Mineralogy of Sulfosalt-Rich Vein-Type Ores, Boliden Massive Sulfide Deposit, Skellefte District, Northern Sweden

Sulfosalt-rich vein-ores, which form an important mineralization type of the Paleoproterozoic metavolcanic-hosted Boliden Au–Cu–As massive sulfide deposit, in the Skellefte district, in northern Sweden, have been investigated by a combination of ore microscopy, electron-microprobe analysis and balanced reactions. The sample material contains a variety of ore minerals, including Au–Ag alloy, gudmundite, selenian galena, kobellite, tetrahedrite, bournonite and laitakarite. The vein ores display structures indicative of extensive deformation. Three distinct decomposition-induced assemblages, which postdate the deformation, have formed at the expense of kobellite: (i) selenian galena + laitakarite, (ii) tetrahedrite + laitakarite, and (iii) bournonite + laitakarite. Both kobellite and bournonite are unusually Se-rich, with concentrations in the range of 3.16–4.55 wt.% and 3.37–4.73 wt.%, respectively. Laitakarite, Bi 4 (Se,S) 3 , is characterized by substantial incorporation of Te, in the range of 0.94–9.64 wt.%. Comparable compositions have not been reported before and are indicative of extensive solid-solution within the system Bi 4 S 3 –Bi 4 Se 3 –Bi 4 Te 3 . Pb, with concentrations in the range of 3.01–4.56 wt.%, constitutes an essential component of laitakarite. The amount of Pb incorporation is approximately constant over the entire compositional range. Construction of balanced reactions for the principal types of decomposition textures demonstrates that these fluid-assisted replacement processes are controlled by individual pairs of immobile elements.