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Mining the brine

The transition to net zero is creating unprecedented demand for metals. Olivia Hogg and Jon Blundy discuss the role for volcanoes as a novel route to a more sustainable future

Words by Olivia Hogg
16 May 2022
Professor Jon D Blundy

White Island, New Zealand, where a large eruption in December 2019 resulted in 22 fatalities, is just one example of the many volcanoes discharging significant quantities of metals into the atmosphere – in this case, 100 tonnes per year of copper and 4.5 kg per year of gold (Edmonds et al., 2018). A significantly greater metal resource is trapped in dense magmatic brines underground.

The era of fossil fuel-derived energy is drawing to a close. Electricity generation via renewable sources is a promising candidate to bridge the gap to a low-carbon future (Fig. 1), but it has created and will continue to generate an unprecedented demand for metals, such as copper, lithium and nickel.

A global-scale transition to an electric economy comes with pervasive socio-political and environmental implications. Conventional mining practices are energy intensive and leave indelible scars on the environment, yet they currently represent the only effective way to supply enough metals to meet rising demand. We cannot halt the growth of economies, and therefore cannot escape the need to secure more metals, but using conventional approaches risks devaluing many of the benefits of the energy transition itself.

To satisfy our continued need for energy, we must change how we mine and what we mine. New research indicates that metalliferous magmatic brines, which are found worldwide beneath dormant volcanoes and above granites, together with co-recovery of geothermal power, may provide a more sustainable solution to the global shortage of key metals.

The mining industry will need to ‘scale-up’ whilst maintaining low emissions

Growing incentives
The Industrial Revolution saw development and growth of steam power, using coal and later petroleum, to fuel industries such as textiles, transport and mining. Fossil fuels quickly became central to richer economies; supplying energy to power electricity networks, internal combustion engines for railway locomotives, road vehicles and ships to trade resources more quickly. Ready access to fossil fuels became synonymous with economic growth. After 1950, the intensive use of fossil fuels spread to economies globally, simultaneously adding to their rapid and ongoing depletion, as well as propagation of a global climate crisis – something that could not have been envisaged 200 years ago.

Renewable energy sources, such as wind, hydroelectric, geothermal and solar, paired with energy storage in batteries, as well as nuclear power are preferred alternatives, and offer a means to alleviate many of the adverse environmental consequences of the use and extraction fossil fuels. However, existing renewable energy infrastructures and technologies are not yet fit for
the anticipated global energy transition.

The UK Government’s Net Zero Strategy (BEIS, 2021) addresses the imperative to develop more low-carbon technologies by 2030 by committing £350 million to the Automotive Transformation Fund, which aims to accelerate electric vehicle use, and £1 billion to offshore wind infrastructure. To meet the associated production goals, a dramatic increase in metal supply is critical. Yet, the UK imports most of its metals, and demand for global supplies is rising, while the security of supply, particularly given recent global political uncertainty, is a grave issue. Is it possible to increase metal supply and sustain a low-carbon approach? Recent environmental opposition to new mines for copper in Ecuador and lithium in Serbia are harbingers of the complex times ahead for a mining industry that is often subject to the same public mistrust as fossil fuel industries.

Figure 1: Global electricity sources for 1990, 2020 and 2050 (predicted). The size of the pie represents total electricity consumption quoted in annual Terawatt hours (TWh). ‘Other renewables’ includes energy storage and geothermal power. From 1990 to 2050, the proportion of electricity derived from renewable sources increases dramatically, and will be coupled with increased demand for economic metals like copper, lithium, cobalt and silver. (Figure based on data provided in McKinsey & Company, Global Energy Perspective 2021, and IEA, World Energy Balances: Overview, IEA, Paris, 2021.)

