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Where does gold come from?



All the gold ever mined would fit into a cube less than 21 metres on each side (Source: bashta/iStockphoto)

The seasons roll around, and when autumn comes, some of the trees take on a gorgeous golden colour. But some Australian trees actually concentrate gold, that precious metal, into their leaves. How did the gold get there? The answer involves exploding stars, asteroids, bacteria, earthquakes, and lots of geology.

All the gold ever mined would fit into a cube less than 21 metres on each side. Gold is a 'precious metal', with a price up around $60,000 per kilogram.

Today, about half of it is used in jewellery, about 40 per cent for financial investments, and about 10 per cent for industrial uses.

Gold has many wonderful properties — it is a very soft and easy to work and turn into jewellery; it is a great reflector of heat and so it's used as a thin film on windows; and it's a great conductor of electricity and pretty inert so it won't corrode when used in electrical circuits. And because it is so inert, back in the old days (several thousand years ago) people would find lumps of gold just lying around on the ground, still shiny and bright after thousands or millions of years out in the weather. But where did it come from?

In the beginning was the Big Bang, some 13.8 billion years ago. At the start, the Universe was so hot that everything was pure energy, and there was no matter. As the Universe expanded and cooled down, the first atoms coalesced. About 90 per cent of them were hydrogen atoms, about 10 per cent helium atoms — with a tiny fraction of other lighter elements. At this early stage, there was no gold anywhere in the Universe.

A few hundred million years after the Big Bang, the first stars were blazing away with their nuclear fires. These nuclear fires forced lighter elements together to make slightly heavier elements, and these nuclear reactions released a huge amount of energy. Gradually, these early stars began making elements such as carbon, nitrogen, oxygen — working their way up through the periodic table towards iron. And still, there was no gold in the Universe.

As these early stars ran out of the lighter elements, they began burning heavier elements. Finally, as they burnt silicon to make iron, they exploded as a supernova, and for a few short moments, each star would release as much energy as all the regular stars in that galaxy put together.

In that cataclysmic explosion, for the first time, atoms of gold were manufactured — and then hurled out into the Universe, along with the other debris from that explosion. It also seems that gold may be created when neutron stars collide.

Over time, vast clouds of gas and dust from these explosions began to coalesce thanks to the attractive force of gravity — and they formed what we called 'stellar nurseries'. A stellar nursery is a location where solar systems are born.

Once one part of the gas and dust had become more concentrated and more dense, its gravity attracted further gas and dust.

About 4.6 billion years ago, a rather special spherical cloud evolved into a slowly spinning flattened disc. This would be our solar system. After about 10,000 years, this disc had gradually separated out into little lumps about 10 kilometres across called planetismals.

Over the next 100,000 to million years-or-so, these 10-kilometre sized planetismals collided and stuck together to form smallish planets ranging in size from Moon-size to Mars-size. Then, over the next 10 to 100 million years, the larger planets would have formed. Specifically, in the case of the Earth, it would have taken about 30 million years.

However, the majority of the gas and dust would have collected at the centre of this spinning disc to form what would become the Sun. Once enough matter had accumulated here, the temperature at the centre of this Sun-to-be would get hot enough for nuclear fusion to happen. The nuclear fires ignited, and the now-hot Sun at the centre of this spinning disc of planets and gas and dust blew the gas and dust away - leaving behind our newborn solar system.

Looking at the early Earth, there would have been no solid crust — instead, the surface was covered by a sea of molten rock. The heavy elements would have melted and sunk — ending up in the iron-rich core. Today, there's enough gold in the core to lay down a half-metre thick crust over our planet.

But, if the heavy metals sank down to the core, how come there's still some gold left near the surface, and how on Earth did it get into Australian eucalyptus trees? Well, I'll talk about that, next time…


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Published 03 December 2013

© 2016 Karl S. Kruszelnicki Pty Ltd