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)

Abstract

Here we present petrographic and field observations, mineral and whole-rock compositions as well as conventional thermobaromerty and thermodynamic modelling of the Sn-mineralized schists near Bockau, Erzgebirge (Germany). The metamorphic mineral assemblage comprises locally abundant cassiterite (up to 2 wt%), which exhibits metamorphic features indicating pre-orogenic Sn enrichment of the sedimentary protoliths. The results imply that the composite schist sequence of Bockau underwent regional metamorphism at 0.8–0.9 GPa and 550 ± 50 °C and a subsequent relatively hot-exhumation path between 600 and 700 °C. Flat compositional zoning profiles of garnet suggest that these elevated temperature conditions were not short-lived but likely lasted for a time period of 20 to 40 Ma leading to the development of retrograde mineral assemblages randomly overgrowing cassiterite. The textural position of cassiterite indicates that its formation is predominantly related to the prograde/collisional stage of the Variscan orogeny (∼340 Ma) and therefore is linked to metamorphic reactions and hence pre-dates widespread LiSn granites of the area (325–310 Ma). The presented observations of the Bockau schists provide the first direct evidence for pre-orogenic Sn enrichment of their sedimentary protoliths. This has two important implications: first, the meta-sedimentary units of the Bockau series as a part of the Frauenbach super unit may host extensive, yet underexplored Sn resources along its entire strike length (> 40 km). Second, melting and assimilation of cassiterite-bearing Bockau schists may have significantly contributed to the formation of Sn-enriched granites within the Saxothuringian Zone of the European Variscides (and elsewhere). Hence, the presence of Sn-enriched (meta) sedimentary units could be an important prerequisite for Sn fertility of a metallogenic region. The ultimate pre-metamorphic source and the cause of sedimentary Sn-enrichment remain poorly constrained.