The behaviour of tin in skarns of the Variscan Erzgebirge – the Geyer skarn system as a case study

This study focuses on the mineralogy and petrography of pro- and retrograde skarn stages at the Geyer Sn system in the Erzgebirge, Germany. These data are used to constrain how tin is distributed amongst silicate minerals and cassiterite, and how given tin-bearing minerals are related to the temporal fluid evolution. Skarns at Geyer are associated to comparatively thin (> 2 m) but laterally extensive marble layers and immediately adjacent mica schists that occur within meta-sedimentary successions of the Saxothuringian basement.

Prograde skarn stage I is characterized by diopside-rich skarnoids with locally reddish garnet. Prograde stage II comprises garnet-pyroxene skarns with massive greenish garnet. The early retrograde stage III is characterized by epidote and amphibole, whereas the late retrograde stage IV is associated with pervasive chloritization. Calc-silicates of stage I have low tin contents and discrete tin minerals are absent. In contrast, garnet of prograde skarn stage II contains up to 0.25 apfu and malayaite occurs locally. Early retrograde epidote and amphibole have elevated tin contents with up to 0.25 apfu and 0.4 apfu, respectively, whereas most of the cassiterite is associated with late retrograde chlorite of stage IV. The retrograde overprint is hence associated with a redistribution and possibly renewed introduction of tin and eventually results in the formation of cassiterite during the late retrograde stage. The observations imply that inheritance of Sn from meta-sedimentary host rocks is negligible. Sn mineralization also post-dates the contact metamorphism (skarnoid) and hence suggest that Sn is mainly introduced by the hydrothermal fluid.