Normalized rare earth element patterns of rock and mineral samples from evolved granitic systems and associated hydrothermal tin deposits frequently show an unusual split into four consecutive curved segments, referred to as tetrads. In the present contribution, the simultaneous occurrence of complementary convex and concave tetrads is described for the first time for vein fluorite, suggesting that this unusual trace-element signature may develop during a single evolutionary stage within the hydrothermal environment. Based on geochemical analysis of vein fluorite, fluid inclusion studies, and thermodynamic modeling, it is shown that fractionation of the rare earth elements can be linked to fluid immiscibility and preferential partitioning of these elements between vapor and coexisting liquid. The findings provide the first direct evidence constraining the geological conditions responsible for the occurrence of the tetrad effect in tin-bearing magmatic-hydrothermal systems and require a reassessment of the current understanding of the origin of tin deposits.

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