Volcanology of the paleoproterozoic Maurliden area and its four unusual massive sulphide deposits, Skellefteå district, Sweden

Abstract: The Skellefte district is one of three major ore provinces in Sweden and contains more than 85 Zn-Cu-Au-Ag massive sulphide deposits. The Maurliden area is situated in the central part of the district and is a subaqueous volcanic dominated area with four Zn-Pb-Cu-Au-Ag sulphide deposits. The stratigraphy is characterised by felsic coherent intrusions, lavas and their clastic facies, sulphide mineralisations and fine to coarse grained stratified to graded volcaniclastic sedimentary rocks. Facies analysis of the Maurliden domain involving mapping and drillcore logging in a number of areas, but mainly in the central mineralised part where most drill-cores and outcrops occur, has led to a better understanding of the palaeovolcanology. Coherent felsic intrusions, lavas and their clastic facies and minor sedimentary and mafic rocks characterize the area. These rocks are tightly to closely upright folded and the massive sulphide deposits are situated in the northern and southern limbs of a major synform. The principal sedimentary facies are; 1) mudstone and sandstone turbidites; 2) pebbly sandstone and sand to siltstone turbidites; 3) graded to stratified breccia/conglomerate and 4) locally derived breccia/conglomerates. The volcanic and sedimentary facies together reflect a dynamic interplay between subsidence and uplift and between volcanism and tectonism. This resulted in adjacent and juxtaposed deep and shallow water depositional environments at different stratigraphic levels. The four massive to network sulphide deposits occur in and above stratigraphic intervals containing sedimentary facies emplaced below wave base, but shallow water facies are widespread and occur close to and stratigraphically below the mineralizations. Based on field relationships, petrographic and textural evidence are all four Maurliden mineralizations interpreted to be hosted in the same strongly quartz-feldspar porphyritic rhyolite. Intrusive contact relationships and high crystal content of this rhyolite suggest that it is an intrusion. The Maurliden sulfide deposits show several features typical of seafloor VHMS such as well-developed stringer zones, metal zonation, potential ore bearing sub-aqueous sedimentary horizon, colloform pyrite and polymetallic composition. However, other features such as intrusive host facies, extensive hydrothermal brecciation, disseminated and vein mineralizations are more typical of porphyry and high sulfidation deposits. The Maurliden deposits with all these different features form a transition between these hydrothermal end members.