The Genesis of the Salt Diapir-Related Mississippi Valley-Type Ba-Pb-(± Zn) Ore of the Slata District, Tunisia: The Role of Halokinesis, Hydrocarbon Migration, and Alpine Orogenesis

The juxtaposition of a Triassic evaporite diapir with the organic matter-rich Fahdene Formation (Albian-Vra-conian) along major faults in the Slata ore district raises the question of the roles played by halokinesis, hydrocarbons, and tectonics in mineralization. The Slata mining district, located in the Tunisian salt diapiric zone, contains Ba-Pb-(± Zn) ore hosted in the Aptian carbonates. The mineralogical paragenetic sequence consists of barite (Ba-1)–galena ± sphalerite ± calcite (Ca-1)–barite (Ba-2) and finally, late calcites (Ca-2 and Ca-3). Fluid inclusions from early barite reveal that it was precipitated from a warm (134°–157°C), H2O-NaCl-KCl-CaCl2, moderately saline (13.3–24.6 wt % NaCl equiv) basinal brine. This fluid is thought to have resulted from the mixing of a deep-seated, hot, metal-bearing fluid with a cooler, dilute SO42−-rich fluid. Early calcite and cogenetic sulfides (galena and sphalerite) precipitated from fluids of similar salinities and temperatures as the barite-forming fluids, but with the additional involvement of hydrocarbons. Sulfur isotope data suggest that thermochemical sulfate reduction of Triassic gypsum was the main source of reduced sulfur for sulfides. Late barite precipitated as a result of the mixing between a Ba-rich, hot, ascending fluid with a cooler, dilute Triassic sulfate-rich fluid in the absence of hydrocarbons. The homogeneous Pb isotope compositions of galena along with the Sr isotope compositions of barite point to a Paleozoic reservoir as the main source of metals with a contribution from the Triassic-Cretaceous rocks. The emplacement of the ore occurred during the Eocene-Miocene Alpine compressional tectonic activity that triggered the circulation of Paleozoic-derived metal-bearing fluids.