Chunky gold nuggets tend to form far underground along fracture lines that run through quartz, but the reason why has never been nailed down.
Now, scientists have proposed an explanation for the effect: the immense forces unleashed by earthquakes squeeze quartz enough to generate electric fields, which in turn drive the formation of the precious deposits.
Lab experiments in Australia found that seismic waves from earthquakes produced strong enough electric voltages in quartz to extract dissolved gold from fluids infiltrating the mineral.
“This mechanism can help explain the creation of large nuggets and the commonly observed highly interconnected gold networks within quartz vein fractures,” the researchers write in Nature Geoscience.
According to World Gold Council estimates, 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.
While scientists understand the broad processes that forge gold nuggets, they have wrestled with a long-unsolved paradox. The mystery is how large nuggets, some weighing tens to hundreds of kilograms, can form when the fluids that seep into quartz veins have gold concentrations around one part per million at best. To form 10kg of gold would require about five Olympic swimming pools of water.
To investigate, Dr Christopher Voisey at Monash University in Melbourne, and others in Australia, immersed lumps of quartz in water that had gold dissolved in it. They then subjected the quartz to the stresses rock experiences during an earthquake.
Quartz is unusual in being the only abundant piezoelectric material on Earth. Its crystal structure is such that when it is squeezed, an electric 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 produced in earthquakes could generate sufficient electric fields in quartz to draw gold out of the surrounding solution. This produced gold nanoparticles on the quartz surface with further gold forming on top.
“Since piezoelectric voltages are instantaneous and leave behind no visible tracer, this can rationalise why gold nuggets commonly appear to be ‘floating’ in quartz veins with no obvious chemical or physical trap,” the researchers write. “We suggest that piezoelectric gold accumulation could be a solution to the longstanding ‘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’ve been unable to explain why in some cases gold gets extremely concentrated and forms very large nuggets,” she said.
“The piezoelectric theory is interesting because it would help to further concentrate any nanoparticles, but also explain why early quartz veins in fault zones are typically barren: you need the quartz veins to be there before you can induce the piezoelectrical effect.
“Understanding the mechanism by which these deposits form can potentially help in targeting rich gold deposits, although in order to translate understanding into real practical applications we would need to know if there are any markers, detectable on Earth’s surface, that this process would leave behind,” she added.
In March, a metal detectorist from Somerset uncovered England’s largest ever gold nugget after turning up late for a dig in Shropshire with a faulty metal detector. Named Hiro’s nugget, the 64.8g lump of metal fetched £12,000 at auction.
