Could quakes explain why gold nuggets are found in quartz veins? Premium

Unlocking the mystery of gold nuggets formation in quartz veins through piezocatalysis during seismic activity.

Sometimes a scientific study comes along that reminds us not all the natural mysteries of this world demand highly specialised knowledge or million-dollar experiments to solve. Instead, they reveal something new by using ideas we were familiar with by high school. Doing this science in this day and age is still limited by access to specific instruments and locations and of course time. Not everyone can do it — but that shouldn’t stop us from being wowed by it.

One such study was published in Nature Geoscience on September 2. It opens thus: “Ore deposits represent natural enrichments of elements compared with their normal distribution in Earth’s crust. Gold deposits stand out by having the highest degree of enrichment, by factors of 1,000 to 10,000 required to make economic deposits … compared with base metals, such as copper, that require ~200x enrichment. Gold nuggets represent the most extreme examples of this gold enrichment. Most nuggets originate from the quartz veins formed in orogenic gold systems found around the world. These systems have had exceptional economic importance throughout human history, representing up to 75% of all gold ever mined.”

(‘Orogenic’ means a large-scale geological process that creates mountains, such as the interaction of the Indian tectonic plate with the Eurasian plate to create the Himalaya.)

For the study, the researchers — all from research institutes in Australia — were curious why most gold nuggets mined in human history were found in orogenic quartz veins.

Scientists know gold isn’t very soluble in fluids. If gold deposits form when the metal condenses out of water in certain locations, we’d need 10 million litres of water just to have 10 kg of gold. So this theory doesn’t present the full picture. Another idea scientists have had is that water could contain more dissolved gold if the gold is present as nanoparticles, but yet others have said there’s no way to explain why a very large quantity of nanoparticles would get out of water at the specific places where miners have found nuggets. Even others have wondered whether the orogenic nugget veins could be formed the same way epithermal vein deposits — which occur up to 1.5 km underground — are formed: when hot, mineral-rich fluids cool, depositing gold, silver, copper, and/or some other metals on the rocks around them. There’s a problem here, too, per the paper: “This mechanism leaves a clear textural and geochemical signature that cannot be applied to most orogenic deposits”.

Where are the large nuggets coming from, then?

It seems the quartz itself might be the answer. Quartz is a piezoelectric crystal: when it is squeezed or its shape is mechanically distorted in some way, it develops a voltage. Since quartz is also an insulator, electrons can’t flow within the crystal in response to this voltage. Instead, the electric field created distorts the electronic properties of the crystal such that charged particles — like electrons — flow from the crystal to an aqueous solution on its surface or vice versa. And if the quartz crystal is continuously distorted back and forth, these charged particles can also keep flowing back and forth.