Abstract Hydrothermal fluids in orogenic and intrusion-related (mesothermal) gold deposits are dominated by saline-aqueous-carbonic fluids, commonly represented by the ternary system H2O-NaCl-CO2. The phase equilibria, thermodynamic properties (PVTx, density, compositions of fluid phases) and quartz dissolution-precipitation behavior in the H2O-NaCl-CO2 system, which are still poorly constrained, are of great importance to understanding the process of hydrothermal gold mineralization. Here, we conducted thermodynamic modeling to constrain the fluid properties under single- and two-phase conditions in the H2O-NaCl-CO2 system at temperatures of 300 to 500 °C and pressures of 0.001 to 3.5 kbar. Our results illustrate thermodynamic controls rooted in the equilibria of the H2O-NaCl-CO2 system. Increasing CO2 and/or NaCl contents shift the solvus to higher pressures and temperatures, expanding the pressure–temperature region of L + V immiscibility. Calculated isopleths of CO2 content in coexisting, immiscible vapor and liquid describe the maximum amount of CO2 that may be present in fluid inclusions through the studied P-T-x ranges. Quartz solubility in the H2O-NaCl-CO2 fluids shows strong dependence on temperature, pressure, and CO2 content, with several potential triggers for vein mineral deposition. Specifically, solubility of quartz generally decreases with decreasing temperature, pressure, and increasing CO2 content both in the single- and two-phase fluids, but exhibits retrograde behavior in the L + V field or at the phase-transition boundary. In detail, the dependence of quartz solubility on pressure is weak at low temperatures (300 °C), and becomes progressively stronger at high temperatures (400 °C and 500 °C), vice versa for temperature dependency at different pressures. The present study reviews two mechanisms of fluid immiscibility and constrain the conditions at which distinct fluid inclusion types, bedding-parallel shear veins and fault-related extension veins are formed in mesothermal gold deposits. Thermodynamic modeling of the solvus of the H2O-NaCl-CO2 system is consistent with the hypothesis that decompression is an efficient mechanism driving fluid immiscibility at specific ranges of temperature and composition, producing coexisting liquid- and vapor-phase fluids, both with low density and relatively low content of CO2. For fluids with high CO2 contents (>10 mol. %), cooling at relatively high pressure may also lead to fluid immiscibility, producing a liquid with low CO2 content in equilibrium with a vapor of medium to high CO2 content. Very CO2-rich, or “pure CO2” fluid inclusions (>90 mol. % CO2, type I) cannot be produced by immiscibility under these conditions, but may rather be the result of decrepitation of primary H2O-NaCl-CO2 fluid inclusions. Within the P-T-x ranges studied here, cooling-dominated phase separation accounts for the formation of fluid inclusions with moderate to high CO2 contents (50–80 mol. % CO2, type IIa). Fluid inclusions with low to moderate CO2 contents (5–30 mol. % CO2, type IIb) can represent the original ore-forming fluids, or can be produced by decompression-dominated phase separation. And H2O-NaCl fluid inclusions (type III) generally represent the latest-stage ore-forming fluids, or can be produced by decompression-induced immiscibility at high temperature. In orogenic (mesothermal) gold deposits, decompression-induced quartz precipitation during pressure fluctuation is dominant in bedding-parallel shear veins. Fault-related extension veins are associated with initial decompression-induced quartz precipitation and subsequent cooling-dominated deposition.

中文翻译：

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石英在咸水碳酸盐流体中的相平衡、热力学性质和溶解度：在造山和侵入相关金矿床中的应用

摘要 造山和侵入相关（中温）金矿床中的热液流体以盐-水-碳流体为主，通常以 H2O-NaCl-CO2 三元体系为代表。H2O-NaCl-CO2 系统中的相平衡、热力学性质（PVTx、密度、流体相组成）和石英溶解-沉淀行为仍然很受限制，对于理解热液金矿化过程具有重要意义。在这里，我们进行了热力学建模，以在 300 至 500 °C 的温度和 0.001 至 3.5 kbar 的压力下限制 H2O-NaCl-CO2 系统中单相和两相条件下的流体特性。我们的结果说明了植根于 H2O-NaCl-CO2 系统平衡的热力学控制。增加 CO2 和/或 NaCl 含量将固溶线转移到更高的压力和温度，扩大 L + V 不混溶的压力 - 温度区域。在共存的、不混溶的蒸气和液体中计算出的 CO2 含量等值线描述了在所研究的 PTx 范围内流体包裹体中可能存在的最大 CO2 量。石英在 H2O-NaCl-CO2 流体中的溶解度显示出对温度、压力和 CO2 含量的强烈依赖性，具有多种潜在的脉矿沉积触发因素。具体而言，石英的溶解度通常随着温度、压力的降低以及单相和两相流体中 CO2 含量的增加而降低，但在 L + V 场或相变边界处表现出逆行行为。详细，石英溶解度对压力的依赖性在低温（300°C）下很弱，在高温（400°C 和 500°C）下变得越来越强，反之亦然对于不同压力下的温度依赖性。本研究回顾了流体不混溶的两种机制，并限制了在中温金矿床中形成不同流体包裹体类型、层理平行剪切脉和断层相关延伸脉的条件。H2O-NaCl-CO2 系统固溶线的热力学模型与以下假设一致，即减压是在特定温度和成分范围内驱动流体不混溶的有效机制，产生共存的液相和气相流体，具有低密度和CO2 含量相对较低。对于 CO2 含量高 (>10 mol. %) 的流体，在相对较高的压力下冷却也可能导致流体不混溶，产生低 CO2 含量的液体与中等至高 CO2 含量的蒸汽平衡。非常富含 CO2 或“纯 CO2”流体包裹体（>90 mol.% CO2，I 型）在这些条件下不能通过不混溶产生，而可能是原始 H2O-NaCl-CO2 流体包裹体爆裂的结果。在此处研究的 PTx 范围内，冷却主导的相分离是形成中等至高 CO2 含量（50–80 mol.% CO2，IIa 型）的流体包裹体的原因。具有低至中等 CO2 含量（5-30 mol.% CO2，IIb 型）的流体包裹体可以代表原始成矿流体，也可以通过减压主导相分离产生。而H2O-NaCl流体包裹体（III型）一般代表最新阶段的成矿流体，或者可以通过减压诱导的高温不混溶产生。在造山（中温）金矿床中，压力波动期间减压引起的石英沉淀在顺层剪切脉中占主导地位。断层相关的延伸脉与初始减压诱导的石英沉淀和随后的冷却为主的沉积有关。