Brines related to Ag deposition in the Zgounder silver deposit (Anti-Atlas, Morocco)

The Zgounder ores occur in sets of quartz-calcite veinlets, cross-cutting the Middle-Precambrian volcanosedimentary formations of Sirwa (Anti-Atlas, Morocco). Two major stages of mineralization are distinguished: i) A first stage is characterized by the deposition of quartz (biotite) together with As-Co minerals. This stage is also characterized by a variety of H2O-CO2-CH4 fluids with moderate densities, interpreted as fluids equilibrated with metasediments. These fluids are trapped under rather high temperatures (around 400-450 degrees C) and a rather wide range of pressures attesting to probable pressure fluctuations from hydrostatic to lithostatic conditions. In addition to pressure and temperature drops, aqueous-carbonic fluids have probably undergone episodic immiscibility, the latter process likely being related to the deposition of metals. ii) The second stage corresponds to the major (Cu-Zn)-Ag(Hg) ore deposition and clearly postdates the As-Co assemblage. Silver deposition occurs after the crystallization of quartz-sphalerite-chalcopyrite veins, but both Cu-Zn and Ag(Hg) ore-bearing fluids are NaCl-CaCl2 brines (24 to 40 wt.% ed. NaCl + CaCl2). Brines occur as several fluid inclusion types CL + V +/- halite cube), and were trapped under minimum temperatures of around 160-200 degrees C. The ion chemistry of the fluid inclusions determined by crush-leach analysis shows that brines display typical features of sedimentary fluids which interacted with evaporites: Na/K ratio ranges from 10 to 12, Na/Ca from 1.9 to 2.4, Na/Li from 220 to 445, Cl/Br from 300 to 380 and Cl/SO4 from 1100 to 2100. Fluids at the origin of the Zgounder Ag mineralization are thus Na-Ca brines unrelated to the earlier As-Co ores. Dilution and slight cooling are the two main driving mechanisms for silver deposition, as in several other examples from Anti-Atlas or Canada.