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Emerald deposits : A review and enhanced classification

Giuliani, G. ; Groat, L.A. ; Marshall, D. ; Fallick, A.E. ; Branquet, Y., Minerals

Emerald deposits : A review and enhanced classification

Giuliani, G. ; Groat, L.A. ; Marshall, D. ; Fallick, A.E. ; Branquet, Y.

Minerals, 2019, 9, 105, 1-63

Although emerald deposits are relatively rare, they can be formed in several different, but specific geologic settings and the classification systems and models currently used to describe emerald precipitation and predict its occurrence are too restrictive, leading to confusion as to the exact mode of formation for some emerald deposits. Generally speaking, emerald is beryl with sufficient concentrations of the chromophores, chromium and vanadium, to result in green and sometimes bluish green or yellowish green crystals. The limiting factor in the formation of emerald is geological conditions resulting in an environment rich in both beryllium and chromium or vanadium. Historically, emerald deposits have been classified into three broad types. The first and most abundant deposit type, in terms of production, is the desilicated pegmatite related type that formed via the interaction of metasomatic fluids with beryllium-rich pegmatites, or similar granitic bodies, that intruded into chromium- or vanadium-rich rocks, such as ultramafic and volcanic rocks, or shales derived from those rocks. A second deposit type, accounting for most of the emerald of gem quality, is the sedimentary type, which generally involves the interaction, along faults and fractures, of upper level crustal brines rich in Be from evaporite interaction with shales and other Cr- and/or V-bearing sedimentary rocks. The third, and comparatively most rare, deposit type is the metamorphic-metasomatic deposit. In this deposit model, deeper crustal fluids circulate along faults or shear zones and interact with metamorphosed shales, carbonates, and ultramafic rocks, and Be and Cr (+-V) may either be transported to the deposition site via the fluids or already be present in the host metamorphic rocks intersected by the faults or shear zones. All three emerald deposit models require some level of tectonic activity and often continued tectonic activity can result in the metamorphism of an existing sedimentary or magmatic type deposit. In the extreme, at deeper crustal levels, high-grade metamorphism can result in the partial melting of metamorphic rocks, blurring the distinction between metamorphic and magmatic deposit types. In the present paper, we propose an enhanced classification for emerald deposits based on the geological environment, i.e., magmatic or metamorphic ; host-rocks type, i.e., mafic-ultramafic rocks, sedimentary rocks, and granitoids ; degree of metamorphism ; styles of minerlization, i.e., veins, pods, metasomatites, shear zone ; type of fluids and their temperature, pressure, composition. The new classification accounts for multi-stage formation of the deposits and ages of formation, as well as probable remobilization of previous beryllium mineralization, such as pegmatite intrusions in mafic-ultramafic rocks. Such new considerations use the concept of genetic models based on studies employing chemical, geochemical, radiogenic, and stable isotope, and fluid and solid inclusion fingerprints. The emerald occurrences and deposits are classified into two main types : (Type I) Tectonic magmatic-related with sub-types hosted in : (IA) Mafic-ultramafic rocks (Brazil, Zambia, Russia, and others) ; (IB) Sedimentary rocks (China, Canada, Norway, Kazakhstan, Australia) ; (IC) Granitic rocks (Nigeria). (Type II) Tectonic metamorphic-related with sub-types hosted in : (IIA) Mafic-ultramafic rocks (Brazil, Austria) ; (IIB) Sedimentary rocks-black shale (Colombia, Canada, USA) ; (IIC) Metamorphic rocks (China, Afghanistan, USA) ; (IID) Metamorphosed and remobilized either type I deposits or hidden granitic intrusion-related (Austria, Egypt, Australia, Pakistan), and some unclassified deposits.

Voir en ligne : https://doi.org/10.3390/min9020105

publié lundi 4 mars 2019