COPPER WAS the first metal worked by human beings. They hammered it into jewellery and ornaments as much as 11,000 years ago. Today, Homo sapiens uses more than 20m tonnes of the stuff a year, much of it in buildings and electrical infrastructure. More will be required in coming decades, to meet the need for widespread electrification brought about by the transition to less carbon-intensive economies. Copper is an essential part of batteries, motors and charging equipment. Solar and wind installations use more copper than their fossil-fuel counterparts, and electric vehicles contain four times more copper than do cars with combustion engines.
This has spurred interest in new sources of the metal, most of which comes at the moment from rocks dug out of vast opencast mines that are then ground up and processed to release the copper they contain, typically about 1% of their mass.
Metal-rich nodules scattered across various parts of the ocean floor are a possibility. But exploiting these brings technological and regulatory difficulties, and is in any case controversial because of the damage it would do to deep-ocean ecosystems. Jon Blundy of Oxford University, however, offers an alternative. This is to extract, from deep under Earth’s surface, the mineral-rich brines from which ores of copper and other valuable metals are generated in the first place. As Dr Blundy points out, “pretty much all of the non-ferrous natural resources that we exploit come ultimately from ancient volcanoes.”
In particular, in 2015, he and his colleagues worked out the chemical details of how copper-sulphide ores form when sulphur-rich gases rise through the plumbing of active volcanoes and encounter metal-rich brines trapped in rocks sitting just above pockets of magma. Modern mining operations dig up examples of these ores that formed millions or billions of years ago. Dr Blundy proposes instead to cut out the middleman and go straight to the deep copper-rich fluids themselves.
As he writes in Open Science, he suspects these are found beneath every active and dormant volcano, though the concentration of copper in the brine concerned will vary from place to place. His evidence comes from electromagnetic surveys carried out on some 40 volcanoes, including Mount Fuji in Japan, Mount St Helens in America and others in Bolivia, New Zealand, the Philippines and elsewhere. These surveys consistently pick up highly conductive zones 2km or more beneath the surface, for which the simplest explanation is the presence of super-salty metal-rich brines. This conjecture is reinforced by analysis of rock samples recovered from such depths under a number of volcanoes. These do indeed contain brines with varying concentrations of copper, as well as other valuable metals including lithium, zinc, gold and silver.
All this suggests that copper could be drilled for commercially in the same way that oil is—except that the boreholes involved would be considerably deeper. That would be difficult, but not out of the question. It would require equipment that could withstand temperatures greater than 400°C and contact with brines ten times saltier than seawater. But the prize would be worth it.
Individual volcanoes would, admittedly, yield only a fraction of the output of a big copper mine. Dr Blundy and his colleagues estimate, for example, that there might be as much as 1.4m tonnes of copper beneath New Zealand’s White Island volcano (pictured above), whereas the world’s largest mines hold tens of millions of tonnes of it. But there are only a handful of such mines, most in mountain ranges near the Pacific coast of the Americas. By contrast, hundreds of volcanoes exist around the world, ready be tapped.
The temperature at which the equipment used would have to operate, moreover, brings an opportunity. The heat involved might be employed to generate electricity—enough to power the drilling operation and perhaps even to yield a surplus. Sucking copper out of Earth’s crust through 2km-long straws might thus be that rare thing in the mining industry, an actual environmental good. ■
A version of this article was published online on July 7th 2021
This article appeared in the Science & technology section of the print edition under the headline “Brine mines”