Study reveals how earthquakes forge large gold nuggets in mere seconds
Imagine cracking open a chunk of white quartz from a gold mine and witnessing vibrant metal streaks within. For over a century, geologists have pondered how gold ends up in such places, attributing it to hot water. But a recent study introduces a groundbreaking twist: earthquakes, with their electrical phenomena, might be the key to gold's presence in quartz veins.
Geologist Christopher Voisey from Monash University, alongside colleagues at CSIRO and the Australian Centre for Neutron Scattering, delved into the intriguing concept of piezoelectricity in quartz. When quartz crystals are subjected to pressure, bending, or twisting, their atomic structure shifts, creating a separation of positive and negative charges, resulting in a voltage across the crystal. This phenomenon is akin to what powers quartz watches, albeit with precise electrical circuits.
The research team focused on fault zones, regions where quartz veins are abundant. These zones experience tectonic plate movement, causing rocks to break, slip, and grind past each other. During earthquakes, stress builds up in the quartz, leading to the release of piezoelectric charges. The scientists posed a crucial question: Can these voltages facilitate the movement of electrons, extraction of gold from solution, and direct attachment of gold to quartz surfaces?
To explore this hypothesis, the researchers conducted controlled laboratory experiments. They immersed quartz pieces in solutions containing dissolved gold, mimicking the hydrothermal fluids found deep underground. By applying mechanical stress to the quartz, simulating the earthquake's effects, they observed the crystal surfaces through high-resolution microscopes.
The results were remarkable. Metallic gold emerged as bright specks, clusters of nanoparticles, and small pseudo-hexagonal crystals on the quartz grains. These findings align with the principles of electrochemical deposition, where dissolved gold ions gain electrons and transform into solid metal on surfaces.
The study also incorporated quartz with pre-existing gold, acting as conductors within the system. When stress created an electric field in the quartz, these metal grains concentrated the field around themselves, fostering the growth of new gold nanoparticles on and around older grains, forming halos and tight clusters. Under these conditions, electric charges facilitated the 'plating' of gold out of solution, resulting in fresh metal coating the quartz surface and thickening the deposits.
Once a tiny 'seed' of gold exists, it becomes the preferred site for further gold deposition during each stress event. Quartz, an electrical insulator, hinders the easy movement of electrons through its interior, making it challenging to initiate nugget growth from scratch. Gold, however, conducts electricity efficiently. Once a small conductive grain forms, it concentrates the electric field at its surface, enabling efficient electron movement where needed.
As reactions progress, the system exhibits a 'rich get richer' pattern, resulting in fewer, larger gold pieces rather than numerous tiny ones. In another set of experiments, the team immersed quartz in a liquid containing gold nanoparticles. When stress was applied, these particles no longer remained evenly distributed. They drifted, gathered, and clumped into larger clusters directly on the quartz surface.
Professor Andy Tomkins, a co-author of the study, expressed awe at the findings. "The results were stunning," he said. "The stressed quartz not only electrochemically deposited gold onto its surface but also formed and accumulated gold nanoparticles. Remarkably, the gold tended to deposit on existing gold grains rather than forming new ones."
This behavior indicates that electric fields around stressed quartz can gather and concentrate mobile gold particles even before they fuse into a continuous grain. In a fault zone filled with quartz veins and gold-bearing fluids, each earthquake temporarily transforms the system into an electrochemical cell.
On certain quartz surfaces, electrons accumulate, and dissolved gold species pick up these electrons, transforming into metallic gold. On other surfaces, complementary reactions occur, with charged ions in the fluid moving to balance the charges. Each 'squeeze'—each earthquake—charges the quartz slightly, driving a small amount of gold plating onto existing grains or fresh nucleation sites.
Dr. Voisey concluded, "In essence, the quartz acts like a natural battery, with gold as the electrode, slowly accumulating more gold with each seismic event."
This study suggests that seismic activity, through the charging and discharging of quartz over geological time, contributes to the close association between gold and quartz and the rare instances of nature forming exceptionally large nuggets. The full study was published in the journal Nature Geoscience.
