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Outokumpu District - Keretti, Vuonos, Luikonlahti |
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Finland |
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Cu Zn Co Au Ag Ni
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Outokumpu copper mine, which commenced operation in 1913 but was exhausted and closed in 1988, is located in the town of Outokumpu in eastern Finland, some 350 km north-east of Helsinki and 100 km west of the Russian border. The Vuonos mine which is 4 km north-east of Outokumpu was discovered in 1965 and mining commenced by open cut in 1972. The Luikonlahti deposit is some 27 km north-west of Outokumpu and was mined from 1971, firstly as an open cut, but then underground.
The Outokumpu district deposits are located within the Fennoscandian Shield, close to the boundary between Proterozoic and Archaean rocks. They lie within the Karelian Schist Belt between the Archaean basement and the Svecofennian island arc domain schists which are dated at 1.93 to 1.80 Ga. All of these sequences are cut by extensive Svecofennian orogenic granitoids and late Precambrian rapakivi granites.
The Karelian Schists are carbon and sulphur rich and occuppy an interval of 0 to 100 km in width between the Svecofennian and the basement Archaean schist, paragneiss, granite gneiss and granulites. The Karelian schist sequence commences with between 0 and 200 to 400 m of the Jatulian Formation comprising a basal sericite quartzite followed by two units of orthoquartzite which are separated by local arkosites and conglomerates. These quartzites are in turn overlain by phyllites, dolomites and black schists and are cut by 2150 Ma dolerite dykes. The Jatulian is followed by the Kalevian sequence of phyllites, mica gneisses and intercalated black schists - the main black 'Karelian schists'. The 'phyllites' are graphitic and sulphide bearing phyllites and greywackes composed of quartz, biotite and plagioclase. The black schists occur in the mica gneisses and phyllites and are generally narrow, from a few to a few tens of metres, and exceptionally up to 500 m thick, with a much higher content of graphite and sulphide than the normal phyllites.
Enclosed within the Karelain schists are numerous ophiolitic meta-serpentinite and serpentinised metaperidotite bodies, which may be separate from or closely associated with 1.95 to 1.97 Ma gabbroic or basaltic rocks. The Outokumpu mineralisation is closely associated with these mafic assemblages. The Outokumpu serpentinites represent mainly dunitic-hartzburgite mantle peridotites in a geological framework that suggests emplacement on the basal parts of major thrust nappes.
The association of serpentinite, dolomite (which has gradational contacts with the serpentinites and is composed of un-banded dolomite and magnesite, believed to represent metasomatic alteration associated with the serpentinite), skarn (including chrome-diopside, chrome-tremolite and zincian chromite with sporadic uvarovite, eskolaite and chromian-epidote and minor pyrrhotite and pentlandite) and quartz-rock (a non-detrital siliceous chemical sediments with low order disseminated pyrrhotite, pentlandite, pyrite and zincian chromite) are common in northern Karelia, and are known as the Outokumpu complex. The serpentintes are conformable with the enclosing mica gneiss and are discontinuous and sinuous and follow the folds in the gneiss around domes of pre-Svecokarelian basement, varying from a few hundred to 2000 m above the domes. These serpentinite lenses have been traced over a strike length of more than 240 km within the region. The associated black schists produce magnetic and electromagnetic anomalies. Regionally, this package contains variable mineralisation, occurrences and deposits of Cu, Ni, Co.
The Outokumpu ores closely follow the folding and structure of the host sequence in gneral and remain close to 200 to 400 m above the footwall mica gneiss. They are hosted by quartz rocks, calc-silicates and black schists. The associated serpentinites are surrounded by a halo of dolomites, calc-silicates, quartz rocks, black schists and mica schists which are interpreted to be alteration products of the serpentinites and/or their wall rocks.
Hydrothermal quartz is the commonest gangue, although serpentinite, dolomite and skarn are also found in contact with the ore, particularly in the hangingwall at the down dip margin of the orebody. In general, the up-dip margin of the orebody is enveloped by quartz rocks, and rarely by mica schist, while the down dip edge penetrates into serpentinites. The ore has sharp contacts with the wall rocks, separated from the serpentinite by chloritised shears, with discordant offshoots penertrating the wall rocks. The ore is almost massive sulphide, with around 58% ore minerals (23.2% pyrrhotite, 21% pyrite, 11% chalcopyrite, 1.7% sphalerite), and quartz the dominant gangue mineral. It occurs as both massive and banded ores. Pyrite rich banded ore predominates in the centre and upper footwall, while pyrrhotite dominant massive ore occurs in the lower edge and near the hangingwall edge, as well as the tapering margins. Banding within the ore is defined by the distribution of sulphides and quartz, and rarely by the skarn minerals. Close to the boundaries the banding parallels the contacts, although in the in the interior, intense folding is visible, in places strong enough to produce breccia ores. Massive ores exhibit weakly developed, un-oriented banded to streaky textures
Three main mines have been operated in the Outokumpu district, Keretti (the main Outokumpu orebody) Luikonlahti 27 km to the NW, as well as Vuonos 4 km to the NE. The Keretti mine exploited the main Outokumpu deposit which is composed of two SW-NE trending, and slightly overlapping tabular bodies, separated by a thrust fault. The south-western body is known as Leitukka and the north-eastern is Kumpu. The total length of the two lenses is 4000 m, although they only average 200 to 400 m in width, tapering at depth to the south-west to 150 m in width. They vary from a few metres in thickness up to 40 m, but average 10 m. They dip at 30 to 60° SE to the north-east and flatten to almost horizontal at depth to the south-west.
