Nature and composition of gold-forming fluids at Umm Rus area, Eastern Desert, Egypt: evidence from fluid inclusions in vein materials

The Umm Rus gold lode is housed along fractures in granitoid-gabbroic rocks, being largely controlled by a NE-SW trending fracture system that affected the Eastern Desert. Mineralogically, the gold lode consists of quartz and carbonate gangue enclosing minor amounts of auriferous pyrite and arsenopyrite. Trace amounts of sphalerite, galena, marcasite and pyrrhotite are also present. The lode can be divided into: (i) Au-poor, pyrite-quartz vein, (ii) Au-rich, pyrite-arsenopyrite-quartz vein and (iii) gangue dominant. Inspection of primary inclusions from the Umm Rus gold lode showed that the ore was formed from CO₂-H₂O-rich fluids (ca. 30–46 mol % CO₂) of low salinity (6.75–7.75 wt. % NaCl equiv.) and alkaline to neutral pH with a density of 0.76–0.85 g/cc. These data are consistent with dissolution of gold as a bisulphide complex. Deposition of Au most likely occurred over a temperature range of 250–300°C and at pressures around 0.35 Kbars. The deposition may have occurred in response to separation of a liquid CO₂-phase from an originally CO₂-H₂O-rich aqueous fluids. The style of mineralization at Umm Rus bears certain resemblances to Au-bearing quartz veins in the Archaean deposits of Canada and Australia and the “Mother Lode” deposits of the U.S.A.