The much-desired transition to a low carbon economy inadvertently creates an increase in the demand for copper and other non-ferrous metals, such as lead, tin, aluminum, and zinc. Higher demand for electric vehicles, charging points, car batteries, inverters, and wind turbines, all require more copper. This is driving earth scientists and engineers to investigate the technological potential of extracting metals from magmatic fluids contained in saline-rich sub-volcanic brines at depths of around 2 km.
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Conventional Mining Processes
Conventional mining, whereby low-grade ore is extracted from open pits, or deep underground, is energy-intensive and environmentally hazardous.
Crushing and grinding produce rock dust, while CO2 emissions are created from the diesel used to power drills, conveyor belts, and transportation. Smelting processes use fossil fuels to extract relatively small quantities of high-grade copper from ore through heating and melting. The land is destroyed and mines may be subject to collapse and flooding.
In contrast, in-situ mining from sub-volcanic brines, offers the possibility of extracting greater quantities of copper more efficiently from large volumes of magma in dormant volcanoes, nearer to the earth’s surface. In-situ mining offers an additional benefit of creating geothermal power as a by-product, with the prospect of making each new project carbon net-zero.
The Potential of Sub-Volcanic Brine
In a new paper, published in Open Science, scientists based in the Department of Earth Sciences at The University of Oxford, discuss the potential of brine for holding precious metals. (Blundy. J, et al. Open Science, 2021).
The paper explains how much of the focus of in-situ mining is on porphyry copper deposits (PCDs), which are associated with the H2O silicic magmas of basalt lava.
Geological stratigraphy studies reveal PCDs are often created in subduction zones and are commonly found to be part of much larger magma systems beneath, at depths of around 1-4 km. Ore deposits are formed when components of magmatic fluids are trapped and cooled, eventually becoming ore minerals in the Earth’s crust.
In-situ sub-volcanic brine mining has the potential for creating greater economic and environmental benefits. Higher quantities of copper can be extracted far more efficiently from greater volumes of magma at shallow depths, saving time and resources, and consequently reducing emissions.
Metals were examined in magmatic brines in two geothermal wells in Lardello, Italy, and Montserrat, in the Caribbean. These contained major elements of Na, K, Ca, Fe, and trace elements of Cu, Zn, and Pb.
Magmatic trapping conditions for brine metals indicate they were originally present at temperatures of 500 to 670 ºC, much higher than the 210ºC of current magma reservoirs.
This is significant because cooling magma is associated with large decreases in the presence of copper, due to dilution processes with meteoric water contained within the cooling magma. In other words, if in-situ sub-volcanic extraction is successful, a significantly greater quantity of copper may be obtained before the magma has cooled and diluted. In turn, this may help satisfy the demand for a complete transition to sustainable transport and a low carbon economy. Recycling copper helps but is not enough on its own to fulfill what is needed globally. One estimate suggests transitioning fully to solar, wind, and tidal power before 2100, will require five times the amount of copper that is currently in worldwide circulation.
Sub-volcanic conduits provide an ideal pathway for fluid to flow and metal-rich brines to accumulate due to having a high porosity state. This provides a mechanism for scientists to potentially concentrate future efforts on extracting and storing magmatic fluids at even greater depths, of 5-6 km in temperatures potentially reaching 500 ºC. Japan is one such country already showing an interest in deeper, hotter extraction from younger magmatic systems.
Interestingly, geologists have also discovered a relationship between magmatic brine and sulfur, whereby sulfide’s poor, yet copper-rich brine tends not to form large PCDs without sulfur additions, for reasons not yet fully understood and still being studied.
In-situ copper mining is not an easy transition from conventional open pit and deep mining. There are two key problems associated with sub-volcanic brine mining. Firstly, the low quantity of copper and other highly sought after yet even rarer non-ferrous metals, such as lithium, may be obtained in some geographically difficult, or hard to reach, volcanic locations. In-situ mining may prove to be locationally, or perhaps politically, difficult to reach for little reward, and therefore undesirable economically.
Secondly, there is the issue of scaling and corrosion of well-bores, which increases with hotter fluids and saline-rich solutions. In-situ mining creates major technical challenges, due to the corrosive nature of the brine solutions being mined. These are problems engineers and materials scientists need to find suitable solutions for.
Opponents of green mining initiatives remain skeptical, claiming that mining locations tend to marginalize indigenous communities, wreak havoc on wildlife, natural ecosystems, and damage biodiversity.
Rising demand for copper has seen the industry in Ecuador alone escalate open-pit mining, with 2.9 million hectares sold between 2017-2018, mainly to Australian investment companies (Burgess. C, 2021).
It can be argued net-zero green mining is needed to reduce open-pit and underground mining, but only if it is a transitional arrangement, and not in addition to increased conventional mining.
Sub-volcanic brine technology certainly has the potential to replace open-pit mining altogether, but only if it can extract the greater quantities of metals scientists forecast. It must also overcome the technical obstacles associated with the geographic and geological location.
Technological solutions by way of new coatings and corrosion-resistant materials to tackle scaling and corrosion also need to be found.
References and Further Reading
Afanasyev. A, Blundy. J, Melnik. O, A. Rust, Sparks, S, Tattitch. B, I. Utkin (2021) The economic potential of metalliferous sub-volcanic brines. Royal Society Open Science. Volume 8. Issue 6. https://royalsocietypublishing.org/doi/10.1098/rsos.202192
Burgess. C, Downes. L (2021) Can 'green mining’ boom save our planet? [Online] Ecologist. Available at: https://theecologist.org/2021/sep/09/can-green-mining-boom-save-our-planet