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International audience; The Bou Jaber ore deposit is one of the numerous diapir-related Pb–Zn–F–Ba deposits of the Dome Zone in Northern Tunisia. Its location is controlled by the regional NE-SW Tajerouine Fault.Ore minerals are hosted in the Late Aptian limestones (Serdj Formation) as open space filling and stratabound replacement bodies. According to Bouhlel et al. (2016), the poly-phase mineralization resulted from the successive activity of three mineral systems, a Pb–Zn, then a barite, and eventually a fluorite deposition system. The latter, from the Late Miocene, is demonstrated in the present study.The fluorite system is characterized by the involvement of oil in the hydrothermal fluids. This oil was produced in the local environment of the deposit from the thermal maturation of the Albian Fahdene black shale source-rock at temperature range of~140 °C–~100 °C. Two brines were involved in the fluorite hydrothermal system. The first one (L1) is a Ca-rich brine (≥20 wt % bulk salinity), with Na/Ca ≤ 0.18, which is thought to have long resided in the basement, before its transfer into the Jurassic reservoir (Upper Nara Formation) and its eventual mobilization at the time of ore deposition. The second brine (L2), less saline (≤14 wt % bulk salinity) is more sodic, with Na/Ca up to 0.53, and represents unmodified brine originated from the nearby Triassic salt. The L1 brine was F-bearing, whereas the L2 brine was associated with oil. Fluorite deposition occurred at the estimated shallow depth of 1.7 km from cooling of the L1 brine due to the first isobaric cooling from ~160 °C to 135 °C under sub-lithostatic conditions (36 MPa). This stage is followed by mixing with the newly incoming L2 brines (transporting oil) along a sub-isochoric decompression path (down to the hydrostatic pressure at 17 MPa) and continuously cooling from 135 °C to 125 °C. The cooling and mixing caused fluorite deposition after which a transient heat advection episode (up to 145 °C) caused late calcite deposition prior to the end of hydrothermal circulation.

International audience; Ain Allega and El Aguiba are among the largest barite-celestite deposits attributed to the Triassic period in Tunisia. They are located in the flysch zone on the eastern edge of the Triassic diapir of Jebel Hamra. The ore body consists of dolomite intensely brecciated and surrounded by marls, clay, gypsum and dolomite which forms the hanging wall of the deposit, and rimmed by Paleocene marls. The ore the surface structure, which in turn controls the mechanism of adsorption and incorporation of minor and trace elements into the growing crystal. For a solid solution, the transitional supersaturation for different growth mechanisms can be significantly different for the two end members. One of the most interesting and intriguing phenomena observed in natural crystals of the (Ba, Sr) SO4 solid solution from this study area is the development of compositional oscillatory zoning, consisting of alternating Ba-rich and Sr-rich layers. The sulfur isotope analyses show significant variability for barite and celestite (from 16.2 to 23 ‰). These values are interpreted as the result of the mixing of two sulfur end-members in the mineralizing fluids, corresponding to thermochemical sulfate reduction of Messinian seawater, together with Triassic sulfate, as sulfur sources. Fluid inclusion studies of celestite show that the BaSO4-SrSO4 solid-solution in both deposits was precipitated from hot saline solution (Th=190±20°C; 16.37 wt. % NaCl equivalent in Ain Allega and 8.2 wt. % NaCl equivalent in El Aguiba). Consequently, the precipitation could be from mixing of basinal brines with magmatic-meteoric fluid. minerals show a cap-rock type mineralization with various forms and types, in particular impregnation in dolomite, breccia cement, replacement of carbonate caprock and open space-filling in the dissolution cavities and fractures. Ore minerals include sphalerite, galena, marcasite and pyrite. Principal gangue minerals are barite, celestite, calcite, dolomite and quartz .The ore minerals are hosted by the Triassic carbonate rocks, which show hydrothermal alteration, dissolution and brecciation. The study of barite-celestite solid-solution shows a bimodal distribution in which the composition is not continuous. Some intermediate compositions are missing; 20-80% SrSO4 for El Aguiba ore deposit and 50-70% SrSO4 for the Ain Allega ore deposit. X-ray diffraction peaks in the BaSO4-SrSO4 series display considerable broadening at intermediate compositions. The unit cell volume varies from 346.97 Å3 in barite (100% BaSO4) to 308.29 Å3 in celestite (100% SrSO4). This variation in cell parameters is attributed to the degree of substitution of barium by strontium since there is less variation between ionic radii of these elements. Consequently, these substitutions caused variation in morphologies, position and d-spacing of peaks (200, 011, 113, 312,122, 112, and 111). Some barite crystals have small-scale, strontium compositional banding which suggests that the crystals responded to regular fluctuations in physiochemical conditions during deposition. The degree of supersaturation in Ba2+ and Sr2+ controls the growth mechanism and consequently