Fluid inclusion, stable isotope and 40Ar/39Ar studies for the Halo-Shakiso emerald deposit of southern Ethiopia

This project investigated the Halo-Shakiso emerald deposit of southern Ethiopia. The deposit is associated with the Halo-Shakiso pegmatite, which is hosted in Neoproterozoic ultramafic rocks. Ar-Ar dating of emerald-bearing phlogopite schist yielded a plateau age of 521.6 ± 0.9 Ma, suggesting a coeval relationship with the Kenticha pegmatite field. Emerald primary fluid inclusions are generally aqueous-carbonic, with salinities of approximately 3.0 wt% NaCl eq. and an XCO2 of 0.06. Fluid incusion microthermometry data, combined with the temperature range of 450 (± 75) °C obtained from stable isotope thermometry, result in pressure constraints of 2.6 (± 1.1) kbar – or depths of 7.8 (± 3.3) km – for emerald precipitation. Emerald channel water δD and calculated δ18O isotope ratios suggest an igneous source for the formational fluids at Halo-Shakiso. As the emeralds are also closely associated with the Halo-Shakiso pegmatite, these findings strongly support the classification of the deposit as a Tectonic-Magmatic-Related emerald deposit.