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Rakkurijarvi |
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Norbotten, Sweden |
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Cu Au Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Rakkurijärvi group of magnetite-copper-gold mineralised breccias, which are hosted within the 2.05 to 1.90 Ga Proterozoic supracrustal sequence of Norbotten, northern Sweden, are located some 8 km SSW of the Kirunavaara iron mine.
The Kirunavaara, Svappavaara, Malmberget and many other major iron ore deposits, as well as Rakkurijarvi, and the Aitik, Viscaria, Pahtohavare and Nautanen Cu deposits of northern Sweden are hosted by Palaeoproterozoic rocks, mainly Karelian (2.5 to 2.0 Ga) and Svecofennian (1.9 to 1.88 Ga). These suites, which extend from northern Sweden into Finland and parts of northern Norway, were deposited in volcanic arc, back-arc basin and rift environments in a supra-subduction setting over an Archaean basement. The preserved Palaeoproterozoic rocks are found in a series of deformed supra-crustal belts of clastic sedimentary and basic to intermediate to felsic volcanic rocks.
The Palaeoproterozoic sequence commences with mainly tholeiitic to komatiitic volcanic rocks of the >1.9 Ga Greenstone Group, overlain by the Middle Sediment Group, locally represented by the basal Kurravaara conglomerate. This in turn is overlain by volcanic and sub-volcanic rocks of the Porphyry Group, which are locally the dominantly andesitic Porphyrite Group and the syenitic and quartz-syenitic Kiruna Porphyries. The latter host the Kirunavaara magnetite-apatite deposit.
The Porphyry Group is intruded by the Haparanda and Perthite calc-alkaline and alkali-calcic monzonite granite suites, associated with multiphase deformation and peak upper greenschist to lower amphibolite facies metamorphism during the Svecofennian orogeny from 1.9 to 1.8 Ga.
The multiphase deformation produced north-south trending, western side up, shear zones, steep to vertical dips of most of the strata, and south-east- plunging folds. A second phase of granitoid magmatism, the Lina suite, intruded these rocks at around 1.79 Ga, followed by the youngest plutonic rocks in the area, the ~1.71 Ga TIB 2 granitoids on the Swedish-Norwegian border.
Both regional and deposit scale scapolite and albite alteration has affected the Palaeoproterozoic rocks of the Kiruna district.
The Rakkurijärvi deposit occurs within a south-east plunging anticline which is bounded along its northern limb by an ENE trending directed shear zone. The Rakkurijärvi deposit is largely composed of sulphide-mineralised breccias within metavolcanic rocks of the Porphyry Group, associated with steeply dipping ENE shears as the Discovery, Tributary and Hangar breccia zones. The subsidiary Conglomerate and HB sulphide zones occur as replacements and disseminations in the Kurravarra conglomerate and Porphyry Group volcanic rocks respectively. Together these zone form a U shaped mineralised complex, open to the west, with Discovery to the NW, Tributary in the NE and two zones including Hangar and another to the SW and SE respectively on the southern limb. The Discovery Zone is characterised by a mineralogy of magnetite-chalcopyrite, whilst the other three zones are principally magnetite-pyrite (Smith et al, 2010). The main magnetite mineralisation in Rakkurijärvi, i.e., the two zones on the southern limb of the U shaped structure, has a strike of 60° and a 70 to 90°SE dip, while the Discovery Zone on the northern limb has a more complex structure with a general strike of 100°, and a dip that varies from 10 to 90°, and a plunge trending 45° to the east (van der Stijl, 2014).
In the Rakkurijärvi area, the Kurravaara conglomerate, which is at the unconformable base of the Porphyry Groups, consists of coarse-grained pebble conglomerates with clasts dominantly of local metavolcanic provenance, but with sparse clasts of polycrystalline quartz, interbedded with meta-arenitic units and rare marble bands. The conglomerates have been typically affected by hydrothermal alteration and replacement by biotite, actinolite, epidote, magnetite, and, rarely, pyrite and minor amounts of other sulphides.
