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Fennoscandian Iron Oxide Copper Gold Province

Sweden

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The Fennoscandian Iron Oxide Copper Gold Province is located within the northern Fennoscandian Shield in the Kola Peninsular of Russia, and in Finland, and northern Sweden and Norway. It largely comprises Archaean and Palaeoproterozoic rocks, with the bulk of known economic mineral deposits restricted to the latter (Billström et al., 2010).

Crustal Setting

  The Meso- and Neoarchaean Kola and Karelian cratons in the east, are composed of 3.5 to 2.9 Ga gneissic tonalite-trondhjemite-granodiorite (TTG), followed by 2.9 to 2.6 Ga greenstones, calc-alkaline volcanic rocks and further TTG magmatism, before consolidation after the last major stage of granitoid intrusion at 2.65 Ga.
  The early Palaeoproterozoic interval, between 2.5 and 1.9 Ga, was characterised by extension/intracratonic rifting and sedimentary basin evolution, and recurrent mantle plume activity, resulting in numerous komatiitic eruptions, layered intrusions (komatiitic, picritic and tholeiitic to calc-alkaline) over the partly concealed Norrbotten Terrane in the west and the contiguous Karelian and Kola blocks to the east. No indication of accretionary phases or formation of major new felsic crust is indicated during this period. The large layered intrusions (hosting significant chromium, nickel, titanium, vanadium and/or PGE mineralisation) represented a major magmatic input between 2.45 and 2.39 Ga, sometimes with associated high grade granulite facies metamorphism of the intruded rocks, while major basaltic and komatiitic stages (with intercalated arenites) which occurred between 2.2 and 2.0 Ga were associated with extensional pulses (Mutanen, 1997; Lehtonen et al., 1998; Rastas et al., 2001). A change took place between 1.96 and 1.75 Ga, during the commencement of assembly of the Nuna/ Columbia supercontinent, when felsic and calc-alkaline andesites and related volcaniclastic sedimentary rocks were deposited in subaerial to shallow water settings, mainly over the Norrbotten Terrane. These rocks were underlain by significant granite, syenoid, dioritoid and gabbroid intrusions. This series involved strong reworking of older crust within the Karelian craton and Norrbotten Terrane. It commenced with 1.90 to 1.86 Ga grey, deformed, metaluminous, tonalite and granodiorite, with associated gabbros, diorites and rare true granites. The Archaean basement, supracrustal rocks and up to 10 km thick pile of Palaeoproterozoic sedimentary and volcanic rocks to the southwest of, and lapping onto the Archaean cratons, were multiply deformed and metamorphosed during the intrusion of these voluminous granitoids and associated gabbros.
  These intrusions were followed by a 1.88 to 1.86 Ga undeformed calc-alkaline quartz monzonite-adamellitegranite suite, and then between 1.85 and 1.75 Ga, by major batholithic developments of S-type granite, migmatite and pegmatite in the core of the amalgamated Norrbotten- Karelian craton. Finally, two pulses of extensive north-south oriented anatectic A- and I-type quartz monzodioritequartz monzonite-adamellite-granite batholiths at ~1.8 and ~1.7 Ga, were generated during another major stage of deformation and metamorphism, extending into, and mainly within, the Svecofennian terrane to the south (Billström et al., 2010 and sources quoted therein). These two sets of granitoids have calc-alkaline to alkali-calcic compositions (Wanhainen and Martinsson, 2010). These intrusive phases were followed by, or overlapped with the interpreted accretion of several volcanic arc complexes, associated with an inferred subduction zone along the SW margin of the Norrbotten and Karelian cratons. Lahtinen et al. (2005) suggest this phase of subduction involved five partly overlapping pulses and the amalgamation of several microcontinents and island arcs onto the southwestern margins of the combined Archaean Karelian craton and Norrbotten Terrane. The major magnetite-apatite deposits at Kiirunavaara and Malmberget, as well as the epigenetic copper-gold deposits of the region, are hosted by the geographically restricted 1.89 Ga Kiirunavaara Group (formerly Kiruna Porphyries) and equivalents that overlie Archaean basement of the Norrbotten Terrane in the Kiruna-Gällivare district (Weihed et al., 2005). The host Kiirunavaara Group volcanic rocks are interpreted to be comagmatic with the 1.88 to 1.86 Ga quartz monzonite-adamellite-granite suite described above. These deposits are located ~250 km to the northeast of the northwest-southeast trending Proterozoic to Archaean basement boundary and inferred subduction zone. Prior to the initiation of subduction, the same district lay within a northeast-trending extensional rift trough (Wanhainen and Martinsson, 2010).
  The volcanic rocks of the Kiirunavaara Group, have trachyandesitic to rhyodacitic compositions, and are taken to represent an early and chemically distinct phase compared to the dominant calc-alkaline andesites and related volcaniclastic sedimentary units of the subduction complex to the southwest. However, recent dating (Storey et al., 2007) suggest they may have started to develop much earlier at ~2.05 Ga, during the extensional phase. For more detail and illustrations, see Billström et al. (2010), and Wanhainen and Martinsson (2010).
Fennoscandian setting

