Oxygen and iron isotope systematics of the Grängesberg Mining District (GMD), Central Sweden

Abstract: Iron is the most important metal for modern industry and Sweden is the number one iron producer in Europe. The main sources for iron ore in Sweden are the apatite-iron oxide deposits of the "Kiruna-type", named after the iconic Kiruna ore deposit in Northern Sweden. The genesis of this ore type is, however, not fully understood and various schools of thought exist, being broadly divided into "ortho-magmatic" versus the "hydrothermal replacement" approaches. This study focuses on the origin of apatite-iron oxide ore of the Grängesberg Mining District (GMD) in Central Sweden, one of the largest iron reserves in Sweden, employing oxygen and iron isotope analyses on massive, vein and disseminated GMD magnetite, quartz and meta-volcanic host rocks. As a reference, oxygen and iron isotopes of magnetites from other Swedish and international iron ores as well as from various international volcanic materials were also analysed. These additional samples included both "ortho-magmatic" and "hydrothermal" magnetites and thus represent a basis for a comparative analysis with the GMD ore. The combined data and the derived temperatures support a scenario that is consistent with the GMD apatite-iron oxides having originated dominantly (ca. 87 %) through ortho-magmatic processes with magnetite crystallisation from oxide-rich intermediate magmas and magmatic fluids at temperatures of 600 °C to 900 °C. A minor portion of the GMD magnetites (ca. 13 %), exclusively made up of vein and disseminated ore types, is in equilibrium with a high-δ18O and low-δ56Fe hydrothermal fluid at temperatures below 400 °C, indicating the existence of a hydrothermal system associated with the GMD volcano.