CATHODOLUMINESCENCE AND TRACE ELEMENT GEOCHEMISTY OF MUZO, COLOMBIAN EMERALDS

Muzo, Colombia is one of the most economically significant emerald deposits in the world, and hosted by hydrothermal breccias in altered black shales, its geologic origin is unusual among emerald deposits. We analyzed Muzo emeralds from the Puerto Arturo, Matefique, Taquendama, and Catedral mines using color SEM—cathodoluminescence (SEM CL) combined with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). We combined these techniques in an attempt to constrain the physical and chemical conditions of crystal growth.

All emeralds observed display a strong red luminescence with complex zoning patterns. Zoning was predominantly concentric around inclusions. Trigonal and hexagonal crystal structures were present, with some images displaying oscillatory zoning. Sector zonation and irregular variations in CL intensity were also observed. Areas of more intense luminescence (bright red) are enriched in V, Cr, Mg, and Na and depleted in Fe relative to areas with less luminescence (dark red). These areas of high trace element concentration and CL intensity correspond to the most intense green coloring of the emeralds. The most common trace elements detected in Muzo emeralds include V (126-7991 ppm), Cr (100-8172 ppm), Mg (1542-7038 ppm), Na (1544-8127 ppm), and Fe (200-680 ppm). In addition, Li (21-61 ppm), Ga (10-44 ppm), Zn (<2-10 ppm), Rb (1-5 ppm), and Sr (<0.05-5 ppm) were present in much lower concentrations; and several elements including B, P, S, K, Ca, Ti, Mn, Ni, Cu, Ge, As, Sn, Sb, Ba, Au, Pb, and Bi were consistently below the detection limit of the LA-ICP-MS.

These results suggest that the Muzo emeralds are compositionally distinct compared to other emerald deposits from around the world. Muzo emeralds have a higher V+Cr to Fe ratios, compared to other deposits. The lack of Fe present in the Muzo emeralds may be responsible for the brilliant green color of these emeralds, as well as for their intensely bright red cathodoluminescence. Muzo emeralds are Zn-poor and have intermediate amounts of Li and Ga relative to world-wide emerald deposits. Li-Zn-Ga ratios differentitate Muzo emeralds from emeralds from Pakistan, Brazil, Nigeria, Zimbabwe, and Zambia. The unusual chemical composition of these emeralds likely reflects the unusual hydrothermal conditions, under which these emeralds form.