Mobilization of tin during metamorphism in the Variscan Orogeny

The Erzgebirge, a major tin (Sn) province in the Variscan orogenic belt, hosts substantial Sn deposits associated with post-kinematic granites, alongside the minor stratabound Sn deposits found in the metamorphic schists of the western Erzgebirge. These schists are derived from Early Ordovician siliciclastic sediments, which were metamorphosed during the Variscan orogeny and subsequently intruded by post-Variscan granites between 325 and 314 Ma. Cassiterite (SnO₂) inclusions and elevated Sn concentrations in metamorphic minerals indicate that Sn was already present in the rocks prior to the intrusion of granites. Early Ordovician sediments preserved in Thuringia typically contain approximately 20 ppm Sn, implying that Sn (reaching locally up to 5000 ppm) was added during prograde metamorphism that started at c. 400 Ma, as confirmed by U-Pb cassiterite dating ^[2]. Retrogression of the schists also resulted in the formation of a second generation of cassiterite. To differentiate the two phases of Sn mobilization and to estimate elements introduced during prograde metamorphism from those mobilized during retrogression, we compare the compositions of the protoliths ^[1]with non-retrogressed schists and of non-retrogressed with retrogressed schists, respectively. During prograde metamorphism there was a marked increase in Si, Fe, and Sn. During retrogression, which involved the formation of chlorite at the expense of biotite, K, Rb and Ba were lost and Sn released from biotite formed a second generation of metamorphic cassiterite. The light elements Li and B were also mobile, with preferential incorporation of Li into biotite during prograde metamorphism and B into chlorite during retrogression. Such enrichment of Sn during metamorphism is important to form source rocks whose partial melting can produce Sn-rich granites.

[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.