The orebody at Vuonos was totally concealed at a depth of 50 to 250 m. It was 3500 m in length and 50 to 200 m wide with an average thickness of 5 to 6 m. Like the main Outokumpu deposit it was thickest and widest near the centre, thinning and dying out towards its margins.
Quartz rock (sub-monominerallic quartzites occurring as parallel beds of variable thickness, separated from serpentinite by dolomite and calc-silicates) is the most common host, although ore also passes through dolomites, calc-silicates and serpentinites. In general the upper margins of the ore is enveloped by quartz rock, while the lower penetrates serpentinite, although there appears to be no contact effects between the ore and wall rocks. Contacts are sharp, and with the serpentinite are often sheared and chloritised. Some offshoots are found in the serpentinites, eg. the 500 m long and 0.5 to 1.5 m thick satellite below the Kumpu lens. At Vuonos the hosts are quartz rock (90%) and black schist (10%), while Luikonlahti is within a suite of quartz rock (60%) and calc-silicate (40%).
Two main categories of sulphide are found at Outokumpu, namely: 1). massive Cu-Zn-Co ores which contain >50% sulphide and 38% SiO2, and are in turn divided into the pyritic and pyrrhotitic ores. The large Keretti deposit (Outokumpu) is pyritic, while Luikonlahti and Vuonos are pyrrhotitic. Vuonos also differs from Keretti in that it has proportionally more Zn and Ag and lesser Cu, Co and Au. 2). disseminated low grade Ni-Cu-Co.
The main minerals are pyrite, pyrrhotite, chalcopyrite and sphalerite with accessory pentlandite, cubanite, mackinawite, magnetite and stannite. The Co is present in the lattice of the pyrite, as Co bearing pentlandite and pyrrhotite and rarely as cobaltite. Breccia ores contain clasts of quartz and pyrite rich material with chalcopyrite and pyrrhotite mobilised into the interstices between clasts. Pyrite porphyroblasts within the pyrrhotite ore were broken during deformation and chalcopyrite, pyrrhotite, sphalerite and occassionally galena were remobilised into fractures in the wall rocks.
The pre-mining ore reserves (mined and remaining) in the Outokumpu district totalled:
50 Mt @ 1.2 to 3.8% Cu, 0.6 to 1% Zn, 0.1 to 0.2% Co, 0.1% Ni, 0.1 to 0.8 g/t Au and 1 to 9 g/t Ag (Loukola-Ruskeeniemi, 1999).
Production from the largest of the three mines, Keretti, was:
28.5 Mt @ 3.8% Cu, 0.24% Co, 1.07% Zn, 0.8 g/t Au, 8.9 g/t Ag, 0.12% Ni, 25.3% S, 28.11% Fe (Loukola-Ruskeeniemi, 1999).
Production at Outokumpu to the end of 1974 was 22.6 Mt @ 3.5% Cu (Isokangas, 1978).
Total production+reserves at Vuonos, 1993 was 6 Mt @ 2.5% Cu, 1.6% Zn, 0.13% Ni, 0.15% Co, 18% S.
Total production+reserves at Luikonlahti, 1993 was 7.7 Mt @ 1.2% Cu, 0.65% Zn, 0.09% Ni, 0.12% Co, 20% S.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1999.
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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Isokangas P 1978 - Finland (extracts on the North Karelia Copper District and the Outokumpu District deposits): in Bowie S H U, Kvalheim A, Haslam H W, (Eds), 1978 Mineral Deposits of Europe IMM, The Min. Soc. v1, Northwest Europe pp 39-53, 90-92
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Loukola-Ruskeeniemi K 1999 - Origin of black shales and the Serpentinite-associated Cu-Zn-Co ores at Outokumpu, Finland: in Econ. Geol. v94 pp 1007-1028
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Mikkola A K, Rouhunkoski P 1980 - The copper deposits and their metallogeny in southern Finland: in Jankovic S, Sillitoe R H (Eds.) 1980 European Copper Deposits Proceedings of an International Symposium held at Bor, Yugoslavia, 18Ð22 September 1979. Belgrade University, pp 180-187
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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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