Essarraj and 4 co-authors reported in 2005 that Ag-ores occur in a specific zone of the Bou Azzer Co–As deposit in the Precambrian basement of the Anti-Atlas belt (Morocco), especially in highly microfractured quartz-depleted diorite. They formed after the main Co–As stage of mineralization, but both ore stages (Co–As and Ag-ore) appear linked to similar immiscible fluids: an hyper-saline Na–Ca brine (5.5–22 wt.%. eq. NaCl and 13.5–18.5 wt.% eq. CaCl2, with Na/Ca ranging from 0.4 to 1.2 during Ag-mineralization) occurring as L + V ± halite fluid inclusions and CH4–(N2) gas dominated fluids. Pressure–temperature estimates for the Ag-stage range from 40 to 80 MPa and 150 to 200°C e.g. at a temperature slightly lower than that of the preceding Co–As stage (200–220°C).
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 favored 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.
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