A geological mystery may have been solved: “Ancient sulphate salt deposits may hold the key to the origin of the giant Kiruna-type iron ore deposits”
A geological mystery that has puzzled scientists for more than a century may finally have been solved: "Ancient sulfate salt deposits may hold the key to the origin of giant Kiruna-type iron ore deposits". Valentin Troll and Ulf B. Andersson are co-authors.
A geological mystery that has puzzled scientists for more than a century may finally have an answer. An international team of researchers from Germany, Sweden, and Spain has uncovered compelling new evidence that ancient sulfate-rich salt deposits played a decisive role in the formation of some of Earth's most important iron ore deposits—a discovery that could reshape scientists' understanding of how these economically vital resources formed.
Iron oxide-apatite (IOA) deposits, also known as Kiruna-type deposits, which are the largest source of iron in the EU and are increasingly important as sources of rare-earth elements. By investigating the world-famous Kiruna mining district in northern Sweden, the researchers analyzed subtle variations in the oxygen isotope composition of the iron-oxide mineral magnetite, the principal iron-bearing mineral in these deposits. These isotopic "fingerprints" revealed an unexpected additional source of oxygen present in the ores: sulfate minerals originating from ancient evaporite deposits left behind when seawater evaporated hundreds of millions to billions of years ago.
The researchers propose that ascending magma assimilated such sulfate-rich sediments deep within the Earth's crust. As the sulfate reacted with the magma, it oxidized dissolved iron, triggering the formation of iron-rich melts and mineral-forming fluids that ultimately concentrated into giant iron ore deposits. The newly identified process offers a unifying explanation for the origin of these large ore bodies, a topic that has been vigorously debated since their discovery well over a century ago.
To determine whether this process was unique to Kiruna, the team also analyzed iron oxide-apatite deposits from around the world. Many deposits—particularly those formed during the Proterozoic Era—display the same distinctive oxygen isotope signature, suggesting that addition of evaporite-derived oxygen is a widespread feature of these ore systems. Remarkably, the isotopic signatures also reflect changes in Earth's atmosphere through geological time, linking the formation of Kiruna-type iron ore deposits to the evolution of the planet's surface environment. Beyond providing new insights into Earth's geological history, the findings could help guide future mineral exploration by highlighting the importance of ancient sulfate-bearing sedimentary basins in the search for large iron oxide-apatite deposits.
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The findings have been published in the journal Nature Communications: Stefan Peters, Dingsu Fend, Valentin R. Troll, Andreas Pack, Fernando Tornos, Ulf B. Andersson, Bernd Lehmann, Tommaso DiRocco (2026) Formation of iron oxide apatite deposits triggered by magmatic assimilation of evaporitic sulfate. doi: https://doi.org/10.1038/s41467-026-75189-0