The sulfur isotope evolution of magmatic-hydrothermal fluids: insights into ore-forming processes
The sulfur isotope evolution of magmatic-hydrothermal fluids: insights into ore-forming processes
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DOI：
10.1016/j.gca.2020.07.042
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发表时间：
2020-11-01
影响因子：
5
通讯作者：
Boyce, Adrian J.
中科院分区：
文献类型：
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作者：
Hutchison, William;Finch, Adrian A.;Boyce, Adrian J.
在岩浆-热液环境中循环的富金属流体形成了一系列具有经济意义的矿床。揭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，而追踪这些过程最广泛使用的工具之一是硫同位素。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pH值、氧化还原和温度的变化）的有价值的信息，但揭示这些相互竞争的过程中哪一个驱动了同位素变化往往具有挑战性。本文利用热力学模型预测了地质上真实的热液流体的S同位素分馏，并试图解开流体来源、物理化学演化和S矿物不平衡的影响。通过模拟一系列流体成分，我们发现S同位素指纹图谱受氧化与还原S物种（SO42-/H2S）的比例控制，并且受温度变化的影响最大。我们发现SO42-/H2S在冷却过程中会发生巨大变化，我们的关键见解是，即使pH恒定且fO(2)固定在特定的矿物氧化还原缓冲液中，单个硫化物或硫酸盐矿物的S同位素也会显示出较大的分馏（高达20‰）。重要的是，虽然通常认为SO42-/H2S在流体演化过程中是恒定的，但我们的分析表明，这对大多数自然系统来说不太可能成立。然后，我们将我们的模型预测与来自斑岩和浅成热液矿床、海底热液喷口和碱性火成岩体的S同位素数据进行比较。我们发现我们的模型准确地再现了斑岩和高硫化度低温热液的S同位素演化，并且大多数需要0 - 5‰的岩浆S源。低硫化度低温热液和海底热液喷口的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，并证实了岩浆S源和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于S同位素研究，因为它们显示了氧化还原、蚀变和矿石矿物学之间的关系，可能对勘探和找矿有用。最后，我们的例子表明，S同位素正演模型是识别S来源、标记不平衡过程和验证岩浆流体演化假设的有力工具。(C) 2020作者。Elsevier Ltd.出版。
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, fO(2) 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 parts per thousand) even when pH is constant and fO(2) 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 parts per thousand. 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. (C) 2020 The Authors. Published by Elsevier Ltd.