Meeting metal demands
How can we secure enough metals to keep up with the resource transition? The reuse of metals from pre-existing manufactured objects will not meet the predicted growth in demand. For example, a report from the World Bank (Arrobas et al., 2017) showed that even an entirely circular economy could not surmount the growing demand for copper. A more practical way to meet rising demand is through mining, yet there are caveats associated with expanding this industry on the requisite scale. By 2050, lithium demand is set to increase by 965%, cobalt by 585%, copper by 7% and silver by 60% compared with current production levels (Arrobas et al., 2017), and yet existing mining operations are extracting ever lower-grade ores and new reserves are harder to find. In recent years, the mining industry has made low-carbon operations a core facet of its identity. However, can we expect the industry to meet rising global demands, whilst simultaneously maintaining a net-zero approach?

Mining giants BHP and Rio Tinto intend to reduce carbon emissions at their own operations by 30% and 15% respectively by 2030, with targets to become entirely carbon-neutral by 2050 (McKinsey Report, 2021). Anglo American has committed to carbon-neutral operations by 2040 through improved precision mining and reductions in water usage. In 2020, Anglo American drew 33% of its global electricity from renewable sources and by 2023 the company aims to increase this to 56% (Anglo American, 2021). Use of electric vehicles on mining sites is increasing: Anglo American attribute 10 – 15% of their operational emissions to mine-site trucks (Anglo American, 2021), so replacing them with fuel cell electric vehicles will significantly reduce emissions. With resource demands set to soar, the mining industry will need to ‘scale-up’ whilst maintaining low emissions.

The reality is that the current infrastructure is insufficient for the required increased level of global production. Europe aspires to roll out low-carbon technologies, such as electric vehicles, but is ill placed to meet the resource needs. Historically, many of the key resources needed for such technologies, including copper, nickel, cobalt and lithium, were mined across Europe. However, proposals to open new, large conventional mines within Europe may face insurmountable environmental opposition. One solution is to look to supplies outside of Europe, with the attendant risk of creating a business-as-usual growth that has unacceptably large environmental impacts. Thus, gains made in Europe
will be offset by activities elsewhere, not least in resource-rich countries hungry for foreign income.

Concerns regarding metal security are leading mining companies to contemplate virgin territory in the deep sea, with some Pacific and European states exploring seafloor mining—presenting new challenges and new environmental risks. In 1972, the Clarion-Clipperton Zone, an abyssal plain between Hawaii and Mexico, was found to host considerable untapped deposits of metals including copper, nickel and manganese (Heffernan, 2019). Thirty years on, such environments are being considered for their mining prospects. Data are scarce, but there is widespread concern that deep-sea mining may impose irreversible damage to poorly understood marine ecosystems and ocean chemistry. Seabed scars from the 1972 field campaign persist today, a portent of the damage that global-scale marine mining projects may inflict.

The majority of non-ferrous metals that we extract are ultimately linked to magma

An unsung source
There is a pressing need to reinvent mining as we know it. The majority of the non-ferrous metals that we extract are ultimately linked to magma. The same processes that create magma in Earth’s crust and mantle, and transport it to the surface as volcanoes, also deliver prodigious quantities of metals dissolved in hot volcanic gases and brines. These processes initiate at depths greater than 5 km within the crust, as magmas begin exsolving volatiles in the form of saline, metal-rich aqueous fluids. Their metal endowment is a veritable who’s who of the Periodic Table, and includes many metals that are critical to the net-zero transition, including copper, lithium and silver.

Figure 2: Compositions of magmatic brines and volcanic gases for representative sodium chloride contents of 50 wt% in brine and 1 wt% in volcanic gas. Metal abundance plotted in wt%. Metals along the x-axis are ordered by increasing abundance in volcanic gases. Boxes mark interquartile ranges and median abundance for a given metal; whiskers denote maximum and minimum ranges; outliers are represented as dots. Across the broad suite of metals, brines show the most enriched metal signatures, up to three orders of magnitude higher than volcanic gases. (Figure based on data compiled from 25 references by O.R. Hogg and available on request from the author.)

