⁴⁰Ar/³⁹Ar Ages and Trace Element Variations in Colombian Emeralds

The Colombian emerald deposits are globally unique because they are hosted in hydrothermally altered black shales on either side of the Cordillera Oriental. We used electron microprobe analysis to study variations in trace element concentrations in transects across emeralds, and used the ⁴⁰Ar/³⁹Ar dating method to determine the age of mica and albite from the Muzo emerald deposits, located on the western side of the Cordillera. EMP analysis of emeralds indicate that higher concentrations of trace metals (Cr, V, Sc, Mn, Mg, and Fe) coincide with lower concentrations of Al, and suggest that these metals substitute for Al in the beryl crystal structure. Deep green color is correlated with high concentrations of Vanadium and Cr, which are the main chromophores proposed by previous studies. Strongly colored samples average 7911 ± 2442 (1σ) ppm V and 4063 ± 893 (1σ) ppm Cr, whereas weakly colored samples average only 1020 ± 788 (1σ) ppm V, and 559 ± 692 (1σ) ppm Cr. The ⁴⁰Ar/³⁹Ar weighted mean ages on three Muzo hydrothermal muscovites in emerald-bearing veins are 62.05 ± 0.63 (2σ) Ma, 30.25 ± 0.25 Ma, and 30.24 ± 0.06 Ma. Three Muzo hydrothermal albite samples yielded less robust total fusion and weighted mean ages, ranging from 95 to 14 Ma. Our muscovite ages indicate two stages of hydrothermal activity at Muzo, around 62 Ma and 30 Ma, in agreement with previous studies