The Siilinjärvi carbonatite complex is located in central Finland close to the city of Kuopio. It is named after the nearby town of Siilinjärvi, located approximately 5 km west of the southern extension of the complex. Siilinjärvi is the second largest carbonatite complex in Finland after the Sokli formation, and one of the oldest carbonatites on Earth at 2610±4 Ma. The carbonatite complex consists of a roughly 16 km long steeply dipping lenticular body surrounded by granite gneiss. The maximum width of the body is 1.5 km and the surface area is 14.7 km2. The complex was discovered in 1950 by the Geological Survey of Finland with help of local mineral collectors. The exploration drilling began in 1958 by Lohjan Kalkkitehdas Oy. Typpi Oy continued drilling between years 1964 and 1967, and Apatiitti Oy drilled from 1967 to 1968. After the drillings, the laboratory and pilot plant work were made. The mine was opened by Kemira Oyj in 1979 as an open pit. The operation was sold to Yara in 2007. The Siilinjärvi apatite mine is the biggest open pit in Finland. Currently the mine comprises two pits; the larger southern Särkijärvi and the smaller northern Saarinen satellite pit. The Särkijärvi pit is approximately 250 m deep, with a bench height of 28 m. The Saarinen pit is located some 5 km north of the main Särkijärvi pit. The overall blast rate at the mine is 600 kt per week, 450 kt from the Särkijärvi pit and 150 kt from the Saarinen pit. Almost all of the glimmerite-carbonatite series rocks are ore-grade rocks; the fenites and crosscutting diabases are waste rocks. There are, however, some late apatite-poor carbonatite veins and certain blocks of carbonatite-glimmerite with < 0.5 wt-% P2O5. The reason why those are barren of apatite is unknown, but it could be related to metamorphism and fluid flow. The Siilinjärvi mine is the only operating phosphorus mine in the European Union. Since 1979, over 400 Mt of rock have been mined, about 65% being ore. By year 2016, the mine had produced 24.7 Mt of the main product, apatite. Ore reserves were 205 Mt in January 2016. The current production is roughly 11 Mt of ore per year, while the average in situ grade is 4.0 wt-% of P2O5. Roughly 85% of the apatite concentrate is processed on-site in Siilinjärvi to produce phosphoric acid and fertilisers, the rest of the concentrate is used in the company's other factories. The by-products are mica and calcite concentrates. Apatite concentrate is produced by flotation in the concentrator near the Särkijärvi pit. The concentrate can be then processed into phosphoric acid using sulfuric acid. The sulfuric acid is currently derived from Pyhäsalmi mine pyrite.
Surrounding rocks
The surrounding bedrock of Siilinjärvi intrusion is Archean, although the boundary between the Archean and Paleoproterozoic bedrock is near-by. The nearest Paleoproterozoic rocks belong to the North Savo Black Schist area. The glimmerite-carbonatite intrusion in Siilinjärvi is located in the south-eastern part of the Iisalmi granite-gneiss terrain. The terrane records both some of the youngest and oldest Archean events in the Fennoscandian Shield, the 2.6 Ga Siilinjärvi intrusion and nearly 3.2 Ga mesosomes found in granulites. Seismic studies have shown that the crustal thickness inside the Iisalmi terrane is unusually thick, some 55–60 km. The terranes thickness is due to several processes, such as thrust-stacking during the Svecofennian collision and post-collisional underplating. At the present erosion level the western part of the terrane is mostly metamorphosed at greenschist facies during the Svecofennian orogeny. The dominating surrounding rock type in the Siilinjärvi area is a granite gneiss with varying texture and to some extent, mineralogy. The main minerals are plagioclase feldspar, quartz, microcline feldspar, biotite, and hornblende. The surrounding granite gneiss extends some 100 km to the north from Siilinjärvi. The Karelian (2.0-1.9 Ga) sedimentary rocks occur in the west and north-west of Siilinjärvi. The rocks are folded mica schist-like gneisses. The gabbros of Lapinlahti and Siilinjärvi are from the Archean Karelian orogeny. A fine-grained quartz-diorite, which intrudes the surrounding granitic gneiss, is located at the north-eastern side of the Siilinjärvi gabbro.
Rock types of the complex
Five different rocks characterise the Siilinjärvi mine: glimmerite-carbonatite series rocks, fenites, diabase dykes, tonalite-diorites, and gneisses. Apatite is associated with the glimmerite-carbonatites. Commonly, carbonatite complexes contain a core of intruded carbonatite which cuts a sequence of phlogopite-rich rocks. In Siilinjärvi, however, the glimmerites and carbonatites are well mixed and occur as subvertical to vertical laminated nearly pure glimmerites and nearly pure carbonatites. The volume of the carbonatite is greater in the center of the intrusion, and rocks near the edges of the body are almost completely glimmerites.
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