Magmatic fluids can advect to the surface where they manifest as metal-rich plumes, emitted from volcanoes like Mount Etna (Italy), which alone discharges some 20 tonnes of copper and 10 kg of gold per day to the atmosphere (Edmonds et al., 2018). Recovering these metals from hot volcanic gas is wholly impractical. However, a much more concentrated metal resource underlies volcanoes. At depths of around 2 km, hot brines separate out from the ascending magmatic fluids. Metal enrichment scales with fluid salinity, and magmatic brines have salt contents reaching 70 wt% sodium chloride (Bodnar and Sanchez, 2014), compared to just 3 wt% sodium chloride in seawater (Kesler, 2005). Consequently, the brines sequester metals, becoming enriched by more than two orders of magnitude in almost all metals relative to their parent magmatic fluid (Fig. 2). An indication of the potential metal resource associated with volcanoes can be gleaned by comparing their global metal output to that from conventional mining (Fig. 3).

Brine mining affords unparalleled advantages compared to conventional methods

Harvesting the bounty
Conventional mining practices utilise ancient volcanic systems, where magmatic activity has long since ceased and brines have deposited their metal load as solid ores. Time and erosion bring these ore bodies closer to the surface where they can be extracted in giant open or underground pits (Fig. 4). The ore grades are typically so low that more than 99% of the rock extracted in this way is waste, resulting in huge tailings piles left at the surface. A promising disruptive concept involves directly mining brines from hot magmatic rocks, such as those beneath dormant volcanic systems or above young granite intrusions. ‘Brine mining’ affords unparalleled advantages compared to conventional methods, because metals are extracted from a concentrated solution, rather than solid rock. Brine mining would eradicate the need for several energy-intensive processes associated with hard-rock ore body refinement, and, if geothermal power is recovered alongside the metals, represents a potentially carbon-neutral method of metal recovery.

Figure 3: Metals produced globally via mining compared to the total atmospheric flux from arc volcanoes (in kg/day). Vertical bars represent the standard error of the mean global volcanic flux. Solid black 1:1 line is plotted. Data lying above this line infer that metals are more enriched in volcanic gas plumes whereas those lying below the 1:1 line indicate that metal production from mines is greater than the amount fluxed from volcanoes. A notable exception is iron, the bulk of whose production derives from non-magmatic sources. Volcanic flux data from Edmonds et al. (2018) and Shinohara et al. (2013). Mined metal production data from Mineral Commodity Summaries 2019, USGS.

Lithium, as an example, is largely mined from hard-rock sources in Australia, by exploiting spodumene minerals hosted in pegmatite deposits. Lithium is also sourced from salar brines in South America, where ancient waters have interacted with and leached lithium from surrounding volcanic deposits. However, the rapid increase in demand for lithium is seeing previously unexplored sources being considered for exploitation. For example, the company Cornish Lithium is actively seeking to co-produce lithium, heat, and water from geothermal waters in Cornwall, UK, in a low-carbon way. Lithium is selectively removed from geothermal waters via Direct Lithium Extraction — a method that, according to Dr Rebecca Paisley, Lead Geochemist at Cornish Lithium, repurposes existing technology that is traditionally used to treat contaminated water and enables lithium extraction in a low-carbon manner, using only a fraction of the water, land and reagent consumption involved in hard-rock or salar brine lithium mining. In Germany, Vulcan Energy proposes to use similar processes to recover hot, lithium-rich brines from beneath the Rhine Valley (Fig. 5). Pilot wells became operational in early 2022, with the prospect of producing the world’s first carbon-zero battery-quality lithium, powered by co-recovered geothermal energy.

Figure 4: Cerro Colorado copper mine, Chile. An example of a large, conventional open-pit porphyry copper mine with a grade of less than 0.5 wt% copper.
The remaining 99.5% of the mined rock is waste, stored in tailings piles at the surface. (Credit: Jon D Blundy)

Could volcanoes be the next frontier in brine mining? Operations, such as those described above, are not targeting fluids of magmatic origin, but geothermal target fluids at temperatures of 250°C or less. As the solute load of brines increases dramatically with increasing temperature, so the economic benefits of brine mining increase with ever-hotter brine sources. Such conditions match those currently being investigated for so-called ‘supercritical’ or ‘superhot’ geothermal power, whereby high-enthalpy fluids at temperatures above the critical point of water (~374 °C) are extracted from depth to the surface. With over 3 million joules per kilogram of fluid extracted, supercritical fluids have over three times the energy of conventional geothermal fluids. Supercritical geothermal power is an area of active research in Iceland, New Zealand, Japan, USA, Italy and Mexico.