Geological summary of the Fennoscandian Shield, northern Norway, Sweden, Finland and Russia, showing the location of the significant magnetite-apatite deposits and IOCG sensu stricto mineralisation. After Billström et al., 2010 and sources quoted therein, and Lahtinen et al., 2011.


District-scale Alteration and Mineralisation

  Large-scale fluid migration of variable salinity, from ~2 Ga, during the multiple stages of pre- and synorogenic magmatism, metamorphism and deformation resulted in regional scapolite, K feldspar-phlogopite, albite and albitecarbonate alteration (Billström et al., 2010). Regional scapolite has been variously attributed to felsic intrusions (ödman, 1957) and to mobilised evaporites from supracrustal successions (Tuisku, 1985; Frietsch et al., 1997; Vanhanen, 2001). Fluid inclusions from areas of regional albitescapolite alteration in Fennoscandia, indicate Ca-Na-Cl brines with salinities of 30 to 40 wt.% NaCl
equiv. and temperatures of 500 to 200°C (Billström et al., 2010).
  Around 40 iron oxide-apatite deposits are known within northern Norrbotten, with ~1600 Mt of ore having been mined from 10 of these during the last 100 years. These deposits are mainly restricted to the volcanic rocks of the Kiirunavaara Group, with very few occurrences known outside the immediate Kiruna-Gällivare area (Billström et al., 2010).
  The Norrbotten - Kiruna Region, hosts the major Kiirunavaara, Malmberget and Gruvberget iron oxide-apatite deposits, and the smaller Viscaria Cu deposit. In the Kiruna district there is a progression upwards from albite-rich sodic alteration at depth, to intermediate potassic (K feldspar-sericite) to sericite-quartz at shallower levels. In the footwall of the main, conformable, tabular magnetite-apatite body at Kiirunavaara, the dominant assemblage is magnetite-albite-actinolite-chlorite, with the density of veins carrying these minerals increasing towards the base of the ore. The orebody is primarily composed of magnetite, with intergrown apatite, actinolite and minor quartz. It comprises a 5 km long by 100 m thick body that persists for at least 1500 m down dip. The ore is bimodal, with high (>1% P) and low (<0.05% P) apatite ore. The apatite-rich ore is locally banded and predominates in the hanging wall and peripheral parts of the deposit, and in varying amounts in the footwall, whereas the apatite-poor variety is found close to the footwall, as slightly irregular and branching bodies of massive and fine-grained magnetite ore (Billström et al., 2010). Magnetite-actinolite veining/ brecciation is found in both the hanging wall and footwall, including large blocks of volcanic wall rocks that may be partially or wholly replaced by actinolite. Blocks of wall-rock within the ore may be replaced by albite or have albitite rims. The deposit follows the contact between the underlying pile of trachyandesitic lava and hanging wall pyroclastic rhyodacite. The hanging wall is locally affected by biotite-chlorite alteration, commonly accompanied by weak enrichment of copper, cobalt and molybdenum. Postore alteration is evident in dykes that transect the deposit, and are altered to K feldspar, sericite and disseminated hematite, while hematite ±quartz, barite and/or fluorite veins cut the ore. The upper and lower contacts of the small, conformable magnetite-hematite (Per Geijer ores) lenses at the top of the rhyodacite are occupied by breccias of volcanic clasts in a hematite, magnetite and/or apatite matrix, with highly K feldspar-sericite-silica altered wall rocks (Billström et al., 2010; Hitzman et al., 1992). Billström et al. (2010), conclude that most features of the Kiirunavaara ores are compatible with both a magmatic magnetite intrusive origin, and over-printing by hydrothermal processes. In the deposit area, there is evidence of pre-ore metasomatic magnetite-albite-actinolite-chlorite alteration and mineralisation, followed by massive, columnar jointed magnetite-apatite ore with the characteristics of a magnetite magma, overprinted by further metasomatic magnetite mineralisation and associated alkalic alteration.
