Humanity’s fascination with gold dates back thousands of years. Gold mining is described in ancient Greek and Roman sources, and the gold rush – especially in the 19th century – played a powerful role in shaping the modern world.
This dense, yellow metal is often found in veins of the rock mineral quartz. That’s because the two condense together from hot fluids underground as a result of changes in temperature, pressure, and chemistry.
Geologists understand this process well, but large nuggets of gold remain a mystery. Gold dissolves in natural fluids at a rate of about one part per million, so how does it get concentrated in lumps weighing tens or hundreds of kilograms?
As we report today in Nature Geoscience, the answer probably lies in the unusual electrical properties of quartz – and what happens when it is put under pressure due to an earthquake.
Quartz under pressure
Quartz is known to be a piezoelectric material. There are not many minerals like this on Earth, and quartz is by far the most abundant mineral.
Piezoelectric materials generate an instantaneous electric charge when put under stress — when a physical force compresses or stretches them. The bigger the force, the bigger the charge.
Not only did we see gold deposit on the surface of quartz, but we also saw it coalesce into nanoparticles. Furthermore, once the process began, gold was more likely to deposit on existing gold particles than on quartz.
This actually makes a lot of sense, since quartz is an electrical insulator and gold conducts electricity. Existing gold particles adopt the electrical potential from the nearby quartz and become the focus of reactions that deposit gold.
Industrial gold plating works in much the same way, the only difference is that we gold plate onto other gold.
Back to the Nuggets
Now that we know how quartz and gold behave in the laboratory, we can think again about geology.
The most impressive gold pieces ever found have been found in quartz veins, where gold-rich fluids flow through faults in earthquake-prone rocks.
During seismic activity, stress on the quartz can generate piezoelectric voltages capable of drawing gold out of these fluids. Once deposited, the gold becomes the focus of further piezoelectric plating as the fluid infiltration continues – so the gold deposits grow larger over time.
Over millions and millions of years, this process would be repeated over and over again. Is this why we see such large gold nuggets in quartz veins like this? We think this must be at least part of the picture.
Christopher Voisey, Research Fellow in the School of Earth, Atmosphere and Environment at Monash University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
