Copper and gold distribution at the Aitik Deposit, Gällivare Area, Northern Sweden

Abstract

Aitik is a disseminated Cu-Au deposit with an annual production of 18 Mt of ore containing 0·38% copper and 0·22 ppm gold. It is situated in northern Sweden in 1·9 Ga Svecofennian, metamorphosed volcanic and sedimentary rocks formed in a volcanic arc environment. The Aitik Deposit entered production in 1968. All geological and geochemical information obtained since then has been used to summarise the distribution pattern for Cu and Au in the deposit. A large-scale vertical metal zoning results from a slight increase of Au with depth. Restricted Cu-rich areas at the surface gradually change into more extensive, medium-grade areas with depth, resulting in approximately the same average Cu content for each level. The vertical metal zoning is accompanied by a lithological change from biotite gneiss to biotite-amphibole gneiss with depth. The mineralised areas trend predominantly north-east and north-south, and plunge north-east (Au) and northnorthwest (Cu). The ore body also shows horizontal metal zoning. A 'high-grade' area (> 0.6% Cu and >. 0.3 ppm Au) of disseminated sulphides is situated in biotite gneiss in the centre of the ore zone. Towards the hanging wall, grades are lower (< 0·4% Cu and < 0·30 ppm Au) and the host rocks are strongly sericite altered, pyrite-rich schists. A 'gold-rich' area (> 0·6 ppm Au) is located within 50 m from the hanging wall, in a residual part of pyrite-rich biotite schist that is strongly K-feldspar, epidote and quartz altered. Another 'gold-rich' area (> 0·6 ppm Au) is situated below 400 m depth on the footwall side of the ore in the northern part of the deposit, where the amphibole content of the host rock is high. Cu-dominated mineralisation is mainly developed close to the footwall side of the ore, where disseminated sulphides and massive veins of chalcopyrite are common. It is suggested that the rich dissemination in the central part of the ore represents the primary mineralisation at Aitik. Later features, such as shear zone related fluid-rock interaction and deformation, caused remobilisation of metals into favourable structural traps, creating local higher-grade zones. The possibility cannot be discounted, however, that the metal distribution has been caused, not only by multistage remobilisation, but also by repeated introduction of metals.