The study, published in Frontiers in Geochemistry, examined mine wastes from two strikingly different legacy mining regions: the town of Cobalt in Ontario, Canada, and three historic mine sites in Cornwall, England. Despite vast differences in geology, ore type, and processing methods spanning a millennium of mining history, the researchers found that cobalt in both locations showed consistently high potential for mobilization — meaning it could readily dissolve and travel into aquatic environments where it might enter the food chain.

“The concentrations we found at Cobalt, Canada were extraordinary,” the research indicates, with high-grade Nipissing tailings containing up to 5,630 milligrams of cobalt per kilogram of material — roughly 900 times higher than the Cornwall sites. Even the low-grade tailings at Cobalt reached 1,230 milligrams per kilogram, dwarfing the averages of 40, 76, and 59 milligrams per kilogram found at Poldice, Wheal Unity, and Dolcoath mines in Cornwall respectively. These concentrations far exceed residential soil quality guidelines set by regulatory bodies in Ontario, Canada, the United States, and the European Union.

The research team, led by Gabriel Ziwa of the Camborne School of Mines at the University of Exeter, employed a sophisticated battery of analytical techniques to understand not just how much cobalt was present, but in what chemical forms. Using X-ray diffraction, scanning electron microscopy, and electron microprobe analysis, they identified the specific minerals hosting the cobalt — a crucial step because a metal locked inside a stable crystal is far less dangerous than one sitting on a mineral surface ready to dissolve at the first change in water chemistry.

At Cobalt, Canada, the picture was dominated by a striking pink mineral called erythrite — a cobalt arsenate that formed as needle-like crystals growing on silicate and carbonate grains throughout the tailings. This secondary mineral, containing roughly 20.8 percent cobalt by weight, proved to be the most widespread cobalt-bearing phase at the site. The researchers also identified primary cobalt minerals including cobaltite, safflorite, and skutterudite, alongside cobalt incorporated into arsenopyrite, loellingite, pyrite, and chalcopyrite. Secondary phases such as arseniosiderite, scorodite, and iron oxyhydroxides also harboured significant cobalt concentrations.

In Cornwall, by contrast, no dedicated cobalt minerals were found at all. Instead, cobalt existed only as a trace component — typically less than one percent by weight — hidden inside common sulfide minerals like pyrite, chalcopyrite, and arsenopyrite, and in secondary phases such as scorodite and iron-manganese oxyhydroxides. Yet this difference in mineralogical hosting made almost no difference to the metal’s behaviour when subjected to sequential extraction testing.

The most alarming result came from the BCR sequential extraction analysis, a standardized European protocol that subjects mine waste to progressively stronger chemical leaches to simulate what might happen under different environmental conditions. In both the Canadian and British samples, an average of 46 percent of the total cobalt was extracted in the very first, mildest step — the “exchangeable” fraction that represents metal most easily dissolved by slight changes in acidity. This means nearly half the cobalt in these wastes is sitting in a form that could be released into water with only minor pH shifts.

Applying the Risk Assessment Code framework developed by Perin and colleagues, the researchers classified cobalt in the Cornwall samples as ranging from high risk to very high risk, with individual samples reaching exchangeable fractions of up to 62 percent. At Cobalt, Canada, values ranged from 28 to 58 percent, placing the metal in medium to very high risk categories. According to the RAC guidelines, metals exceeding 50 percent in the exchangeable fraction are considered to easily enter the food chain — a classification that directly implicates these mine wastes as potential vectors for human cobalt exposure through crops, livestock, and water supplies.

The chemistry behind this mobilization is well understood. At Cornwall’s acidic sites, where paste pH values ranged from 3.72 to 5.77, sulfide minerals like pyrite generate acid through oxidation, and any cobalt-bearing phases dissolve readily, releasing cobalt ions and arsenic oxyanions into solution. At Cobalt, Canada, the near-neutral pH of 7.50 to 7.84 currently stabilizes erythrite within its thermodynamic stability field — but the researchers warn this equilibrium is fragile. A shift toward acidity, whether from changing oxidation conditions or altered hydrology, could dissolve the erythrite and release its cobalt payload. The iron and manganese oxyhydroxides that scavenge cobalt in both locations are themselves thermodynamically unstable under reducing conditions, meaning buried or waterlogged wastes could also release their stored metal.