The economic benefits of brine mining increase with ever-hotter brine sources

Although supercritical geothermal power faces some obvious technical challenges, this is not uncharted territory; more than 25 years ago Japan drilled a 3.7-km-deep geothermal well into a hot, 90,000-year-old granite at Kakkonda (Fig. 6), where they recovered small quantities of unusually metal-rich, 520 °C brine (Saito et al., 1998). Since then technology has advanced, with new developments in drill bits, drilling muds, well-bore casings and well-head equipment. At Larderello (Tuscany, Italy), for example, the Venelle-2 Well recently reached 2.9 km depth, penetrating fluid-bearing rocks at over 500 °C (Petty et al., 2020). Technological advances are reducing significantly the costs of drilling deep, hot wells, which in turn impacts the economics of geothermal energy as a source of baseload green power.

Figure 5: Stages involved in Conventional Lithium Extraction Process versus proposed methods of Geothermal Lithium Extraction. Figures quoted are per tonne of lithium hydroxide produced. Conventional lithium extraction involves hard rock open-pit mining. Rock containing ore must be ground with machinery powered by fossil fuels. Geothermal Lithium Extraction can be powered by geothermal heat associated with hot lithium-bearing fluids, reducing associated CO2 emissions. (Figure based on publicly available data provided in the Minviro Life Cycle Analysis for Vulcan Energy, 2020.)

Getting fluids to flow through rocks at supercritical conditions is an issue for both geothermal power and brine mining. At high temperatures within Earth, rocks behave in a ductile, rather than brittle, fashion, inhibiting the formation of fractures and reducing permeability. One solution to this problem is reservoir stimulation, a process at the heart of enhanced geothermal systems. This approach typically entails injecting fluids into the reservoir to promote fractures and permeability development, but at rates too low to trigger seismicity. Encouragingly, recent research from Japan, shows how stimulated ductile reservoirs tend to form ‘cloud-fracture networks’ consisting of many tiny, permeability-enhancing microcracks, rather than large seismogenic fault-like cracks (Watanabe et al., 2019). Whether dense brines can be recovered in this way, alongside lower-density supercritical fluids, remains to be seen.

The next challenge for brine mining is recovering solute-rich fluids to the surface without extensive scaling of the wells themselves because the solutes precipitate during ascent. In the geothermal industry, scaling is a serious impediment to efficient fluid extraction that can impact the longevity of a power plant. Conversely, to the miner, scales represent unusually high-grade ‘ore’ of a type rarely encountered in nature. For example at Kakkonda, well-bore scales contain up to 13% per cent weight of copper, 20% zinc and 20 ppm gold, not to mention a wealth of other valuable metals. Ensuring that this polymetallic bounty is brought to the surface, rather than precipitating en route is crucial. Designing novel materials that can sequester metals from hot fluids at the bottom of the well, and development of well-casing materials that inhibit scale nucleation, are just two possibilities under consideration.

In conventional mining, so-called ‘behemoth’ deposits, with more than 60 million tonnes of copper, have dominated the global raw materials supply chain for decades. There is increasing recognition that some smaller, high-grade deposits may be economically viable (and commercially ‘agile’), while many critical metals are found as by-products of mines targeting more abundant metals. Behemoth deposits hold considerably more metal than is likely to be hosted beneath a single volcano, but whereas behemoth deposits are exceedingly rare and hard to find, volcanoes are abundant and easy to spot. Around 2,000 volcanoes globally have the potential to become future brine mines. Interestingly, of the 44 European states, at least 15 have active or dormant volcanoes under their jurisdiction that would be potentially suitable for the simultaneous extraction of metals and geothermal power.

