Metamorphic origin of stratiform cassiterite mineralization in the Schwarzenberg – Aue district – Clues to the metamorphic history and pre-orogenic Sn enrichment of the Erzgebirge (Germany)

The Erzgebirge hosts numerous stratiform tin occurrences, which are located along the same structural level within the Variscan orogenic belt. One of these tin deposits, the Bockau tin occurrence, is located in the area between Aue-Bad Schlema – Bockau – Zschorlau with alternating sequences of quartzite and metaschist. This layered sequence records geochemical signatures of intensely weathered sedimentary rocks. Their protolith probably formed during the Ordovician at the passive continental margin of Gondwana. The mineral assemblage consists of quartz + biotite + garnet + muscovite + andalusite + chlorite + cassiterite + accessory-phases. Cassiterite grains are concentrated in layers concordant to the foliation plane, occur as inclusions within foliation parallel biotite, and are overgrown by garnet blasts. These textures indicate that cassiterite has been present prior to the Variscan continental collision. The frequent occurrence of cassiterite also correlates with the high Sn-contents of the tin bearing metasediments (mainly ranging from < 50 to 2000 ppm). Different methods of conventional thermobarometry and pseudosection modelling were applied to reconstruct the regional metamorphic overprint at 550 ± 50 °C / 8 - 9 kbar and a subsequent thermal peak of at least 600 °C. These results are characteristic for the regional metamorphic conditions reached along the north-western border of the Erzgebirge, though the maximum temperatures reached are slightly higher than previously thought. There is a lack of evidence for a post-magmatic / metasomatic history of the Bockau tin occurrence by the applied methods and petrographic observations.