Visual Capitalist, a producer of science and economics infographics, has recently published part two of a five part series on the science and economics of gold. The second part of the series, which deals with the formation of gold deposits and the techniques used to mine them, provides effective visual ways of thinking about gold that both newcomers to mining and seasoned experts will find interesting.

Where does gold come from? It’s found in a variety of settings on Earth, including placer deposits, where gold eroded from surface features is buried along with sand and gravel, and in veins, where hot fluids related to volcanism once circulated.

But how did all this gold end up on Earth in the first place?

Astrophysicists have long known that most heavy elements in the universe are formed within stars. Gold is one of the heavier elements, and scientists have suggested that it could only form in large quantities as the result of a collision between two neutron stars (more on that here).

The cloud of matter left over from such a collision, which includes gold, may eventually coalesce into a planetary system complete with a new star.

This theory might lead you to believe that the gold we mine today was part of the earth at the time of its formation. It turns out that isn’t the case. Here’s why:

When the earth formed, it contained enough gold to form a 4 metre thick layer over the entire planet. Unfortunately, that’s not what happened.

In a process called differentiation, elements were able to separate into layers by density (think of a layer of oil separating from a layer of water). Since gold is heavier than most elements on Earth, it sank deep into the core along with the other metals.

That gold is still there today. Although it represents far more gold than will ever be mined, it is completely inaccessible in the core of our planet.

Fortunately, more gold was on the way. During the late heavy bombardment, the earth was struck by billions of meteors. It’s thought that about 20 billion tons of gold were delivered by the bombardment. Since the earth’s crust had cooled at that point, the gold was deposited near the surface and didn’t sink to the core.

Once the gold was on the surface, a variety of geologic forces, most related to the flow of hot, mineral-laden fluids, concentrated it into relatively high grade deposits, which we mine today.

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Image: Visual Capitalists