The fate of Sn in the evolution of a skarn system - the Geyer skarn (Erzgebirge) as a case study

Despite the fact that skarns of the Erzgebirge host significant Sn resources, many well-known and mineralized skarn occurrences in the Erzgebirge remain poorly documented in the scientific literature. This study focuses on the mineralogy and petrography during the pro- and retrograde skarn stages at the Geyer skarn deposit. We also aim at understanding how tin is distributed during late-magmatic cooling and fluid evolution between silicate minerals, cassiterite, skarn-forming and greisen-forming fluids.

The metamorphic rock series hosting the skarn layers is lithologically diverse and are underlain and intruded by variscian granites. In the Geyer deposit, three skarn horizons are distinguished, characterized by subparallel skarn layers up to two meters in thickness.

We selected ~100 samples from nine drill cores from previous exploration campaigns for detailed petrographic investigation of the complex and diverse mineral assemblage at Geyer. Preliminary trace element analyses show the distribution and the evolution of tin in silicate skarn minerals. The prograde skarn assemblage comprises white to pale green clinopyroxene (diopside-hedenbergite series), red and green garnet (grossular-andradite series), minor wollastonite and vesuvianite, being accompanied by magnetite, scheelite and malayaite. Garnet occurs in several generations (with successively decreasing Sn-content) and may occur as euhedral crystals or as colloform anhedral masses that pervasively replace the protolith or occur as vein-infill. Subhedral clinopyroxene often appears densely packed. The skarns often exhibit a skarnoid texture, commonly preserving the original fabric of the replaced metasediments. Coarse-grained, isotropic metasomatic textures are also recognized, but are less abundant and may represent overprinted carbonate-rich layers. The early retrograde stage comprises amphibole, chlorite and Sn-rich epidote group minerals, which occur as veins or replace prograde skarn minerals. Chalcopyrite, sphalerite and arsenopyrite are associated with this retrograde stage. In the late retrograde stage, cassiterite occurs together with fluorite, calcite and quartz.

Understanding the mineral sequence, the occurrence of tin minerals, and the incorporation of tin into various silicate minerals provides valuable information about the formation of the skarns and their economic potential.