The genetic relationship between rhyolitic volcanism and Zn-Cu-Au deposits in the Maurliden volcanic centre, Skellefte district, Sweden volcanic facies, lithogeochemistry and geochronology

Sammanfattning: The Skellefte district is one of three major ore provinces in Sweden and contains more than 80 Zn-Cu-Au-Ag massive sulphide deposits. The Maurliden area in the central part of the district represents a subaqueous, constructional and relatively topographic high silicic volcanic centre. It hosts four sulphide deposits and was active during around 1885 Ma. The tholeiitic to transitional Maurliden volcanic centre developed in an extensional arc region in the Skellefte district that probably developed on an immature (relatively thin) continental arc crust. The four Maurliden Zn- Cu-Au massive to stringer sulphide deposits are hosted within the same quartz-feldspar porphyritic pumice breccia-sandstone unit (QFP pumice unit) and were formed by replacement and infilling of this unit together with seafloor precipitation/exhalation. The QFP pumice unit was generated by explosive subaqueous eruptions and was sedimented on the seafloor as a succession of subaqueous mass-flow pulses. Field relationships and geochemistry indicate a close relationship between massive sulphide deposition and host rock in composition, intensity of volcanism and sedimentation during the evolution of the Maurliden volcanic centre. The volcanic activity before ore deposition was dominated by felsic extrusive volcanism associated with both terrestrial to shallow marine breccia-conglomerates and siltstone-sandstone turbidites deposited below wave base. At the same time or slightly after this stage, there was a stage of quartz-feldspar porphyritic rhyolitic extrusive and explosive volcanism. At this stage the ore-host, the QFP pumice unit, was subaqeously erupted and emplaced. The waning stage of the felsic volcanic activity was accompanied by deposition of massive sulphide and siltstone-sandstone. Most mafic rocks are post-ore dykes. The similar geochemical composition of the felsic rocks suggests that they were derived from a common magma source. Trace element and rare earth element characteristics suggest that these magmas were relatively primitive. The coeval felsic volcanic activity and the strong facies control on massive sulphide deposition, suggest that massive sulphide deposition was intimately linked to the magmatic evolution of the Maurliden volcanic centre. The QFP pumice unit has a strong control on the location of sulfide mineralization in the Maurliden area, and represents a new VMS horizon in the central Skellefte district and it is possible that this horizon occurs in other unexplored parts of the Maurliden domain.