Scientists at the University of Oxford are proposing the idea of sustainably extracting copper, gold, zinc, silver and lithium from brines trapped in porous rocks at depths of around 2 kilometres below dormant volcanoes.
In a paper published in the journal Open Science, the researchers explain that the gases released by magma beneath volcanoes are rich in metals. 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.
According to the team led by petrologist Jon Blundy, this trapped subterranean brine is a potential ‘liquid ore’ containing a slew of valuable metals, including gold, lithium and several million tonnes of copper, all of which could be exploited by extracting the fluids to the surface via deep wells.
Employing this method could potentially reduce the cost of mining and ore processing. In addition, since geothermal power would be a significant by-product of this green-mining approach, operations would be carbon-neutral.
“Active volcanoes around the world discharge to the atmosphere prodigious quantities of valuable metals,” Blundy said in a media statement. “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.”
To reach this conclusion, the researcher and his team at Oxford joined forces with Russian colleagues and worked on drill cores from a number of deep geothermal systems located in Japan, Italy, Montserrat, Indonesia and Mexico.
Using volcanology, hydrodynamic modelling, geochemistry, geophysics and high-temperature experiments, they were able to confirm their predictions of metal-rich brines.
The scientists say 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 such as Chile, the DRC, or the US, as it is currently because volcanoes exist all around the world.
There are risks to this proposal, though. The main ones are related to the technology that has to be used as the process involves drilling into rock at 2 kilometres depth and at temperatures of more than 450°C. On top of this, the extracted fluids are corrosive, which places limits on the types of drilling materials and they tend to dump their metal load in the well-bore, a problem known as ‘scaling.’
These limitations mean that more research needs to be done around the dynamics of fluid flow and pressure-temperature control in the well-bore and that there will be a need to develop resistive coatings to prevent well-bore corrosion.
Luckily, many of these challenges are already being addressed through deep, hot geothermal drilling projects. In some cases these projects have reached temperatures over 500 °C; and occasionally they have tapped into small pockets of molten rock, for example in Iceland and Hawaii.
The latter challenge, however, is being addressed already as the Oxford team has patented an idea for fluid extraction that guarantees that the fluids continue to flow into the well once drilled, taking into account the permeability and porosity of hot, ductile rock.
Whether there is a risk of triggering volcanic eruptions, the researchers say it is very small, but must be assessed even though they are not planning to drill into magma itself, but into the hot rocks above the magma chamber, which greatly reduces the risk of encountering magma.
The scientists 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 associated with the technique and will herald a new advance in the understanding of volcanoes and their bounty of energy and metals.
In their view, a working ‘brine mine’ could be 5-15 years away, depending on how well the challenges can be addressed.