- GeoRef, Copyright 2008, American Geological Institute. Abstract, Copyright, Society of Economic Geologists
After the Variscan orogeny, from the upper Carboniferous to the Quaternary syn(dia)genetic and epigenetic mineralization occurred in the mid-European basin. Uranium, base metal, Sr-F-Ba, Fe-Mn, and Au-Sn placer deposits are strata bound within clastic, volcaniclastic, and calcareous rocks. Some vein-type and veinlike mineralization of post-Variscan age is found in the uplifted basement blocks and is spatially related to late Variscan and late Alpine unconformities.Post-Variscan mineralization may be subdivided into seven types with respect to the deposited environment of their host rocks: (1) saprolite-peneplain (Cu, Pb, Zn, F, Ba, Fe, Mn), (2) alluvial fan (Fe, Sn, Au, U, Pb, Zn, F, Ba), (3) braided streams-bed load fluvial systems (Pb, Zn, U, F, Ba), (4) meandering to anastomosing streams-mixed load fluvial systems (U, Pb, Cu), (5) high-energy shore zone of lake margins or coastal regions (Fe, Pb, Cu, U), (6) low-energy lake centers or near-shore marine deposits (lagoons) (Fe, Pb, Zn, Cu, U), and (7) mixed terrigenous calcareous shelf deposits (F, Sr, Zn, Pb).There were various sources of metals for the strata-bound mineralization. During the late Paleozoic and Lower Triassic the metals were supplied from volcanites and volcaniclastic rocks of calc-alkaline affiliation (volcanic arc ?). The Cu-Pb-Zn mining districts in the Kupferschiefer are located above or close to a suture zone that delineates a former active plate margin with endogenous Cu-Mo deposits. Following down the stratigraphic column in the footwall of the Kupferschiefer, the elements increase in reverse order of deposition. From the Lower Triassic to the Upper Jurassic, metamorphic and granitic rocks in Variscan basement highs became exposed to chemical weathering and acted as the metal source. Denudation of the Variscan basement is mirrored by the element spectrum of Triassic strata-bound deposits, commencing with Pb-Zn (gneisses) and leading to U deposits (granites). From the Lower Cretaceous onward some of the metals in the mineralization were derived from the basement. Most of the mineralization, however, originated from intrabasinal-intra-Mesozoic redeposition (e.g., ironstones).Five metallogenic sequences may be delineated, four of which start with mineralization in a fluvial environment and terminate with precipitation of Sr and F in a mixed clastic-carbonate shelf environment (sabkha). Metallogenic sequences reflect marine transgression and regression, which may be correlated with different rifting and spreading stages in the Atlantic Ocean.Two major unconformities, one caused by the Variscan uplift, the other by Subhercynian and Laramide movements, may be traced across the basin. Supergene (e.g., U, Mn-Fe) and hypogene (e.g., Pb-Zn) mineralizations are directly related to these first-order geohydraulic planes. Minor unconformities and erosional surfaces also influenced the development of host environments of strata-bound ore deposits and the termination of metallogenic sequences. Low relief erosional surfaces favored restricted water circulation and resulted in the formation oftransgressive black shales (e.g., Kupferschiefer). The development ofa sabkha and its related mineral deposits also requires the presence of a low topography as well as an arid climate and resultant strong evapotranspiration. Karstification, essential to mid-European Mississippi Valley-type deposits, commenced more or less contemporaneously with the evolution of tidal fiats on these erosional surfaces in a supratidal zone and persisted until the hydrology was changed by a new transgression. Vein-type mineralization related to these unconformities was at least in part supplied by sulfur and strontium from sabkha-derived evaporites, as suggested by Sr and S isotope analyses.It is difficult to define the time of formation ofpost-Variscan ore mineralization, except for syngenetic Fe-Mn, U ores (Tertiary) and Fe (Cretaceous) and Sn placers (Quaternary). The bulk of strata-bound ore mineralization in the mid-European basin formed by diagenetic and epigenetic processes. The few age data and some evidence from basin subsidence suggest that there were two intervals of diagenetic and/or epigenetic ore mineralization. The period from Lower Triassic to upper Dogger correlates with early rifting in the central parts of the Atlantic Ocean. Another set of data which derived from U-F deposits in the Upper Cretaceous and Mississippi Valley-type deposits attests to a thermal and fluid influx from deepseated rifts (Rhine graben, Ohre rift) during the Late Tertiary.