The Colombian emerald fields are located on the eastern and western flanks of the Eastern Cordillera of Colombia and account for 50 to 70% of annual global supply. Although there are additional sources for emeralds globally, Colombian emeralds are characterised for their superior transparency.
Emeralds were formed at moderate geological temperature and pressure by the metamorphic processes that created the Eastern Cordillera of the Colombian Andes.
The Eastern Cordillera is located in the northern Andean orogenic belt in the central portion of Colombia and is a fold and thrust belt that has been active since the middle Miocene, squeezed and thrust over its two foreland basins.
Originally a large Mesozoic basin, the Cordillera is composed of interbedded sandstones, limestones, black shales and evaporites (Cheilletz and Giuliani, 1995). The perspective formations in the district are the Muzo and Rosablanca which are lithologically similar.
These formations are comprised of sequences of limestones, shales, siltstones and sandstones of variable thickness. The Rosablanca unit is significantly thicker throughout the basin and contains a greater amount of limestone interbedded with the shales.
Emeralds are typically found in calcite-dolomite veins with minor pyrite, Ankerite, Albite, and quartz. These veins are the product of an exotic hydrothermal fluid and metasomatic fluid interaction followed by deposition along structures created by the tectonic activity in the belt.
Unlike other global emerald sources, Colombian emeralds are not directly related to igneous activity.
Circulating saline fluids mobilised beryllium from sedimentary source rocks. Crystals grew in specific local structural ‘traps’ within faults and breccias.
The metasomic fluids appear to be derived from particular shale formations, which are likely the source of Vanadium which is the cause of the vibrant colour characteristic and could be why only formations of a certain composition are perspective target horizons for emerald mineralization.