  The Palaeoproterozoic Aitik hybrid porphyry copper-gold/ IOCG deposit is located ~200 km northeast of the northwest-southeast-trending Archaean-Proterozoic palaeoboundary in the Fennoscandian shield. The deposit, which is located ~16 km southeast of Malmberget, is considered to have formed at ~1.9 Ga, in a volcanic arc environment over the cratonic margin, related to distal subduction of oceanic crust from the southwest, beneath the Archaean craton. The country rocks comprise metamorphosed intermediate volcanic and clastic sedimentary rocks that were intruded by plutonic rocks of granitic, dioritic and gabbroic composition. Aitik is associated with the major, long-lived, northnorthwesttrending Nautanen deformation corridor of multiple shear zones (Wanhainen and Martinsson, 2010). The rocks at Aitik have experienced at least two metamorphic events (Bergman et al., 2001), and four main phases of alteration (Wanhainen and Martinsson, 2010). An early, pre-metamorphic potassic porphyry alteration took the form of replacement of amphibole by biotite, and microcline growth in the groundmass. The second phase comprises minerals characteristic of amphibolite facies peak metamorphic conditions (e.g., amphibole and garnet), followed by a third assemblage indicative of retrograde conditions (e.g., biotite, sericite, chlorite, epidote and calcite) that are widespread and common within the groundmass of most of the rocks. Chlorite and sericite are abundant in the footwall and hanging wall rocks, while biotite and sericite alteration dominate in the ore zone. The fourth and final phase is characterised by K feldspar, magnetite, scapolite, amphibole, tourmaline, garnet, muscovite, apatite, allanite and quartz, occurring locally within all rock types, together with chalcopyrite and pyrite.
  A metamorphosed quartz monzodioritic intrusion, related to the early porphyry copper-gold mineralisation, is situated in the footwall of the deposit. The ore zone comprises biotite and quartz-muscovite-sericite schists towards the footwall and hanging wall respectively, with disseminated and quartz-stockwork hosted chalcopyrite and pyrite. High salinity fluids (30 to 38
equiv. wt.% NaCl + CaCl2) were released during emplacement of the intrusion at ~1.89 Ga, resulting in the observed mineralisation, veining and alteration of the intrusive and surrounding volcaniclastic rocks. Remnants of the primary porphyry copper mineralisation are best preserved in the footwall intrusion, in intrusive units within the volcaniclastic rocks of the ore zone, and in quartz stockworks at the margins of the quartz monzodiorite. An overprinting mineralisation and alteration event characteristic of an iron oxide-alkali altered mineralised system occurred ~100 m.y. later, accompanying compression, monzonitic-granitic magmatism, ductile deformation, and block movements across northern Norrbotten. Magnetite and sulphide enrichments, are found within the deposit, locally occurring as disseminations and within late veins of mainly amphibole, K feldspar, tourmaline, garnet, quartz and epidote, together with late scapolite alteration (Wanhainen and Martinsson, 2010 Billström et al., 2010).
  These mineral assemblages are associated with regionally extensive sodic-calcic alteration which is distributed throughout the entire northern Norrbotten ore province, although extensive Phanerozoic and superficial cover do not allow the continuity of this alteration to be established. However, similar, characteristic, scapolite-, amphibole and K feldspar-rich mineral assemblages are extensively developed throughout the province, overprinted by intense K feldspar alteration in areas of copper-mineralisation and along deformation zones (Edfelt, 2005).
  This implies that the widely circulated fluids responsible for iron-oxide copper-gold and related mineralisation, and extensive sodic-calcic alteration in the region during this tectonic event, also affected the Aitik deposit, and probably involved addition of copper and gold. This late mineralising fluid was highly saline (30 to >60
equiv. wt.% NaCl + CaCl2) and contained solids of ferropyrosmalite and hematite. The preceding was summarised from Wanhainen and Martinsson (2010).
  Other deposits and mineralised occurrences in the province include Pahtohavare and Rakkurijärvi

The most recent source geological information used to prepare this summary 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.


  References & Additional Information
   Selected References:
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
Porter T M,  2010 - Current Understanding of Iron Oxide Associated-Alkali Altered Mineralised Systems: Part II, A Review: in Porter T M, (Ed),  2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide   v.3 pp. 33-106
Wanhainen C and Martinsson O,  2010 - The Hybrid Character of the Aitik Deposit, Norrbotten, Sweden: a Porphyry Cu-Au-Ag(-Mo) System Overprinted by Iron-Oxide Cu-Au Hydrothermal Fluids: in Porter T M, (Ed),  2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.4 pp. 415-426


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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