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Triple oxygen isotope compositions of iron oxide-apatite deposits – a window into ore formation processes and the ancient atmosphere

Iron oxide - apatite (IOA) deposits are magnetite- and apatite-rich rock assemblages that are hosted by intermediate to felsic magmatic rocks, and are frequently mined for iron. The geological formation mechanisms of IOA deposits were suggested to involve either high-temperature silicate, iron oxide and/or sulfate melts or, alternatively, hydrothermal processes at more moderate temperatures. Here, we show that magnetite and apatite samples from the type locality of the deposits in the Kiruna district, northern Sweden, contain up to tens of atom percent oxygen from evaporitic sulfate, as is demonstrated by their anomalously low Δ′17O values. In order to explain this observation in conjunction with field evidence for an igneous origin of the deposits from Kiruna, we propose that the IOA assemblage of Kiruna crystallised from iron-rich sulfate melts and/or from sulfate-rich iron oxide melts. The iron-rich sulfate melts would have formed when evaporite-rich sediments melted by anatexis and scavenged iron from magmatic sources, whereas sulfate-rich iron oxide melts could have formed, e.g., when iron oxide liquids exsolved from silicate magmas and then assimilated evaporites. An inventory study shows that several other Proterozoic and Cambrian IOA deposits have anomalously low Δ′17O values similar to Kiruna, whereas post-Cambrian IOA deposits, in contrast, have more moderate Δ′17O values. IOA deposits may therefore ubiquitously contain evaporite-derived oxygen, with variations in the lowermost Δ′17O values of the deposits reflecting the changing isotope composition of atmospheric O2 through time; and therefore providing an unexpected window into global bioproductivity and atmospheric pCO2 levels of the ancient Earth.


Stefan T.M. Peters1, Dingsu Feng2, Valentin Troll3, Andreas Pack2, Ulf B. Andersson4, Fernando Tornos5, Bernd Lehmann6, Tommaso di Rocco2
1Leibniz-Institut zur Analyse des Biodiversitätswandels, Abt. Mineralogie; Museum der Natur Hamburg, Grindelallee 48, 20146 Hamburg, Germany;Georg-August-Universität Göttingen, Abt. Geochemie und Isotopengeologie, Goldschmidstraße 1, 37077 Göttingen, Germany; 2Georg-August-Universität Göttingen, Abt. Geochemie und Isotopengeologie, Goldschmidstraße 1, 37077 Göttingen, Germany; 3University of Uppsala, Department of Natural Resources & Sustainable Development, Villavägen 16, Uppsala, 75236, Sweden; 4Luossavaara-Kiirunavaara AB, Research and Development, FK9, 981 86, Kiruna, Sweden; 5Instituto de Geociencias (IGEO, CSIC-UCM), Dr Severo Ochoa, 7, 28040 Madrid, Spain; 6Technical University of Clausthal, Adolph-Roemer-Str. 2a, 38678 Clausthal, Germany
GeoMinKöln 2022