FLUID EVOLUTION AT THE NORTH AMERICAN EMERALD MINE, HIDDENITE, NORTH CAROLINA

The North American Emerald Mine (NAEM), located near Hiddenite, North Carolina, produced 60,000+ carats of emeralds between 1995 and 2012, when the mine went on standby and operations were temporarily ended. The NAEM is located at the former Rist mine site that had produced emeralds since 1882, and is considered to be the most significant North American emerald locality. The emeralds are found in Alpine fissure-type quartz veins hosted by Silurian(?) migmatitic metasedimentary rocks interlayered with calc-silicate rocks that reached upper amphibolite facies (sillimanite grade). These rocks are part of the Brindle Creek thrust sheet (Cat Square Terrane) that overlies metasedimentary and meta-igneous rocks of Silurian to late Proterozoic age.

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.