Abstract
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 formation of episyenite-bound and vein-type U mineralization in the Variscan basement, whereby U was leached from granites and Silurian sedimentary rocks and their metamorphic equivalents and was deposited in local episyenite and vein-type mineralization (e.g., districts of Aue-Niederschlema, Jáchymov, Příbram, Forez, and Salamanca). Late-Variscan erosion of the orogenic belt resulted in the redistribution of U from Variscan metamorphic rocks and granites, but also the Silurian black shales, into late Carboniferous and Permian sedimentary basins. Permian mobilization of fault zones controled the availability of U-rich source rocks for leaching, the transport of U into the basins, and the distribution of organic-rich sediments that may trap U in organic-rich layers (e.g., Döhlen, Lodève). The Permian U redistribution represents the major deposit-forming event throughout Variscan Europe, whereby hydrothermal U mineralization in episyenites and veins in the Variscan basement and low-temperature U mineralization in sandstones and organic-rich sediments are different expressions of the same tectonic process, i.e., the development of the Central European Extensional Province. Later tectonic reactivations of these fault zones during the opening of the Tethys and North Atlantic oceans and during the Alpine Orogeny mainly resulted in the local redistribution of U mineralization along the same fault zones at c. 180 Ma, 150 Ma, and 120 Ma, and more recently during the Alpine orogeny, in part with addition of other metals (e.g., Ag, Bi, Co, Ni, As) leached from the local wall rocks. Post-Permian intense chemical weathering, mostly in the Triassic and the Cretaceous, resulted in (i) U redistribution and additional U concentration within near-surface mineralization (e.g., Ronneburg, Aue-Niederschlema), (ii) leaching from near-surface U-rich rocks followed by entrapment in sediments to form lignite-type (e.g., black shales, peats) and roll-type mineralization (e.g., Königstein), and (iii) the dispersion of U in the supergene environment. The rocks of the Variscan belt host a history of more than 400 Ma of U redistribution, starting with the deposition of early Silurian U-rich black shales. The formation of these black shales is climatically controlled. Their spatial distribution eventually controls the spatial distribution of younger mineralization that is generally closely related with the tectonic development and reactivation of the basement and the redistribution by magmatic, hydrothermal, and erosional processes. Basement faults are pathways for fluids and their reactivation controls the formation and inversion of basins and the uplift of basement blocks, eventually linking the endogenic U cycle (magmatic and hydrothermal rocks) with the exogenic U cycle (making U-rich rocks available for erosion and leaching and creating traps to precipitate mobilized U). The timing of post-Variscan U mineralization is controlled by these tectonic reactivations, whereas supergene enrichment or redistribution is particularly efficient in periods of intense chemical weathering.
