Dr Karl › Dr Karl's Great Moments In Science

How on Earth does gold get into gum trees?

Gold originally came from exploding stars, but there are many layers to the story of how this precious metal found its way into eucalyptus trees. Dr Karl unearths more golden nuggets of geochemistry.

Last time I talked about how gold was made when stars exploded. And, down here on Earth, gold has turned up in the strangest of places such as Australian eucalyptus trees.

Now, immediately after our planet formed, the Earth's entire surface was an ocean of molten rock. Within some five millions years practically all of the iron sank, dragging gold and other precious metals with it to the core. There are enough precious metals down there to cover the surface of our planet with a layer some four metres thick.

First, how did the gold get back to the surface of the Earth? It didn't.

Another batch of gold got delivered in the so-called 'Late Heavy Bombardment' — now, this is still a new theory.

After the Earth first coalesced from the dust cloud there was an initial period of impacts by moon-sized objects.

This was followed by a 500-million-year period of small impacts and then the Late Heavy Bombardment some 4.1 to 3.8 billion years ago. The young Earth, at only half-a-billion years old was sprinkled with heavy and precious metals delivered by some 200 billion billion tonnes of incoming asteroids in the Late Heavy Bombardment. This added more heavy metals, including gold, but only to the crust.

Today, gold is mostly found in two situations. There is the primary gold in so-called hydrothermal quartz veins or gold veins; and then there is secondary placer gold.

It seems as if placer gold has been formed by erosion of a primary quartz gold vein producing gold dust, gold flakes and small nuggets of gold. This secondary placer gold is found in loose deposits of sand and gravel. Because the gold is much denser than the sand and gravel it's easy to separate out as in panning for gold.

Now how did the gold get into the hydrothermal quartz veins in the first place?

One recent and interesting theory is that the gold was precipitated in the veins by earthquakes.

This theory says the hydrothermal veins formed during periods of mountain building over the last three billion years.

Very large volumes of water carrying dissolved gold flowed inside seismically active faults. An earthquake would make these faults suddenly expand. According to the simulations a magnitude 2 earthquake would increase the volume inside a fault by 130 times. While a magnitude 6 earthquake would cause an expansion of some 13,000 times. This would create a massive drop in pressure and any liquid present in the vein would suddenly turn into a low density vapour. Then quartz and various trace elements including gold would suddenly precipitate as a solid. Instantly a quartz vein enriched with gold had been deposited, a mini gold mine in waiting.

In geologically active areas, repeated cycles of earthquakes over periods shorter than 100,000 years would increase the levels of gold in the veins by 1000 times from several parts per billion to several parts per million. It's estimated that this process of earthquakes causing flash vaporisation could have caused over 80 per cent of the world's economically viable gold vein deposits.

Now let's add life to the story.

In some cases when you look at the gold with an electron microscope it looks like piles of bacteria encased in gold. We have found what appears to be lacy patterns of bacteria in 2.8-billion-year-old South African gold and in 220-million-year-old Chinese gold.

In 2006, we found a bacterium that existed as part of a living biofilm on gold and which could survive in concentrations of gold that would kill other bacteria. Sometimes the bacteria would accumulate the gold in specific areas internally, and other times it would be entirely externally covered by gold.

In 2013, we discovered another bacterium that manufactured a protein that would grab any environmental gold and dump it as a precipitate. At this stage, we don't know how much gold accumulation we can blame on the bacteria.

But finally we can get back to the gum trees.

In Western Australia we have found eucalyptus trees that can extract dissolved gold from water. This gold comes from gold deposits some 40 metres below the surface and as the tree sucks the water up, it also sucks up the gold. The gold is present as tiny 8-micron-wide particles inside the cells of the tree. The highest concentrations are on the extremities of the tree, such as the distant leaves, and again, probably like the bacteria, the gold is a toxin that the tree is trying to get rid of.

And that's the story of how gold found its home among the gum trees …

^ to top