Ore formation in porphyry Cu-Au-(Mo) systems involves the exsolution of metal-bearing fluids from magmas and the transport of the metals in magmatic-hydrothermal plumes that are subject to pressure fluctuations. Deposition of ore minerals occurs as a result of cooling and decompression of the hydrothermal fluids in partly overlapping ore shells. In this study, we address the role of vapor-like fluids in porphyry ore formation through numerical simulations of metal transport using the Gibbs energy minimization software, GEM-Selektor. The thermodynamic properties of the hydrated gaseous metallic species necessary for modeling metal solubility in fluids of moderate density (100–300 kg/m³) were derived from the results of experiments that investigated the solubility of metals in aqueous HCl- and H₂S-bearing vapors. Metal transport and precipitation were simulated numerically as a function of temperature, pressure, and fluid composition (S, Cl, and redox). The simulated metal concentrations and ratios are compared to those observed in vapor-like and intermediate-density fluid inclusions from porphyry ore deposits, as well as gas condensates from active volcanoes. The thermodynamically predicted solubility of Cu, Au, Ag, and Mo decreases during isothermal decompression. At elevated pressure, the simulated metal solubility is similar to the metal content measured in vapor-like and intermediate-density fluid inclusions from porphyry deposits (at ~200–1,800 bar). At ambient pressure, the metal solubility approaches the metal content measured in gas condensates from active volcanoes (at ~1 bar), which is several orders of magnitude lower than that in the high-pressure environment. During isochoric cooling, the simulated solubility of Cu, Ag, and Mo decreases, whereas that of Au reaches a maximum between 35 ppb and 2.6 ppm depending on fluid density and composition. Similar observations are made from a compilation of vapor-like and intermediate-density fluid inclusion data showing that Cu, Ag, and Mo contents decrease with decreasing pressure and temperature. Increasing the Cl concentration of the simulated fluid promotes the solubility of Cu, Ag, and Au chloride species. Molybdenum solubility is highest under oxidizing conditions and low S content, and gold solubility is elevated at intermediate redox conditions and elevated S content. The S content of the vapor-like fluid strongly affects metal ratios. Thus, there is a decrease in the Cu/Au ratio as the S content increases from 0.1 to 1 wt %, whereas the opposite is the case for the Mo/Ag ratio; at S contents of >1 wt %, the Mo/Ag ratio also decreases. In summary, thermodynamic calculations based on experiments involving gaseous metallic species predict that vapor-like fluids may transport and efficiently precipitate metals in concentrations sufficient to form porphyry ore deposits. Finally, the fluid composition and pressure-temperature evolution paths of vapor-like and intermediate-density fluids have a strong effect on metal solubility in porphyry systems and potentially exert an important control on their metal ratios and zoning.

中文翻译：

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类蒸汽流体是否是用于形成 Cu-Au-Mo 斑岩矿的可行矿石流体？

斑岩 Cu-Au-(Mo) 系统中的矿石形成涉及含金属流体从岩浆中出溶以及金属在受压力波动影响的岩浆热液羽流中的迁移。矿石矿物的沉积是由于部分重叠的矿壳中的热液流体冷却和减压而发生的。在这项研究中，我们通过使用 Gibbs 能量最小化软件 GEM-Selektor 对金属输运进行数值模拟，来解决蒸汽状流体在斑岩矿形成中的作用。模拟金属在中等密度 (100–300 kg/m ³ )流体中的溶解度所必需的水合气态金属物质的热力学性质来源于研究金属在 HCl- 和 H ₂水溶液中的溶解度的实验结果S-轴承蒸气。金属传输和沉淀作为温度、压力和流体成分（S、Cl 和氧化还原）的函数进行了数值模拟。模拟的金属浓度和比率与在来自斑岩矿床的蒸汽状和中等密度流体包裹体以及来自活火山的凝析气中观察到的金属浓度和比率进行了比较。在等温减压过程中，Cu、Au、Ag 和 Mo 的热力学预测溶解度降低。在升高的压力下，模拟的金属溶解度与在斑岩沉积物（约 200–1,800 bar）的蒸气状和中等密度流体包裹体中测得的金属含量相似。在环境压力下，金属溶解度接近活火山气体冷凝物中测量的金属含量（约 1 巴），这比高压环境下低几个数量级。在等容冷却过程中，Cu、Ag 和 Mo 的模拟溶解度降低，而 Au 的溶解度在 35 ppb 和 2.6 ppm 之间达到最大值，具体取决于流体密度和成分。类似的观察结果来自蒸汽状和中等密度流体包裹体数据的汇编，显示 Cu、Ag 和 Mo 含量随着压力和温度的降低而减少。增加模拟流体的 Cl 浓度会促进 Cu、Ag 和 Au 氯化物物种的溶解度。钼溶解度在氧化条件和低 S 含量下最高，金溶解度在中等氧化还原条件和高 S 含量下提高。蒸气状流体的 S 含量强烈影响金属比率。因此，随着 S 含量从 0.1 重量％增加到 1 重量％，Cu/Au 比值下降，而 Mo/Ag 比值则相反；当 S 含量 > 1 wt% 时，Mo/Ag 比率也会降低。总之，基于涉及气态金属物种的实验的热力学计算预测，蒸气状流体可以传输和有效沉淀浓度足以形成斑岩矿床的金属。最后，类蒸汽和中等密度流体的流体成分和压力-温度演化路径对斑岩系统中的金属溶解度有很强的影响，并可能对它们的金属比例和分带产生重要的控制。Mo/Ag 比率也降低。总之，基于涉及气态金属物种的实验的热力学计算预测，蒸气状流体可以传输和有效沉淀浓度足以形成斑岩矿床的金属。最后，类蒸汽和中等密度流体的流体成分和压力-温度演化路径对斑岩系统中的金属溶解度有很强的影响，并可能对它们的金属比例和分带产生重要的控制。Mo/Ag 比率也降低。总之，基于涉及气态金属物种的实验的热力学计算预测，蒸气状流体可以传输和有效沉淀浓度足以形成斑岩矿床的金属。最后，类蒸汽和中等密度流体的流体成分和压力-温度演化路径对斑岩系统中的金属溶解度有很强的影响，并可能对它们的金属比例和分带产生重要的控制。