Arenaceous layers within the conglomerate unit are predominantly composed of pink feldspar fragments within a matrix of finer rock fragment and hydrothermal minerals and a preserved relict sedimentary bedding and cross lamination. Clasts within the conglomerate have been partially altered to actinolite and magnetite. Within the deposit area the meta-volcanic rocks of the Porphyry Group have been affected by albite and silica alteration and are mainly pink to grey, feldspar phyric, porphyritic igneous rocks with a fine-grained matrix composed of plagioclase, quartz, biotite, sericite, and chlorite with accessory amounts of ilmenite, rutile, and magnetite. Partial biotite metasomatism usually accompanies zones of scapolitsation.
Minor units of calcite marble and calc-schists occur throughout the Porphyry Group, dominantly composed of biotite-tremolite marble with local significant quartz and epidote. In some places, particularly at the boundary between marble and intact biotite schist, a breccia of biotite schist with a carbonate matrix occurs, with the development of a more iron-rich amphibole in the matrix, locally accompanied by chalcopyrite and pyrite. Skarn containing magnetite is also developed locally along these contacts.
Brecciation has affected all the lithologic units of the Porphyry Groups, with magnetite breccias forming the main host rock of Cu-Au mineralisation. The breccias range from distal jigsaw textures to lithic breccias close to the core of the main mineralised zone.
The outer, least altered brecciated lithologic units in the meta-volcanic rocks comprise pink porphyry, with a fracture fill and matrix of biotite, actinolite and occasionally calcite, with minor magnetite and sulphides.
Breccias showing some degree of clast transport and rotation grade into jigsaw breccias. Secondary biotite rims clasts, with the proportion being higher in association with incipient magnetite alteration, and red feldspar overprinting some of the altered clasts. Tremolite and actinolite are locally developed in biotite schist within the shear zones.
Inwardly, these breccias grade into lithic breccias with a high degree of clast transport and rotation, through lithic-magnetite breccias to magnetite breccias. The matrix generally comprises actinolite and magnetite with minor quartz, epidote, albite, scapolite, calcite, and magnetite, or chlorite and magnetite. Clasts are commonly rimmed by actinolite-magnetite alteration, with local complete replacement of some clasts.
The main magnetite breccias comprise fine-grained (<1 mm) brecciated, magnetite rock in which the majority of clasts are from 20 to 30 cm in diameter containing minor actinolite. These matrices show some zonation throughout the magnetite breccias. Those adjacent to relatively unfractured volcanic rock have a matrix of calcite, actinolite and chlorite with minor epidote and biotite and accessory apatite, allanite, and titanite. Towards the centre of the brecciated zones the matrix consists of albite, actinolite and euhedral magnetite, with later cement.
The first alteration stage within the metavolcanic rocks appears to have been pervasive sodic albite alteration and silicification associated with the destruction of primary igneous biotite to produce a pink-red porphyry. In the shear zone, the original porphyritic nature of the rock is obliterated to produce albite-biotite schists with minor scapolite and magnetite or albite-actinolite and albite-actinolite- magnetite schists.
The albite alteration is overprinted by magnetite-actinolite, with the progressive alteration of lithic clasts to massive magnetite breccias. Within the breccias, actinolite is developed on the rims of clasts; albite and scapolite infill vug space with an accessory mineral assemblage which includes apatite, titanite, and allanite. Calcic and potassic minerals overprint the early albite and magnetite-actinolite-albite assemblages, including K feldspar, scapolite, and biotite in matrix and replacing feldspar phenocrysts. Outside of the main breccia zones this stage occurs as albite-magnetite-actinolite altered porphyry.