Figure 6: An example of a conventional (50 MW) geothermal power plant (Kakkonda, Japan) that is being considered for upgrading to ‘supercritical’ status by accessing fluids at depths of up to 4 km and temperatures of 450 °C. Kakkonda was the site of geothermal drilling to 3,729 m depth and temperatures over 500 °C in 1995. (Credit: Jon D Blundy)

A unique perspective
We cannot eradicate the demand for energy and metals because they are, and continue to be, the means for economies to develop and prosper. It would be hypocritical to demonise mining, whilst relishing the economic benefits that it delivers. And yet, that is commonly the case, not least because metal extraction and consumption are usually decoupled geographically – a legacy in many ways of colonial times and the Industrial Revolution. As with so much of what we consume, it is vital to acknowledge the link between resources and their supply chain.

We propose that one solution to the rapidly emerging energy crisis may be to mine metals in an unconventional way that offers a less environmentally impactful and potentially carbon-zero approach. There is a growing awareness that magmatic brines have the potential to resolve the resource paradigm in which we find ourselves. Investing time in technological development and broadening our understanding of volcanic systems, including drilling into them, is central to evaluating how we can simultaneously harness geothermal power and metals, and so better equip us for the energy transition. In many ways, the storage of magmatic fluids in underground porous rock resembles oil-and-gas reservoirs, meaning that existing hydrocarbon expertise could be readily repurposed in the hunt for brine reservoirs.

As Earth scientists, we have a unique perspective that allows us to address the critical resources challenges ahead. This includes improved strategies for finding resources, and the development of cleaner and more efficient extractive and refinement processes, while at the same time considering mutual global ambitions to deliver carbon-negative energy and building resilience across Europe towards the ever-changing nature of our supply chain.

Olivia R Hogg
PhD Student, University of Cambridge, UK
email: orh23@cam.ac.uk
twitter: @olivia_hogg

Prof Jon D Blundy FRS
Royal Society Research Professor, University of Oxford, UK
email: jonathan.blundy@earth.ox.ac.uk

The authors would like to thank Dr Rebecca Paisley for discussion and comments.

Further reading
Anglo American. (2021) Climate Change Report
Arrobas, D.L.P. et al. (2017) The Growing Role of Minerals and Metals for a Low Carbon Future. World Bank Group
BEIS (2021) Net Zero Strategy: Build Back Greener. UK Government policy paper
Bewick, D. (2021) Big miners’ capital discipline is good news for investors. The Economist
Blundy, J. et al. (2021) Royal Society Open Science 8.6: 202192
Bodnar, R.J. et al. (2014) Treatise on Geochemistry (2nd Ed) Elsevier, 119, 142
Bowell, R. (2017) Elements 13(5), 297–298
Edmonds, M. et al. (2018) Nat. Geosci. 11(10), 790-794
Green Alliance. (2021) Critical point: Securing the raw materials needed for the UK’s green transition
Heffernan, O. (2019) Nature, 571(7766), 465-469.
IEA. (2021) World Energy Balances: Overview, IEA, Paris
Kesler, S.E. et al. (2015) Mineral Resources, Economics and the Environment. Cambridge University Press
Lezak, S. et al. (2019) Low-Carbon Metals for a Low-Carbon World: New Energy Paradigm for Mines. Rock Mountain Institute 6
McKinsey & Company. (2021) Creating the carbon-zero mine
McKinsey & Company. (2021) Global Energy Perspective 2021
Petty, S. et al. (2020) Path to Superhot Geothermal Energy Development. GRC Annual Meeting 2020
Reinsch, T. et al. (2017) Geothermal Energy 5(1), 1-25
Saito, S. (1998) Geothermics 27(5-6), 573-590
Shinohara, H. (2013) J. Volcan. Geotherm. Res. 268, 46-63
The US Geological Survey. (2019) Mineral Commodity Summarie
Thompson, J.F.H. (2018) Elements 14(5), 298
Vulcan Energy. (2020) Minviro Life Cycle Analysis for Vulcan Energy


This Feature links to the Science Snapshot Mining the Brine

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