Is metamorphic Sn redistribution an important step in the generation of giant Sn deposits?

• Applicants: Privatdozent Dr. Johannes Glodny; Privatdozent Dr. Uwe Kroner; Professor Dr. Rolf Ludwig Romer
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 521230495

Important primary Sn deposits are bound to geochemically highly specialized S-type granites that are characterized by extreme fractional crystallization under reduced conditions. Tin enrichment, however, is not restricted to magmatic and hydrothermal processes at the emplacement level, but seems to be strongly influenced by the type of protolith and the melting conditions. For instance, muscovite-dehydration (low-T) melting partitions Sn into the restite, whereas biotite-dehydration (high-T) melting transfers Sn into the partial melt. Strong enrichment of Sn in the partial melt requires loss of early melts, which are particularly voluminous in protoliths that had experienced intense chemical weathering. Similarly, prograde metamorphic redistribution of Sn also may result in the enrichment of Sn in the protolith. Traditionally, metamorphic mobilization of Sn is considered not to be relevant. There are, however, Sn-rich metamorphic minerals in some skarns (e.g., Hämmerlein) and some quartz-mica-schists (e.g., Bockau and Aue) of the Erzgebirge that are clearly older than the emplacement of local Variscan Sn-specific granites, implying that Sn was indeed mobile during metamorphism. Thus, the question is whether such mobilization may be essential for protolith enrichment and for the later formation of magmatic Sn deposits. This project focuses on Sn-rich quartz-mica-schists (typically 200 ppm Sn, but on dm-scale up to 5000 ppm Sn) of the Bockau area and local quartz-cassiterite mineralization within these schists (with >1wt% Sn) addressing the following questions: (i) There are several textual and structural types of cassiterite. Under which P-T-d conditions of their hosts was Sn mobile and did form the various types of cassiterite. What is the U-Pb age of the various types of cassiterite; (ii) Which elements were mobile (added or lost) together with Sn and what is the source of these metals. The problem is addressed using the chemical composition of unmineralized and variably mineralized mica schists to characterize element mobility, and the Li, B, Sr, Nd, and Pb isotopic compositions of these rocks to constrain the source; (iii) Did the chemical composition of the fluid change over time and does the composition of the fluid influence whether Sn substitutes in silicate minerals or forms a phase of its own? To address this question, we use the chemical zonation of biotite, garnet, and (whenever possible) cassiterite. Similar stratiform Sn mineralization (but with sulfide minerals) occurs in corresponding tectonic units in Poland and the Czech Republic. Comparison with these small deposits is used to distinguish between general and local mineralization features. A key question of the project is to investigate whether metamorphic mobilization of Sn in the pre-collisional accretionary wedge is a process that makes the difference between a small Sn mineralization and a major Sn deposit in granites that formed by melting of meta-sedimentary rocks.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2238: Dynamics of Ore-Metals Enrichment - DOME