The Hybrid Character of the Aitik Deposit, Norrbotten, Sweden:
A Porphyry Cu-Au-Ag(-Mo) System Overprinted by Fe-oxide Cu-Au Hydrothermal Fluids
by
Christina Wanhainen, Olof Martinsson, Department of Chemical Engineering and Geosciences, Division of Geosciences, Luleå University of Technology, Luleå, Sweden
in Porter, T.M. (Ed), 2010 - Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, Advances in the Understanding of IOCG Deposits; PGC Publishing, Adelaide. v. 4, pp. 415-426.
ABSTRACT
The Aitik Cu-Au-Ag(-Mo) mine in northern Sweden is the biggest open pit operation in northern Europe, and one of Europe's largest metal producers. The open pit is 3 km long, 930 m wide and 405 m deep. Approximately 480 Mt of ore averaging 0.4% Cu, 0.2 g/t Au and 4 g/t Ag has been produced, with the metal production in year 2007 being 50 487 t of copper, 1.178 t of gold, and 42.301 t of silver.
The Palaeoproterozoic Aitik porphyry Cu-Au-Ag(-Mo) deposit and its host rocks, are situated approximately 200 km north of the Archaean-Proterozoic palaeoboundary in the Fennoscandian shield. They are considered to have formed in a volcanic arc environment related to subduction of oceanic crust beneath the Archaean craton at c. 1.9 Ga. A quartz monzodioritic intrusion related to the formation of porphyry copper mineralisation is situated in the footwall of the deposit. High salinity fluids (30-38 eq. wt% NaCl + CaCl2) responsible for chalcopyrite-pyrite mineralisation was released contemporaneously with quartz monzodiorite emplacement and quartz stockwork formation at c. 1.89 Ga, and caused potassic alteration of the intrusive and surrounding volcaniclastic rocks. Remnants of this 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 of Fe-oxide Cu-Au character occurred about 100 Ma later, when eastward subduction resulted in compression, monzonitic-granitic magmatism, ductile deformation, and block movements in northern Norrbotten. Extensive deformation of rocks and redistribution of metals occurred. Magnetite enrichment locally found within late veins of mainly amphibole, K-feldspar, tourmaline, garnet, quartz and epidote, together with late scapolite alteration within the deposit, implies that fluids responsible for Fe-oxide Cu-Au mineralisation and extensive Na-Ca 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->60 eq. wt% NaCl + CaCl2) and contained solids of ferropyrosmalite and hematite.
Studies of the Aitik deposit reveal a multiphase origin of the ore, with multiple sources of the ore fluids and peaks of mineralisation around 1.9 and 1.8 Ga.
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