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Suurikuusikko, Kittila Mine |
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Finland |
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Au
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Super Porphyry Cu and Au
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The Suurikuusikko orogenic gold deposit at the Kittilä mine is located in the Lapland Belt of central Lapland, ~50 km NE from Kittilä in northern Finland (#Location: 67° 54' 5"N, 25° 23' 27"E).
The deposit was discovered in 1986, with open pit and underground exploitation commencing in 2007 as the Kittilä Mine. The total ore tonnage mined to the end of 2011 was 3.054 Mt @ 5.12 g/t Au for 10.720 t of contained gold. Reserves and resources to the same date totalled 55.50 Mt @ 4.13 g/t Au (GTK, 2012).
Regional Setting
The Suurikuusikko deposit lies within the Fennoscandian/Baltic Shield of northern Finland, which also extends into Norway, Sweden and northwestern Russia. The oldest rocks of the Fennoscandian Shield are of Archaean age, located in the northeast, in the Kola Peninsula, Karelia and northeastern Finland, and comprise mainly gneisses and subordinate greenstone rocks dated at 2.5 to 3.0 Ga. This area is partially covered by Karelian Palaeoproterozoic rocks ranging in age from 2.5 to 1.9 Ga and the Lapland granulite belt, dated at 1.9 Ga. Only minor areas of Archaean rocks outcrop in northernmost Sweden, although Archaean crust probably underlies much of that area.
Most of northern and central Sweden, and NW and SW Finland, including the Kittilä region, belong to the Svecofennian province, which consists of metasedimentary and metavolcanic rocks and several intrusive generations that were formed and deformed by the 1.9 to 1.75 Ga Svecofennian orogeny. These rocks host the metallic deposits of the Skellefte and Norrbotten districts in Sweden, and are commonly intruded by younger 1.65 to 1.5 Ga Rapakivi granites, and are overlain by younger 1.5 to 1.2 Ga Jotnian sandstones.
The 1.8 to 1.65 Ga Trans-Scandinavian igneous belt in the west of the Svecofennian province, is composed of largely undeformed granitoids and associated intrusions, and was emplaced in at least three different episodes during this period, extending from southern Sweden to northern Scandinavia.
The Southwestern gneiss, or Sveconorwegian province, is located to the west of the Trans-Scandinavian igneous belt. It comprises rocks that were formed during the 1.70 to 1.55 Ga Gothian orogeny, but that were intruded by several generations of intrusions and underwent a complex evolution between 1.7 and 0.9 Ga. In western Norway, these gneisses were again deformed during the ~400 Ma Caledonian orogeny.
The Scandinavian Caledonides, which extend through most of Norway and include adjacent parts of Sweden, are composed of Neoproterozoic to Silurian metasedimentary and metavolcanic rocks, thought to have been deposited in the Iapetus Ocean between 700 to 400 Ma. Together with slices of older basement, these rocks were thrust several hundred km eastwards over the edge of the Fennoscandian Shield during the Caledonian orogeny.
District and Deposit Geology
The Suurikuusikko deposit is hosted by Palaeoproterozoic rocks of the Svecofennian province. It lies within mafic volcanic and sedimentary rocks ascribed to the Kautoselkä or Porkonen Formation of the >2000 Ma Kittilä Group of the Palaeoproterozoic Central Lapland Greenstone Belt.
In this part of northern Finland, bedrock is typically covered by a 1 to 10 m thick, but uniform blanket of unconsolidated glacial till, with only scarce and irregularly distributed bedrock exposures. There are no natural bedrock exposures in the immediate vicinity of the gold mineralisation in the Kittilä mine area. Overburden thickness varies along the main north-south axis of the deposit, from 2 m in the south, to 7 m in the north, but reaches 15 m to west. It is characterised by well graded and packed till containing angular boulders of underlying bedrock, and is, in turn, overlain by discontinuous sand and gravels formed from scouring and washing of the till unit and by a peat layer 1 to 2 m thick.
In the mine area, the Kittilä Group comprises volcanic and sedimentary rocks metamorphosed to greenschist assemblages (chlorite-carbonate), which strike north to NNE and are near-vertical. The volcanic rocks have been subdivided into iron-rich and magnesium-rich tholeiitic basalts. The iron-rich tholeiitic basalts which represent the oldest rock unit, are ascribed to the Kautoselka Formation, and occur primarily in the western part of the deposit area. The eastern portion of the deposit area is characterised by rocks of the Vesmajarvi Formation composed of magnesium-rich tholeiitic basalt, coarse volcaniclastic units, graphitic schist and minor chemical sedimentary rocks.
