Copper is one of the most versatile and exploited metals in the world, thanks to its electrical and thermal conduction properties. It’s also paramount for low carbon technologies; an electric car needs four-and-a-half-times more of the metal than a standard internal combustion engine vehicle.
Yet, while mammoth copper mines, such as the Chilean giants of Escondida and Andina, exist, there remain concerns as to the metal’s long-term viability. Copper demand is forecast to exceed current available resources within a few decades, with much of the ‘low hanging’ deposits already discovered and mined to exhaustion.
This has prompted scientists to seek out new ways to identify smaller, deeper deposits containing the versatile metal. One such group is from the University of Exeter, whose work at the geologically unique area of the Yerington District of Nevada, US has uncovered new markers that can help identify critical porphyry copper deposits.
Unique deposits
Porphyry copper deposits – which make up the high tonnage low grade resource at mines like Escondida and Andina and make up three-quarters of the world’s total copper – develop several kilometres below the Earth’s surface, above even deeper large magma chambers.
These depths make them hard to study but in Yerington, the tilting of the upper crust has exposed a cross-section through four porphyry copper deposits and their host rocks, providing unique exposure of this geological formation.
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Lead scientist Lawrence Carter and his colleagues studied the deposits for specific textures and other markers, which can tell geologists about the formation of these porphyry deposits and may give an early indication of their location.
“Through a very meticulous study of different rocks around these deposits, we started to understand how specific textures can be used to navigate a system’s architecture, and help geologists determine where porphyry-style mineralisation may be – essentially acting as exploration markers that copper could be found there,” explains Carter, who conducted the work as part of his PhD studies.
The textures the researchers uncovered relate to specific physical processes that occurred when the rocks and mineral deposits were forming, and the search for these textures can be used as a “toolkit” when exploring for new mineral deposits.
“Porphyry deposits are extremely rare, needing a series of specific conditions and events to trigger the vast concentration of mineralising fluids,” explains Carter, demonstrating how the unique conditions needed for the deposits to form made them particularly rare, and impeded study. Historically, understanding of such textures has been disjointed because they are often small, poorly exposed, or are simply not recognised when encountered.
Yet the accessibility of the deposits at the Yerington site means this uniqueness is actually a blessing, as Carter explains: “These textures capture some pretty wacky things that have happened beneath the Earth’s surface, and that’s what exploration geologists are looking for – some irregular event that has occurred.”
Building a textural toolkit
The ‘textural toolkit’ can be used by geologists in early field-based campaigns as well when logging drill core in more advanced projects. This is ”just a geologist on the ground looking at them” says Carter, which he says is ”cheap and easy” as opposed to other exploration techniques that can be invasive and expensive.
“These textures can guide geologist to where those more expensive invasive techniques should be used,” he adds. “[The features of the textures are] obviously important for geologists because by using these textures, we can guide them to where to look more specifically, where to drill holes.”
To develop this textural toolkit, the scientists collected field samples and performed micro textual studies using technology such as scanning electron microscopes, cathodoluminescence imaging, and QEMSCAN automated mineralogy at the Camborne School of Mines lab in Cornwall, UK.
The research was financed by NERC GW4+ DTP, which contributes towards PhD funding, the Society of Economic Geologists Foundation and the NERC highlight topic ‘FAMOS’, which is supported by mining majors such as Rio Tinto and BHP.
Another recent study by scientists from the University of Geneva found that porphyry deposits are formed as a result of mechanisms very similar to those that cause large volcanic eruptions. As such it concluded that large reserves of copper are born of failed eruptions.
The scientists used data and figures provided by the mining companies and on those collected in the field and in the laboratory by numerous researchers, combined with petrological and geochemical models. They concluded that findings could open new avenues for the development of geological, mineralogical and geochemical tools for future successful exploration of the largest porphyry copper deposits on Earth.
Carter says this research can complement that of his and his colleagues. Going forward, he hopes to secure more funding to create an ”elaborate guide to rock textures”, cataloguing other textures that have previously been identified, and looking for new ones. He is also working on an EU Horizon 2020 project called GREENPEG, developing exploration toolkits for lithium and rare earth element pegmatite deposits.
Changing the face of exploration
Porphyry-type deposits – named after a magmatic rock that contains copper – do not only contain copper, which is also used to produce all types of wires and electrical connectors, but large amounts of gold and molybdenum, a chemical element that is vital in the industrial sector due to its use in steel alloys. As a result, the ability to locate and identify these deposits accurately could be of significant benefit for a number of industries.
Indeed, these studies can help inform the changing landscape of mineral exploration and discovery. As most of the easy to access deposits have now been exploited, undiscovered resources are now largely expected to be found in more remote and difficult to access locations.
As such, industry has started to prioritise the use of technology for exploration, following in the footsteps of the oil and gas sector, says Carter. This can help to cut exploration costs and timeframes.
“Exploration has changed in recent years, with a drive towards using remote sensing and satellite technology, and in part a move away from traditional field geology,” he explains.
“But I think our research shows this type of fieldwork – boots on the ground – will always be needed because you can’t look inside nooks and crannies from afar. Technologies are really complimentary, but not a replacement, to the geologist actually doing proper geology.”
