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Address : Geological Survey of Finland, Finland
Celebration date : 4 Saturday October 2008
Author name : Kousa, Jukka; Luukas, Jouni; Nikander, Jarmo; Kontinen, Asko; Sorjonen-Ward, Peter

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  • Article title : Paleoproterozoic volcanic-related mineral systems along the Karelian cratonic boundary in Finland
    Article type : Tectonic & Seismotectonic
    Location : International Geological Congress,oslo 2008

    Fulltext :

    Paleoproterozoic rifting and orogenic deformation at the Karelian craton boundary provided a favorable geodynamic environment for a range of hydrothermal and magmatic-related mineral deposits, some of which are associated with or hosted by volcanic rocks. Several episodes of rifting and accompanying magmatism are documented commencing with layered intrusive complexes are dated to 2.44-2.39 Ga; while these are associated with bimodal volcanism, eruptive settings were apparently subaerial. Widespread 2.2 Ga mafic sills, despite extensive hydrothermal albitic alteration, lack eruptive equivalents. The next phase of magmatism, at 2.11 Ga, spans the transition from a shelf to a subsiding basin regime; evidence is compelling, though not conclusive, that the Hammaslahti Cu-Zn-Au deposit represents a distal subseafloor hydrothermal system related to contemporaneous mafic magmatism. However, the Talvivaara Ni deposit, hosted by similar turbiditic sediments and black shales, shows no spatial correlation with volcanic activity. Submarine bimodal volcanics dated at 2.06 Ga, broadly coincident with the mineralized polymetallic Kevitsa (and Bushveld!) and Fe-Ti-V Otanmaki intrusive complexes, have no known mineral potential. Ultimate breakup of the craton, and formation of the Jormua ophiolite at 1.95 Ga provides the context for the Outokumpu Cu-Co-Zn-Ni-Au deposits; recent studies demonstrate that host metaperidotites are exhumed depleted mantle lithosphere, and even if hydrothermal alteration and metal enrichment reflects subseafloor convective processes, the proportion of volcanic rocks in the system is trivial.
    Several discrete mineralizing events are now recognized within the framework of Svecofennian orogenic deformation. Two distinct phases of volcanism have been defined, with the two largest zinc deposits in Finland belonging to the earlier stage: the Pyhasalmi Zn deposit, now in its 50th year of production, lies within a bimodal volcanic sequence dated at 1.93-1.92 Ga; despite metamorphism and intense deformation, alteration geochemical and isotopic signatures clearly indicate a submarine VHMS affinity. The Vihanti deposit is marginally younger, and differs in showing evidence for a subseafloor or still later replacement origin. Both deposits were deformed and metamorphosed soon after formation, with peak metamorphism between 1.88-1.89 Ga. This time interval also coincided with widespread bimodal volcanism and sedimentation, with complementary granitoid and gabbroic intrusions, many of which are preserved at a relatively low strain state, suggesting generation of a mosaic of post-collisional transtensional basins, within a margin-parallel strike-slip kinematic framework. It is this younger event which is more likely to correlate with volcanism in the Skellefe district. Although late orogenic intrusive-related nickel and gold deposits are known from this region, there has been relatively little exploration for VHMS deposits in these rocks.

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