What makes this research particularly significant is its implication for the future. Cobalt demand is projected to surge as electric vehicle adoption accelerates globally, and governments worldwide have designated it a strategic resource. Yet the environmental behaviour of cobalt in mine waste has been remarkably understudied compared to metals like arsenic or lead. The Exeter team’s finding that cobalt behaves similarly across such geologically different settings — a five-element silver-cobalt-nickel vein deposit in Precambrian rocks versus polymetallic tin-copper-arsenic veins in Devonian-Carboniferous host rocks — suggests that the metal’s mobility in mine waste may follow predictable patterns worldwide. If confirmed by further research, this would mean that every new cobalt mine opening today is generating waste with the same latent capacity to contaminate water and food chains that century-old sites in Canada and England are only now revealing.

Subject of Research: Geochemistry and mineralogy of cobalt mobility in legacy mine wastes from Cobalt, Canada and Cornwall, England

Article Title: Geochemistry and mineralogy of Cobalt in mine wastes: examples from Cobalt, Canada and Cornwall, England

Article References: Ziwa, G., Crane, R., & Hudson-Edwards, K. A. (2025). Geochemistry and mineralogy of Cobalt in mine wastes: examples from Cobalt, Canada and Cornwall, England. Frontiers in Geochemistry, 3, Article 1543695. https://doi.org/10.3389/fgeoc.2025.1543695

Image Credits: AI Generated

DOI: 10.3389/fgeoc.2025.1543695

Keywords: cobalt, mine waste, tailings, geochemistry, mineralogy, erythrite, sequential extraction, mobility, critical raw materials, contamination, Cornwall, Ontario

The mines at Crocette and Pestarena tell a story that stretches back to the end of the thirteenth century, when the first official documents recorded underground gold mining in the valley. Operations continued into the mid-twentieth century, ending at Pestarena in 1961 as ore grades declined and costs rose. The gold was locked in quartz-carbonate veins hosted in paragneisses and micaschists at Pestarena and in orthogneisses at Crocette, where it occurred alongside sulfide minerals, chiefly pyrite and arsenopyrite, with lesser galena, sphalerite, pyrrhotite and chalcopyrite. Extraction relied on mercury amalgamation and, from the late 1880s, cyanidation, both of which generated enormous quantities of waste rock and fine-grained tailings that were dumped around the processing plants with little regard for the consequences.

That carelessness left a lasting mark. Sulfide minerals in the exposed waste oxidize when they meet air and water, generating acid or neutral mine drainage that mobilizes potentially toxic elements into soils, groundwater and streams. Previous investigations had already documented alarming conditions: soils near Crocette with arsenic concentrations ranging from 145 to more than 40,000 milligrams per kilogram, a natural background value for the area calculated at 477 milligrams per kilogram against an Italian regulatory threshold of just 20 to 50, and surface waters in the most contaminated samples reaching 279 micrograms of arsenic per liter, far above the Italian limit of 10. A 2025 study by members of the same team found that three quarters of groundwater and surface water samples at the two sites were contaminated with arsenic, with other elements exceeding thresholds in groundwater downstream of the tailings.

To build a complete picture, the researchers merged decades of earlier geochemical and mineralogical data with a new sampling campaign conducted in May and July 2024, collecting 28 fresh samples of waste rock, tailings, soil and sediment from depths of up to 30 centimeters. The new material was analyzed at an accredited laboratory using a four-acid digestion followed by inductively coupled plasma emission spectroscopy and mass spectrometry, covering more than fifty elements, while mineralogy was resolved with X-ray powder diffraction and scanning electron microscopy. Because decades of erosion and dispersion have blurred the boundary between waste and native soil, the team classified every sample under two scenarios: a conservative one counting only material clearly located on waste dumps, and a broader one that also included adjacent, vegetation-free or chemically distinctive material, mirroring the distinction between indicated and inferred resources in resource estimation.

The mineralogical work revealed the fingerprints of intense chemical weathering. Alongside the dominant quartz, feldspars and micas inherited from the host rocks, the waste contains secondary minerals that record the breakdown of the original sulfides: scorodite, formed from the alteration of arsenopyrite; jarosite, precipitated as pyrite and pyrrhotite oxidize; and alunite, produced during the alteration of aluminosilicates. Electron microscopy also identified grains of monazite-(Ce), a phosphate mineral that can carry up to seventy percent rare earth oxides, dominated by cerium with significant lanthanum, praseodymium and neodymium, along with traces of residual arsenopyrite, ilmenite, zircon and barite. Scheelite, a calcium tungstate and one of the world’s principal tungsten ores, appeared among the minor minerals, a hint of the critical metals hiding in plain sight.

