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Kevitsa |
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Finland |
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Ni Cu PGE PGM Co Au
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Super Porphyry Cu and Au
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The Kevitsa Ni, Cu, Co, Au, PGE deposit is located some 142 km north-northeast of Rovaniemi, the capital of Finnish Lapland in northern Finland, approximately 800 km north of Helsinki and 100 km noth-east of Kemi on the Baltic coast of the Gulf of Bothnia. (#Location: 67° 42" N, 29° 58" E).
The Kevitsa deposit lies within the Baltic Shield which underlies most of Finland and is the ancient core of most of Northwestern Europe. It comprises older Archaean basement gneisses, late Archaean greenstone belts and major igneous intrusions. Towards the end of the Archaean, igneous activity generated an abundance of layered intrusions, one of which, the Kevitsa intrusion, hosts the Kevitsa deposit.
Mineralisation had first been indicated in the area in outcrop and river float boulders in the 1960s. Reconnaissance exploration work was undertaken by Outokumpu in the 1970s and initial diamond drilling was undertaken by GTK, the Finnish Geological Survey, in 1984. Ground magnetic, gravity and electromagnetic geophysical surveys and glacial till sampling was carried out between 1984 and 1987 by GTK, who discovered Ni-Cu mineralisation in diamond drilling in 1987. Further diamond drilling, trenching, airborne geophysical surveys and till sampling continued through to 1995, whilst in 1996 to 1998 Outokumpu Metals and Resources undertook further drilling and processing test work. In 2000, the project came under the control of Scandinavian Minerals, and in 2008 passed to First Quantum Minerals Ltd. (FQM). Mine construction commenced in 2010 with the first commercial production was in 2012. In 2016 FQM sold the Kevitsa Mine to Boliden Mineral AB who commissioned an expansion in 2020 (Berthet, 2020).
The Kevitsa igneous complex lies within the Central Lapland Greenstone Belt of the Fennoscandian Shield. The greenstone belt is composed of Palaeoproterozoic volcano-sedimentary rocks that have been divided to seven stratigraphic groups (Räsänen et al., 1996). These are, from oldest to youngest, the Salla, Onkamo, Sodankylä, Savukoski, Kittilä, Lainio and Kumpu Groups. The Savukoski Group supracrustal rocks envelop the Kevitsa intrusion. This group represents a major marine transgression, and is dominated by black schists, phyllites, micaceous arkose, tuffites, mafic and felsic metavolcanic rocks and an uppermost unit of ultramafic metavolcanics. According to Räsänen et al. (1996) these rocks are polydeformed and thrusted, resulting in overturning and structural repetition of the stratigraphy. There are three major ductile deformational events within the greenstone belt, with simultaneous and later shear zones that are related to regional structures (Hölttä et al., 2007).
The Kevitsa Intrusion, which has been dated at 2058 ±4 Ma (Mutanen and Huhma, 2001), is ~3.5 km north-south by 5 km east-west and is roughly circular in outcrop. It comprises ultramafic olivine pyroxenites and peridotites in the northeast representing the lower to middle sections of the intrusion. It overlies a basal pyroxenite and gabbro, and is capped by pyroxenite. These facies are overlain by gabbro that is exposed in the west and central sections, and is predominantly a granophyre in the south. There is a dunite unit in the middle of the deposit, which is disconcordant to magmatic layering, and a similarly crosscutting mass near the base of the intrusion. The Kevitsa Intrusion is characterised by internal layering defined by changes in composition. Xenoliths are common in the ultramafics and within the orebody. They are variable in size and composition, and are typically sedimentary, mafic or ultramafic rocks. Several mafic dykes cut in the intrusion, ranging in age, but are not voluminous. The deposit and surrounding country rocks have been subjected to several tectonic and metamorphic events (Hölttä et al., 2007). The NNE-SSW trending Satovaara fault, and other associated structures have deformed the eastern margin of the Kevitsa intrusion whilst smaller scale structures with a similar trend are mapped within the intrusion (Berthet, 2020).
Much of the intrusion is partially covered by a thin, up to 5 m thick, discontinuous layer of glacial till with bedrock exposed on hills. In boggy areas, 1 to 5 m of peat has been developed on over the till. Where mineralisation outcrops, an oxidised zone of a few metres in thickness has been developed above the water table.
