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Benbow
Mining Camp, 1932.
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Mouat
mine tram terminal circa 1950. |
Stillwater
Mine in recent picture.
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Part 2: Exploration for Platinum-group elements
Sulfide-bearing samples, collected by
Art Howland and J.W. Peoples during the early 1930s, were analyzed
for PGE’s by INCO,
and the results reported by Howland and others (1936). In one
of the samples from layered gabbro of the Banded series, they
reported the presence of Sperrylite and a mineral they called
stibiopalladinite. They compared this part of the complex to
the Merensky Reef of the Bushveld Complex. These important observations
did little to encourage the PGE exploration until the early 1960s,
when at least two independent groups began to examine the PGE
potential of the Stillwater Complex. Page and Jackson (1967)
found platinum-group minerals in samples of chromite that had
been collected over the previous years. During the summers of
1967, 1978 and 1969, they submitted many rocks from the complex
for PGE analysis. However, except for samples from the lower
chromitites the results were not encouraging.
In 1961, H. K. Conn of the Johns-Manville Corporation attended
the Commonwealth Mining Congress in South Africa and visited
the Rustenburg platinum mines on one of his tours. This visit
sparked his imagination that there might be a zone similar to
the Merensky Reef in the Stillwater Complex because he was already
familiar with the complex and the work of Howland and others.
(1936) regarding PGEs in the complex. Nothing was done until
1967, when E.L. Mann and S. G. Ellingwood visited the Stillwater
Complex and initiated a modest program of soil sampling and reconnaissance
mapping. Results were generally disappointing, and Johns-Manville
might well have walked away from the area after the first summer,
were it not for one set of samples displaying weak Pt, Pd, Cu,
and Ni values. Field exploration continued each year, with only
minimal encouragement until 1973, when Johnson-Manville geologists
discovered the major Stillwater PGE zone, commonly referred to
now as the J-M Reef.
After trenching and drilling confirmed the lateral continuity
and character of the the mineralization, claim staking accelerated.
By the summer of 1975, the Johns-Manville Corporation held claims
over all but about 1.5 miles of the 28-mile strike length of
the J-M Reef.
Other areas distinct from the J-M Reef were also found to have
satisfactory recoveries. In 1974, the Janet 50 zone and on outcrops
on the West Fork of the Stillwater River were found with PGEs.
On August 22, 1974, 3,600 feet of strike length ore grade material
was delineated on the West Fork, and on December 15, an exploration
adit was begun. The exploration successfully define the tabular
nature of the deposit, but water flow from an intersected fault
zone forced the rift to be cemented. By 1977, exploration had
confirmed the presence of a zone 3.4 miles long by 6.9 feet wide,
which averaged 22.3 g of platinum and palladium per ton, in a
ratio of 1/3.5. Metallurgic tests provided recoveries of 85 percent
of the platinum and palladium, and 69 percent of the nickel and
75 percent of the copper from test samples.
Exploration continued, and in 1979 the Chevron Resources Company
joined the Johns-Manville Corporation to form the joint-venture
company Stillwater PGM Resources. By autumn 1982, extensive additional
exploration and testing again confirmed the broadly tabular continuity
of the ore package with encouraging ore grades. Furthermore,
a test stope showed that shrinkage stoping would be an acceptable
mining method.
During this aggressive exploration period by Johns-Manville,
The Anaconda Minerals Company was in 1977 recovering from its
losses in Chile, and reviewed the company’s land ownership
in the Stillwater complex. Regional mapping led to their discovery
of the PGE bearing reef on the GAY claim group, and much drilling
and testing was initiated. In spring 1982, the Anaconda Minerals
Company and Stillwater PGM Resources began negotiations to form
a joint venture on a land package centered on Anaconda’s
claims on the J-M Reef on the Stillwater River Valley. Stillwater
PGM Resources agreed to add flanking claims to those held by
Anaconda, thus extending the land package to a 6-mile strike
length.
Agreement was reached to form the Stillwater Mining Company,
a Chevron-Manville-Anaconda partnership, with Chevron Resources
Company as the managing partner. A program to prove reserves
and determine grade was initiated. Detailed field mapping was
continued, augmented by a surface drilling program.
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