The contact between the Kautoselka and Vesmajarvi formations is represented by a transitional zone, the Porkonen Formation, comprising mafic tuffs, graphitic metasedimentary rocks, black chert and banded iron formation, which varies from 50 and 200 m in thickness between. This formation is characterised by strong heterogeneous penetrative strain, narrow shear zones, breccia zones and intense hydrothermal alteration (carbonate-albite-sulphide) and by the gold mineralisation on the Suurikuusikko deposit. Minor mafic dykes intrude the Porkonen Formation, two of the more extensive of which are mafic in composition and have been traced across several hundred metres of strike length (north-south). These dykes range in thickness from a few, up to 65 metres.
The Porkonen Formation, which includes the Suurikuusikko Gold Trend, is the main host for the gold mineralisation within the Kittila Mine. The distribution of sub-units within the formation is very irregular, attenuated by intense deformation, and consequently are difficult to correlate between drill holes. The internal geometry of the Porkonen Formation is very complex and exhibits features consistent with those observed in major brittle-ductile deformation zones such as heterogeneous strain, strong planar cleavages, oblate strain features, narrow shear zones, striated slip surfaces, transposition and minor disrupted folding, suggesting the unit represents a significant structural discontinuity where strain was concentrated during regional deformation. The Suurikuusikko Trend shear zone is clearly evident airborne geophysical data images, and has been traced by drilling over a strike length of >15 km along a NNE trend.
Mineralisation
The known gold mineralisation in the Kittilä Mine is almost exclusively refractory, occurring as sulphide-rich rock (up to 30% or more disseminated sulphides, but averaging 10%) with variable textures, alteration and deformation characteristics. The main sulphide minerals are arsenopyrite, arsenic-rich pyrite and subordinate amounts of pyrrhotite. Gold particles are locked inside fine-grained arsenopyrite (~73% of the gold) or arsenic-rich pyrite (~23% of the gold). Only very minor gold occurs as very fine free grains in the hydrothermally altered rock.
The sulphides are commonly associated with abundant carbon (amorphous or graphite) and intense silica, albite and carbonate alteration. The mineralisation is found primarily as replacement bodies that were emplaced during active brittle-ductile deformation. Deformation continued after the mineralisation, resulting in overprinting deformation which modified the geometry of the gold replacement zones, although despite the variability of form and style, the resulting gold mineralisation zone is generally very consistent. The gold mineralisation is generally very easily distinguished visually, primarily because of its arsenopyrite content and visible alteration patina, whereas the original host rock is often very difficult to determine. Therefore, the continuity and the boundaries of the auriferous zones are defined and inferred from alteration, sulphide content and assay data.
Gold mineralisation is largely confined to a tabular zone of the Suurikuusikko Trend extending over 15 km in strike length. Within this area, eight main auriferous zones have been outlined over a 4.5 km strike length, defining the Kittilä mine. These zones each have reliable continuity, grade and thicknesses. From south to north they are:
• Ketola, the southernmost zone, tested over a strike length of 960 m, containing seven lenses. The bulk of the mineral resource is between surface and 200 m depth. The average thickness of these lenses is 4.5 m, ranging between 3 and 9 m.
• Etela, with a 400 m strike length, tested to a depth of 400 m. The average thickness of the four individual lenses of this zone is 3.1 m, ranging from 3 to 4 m, although some lenses overlap and merge to produce a greater combined thickness.
• Suuri (Western, Central and Eastern subzones), the most important zone in 2010. Each of the three subzones is composed of several subparallel lenses, each of which can occur (alone, in pairs or together) within a 100 to 200 m wide corridor within the main Suurikuusikko structure. These lenses have been traced almost continuously for over 1300 m north-south and 300 to >600 m vertical extent, with three to five sub-parallel, irregular, tabular lenses of mineralisation generally present together. Gold mineralisation occurs in 33 lenses. The average thickness of the lenses is 5.6 m, ranging from 3 to 32 m. Where lenses merge together the combined thickness is greater. The lowest level of the Suuri Zone is at 675 m. The next level (700 m) below belongs to Suuri/Roura Extension zone.
