Volcanic facies and hydrothermal alteration at the Norra volcanigenic massive sulfide deposit : Storuman area, Sweden

Sammanfattning: The Skellefte District, with a long and rich history of base metal extraction that commenced asearly as 1924, is rated amongst the leading mining provinces in Sweden and Europe. The districtrepresents a Paleoproterozoic volcanic arc hosting more than 85 known (gold-bearing) VMSdeposits and several economic grade orogenic gold occurrences.The Barsele property lies at the intersection of the Skellefte District and the Lycksele-Storuman(Gold Line) metallogenic trends. The Project includes the Central-Avan-Skiråsen intrusionhostedgold deposits and the Norra VMS deposit.The mineralizations at Barsele area were discovered by Terra mining in 1980, and since then thearea has been the focus of various exploration companies including MinMet, NorthlandResources, Orex Minerals and Agnico Eagle. Despite these historic interests, only little work hasbeen done to combine volcano-sedimentary facies studies with lithogeochemistry to betterunderstand the setting and genesis of the deposits.This study intends to resolve the stratigraphic architecture of the Norra volcano-sedimentarysuccession and establish a better understanding of the alterations caused by fluid system whichformed VMS mineralization. This was performed by integrating the results obtained fromvolcanic facies logging of two profiles and immobile-element lithogeochemical investigationswhich were supplemented by petrographic analysis.On the basis of volcanic facies analysis it was revealed that, the massive to semi-massive sulfidemineralization at Norra is hosted within a sub-vertically dipping volcano-sedimentarysuccession. The footwall is comprised of sheared mafic and felsic porphyritic intrusions and lavas,foliated sulfidic and graphitic black shales and mudstones. The hanging wall is generallydominated by feldspar-porphyritic basaltic-andesite and dacitic intrusions.Lithogeochemical studies indicate that the coherent volcanic rocks belong to three distinctmagma series. Feldspar+quartz-porphyritic dacite and basaltic-andesites are comagmatic, withtransitional to calc-alkaline affinity, whereas feldspar-porphyritic basalt, which encloses themineralized facies have a tholeiitic affinity. Post mineralization doleritic dykes belong to a thirdseparate magmatic suite of transitional affinity. The mineralization occurs close to the lithologicalcontact between the dacitic intrusion at the stratigraphic footwall and the feldspar-porphyriticbasalt. The contact also marks a chemical transition from a footwall of transitional to calc-alkalinemagmatic affinity to tholeiitic hanging-wall. Structural interpretation using chemostratigraphiccorrelation indicate that the ore lens sits on the north-eastern limb of an upright syncline.Clast-rich massive to semi- massive sulfide ore textures, and the associated style and intensity ofalteration suggest that subseafloor replacement was the dominant type of VMS mineralization.