Metamorphic mobilization of tin in the Erzgebirge

Primary tin (Sn) deposits are commonly associated with highly fractionated, reduced granites derived from melting of evolved crustal rocks. While extreme fractionation is necessary to generate Sn-granites from low-Sn siliciclastic sediments (~5–10 ppm Sn), melting of Sn-enriched source rocks can yield Sn-rich magmas with minimal fractionation. We present an example of Sn-rich mica schists from the Bockau area, Western Erzgebirge that underwent metamorphic Sn enrichment prior to post-orogenic Sn-granite intrusion^[2]. Strata-bound Sn mineralization in the schists average 50–200 µg/g, with local concentrations up to 1 wt%. Microfabric analysis shows that Sn mobilization was linked to progressive deformation. Tin occurs in prograde minerals—biotite and garnet—as structurally bound Sn and as cassiterite (SnO₂) inclusions. A second phase of Sn mobilization is represented as secondary cassiterite inclusions during the retrogression of biotite to chlorite. Comparative whole-rock geochemical analysis of the protoliths^[1], Sn-rich non-retrogressed, and retrogressed schists reveal continuous Si addition throughout metamorphism and significant Fe and Sn enrichment during prograde metamorphism, accompanied by depletion of K, Rb, Ba and a relative decrease in the immobile elements, Al, Ti and Zr. Additionally, Li, B concentrations increased during the prograde and retrograde stages, respectively.

[1] Romer, R.L., and Hahne, K., 2010. Life of the Rheic Ocean: Scrolling through the shale record. Gondwana Research, 17(2-3), pp. 236–253

[2] Romer, R.L., Kroner, U., Schmidt, C., and Legler, C., 2022. Mobilization of tin during continental subduction-accretion processes. Geology, 50(12), pp. 1361–1365.