Scientists discover how gold reaches Earth's surface from deep underground

Gold does not just sit on Earth’s surface in neat little nuggets, ready to be picked up or panned by lucky prospectors. That’s not how it works – not even close.

Most gold is trapped in rocks deep in Earth’s mantle, which is the super-hot layer sandwiched between the crust we walk on and the planet’s core.

Scientists have been trying for years to figure out how gold escapes from the mantle and ends up in deposits that miners can dig up.

Now, a new study has identified a big piece of the puzzle involving unusual chemistry occurring 30 to 50 miles beneath the surface.

Gold in Earth’s mantle

Picture the edge of an ocean where the seafloor is slowly sliding under a continent. Geologists call these spots subduction zones, and they act as recycling centers for old oceanic crust.

As that old seafloor sinks, it drags along water and chemicals that became trapped in the rocks over millions of years.

Here is where it gets interesting. When the sinking slab reaches depths of 30 to 50 miles, the pressure and heat squeeze out super-hot, salty fluids.

These fluids rise into the mantle rock above, helping create the magma that feeds the volcanoes around the Pacific. Sometimes, if conditions align, these fluids also carry large amounts of gold.

Adam Simon, a professor of Earth and environmental sciences at the University of Michigan, and his team recently published their findings in the Proceedings of the National Academy of Sciences.

“This thermodynamic model that we’ve now published is the first to reveal the presence of the gold-trisulfur complex that we previously did not know existed at these conditions,” Simon explained.

“This offers the most plausible explanation for the very high concentrations of gold in some mineral systems in subduction zone environments.”

Gold travels from the mantle with sulfur

Left alone, gold is essentially a homebody. It does not dissolve easily and prefers to stay put. But pair it with the right partner – sulfur, specifically – and suddenly it is ready to travel.

The research team built a computer model that tracks the heat, pressure, and chemical reactions happening deep underground. What they found changes how we think about gold transport.

When those fluids from the sinking oceanic plate hit the mantle, they change its whole personality. They make it more oxidizing, which means it becomes better at removing electrons from other atoms. Think of it as switching the mantle from calm to aggressive mode.

In this aggressive, oxidized mantle, sulfur shape-shifts into forms it normally would not take.

One of these forms creates a molecule in which one gold atom links with three sulfur atoms in a chain. This gold-trisulfur complex is essentially gold’s first-class ticket to the surface.

Without this complex, gold stays locked in minerals. With it, huge amounts of gold can dissolve in those hot fluids and ride them upward.

We are talking grams of gold per cubic meter of fluid – thousands of times more than is normally scattered through mantle rocks.

Global subduction zones

Not every subduction zone produces gold deposits, and now we know why.

You need the right ingredients: enough oxidizing fluids from the sinking plate, water to create the proper fluid-transport system, and temperatures and pressures that hit the sweet spot for forming that gold-trisulfur complex.

The study found that water is not optional – it is absolutely essential. Systems with water can move far more gold than dry ones. Water is the delivery truck that makes everything work.

When all these ingredients come together, the mantle hands off its gold to rising magmas.

These magmas rise toward the surface, cool, and deposit their golden cargo into cracks and veins. After a few million years, a deposit worth mining can form.

Connecting the dots

“On all of the continents around the Pacific Ocean, from New Zealand to Indonesia, the Philippines, Japan, Russia, Alaska, the western United States and Canada, all the way down to Chile, we have lots of active volcanoes,” Simon explained.

“All of those active volcanoes form over or in a subduction zone environment. The same types of processes that result in volcanic eruptions are processes that form gold deposits.”

This is not just interesting science – it is also practical. Now that geologists understand this process, they can be smarter about where to look for gold.

Find an old subduction zone where the conditions were just right, and you might find the next big deposit.

Settling the argument

Scientists have been debating whether this gold-trisulfur complex could exist under mantle conditions. Some said yes; others said no.

This study effectively ends that argument by showing not only that it exists, but that it is probably the main way gold moves from the deep Earth to the surface.

“These results provide a really robust understanding of what causes certain subduction zones to produce very gold-rich ore deposits,” Simon noted.

“Combining the results of this study with existing studies ultimately improves our understanding of how gold deposits form and can have a positive impact on exploration.”

It turns out that subduction zones are not just garbage disposals for old ocean floor. They are chemical factories that remix sulfur, water, and heat into a system that can move gold from the deep Earth to places where we can mine it.

When the chemistry is right, Earth essentially runs its own gold-concentration process, creating the deposits that have fascinated humans since we first spotted that distinctive yellow gleam in a riverbed.

The full study was published in the journal Proceedings of the National Academy of Sciences.

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