The Skellefteå region consists of Paleoproterozoic-aged rocks that host several world-class volcanogenic massive sulphide copper, zinc, and lead deposits that have been worked on for nearly a century. The Skellefteå district lies within a large and ancient cratonic block named the Fennoscandian shield. The Fennoscandian shield spans much of Finland and northwestern Russia, extending further westward throughout Sweden and Norway.
Mineralization in the Skellefteå region is focused within and around a regionally extensive, west- to northwest-trending structural feature named the Skellefteå belt. The Skellefteå belt is 120 km long and 30 km wide and consists of deformed and metamorphosed volcanic, sedimentary, and igneous rocks that are all Paleoproterozoic in age. Deformation and metamorphism is attributed to the Paleoproterozoic-aged Svecokarelian orogeny that occurred around 1.88-1.8 Ga. Metamorphism associated with the Svecokarelian orogeny and ranges in intensity from greenschist to amphibolite facies.
The rocks of the Skellefteå belt are observed to have undergone two major shortening events and metamorphism during the Svecokarelian orogeny. The first of the major shortening events resulted in folding and shearing: folding consisting of vertical to upright isoclinal folds with east- to northeast-striking axial planes, while shear zones are oriented sub-parallel to the axial planes of the folds. The later shortening event produced structures mainly dominated by shearing, with only minor folding coaxially overprinting the earlier generation of folding. These late-stage shears appear to play a crucial role in the formation of the deposit.
The Björkdal gold deposit is a lode-style, sheeted vein deposit that is hosted within the upper-portions of the Skellefteå Group lithologies as they are found at Björkdal (as described above). Gold is found within quartz-veins that range in thickness from less than a few centimetres in width, to over several decimetres in width. These veins are usually observed as vertical to sub-vertical dipping veins that strike between 000° and 090°, with the majority of veins occurring with a strike between 030° and 060°. The veining is locally structurally complex, with many cross-cutting features as well as thin quartz veinlets which introduce mineralization into the wall rocks proximal to the main quartz veins.
Gold-rich quartz veins are most often associated with the presence of minor quantities of sulphide minerals such as pyrite, pyrrhotite, marcasite, and chalcopyrite alongside more common non-sulphide minerals such as actinolite, tourmaline and biotite. Scheelite and bismuth-telluride compounds (i.e., tellurobismuthite and tsumoite) are also commonly found within the gold-rich quartz veins and are both excellent indicators of gold mineralization.
Gold occurs dominantly as free gold, however, gold mineralization is also associated with Bi-telluride, electrum and pyroxenes. Silver is seen as a minor by-product of the Björkdal processing plant, however, very little is known about its deportment within the deposit, although it is assumed to be associated with electrum.