Siilinjärvi carbonatite
The Siilinjärvi carbonatite complex is located in central Finland close to the city of
The Siilinjärvi apatite mine is the biggest
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
The Siilinjärvi mine is the only operating phosphorus mine in the
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.[9]
The
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
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.[16]
Rock types of the complex
Five different rocks characterise the Siilinjärvi mine: glimmerite-carbonatite series rocks, fenites,
Commonly,
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
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
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.[24]
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.
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.[27] 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.[28]
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.[29] The fenites are also found as xenoliths in the glimmerite-carbonatites.[30] The most common fenite type is a reddish or greenish grey rock with varying grain size.[31] 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.[32]
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.[33] 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.[34] 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.[35]
The mela
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).[38]
Micas
The most common mica mineral in Siilinjärvi complex is tetraferriphlogopite, which comprises 65% of the intrusion. Some glimmerites contain over 90% tetraferriphlogopite. The colour of the mineral is black or greenish black, dark brown or reddish brown. The colour is dependent on the host rock and the intensity of the deformation of the rock. The reddish brown mica usually occurs with the carbonate-glimmerites and the black mica occurs with the glimmerites.
Phlogopite occurs as disseminated flakes, tabular crystals and lamellar or foliated aggregates. The grain size of the micas varies from only a couple of µm to several centimetres, the average size is 1–2 mm in diameter.[41] The phlogopite is altered into brown biotite-phlogopite in the shear zones, and in the most intensely sheared zones, into biotite and chlorite.[42] The most common inclusion mineral in micas is magnetite, but generally the inclusions are rare. Some zircon inclusions can also be found.[43]
Carbonates
The dolomite of Siilinjärvi is yellowish or brownish white and it is hard to distinguish it from calcite. The most common form of dolomite is roundish anhedral grains with diameter of 0.2-0.4 mm. The dolomites are also found as large, almost euhedral, grains with diameter of 4–6 mm. Other common textures are myrmekite and exsolution lamellae with calcite. Euhedral grains are only found in carbonatites.[44] The microprobe studies of Siilinjärvi dolomite show homogeneous compositions with low FeO-, SrO- and MnO –content.[45]
Apatites
The apatite in Siilinjärvi is mainly fluorapatite, but also carbonate-fluorapatite can be found.[46] The ore-bearing rocks of Siilinjärvi contain roughly equal amounts (about 10%) of light green to grey apatite. The amount of fluorine is about 2-4 wt-% in the Siilinjärvi apatite.[47] The apatites of the mine contain quite high amounts of SrO, and sometimes also CO2. Apatite is found in companion with mica in mica rich rocks and with calcite, dolomite or mica in carbonate-rich rocks.[48]
Typically, the apatite occurs as rounded grains or as
Amphiboles
The most common amphibole in Siilinjärvi is blue-green
Accessory minerals
Magnetite is the most common accessory mineral in the ore rocks, and constitute usually less than 1 vol% of the ore. It is mostly found in the glimmerites.
Geological structures
The dominant foliation dip direction in the Särkijärvi area is almost N-S (265-275°) and it dips nearly vertically (85-90°) towards west. The strike of the foliation is also the dominant direction of shearing. Other shearing trend is north-west to south-east, but it is weaker. This direction is also the dominating direction of the diabases.[56]
Shearing is a common feature in the main Siilinjärvi ore body and the contact zone between the country rock and ore body. There are also contact zones which show the primary magmatic contact. Paleoproterozoic diabase dykes crosscut the sheared zone. At least two stages of deformation can be found in the Siilinjärvi complex rocks. The deformation took certainly place during the Svecofennian orogeny, but other earlier stages of deformation may have occurred.[57]
References
Sources
- Al-Ani, T. 2013. Mineralogy and petrography of Siilinjärvi carbonatite and glimmerite rocks, eastern Finland. Geological Survey of Finland, Archive Report, 164.
- Härmälä, O. 1981. Siilinjärven kaivoksen mineraaleista ja malmin rikastusmineralogisista ominaisuuksista. Master's thesis, University of Turku, Department of Geology and Mineralogy. 121 p.
- Korsman, K., Korja, T., Pajunen, M., Virransalo, P., & GGT/SVEKA working group. 1999. The GGT/SVEKA transect: structure and evolution of the continental crust in the Paleoproterozoic Svecofennian orogen in Finland. International Geology Review 41, 287–333.
- Lukkarinen, H. 2008. Siilinjärven ja Kuopion kartta-alueiden kallioperä. Summary: Pre-Quaternary rocks of the Siilinjärvi and Kuopio map-sheet areas. Suomen geologinen kartta 1:100 000: kallioperäkarttojen selitykset lehdet 3331, 3242. Geological Survey of Finland. 228 p.
- Mänttäri, I. & Hölttä, P. 2002. U–Pb dating of zircons and monazites from Archean granulites in Varpaisjärvi, Central Finland:: Evidence for multiple metamorphism and Neoarchean terrane accretion. Precambrian Research 118, 101–131.
- O’Brien, H., Heilimo, E. & Heino, P. 2015. The Archean Siilinjärvi carbonatite complex. In: Maier, W., O’Brien, H., Lahtinen, R. (Eds.) Mineral Deposits of Finland, Elsevier, Amsterdam, 327–343.
- Puustinen, K. 1971. Geology of the Siilinjarvi carbonatite complex, Eastern Finland. Geological Survey of Finland. Bulletin of the Geological Society of Finland 249, 43 p.
- Salo, A. 2016. Geology of the Jaakonlampi area in the Siilinjärvi carbonatite complex. Bachelor's thesis, Oulu Mining School, University of Oulu. 27 p.
- Sorjonen-Ward, P., & Luukkonen, E. J. 2005. Archean rocks. In: M. Lehtinen, P.A. Nurmi, O.T. Rämö (Eds.), Precambrian Geology of Finland—Key to the Evolution of the Fennoscandian Shield, Elsevier, 19–99.
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