• Suuri/Roura Extension, which is the 1700 m long downward extension to >1100 m depth of the Eastern envelope of the Suuri Zone, comprising several parallel lenses that also continue northward below the Roura-C zone. Mineralisation occurs as 19 subparallel, irregular, tabular lenses, each with an average thickness of 6.7 m, ranging between 3 and 35 m. The ore lenses appear to decrease in thickness and grade to below cut-off in the south.
• Roura-C (Central Rouravaara), which extends for 750 m along strike, ti a depth of ~650 m. Gold mineralisation occurs in 21 lenses, which individually average 4.8 m in thickness, ranging between 3 and 18 m, although where one or more overlaps and merge may be thicker.
• Roura-N (North Rouravaara), which extends over a 670 m north-south strike length and to a depth of >600 m. Gold mineralisation occurs in 13 lenses, which individually average 3.9 m in thickness, ranging between 3 and 9 m, although where one or more overlaps and merge may be thicker.
• Rimpi-S, which extends over 1380 m north-south and over a vertical interval of >600 m. Gold mineralisation occurs in seven lenses, which individually average 4.3 m in thickness, ranging between 3 and 9 m, although where one or more overlaps and merge may be thicker.
• Rimpi-N (South and North Rimminvuoma), the northernmost auriferous zone currently known (2010) along the main Suurikuusikko Trend within the Kittilä Mine are, extending over a strike length of 450 m and over a vertical interval of 400 to 600 m.
At the end of December 2009, the mineral reserve and mineral resource models included a total of 76 distinct auriferous lenses within these seven zones. The average content of sulphide was around 10%, ranging from 2 to 30%, with no major differences in terms of mineralogy. Ketola and Etela are isolated on the southern margin of the trend, separated by < 400 m gaps, although the remaining zones are largely contiguous.
Reserve and resource estimates as of 31 December, 2013 were as follows (Agnico Eagle published reserve-resource release, 2014):
proved ore reserve - 1.104 Mt @ 4.27 g/t Au (underground);
probable ore reserve - 30.520 Mt @ 4.65 g/t Au (underground);
measured mineral resources - 0.511 Mt @ 2.69 g/t Au (underground);
indicated mineral resources - 10.519 Mt @ 2.79 g/t Au (underground);
inferred mineral resources - 7.265 Mt @ 4.12 g/t Au (underground);
inferred mineral resources - 0.257 Mt @ 4.0 g/t Au (open pit);
Note: reserves are not included in resources.
Patten et al. (2023) claim Kittilä is the largest Au mine in Europe with ~302 t Au 'reserves'.
Note: This summary is largely drawn from Doucet, D., Girard, D., Grondin, L. and Matte, P., 2010 - Technical Report on the December 31, 2009, Mineral Resource and Mineral Reserve Estimate and the Suuri Extension Project, Kittila Mine, Finland, Prepared for Agnico-Eagle Mines Limited, 126p.
The most recent source geological information used to prepare this decription was dated: 2010.
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.
Suurikuusikko
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Selected References:
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Niiranen, T., Lahti, I. and Nykanen, V., 2015 - The Orogenic Gold Potential of the Central Lapland Greenstone Belt, Northern Fennoscandian Shield: in Mineral Deposits of Finland Chapter 10.2, pp. 733-752. doi.org/10.1016/B978-0-12-410438-9.12001-1
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Patten, C.G.C., Molnar, F., Pitcairn, I.K., Kolb, J., Mertanen, S. and Hector, S., 2023 - Multi-source and multi‑stage metal mobilization during the tectonic evolution of the Central Lapland Greenstone Belt, Finland: implications for the formation of orogenic Au deposits: in Mineralium Deposita v.58, pp. 461-488. doi.org/10.1007/s00126-022-01133-z.
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Sayab, M., Molnar, F., Aerden, D., Niiranen, T., Kuva, J. and Valimaa, J., 2020 - A succession of near-orthogonal horizontal tectonic shortenings in the Paleoproterozoic Central Lapland Greenstone Belt of Fennoscandia: constraints from the world-class Suurikuusikko gold deposit: in Mineralium Deposita v.55, pp. 1605-1624.
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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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