| Summary: | Silurian organic-rich black shales are widespread and serve as basic source for most U mineralization throughout the entire Variscan Orogen of Central and Western Europe. In particular the lowermost unit of Silurian black shales is regionally strongly enriched in U. The strong U-enrichment of these shales is the result of the Hirnantian glaciation that removed old near-surface rocks that already had lost most leachable U during long-lasting alteration and brought rocks with leachable U to the surface. Sea level rise due to the melting of the Hirnantian ice sheet inundated the glacially modeled continent margin and resulted in regionally restricted oceanic circulation and anoxic conditions that are related to both low initial oxygen levels and sudden increase of organic production. Organic-rich shales on the Gondwana shelf developed particularly high U content as they scavenged U from the surface runoff that was leached after deglaciation from the newly exposed drainage area. The present spatial distribution of Silurian U-rich black shales within the Varisan orogenic belt is largely controlled by large-scale tectonic processes operating during the approach and eventual collision of Gondwana and Laurussia, leading to the formation of western Pangea. During the Variscan orogeny, these U-anomalous rocks became stacked and folded at the margins of some low-strain domains of the orogenic belt and exposed to metamorphism and in part to crustal melting – contributing to the formation of U-fertile post-kinematic granites – in the high-strain domains of the orogenic belt. There are no U deposits that are directly related to the emplacement of these granites. Variscan granites, however, are the most important source of leachable U for younger, major U mineralization. Late-Variscan reorganization of the plate movement between Gondwana and Laurussia resulted in an extensional tectonic regime throughout Europe with important Permian sedimentary basins. Heat input and fluid migration along Permian fault zones resulted in the ...
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