Controls on the enrichment and transport of As and Sb in magmatic fluids: experimental studies and implications for hydrothermal Au-Ag-As-Sb deposits

世界级岩浆热液矿床的成因在很大程度上是由深部岩浆源对金属的有效提取及其通过岩浆挥发物向上覆环境的运输控制的。然而，在所谓的“岩浆-热液转变”中发生的过程，如岩浆脱气和结晶，仍然知之甚少。因此，要对这些过程进行可靠的定量理解，就需要更好地限制岩浆源和溶解挥发物之间的金属分配，以及金属配合物在输送岩浆挥发相中的稳定性。具体来说，量化压力、温度、氧化还原状态和化学变化如何影响成矿流体中金属的分配和形态是至关重要的。然而，在岩浆热液条件下（300-800⁰C, 0.5-3 kbar）的实验数据相当有限，这是由于采样不淬灭挥发相相关的特殊实验挑战。本提案将通过应用多学科实验方法将实验和分析工作与热力学建模相结合来解决这一知识差距。我们将特别关注As和Sb的行为，以及它们如何影响贵金属Au-Ag的迁移和转移，它们与大量热液矿床有关。通过在热液热压釜中采用基于原位同步加速器的x射线方法和作为合成流体包裹体（SFI）的非原位流体包裹体相结合的方法来探测与硅酸盐熔体平衡的挥发物（流体）。在与岩浆-热液转变相关的P-T条件下获得的光谱数据将确定金属的形态，并允许对元素从熔体到挥发相的转移进行热力学描述。获得的数据将用于构建传质和热力学模型，以评估花岗岩侵入物和溶解挥发物作为as - sb -Au- ag金属来源在形成一系列热液矿床中的作用，从浅成低温热液到卡林型Au/Ag-As-Sb矿床。

The genesis of world-class magmatic-hydrothermal ore deposits is largely controlled by the efficient extraction of metals from a deep magma source and their transport by magmatic volatiles to superjacent environments. However, processes occurring at the so-called ‘magmatic-hydrothermal transition’, as magmas degas and crystallize, remain still poorly understood. Developing a robust quantitative understanding of these processes thus requires better constrains on the partitioning of metals between the magma source and the exsolved volatiles, and on the stability of metal complexes in the conveying magmatic volatile phase. Specifically, it is critical to quantify how changes in pressure, temperature, redox state and chemistry affect the partitioning and speciation of metals in ore-forming fluids. Yet, the experimental data is rather limited at magmatic-hydrothermal conditions ( 300-800 ⁰C, 0.5-3 kbar) due to exceptional experimental challenges associated to sampling unquenchable volatile phases. This proposal will address this gap of knowledge by applying a multidisciplinary experimental approach combining experimental and analytical work with thermodynamic modelling. We will particularly focus on the behaviour of As and Sb, and how they affect the mobility and transfer of precious metals Au-Ag, with whom they are associated in a large number of hydrothermal deposits. Volatiles (fluids) equilibrated with silicate melts will be probed by a combination of in situ synchrotron-based X-ray methods in a hydrothermal autoclave and ex situ fluid entrapment as synthetic fluid inclusions (SFI). Spectroscopic data acquired at P-T conditions relevant for the magmatic-hydrothermal transition, will identify the speciation of metals and allow thermodynamic description of the transfer of elements from the melt to the volatile phase. The obtained data will be employed to construct mass transfer and thermodynamic models to assess the role of granitic intrusions and exsolved volatiles as a source of As-Sb-Au-Ag metals in the formation of a range of hydrothermal ore deposits, from epithermal to Carlin-type Au/Ag-As-Sb deposits, at shallower depths.