Fluid evolution in the El-Sid gold deposit, Eastern Desert, Egypt

Auriferous quartz (±carbonate) veins in the El-Sid mine cut through the western margin of the Fawakhir granitic intrusion and the immediate country ophiolites. Gold mineralization is spatially and temporally associated with ENE-WSW fault/shear zones developed late in the deformational history of the area. Field and microscopic studies suggest two distinct ore stages; namely an early gold-Fe-As-sulphide, and a late gold-base metal mineralization. New microthermometric and Raman data suggest gold deposition as a result of a complex history of fluid immiscibly, dilution of low-salinity aqueous-carbonic fluids in the early mineralization stage, while wall-rock alteration and pressure loss precipitated Au from intermediate-salinity aqueous ore fluids during the late stage. Fluid inclusion isochoric reconstructions, combined with oxygen and sulphur isotope data, indicate conditions of 320±20 °C and 1.3±0.2 kbar for the early gold-Fe-As-sulphide mineralization, and c. 200±15 and 0.6±0.9 kbar for the late gold-Zn-Pb-Cu-sulphide stage. The clockwise evolution path in pressure-temperature space likely documents gold mineralization under post-peak metamorphic conditions. The calculated sulphide δ³⁴S^H2S equilibrium values 29.04‰ to -4.75‰, may refer to a variable redox state of sulphur in the ore fluids from the early to late mineralization stages. The stable isotope signature of the vein quartz and calcite suggest mixed magmatic and metamorphic fluid sources (δ¹⁸O H₂O = +4.9‰ to +7.4‰). Unusually low δ¹³C values of calcite in the late mineralization (213.9‰ to 214.7‰) may reflect input of magmatic CO₂ and/or oxidized carbonaceous material in the infiltrating fluid.