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Platinum Group Elements in Finland

The exploration for platinum group metals (PGM) began in the late 1970s as part of a worldwide boom in the search for these metals, driven largely by improvements in analytical techniques that allowed routine analysis of PGE in exploration samples. However, the earliest documentation concerning PGE in Finland goes back to the mid-1960s, when Outokumpu assayed a Cu-Ni sulphide-bearing gabbroic outcrop sample from Koillismaa, which gave sub-economic values for PGE and gold. At the same time, a 3.4-m-long intersection averaging 0.59 g/t Pt, 0.84 g/t Pd, 0.77% Cu and 0.53% Ni was discovered in the Yli-Portimojärvi deposit (Portimo Layered Complex). Exploration in the Koillismaa area led to two open-pit test mines (Porttivaara and Kuusijärvi), but only copper and nickel were recovered. Both Suhanko and Koillismaa are still the focus of intense exploration.

Documented principal PGE deposits are almost exclusively confined to a roughly east-west trending, 300-km-long belt in northern Finland. Known as the Tornio-Näränkävaara Belt, it represents a major failed rift system into which large volumes of mafic magma were intruded ca 2.45 Ga into the upper crust. The mafic intrusions in the belt have a late Archean felsic gneiss terrain on their southern side and a Paleoproterozoic volcano-sedimentary sequence on their northern side.

All PGE mineralisation types characteristic of layered mafic intrusions can be found in the Tornio-Näränkävaara Belt. These include: accumulations of PGE-enriched base metal sulphides in the lower parts of the intrusions; stratiform PGE enrichments higher in the cumulate sequences; and offset PGE-base metal deposits below the intrusions.

Three potentially world-class reef-type PGE deposits have been recognised in two separate intrusions: Narkaus and Penikat. These reefs, Paasivaara, Sompujärvi, and Siika-Kämä Reefs, are some of the first reported in the world outside South Africa and Montana in early 80s. In addition to these, two other reef-type deposits are included, Ala-Penikka and Rytikangas Reefs from Penikat and Suhanko Intrusions respectively. Along with the appraisals of these reefs, the economic significance of some lower-grade but high-tonnage sulphide PGE became apparent. Such deposits, also called contact-type deposits, occur at the base of the layered mafic intrusions.

Table 1. Largest PGE deposits in term of contained metals.

Concentration Contained metals
 
Tonnes
Mt
Pt
g/t
Pd
g/t
Rh
g/t
Au
g/t
Cu
wt.%
Ni
wt.%
Pt
‘000 oz
Pd
‘000 oz
Rh
‘000 oz
Au
‘000 oz
Cu
‘000 t
Ni
‘000 t
Reef types
Siika-Kämä
43.1
0.72
2.70
 
0.11
0.21
0.08
998
3,741
 
111
47.4
34.5
Sompujärvi
6.7
3.08
5.36
0.38
0.10
   
663
1,155
81.9
21.5
   
Paasivaara
5.0
4.04
2.58
0.08
0.61
0.28
 
649
414
12.6
98
14
 
Contact types
Ahmavaara
187.8
0.17
0.82
 
0.10
0.17
0.07
1,028
4,978
 
605
328
129
Konttijärvi
75.2
0.27
0.95
 
0.07
0.10
0.05
655
2,304
 
177
73.0
34.7
Keivitsa* 208 0.19 0.14   0.11 0.42 0.28 1,261 954   743 881 579

*unclassified deposit type


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The Penikat Intrusion is about 23 km long and up to 5 km wide, and is composed of a lowermost, predominantly ultramafic sequence (one fifth of the total) followed by gabbroic upper sequences. The full stratigraphy has been divided into five megacyclic units, each interpreted to represent a major new influx of mafic magma. There are two significant PGE-enriched layers, the Sompujärvi (SJ) and Paasivaara (PV) Reefs, located at the bottom and top of megacyclic unit IVrespectively. The SJ is 1 m thick on average, having generally 5 - 10 g/t precious metals, but having much higher concentrations locally (see Kirakkajuppura and Sompujärvi Reef north). The SJ Reef is encountered throughout the entire 23 km length of the Penikat Intrusion and has been test mined close to its northern end at Kirakkajuppura. The strike length of the PV Reef included in the resource estimate is 15 km.

The Suhanko Intrusion of the Portimo Complex hosts large contact-type PGE and base-metal enrichments in its marginal series, Ahmavaara, Suhanko, Vaaralampi, Niittylampi, Yli-Portimojärvi, and Konttijärvi. These contact-type deposits show exceptionally high PGM concentrations in places, relative to what is typically found in basal sulphide occurrences. They are represented by the Konttijärvi and Ahmavaara deposits, which in some respects resemble the Platreef of the Northern Bushveld Complex. Both Ahmavaara and Konttijärvi have been test mined.

The offset Cu-Pd deposits (Kilvenjärvi, Kilvenlatvalampi and Kilvenjoki) found below the Narkaus Intrusion of the Portimo Complex are structurally complex PGE+Au occurrences typically related to chalcopyrite veins and sulphide disseminations, but in places showing high grades with only little sulphides. The chalcopyrite veins are reported to contain up to 100 g/t PGE+Au.

The marginal zone of the Western Intrusion of the Koillismaa Complex is virtually mineralised throughout the entire c. 100 km long strike length. PGE grades vary but reach locally those of the Portimo Complex. Lavotta, Kuusijärvi, and Rusamo represent more base metal enriched mineralisation type while the Kaukua and Murtolampi have relatively higher precious metal grades. The Koillismaa Complex is regarded to have similar potential for principal PGE deposits as the Penikat and Portimo Complexes in the same Tornio-Näränkävaara Belt.

The Keivitsa and Nuottijärvi Intrusions are exceptions, representing PGE enrichments in younger intrusions, 2.06 Ga and 1.90 Ga in age, respectively. The PGE-bearing copper-nickel sulphides at Keivitsa were found in the mid-1980s during exploration by GTK. This high tonnage, low-grade deposit, contains pipe-like structures with grades of 1.01% Ni, 0.21% Cu and 2.4 g/t PGE. Numerous smaller PGE-bearing nickel deposits in komatiites and komatiitic basalts have been discovered in the Archaean greenstone belts in eastern Finland (Vaara, Hietaharju, Peura-Aho, and Kauniinlampi. Two examples of small PGE enrichments from southern Finland have also been entered in the database (Kalkkinen and Kätkytsaari). The database also includes some ‘first observation’ types of target like Näätävaara and Lomalampi.


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Last updated: 14.11.2014

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