The microbial controls on the deposition of Pb-Zn minerals in carbonate-hosted Tunisian ore deposits

The Ain Allega, Mjar Hannech and Sidi Driss Pb-Zn-Ba-Sr-As-(Hg) deposits in northern Tunisia are hosted in a post-nappe anticline with a core of a Triassic evaporite diapir affected by the NE-SW-trending Ghardimaou-Cap Serrat and E-W lineament. The ore minerals show different styles; particularly, impregnation in dolomite, cement of breccias, replacement ore and open space filling in the dissolution cavities and fractures. Ore minerals include sphalerite, galena, marcasite, pyrite and cinnabar. Principal gangue minerals are composed of barite, celestine, calcite, dolomite and quartz. The orebodies are hosted by Triassic carbonate rocks (Ain Allega ore deposits), Cretaceous carbonate rocks (Mjar Hannech ore deposits) and by the volcanoclastic layers (Sidi Driss ore deposits). These host rocks show hydrothermal alteration, dissolution and brecciation. Sphalerite in the carbonate-hosted Zn-Pb deposits in Tunisia occurs as nano-size microglobular blebs and peloids and forms finely-laminated bands with wavy and even ripped up features. Peloids are present in collapse breccias of karst cavities. Peloids (5-100 mu m) and bacterial filaments (1 mu m long) locally form the common micro-texture of the sulfide mineralization. The core and the shell of peloids are composed of different minerals; there are 1-3 shells and each shell is 1-1.5 mu m thick, commonly with coarse crystals. These textures probably represent fossil microbial mats as in-situ sulfate-reducing bacteria products and are similar to nano-textures observed in bacterial biofilms. Electron microscopy, combined with major element changes (Zn, Pb, As, Fe, and Cd) and sulfur isotope values suggest that sphalerite nano-textures and nano-spheres are associated with fossilized bacterial-mat. The delta S-34 values of sphalerite (-43.9 to +22.38 parts per thousand) show that the microbial production of H2S controlled sphalerite precipitation. The biogenic nano- to macro-textured sphalerite and sulfur isotope data suggest that microbes had an important role in the genesis of carbonate-hosted Zn-Pb deposits in Tunisia during the Miocene.