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International audienc

The Zgounder ores occur in sets of quartz-calcite veinlets, cross-cutting the Middle-Precambrian volcanosedimentary formations of Sirwa (Anti-Atlas, Moroeco). 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 H20-COTCH4 fluids with moderate densities, interpreted as fluids equilibrated with metasediments. These fluids are trapped under rather high temperatures (around 400-450°C) and a rather wide range of pressures attesting to probable pressure f1uctuations from hydrostatic to lithostatic conditions. In addition to pressure and temperature drops, aqueous-carbonic tluids have probably undergone episodic immiscibility, the latter process likely being related to the deposition of metals. ii) The sec(md stage eorresponds 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-CaCI2 brines (24 to 40 wt. % eq. NaCI + CaCI2). Brines occur as several tluid inclusion types (L + V ± halite cube), and were trapped under minimum tem peratures of around 160-200 oe. 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: NalK 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/S04 from 1100 10 2100. Fluids at the origin of the Zgounder Ag mineralizalion are thus Na-Ca brines unrelated to the earlier As-Co ores. Dilution and slight cooling are the two main driving mechanisms for silver de position, as in several other examples from Anti-Atlas or Canada.

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Au-bearing quartz veins result generally fr om a complex succession of stages of quartz crystallization in relation with long lived deformation. These stages may be related to dynamic rock-fluid interaction, and espe cially to the pressure variations of a suprahydrostatic fluid. Using a multidisciplinary approach including a detailed characterization of quartz microdomains and related fluid inclusions, and the determination of composition-molar volumes for each stage of fluid migration, a reconstruction of the changes in fluid pressure throughout the crack healing or filling has been carried out on the example of Hercynian quartz veins and more especially of Montagne Noire quartz veins (French Massif Central). The trapped fluids in specific networks of healed microcracks frequently show extreme fluctuation in their density although some chemical variables (salinities, CH4 / COz ratio) could display little changes. Such fe atures result primarily from changes in fluid pressure in relation with tectonic activity or changes in the structural levels. However, some associated or superimposed processes could result in additional changes in the fluid density, complicating the reconstruction of the P-T paths : i) the local fluid immiscibility which could be accompanied or not by heterogeneous fluid trapping, ii) superimposed fluid trapping in the same quartz grains due to intense and repeated microfissuring which could cause locally post-trapping changes, iii) water leakage linked to ductile deformation of quartz. The use of selected fluid inclusion data obtained on quartz domains apparently preserved from any of these disturbances thanks to the use of SEM cathodoluminescence, appears especially useful i) to give quantitative constraints on pressure changes occurring during quartz vein deformation, ii) to test models depicting the control of fluids on deformation of rocks, especially the necessary fluid pressure (Pt) variations to get fluid-pressure-ac tivated systems, iii) to test the models of gold deposition, involving fluid phase separation. The limitations and advantages to the use of fluid inclusions to reconstruct P changes is discussed. The approach applied to the Montagne Noire vein sampIes shows that fluid inclusions may depict considerable changes in Pf and structural levels. The estimated conditions of an early microfissuring-healing event affecting the milky quartz are the fo llowings : minimal pressures (homogenization pressure, Ph) : 260-450 MPa; maximal and minimal trapping pressures assuming a temperature range of 350-400°C : 400-550 MPa, and 260-290 MPa, respectively at Malabau, and 250 MPa, 150 MPa (at 300-350T) at Cabrespine; maximal depth in the 10-19 km range depending on the hypotheses. This stage fo llows arsenopyrite deposition, but precedes later microfissuring associated with a Au-Bi-Cu stage which occurred under much lower P-T conditions.