Abstract

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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.