Shear-zone hosted gold mineralization of the Arabian–Nubian Shield: devolatilization processes across the greenschist–amphibolite-facies transition

Orogenic gold ores of the Arabian–Nubian Shield are structurally controlled by the Najd Fault System. The Najd Fault System controlled the exhumation of the metamorphic complexes and, as such, there is a genetic relationship between the metamorphism and the formation of the orogenic gold ores. In order to constrain this genetic relationship, field observations, petrography, geochemistry and fluid inclusions of four mines from the Egyptian side of the shield are presented. The studied gold-bearing dykes and veins are structurally controlled where the gold-bearing fluid precipitated in pre-existing second-order and third-order extensional faults of the major NW–SE Najd Fault System. Fluid inclusions indicate that the gold was precipitated at shallow- to medium-crustal levels, equivalent to a temperature range of 250–350°C, and from low salinity metamorphic fluids, possibly mixed with magmatic/meteoric water. Thermodynamic modelling suggests that gold-bearing fluids were generated due to metamorphic devolatilization processes across the greenschist–amphibolite-facies transition of ophiolitic and metasedimentary source rocks. The Najd Fault System enables the vertical transport of gold-bearing fluids from the source region to the depositional sites. Decreasing the temperature of the fluid is required to precipitate the gold. However, the gold precipitation process needs to be buffered by Fe-bearing wall rocks.