Geological, Mineralogical and Geochemical Characterization of Emerald Deposit from Halo area
Creators: name: Mot, Muaz
Publisher: name: National Academic Digital Repository of Ethiopia
Year: 2020

description: In the years 2011-2012, the first Ethiopian emeralds reached the international gemstone market.<br> In addition to localized artisanal emerald mining in two areas, two companies started production<br> at open pit mining operations close to Web village in Southern Ethiopia in 2017 and Halo in<br> South-central Ethiopia in 2016. Little geological information on the two deposits has been<br> published so far and the current study provides new mineralogical and geochemical data to allow<br> a genetic interpretation of those two emerald deposits in Ethiopia using integrated approach of<br> geological mapping and sampling, petrographic analysis, and whole-rock geochemical analysis.<br> The country rocks for both emerald deposits are Neoproterozoic units of highly deformed metaultramafic sequences embedded in amphibolite and biotite-hornblende gneisses metamorphosed<br> under prograde metamorphic conditions reaching amphibolite facies (600-650Cº and ~6kbar).<br> This metamorphic event is locally associated with a strong metasomatism affecting ultramafic<br> units selectively. The early metasomatic event leads to the formation of phlogopite units<br> (glimmerites) through hydration, and the introduction of K2O, Rb, Ba, and P2O5. A second,<br> retrograde metamorphic event causes locally chloritization and sericitization mainly along brittle<br> faults or shear zones as well as in the contact zone of pegmatites. Contact zones of pegmatites<br> show evidence of a second metasomatism, which is crucial in the formation of emeralds in the<br> glimmerite unit.<br> Emeralds are hosted in late rare element bearing granitic pegmatites and in glimmerite<br> immediately in contact with those pegmatites. Geochemically, the pegmatites introduce the<br> elements Be, Nb, and Ta into the contact zone but only Be leads to the formation of emeralds by<br> utilizing the available Cr in the glimmerite for attractive coloration. The mineralization of the<br> Halo emerald can be classified as a tectonic-magmatic-metasomatic type, which is related to<br> granitic intrusions with pegmatite in contact with mafic-ultramafic rocks.<br> At the location of Halo, the rare element bearing peraluminous pegmatites are cross-cutting the<br> regional foliation of the country rocks and show only minor dynamic recrystallization in quartz.<br> At the Web location, the emerald bearing pegmatites are concordant with respect to the regional<br> foliation and are affected by the regional scale ductile folding and shearing deformation.<br> Emeralds from Web tend to show lower clarity ratings and stronger effect of post-formational<br> deformation like an en-echelon alignment of fluid inclusion trails, tight joint systems, and brittle<br> fracturing.<br> The emerald formation in Southern Ethiopia is related to three crucial geological factors: (I) The<br> presence of a highly deformed and metamorphosed ultramafic sequence; (II) Selective<br> metasomatism to convert the meta-ultramafics to glimmerites by the introduction of K, Rb, and<br> Ba; (III) The concordant or cross-cutting intrusion of the rare element bearing pegmatites with<br> suitable Be-content which can invade glimmerites in contact with the pegmatites. Coloration for<br> high-quality emeralds depends on the high chromium content of the glimmerites