Mineral and fluid inclusion study of emeralds from the Lake Manyara and Sumbawanga deposits, Tanzania

Mineral and fluid inclusions were investigated in Tanzanian emeralds sampled in the Lake Manyara and Sumbawanga deposits. Microthermometry and Raman microprobe analyses were applied for this study. Primary and pseudosecondary H₂O-CO₂ inclusions, with numerous daughter solid phases, are common in the emeralds from the Lake Manyara deposit. Magnesite, Mg-calcite, aragonite, dolomite, calcite, nahcolite, quartz and chrysoberyl were identified as trapped solids in fluid inclusions. Similar mineral inclusions were also found in the emeralds themselves. The composition of the trapped fluid present at emerald growth is estimated to be a carbonic-rich solution with chloride content of about 6 wt% NaCl equiv. The P-T conditions of emerald growth are as follows: T=370-470°C and P= 3.0-7.0 kbar. Mineral inclusions of phenakite, euclase, bertrandite and helvite are common in the emeralds of the Sumbawanga deposit. CO₂-rich inclusions with an aqueous phase (usually below the detection limit), and CaCl₂-rich inclusions with salinity of up to 17 wt% NaCl equiv. are related to emerald growth. The trapping of fluid inclusions postdates the growth of mineral inclusions. The P-T conditions of emerald growth are as follows: T= 220-300°C and P= 0.7-3.0 kbar.