Abstract Metal-rich fluids that circulate in magmatic-hydrothermal environments form a wide array of economically significant ore deposits. Unravelling the origins and evolution of these fluids is crucial for understanding how Earth’s metal resources form and one of the most widely used tools for tracking these processes is sulfur isotopes. It is well established that S isotopes record valuable information about the source of the fluid, as well as its physical and chemical evolution (i.e. changing pH, redox and temperature), but it is often challenging to unravel which of these competing processes drives isotopic variability. Here we use thermodynamic models to predict S isotope fractionation for geologically realistic hydrothermal fluids and attempt to disentangle the effects of fluid sources, physico-chemical evolution and S mineral disequilibrium. By modelling a range of fluid compositions, we show that S isotope fingerprints are controlled by the ratio of oxidised to reduced S species (SO42−/H2S), and this is most strongly affected by changing temperature, fO2 and pH. We show that SO42−/H2S can change dramatically during cooling and our key insight is that S isotopes of individual sulfide or sulfate minerals can show large fractionations (up to 20‰) even when pH is constant and fO2 fixed to a specific mineral redox buffer. Importantly, while it is commonly assumed that SO42−/H2S is constant throughout fluid evolution, our analysis shows that this is unlikely to hold for most natural systems. We then compare our model predictions to S isotope data from porphyry and epithermal deposits, seafloor hydrothermal vents and alkaline igneous bodies. We find that our models accurately reproduce the S isotope evolution of porphyry and high sulfidation epithermal fluids, and that most require magmatic S sources between 0 and 5‰. The S isotopes of low sulfidation epithermal fluids and seafloor hydrothermal vents do not fit our model predictions and reflect disequilibrium between the reduced and oxidised S species and, for the latter, significant S input from seawater and biogenic sources. Alkaline igneous fluids match model predictions and confirm magmatic S sources and a wide range of temperature and redox conditions. Of all these different ore deposits, porphyry and alkaline igneous systems are particularly well-suited to S isotope investigation because they show relationships between redox, alteration and ore mineralogy that could be useful for exploration and prospecting. Ultimately, our examples demonstrate that S isotope forward models are powerful tools for identifying S sources, flagging disequilibrium processes, and validating hypotheses of magmatic fluid evolution.

中文翻译：

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岩浆-热液流体的硫同位素演化：对成矿过程的洞察

摘要 在岩浆-热液环境中循环的富含金属的流体形成了大量具有经济意义的矿床。解开这些流体的起源和演化对于了解地球金属资源的形成方式至关重要，而硫同位素是追踪这些过程最广泛使用的工具之一。众所周知，S 同位素记录了有关流体来源及其物理和化学演化（即改变 pH、氧化还原和温度）的宝贵信息，但要解开这些竞争过程中的哪一个驱动了同位素变异通常具有挑战性. 在这里，我们使用热力学模型来预测地质现实热液流体的 S 同位素分馏，并试图解开流体源的影响，物理化学演化和 S 矿物不平衡。通过对一系列流体成分进行建模，我们表明 S 同位素指纹受氧化与还原 S 物质的比例 (SO42-/H2S) 控制，而这受温度、fO2 和 pH 值变化的影响最大。我们表明 SO42−/H2S 在冷却过程中会发生显着变化，我们的关键见解是，即使 pH 恒定且 fO2 固定在特定矿物氧化还原缓冲液中，单个硫化物或硫酸盐矿物的 S 同位素也可以显示出较大的分馏（高达 20‰） . 重要的是，虽然通常假设 SO42-/H2S 在整个流体演化过程中是恒定的，但我们的分析表明，这不太可能适用于大多数自然系统。然后我们将我们的模型预测与来自斑岩和浅成热矿床的 S 同位素数据进行比较，海底热液喷口和碱性火成体。我们发现我们的模型准确地再现了斑岩和高硫化超热流体的 S 同位素演化，并且大多数需要 0 到 5‰ 之间的岩浆 S 源。低硫化超热流体和海底热液喷口的 S 同位素不符合我们的模型预测，反映了还原和氧化 S 物种之间的不平衡，对于后者，来自海水和生物源的大量 S 输入。碱性火成岩流体与模型预测相匹配，并确认了岩浆 S 源以及广泛的温度和氧化还原条件。在所有这些不同的矿床中，斑岩和碱性火成岩系统特别适合 S 同位素调查，因为它们显示了氧化还原、对勘探和勘探有用的蚀变和矿石矿物学。最终，我们的例子表明 S 同位素正向模型是识别 S 源、标记不平衡过程和验证岩浆流体演化假设的有力工具。