Fe-Cu (Ag, Au) ore deposits and thermodynamic conditions of the mineralizing hydrothermal fluids of the Chouichia abandoned mine (Northern Tunisia): mineral geothermometers and occurrence evidences of native gold traces, silver and Ag-related minerals

The Cu-Fe ore deposits of the Chouichia abandoned mine are considered among the most complex mineralizations in the northwest Tunisia due, in particular, to their multi-phased poly-metallic character. The deposits fit into the front of the North African thrust domain of Alpine and Atlas orogenies. The mineralization is associated with hydrothermal fluid circulations through the Fej-Et-Tamer deep-rooted accident. Petrography, microanalysis by transmission electron microscopy and scanning electron microprobe allowed to determine the mineral association, the paragenetic phases and to retrace the thermodynamic conditions of the mineralizing fluids. Five mineralizing phases are defined. Native silver, silver minerals (proustite-pyrargyrite, acanthite-argentite), native gold, molybdenite, cinnabar, pyrrhotite, barite and bornite are first observed. Native silver is frequently in association with silver minerals during the later mineralizing phases. Native gold is observed in polished sections in intergranular vug of siliceous facies or as inclusions in quartz. Sulfides and sulfosalts can be enriched in many metallic elements (Bi, Hg, Ag, Au, Sn). The calculated empirical formulae for the Fahlore’s solid solution (tetrahedrite-tennantite) show high Cu values reaching 11.832 apfu for tetrahedrite. According to the literature (Repstock et al. 2016 ; Shapovalova et al. 2019 ), such a Cu-excess makes the tetrahedrite from the Chouichia deposits the richest in copper so far cited. The thermodynamic approach using minerals/mineral associations as geothermometers suggests mineralizing fluid temperatures, for the main vein phases, between 200 and 500 °C providing a subsurface heat source. The last mineralizing phase is marked by native silver, silver minerals and by cooling below 178.85 °C.