a localized
equivalent to the Merensky reef, and at
the Tweefontein locality, a few kilometers
to the south of the Sandsloot mine,
the mineralized sequence is relatively
thin and includes both chromitite
stringers and a pegmatoidal pyroxenite
(Viljoen and Schurmann, 1998).
We find the importance of footwall
contamination to the Platreef has been
overstated, in part as earlier studies
were restricted to an area where the
floor rocks are dolomite (White, 1994).
A new terminology may assist with
explaining this: the sequence below the
(barren) Main zone should be categorized
as a Platreef unit rather than the
“Platreef.” This consists of gabbronorite,
feldspathic websterite, feldspathic
harzburgite, and reconstituted feldspathic
pyroxenite, the latter revealing a
pegmatoidal or glassy texture. These
rock units are in discordant intrusive
relationship with one another and with
the overlying Main zone. Thus the
sequential stratigraphy (A, B, and C
reefs of earlier workers) is inappropriate,
yet despite this, a Main mineralized
layer (analogous to the “B” reef?) can
generally be identified. At the Akanani
locality, where deep drilling has
revealed a relatively thick Platreef unit
downdip from the open pit Sandsloot
deposit, we found PGE to be most abundant
in sinuous layers of harzburgite
and, to a lesser extent, reconstituted
pyroxenite; the gabbronorite and feldspathic
websterite are relatively weakly
mineralized.
The pipe deposits
The pipe deposits are no longer of commercial
interest and, paradoxically,
have a negative affect on reef-type
mines, as they disrupt the layered wall
rocks. PGE were concentrated in small
core zones (max diam 24 m) of coarsegrained,
iron-rich dunite and wehrlite
(Wagner, 1929), now mostly mined out.
The bulk of each pipe (diam >300 m),
however, is dominated by barren magnesian
dunite. Barren outer envelopes
also occur, and our unpublished mapping
at Mooihoek demonstrates this
component is even more extensive than
the magnesian dunite. Bushveld pipes
that do not reveal this zonation are typically
barren (Viljoen and Scoon, 1985).
LOW-GRADE PGE DEPOSITS
An understanding of the economic
deposits is incomplete without mention
of low-grade ores in the Bushveld. These
include all the chromitite layers located
below the UG2 reef, as well as the
Pseudoreefs that are situated between
the UG2 and Merensky in the northwestern
Bushveld. The Bushveld
chromitites reveal regular upward
trends in both their composition (e.g.,
decrease in the Cr/Fe ratio) and the PGE
grade and tenor (Scoon and Teigler,
1994). These chromitite layers, however,
report very low contents of sulfide. These
data are important as they demonstrate
that the UG2 and Merensky reefs do not
occur in isolation but represent the culmination
of a general upward increase
in the PGE content of mineralized layers
within the Rustenburg Layered Suite.
This trend is disrupted by the low-grade,
harzburgitic Pseudoreefs. The bifurcation
and subsequent elimination of discrete
layers of harzburgite (bounded top
and bottom by stringers of chromitite) is
indicative of rejuvenation of more primitive
magmas that were intruded laterally
and which may be interpreted as
precursors to the Merensky reef (Scoon
and De Klerk, 1987).
GENESIS AND
CONCLUDING REMARKS
The holistic approach advocated by
Wagner (1929) is important to our
understanding of the PGE deposits in
the Bushveld Complex. There is a
marked spatial association of PGE with
chromitite and other ultramafic rocks
that have sharply defined and demonstrably
discordant basal contacts, and
not uncommonly sharp upper contacts.
Norite-anorthosite is almost invariably
barren, except in isolated cases. These
relationships, in addition to the restriction
of economic ores to the Upper
Critical zone, are unlikely to be coincidental.
Rather than the model of Camp -
bell et al. (1983), in which the resident
magma column provides the PGE, we
believe field relationships and mass balance
considerations are consistent with
the “lateral mixing hypothesis” of
Scoon and Eales (1989) and Scoon and
Teigler (1994). Thin, hot layers of fresh,
ultramafic magma, enriched in PGE
(and chromite: Eales, 2000) streamed
laterally into the different chambers of
the intrusion. In the case of the
Merensky reef, as well as some other
units, the new, U-type magma was
intruded into an earlier-formed crystalline
substrate of
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norite-anorthosite
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