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.

在岩石和矿床形成期间重建氧化还原条件通常仍然是一项具有挑战性的任务，特别是如果无法确定缓冲氧逸度 ƒO _{2 的}反应。这些信息对于众多矿床类型的遗传模型至关重要，因此长期以来一直存在着近年来不断增加的研究活动，以解决高温流体中金属的络合和氧化还原状态问题。在这项研究中，我们限制ƒO ₂为在主矿石矿物锡石，的SnO初级流体夹杂物的组成锡矿形成期间₂。我们表明，来自世界各地（来自热液脉、格雷森、矽卡岩矿床和伟晶岩）的分析锡石样品在 ƒO₂在 QFM（石英-铁橄榄石-磁铁矿）和 NNO（氧化镍-镍）之间，对于某些矿脉/灰泥矿床，氧化条件高达 MH（磁铁矿-赤铁矿）。这与原位拉曼光谱实验一起支持了以下假设：在许多（如果不是大多数）形成锡矿的热液流体中，Sn(IV) 优于 Sn(II)，这意味着氧化还原反应的作用比先前地层中假设的要小热液锡矿床模型。