Phase equilibria, thermodynamic properties, and solubility of quartz in saline-aqueous-carbonic fluids: Application to orogenic and intrusion-related gold deposits
Geochimica et Cosmochimica Acta(2020)
摘要
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.
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关键词
H2O-NaCl-CO2,Thermodynamic properties,Quartz solubility,Fluid inclusions,Mesothermal gold deposits
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