The Kilkile Emerald; Genesis, Mineralogy, Chemistry and Mining; Kilkile, Shakiso- Southern Ethiopia
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Date
2019-09-04
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Addis Ababa University
Abstract
This study provides quantitative geochemical, petrological, and mineralogical data on
major rock types and emeralds of the kilkile area. The meta-basic rocks containing
740ppm Cr provided the amount necessary for emerald mineralization. Emeraldhosting
biotite schist is confined to the contacts of quartz- feldspar- muscovite
pegmatite dykes/veins with meta-basics. The pegmatite dykes are genetically related to
a fertile granite pluton within the Adola district. The formation of biotite schist from
meta-basic rocks is associated with the introduction of pegmatitic components in to the
meta-basics, causing enrichment with the pegmatitic composition. Quartz-feldsparmuscovite
pegmatites of the kilkile area belongs to the rare-element pegmatites of the
LCT family with common beryllium enrichment (12ppm). These pegmatite
dykes/veins intruded the N-S orienting regional structures. The emeralds have higher
MgO (0.26-4.08 wt. %), Fe2O3 (1.39-2.18 wt. %), Al2O3 (17.7- 20.1 wt. %), CaO (1.25-
3.85 wt. %), K2O (0.07-1.7 wt. %), P2O5 (0.02-1.88) and Be (527-1000 ppm) and lower
Cr2O3 (0.01-0.09 wt. %) and SiO2 (59.13-61.79 wt. %) contents. The relatively higher
value of iron and magnesium in the kilkile emerald might attribute for the
ferromagnesian nature of the meta-basic rocks or aggressiveness of the brine solution
in liberating Mg and Fe from meta-basic rocks. The low chromium content in the metabasic
rocks contribute for chromium deficiency in the emerald. The high value of
Fe2O3 depresses the green colour in addition to low Cr2O3. Compared to the Zambian,
the meta-basic rocks in the case of Ethiopia have low magnesium and iron content.
However, the relatively higher value of iron and magnesium in Ethiopian emerald
might contribute for relative competency of magnesium and iron in occupying the
aluminium site in the crystal structure of beryl. The higher value of K2O in the
Ethiopian emeralds than in Zambian might suggest the role of potassium metasomatism
at the pegmatite and meta-basic contact. The genesis of Ethiopian emerald holds the
classical model for emerald mineralization, the condition in which pegmatites interact
with the surrounding meta-basic rocks. The current mining activity at kilkile area
requires vertical to horizontal ratio chances due to the overlapping of mining progress
with direction with the dipping of the host rock units. Therefore, low vertical to
horizontal ratio is recommended for the western berms of the mining pits.
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Keywords
Kilkile Emerald, Genesis, Mineralogy, Chemistry and Mining, Kilkile, Shakiso- Southern Ethiopia