The occurrence of critical metals in mine waste: not all that glitters is gold - a case study from the Bäckegruvan Fe-Cu mine tailings, Bergslagen, Sweden

With the ongoing green transition and current geopolitical instabilities, the EU has increased focus on domestic sources of critical metals (CM), including secondary resources such as abandoned mine waste. As part of the FutuRaM project (101058522) to establish methodologies to better understand waste streams of secondary raw materials in the EU, several former mine waste repositories studied for secondary raw material potential. The abandoned Bäckegruvan Fe ± Cu mine tailings, in the Bergslagen mining district of southern central Sweden, provide a case study for examination of critical metal potential in mine waste.

As part of a Swedish governmental directive, surficial sampling of abandoned mine waste was conducted throughout Sweden, which demonstrated Bäckegruvan contained high levels of Co and rare earths (REE) withing the tailings. Further investigations, including additional sampling, drilling at depth provided several hundred samples to quantify the potential resources at Bäckegruvan. Bulk geochemical analyses of the collected samples for CM identified average concentrations of 610 ppm Co and 1479 ppm REE within the tailings. Combining these data with advanced geophysical surveys to calculate volume enabled the conservative estimate of 5,700 tonnes of REE and 1200 tonnes of Co in the 5.3 Mt of tailings (Camitz et al., 2024).

The estimates provided in (Camitz et al., 2024) do not consider the importance of mineralogy when considering extractive potential. Investigations of  select samples were conducted using SEM-EDS and XRD. These methods identified numerous host phases for REE, where the most abundant included törnebohmite, and gadolinite-Ce which lack economic recovery methods. Minor bastnäsite, allanite, monazite and xenotime were observed. Only rare, small grains of Co-bearing mineral phases (e.g. cobaltite ) were identified in the samples. SEM-EDS identified pyrrhotite as the predominant host of Co, with concentrations between 0.2 - 2.0 wt%. In pyrrhotite Co likely occurs as lattice substitution, while the higher weight percentages may indicate nano-scale inclusions of Co-minerals in the pyrrhotite. Similar “missing” Co has been observed in pyrrhotite at the Zinkgruvan mine in Bergslagen (Hjorth, 2022). This makes any recovery of Co from this mine waste uneconomic.

This study has demonstrated that, despite apparently promising levels of REE and Co shown by geochemical analysis at Bäckegruvan, at present economic recovery is unlikely. With the focus on EU domestic production of CM resources as laid out in the Critical Raw Materials Act, including re-mining of secondary resources, this study reinforces the need for detailed studies of mine waste for geological feasibility before inclusion in any national resource estimate, or strategic stockpile.

References:
Camitz, J., Rauséus, G., Jönberger, J., Persson, L., Sopher, D., & Bastani, M. (2024). Secondary resources directive: Characterisation of mining waste in central and southern Bergslagen, Sweden (SGU-rapport 2024:03). Sveriges Geologiska Undersökning.

Hjorth, I. (2022). Characterization of the cobalt content in zinc ore from Zinkgruvan, Sweden (Master’s thesis, Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering). DiVA – Digitala Vetenskapliga Arkivet.