Siilinjärvi carbonatite
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.
[1] The carbonatite complex consists of a roughly 16 km long steeply dipping lenticular body surrounded by granite gneiss.
The current production is roughly 11 Mt of ore per year, while the average in situ grade is 4.0 wt-% of P2O5.
The nearest Paleoproterozoic rocks belong to the North Savo Black Schist area.
[9] The glimmerite-carbonatite intrusion in Siilinjärvi is located in the south-eastern part of the Iisalmi granite-gneiss terrain.
[12] 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.
[13] The dominating surrounding rock type in the Siilinjärvi area is a granite gneiss with varying texture and to some extent, mineralogy.
A fine-grained quartz-diorite, which intrudes the surrounding granitic gneiss, is located at the north-eastern side of the Siilinjärvi gabbro.
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.
These carbonatite rocks are more abundant in the center of the intrusion and they occur as thin veins in glimmerite.
[20] The main minerals of the ore rocks are tetraferriphlogopite, calcite, dolomite, apatite and richterite.
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.
[26] The carbonatitic rocks (> 50% carbonates) in Siilinjärvi are brecciated and are mostly composed of calcite, dolomite and apatite.
[30] The most common fenite type is a reddish or greenish grey rock with varying grain size.
[35] 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.
[36] It is located in the northern part of the complex and is possibly related to the same intrusive event as the carbonatite.
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).
[39] Phlogopites show very strong red-brown to pinkish yellow reverse pleocroism, which is due to high Fe3+ contents.
[40] The phlogopite of Siilinjärvi is sold as soil conditioner under the trade name “Yara biotite”.
Phlogopite occurs as disseminated flakes, tabular crystals and lamellar or foliated aggregates.
[44] The microprobe studies of Siilinjärvi dolomite show homogeneous compositions with low FeO-, SrO- and MnO –content.
[46] The ore-bearing rocks of Siilinjärvi contain roughly equal amounts (about 10%) of light green to grey apatite.
The hexagonal rods and cross sections are sparse in deformed areas, where the grains are disintegrated and broken.
[50] The most common amphibole in Siilinjärvi is blue-green richterite, which forms about 5% of the overall volume of the intrusion and usually less than 15 vol-% of the glimmerites.
However, the grain size varies quite a lot, and large crystals with diameter of several centimeters are not uncommon.
[53] Sulfide minerals represented in the ore are pyrite, pyrrhotite, and lesser amounts of chalcopyrite.
[54] Barite, strontianite, monazite, pyrochlore, zircon, baddeleyite, rutile and ilmenite have been identified in Siilinjärvi as rare accessory minerals.
