Where Does Gold Come From?

We’ve looked at gold’s history and multiple uses. Now…where does it come from?

No one knows for sure, but the answer lies in the stars.

The PGE iridium is found in the KT boundary that marks the mass extinction event that killed off many dinosaurs.  Source
The KT boundary contains iridium, a platinum-group element. Source

Where is gold found?

Gold and platinum are both rare elements, but they’re found in different places.

Platinum-group elements (PGEs) are so closely associated with impact craters that field researchers can identify a probable impact by the presence of PGEs, even if weathering has hidden or erased the crater walls and floor.

Gold, on the other hand, is found in a wide variety of geologic environments and in many different types of rock. There is no clear association with impact craters, although a lot of people claim one exists in South Africa at the 2-billion-year-old Vredefort astrobleme, where the richest gold deposits on Earth are found.

It isn’t a true association, though.

The deposits that contain gold (the yellow layer in the diagram below) were actually laid down and then covered by lava and other rocks almost a million years before the impact happened.

Cross-section of the Vredefort impact crater. Source: Oggmus

The impact raised the buried layers up close enough to the surface that we could find them, but it didn’t bring the gold in. That was already in place.

Geologic processes tend to concentrate the elemental ore in lodes (embedded in rocks) or placers (deposited by running water). The Witwatersrand deposits are placers and account for almost half of the gold ever mined on Earth.

Gold has also been mined in northern Africa and, for that matter, on every continent. What’s missing is Rumpelstiltskin, who knew how to turn straw into gold.

Where did Earth’s gold come in the first place? To answer that, we must look to the stars.

Star seed

Gold is a transitional metal, element number 79 on the periodic table.

Gold (Au) is heavier than iron (Fe).  Source
Gold (Au) is heavier than iron (Fe). Source

Scientists believe that the elements through iron form inside stars through nucleosynthesis. This releases energy (i.e., the star “burns”) until elements with 26 protons (iron) exist.

After that, the star is out of fuel and likely will explode as a supernova.

One explanation for the existence of heavier elements, including gold, is that they formed in the higher pressures and temperatures of the exploding star.

Not shown in this star about to collapse and explode:  Any element heavier than iron.
Not shown in this star about to collapse and explode: Any element heavier than iron.

However, there is no consensus on the precise details of how this happens. The US National Research Council has listed the formation of elements between iron and uranium as Number Three on its list of the top 11 physics questions for the 21st century.

Some of the more layman-friendly discussions about this that I’ve come across online indicate that neutron capture and red giants may be involved.

There is an added wrinkle when it comes to gold. According to NASA,

The relative average abundance [of gold] in our Solar System appears higher than can be made in the early universe, in stars, and even in typical supernova explosions.

These NASA experts suggest it can be explained by a collision of two neutron stars – an event that would produce, in their words, “one of the most powerful explosions in the universe.”

Oh, and such a collision can also create a black hole, according to supercomputer models:

Not in Michael Bay’s wildest dreams…

The late veneer

So…how did gold then get to Earth? Scientists are still arguing about that, but the most popular explanation to date is the “late veneer” theory.

During the formation of the Solar System, many scientists say, Earth (which is a space object as well as humanity’s home) had its own share of gold and other very dense, iron-loving elements (siderophiles) from the stars.

Soon after the planet first took shape, the siderophiles sank, along with iron, when Earth’s core formed (PDF).

Obviously there is still quite an abundance of iron near the surface today, not to mention gold and other siderophiles. This happened after the core formed, these scientists say, when a late heavy bombardment of meteorites left a “veneer” of siderophiles.

A lot of questions still remain about the origin of gold and its presence on Earth. The Vredefort impact 2 billion years ago uncovered massive gold deposits, but it didn’t bring gold to planet Earth.

Gold lacks the association with impact structures that the platinum-group elements have. Probably it was born in a star and then blown out to seed the Universe. Some of it ended up in the planetary nebula that eventually condensed to form the Solar System.

It’s quite an impressive history to ponder, the next time you use a smart phone, watch an astronaut in space, or wear some gold jewelry.



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