Ni-Cu-(PGE) mineralisation is located in the middle of the ultramafic unit of the intrusion, and it is typically hosted by olivine-websterite and its variants. Overall, the host has been described as a clinopyroxene-dominated rock with 0 to 30% orthopyroxene, 5 to 25 % olivine and 0 to 10 % plagioclase. The layered rocks have subtle visual and geochemical differences, with the distribution and form of the observed mineralogical and geochemical variations interpreted to represent multiple magmatic pulses. No internal contacts are observed within to these pulses, although frequently the base of one pulse (olivine websterite) will grade relatively sharply into the upper part of the underlying pulse (plagioclase bearing olivine websterite). The layers within the intrusion are irregular in shape. Geochemically, differentiation within the pulses is most clearly demonstrated by Al2O3 contents. Luolavirta et al. (2017) proposed that the Kevitsa magma chamber was initially filled by a single continuous flow of basaltic magma, followed by differentiation in an almost closed system. Subsequently, new magma pulses were repeatedly injected into the interior of the intrusion in a dynamic, i.e., 'open' system, thus forming the sulphide orebodies. This model would explain the contrasting intrusive stratigraphy in the different parts of the intrusion, which likely reflects differing emplacement histories (Berthet, 2020).
The most widespread alteration within the mineralised Kevitsa intrusion involves amphibole replacement of ferromagnesian minerals, which is typically pervasive and generally has sharp, non-gradational boundaries. This pervasive amphibole alteration is often accompanied by millimetre- to metre-scale carbonate or carbonate-quartz veining. The earliest alteration at Kevitsa involved the replacement of olivine by dark serpentine. Magnetite occurs as both a primary, and as a secondary mineral associated with other alteration styles, and is found as veins in serpentinite and carbonate alteration. Epidote alteration is associated with rodingite dykes. Actinolite-chlorite alteration is apparently associated with crosscutting structures. Narrow actinolitic selvedges commonly bracket carbonate ±quartz veins, whilst wider developments of green actinolite also occurs as a distinctive vein set. Strong talc-carbonate alteration is associated with shear zones, late fractures and as veins, and ranges from selective replacement of ferromagnesian minerals to pervasive alteration of the whole rock (Berthet, 2020).
The deposit has a higher grade core which outcrops as an irregular, roughly circular mass about 300 to 400 m in diameter, plunging at ~50° to the southwest. Metal grades tend to decrease gradually away from this core in all directions. Although a number of styles of mineralisation have been recognised, two types predominate within the orebody, namely:
i). The Main or Ni-Cu type which constitutes more than 90% of the >0.2% Ni mineralisation of the Keivitsa deposit to a depth of 330 m, forming a zone that is up to 300 metres thick in the centre of the deposit. It has a high sulphide content, high Ni and Cu grades, significant PGE and gold contents and high Ni:S, Ni:Co and Ni:Cu ratios.
ii). The Ni-PGE type, which mainly occurs near surface above the other mineralisation styles in relatively narrow vertical shoots in the centre of the deposit. Its sulphide content is low, although the Ni values in the sulphide fraction are the highest in the deposit, whilst the associated silicate minerals also have very Ni-rich and PGE contents, the highest of the deposit. It has a higher Ni:Co ratio than the 'Main Type', but in general, a lower sulphur and copper content.
Both ore types have the same gangue mineralogy and the same pyrrhotite, pentlandite and chalcopyrite assemblage, as detailed below. As the sulphur content decreases the pyrrhotite content diminishes, pentlandite obtains a higher nickel content and the iron deficient nickel sulphides, millerite and heazlewoodite begin to appear.
The economic Ni-Cu-PGE mineralisation at Kevitsa is largely disseminated, with some minor semi-massive sulphide veins. Overall, the mineralised zone has an irregular geometry, cut by several faults which cause local offsets. It occurs within the olivine-pyroxenite of the ultramafic zone of the Kevitsa Intrusion, and contains up to 5% sulphide, the majority of which are found as fine to medium granular masses, disseminated interstitial to the silicate mineral crystals. The silicate mineralogy is predominantly composed of olivine and orthopyroxene with finely disseminated, 100 to 500 µm sulphides. The dominant mineralisation is Ni-Cu, comprising 95% of the deposit, enclosing smaller domains that can be differentiated on the basis of Cu and Ni sulphide grades and the varying PGE content. Ni-PGE mineralisation occupies a relatively smaller volume. Sulphides comprise more than 95% pentlandite, chalcopyrite, pyrrhotite, troilite and pyrite. Chalcopyrite and pentlandite are the principal economical minerals, although pyrrhotite is the most common sulphide. Other copper and nickel sulphides include cubanite, millerite and heazlewoodite, with minor associated gold, cobalt and PGE concentrations. In unaltered rocks, the sulphide and silicate grains are smooth, but in amphibole altered rocks those boundaries are irregular and serrated. Chalcopyrite is usually present as large anhedral grains, sometimes with associated cubanite and talnakhite, and as fine intergrowths within the gangue silicates. Pentlandite can be coarse-grained and sub-euhedral, as well as smaller intergranular grain bands between silicates and pyrrhotite, and 'exolution flame' inclusions within pyrrhotite or pyrite of very fine grain size. In addition to pentlandite, nickel also occurs in the crystal lattice of some silicate minerals such as olivine, clinopyroxene and tremolite. The nickel in silicates is not recoverable in the metallurgical process and therefore sulphide nickel is analysed by selective leach method. Pd and Pt typically occur as sulphosalts, such as arsenides and tellurides. Kojonen et al. (2008) note that more than half of the PGE carrying minerals occur as inclusions in amphibole, serpentine and chlorite. PGE carrying minerals which are related to sulphides occur mostly on sulphide grain boundaries, inclusions in sulphides or in late fracture fillings in pentlandite (Berthet, 2020).
