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.Kouvo, O., 1984. GTK internal report to H. Lukkarinen, 4 p. 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.O'Brien et al. 2015
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.O'Brien et al. 2015 The Saarinen pit is located some 5 km north of the main Särkijärvi pit.Salo 2016
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.O'Brien et al. 2015
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.O'Brien et al. 2015 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.Salo 2016 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.O'Brien et al. 2015
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.Lukkarinen 2008
The glimmerite-carbonatite intrusion in Siilinjärvi is located in the south-eastern part of the Iisalmi granite-gneiss terrain.Härmälä 1981 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.Mänttäri & Hölttä 2002 Seismic studies have shown that the crustal thickness inside the Iisalmi terrane is unusually thick, some 55–60 km.Korsman et al. 1999 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.Sorjonen-Ward & Luukkonen 2005
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.Puustinen 1971 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.Härmälä 1981
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.Härmälä 1981
Rock types of the complex
Five different rocks characterise the Siilinjärvi mine: glimmerite-carbonatite series rocks, fenites, diabase dykes, tonalite-diorites, and gneisses.O'Brien et al. 2015 Apatite is associated with the glimmerite-carbonatites.Härmälä 1981
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.O'Brien et al. 2015
Ore-bearing rocks The central ore body comprises glimmerites and carbonatites. The phlogopite-rich ore-bearing rocks vary from almost pure glimmerite to carbonate-glimmerite and silicocarbonates. Carbonatites, which contain more than 50% carbonates, represent only about 1.5 vol% of the main intrusion. These carbonatite rocks are more abundant in the center of the intrusion and they occur as thin veins in glimmerite. The ore body also contains blue-green rocks which are composed up to 50 modal% of richterite.O'Brien et al. 2015 The main minerals of the ore rocks are tetraferriphlogopite, calcite, dolomite, apatite, and richterite. Zircon, magnetite, pyrrhotite, chalcopyrite, and pyroxenes occur as accessory minerals. The apatite is fluorapatite and the amount of CO2 varies.Härmälä 1981
Glimmerite is intensely foliated, greenish black, dark, or reddish brown rock (depending on the dominating mica mineral) containing 0-15 % carbonate minerals. The oriented rocks are fine- to medium-grained and usually porphyritic. The matrix is composed of fine-grained, aphanitic phlogopite and the porphyroclasts are tabular phlogopite grains. The fine-grained glimmerites are often more equigranular. The mineral composition of the glimmerites are on average 82% phlogopite, 8% apatite, 7% amphiboles, 2% calcite, and 1% dolomite. At some areas, the apatite content is so high that the rock is named as apatite rock (at least 25% apatite). Apatite occurs as grand sized grains in these rocks, and the diameter of the crystals can be up to several decimeters.Härmälä 1981 Accessory minerals of the glimmerites include ilmenite, magnetite, and pyrochlore.Al-Ani 2013
The carbonate-glimmerites are lighter coloured rocks compared to the pure glimmerites. That is obviously because of the carbonate content (15-25 % carbonate minerals), but also because of the lighter, reddish brown colour of the mica. They are less oriented than the glimmerites and more equigranular. The grain size is medium. The mineral composition of the carbonate-glimmerites are on average 64% phlogopite, 10% apatite, 10% calcite, 9% dolomite, and 7% amphiboles.Härmälä 1981
The silicocarbonates contain 25-50 % carbonate minerals and are quite light coloured, the shade is dependent on the mica colour. The texture is quite similar to the carbonate-glimmerites, excluding the areas, where the carbonates and micas are banded and occur as their own phases. The average mineral composition is 46% phlogopite, 22% dolomite, 19% calcite, 9% apatite, and 4% amphiboles, although the amount of calcite should be higher than the one of dolomites.Härmälä 1981 The accessory minerals of the silicocarbonates include strontianite, barite, zircon, ilmenite, and magnetite.Al-Ani 2013
The carbonatitic rocks (> 50% carbonates) in Siilinjärvi are brecciated and are mostly composed of calcite, dolomite, and apatite. The accessory minerals include phlogopite, ilmenite, and magnetite.Al-Ani 2013 Generally, the dolomite content of the carbonate rocks varies quite a lot. The content is mostly very low and the rock consist mainly of calcite but, in some areas, the dolomite content can be as high as 50%. The Siilinjärvi carbonates are light grey, white, or slightly reddish fine- to medium-grained rocks with average grain size about 0.9-1.2 mm. These rocks occur commonly as vertical dykes.Härmälä 1981
Fenites Fenites surround the ore-bearing rocks in the Siilinjärvi complex. They were formed metasomatically when the carbonatite-glimmerite rocks intruded into the granite gneiss host. The fenites consist mainly of perthitic microcline, richterite amphibole, and pyroxene, but there also are a wide variety of fenite types that include minerals like pyroxene, amphibole, carbonate, quartz, apatite, and quartz-aegirine.O'Brien et al. 2015 The fenites are also found as xenoliths in the glimmerite-carbonatites.Härmälä 1981 The most common fenite type is a reddish or greenish grey rock with varying grain size.Puustinen 1971 Microcline content of the fenites is on average about 50% and the microcline is abundant in perthite. The amount of plagioclase varies much more, and the highest percentages found are about 20-30 %. Anorthoclase content on individual plagioclase grains is 10-15 %. Amphibole percentage is 0-30 % and pyroxene percentage 0-15 % of the rock. Some fenite types contain as much as 15% biotite.Härmälä 1981
Cross-cutting dykes Basaltic diabase dykes crosscut the entire Siilinjärvi complex. Their width varies from a couple of centimetres up to 60 metres. The diabase dykes have a very distinct northwest–southeast or north-northwest–south-southeast vertical orientation.O'Brien et al. 2015 The diabases are dark green, almost black aphanitic rocks without macroscopic orientation. The hornblende content of the Siilinjärvi diabases is 50-70 %, and plagioclase content is 25-40 %. The hornblende is altered to biotite in contact zones, and the plagioclase is albitic. The altered margins of the hornblende dyke are about 50 cm wide. The accessory minerals include titanite, epidote, pyrite, apatite, quartz, and zircon.Härmälä 1981 The preliminary studies show that there are at least three different diabase varieties: calcite-bearing, sulfide-bearing, and barren diabase. The sulfide content is higher in the more sheared rocks.O'Brien et al. 2015
The melasyenite, which crosscuts all the other parts of the complex but the diabase dykes, is composed of alkali feldspar, biotite, alkaline amphibole, apatite, and magnetite. The mafic melasyenite dyke is 4 km long and 20–30 m wide and appears to have a lamprophyric character.Puustinen 1971 It is located in the northern part of the complex and is possibly related to the same intrusive event as the carbonatite.O'Brien et al. 2015
Minerals of the Siilinjärvi intrusion Most common minerals of the Siilinjärvi intrusion are micas, carbonates, apatites, and amphiboles. The average composition of the Siilinjärvi ore is 65% phlogopite (including tetraferriphlogopite), 19% carbonates (calcite/dolomite ratio 4:1), 10% apatite (equivalent to 4% P2O5 in the whole rock), 5% richterite, and 1% accessory minerals (mainly magnetite and zircon).O'Brien et al. 2015
Micas