Green mining could pave the way to a net-zero future as the world aims to limit the impacts of climate change, a study has found.
Scientists from the Department of Earth Sciences at the University of Oxford have demonstrated how it is possible to directly extract valuable metals from hot salty fluids (brines) trapped in porous rocks at depths of about 2km below dormant volcanoes.
They propose this radical green-mining approach to provide essential metals for a net-zero future – copper, gold, zinc, silver and lithium – in a sustainable way.
“Getting to net zero will place unprecedented demand on natural metal resources, demand that recycling alone cannot meet,” said Professor Jon Blundy, lead author of the report.
“We need to be thinking of low-energy, sustainable ways to extract metals from the ground. Volcanoes are an obvious and ubiquitous target.”
Volcanoes could house key valuable metals to accelerate green mining
Magma beneath volcanoes releases gases that rise towards the surface that are rich in metals, the study explains.
As the pressure drops, the gases separate into steam and brine. Most metals dissolved in the original magmatic gas become concentrated in the dense brine, which in turn gets trapped in porous rock.
The less-dense and metal-depleted steam continues up to the surface, where it can form fumaroles, such as those seen at many active volcanoes.
The scientists’ new paper, published in Open Science, reveals how the trapped, subterranean brine is a potential “liquid ore” containing a slew of valuable metals, including gold, copper and lithium, that could be exploited by extracting the fluids to the surface via deep wells.
Their models show the brines potentially contain several million tonnes of copper, which is a key metal for making the transition to net zero, due to its importance in electricity generation and transmission, and electric vehicles.
Green mining a scientific and engineering challenge in the drive to a net-zero future, says Prof Blundy
The paper also shows how geothermal power will be a significant by-product of a green-mining approach, meaning that operations at the well-head will be carbon-neutral.
Conventional mining extracts metals, such as copper, from deep pits or underground mines in the form of solid ores that then need to be crushed and processed.
In the case of copper, more than 99% of the crushed rock is waste. Such mines are environmentally impactful, very expensive to construct and decommission, produce huge tailings piles of waste rock, and are very energy-demanding and CO2-producing.
The paper claims the prospect of extracting metals in solution form from wells reduces the cost of mining and ore processing, while also exploiting geothermal power to drive operations, which vastly reduces the environmental impact of metal production.
“Active volcanoes around the world discharge to the atmosphere prodigious quantities of valuable metals,” said Prof Blundy. “Some of this metal endowment does not reach the surface, but becomes trapped as fluids in hot rocks at about 2km depth.
“Green mining represents a novel way to extract both the metal-bearing fluids and geothermal power, in a way that dramatically reduces the environmental impact of conventional mining.”
The research is part of an international effort between the UK and Russia that uses volcanology, hydrodynamic modelling, geochemistry, geophysics and high-temperature experiments.
The team has worked on drill core from a number of deep geothermal systems in Japan, Italy, Montserrat, Indonesia and Mexico to confirm their predictions of metal-rich brines.
“Green mining is a scientific and engineering challenge that we hope that scientists and governments alike will embrace in the drive to net zero,” said Prof Blundy.
The study notes that geophysical surveys of volcanoes show that almost every active and dormant volcano hosts a potentially exploitable “lens” of metal-rich brine. This means that metal exploration may not be limited to relatively few countries as it is currently (Chile, US, Peru, China, Democratic Republic of Congo etc.), owing to the ubiquity of volcanoes around the world.
Brine mine could be between five and 15 years away, say scientists
The principal risks are technological, as the process involves drilling into a rock at 2km depth and at temperatures of more than 450C, according to the paper.
It highlights that the extracted fluids are corrosive, which places limits on the types of drilling materials.
“The extracted fluids tend to dump their metal load in the well-bore, a problem known as ‘scaling’,” the study added.
“Preventing scale formation will require complex thinking about the dynamics of fluid flow and pressure-temperature control in the well-bore. Preventing well-bore corrosion will require developments in materials science to create resistive coatings.”
According to the scientists, many of these challenges are already being addressed through deep, hot geothermal drilling projects. In some cases, they said these projects have reached temperatures above 500C and occasionally they have tapped into small pockets of molten rock, in places such as Iceland and Hawaii.
Ensuring the fluids continue to flow into the well once drilled is a complex problem and the permeability and porosity of hot, ductile rock is a challenging field, the study notes.
The scientists have already patented an idea for fluid extraction. They say that the risk of triggering volcanic eruptions is very small, but must be assessed.
The team are not planning to drill into magma itself, but into the hot rocks above the magma chamber, which they claim greatly reduces the risk of encountering magma.
They have spent the last five years “de-risking” the concept, and are now ready to drill an exploratory well at a dormant volcano. This will clarify many of the risks and challenges described and will herald a new advance in the understanding of volcanoes and the energy and metals they possess.
“Continuing the de-risking work, which we are pursuing on many fronts through an international collaboration, is important,” said Prof Blundy. “Likewise, we need to identify the best test-case volcano to drill an exploration well.”
The scientists believe a working “brine mine” could be between five and 15 years away, depending on how well the challenges can be addressed.
