Platinum in Arizona
Circular No. 3, August, 1980
by Ken A. Phillips, Chief Engineer
Platinum (in reality the entire platinum group of elements) has been and
continues to be the subject of much discussion among miners in Arizona. Do
the metals exist in Arizona? Are they in mineable quantities, if they
do exist? Are they now produced? What is the significance of the reports
of theft of platinum from Inspiration Copper Company?
The platinum group of metals comprises platinum, palladium, rhodium,
iridium, osmium, and ruthenium, of which platinum is the most
important. Of these metals, osmium is generally alloyed with iridium. The
platinum group metals occur chiefly as natural alloys of various
proportions and to a much lesser extent as sulfides and arsenides. All of
these metals are rare and expensive. They possess individual properties
that make them uniquely attractive for certain uses in highly developed
technology and for use in jewelry and for decorative purposes.
Platinum and palladium are the most abundant and most widely used of
the group; the other four metals are mainly used as alloy modifiers with
platinum or palladium. The high melting points, corrosion resistance, and
catalytic properties of these metals have many industrial applications.
There has been minor past production of the platinum group metals from
California and Alaska, but present domestic production, mostly as a
by-product in the refining of copper, is insignificant when compared
with domestic consumption.
Do the platinum group metals exist in rocks in Arizona?
The answer to this question is a qualified,
"Yes." Tiny or trace amounts of the platinum
group elements are distributed throughout the earth's crust. The average
is reported to be about 0.00016 troy ounces per ton for platinum and
about twice that for palladium. The others are about five times as rare
as platinum. The platinum group elements are found mostly in geologic
association with mafic and ultramafic rocks, such as peridotite, norite,
and dunite. They may also be found in placer deposits derived from such
rocks, if the rocks carried the minerals.
Ultramafic rocks are very uncommon in Arizona. Thus, it is probable that
the average platinum group element content of rocks in Arizona would
be less than the crustal average for the world. However, the geologic
history of Arizona encompasses a few billion years and the possibility
of the past occurrence of such rocks containing platinum elements,
does exist. If such rocks have been destroyed through weathering or
alteration, it is conceivable that contained platinum mineral grains
might find their way into a lens of schist or placer gravels. This could
result in the rare detection of platinum in an assay sample and the fact
that subsequent samples from the same location would not necessarily
show platinum group elements.
Do the platinum minerals occur in mineable quantities, if they do exist?
The answer to this question is a qualified,
"No." To our knowledge there has never been production
of platinum from Arizona. The definition of ore is rock that can be
mined and will yield its valuable mineral at a profit. To be classified
as platinum ore, the platinum content would have to make up a significant
portion of the value of the ore. The recovery of trace amounts of platinum
group metals from other ores in the final stages of refining does not
make the rock platinum ore. Although no one can predict the future, the
present knowledge of the geology, mineralogy and economics of platinum
and Arizona are not conducive to the discovery of producible quantities
of platinum group metal ores in Arizona.
Are platinum group metals now produced from newly mined rock
(not platinum ore) in Arizona?
The answer is, "Yes." Platinum group elements are
recovered from anode slimes produced during the refining of copper. Emphasis
must be placed on the term "refining" of copper. Slimes are not
produced at the smelter stage of copper production. In addition, the process
must be electrolytic and not fire refining.
As an example, one ton of anode slimes is recovered from 178,200 tons
of ore at the electrolytic refinery of Inspiration Consolidated Copper
Company at Inspiration, Arizona. At this rate, the following figures
can be calculated based on known concentration ratios and recoveries
(custom ore to the refinery is also included in this calculation):
|66 tons of copper ore
|| = 1 ton of copper concentrate
| 3 tons of concentrate(198 tons of ore)
|| = 1 ton of copper anode
|900 tons of anode (178,200 tons of ore)
|| = 1 ton of cathode slime
|2 tons of slime(356,400 tons of ore)
|| = 1 ounce of platinum
In addition to the platinum, some palladium and other precious metals are
recovered. By extrapolation, the copper ore assays as follows:
||= 0.0000029 tr. oz/ton of ore
||= 0.000029 tr. oz/ton of ore
||= 0.0026 tr. oz/ton of ore
||= 0.025 tr. oz/ton of ore
||= 0.5% or l0 lb/ton of ore
Such values of platinum, palladium and gold are determined by mathematical
extrapolation because they are too low in value to assay directly by normal
methods. It is safe to assume then that if this concentration of platinum,
palladium, gold and silver did not happen automatically during refining
(purification of copper), it would not and could not be recovered.
What is the significance of the theft of platinum from
Inspiration Consolidated Copper Company?
The media has been publishing (May -July, 1980) reports of
copper and platinum thefts worth millions of dollars from Inspiration
Consolidated Copper Company. Included specifically in the reports is the theft
of 4899 ounces of platinum. In a report from Inspiration dated June 26, 1980,
data was supplied on a reported theft of platinum from their laboratory. Quoting
that report: "On November 18, 1974, 48.99 ounces of platinum, in the form
of a platinum cathode, was stolen from the Analytical Lab." This may be
where 4899 ounces reported in the newspapers came from if a decimal point was
The same report went on to say that if all the platinum were extracted
from the anode slimes produced by Inspiration, it would take 167 years
to recover 5,000 troy ounces of platinum. The platinum, palladium, gold
and silver are extracted from the slimes at custom refiners outside
- Platinum and platinum group elements do occur in Arizona.
- They are not known to occur in sufficient concentration to justify their
- They are recovered, but only when concentrated nearly 200,000 times in a
process that is necessary to refinement of copper. (If the anode could be used
without electrolytic refining, the precious metals would not be worth
- The report of a theft of large amount of platinum from Inspiration
Consolidated Copper Company appears to be a typographical error.
Platinum group metals will continue to be the subject of discussion
and arguments among Arizona miners. The platinum group metals have
very high prices and it is the dream of most prospectors to find ores
of very valuable minerals. That dream encourages prospectors to keep
looking and that is how valuable mineral deposits are found. However,
an erroneous report of high concentrations of platinum group metals (or
any other metals, if in error) in ores will cause a prospector to spend
nonproductive time and money trying to prove a mine or develop exotic
extraction methods. When that happens, everyone loses, especially the
prospector. The prospector should always double or triple check assay
reports showing ore grade concentration of platinum group metals.
Erroneous reports of the presence of platinum group metals often stem from
the difficulties in detecting them. Assays for platinum and palladium,
even for amounts that could constitute ore, are much more difficult to
determine than for gold. Even reliable and experienced assayers have
been deceived into reporting nonexistent platinum. On the other hand,
it is not likely to be missed if present and looked for.
A number of metals and minerals have been mistaken for platinum group
metals due to some of their properties. Among them are "chilled"
birdshot (lead alloyed with arsenic), lead fragments from other sources,
specular hematite in small flakes, and old amalgam. Any of these materials
may be found in some panned concentrates. If the sample has been roasted
in the presence of carbon, metallic iron or other metals may have been
formed and might be reported as platinum. All of these materials may be
surprisingly resistant to acid tests. Even some gold-silver alloys are
nearly insoluble in aqua regia.
Because of the difficulty in detecting the platinum group metals and
because of their probable scarcity in Arizona and elsewhere, caution is
advised in accepting any unconfirmed reports of their presence.