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

Abstract

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 μm in diameter). Free gold appears mainly as inclusions in quartz and as microscopic inclusions (<5 μm) 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 H 2O–CO 2 + NaCl types. In most cases, these fluid inclusions co-exist in individual samples and show various CO 2 phase volume proportions at 40 °C. Co-existing H 2O-rich liquid and CO 2-rich vapour fluid inclusions homogenized into liquid and carbonic vapour phases, respectively, over the same temperature range (270–490 °C). The petrographic observations and microthermometric data suggest that fluid inclusions in quartz intimately associated with the early and main mineralizing events were trapped during phase separation of an originally homogenous H 2O–CO 2 liquid, with low salinity (2.8–8.2 wt.% NaCl equiv.) and high density of 0.8–0.9 g/cc (corresponding to 15–76 mol% CO 2). These data are consistent with transportion of gold as a bisulphide complex. Gold deposition occurred over a temperature range of 270–430 °C and pressures of 160–272 MPa (∼6–11 km depths), likely due to decreases in sulphur activity (10 −6.5–10 −12.2) accompanying fluid unmixing. Gold deposition in the Atud diorites was related to fluid phase separation, sulphidization and carbonatization of host dioritic rocks during hydrothermal alteration and mineralization. The diorites are considered to have acted as preferential sites for fluid flow and ore precipitation due to their brittle nature during regional deformation.