Tectonic setting and metallogeny of the Kiruna greenstones

• Main Author: Martinsson, Olof
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Luleå tekniska universitet, Geovetenskap och miljöteknik 1997
• Subjects: Geology; Geologi; Kiruna; Lofoten; Norway
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-26343

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 ...