Chunky gold nuggets tend to form far underground along fault lines that run through quartz, but the reason for this has never been nailed down.
Now scientists have proposed an explanation for the effect: the tremendous forces unleashed by earthquakes push quartz enough to generate electric fields, which in turn drive the formation of the precious deposits.
Laboratory experiments in Australia found that seismic waves from earthquakes produced strong enough electrical stresses in quartz to extract dissolved gold from fluids infiltrating the mineral.
“This mechanism may help explain the creation of large nuggets and the commonly observed highly interconnected gold networks within quartz vein fractures,” write the researchers. Nature Geoscience.
According to estimates from the World Gold Council, between 2,500 and 3,000 tonnes of gold are mined each year. Most nuggets come from quartz veins, which account for about three-quarters of all the gold ever mined.
As scientists understand the broad processes that forge gold nuggets, they have grappled with a long-unsolved paradox. The mystery is how large nuggets, some weighing tens to hundreds of kilograms, can form when the fluids seeping into quartz veins have gold concentrations of about one part per million at best. To form 10 kg of gold, about five Olympic swimming pools of water would be required.
To investigate, Dr Christopher Voisey at Monash University in Melbourne, and others in Australia, immersed lumps of quartz in water with gold dissolved in it. They then subjected the quartz to the stress experienced by rock during an earthquake.
Quartz is unusual because it is the only abundant piezoelectric material on Earth. Its crystal structure is such that when it is pressed, an electrical voltage forms across it. The effect is used in piezoelectric lighters that produce a spark at the click of a button.
Voisey and his colleagues found that the stresses and strains caused in earthquakes can generate sufficient electric fields in quartz to pull gold out of the surrounding solution. This produced gold nanoparticles on the quartz surface with further gold forming on top.
“Since piezoelectric stresses are instantaneous and leave no visible tracer, this may rationalize why gold nuggets commonly appear to ‘float’ in quartz veins without an obvious chemical or physical trap,” the researchers write. “We propose that piezoelectric gold accumulation may be a solution to the long-standing ‘gold nugget paradox’.”
Dr Taija Torvela, a structural geologist at the University of Leeds who was not involved in the research, called the paper “thought-provoking”.
“It is true that we could not explain why gold in some cases becomes extremely concentrated and forms very large lumps,” she said.
“The piezoelectric theory is interesting because it will help to further concentrate any nanoparticles, but also explains why early quartz veins in fault zones are typically barren: you need the quartz veins to be there before you can cause the piezoelectric effect.
“Understanding the mechanism by which these deposits form may help target rich gold deposits, although to translate understanding into real practical applications, we will need to know if there are any markers, detectable on the Earth’s surface, that indicate this process will leave behind. ,” she added.
In March, a metal detector from Somerset uncovered England’s biggest ever gold nugget after turning up late for a dig in Shropshire with a faulty metal detector. The 64.8g lump of metal, called Hiro’s nugget, fetched £12,000 at auction.
