Seismic activity may explain large gold nugget mystery

A groundbreaking study published in Nature Geosciences suggests that seismic activity could finally unravel the mystery behind the formation of large gold nuggets found beneath the Earth's surface. 

Seismic activity, such as earthquakes, might finally unravel the mystery of how large gold nuggets form beneath the Earth's surface, according to a new study published in Nature Geosciences. Most gold nuggets are believed to originate from quartz veins within orogenic gold systems, which have been economically significant throughout history, accounting for up to 75 per cent of all gold ever mined. However, the process behind the formation of these large nuggets has long puzzled scientists, Caliber.Az reports, citing foreign media.

The hydrothermal fluids that transport gold through the Earth's crust typically have very low gold concentrations (around 1 part per million), while gold concentrations in quartz veins can surpass 50 per cent, sometimes leading to the creation of nuggets weighing hundreds of kilograms. This discrepancy has been a longstanding paradox. Dr. Chris Voisey, a geologist from Monash University and the lead author of the study, explains that the conventional theory suggests gold precipitates out of these fluids as they cool or undergo chemical changes, becoming trapped in quartz veins. 

However, this explanation doesn’t fully account for the formation of large gold nuggets, given the low gold concentration in these fluids. Dr. Voisey and his team explored a new hypothesis: the role of seismic activity in the formation of gold within orogenic quartz veins. They proposed that the cyclic stress caused by earthquakes could induce a mechanism known as piezoelectricity — where materials like quartz generate an electric charge under stress — potentially aiding in the accumulation of gold in these veins. 

To test their hypothesis, Dr. Voisey and his team simulated earthquake conditions in a laboratory setting and found that the stress applied to quartz can generate sufficient voltage to electrochemically deposit gold onto its surface. Quartz is unique among common minerals on Earth because it forms crystals without a center of symmetry. When quartz is distorted under stress, its non-centrosymmetric nature generates an electrical potential across the crystal, directly proportional to the mechanical force applied, the study explains. 

"The results were astonishing," stated co-author Professor Andy Tomkins from Monash University. "The stressed quartz not only electrochemically deposited gold onto its surface, but it also formed and accumulated gold nanoparticles. Remarkably, the gold showed a preference for depositing on existing gold grains rather than creating new ones." 

The study notes that gold, being a conductor, contrasts with quartz, an insulator. Once some gold is deposited, it becomes a focal point for further accumulation, effectively "plating" the gold grains with more gold.

"Our discovery offers a plausible explanation for how large gold nuggets form in quartz veins," said Dr. Voisey, likening the quartz to a natural battery and the gold to an electrode that gradually accumulates with each seismic event. The researchers believe that this new insight not only resolves a longstanding geological puzzle but also underscores the connection between Earth's physical and chemical processes.