Tin isotope composition as a fingerprint for the evolution of granitic ore deposits

Understanding the formation of Sn deposits, with cassiterite (SnO₂) commonly being the predominant ore mineral, is crucial for efficient mining and ensuring sustainable Sn utilization. Since cassiterite crystallization is accompanied by redox change from mainly Sn²⁺ in silicate melts (or fluids) to Sn⁴⁺ in SnO_2, and heavier tin isotopes preferentially bond to Sn⁴⁺, studying Sn isotope compositions may help track the redox conditions in the melt and/or fluid during Sn enrichment and cassiterite crystallization.

As a case study, we comparatively investigated the Sn isotope and trace element evolution of cassiterites from the rare metal granite system of Argemela (Portugal) and the greisen system of Sadisdorf (Germany). While in Argemela, Sn transport and cassiterite formation are thought to have mainly occurred by melts, metal transport and enrichment in Sadisdorf occurred during fluid-assisted processes.

Tin isotope and trace element analyses of individual zones reflecting different crystallization stages of the cassiterite grains were conducted in situ using femtosecond-LA-MC-ICP-MS. This method reveals conditions and evolution of the metal transport medium during cassiterite crystallization and ore formation.

Preliminary results indicate that cassiterite crystals from Sadisdorf have frequently high W concentrations and a trend from lighter Sn isotope ratios in the core towards heavier values at the rim, indicating increasing oxidizing conditions during crystallization. In contrast, crystals from Argemela mostly show high Nb-Ta concentrations and decreasing Sn isotope values from core to rim, possibly explained by Rayleigh crystallization. The ore-forming conditions in both localities will be further tackled by the investigation of Li isotopes in Li-micas.