We performed experiments to determine the solubility of Au and Pd in magmatic aqueous fluids as a function of oxygen fugacity (ƒO₂), temperature (T), pH and total chloride concentration (Cl_total). Experiments were conducted at 800–1000 °C and 200 MPa in an externally-heated rapid-quench Molybdenum-Hafnium Carbide (MHC) cold-seal pressure vessel assembly. We employed a synthetic fluid inclusion (SFI) technique to entrap equilibrated, hydrothermal fluids in response to in situ fracturing of quartz cylinders at experimental run conditions. The solubility of Au and Pd both have positive relationships with ƒO₂, temperature, acidity and chlorinity. Concentrated aqueous brines containing 63 wt. % NaCl can dissolve wt. % levels of Au (∼1.2 wt. %) and Pd (∼1.7 wt. %) at metal saturation in relatively oxidized conditions, 1.44 log units above the Ni-NiO oxygen buffer (NNO+1.44), and mildly acidic pH at 900 °C and 200 MPa. Thermodynamic modeling of experimental results suggests that Au is mainly transported as AuCl_(aq) at high pH and low Cl_total conditions, whereas HAuCl_2(aq) and potentially AuCl₂^–_(aq) predominates at low pH and high Cl_total conditions. Results from thermodynamic modeling also suggest Pd is mobilized in significant contributions by both PdCl_2(aq) and PdCl₃^–_(aq) with the latter gaining predominance in response to increasing Cl_total. Calculated fluid/melt partition coefficients for Au and Pd in low-density, magmatic vapors at 1000 °C and 200 MPa suggest that Pd may experience fractionation from Au in porphyry Au-Cu (±Pd, Pt) systems due to the restricted compatibility of Pd in the fluid phase (requiring strongly acidic and substantially high ƒO₂ conditions). Moreover, high-density, concentrated aqueous brines facilitate the compatibility of Pd in the fluid phase which may be important with respect to the formation of platinum-group element (PGE)-enriched horizons in layered mafic intrusions (e.g., J-M Reef, Stillwater Complex, U.S.A.). The potential for magmatic, near-neutral pH, high-salinity brines to dissolve significant amounts of Pd as Pd(II)-chloride complexes (∼400 to ∼900 µg/g) well below the NNO buffer suggests that such fluids may be responsible for late-stage hydrothermal remobilization of Pd within mafic-ultramafic igneous environments (e.g., Cu-Ni-PGE footwall deposits and low-sulfide PGE deposits in the Sudbury Igneous Complex, Canada).

我们进行了实验，以确定 Au 和 Pd 在岩浆含水流体中的溶解度作为氧逸度 (ƒO ₂ )、温度 ( T )、pH 和总氯化物浓度 (Cl _total )的函数。实验是在 800–1000 °C 和 200 MPa 下在外部加热的快速淬火碳化铪 (MHC) 冷密封压力容器组件中进行的。我们采用合成流体包裹体 (SFI) 技术捕获平衡的热液流体，以响应石英圆柱体在实验运行条件下的原位压裂。Au 和 Pd 的溶解度均与 ƒO ₂呈正相关、温度、酸度和氯度。含有 63 重量份的浓缩盐水。% NaCl 可以溶解wt。在相对氧化的条件下，金属饱和度下 Au（~1.2 wt.%）和 Pd（~1.7 wt.%）的 % 水平，比 Ni-NiO 氧缓冲液 (NNO+1.44) 高 1.44 log 单位，以及 900 时的弱酸性 pH °C 和 200 兆帕。实验结果的热力学模型表明，Au在高 pH 值和低 Cl_总量条件下主要作为 AuCl _(aq)传输，而 HAuCl _2(aq)和潜在的 AuCl ₂ ^– _(aq)在低 pH 值和高 Cl_总量条件下占主导地位。热力学模型的结果还表明 Pd 在 PdCl _2(aq)和 PdCl ₃ ^– _(aq)，后者随着 Cl _{total 的}增加而获得优势。在 1000 °C 和 200 MPa 的低密度岩浆蒸汽中计算的 Au 和 Pd 的流体/熔体分配系数表明，由于斑岩型 Au-Cu (±Pd, Pt) 系统中 Pd 的相容性受限，Pd 可能会从 Au 中分馏流体相中的 Pd（需要强酸性和相当高的 ƒO ₂状况）。此外，高密度、浓缩的盐水有利于 Pd 在流体相中的相容性，这对于层状镁铁质侵入体（例如，JM Reef、Stillwater Complex）中富铂族元素 (PGE) 层的形成可能很重要， 美国）。岩浆、接近中性 pH 值、高盐度的盐水以远低于 NNO 缓冲液的 Pd(II)-氯化物复合物（~400 至~~900 µg/g）的形式溶解大量 Pd 的潜力表明，这些流体可能是原因用于在镁铁质-超镁铁质火成岩环境（例如，加拿大萨德伯里火成岩复合体中的 Cu-Ni-PGE 下盘矿床和低硫化物 PGE 矿床）中 Pd 的后期热液再动员。