The Aitik copper mine, which is located 15 km east of Gallivare in Norbotten, northern Sweden, has been Europes largest copper producer.
The deposit was discovered in the early 1930s but was not worked on a large scale until bulk-mining technology made exploitation feasible in the 1960s. Production commenced in 1968 at a rate of 2 Mt/y of ore, with progressive expansions to an output in the mid 1990's of approximately 19 Mt of ore per year for approximately 0.2 Mt per annum of concentrates.
The orebody is situated approximately 200 km north of the Archaean-Proterozoic palaeoboundary in the Fennoscandian shield and is hosted by Paleo- to Mesoproterozoic metamorphosed Svecofennian sediments and intrusives surrounded by granitic intrusions, within a supracrustal metamorphosed shear zone of Precambrian age.
Although it has been multiply deformed, metamorphosed and hydrothermally altered, the Aitik deposit exhibits features characteristics of both porphyry copper and iron oxide-copper-gold (IOCG) styles of mineralisation, indicating that it is of mixed origin, with a major part of the deposit originating from an early porphyry copper phase and a second, minor part, originating from an overprinting IOCG stage (Wanhainen, 2005).
The Aitik intrusion, a subvolcanic, multiphase calc-alkaline, intermediate composition intrusion is found in the footwall of the Aitik deposit. This metamorphosed intrusion, which is geochemically classified as a porphyritic quartz monzodiorite, belongs to the 1.9 Ga Haparanda suite of granitoids, which were formed in a compressional environment in a volcanic arc setting, corresponding to the period of subduction recorded further south in the Skellefte district. It includes younger, but comagmatic, phases of micro-quartz monzodiorite and diorite, and is suggested to represent a cupola protruding from a larger pluton at depth. The Aitik intrusion is weakly mineralised, with disseminations and veinlets of chalcopyrite, pyrite and magnetite. It has been subjected to regional metamorphism, deformation and dominantly potassic hydrothermal alteration.
A porphyritic rock with rich dissemination of sulphides is locally found within the main ore zone which comprises strongly altered and deformed biotite schist to gneiss towards the footwall and quartz-muscovite (sericite) schist towards the hanging wall. This porphyritic rock strongly resembles the footwall quartz monzodiorite, both texturally and compositionally, and is interpretted to represent an apophyses protruding from the upper part of the footwall intrusion.
Porphyry style chalcopyrite-pyrite mineralisation in a quartz stockwork was deposited contemporaneously with the emplacement of the quartz monzodiorite at around 1.89 Ga, accompanied by potassic alteration of the intrusive and surrounding volcaniclastic rocks. The potassic alteration is manifested by early biotite and K-feldspar, closely associated with disseminated sulphides and magnetite. In addition to the potassic alteration, sericite is abundant, accompanied by garnet porphyroblasts, quartz and pyrite
The Aitik IOCG mineralising event is interpreted to have occurred approximately 100 Ma later, when eastward subduction produced compression, monzonitic-granitic magmatism, ductile deformation, and tectonic block movements in Northern Norrbotten and at Aitik was responsible for extensive deformation and redistribution of metals. Magnetite enrichment is locally evident in late veins of mainly amphibole, K-feldspar and epidote, together with late scapolite alteration within the deposit, and the addition of Cu and Au. This phase of mineralisation appears to be related to the extensive Na-Ca alteration in the region during this tectonic event.
Tectonically remobilised sulphides have contributed to the formation of a copper-rich area in the strongly deformed rocks towards the footwall of the ore. The distribution of gold in the Aitik ore body correlates to a certain degree with that of copper, but gold-rich areas corresponding with high quartz vein frequency and high pyrite content, have also developed by remobilization. The ore body also shows a vertical metal zoning, with gold-rich areas increasing with depth and restricted copper-rich areas being replaced by more extensive, medium-grade areas at depth.
Pre-mining reserves and resources totalled over 150 Mt @ 0.4% Cu, 0.2 g/t Au.
However, production to late 2004 was 424 Mt @ 0.4% Cu, 0.2 g/t Au, 4 g/t Ag.
Production in the year of 2004 was 64 800 t Cu, 2.035 t Au, and 45.110 t Ag.
Total production to the end of 2011 was: 561 Mt @ 0.37% Cu,
Proven + probable ore reserves at 31 December, 2011, were: 710 Mt @ 0.25% Cu, 1.6 g/t Ag, 0.15 g/t Au,
Measured + indicated + inferred resources as at 31 December, 2010, were: 1670 Mt @ 0.18% Cu, 1.0 g/t Ag, 0.11 g/t Au.
Source: Boliden website, 2012.
(Source: Porter Geoconsultancy, http://www.portergeo.com.au/, 2012)