U-Pb ages of ferberite, chalcedony, agate, 'U-mica' and pitchblende: constraints on the mineralization history of the Schwarzwald ore district

摘要

This study deals with the use of new and unusual geochronometers applied to a variety of mineralizations in the Schwarzwald region of southwest Germany. In detail, we present new U-Pb age data of ferberite (from Clara Mine), chalcedony (from Silberbrunnle Mine), agate (Geisberg), carneol ("Markgrafler Land"), and confirm earlier geochronologic work on multistage U-mineralizations (Wittichen and Menzenschwand). Using these various geochronometers, we show both their usefulness, precision and robustness by comparing our age data to earlier work, and we can augment existing data with our new dataset to draw a picture of the timing of late- to post-Variscan hydrothermal mineralization in the region. The combined dataset reveals that vein-type mineralizations in the Schwarzwald have formed episodically over the last 300 Ma with distinct peaks of mineralization around 300 Ma (late-Variscan), several periods between 200 and 100 Ma (possibly related to the opening of the North Atlantic Ocean in the Jurassic and early Cretaceous), and around 40-20 Ma (related to the opening of the Rhinegraben). This pulsed formation of hydrothermal deposits is not unique for the Schwarzwald, as it has also been documented for the more richly mineralized Erzgebirge and other parts of post-Variscan Europe. The wide age range found within some districts or vein systems does not represent an analytical artefact, but reflects multiple reactivations of the same structures associated with changes in the stress distribution at continental margins. Comparison with the Erzgebirge shows, however, that the intensity of mineralization in a given time period shows regional differences, which reflect differences in regional tectonics.