• journal_title：European Journal of Mineralogy
• Contributor：Samira Essarraj ; Marie-Christine Boiron ; Michel Cathelineau ; David A. Banks ; Abdelmajid El Boukhari ; Mohamed Youssef Chouhaidi
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：1998-
• Format：text/html
• Language：en
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：10
• issue：6
• firstpage：1201
• section：Articles

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 H ₂ O-CO ₂ -CH ₄ 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-CaCl ₂ brines (24 to 40 wt.% eq. NaCl + CaCl ₂ ). Brines occur as several fluid inclusion types (L+V+ or -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/SO ₄ 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.