VARIABLE COMPOSITIONS OF FLUIDS IN EMERALD DEPOSITS

Emerald deposits form under a wide variety of genetic conditions, but all are associated with either (1) pegmatite or granite bodies, or (2) thrust-shear zones that involve Cr(V)-bearing rocks that are disconnected from pegmatites. As part of a larger, ongoing study to categorize fluid compositions associated with various types of mineral deposits, we have conducted a detailed examination of fluids associated with emerald deposits. This information can be used to distinguish between synthetic and natural emeralds, and the provenance may be determined in some cases based on the fluid compositions, including the δ¹⁸O vs δD isotopic composition of H₂O from channels in the crystal structure. For example, emeralds from Columbia contain a distinct suite of daughter minerals in fluid inclusions that distinguish these stones from those from other locations, and from synthetic emerlads.

The composition of fluids in inclusions in natural emeralds shows wide variability that reflects the environment of formation. For example, some pegmatite-related deposits such as Dyakoy, China (Xue, G., 2010 Econ. Geol.) are characterized by low-salinity H₂O-NaCl fluids, whereas fluids in the Rila deposit, Bulgaria (Alexandrov P., 2001, Econ. Geol.) also contains CO₂. The Delbegetey, Kazakhstan granite-related emerald deposit, is characterized by H₂O-CO₂-NaCl fluids containing up to 10 mol% CH₄, with salinities between 4-10 wt. % NaCl (Gavrilenko, E.V., 2006, Mineral. Mag.). The shear zone-hosted Coscuez, Chivor, Vega and San Juan (Colombia) deposits are characterized by high salinity, H₂O-NaCl-CaCl₂-KCl-CO₂-N₂ fluids (Cheilletz, A., 1994, Econ. Geol.; Giuliani, G., 1995, Eur. J. Mineral.). Some such as the Franqueira granite pegmatite deposit (Fuertes-Fuente, M., 2000, Can. Mineral.) show a fluid evolution that started from H₂O-CO₂-NaCl±CH₄ that evolved to a volatile-free low salinity H₂O-NaCl fluid.