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Holly Stein, Colorado State University (United States)
Bernard Bingen, Geological Survey of Norway (Norway)
Gang Yang, Colorado State University (United States)
Peter Ihlen, Geological Survey of Norway (Norway)
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The Eidsvoll Au deposit in SE Norway, once a major producer in northern Europe, is of historic and geologic interest. Located in the Mjøsa-Vänern mineral belt (MVMB), with other base-metal and porphyry-style deposits [1], Eidsvoll Au mineralization has long been associated with the SE-trending Sveconorwegian Mylonite Zone (MZ), a continental-scale terrane boundary along the SW margin of Fennoscandia. The MVMB obliquely oversteps the MZ, however, and its ore-hosting structures differ from the MZ. Thus, a relationship between the MZ and MVMB mineralization is not supported. Re-Os dating unequivocally shows this to be the case, and illustrates new-found potential for dating not only pyrite, but hydrocarbon associated with gold deposits.
Re-Os dating of two generations of pyrite, Re-Os dating of carbonaceous stringers (coalblende), and U-Pb dating of hosting augen gneiss at the Eidsvoll Brustad mine define a complex shallow crustal history marked by mineralized post-Sveconorwegian brittle structures. All structural stages of Eidsvoll mineralization postdate Sveconorwegian greenschist facies mineralogy in the host rocks [2].
U-Pb zircon dating of the Brustad augen gneiss at the mine provides a precise age of 1668 Ga, similar to a previous 1674 ± 10 Ga age [3]. A precise 8-point Re-Os isochron age of c. 870 Ma was obtained for cubic pyrite samples hosted in early quartz-sulfide-Au veins cutting the Brustad gneiss. A second generation of fine-grained cataclastic pyrite with 187Re/188Os ratios 4000 yields Re-Os model ages of c. 440 Ma. We suggest that the Caledonian orogenic wedge resulted in cataclasis and recrystallization of earlier pyrite at Eidsvoll, liberating Au and producing chalcopyrite. Re-Os ages for simple fracture-coatings of hydrocarbon yield Miocene ages, suggesting much later hydrocarbon migration into Eidsvoll rocks, likely from Paleozoic source rocks in S Norway.
The 870 Ma NE-trending pyrite-bearing brittle structures at Eidsvoll are parallel to the NE-trending Kristiansand-Porsgrunn shear zone (KPSZ) which separates the Bamble and Telemarkia terranes SW of the intervening Permian Oslo rift. An 11 m.y. history (891-880 Ma) of extensional ductile deformation is recorded along the KPSZ by 40Ar/39Ar dating of mica fish [4]. We propose that 891-880 Ma extensional reactivation of the KPSZ, 870 Ma fault-hosted Au mineralization at Eidsvoll, and 850 Ma NE-trending monzonorite dikes (Hunnedalen) in SW Norway [5], are evidence for a SE-trending extensional stress regime after the Sveconorwegian orogeny. In sum, 870 Ma, 440 Ma, and Miocene ages at Eidsvoll chart a long-term, episodic post-Sveconorwegian faulting history in the SW Fennoscandian shield.
References Cited: [1] Stein et al (2000) Econ Geol 95:1657-1671; [2] Telstø (2005) PhD thesis 2005:129 (NTNU); [3] Alm et al (2002) GFF 124:45-61; [4] Mulch et al (2005) EPSL 233:372-390; [5] Walderhaug et al (1999) EPSL 169:71-83.
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