Geochemistry of emerald from the Colombian Emerald belts: Genetical implications

Colombian emeralds are widely recognized for their unique gemological and geological features; The first one involves color, which depend on the proportions of their minor and trace elements. The latter is associated with its genesis, which implicates hydrothermal brine fluids affecting the sedimentary host rock. Thus, to propose an emerald finger-print for each belt, we use geochemical methods as Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), ternary and binary diagrams, and principal component analysis (PCA) in 258 emerald samples; 117 from the eastern emerald belt (EEB) and 141 from the western emerald belt (WEB).

The chromophores elements relation (Cr, V, and Fe) is used to differentiate emeralds from different geographical proveniences, that approach is valid with Colombian emeralds. Besides, in this study, we notice differences betwixt the V/Cr relationship in both belts: in the EEB, the proportion is lower than 0.75, and in the WEB, this proportion is upper than 0.75, and in most of the cases is over 1. This tendency is also remarkable in the ternary plot that includes iron.

Furthermore, other chemical elements have a particular behave depending on the belt, is the case of Na and Mg; in this study, we introduce a binary plot as a new method where the linear relation between those elements permit expressing a mathematical equation to distinguish each belt (Na=0,8Mg+362), where there is a cut-off point in Mg=3225 and Na=2940, the data lower of that point represent samples of the EEB, and the data upper come from the WEB.

In detail, the basin environment of each belt controlled the abundance of heavy metals; in the WEB, the quantity of those elements is higher. Evidence of this is the higher volume of sulfides and oxides. Except for the Cr content, this variant could be due to a previous stage of mineralization that includes Cr-rich muscovites. In the EEB, the Na and K content in the emeralds is lower because the rock suffers feldespatic enrichment in layers (Santa Rosa and Chivor formations). Owing to that, the temperature is enough to generate a massive albitization process affecting the original lithology of the stratigraphic sequence.