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Abstract

Reconstruction of the redox conditions during formation of rocks and ore deposits often remains a challenging task, particularly if no reactions buffering the oxygen fugacity ƒO2 can be identified. This information is crucial for genetic models of numerous ore deposit types, and consequently there has been long-standing and in recent years increasing research activity addressing complexation and redox state of metals in fluids at high temperatures. In this study, we constrain ƒO2 during tin ore formation from the composition of primary fluid inclusions in the principal ore mineral cassiterite, SnO2. We show that the analysed cassiterite samples from worldwide locations (from hydrothermal veins, greisens, skarn deposits, and pegmatites) precipitated at ƒO2 between QFM (quartz-fayalite-magnetite) and above NNO (nickel-nickel oxide), with more oxidizing conditions up to MH (magnetite-hematite) for certain vein/greisen deposits. This, together with in situ Raman spectroscopic experiments, supports the hypothesis that Sn(IV) prevailed over Sn(II) in many if not most tin-ore forming hydrothermal fluids, which implies that redox reactions play a smaller role than assumed in previous formation models of hydrothermal tin deposits.