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Kemi

Finland

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The Kemi chromite deposit is located near the town of Kemi on the Gulf of Bothnia coast in western Finland (#Location: 65° 47' 3"N, 24° 42' 36"E).

The deposit is hosted by a north-east trending, 200 to 2000 m thick and 70° dipping Palaeoproterozoic (2.44±0.16 Ga) layered mafic intrusion that extends down dip for at least 2 km.   It occurs as a single sill-like intrusion.   The lower part of the intrusion is around 500 m thick and is composed of massive peridotite and pyroxenite cumulates and a series of continuous chromitite layers.   Specifically, the cumulate sequence commences with a basal bronzite-chromite cumulate, grading upward into olivine + bronzite-chromite cumulate, followed by the main chromitite.   The pyroxenitic and peridotitic cumulates below the main chromitite layer contain several 0.5 to 1.5 m thick chromite-rich interlayers.  Olivine and chromite have also been observed in the upper part which is generally well layered (to thinly banded) and where plagioclase is the predominant cumulus phase, either alone or with pyroxenes.   No well-developed marginal series with a fine-grained chilled margin is evident. Instead the contact with the basement gneiss is irregular, with pronounced interaction between the intrusion and the basement, with the basal contact represented by a megabreccia.   The footwall of the intrusive body is composed of Archaean granitoids.

A continuous chromitite layer extends over the full 15 km length of the intrusive near the base of the lower part, varying from a few mm up to 90 m in thickness.   A number of other thinner chromitite bands are found above the main ore band.   The mineable section of the chromitite layer extends over a strike length of 4.5 km and comprises eight separate orebodies.   The lower part of the main chromitite is massive, but becomes banded in the upper sections.

The intrusive has a shallow funnel shaped, i.e., with a thicker central section within the plane of the intrusion, flanked by thinner wings. This funnell shaped profile is evident in the current surface exposure, with a central thickening that plunges at 70° to the NW.   The chromitite cumulus layer is thickest in the middle of the intrusion, while the ore zone is also intensely fractured and altered which has degraded the mineralisation.

Chromite grains of the chromite-rich layers contain silicate inclusions which are rich in alkalies and silica, with albite being one of the most common minerals in these inclusions.

The main ore deposit averages 40 m in thickness over a strike length of ~3 km with an average grade of 29% Cr2O3. It is divided into four contiguous zones, from west to east, as follows (Outokumpu Oy presentation, 2014):
• Surmaoja - with proved reserves of 18 Mt plus resources of 13 Mt down dip, and
  a further 11 Mt on the western margin in the Nuottijärvi-Matilainen-Mätäsoja section;
• Elijärvi - Viia - with proved reserves of 32 Mt and down plunge resources of 67 Mt;
• Viianmaa - with resources of 5 Mt;
• Perukka - with resources of 2 Mt;
Resources are estimated to a depth of 1 km below the surface.

The deposit was discovered in 1959, and a decision taken to commence mining in 1964. Production started in the open pit in 1968. An underground mine was constructed from 1999 to 2003. All the ore production has been from underground since 2006. Underground mining to 2014 has mainly been within the Elijärvi - Viia section to a depth of 500 m.

Open pit reserves in 1989 amounted to 40 Mt @ an average grade of 26.6% Cr
2O3 and a Cr/Fe ratio of 1.53, within an overall resource of 110 Mt.

Statistics in 2002 were:
    Mined ore - 22 Mt @ 26% Cr
2O3
    Open pit reserve - 70 Mt @ 26% Cr
2O3
    Mineral resource - 144 Mt @ 26% Cr
2O3
    Total mined+reserve+resource - 236 Mt @ 26% Cr
2O3

For detail see the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2012.     Record last updated: 8/1/2015
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Kemi

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Alapieti T T, Kujanpaa J, Lahtinen J J, Papunen H  1989 - The Kemi stratiform Chromitite deposit, northern Finland: in    Econ. Geol.   v84 pp 1057-1077
Yang, S.-H, Hanski, E., Li, C., Maier, W.D.,Huhma, H., Mokrushin, A.V.,Latypov, R., Lahaye, Y., O Brien, H. and Qu, W.-J.,  2016 - Mantle source of the 2.442.50-Ga mantle plume-related magmatism in the Fennoscandian Shield: evidence from Os, Nd, and Sr isotope compositions of the Monchepluton and Kemi intrusions: in    Mineralium Deposita   v.51, pp. 1055-1073.


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