Penetration of surface-evaporated brines into the Proterozoic basement and deposition of Co and Ag at Bou Azzer (Morocco): Evidence from fluid inclusions

Chlorinity, cation (Na/Ca ca. 2.2) and halogen ratios (Cl/Br from 300 to 360) are typical of deep basinal brines, especially of surface-evaporated brines that have exceeded halite saturation. The primary brines were modified by fluid-rock interaction during burial and migration through the basement. Ag-deposition was probably favoured by dilution and cooling due to the mixing of brines with less saline fluids. Similarities between the Ag-brines from Bou Azzer, Zgounder and Imiter suggest a regional scale circulation of basinal brines during extension probably later than the Triassic, during the early stages of rifting of the Atlantic.