FLUID EVOLUTION AT THE NORTH AMERICAN EMERALD MINE, HIDDENITE, NORTH CAROLINA
Emeralds at the NAEM occur in cavities in close association with quartz, muscovite and carbonate, and fluid inclusions (FI) are abundant in emerald, quartz and carbonate minerals. When examined at room temperature, FI in emerald contain two fluid phases. Raman analysis showed that the “bubble” was liquid carbon dioxide with an average density of ~0.74 g×cm-3. Based on room temperature phase ratios, the fluid has a composition of ~80-90 mole% CO2 and 10-20 mole% H2O. FI in spatially associated quartz also contain two fluid phases when observed at room temperature, and the vapor bubble contained CO2. However, the density of the CO2 phase was much lower (~0.1 g×cm-3) compared to FI in emerald. Carbonates also contained two fluid phases at room temperature, and a third phase (CO2 vapor) formed during cooling at about 10°C, indicating that one of the fluids at room temperature is liquid CO2 and the other is liquid water. The FI cannot be heated to homogenization owing to decrepitation from the high internal pressures generated in CO2-rich FI during heating. However, Ca/Mg partitioning data for carbonate minerals indicate a formation temperature of ~250°C. Assuming that the trapping temperature for the FI in emeralds is the same as the temperature of formation of the closely associated carbonates, PVTX data for the system H2O-CO2 indicate a minimum pressure of formation of ~1.1 kbar. CO2-rich fluids are characteristic of medium-grade metamorphic environments, suggesting that emerald formation and regional metamorphism are related.