Fluid Inclusion Characterization of Auriferous Quartz Veins in The Akobo Gold Deposit, Southwestern Ethiopia

The Akobo gold deposit is situated in the NW-SE striking Surma shear zone of Western Ethiopian Precambrian terrain. Gold occurrence in this area is associated with structurally controlled quartz veins. The auriferous quartz veins are hosted in felsic metavolcanics and meta mafic-ultramafic rocks. Gold occurs mainly as discrete grains in the vein and within vein quartz fractures. This paper discusses the nature and evolution of CO2-bearing hydrothermal fluids responsible for gold deposition based on fluid inclusions study and Raman spectrometry. Fluid inclusion petrography and microthermometry of vein quartz samples were carried out at the Fluid Inclusion Laboratory, Department of Earth Sciences, IIT Roorkee, India. Raman spectroscopy for selected carbonic monophase and aqueous carbonic fluid inclusions was carried out at the Fluid Inclusion Laboratory of Wadia Institute of Himalayan Geology, Dehradun, India. Primary carbonic and aqueous carbonic fluid inclusions were further studied in the current study. Two types of primary inclusions were recognized based on the phase content, phase transitions during heating and freezing, and Raman spectra. These are, Type-I: carbonic monophase (CO2±CH4±N2) and Type-II: aqueous-carbonic (H2O-CO2±CH4±N2) inclusions. Type-I inclusions show TMCO2 varying from -62°C to -57°C and THCO2 in the range of -27.9°C to 22.8°C. The CO2 density ranges from 0.74 to 1.07g/cm3. Type-II inclusions have a melting temperature of CO2 (TM CO2) ranging from -61.2°C to -56.7°C and the temperature of homogenization of CO2 (TH CO2) ranges from -0.8°C to 22.3°C. Clathrate melting temperatures (TM clath) range from -9.2°C to 9.5°C, indicating salinities around 1.04 to 23.69 wt.% NaCl equivalent. Total homogenization (THtot) occurred between 185°C and 450°C. The presence of CH4 and N2 causes a decrease in TMCO2 in both types of inclusions. Medium to high temperature, low to medium salinity, and low-density CO2 fluids in the study area are similar to fluids involved in orogenic lode gold mineralization worldwide.