Tectonic setting and metallogeny of the Kiruna greenstones

Sammanfattning: The Paleoproterozoic Kiruna Greenstones belong to a large c. 2.1 Ga tholeiitic province in the north-eastern part of the Baltic Shield. The mainly basaltic volcanism was related to an event of continental rifting, which ended up with continental rupture. The occurrence of komattites, picrites and thick piles of tholeiites in the northern parts of Sweden, Norway and Finland is in contrast to the sedimentary dominated areas in central-eastern Finland. This suggests the location of a mantle plume to the northern area, which generated the large volumes of mantle melts. The stratigraphical record of the well preserved Kiruna Greenstone Group demonstrates a change from initial clastic sedimentation, evaporate deposition and WPB-type volcanism to later extensive volcanism of flood basalt character. Subsequent crustal thinning generated MORB-type magmas by decompressional mantle melting. The later development of a subaqueous basin was accompanied by a change to explosive volcanism, and large amounts of volcaniclastic material was formed by Surtseyan eruptions. During ocean opening along a line from Ladoga to Lofoten a NNE-directed failed rift-arm was formed. This is expressed by rapid basin subsidence and voluminous eruption of MORB-type pillowlava, which created an anomalous environment of local extent within the greenstone domain. Basin shoaling and subsequent uplift and erosion of the rifted margin marks an end of the rift event. Two different types of economic sulfide deposits occur in the Kiruna Greenstones, syngenetic Cu-(Zn) ores of exhalative origin (Viscaria-type), and epigenetic Cu-Au ores (Pahtohavare-type). Both types are formed from highly saline hydrothermal fluids, but they are clearly different in metal association, ore related alteration and ore character. Conspicuous for the Viscaria-type is the occurrence of stacked blanket-shaped mineralizations of magnetite and sulfides, and the layered structure of high-grade Cu-ore, which is explained by repeated exhalative activity and deposition of the ores in brinepools. The most productive ore was formed in association with the main stage of basin subsidence and MORB-type volcanism in the failed rift. Faults parallel with the rift axis acted as channels for the ore fluids, and controlled the shape and location of brine-pool in which the ore was precipitated. Ores of the Pahtohavare-type have formed in zones of active ductile to brittle shearing. In detail the location of ores are mainly lithologically controlled by black schists, which has acted as chemical traps. The ores are hosted by albite felsites, and surrounded by characteristic zones of scapolite-biotite alteration. Calcopyrite and pyrite are the main ore minerals and they occur mainly as veinlets, veins and matrix to brecciated albite felsite. Ferro-dolomite is a characteristic ore related mineral formed in several generations from early dissemination in albite felsite, gangue to ore minerals and late barren veins. The Viscaria and Pahtohavare deposits are different in many respects to typical massive sulfide deposits and Au-ores in most other greenstone terrains. This is mainly due to their formation from highly saline solution, which is a common feature of both exhalative and epigenetic sulfide deposits formed in continental rift environments. Thus, the existence of evaporitic sediments at the base of the Kiruna Greenstones may be of major metallogenetic importance for this region, serving as a source for saline hydrothermal fluids.