Contrasting Sn isotope signatures in cassiterites of melt- and fluid-dominated granitic ore systems

Cassiterite (SnO₂) is the main tin ore mineral. Hence, understanding the controlling factors for cassiterite crystallization/precipitation are a crucial basis for developing ore deposit models for exploration and mining.

The precipitation of cassiterite is inherently linked to a change in Sn speciation, from Sn²⁺ in silicate melts or hydrothermal fluids to Sn⁴⁺ in the oxide mineral. The preferential enrichment of heavier Sn isotopes in oxidized Sn⁴⁺ species makes variations in Sn isotope ratios a promising tool allowing to constrain redox conditions during transport, concentration, and deposition. To better understand Sn-isotope fractionation in ore-forming environments, we examined cassiterite from a magmatic and magmatic-hydrothermal occurrence: the Argemela rare-metal granite system in Portugal and the Sadisdorf greisen system in Germany, because they represent different environments of Sn mobilization and deposition, namely mainly melt-driven at Argemela (magmatic cassiterite) and fluid-driven at Sadisdorf (hydrothermal cassiterite). High-resolution, in situ measurements of Sn isotopes and trace elements were carried out on distinct growth zones within individual cassiterite crystals using UV femtosecond laser ablation multi-collector ICP-MS. The results reveal clear contrasts between magmatic and hydrothermal cassiterite. Hydrothermal cassiterite from Sadisdorf commonly displays elevated W contents and an increase in δ^124/117Sn values from core to rim, suggesting that oxidation occurred during precipitation. At Sadisdorf, vein-hosted cassiterite shows a spatial trend from positive δ^124/117Sn values in proximal greisen to negative δ^124/117Sn values in more distal veins. This systematic difference suggests progressive reduction along the fluid flow path, recorded in the Sn isotope signatures. In contrast, cassiterite crystals from Argemela are enriched in Nb and Ta, and some grains show decreasing δ^124/117Sn values toward their crystal rims, which can be explained by Rayleigh crystallization.

These preliminary findings indicate that Sn isotopes are a suitable tracer of redox conditions and processes during Sn transport and cassiterite crystallisation. Ongoing Li isotope analyses of Li-bearing micas will provide additional constraints on the nature of the fluid and ore-forming conditions in both granitic systems.