Weathering processes and critical minerals distribution in VMS tailings from Storwartz, Norway

• Storwartz tailings contain up to 10 wt% sulfides in unoxidized zones. • Stratigraphic heterogeneity reflects deposition and hydrological controls. • Critical metals (Cu, Zn, Co) persist in deeper, unweathered tailings layers. • Low volume and poor mineral liberation hinder reprocessing feasibility. Historical mining activities have left behind substantial amount of sulfidic mine tailings that present major environmental and human health risks. These tailings were disposed of using inadequate disposal practices. A thorough understanding of the chemical and mineralogical composition of these tailings is crucial for assessing their environmental impact and devising effective remediation strategies. This knowledge can help transform these tailings from waste into valuable secondary resources. The historical Storwartz mine tailings are located in the prolific Røros mining district, Norway with over 300 years of mining history and are part of a UNESCO heritage site. The tailings result from flotation of volcanic-hosted massive sulfide (VMS) ores from the beginning of the 20th century and were disposed indiscriminately near the processing plant. This has resulted in extensive weathering of the tailings and decades of acid rock drainage seeping into the lake Djupsjøen several kilometers downstream. This study, focusing on the mineralogy, explores the alterations and current composition of the tailings including their content of critical raw materials and models the basic processes impacting the composition as a result of low temperature weathering. Results show that the tailings have a highly variable composition in 1 m intervals of push core samples. Economically important elements such as zinc, copper, and cobalt reach concentrations of up to 2.8 wt%, 0.5 wt%, and 145 ppm respectively. For environmentally concerning elements, lead is present at concentrations of up to 2470 ppm in some samples. Mineralogically, the tailings are predominantly composed of quartz and chamosite (up to 70 wt% of modal mineralogy), with minor amounts of albite, k-feldspar, biotite, garnet and sulfides. Sulfide minerals, including pyrrhotite, pyrite, sphalerite, and chalcopyrite, are restricted to the deeper, unoxidized layers of the tailings, with notable enrichment at the transition zone between oxidized and unoxidized tailings in the cores. This mineralogical zonation reflects progressive sulfide oxidation, carbonate dissolution and silicates consumption driven by fluctuating redox conditions and water table dynamics. While it is unlikely that the Storwartz mine tailings will be reprocessed due to their modest volume (less than 1 Mt) and UNESCO heritage status, the knowledge gained from this study provides valuable insights into the mineralogy and post-production history of tailings worldwide. On a global scale, our study demonstrates the environmental impact and the potential revalorization of unsupervised historical tailing deposits.