Chapter 4.3 - The Archean Siilinjärvi Carbonatite Complex

Abstract

The Siilinjärvi carbonatite complex is named after the nearby village of Siilinjärvi, located some 5 km west of the southern extension of the complex. It is one of the oldest carbonatites on Earth, and the oldest presently being mined. Due to its age, it has been metamorphosed, particularly during the 1.8 Ga Svecofennian event, and intruded by several generations of younger, mafic to intermediate dikes, and a tonalite-diorite intrusion. Despite this overprint, the bulk of the rocks of the complex show well-preserved igneous textures, primary magmatic compositions of constituent minerals, and retain most of their primary isotopic compositions, which point to a mantle derivation. As such, the carbonatite and associated rocks provide a rare opportunity to constrain the mineralogy and geochemistry of the Archean mantle. Dominant rock types in the complex include carbonatite, silicocarbonatite, carbonatite-glimmerite, and glimmerite (mainly tetraferriphlogopite) within the main intrusion, all of which are surrounded by a halo of metasomatically produced fenites. Primary magmatic saline aqueous fluids rich in alkali elements have been identified in fluid inclusions in zircon and apatite from the carbonatite, explaining the formation of the well-developed metasomatic fenite syenite halo. Mining for apatite as a source for phosphorus began in 1979 by Kemira Oy, but since 2007 the deposit has been under the ownership of Yara International ASA, presently producing about 11 Mt of ore per year combined from the main Särkijärvi pit in the south, and the satellite Saarinen pit in the north.