Of all of the valuable metals which can be found on Earth, by far the grouping which
has the most unusual occurrences are platinum group elements, which include this grouping
of 6 elements.
For example, let's look at the fascinating elements of platinum and palladium.
Both of these elements are more common than gold in the Earth's crust, yet, platinum group
elements are much more rare to find in concentrations high enough to mine for a profit than gold.
This is why there are 127 active gold mines in the United States and yet only 2 mines
which produce platinum or palladium.
These two mines can be found within the state of Montana at the East Boulder Mine and more
famous Stillwater Mine.
Altogether, these mines produce around 475,800 ounces of palladium and 134,200 ounces of
platinum each year.
For those who were not aware, platinum ore bodies are generally quite weird, which instead
of occurring in a large solified magma chamber or the altered rock above it which could be
hundreds of meters to more than a kilometer thick, instead occur in very long intrusions
which are both narrow and thin.
For example, at the Stillwater mine complex, the main target zone for platinum and palladium
mining is a 24.9 mile or 40 kilometer long layered 2.7 billion year old igneous intrusion
which is generally only 6.9 feet or 2.1 meters wide and only 7.9 feet or 2.4 meters thick.
There are two competing models which attempt to explain how such a weird intrusion can
form, which are colloquially referred to as "sinkers" and "floaters".
I was asked by a previous employee of this mine which model I thought was correct.
Before I state my conclusion, let us look further into the exact ore body.
While this long and thin ore body does contains some distinct platinum and palladium ore minerals
such as braggite and sperrylite, these are only minority sources of the valuable elements.
As instead, most of the platinum and palladium occur as inclusions within or around the three
sulfide minerals which are pyrrhotite, chalcopyrite, and pentlandite.
In each of these minerals, the amount of platinum and palladium within them might be as high
as 5% by weight.
Altogether, this creates an average grade of 15 to 20 grams of platinum and palladium
per ton of ore.
However, the intrusion which formed this vast ore body wasn't so thin.
Rather, the thin 2.4 meter thick orebody merely represents a singular lower layer in a much
thicker intrusive rock that formed, being largely comprised of what is known as a cumulate
rock.
Cumulate rocks can form due to density sorting, where lower density crystals can rise to the
top of a magma chamber while higher density crystals can sink to the bottom of a magma
chamber.
Crystallization may also occur along the walls of a body of magma, as the wallrock is colder
than the warmer body of magma, with minerals crystalizing in situ, that is, against the
wallrock without being remobilzed.
During in situ crystallization, occasional sulfide minerals are emplaced, with these
sulfide minerals subsequently sucking elevated platinum, palladium, and rhodium in the komatiite
magma which is naturally enriched in these 3 elements, placing them into and around the
crystals.
Thus, the whole "floater and sinker" versus in situ debate revolves around whether the
platinum and palladium ore was deposited through the fractioning of magma caused my minerals
being removed from the melt by gravity, either sinking or floating based on the mineral's
density relative to the melt, or if it was formed by direct contamination of the adjacent
silicate rock.
The reason why there has been a debate for so long is that while the sinker theory seems
to best predict where ore will end up, it does not perfectly fit the actual placement
of ore 100% of the time.
Meanwhile, the floater theory is a worse fit overall, but can seemingly predict where select
amounts of ore will end up.
So, which of these three theories do I think is correct?
The truth is it depends on the deposit, as a combination of the three is generally correct,
with generally one of the three processes being dominant.
For example, The Skaegaard intrusion in Greenland is commonly believed to have formed in a closed
system, and has an oval shape, with a single flow of magma largely crystallizing via in
situ crystallization.
Meanwhile, the Stillwater and Bushveld complexes are open systems in which multiple lava flows
lead to multiple layers of minerals and rocks being formed repeatedly based on the addition
of new magma flows of different compositions.
In such a system, cumulate rocks are formed that are layered and horizontal, and sinking
and floating may have been a more important mechanism than in situ crystallization.
As a final note, I would like to thank my new YouTube member Elizabeth Warman for supporting
this channel!