This chapter assesses the nature of ore forming fluids and ensuing model for the evolution of gold mineralization in the Birimian in West Africa as against previous model that highlighted and focused on comparison of the early Proterozoic Birimian greenstone belts to that of the Archaean greenstone belts. Studies suggest that the fluid was formed at a minimal temperature between 370 and 410 °C and likely maximum pressures at 2 to 4 kbars and was originally one-phase, liquid, homogeneous and the salinity was ± 5 wt.% equiv. NaCl. Due to cooling and decrease in pressure, unmixing into two immiscible fluids as CO2-rich (CO2-H2O) and H2O-rich (aqueous) took place. The CO2-rich and H2O-rich phases had ≈0 wt.% equiv. NaCl and ≈16 w.t% equiv. NaCl respectively and trace gases in the auriferous fluid were CH4 and N2 in decreasing order respectively. The gold was transported mostly as Au (HS)2ˉcomplex in a highly reducing fluid of pH 5.8. Gold precipitation in the wall rocks occurred by complex destabilization forming sulphides and carbonates associated with the CO2-rich fluid phase whilst the auriferous quartz veins deposition was due to the complex weakening by the phase separation of the fluid into CO2-rich and H2O-rich phases with falling temperature. The mineralizing fluid was metamorphic formed at a great depth. This chapter gives a clearer understanding of the intriguing factors that control gold deposition in greenstone belts in the absence of Banded Iron Formation (BIF) and komatiites.