Emerald Deposits: A Review and Enhanced Classification

Abstract

Although emerald deposits are relatively rare, they can be formed in several different, butspecific geologic settings and the classification systems and models currently used to describeemerald precipitation and predict its occurrence are too restrictive, leading to confusion as to theexact mode of formation for some emerald deposits. Generally speaking, emerald is beryl withsufficient concentrations of the chromophores, chromium and vanadium, to result in green andsometimes bluish green or yellowish green crystals. The limiting factor in the formation of emeraldis geological conditions resulting in an environment rich in both beryllium and chromium orvanadium. Historically, emerald deposits have been classified into three broad types. The first andmost abundant deposit type, in terms of production, is the desilicated pegmatite related type thatformed via the interaction of metasomatic fluids with beryllium-rich pegmatites, or similar graniticbodies, 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 ofgem quality, is the sedimentary type, which generally involves the interaction, along faults andfractures, of upper level crustal brines rich in Be from evaporite interaction with shales and otherCr- and/or V-bearing sedimentary rocks. The third, and comparatively most rare, deposit type is themetamorphic-metasomatic deposit. In this deposit model, deeper crustal fluids circulate along faultsor shear zones and interact with metamorphosed shales, carbonates, and ultramafic rocks, and Beand Cr (±V) may either be transported to the deposition site via the fluids or already be present inthe host metamorphic rocks intersected by the faults or shear zones. All three emerald depositmodels require some level of tectonic activity and often continued tectonic activity can result in themetamorphism of an existing sedimentary or magmatic type deposit. In the extreme, at deepercrustal 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, andgranitoids; degree of metamorphism; styles of minerlization, i.e., veins, pods, metasomatites, shearzone; type of fluids and their temperature, pressure, composition. The new classification accountsfor multi-stage formation of the deposits and ages of formation, as well as probable remobilizationof previous beryllium mineralization, such as pegmatite intrusions in mafic-ultramafic rocks. Suchnew 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-relatedwith 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) Metamorphicrocks (China, Afghanistan, USA); (IID) Metamorphosed and remobilized either type I deposits orhidden granitic intrusion-related (Austria, Egypt, Australia, Pakistan), and some unclassifieddeposits.

Geology of Corundum and Emerald Gem Deposits: A Review

“…Re-search by Hewton et al (2013) showed that the elements necessary to form beryl were liberated by inorganic thermochemical sulfate reduction via the circulation of warm basinal brines through siliciclastic, carbonate, and evaporitic rocks (figure 17). The Mountain River green beryl occurrence thus represents a variant of the Type IIB emerald deposit in the classification scheme of Giuliani et al (2019) and suggests the potential for Colombian-type emerald mineralization in northwestern Canada.…”

“…Emerald deposits are found on all five continents (figure 10) and range in age from Archean (2.97 Ga for the Gravelotte deposit in South Africa) to Cenozoic (9 Ma for the Khaltaro deposit in Pakistan) (figure 11). Giuliani et al (2019) introduced a new classification scheme in which emerald deposits are divided into two main types depending on the geological environment, and further subdivided on the basis of host rock (table 1) (Behling and Wilson, 2010). The emerald deposits are hosted by Cr-rich (3,000 to 4,000 ppmw) talc-chlorite ± actinolite ± magnetite metabasic rocks of the Muva Supergroup, which have been identified as metamorphosed komatiite (Seifert et al, 2004).…”

“…At the Panjshir Valley in Afghanistan (and Davdar in China) the host rock is a metamorphosed sedimentary rock and the formational fluids are metasomatic-hydrothermal in origin. At Hiddenite in northeastern North Carolina (United States), the host rock is a migmatite and the fluids are hydrothermal (Giuliani et al, 2019).…”

Combining Fourier transform infrared and Raman spectroscopies with Gaussian deconvolution: An improved approach for the characterization of emeralds

“…Ionic substitutions can occur between ions with the same charge or with different charge. For example, the substitution of divalent cations for Al is coupled with the substitution of a monovalent cation for a vacancy at a channel site . In addition, water molecules can accommodate inside the hollow channels within the rings of silica tetrahedral.…”

“…For example, the substitution of divalent cations for Al is coupled with the substitution of a monovalent cation for a vacancy at a channel site. [17,18] In addition, water molecules can accommodate inside the hollow channels within the rings of silica tetrahedral. In fact, notwithstanding beryl is nominally anhydrous, it is widely reported that natural and hydrothermal emeralds contain H 2 O or OH groups in the structural channels.…”