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

JULY 2009 No 78 SEG NEWSLETTER 15
FIGURE 4. Photograph of Dr Hans Meren -
sky “pointing out a diamond in matrix
from Alexander Bay (from Lehmann, 1955).
when a sample panned by Andries
Lombaard from an ephemeral stream
on his farm Maandagshoek, located
some 40 km from Lydenburg, was dispatched
in June 1924 to the Johannes -
burg office of Merensky. The assay
reported native Pt and Au, together
with iron oxide and traces of Rh and Ir.
Merensky immediately undertook a
field visit, during which he requested
Lombaard, an experienced gold
prospector, together with his cousins
Schalk and Willem Schoeman, to continue
the search. The “Lydenburg
Platinum Syndicate” was formed by
Merensky, privately funded by close
friends, with the objective of locating
alluvial and hard-rock PGE ores. After
acquiring some mineral rights titles,
Merensky returned to the field to find
that the Schoeman brothers had
panned Pt in soils to the east of the
stream. Three days later, on August 15,
1924, the Syndicate located Pt in outcrops
of “dark, lustrous crystalline
pyroxenites and ultrabasic rocks” on
Mooihoek to the east of Maandagshoek.
The mineralization occurred in a discordant
body (pipe) over which the syndicate
had to apply for claims. Discovery
of the Driekop pipe is credited to Willem
Schoeman, who recollected seeing similar
rocks on a small hill to the north of
Maandagshoek. The Syndicate also
located the low-grade Twyfelaar pipe,
but the Onverwacht pipe was discovered
by Rand Mines (in October 1924),
with geologist F.W. Blaine undertaking
the field program. No additional platiniferous
pipes have been discovered in
the Bushveld, despite the subsequent
recognition of many thousands of discordant
bodies!
In September 1924, the Syndicate
made the “far more important finding”
of a layered reef at Maandagshoek. This
was initially credited to Lombaard, as
he undertook the rock chip sampling,
but the prospecting team insisted it be
named the “Merensky reef.” Merensky
was aware that the pipes were of limited
size and was convinced that layered
rocks, specifically ultramafics with secondary
Cu, were a far more important
target. Wagner (1929) referred to the
Merensky reef as the “Mother Lode”
despite the pipes being successfully
mined, and despite problems with
exploitation of the Merensky reef as
described below. The Syndicate delineated
the reef over much of the eastern
limb, and several months later found
the reef in the western limb; Merensky
also assisted with discovery of the
Platreef in the northern limb. Addi -
tional funding was acquired and the
company was renamed “Lydenburg
Platinum Ltd.” Evaluation of the three
pipes, the Merensky reef, and some
alluvial concentrations was undertaken,
and many of the trenches and underground
workings from this period can
still be examined, including several reef
declines and drives at the Winnaarshoek
locality (Mitchell and Scoon, 2007). In
1925, the company was purchased by
the Gold Fields group and floated on
both the JSE and LSE during a shortlived
boom when the Pt price was five
times that of gold, driven by shortages
and stockpiling.
It is interesting that Merensky’s parents,
who were German missionaries,
passed through the discovery area while
fleeing an uprising in Sekhukhuneland
prior to settling at Botshabelo, not far
from the eastern limb of the intrusion,
where Hans was born and lived as a
young child. Merensky played a pivotal
role in a number of additional discoveries,
including the west coast diamond
fields, the chromite deposits at Jagdlust,
the apatite orebody at Phalaborwa, and
the southern extension of the Wit -
watersrand gold fields, and several of
the Lombaard and Schoeman families
became successful geologists.
EXPLORATION METHODOLOGY
Our research has led us to conclude that
South African geologists, including
Merensky, had speculated for many
years about the possibility of finding Pt
in ultramafic rocks of the Bushveld
Complex. Merensky first sampled and
assayed rocks from the eastern limb in
1904, including chromitite layers. The
association of PGE with the Bushveld
chromitites was discovered by Bettel
(1925) in 1906 and it should be noted
that chromite was mined from the
Bushveld long before the platinum discoveries,
including at the Winterveld
mine, where the Onverwacht pipe is
located. Merensky also provided samples
for Hall and Humphrey (1908),
who reported that one chromitite layer
yielded a grade >6 g/t (the UG2?). This
did not constitute a “discovery,” as the
fineness of the PGM meant extraction
problems were not resolved for many
years (Vermaak, 1985). Discovery of the
nickel-rich Vlakfontein pipes in 1923
was also important, and the association
of Pt and chromitite as well as similarities
between the pipes and the deposits
of the Russian Urals were widely discussed.
During announcement of the
discoveries, Merensky was supported by
both A. L. Hall and P. A. Wagner, an
important point as financial scandals
resulting from earlier “discoveries,”
together with problems with the
Waterberg vein deposits, had hampered
previous investment. In summary, we
recognize three important components
to the exploration methodology: knowledge
of field relationships, an exploration
model focusing on ultramafic
rocks, and use of stream sediment sampling
despite the polygenetic source
(Oberthur et al., 2004).
SOME EARLY MINING HISTORY
Prior to mining of the Bushveld ores,
the main supply of Pt was from alluvial
deposits, mostly located in Russia. The
Bushveld pipes are the oldest underground
Pt mines (Onverwacht was
opened in 1925), with the exception of
small-scale workings at Solovyov Hill in
the Urals. The pipes were evaluated by
core drilling and trial mining. They
yielded spectacular grades, notably at
Onverwacht, where resources were initially
calculated as 55,000 t at 16 g/t
PGE (>90% Pt) to a depth of 76 m (mining
eventually attained a depth of 320
m). They proved relatively easy to mine
and process, as ore minerals, dominated
by sperrylite and Pt-Fe alloy, were
coarse-grained and amenable to gravity
concentration (Wagner, 1929).
Production costs amounted to half the
Pt price. In comparison, development
of the Merensky reef proved far more
difficult, owing to higher mining costs,
metallurgical problems caused by oxidation
of near-surface
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