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

18 SEG NEWSLETTER No 78 JULY 2009
... from 17
(Mitchell and Scoon, 2007), an important
consideration if, as suggested by
Eales et al. (1986), the lowermost part of
the Main zone was emplaced after the
reef. A similar hypothesis is applicable
to the Platreef: U-type magma intruded
earlier-formed gabbronorite and websterite,
forming mineralized harzburgite
and pegmatoid, which in turn was
emplaced prior to the Main zone.
Chromitite layers formed by mixing of
U-type and more evolved magma (Atype),
a hypothesis presented by Sharpe
and Irvine (1983), albeit we have suggested
the latter, at least in the Upper
Critical zone was derived by partial
melting of plagioclase cumulates under
the influence of the new influxes of Utype
magma. The spectacular grade of
some chromitite stringers, as well as
PGE-bearing chromitites with low sulfide
contents, suggests two processes—
nucleation of PGM triggered by crystallization
of copious amounts of chromite
and S-saturation—were necessary to
generate economic deposits. U-type
magmas also intruded the complex in
vertical conduits (Scoon and Mitchell,
2004b). Partial melting of earlierformed
reefs, triggered by heat associated
with the magnesian dunite, generated
a PGE- and iron-rich melt which
drained downward into the cores of specific
pipes, a hypothesis supported by
similarities with so-called replaced
Merensky reef (reef invaded by discordant
iron-rich ultramafic pegmatite:
Scoon and Mitchell, 2004a). In summary,
we find that hydrothermal fluids
are of little importance in formation of
PGE deposits in the Bushveld, as discussed
by Barnes and Campbell (1983),
and recognition of orthomagmatic processes
is fundamental. The uniquely
layered Upper Critical zone, containing
the world’s premier PGE ores, resulted
from episodic replenishment by U-type
magmas that persisted long after the
onset of the crystallization of noriteanorthosite.
ACKNOWLEDGMENTS
Discovery and Geology of the PGE Deposits of the Bushveld Complex, South Africa (Continued)
We acknowledge reviews by Wolfgang
Maier and an anonymous reviewer,
and are particularly grateful to the former
for his constructive comments. We
pay tribute to Hugh Eales for his mentorship
early on in our careers, and for
his ongoing interest in our research.
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