CHEMICAL CLASSIFICATION AND INSIGHTS INTO THE FORMATION OF EMERALD DEPOSITS

The chemistry of emeralds from 24 deposits in 10 countries was determined by Instrumental Neutron Activation Analysis (INAA). The INAA data were augmented by additional high sensitivity measurements from the literature. The chemical data were grouped on the basis of the type of emerald deposit: Type I directly related to igneous processes, Type IIB formed by low to moderate temperature solutions in sedimentary sequences, and Types IIA, IIC, and IIB related to metamorphic processes (classification of Giuliani et al. 2019). Chemical discriminant diagrams using Na, K, Rb, Cs, Mg, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, As, and Sb were found to reliably differentiate emeralds from these different types of deposits.

On most classification diagrams, the metamorphic emerald deposits (IIA, IIC, IID) plot between the IA (igneous origin) and IIB (low to moderate temperature sedimentary) emerald deposits. Type IA emeralds have higher Cs, Rb, Fe, Co, As, and Sb compared to Type IIB emeralds. These differences reflect the origin of the emerald-forming solutions – Type IA derived from igneous sources while Type IIB are derived from evaporite/black shale interactions. The chemistry of emeralds formed in metamorphic environments is largely controlled by interactions between the emerald-forming solutions and the host rock(s). There is limited rare-earth element (REE) data for emeralds, but the data of this study (and from the literature) suggest that for the Type I emerald deposits the REEs reflect the chemistry of the magmatic systems that give rise to the emerald-forming solutions while in the case of the Type II emerald deposits the REEs reflect the composition of the rocks that equilibrated with the emerald-forming solutions. Likewise, REE data for metamorphic emeralds also reflect the chemistry of the host rocks.

Emeralds deposits formed in mafic to ultramafic rocks are characterized by high Cr abundances (V/Cr <<1) and those formed in sedimentary and felsic igneous rocks by high V abundances (V/Cr >1). In most cases, relative to the host rock(s), V tends to more enriched in emeralds than Cr. This differential enrichment can be extreme in the case of the high V emeralds. There is some evidence that the relative abundance of Cr and V in solutions is sensitive to redox conditions which may be an explanation for extreme V/Cr ratios (>>1).