Biotite-scapolite schists are the dominant host rock types to the mineralised lithic and magnetite breccias of the Discovery zone, characterised by the formation of biotite rims on clasts, filling of fractures by scapolite in the breccias, while biotite-actinolite veins also reflect this stage of alteration. Quartz and calcite are associated with K feldspar and epidote veining representing a later stage of fluid flow associated with the deposition of the main sulphide assemblage. Epidote overprints all the previous stages and, together with minor calcite, replaces both phenocrysts and matrix in the metavolcanic sequence, commonly associated with reddening of relict feldspar.
The main sulphide deposition occurred syn- to post-calcite deposition. The main sulphide phases are chalcopyrite and pyrite, with rare molybdenite. In the strongly mineralised magnetite breccias, pyrite and chalcopyrite either form the breccia matrix or occur as late-stage filling in the matrix and fractures, with chalcopyrite succeeding pyrite deposition. Sulphides are also found in the lithic and jigsaw breccias, most commonly as a late matrix fill associated with calcite and locally magnetite. Minor hematite replacement of magnetite is locally associated with matrix sulphides within magnetite breccia. chalcopyrite and pyrite occur both as a replacement of clasts and in the matrix within mineralised Kurravaara conglomerate. Native copper and secondary copper carbonates occur in the most weathered portions of the deposit as a result of Pre-Quaternary weathering.
In summary, mineralisation is spatially associated with early sodic-calcic alteration (albite-actinolite- scapolite), which was overprinted by potassic alteration (K feldspar-scapolite-biotite) and finally low-temperature potassic alteration, largely characterised by calcite alteration and veining. The deposition of chalcopyrite accompanied the late stages of potassic alteration and the formation of calcite in veins and breccia matrices.
The timing of mineralisation is consistent with an association with the Perthite-Monzonite and Haparanda Suite granitoids and regional metamorphism in the area.
The deposit was described as comprising "a small open-pittable copper-gold deposit of modest grade" (Lundin Mining, 2008).
Billström et al. (2010) quote a 'size' of 1.4 Mt @ 0.25% Cu for the Rakkurijärvi prospect. Smith et al. (2010) described best intersections of 43 m of 0.83% Cu, 0.05 g/t Au and 40.4 m of 1.41% Cu, 0.33 g/t Au.
Kiruna Iron AB, reported a JORC compliant Inferred Mineral Resource of 69.7 Mt of 28.5% Fe for the magnetite mineralisation at Rakkurijärvi, and 10.9 Mt of 38.8% Fe, 0.31% Cu in the Discovery Zone, both at a cut-off grade of 20% Fe (Lindholm 2012 quoted by van der Stijl, 2014).
The description is largely summarised from Smith et al., 2007
The most recent source geological information used to prepare this decription was dated: 2014.
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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Billstrom, K., Eilu, P., Martinsson, O., Niiranen, T., Broman, C., Weihed, P., Wanhainen, C. and Ojala, J., 2010 - IOCG and Related Mineral Deposits of the Northern Fennoscandian Shield: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide v.4 pp. 381-414
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Smith M, Coppard J and Herrington R, 2010 - The Geology of the Rakkurijärvi Copper-Prospect, Norrbotten County, Sweden: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide v.4 pp. 427-440
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Smith M, Coppard J, Herrington R and Stein H, 2007 - The Geology of the Rakkurijarvi Cu-(Au) Prospect, Norrbotten: A New Iron Oxide-Copper-Gold Deposit in Northern Sweden: in Econ. Geol. v102 pp 393-414
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Storey, C.D. and Smith, M.P., 2017 - Metal source and tectonic setting of iron oxide-copper-gold (IOCG) deposits: Evidence from an in situ Nd isotope study of titanite from Norrbotten, Sweden: in Ore Geology Reviews v.81, pp. 1287-1302.
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van der Stijl, I., 2014 - Mineral Chemical Comparison between Rakkurijoki, Rakkurijärvi and Discovery Zone FeOx Occurrences, Kiruna District, Northern Sweden: in A Master of Science thesis presented to, Lulea University of Technology, Department of Civil, Environmental and Natural Resources Engineering, 93p.
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