Deciphering the metal stable isotope record of Sn-W ore deposits: a complementary approach based on experiments and case studies

锡钨矿床中金属的富集、迁移和结晶作用是在高度演化的岩浆热液系统中熔体和流体驱动过程的复杂组合的结果。然而，成矿的各个阶段的确切条件和控制参数尚未得到很好的约束。除了结构和经典的地球化学和矿物学研究，金属的稳定同位素分馏可以提供重要的信息，在金属运输过程中的关键变化，最终导致金属沉积，因为它们的分馏关键取决于它们的键合环境的变化。因此，只要金属键合变化和所产生的同位素分馏之间的关系被很好地理解，或者甚至被校准，同位素特征可以识别金属传输和结晶的条件。 在这里，我们提出了一个互补的方法，结合实验和案例研究（1），以更好地理解和校准熔体流体和锂云母之间的锂同位素分馏精心设计的实验室实验和（2）应用锂和锡同位素代理两个已经很好地表征，但化学和结构不同的花岗岩锡钨锂矿床（萨迪斯多夫，Erzgebirge和Argemela，葡萄牙）。对于Li同位素（主要是Li云母）和Sn同位素，将使用飞秒激光消融（LA-）MC-ICP-MS原位分析锂云母和锡云母。我们期望从这两种同位素系统获得互补信息：锂同位素对熔体出溶过程中流体的性质和断裂作用敏感（第一和第二沸腾）或之间的流体（蒸汽-盐水），这将是实验校准。在蒸汽-盐水分离过程中或在水芒硝结晶过程中，预计Sn同位素主要由于氧化还原变化（从Sn 2+到Sn 4+）而发生碎裂（如文献中的几项研究所示）。在汉诺威已经建立了本研究计划的实验和分析装置，并将Li和Sn同位素系统应用于Sn-W-Li矿床中的寄主矿物，预计将提供有关金属迁移和结晶条件的非常有价值的信息。如果时间允许，我们计划建立原位钨同位素分析的黑钨矿与LA-MC-ICP-MS，验证了高精度的解决方案双穗分析相同的黑钨矿晶体（在科隆）。由于钨同位素分馏过程中的黑钨矿，这是经常与辉绿石在热液脉，不受氧化还原过程的控制，钨的同位素组成可以让我们约束的作用，降低温度在成矿演化。

Enrichment, transport and crystallisation of metals in Sn-W deposits results from a complex combination of melt- and fluid-driven processes in highly evolved magmatic-hydrothermal systems. However, the exact conditions and controlling parameters in the various stages of ore formation are not yet well constrained. In addition to structural and classical geochemical and mineralogical investigations, the stable isotope fractionation of the metals may provide important information on crucial changes during metal transport that finally result in metal deposition, as their fractionation critically depends on changes in their bonding environment. Thus, provided that the relationship between metal bonding changes and resulting isotopes fractionation is well understood, or even calibrated, isotopic signature may fingerprint the conditions of metal transport and crystallisation. Here, we propose a complementary approach, combining experiments and case studies (1) to better understand and calibrate the fractionation of Li isotopes between melt-fluid and Li-micas in well-designed laboratory experiments and (2) to apply the Li and Sn isotope proxy to two already well-characterized, but chemically and structurally distinct granitic Sn-W-Li deposits (Sadisdorf, Erzgebirge and Argemela, Portugal). For Li isotopes (mostly) Li micas and for Sn isotopes cassiterite will be analysed, both in situ with femtosecond laser-ablation (LA-) MC-ICP-MS. We expect complementary information from these two isotope systems: Li isotopes are expected to be sensitive to the nature of the fluid and to fractionate during melt-fluid exsolution (first and second boiling) or between to fluids (vapour-brine), which will be experimentally calibrated. Sn isotopes are expected to mostly fractionate as a result of a redox change (from Sn2+ to Sn4+), during vapour-brine separation or during the crystallization of cassiterite (as indicated by several studies of the literature). The experimental and analytical set up for the investigations planed in this study are already established in Hannover and application of the Li and Sn isotope systems to their host minerals in Sn-W-Li deposits is expected to provide very valuable information on the conditions of metal transport and crystallization. If time allows we plan to establish in situ W isotope analyses of wolframite with LA-MC-ICP-MS, validated with high-precision solution double spike analyses of the same wolframite crystals (in Cologne). As W isotope fractionation during the crystallization of wolframite, which is frequently associated with cassiterites in hydrothermal veins, is not controlled by a redox process, the isotope compositions of W may allow us to constrain the role of decreasing temperatures during the metallogenic evolution.