What is the Siilinjärvi carbonatite complex?

The Siilinjärvi carbonatite complex is a massive, ancient geological formation located in central Finland, dated to approximately 2.6 billion years old. Discovered in 1950, it hosts the largest open-pit mine in Finland and serves as the sole operating phosphorus mine in the European Union. Almost all of its glimmerite-carbonatite series rocks function as ore-grade material, yielding valuable apatite used to produce phosphoric acid and fertilisers.

What we know

• The Siilinjärvi carbonatite complex is one of the oldest carbonatites on Earth, aged at 2610±4 Ma, and is situated within a region shaped by a mix of ancient and young geological materials typical of Finland. [2, 4]
• The complex was discovered in 1950 by the Geological Survey of Finland with assistance from local mineral collectors, and its exploration drilling was subsequently advanced by multiple companies over the following decades. [1, 2]
• The Siilinjärvi mine is currently the only operating phosphorus mine in the European Union, utilizing surface mining techniques to extract ore from open pits. [2, 5]
• The carbonatite complex consists of a roughly 16 km long, steeply dipping lenticular body defined by a mineralogy characteristic of carbonatites, surrounded by granite gneiss. [2, 3]

Still open

• The exact reason why certain late carbonatite-glimmerite blocks and veins are barren of apatite with less than 0.5 wt-% P2O5 remains unknown, though it may be linked to metamorphism and fluid flow.

Where to go next

• the bigger pictureWhat are carbonatites and how do they form?
• how it workedHow is apatite extracted using open-pit mining?
• whereWhat characterizes the geology of Finland?
• compared withHow does the Sokli formation compare to Siilinjärvi?
• whenWhen did the exploration drilling at Siilinjärvi take place?
• an unexpected connectionHow does a Finnish phosphorus mine rely on a pyrite mine?The sulfuric acid used on-site at Siilinjärvi to process apatite concentrate into phosphoric acid is derived directly from pyrite sourced from the Pyhäsalmi mine.

Sources

• [1]Siilinjärvi carbonatite (Wikidata Q11893397) · CC0 1.0
• [2]Siilinjärvi carbonatite · CC BY-SA 4.0
• [3]Carbonatite · CC BY-SA 4.0
• [4]Geology of Finland · CC BY-SA 4.0
• [5]Surface mining · CC BY-SA 4.0