When the researchers normalized their measured concentrations against the composition of the upper continental crust, the scale of the enrichment became striking. At Pestarena, arsenic in the waste is a median of roughly 1,700 to 2,000 times more abundant than in average crustal rock, while bismuth, tungsten and antimony show median enrichments of about 100, 35 and 15 respectively, with slight enrichment of beryllium and of the light rare earth elements lanthanum, cerium, praseodymium and neodymium. At Crocette the picture is even more dramatic: arsenic reaches a median enrichment of around 4,000 times crustal values, followed by tungsten at 50, antimony at 40 and bismuth at 26. These figures matter because arsenic, bismuth, antimony and tungsten all appear on the European Union’s list of critical raw materials, the strategic elements underpinning semiconductors, photovoltaics, clean energy technologies and defense applications.

The same waste that concentrates these valuable elements also drives the environmental damage, and the team quantified that damage using standard contamination indices. River sediments collected away from the mine workings proved uncontaminated, but sediments from the drainage channels of the mine tunnels and the tailings outflows showed heavy to extreme arsenic contamination, moderate to heavy lead contamination and high ecological risk, driven overwhelmingly by arsenic. In the surrounding soils, the geo-accumulation index placed Pestarena in the heavily to extremely contaminated category for arsenic, with a mean value of 3.5 and individual samples exceeding 5.0, alongside moderate lead contamination, while Crocette soils fell in the moderately to heavily contaminated range. The potential ecological risk index, which weights each element by its toxicity, averaged 296.4 at Pestarena and 251.1 at Crocette, both within the moderate risk category, with arsenic contributing the dominant share, followed by lead and cobalt.

What elevates the study beyond a conventional contamination survey is its economic framing. By estimating the volumes of waste rock and tailings at each site from field observations and stratigraphic data, and applying bulk densities of 1.5 grams per cubic centimeter for waste rock and 1.8 for tailings, the researchers calculated the residual quantities of arsenic, bismuth, antimony and tungsten still locked in the dumps. They then multiplied these quantities by plausible recovery rates of 60 to 80 percent, consistent with efficiencies of 77 to 99 percent reported for tungsten, 81 to 99 percent for arsenic, up to 98 percent for antimony and 76 to 99 percent for bismuth in comparable secondary resources, and by average market prices from 2000 to 2021. Arsenic emerges as the most abundant target, with bismuth, antimony and tungsten present in significant quantities, although the authors stress that these preliminary estimates cannot by themselves establish economic feasibility, since remediation, processing and residue disposal costs must all be weighed against revenues.

The technical pathways for such recovery already exist. Published case studies describe arsenic and antimony extraction from antimony flotation waste by acid leaching, the recovery of antimony and bismuth from copper smelter flue dusts through acid chloride leaching and selective precipitation, the stabilization of arsenic as crystalline scorodite during copper electrorefining, and the recovery of tungsten and arsenic from tungsten-bearing residues using alkaline leaching with pressure oxidation, deep eutectic solvents paired with electrodialysis, or acid leaching followed by solvent extraction. Arsenic occupies a paradoxical position in this landscape: it is both a valuable resource, essential for gallium arsenide semiconductors and electronic technologies, and one of the world’s most notorious poisons, capable of causing severe chronic health effects through contaminated water and food. Any recovery scheme must therefore be paired with safe long-term management of the arsenic left behind, for example by precipitation as scorodite or vitrification, to prevent the problem simply migrating from one waste heap to another.

The broader message is one of reframing. Within the European Union, mining and quarrying generated nearly 500 million tonnes of waste in 2022, the second largest share of any economic activity, and the Circular Economy Action Plan together with the Critical Raw Materials Act now explicitly encourages treating old waste heaps as anthropogenic deposits rather than burdens. The Anzasca Valley study suggests that remediation and resource recovery need not be opposing goals: excavating and processing contaminated waste can simultaneously reduce ecological risk, clean up soils and water, and supply strategic materials that Europe currently imports at geopolitical peril. The authors caution that further work is needed to characterize the materials more precisely, optimize treatment processes and evaluate the full economic and environmental sustainability of recovery strategies. But their central conclusion is clear: the mountains of waste left behind by a thousand years of Alpine gold mining may yet prove to be among the most valuable things those mines ever produced.

Subject of Research: Environmental impact assessment and critical raw material recovery from abandoned gold mining waste in the Western Alps

Article Title: From environmental burden to resource potential: environmental impact evaluation and Critical Raw Materials recovery from abandoned gold mining waste in the Western Alps (NW Italy)

Article References: Zaniboni, L., Cavallo, A., De Luca, D. A., Lasagna, M., Padoan, E., Martin, M., & Dino, G. A. (2026). From environmental burden to resource potential: environmental impact evaluation and Critical Raw Materials recovery from abandoned gold mining waste in the Western Alps (NW Italy). Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38241-6

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38241-6

Keywords: critical raw materials, arsenic, gold mining, mine waste, tailings, circular economy, environmental contamination, ecological risk, Western Alps, Italy, tungsten, antimony