TRACE ELEMENT VARIATIONS AND AGE OF COLOMBIAN EMERALDS

The Colombian emerald deposits are recognized as the most economically significant emerald deposits in the world, and are hosted in hydrothermally altered black shales on either side of the Cordillera Oriental--a setting unique compared to other known emerald deposits. As such, uncertainty persists in the path or source of the trace metals (namely chromium and vanadium) which are responsible for the brilliant green color in these emeralds. We have used electron microprobe analysis to study variations in trace element concentrations in transects across Colombian emeralds, and used the ⁴⁰Ar/³⁹Ar method to determine the age of the Muzo emerald deposits, located on the western side of the Cordillera Oriental.

EMP analysis of lightly colored emerald crystals from the Puerto Arturo, Matefique, Taquendama, and Catedral mines indicate that higher concentrations of trace metals (Cr, V, Sc, Mn, Mg, and Fe) coincide with lower concentrations of aluminum, and suggest that these metals substitute for aluminum in the beryl crystal structure. In addition, deeper green color in the samples (in particular from Matefique) is correlated with higher concentrations of vanadium and chromium, while other trace metal concentrations show very little variation with color, suggesting V and Cr are the main chromophores. More strongly colored samples contain an average of 7910.5 ± 2442.45 (1s) ppm V and 4062.83 ± 853.59 (1s) ppm Cr, while weakly colored samples show only 1044.89 ± 782.55 (1s) ppm V and 574.43 ± 686.68 (1s) ppm Cr. In future, we plan to further investigate the source of these trace metals.

We obtained ⁴⁰Ar/³⁹Ar weighted mean ages on hydrothermal mica (muscovite) found in wall rock at the margins of emerald-bearing veins. The ages of two mica samples from the Muzo mine are 62.05 ± 0.40 Ma (2s) and 60.54 ± 0.63 Ma (2s). Previous ⁴⁰Ar/³⁹Ar ages from the western Cordillera Oriental include 35-38 Ma at Coscuez and 32 Ma at Muzo (Cheillitz et al., 1994). However, ages from the eastern Cordillera Oriental reported by Cheillitz et al. (1994) are 65 Ma, and Ordonez et al. (2001) have obtained 61 to 67 Ma Rb/Sr ages for emeralds from both the eastern and western Cordillera Oriental. Our ages indicate that hydrothermal activity affected the Muzo area around 61 to 62 Ma as well, and establish the need for additional isotopic dating of the Muzo emerald deposit.