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The Skellefte district in northern Sweden consists mainly of pyrite-rich massive sulphide deposits intercalated in an early Proterozoic volcano-sedimentary sequence. The volcanic activity was predominantly submarine, and gave rise to abundant felsic rocks with minor mafic volcanic rocks and rare andesitic volcanism. A few dykes and volcanic flows of high Mg-Cr basalts, probably komatiitic, are also found within the volcanic pile.The geochemistry of the volcanic rocks is of subalkaline character and displays slightly tholeiitic to calcalkaline differentiation trends. The Ti, Zr, Y, Cr and rare-earth contents of the basaltic and andesitic rocks are consistent with a subduction related volcanic arc setting.The bimodal nature of the volcanic suite, the presence of highly primitive basaltic rocks and the abundance of felsic pyroclastic material suggests that rifting occurred within the volcanic arc. Long periods of tensional stress predominated and favoured the extrusion of large volumes of felsic pyroclastics and the formation of tectono-volcanic depressions. The convergence in time and space of a thick volcanic sequence with high permeability, a high geothermal gradient related to the rising of basaltic magma, and synvolcanic faulting resulted in extensive hydrothermal activity. The hydrothermal activity caused regional alteration of the volcanic rocks and locally deposition of pyrite-rich massive sulphide deposits within local, tectonically controlled, depressions.

The Boliden-Lftngdal area in the eastern part of the Skellefte district consists of an early Proterozoic volcanic-sedimentary sequence which includes massive sulfide deposits. Both the ores and their country rocks have been strongly folded and metamorphosed in the lower amphibolite facies and intruded by synorogenic (1,890 m.y.) and late orogenic (1,750 m.y.) granitoids.The calc-alkaline/tholeiitic volcanic suite displays petrological and geochemical features similar to Phanerozoic subduction-related volcanic arcs. Its bimodal character suggests that extensional forces dominated in the arc during long periods, probably producing incipient rifting. In connection with this tensional stress large volumes of felsic pyroclastic and subordinate lava flows were extruded and large-scale tectono-volcanic depressions formed (e.g., calderas). This event, with predominantly felsic volcanism, culminated with the extrusion of genetically unrelated mafic and ultramafic rocks. Most of this activity occurred as submarine volcanism.The convergence in space and time of a thick volcanic sequence with high permeability, synvolcanic faulting, and a high geothermal gradient produced by the magmatic activity favored the starting of a convective hydrothermal system where the fluid was modified seawater. The hydrothermal activity produced extensive alteration in the volcanic rocks under low water/ rock mass ratio conditions, changing their original chemical compositions and producing orebearing hydrothermal solutions. The geochemical data suggest that most of the Cu and Zn were leached from the volcanic rocks whereas the provenance of the Pb is more uncertain because it is normally enriched in these rocks. When the ore-bearing hydrothermal solutions reached the sea floor, they caused the deposition of massive sulfide deposits within local tectono-volcanic depressions.

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