|IOCG and Related Mineral Deposits of the Northern Fennoscandian Shield|
Kjell Billström, Museum of Natural History, Laboratory for Isotope Geology, Stockholm, Sweden, Pasi Eilu, Geological Survey of Finland, Espoo, Finland, Olof Martinsson, Department of Geosciences, Luleå University of Technology, Luleå, Sweden, Tero Niiranen Geological Survey of Finland, Rovaniemi, Finland, Curt Broman, Department of Geology and Geochemistry, Stockholm University, Sweden, Pär Weihed, Christina Wanhainen, Department of Geosciences, Luleå University of Technology, Luleå, Sweden and Juhani Ojala, Geological Survey of Finland, Rovaniemi, Finland
in - Porter, T.M. (ed.), 2010 - Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, v. 4, Advances in the Understanding of IOCG Deposits; PGC Publishing, Adelaide. pp. 381-414.
The northernmost Fennoscandian shield comprises Archaean and Palaeoproterozoic rocks. Unlike most other shield areas, economic mineral deposits are largely restricted to its Palaeoproterozoic parts. The latter are characterised by intracratonic basin evolution between ca. 2.5 and 2.0 Ga, involving recurrent mantle hotspot activity with numerous layered intrusions, komatiite and picrite eruptions, but no signs of accretionary phases or formation of major new felsic crust. Accretion and continent-continent collision followed from ca. 1.9 to 1.8 Ga, during the Svecofennian orogeny.
A range of mineralisation styles are hosted by extensive ca. 2.5 to 2.0 Ga greenstone belts and younger, subduction-related 1.9 to 1.8 Ga Svecofennian intrusive and extrusive settings. These mineralisation styles partially overlap, and individual deposits may not readily be placed into genetic classification schemes. A provisional grouping of observed mineralisation styles comprises (1) stratiform-stratabound sulphide, (2) apatite-iron, (3) skarn-related iron and BIF, and (4) epigenetic(±syngenetic?) Au and Cu-Au deposits. The descriptive section of this paper also highlights features that may relate to orogenic gold, IOCG and 'atypical metal association' categories of mineralisation.
The assumption made is that the deposition of a diverse range of ore deposits was made possible by a long and complex geological evolution. This involved an initial (sowing) stage where iron, and to some extent copper and gold, were concentrated during 2.3 to 2.1 Ga (Karelian) rock-forming processes. Following this, ore elements were mobilised during two younger (Svecofennian) stages at 1.92 to 1.87 and 1.85 to 1.79 Ga, respectively. The latter were triggered by metamorphic and magmatic episodes, and fluids liberated during these stages precipitated IOCG and related deposits when fluids met structural and chemical traps in suitable host rocks. Ore fluids are generally saline, and their development probably involved incorporation of evaporites and, at least locally, also felsic magmatism may have played a role.
Skarn-related mineralisation, hosted by ca. 2.1 Ga greenstones, occurs both as a BIF type in Sweden (formed at around 2.1 Ga), and as a gold-copper enriched variety (the result of Svecofennian epigenetic processes) in the Kolari region of Finland. The huge Kiirunavaara deposit is the type example of apatite iron ores, and is here considered to have formed from a magma at ca. 1.88 Ga, although it also has features best explained by a magmatic-hydrothermal overprint. A younger, less prominent, stage of apatite iron ore formation took place at approximately 1.78 Ga. Epigenetic gold and copper-gold deposits are particularly hard to classify as these show mixed ore characteristics, and to some extent this is likely to be due to multiple mineralisation stages (cf. the huge, low grade Aitik deposit in Sweden which is interpreted to be a hybrid porphyry-IOCG-type of ore). Structurally controlled, orogenic gold mineralisation is common in the Central Lapland greenstone belt, although there are also gold deposits with enhanced contents of e.g., copper, cobalt and uranium (e.g., at Saatopoora). The latter, sometimes referred to as being of an 'atypical metal association' type, could potentially also include syngenetic mineralisation (e.g., at Juomasuo). The range of epigenetic (±syngenetic) gold and copper-gold deposits could possibly be related to a vague east-west trend defined by gold-rich deposits in the east (Finland), followed by IOCG (copper±gold) and more iron-dominant ore types near the Finnish-Swedish border and further west into Sweden.
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