Environmental Geochemistry of Sulfide-Bearing Iron Oxide Deposits in Scandinavia: Importance of Gangue Minerals and Sulfide Crystallinity in the Delay of Acid Generation

Sulfide-bearing iron oxide deposits consisting of magnetite and silicates are common within the greenstones of north-west Finland and northern Sweden. These iron oxide deposits have variable copper, gold, and uranium content and occur in association with tuffite, black schist, and dolomitic marble. The deposits have a resource size of up to 145 Mt and an iron content of 35%–50% (e.g., Stora Sahavaara). The total sulfur content of these deposits is typically in the range of 1%–5% but can have exceptional values up to 20.8%, and disseminated pyrite, pyrrhotite, and chalcopyrite are commonly present. The prediction of acid rock drainage and metal leaching potential requires a detailed understanding of the site-specific rates and mechanisms of weathering. This has been obtained through geochemical (multi-element analysis and acid–base accounting) and mineralogical characterization testing undertaken on representative materials, including multi-element analysis, acid–base accounting, net acid generation testing, and humidity cell testing. Despite the high sulfide content and low neutralizing potential of most rock types found in these deposits, the humidity cell tests showed a delayed onset to acid generation, which is primarily attributed to sulfide crystallinity and mafic silicate dissolution leading to slow oxidation and reaction rates. The need for long-term kinetic testing is evident from the Hannukainen amphibole and schist rock types. This study provides an overview of the environmental geochemistry of the skarn-hosted sulfide-bearing iron oxide deposits in Scandinavia. These deposits show potential for acid generation but due to the buffering reactivity of mafic silicates and the high crystallinity of the sulfides, the rate of acid generation is slow and the onset of these conditions delayed by mineral buffering.