Fluid inclusions in the mesozonal gold deposit at Atud mine, Eastern Desert, Egypt

Gold mineralization at the Atud mine occurs as fracture-filling auriferous quartz veins hosted in Neoproterozoic dioritic rocks and along their contact with metagabbro. Gold mineralization is associated with metasomatic alteration zones around shear zones and quartz–carbonate vein arrays. The mineralized veins consist of quartz, carbonate and albite gangue enclosing minor amounts of pyrrhotite, arsenopyrite, pyrite and sphalerite. Trace amounts of galena, chalcopyrite, magnetite and rutile are also present. Paragenetically, the mineralization is divided, with decreasing temperature, into three stages, namely: (1) Early (Au-poor, pyrrhotite– arsenopyrite–pyrite–quartz vein); (2) Main (Au-rich, sphalerite–arsenopyrite–pyrite–galena chalcopyrite–quartz vein); and (3) Late (quartz–carbonate–pyrite galena). Gold (15.6–36.2 at. % Ag) is present mainly as discrete grains of native gold (<5–20 lm in diameter). Free gold appears mainly as inclusions in quartz and as microscopic inclusions (<5 lm) in arsenopyrite and pyrite. Gold also occurs in fractures and grain boundaries of pyrite, arsenopyrite and base metal sulphides. These occurrences of gold indicate that several influxes of gold and/or stages of remobilization took place. Based on temperatures inferred from arsenopyrite compositions by electron microprobe, the estimated temperatures for Early and Main mineralized stages reach 340–430, and 273–368 °C, respectively. The sulphur activity (atm) of ore formation at the Atud deposit was estimated for each stage as 10 6:5 –10 9:6, and 10 8 –10 12:2, respectively. Fluid inclusions in quartz intimately associated with the mineralization are dominated by aqueous H2O–CO2 þ NaCl types. In