Gold–sulfide mineralization in the Sir Bakis mine area, Central Eastern Desert, Egypt

The Sir Bakis area in the central Eastern Desert of Egypt hosts orogenic type gold deposits in shear-related, structurally controlled, quartz veins. These NNW–SSE-trending veins cross-cut Precambrian metavolcanics and granitoids of the Arabian Nubian shield that formed during the Pan-African Orogeny (780–610 Ma). The primary ore mineral assemblage consists of pyrite (As < 3.6%), chalcopyrite (Zn < 1.5%), galena (Zn < 0.2%, Te < 0.48%), sphalerite (7.9–8.9% Fe), magnetite (Ti, Cr, and Mn below detection), and gold (9–20% Ag). Gold also occurs as a texturally late mineral along with hessite (< 7% Au), silver, electrum (Au0.63Ag0.32), and covellite (Zn < 2.4%; Ag < 2.2%) that develop along rims and fractures of primary minerals. Three different types of fluid inclusions in vein quartz were observed: Type A are 3-phase (2 liquids + gas) solitary inclusions; Type B are 2-phase (liquid + gas) inclusions in clusters or isolated, and Type C are 2-phase (liquid + gas) inclusions in healed fractures mostly crossing grain boundaries. Microthermometric measurements show that eutectic melting points (Tme) for type A inclusions fall between − 63 and − 51 °C (most ~ − 56 °C), whereas type B inclusions have four distinct modes at − 66°, − 55°, − 35°, and − 22 °C. Type A inclusions record average clathrate melting temperatures of ~ 9.3 °C. All inclusions homogenize consistently through the disappearance of the vapor, with final homogenization temperatures (Th) for type B and C inclusions recorded at 263 and 240 °C, respectively. Analysis of microthermometric measurements on early type A and B fluid inclusions suggests that low salinity, CH4-bearing, aqueous-carbonic fluids (XH2O = 0.38–0.61; XCO2 = 0.26–0.53; XCH4 = 0.05–0.1; XNa+  = 0.005–0.013) were among the earliest trapped (in type A), followed by mostly aqueous, either CO2-bearing or CO2-free fluids (in type B inclusions; XH2O = 0.61–0.99; XCO2 < 0.04; XNa+  = 0.0006–0.025; XMg2+  = 0.01–0.13, XCa2+ < 0.0014, XCl− < 0.26). Ore-bearing fluids were likely CH4-bearing, aqueous-carbonic fluids similar to those trapped in type A inclusions. These fluids were likely metamorphic in origin, and leached Au from ultramafic and mafic ophiolitic rocks, carrying it in the form of bisulfide complexes. Ore minerals were precipitated upon oxidation as the fluids migrated to shallower crustal levels through the deep seated shear zones. Sphalerite thermobarometry and microthermometric analysis of early inclusions suggest that the primary ore mineral assemblage formed at T 280–300 °C, P > 4 kbar. Textural analysis of vein minerals and microthermometric measurements suggest that pyrite and galena likely recrystallized with quartz during late-stage deformation, leading to the reprecipitation of Au and Ag at T = 220–150 °C, P < 1 kbar. The two-stage evolution of Sir Bakis ore is consistent with genetic models proposed for many orogenic Au deposits that invoke subduction/collision followed by extension, and with the tectonic history of the central Eastern Desert which records regional metamorphism and intrusion of syncollisional magmas, followed by crustal extension, shearing, and post-orogenic magmatism.