The renewable energy sector is booming, and as demand for clean energy rises, so too does demand for the metals it relies on—copper and nickel chief among them. As the world continues scaling up renewables to meet the goals of the Paris Agreement, demand will almost certainly top supply in the coming decades. To address this future gap, an international group of researchers and the mining giant BHP teamed up to identify the processes that determine whether or not these metals make it into accessible deposits in the crust.
In a new study, published recently in Nature Communications, the team identified a “Goldilocks zone” at the base of the crust: If it’s too hot or too cold, the metals stay put in the mantle, but when the temperature is just right, they can pass through to the crust. The research, funded by the Natural Environment Research Council, is part of a larger project known as FAMOS (From Arc Magmas to Ores), whose ultimate goal is to help the mining industry develop better exploration tools and techniques.
Nickel, cobalt, and lithium are all critical components of batteries for electric vehicles, whereas copper is necessary for charging stations, solar panels, and wind turbines. “Copper is hugely important because basically anything that uses electricity has copper wire,” said David Holwell, an associate professor at Leicester University and lead author of the new study.
These metals are sourced from deep below Earth’s surface in the mantle, where plumes of buoyant rock arise and tectonic plates collide and tear apart. These processes produce magma that then rises through volcanic plumbing systems toward the surface.
“We have essentially ‘gateways’ that this magma has to go on…to take metals from the mantle source up into the upper crust and form deposits which can then be exploited to feed society’s needs,” Holwell said.
One of those gateways appears to be at the base of the crust, where magma from the mantle often pools. These magmas are largely silicate in composition but sometimes contain dense sulfides in which metals like gold, nickel, cobalt, and copper concentrate. If these sulfides remain warm enough courtesy of ongoing magmatic activity or are later reheated after crystalizing, they may be able to continue rising into the crust. “It was generally thought that [this type of gateway] was a sulfide trap,” Holwell said, not a gateway. In a trap, he explained, “the metals will stay there, and they will not go up into the upper crust.”
A Natural Laboratory
It’s hard to study this gatekeeping process up close for the same reasons it’s impossible to access these metals before they make their way into the crust: The mantle-crust boundary is deep—20, 30, even 40 kilometers down—and is inaccessible with current drilling technology. The team got around this by traveling to a “natural laboratory” in the Italian Alps where tectonic forces long ago rotated the crust 90°, exposing the former boundary between the mantle and the crust.
Using a scanning electron microscope to investigate samples of the ancient crust, Holwell and his colleagues saw veins and networks of copper sulfide amid the surrounding silicate rock, suggesting that this material was mobile before it eventually solidified. Nickel sulfide in the samples, meanwhile, had remained crystallized with the main rock mass. Using thermodynamic models, the team showed that temperature was key in controlling which sulfides were liquefied and which weren’t. Below about 1,100°C, they suggest, the sulfides would have all remained in solid crystal forms and locked in place at the mantle-crust boundary. However, if the temperature rose to between about 1,100°C and 1,200°C, nickel, cobalt, and iron sulfides would remain solid while copper and gold sulfides became liquid.
“If it’s liquid, that makes it potentially mobile,” Holwell said. “So we have a scenario here where our gateway can potentially open, and if the magmas can then flow upwards into the upper crust, they can carry that copper and gold with them.”
The key word here is potentially, according to Jamie Wilkinson, a professor at Imperial College London. Although this study identifies a process by which some metals can be mobilized, it doesn’t explain how the dense copper and gold sulfide melts are actually transported upward. More work is needed to understand how silicate magmas carry the liquid sulfides up through the crust, Wilkinson said.
Wilkinson, though not an author on this study, leads the FAMOS project. “This is actually the second gate that we identified as critical,” Wilkinson said, “the first one being what happens in the melting region [of the deep mantle] itself.” And there may be others. When it began, project members identified five potentially important gateways that may influence how metal-rich sulfides migrate up to and through the crust.
“The aim of the project was to understand the systems better in order for us to, from an exploration point of view, predict where we might find more” of the ore deposits, Holwell said. “If we know more about what we’re searching for, then we’re going to spend less time, effort, money, and energy doing that exploration.”
—Kate Wheeling (@katewheeling), Science Writer