How do we extract resources in a socially responsible way?

Christoph Heinrich, a professor at ETH’s Institute of Geochemistry and Petrology, is keen to emphasise one thing right from the start: “It’s simply wrong to talk about final limits to resource availability – at least for metals.” Although he acknowledges that raw materials such as copper, gold and zinc are getting harder to extract, he says that we won’t be running short of these metals in a physical sense any time soon.

Heinrich’s principal concern is not scarcity, but rather the environmental impact of resource extraction. “It makes sense to use metals sparingly and recycle them where possible. But that’s not because those metals are running out – the real issue is that metal extraction from rocks costs energy and impacts on the environment. Therefore, you have to find a place where metals have accumulated naturally.” Heinrich argues that the limiting factor in longterm resource supply is our ability to locate geological ore deposits. He also stresses the challenges of finding socially responsible ways of extracting resources and of developing environmentally friendly methods of processing ore. Heinrich is convinced that we can resolve these challenges, and so he does not like the term “resource curse” at all. “It’s unnecessarily negative,” he says. “Mineral resources are a necessity, and they represent a valuable asset for many countries,” he insists. “The problem isn’t natural resources, it’s people’s behaviour. The fact is: we need rare elements and specialised materials to solve what is arguably today’s biggest environmental problem – achieving sustainable energy conversion without causing catastrophic changes in the earth’s climate.

Metals from four kilometres down

Heinrich has reasons for his assertions. Having spent many years studying the geology of metal deposits in his research work, he is still intrigued by the question of why these elements accumulate in certain places when, on average, they are distributed so sparsely in the crust. He investigates the geological process of metal enrichment, because this knowledge is needed to understand their uneven distribution and to search for new ore deposits. The importance of this issue lies in the fact that most of the deposits containing metals in workable quantities close to the earth’s surface have already been discovered – and are being mined out rapidly. But a few kilometres down there are still plenty of places where scientists can reasonably expect to find ample quantities of metals in rich deposits. The problem is that these are much harder to find than deposits exposed at the surface.

Heinrich predicts a trend towards ore extraction from underground mines, which he also argues makes sense from an ecological perspective. Large-scale ore extraction in surface mining takes up giant swathes of land and consumes huge amounts of energy. That’s because you can’t simply dig a vertical hole in the ground: to get to the deeper deposits, you first have to remove tons of unusable rock from around the ore zone. Depending on the shape of the metal-enriched zone, it therefore becomes more economical and energy-efficient to extract the ore underground. From a technical perspective that’s already feasible – in fact the deepest mines extend down to four kilometres below the earth’s surface.

Finding new deposits that are hidden at great depths will require a better understanding of what creates these zones in the first place, and that’s where Heinrich’s research comes in. His work revolves around two key approaches. The first one is the geological characterisation of existing deposits – a method that provides insights into the geometry of metal distribution – combined with accurate dating of past geological events, which supplies a precise history of when large-scale ore-forming processes such as magmatism were active. The second focus of Heinrich’s research is to examine the physical and chemical processes that cause metals to accumulate.

Heinrich is particularly interested in deposits that form beneath volcanoes, such as those found in the Andes. These volcanoes are situated near subduction zones where an oceanic plate sinks beneath a continental plate. Magmatic bodies form in the crust above the subduction zone, and as the magma rises, it emits hot, highly saline fluids: solutions that also carry abundant metal and sulfur. These fluids transport the metals from the magma to the surrounding rock. But this mobilization alone is not sufficient: for a deposit to form, you need a second chemical process that precipitates the metals in the form of ore minerals in a confined rock volume.

Over the past few years Heinrich and his group have been investigating small fluid inclusions in minerals contained in the rocks around ore deposits. These inclusions help researchers identify the conditions in which the minerals were formed and the chemical properties that contributed to their precipitation. Besides investigating these chemical processes, the experts are also taking a closer look at the physical transport of fluids through fractured rocks. Their results show that there are two opposing forces that provide the key to the concentrated accumulation of metals: the hot fluid from the magmatic body, and the cold groundwater that seeps into the rock from the surface. The interface between these two fluid regimes defines the zone where metals are deposited in concentrated form.

To understand these processes quantitatively, Heinrich’s group has developed a numerical model that simulates the large-scale circulation of surface water as well as the fluid expulsion from the magma. “At the moment it’s still a generic model that we’re using to depict the conditions in a generalised form,” Heinrich says. “But now we want to tailor the model more closely to real conditions, to enable the modelling of specific situations in the future.”

Yet even the rough model they’ve developed so far confirms that the crucial zone for ore formation is confined to a depth range between two and five kilometres. Experts therefore have good reason to suspect that there are plenty more as-yet undiscovered deposits at these depths. “There’s a key question for us as geologists,” says Heinrich: “How can we determine if there’s a workable deposit underground if there are hardly any traces left on the surface of those long-gone fluid processes?”

A sample case in Eastern Europe

Heinrich plans to apply the scientific work of his team as part of a European “Horizon 2020” project that also includes researchers from Geneva, France, England and Germany. The goal of the project is to obtain a clearer understanding of how deposits formed in Eastern Europe, from Romania to Turkey. “Computer models obviously can’t replace the traditional methods of geological and geophysical exploration,” Heinrich explains, “but they can provide new insights into ore formation and help us locate deeply buried deposits more efficiently.” There are good reasons why the researchers chose Eastern Europe for this project: during the most recent major phase of mountain formation – which is when the Alps were formed – this region was situated on the northern shore of the Tethys Ocean. The plate motions led to the formation of a subduction zone with magmatic activity similar to the Andes. This set the stage for the formation of ore deposits in which important metals such as copper, lead, zinc, gold and silver accumulated, along with many other rare metals.

The science developed by the ETH researchers can also be used to investigate the formation of ore deposits on the ocean floor. Recently discovered metal deposits off the coast of New Zealand are also attributable to their volcanic surroundings, but they owe their current position to a quite different process. The calculations done by Heinrich’s team suggest that the metal-rich fluids from the magma are too heavy to rise to the ocean floor, so they tend to accumulate beneath it.

Salty seawater plays an essential role by washing the metal-rich solutions to the ocean floor after submarine magma activity subsides, forming hydrothermal vents (known as black smokers) that are particularly rich in copper and gold. “Thanks to our model we now have a more detailed understanding of the factors that shape these events, which could contribute significantly to the exploration of untapped resources for the future,” sums up Heinrich.

This article is published in collaboration with ETH Zurich. Publication does not imply endorsement of views by the World Economic Forum.

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Author: Felix Würsten is working as editor in the Public Relations team at ETH Zurich.

Image: An Afghan man carries supplies for the miners to the emerald mines in the mountains. REUTERS/Ahmad Masood.