Where exposed at the surface, the mineralisation has been oxidised to a depth of a few metres above the water table.
Pre-feasibility Reserves and Resources (First Quantum, 2008) were:
Proved + Probable Reserve (0.18% Ni cut-off) - 66.8 Mt @ 0.295% Ni, 0.427% Cu, 0.014% Co, 0.141 g/t Au, 0.196 g/t Pd, 0.303 g/t Pt;
Resources to a depth of 1000 m (First Quantum, 2008):
Measured + Indicated Resource (0.2% Ni cut-off) - 141 Mt @ 0.30% Ni, 0.42% Cu, 0.01% Co, 0.12 g/t Au, 0.18 g/t Pd, 0.28 g/t Pt;
Inferred Resource (0.2% Ni cut-off) - 291 Mt @ 0.29% Ni, 0.46% Cu, 0.01% Co, 0.09 g/t Au, 0.09 g/t Pd, 0.12 g/t Pt;
Measured + Indicated Resource (0.1% Ni cut-off) - 287 Mt @ 0.22% Ni, 0.29% Cu, 0.01% Co, 0.09 g/t Au, 0.13 g/t Pd, 0.20 g/t Pt;
Inferred Resource (0.1% Ni cut-off) - 544 Mt @ 0.22% Ni, 0.32% Cu, 0.01% Co, 0.07 g/t Au, 0.08 g/t Pd, 0.09 g/t Pt;
Since commercial production commenced in 2012, to the end of 2020, 63.79 Mt of ore had been mined at Kevitsa, yielding the following recovered tonnages in concentrate of: 182 771 tonnes of Cu; 89 996 t of Ni; 3978 t of Co; 4.511 t of Au; 10.779 t of Pt and 8.082 t of Pt, from the 64.235 Mt of ore milled (Berthet, 2020)
Remaining Ore Reserves and Mineral Resources at 31 December, 2020 (Boliden Kevitsa Mineral Resources and Mineral Reserves Report, 2020) were:
Proved + Probable Reserve - 128 Mt @ 0.21% Ni, 0.32% Cu, 0.01% CoS, 0.09 g/t Au, 0.12 g/t Pd, 0.18 g/t Pt;
Measured + Indicated Resource - 175 Mt @ 0.22% Ni, 0.33% Cu, 0.01% CoS, 0.07 g/t Au, 0.08 g/t Pd, 0.4 g/t Pt;
Inferred Resource - 4 Mt @ 0.12% Ni, 0.22% Cu, 0.01% CoS, 0.03 g/t Au, 0.02 g/t Pd, 0.06 g/t Pt;
NOTE: Mineral Resources are reported exclusive of Ore Reserves.
Much of this summary is drawn from information in Berthet, L., 2020 - Boliden Summary Report, Resources and Reserves, 2020, Kevitsa Mine, 37p.
The most recent source geological information used to prepare this decription was dated: 2020.
Record last updated: 8/1/2022
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.
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Selected References:
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Begg, G.C., Hronsky, J.A.M., Arndt, N.T., Griffin, W.L., O Reilly, S.Y. and Hayward, N., 2010 - Lithospheric, Cratonic, and Geodynamic Setting of Ni-Cu-PGE Sulfide Deposits: in Econ. Geol. v.105, pp. 1057-1070.
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Luolavirta, K., Hanski, E., Maier, W. and Santaguida, F., 2018 - Whole-rock and mineral compositional constraints on the magmatic evolution of the Ni-Cu-(PGE) sulfide ore-bearing Kevitsa intrusion, northern Finland: in Lithos v.296-299, pp. 37-53.
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Luolavirta, K., Hanski, E., Maier, W., Lahaye, Y., O Brien, H. and Santaguida, F., 2018 - In situ strontium and sulfur isotope investigation of the Ni-Cu-(PGE) sulfide ore-bearing Kevitsa intrusion, northern Finland: in Mineralium Deposita v.53, pp. 1019-1038.
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Santaguida, F., Luolavirta, K., Lappalainen, M., Ylinen, J., Voipio, T. and Jones, S., 2015 - The Kevitsa Ni-Cu-PGE Deposit in the Central Lapland Greenstone Belt in Finland: in Mineral Deposits of Finland, Chapter 3.6, pp. 195-210.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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