SULFIDE GEOCHEMISTRY AND GENESIS OF CHOUICHIA AND AIN EL BEY COPPER-DEPOSITS IN NORTHWESTERN TUNISIA

The Chouichia and Ain el Bey copper veins that occur in the Eastern At las fold belt in northwestern Tunisia, are hosted in Upper Cretaceous- Paleogene sedimentary sequences in a regional transcurrent shear zone. Paragenetic assemblages were formed during four distinct stages all o f which are separated by fracturing and brecciation: Stage 1 consists of low temperature siderite (160-180 degrees C) formation in associati on with pyrite, including framboidal pyrite and marcasite. Stage 2 inc ludes pseudomorphing of marcasite by As-rich pyrite and arsenopyrite, and formation of chalcopyrite at higher temperatures (200-300 degrees C) from S-rich, Fe-Cu-bearing fluids; As contents in individual pyrite and arsenopyrite crystals increase markedly in the rims relative to t he centers, thus indicating non-equilibrium conditions. Stage 3 involv es fracturing and brecciation predating deposition of enargite, luzoni te and tennantite at Ain el Bey, and famatinite and tetrahedrite at Ch ouichia, from As-Sb-Bi-bearing ore-forming fluids; tennantite-tetrahed rite series exhibit iron and copper-excess replacements in tetrahedral sites interrelated with Cu-Fe interactions (electron transfer). In St age 4 fracturing was followed by calcite formation in voids. Comprehen sive data was obtained from scanning electron microprobe (SEM) and mic roprobe chemical analyses of minerals, geothermometry using sulfur iso topes, As contents in arsenopyrite crystals and fluid inclusions in si derite and calcite, support an input of magmatic hydrothermal ore-form ing fluids, although contamination by sedimentary sulfur were also ide ntified.