Ancient microbes formed Earth's biggest hoard of gold
Where there was life, there may be gold. Some of the
oldest life forms may have played a crucial role in the formation of
what's now the Earth's largest known gold reserve.
The process could only have taken place
during a window of opportunity after life on land came to being and
before it created the planet's oxygen-rich atmosphere. This means such
gold deposits could not be formed today – but it potentially gives us a
new way to find them.
Approximately 40,000 metric tonnes of gold have been mined from the Witwatersrand Basin
in South Africa since its discovery in 1886. That's three times more
than from any other single source and roughly half the gold ever been
mined on Earth.
The gold accumulated 3 billion years ago. But how it did so has been a matter of geological debate.
Rich pickings: the Witwatersrand Basin in South Africa (Image: James L. Stanfield/Getty)
Golden eruption
There's little doubt that the gold first
reached the Earth's surface through the erosion of gold-bearing veins in
a granite mountain range called the Kaapvaal Craton in what is now
north-east South Africa. The precious metal came up with the lava that
formed the mountains.
But it is unclear how huge quantities of
this gold ended up several hundred kilometres to the south west in the
Witswatersrand basin.
The prevailing theory is that metallic
gold fragments were eroded from the Craton mountains and transported by
rivers that dumped their cargo in shallow lakes overlying what is now
the gold-rich basin.
"The idea is that it arrived by mechanical
transportation together with sediments, like gold washing into creeks
and gullies," says Christoph Heinrich of the Swiss Federal Institute of Technology in Zurich, Switzerland. He disagrees.
Heinrich argues that the gold was first
dissolved chemically by volcanic rain and then washed to the basins by
river. It was then precipitated out of the water by mats of microbes
growing in shallow pools of the Basin.
"We don't know if the gold precipitated
out during life or after they died, but basic chemistry tells us that
organic life reduces gold chemically from the ionic to the elemental
form," Heinrich says.
No oxygen
Central to his theory is that 3 billion years
ago, the atmosphere was still largely free of the oxygen that was
produced half a billion years later by algae and cyanobacteria during
the Great Oxidation Event.
Before this event, the air was full of
gases containing sulphur – such as hydrogen sulphide – that were pumped
out by volcanoes and rained down on the mountains in acid rain. The gold
would have formed soluble complexes with sulphur, which were then
absorbed in water and finally separated by the primitive microbes, leaving behind today's rich legacy of elemental gold.
This was only possible because there was
so little oxygen in the atmosphere at the time, says Heinrich. If oxygen
had been there, it would have reacted with and destroyed the
sulphide-based gold complexes before they could reach the shallow,
life-bearing pools. "Oxygen would have 'killed' the sulphur compounds
that carried the gold," he says.
"The billion-dollar question is whether the same process created other gold deposits," says Heinrich.
If it did, then it may provide new
geological clues for today's gold prospectors. Currently, he says that
searches tend to focus on gravel-rich areas like those that gave rise to
the Californian gold rush. But if gold was transported in solution
rather than as grains, and later extracted by early life forms, this may
open up new ways to trace untapped deposits, he says.
So where would he look for gold? "I would
look for carbon because if I'm right, carbon is an essential part of
it," says Heinrich. "So I would maybe look for carbon-rich shale in the
same lake-type environments but without the gravel."
Heinrich's theory is worth serious consideration and further testing, says Jan Kramers
of the University of Johannesburg in South Africa. "It works well with
the now well-established observation that the atmosphere was not
oxidising and the rain was more acidic than today, and it's hard to
fault the arguments for transport of gold in solution under these
conditions, followed by precipitation in the presence of organic
matter."
Journal reference: Nature Geoscience, DOI: 10.1038/NGO2344
Aucun commentaire:
Enregistrer un commentaire