Now let's look at what seems to be a straightforward problem with a straightforward answer, and then consider the complexities that make prediction difficult. We will see that once again, the individual elements in the system behave unpredictably.
Some years ago, the woman who later became my wife was practicing her panning technique in the Don River, in Toronto. (She was about to to recover gold grains from some of our fine concentrates from a West African sampling program).
She got an unpleasant surprise from the gravels at the first bend north of the Viaduct, where the river runs up against the railroad tracks.
The pan was full of mercury.
What happened after alerting the authorities was somewhat amusing but a digression from our current discussion (complex indeed, but not entirely unpredictable). So it has been omitted.
A few years later she carried out a sampling program looking for mercury in the Don River. The samples were collected in locations as advised by another local geologist, who had many years of experience in placer operations worldwide. I also helped with some of the heavy lifting.
Samples were collected in the winter, when the chances were greatest of finding elemental mercury.
Elemental mercury was indeed recovered. Some of the droplets were beautiful. Also noted many interesting textures where mercury had alloyed with other metals.
Electron microscope imagery of a mercury droplet (left). Scale bar
is 100 microns in lenght. Detail of droplet (right) showing spherical
textures on mercury-amalgam plates. The small "bubbles" at upper
left are due to electric charges building on the mercury droplet while
under the electron beam.
Small spheres of mercury amalgam were common at many sites in the Don River, but the richest sites were at the location of old landfills.
There is a lot ornamentation on some of these spheres--but am not sure of the causes of it. I would note that we see the same thing on iron spheres of about the same size that we find in West Africa.
There were no surprises during the study--no pans full of mercury. Concentrations at several sites multiplied by volume of sediment in river divided by total surface area of the drainage basin resulted in a mass balance calculation consistent with the amount of mercury expected purely from rainfall.
It turns out that the mercury surprise found earlier came from a spill of sewage from the North Leaside treatment plant in August of 1997 (a few days before the panning expedition). The references to the spill seem to have vanished from the internet, but the article by McAndrew (1997) in the Toronto Star covered it.
At the time we calculated that given the size of the spill, the amount of mercury that may have entered the river could have been in the hundreds of kg, while still satisfying the government regulations on mercury concentration in discharged effluent. Such an even released many times more mercury into the river than enters through rainfall (I hesitate to call it "natural" as most atmospheric mercury results from coal burning).
The local authorities monitor mercury levels in the Don River by measuring mercury content of all of its feeder streams, but make no measurements within the Don itself. The sewage spill went directly into the river, so it is no surprise that our Dear Leaders had no idea of its presence until they were told of my wife's discovery. Now who would have predicted that?
Mercury use is endemic among artisanal miners. While I did not find it in use in the coastal region of Ghana (where some of the gold is very coarse), Company representatives did find "white gold" in sands and gravels in the Pra River.
By white gold, I mean an amalgam of gold and mercury. Such a finding means that somebody is using mercury to amalgamate the fine gold grains (it makes them stick together), but is also somehow still losing some of it off the end of the sluice.
The hazard of course is that by breathing the fumes you end up like the Mad Hatter.
"Would you like a drop of mercury with that?"
So we used to show off this bag of white gold at shareholder meetings, and somehow after each meeting there seemed to be less of it than before.
One shareholder scoffed that the grains in the bag weren't gold. We let him take one home. I explained that it was amalgamated with mercury and that he shouldn't heat the grain up. He told me he didn't believe me.
He phoned back very excitedly the next day. It was amazing! He put the grain on his stove burner, turned it on, and the grain turned to gold.
And a little more mercury entered the Toronto atmosphere. Now who would have predicted that?
McAndrew, B., 1997. Don River spill may threaten city beaches. Toronto Star, August 19, 1997.