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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; The Djebel El Hamra Pb-Zn-Ba-Sr (Hg) deposits in northern Tunisia are hosted in a post-nappe anticline with a core of a Triassicevaporite diapir affected by the NE–SW-trending Ghardimaou-Cap Serrat lineament. Three stages of mineralization occurred inthe Triassic dolostone: stages I and II caused alternating deposition of sulfate (Ba, Sr) and sulfide (sphalerite, galena) minerals;stage III formed late-stage calcite-marcasite-cinnabar. Zebra textures record the syntectonic transition from compression toextension in the Late Tortonian-Messinian interval. Two fluid end-members were involved in sulfate deposition: one lowsalinity(L1, ~3 wt% eq. NaCl) fluid, probably from a meteoric origin, and a Na-Ca-Cl brine (L2, ~22 wt% eq. NaCl) solutionwhich originated from the Triassic diapiric source. A third end-member fluid (L3) with long residence time in the basement wasalso involved in the Pb-Zn deposition. The δ34S values froma cluster of sulfates around +16‰, show a Triassic evaporate source.The sulfur in sphalerite resulted from bacterial sulfate reduction (BSR); however, crystallization in a closed system resulted in arange of δ34S between +1.6 and +26.5‰. The δ34S values in galena (−28.4 to +8.2‰) are consistent with a BSR and thermochemicalsulfate reduction (TSR) origin of the sulfur. Secondary ion mass spectrometry (SIMS) lead isotope data in galena(207Pb/204Pb: 15.595 to 16.193, 206Pb/204Pb: 18.673 to 18.939, 208Pb/204Pb: 38.330 to 40.572) point to local contributions(sedimentary and Cenozoic magmatic rocks) to the main source from the Precambrian basement. Ore deposition occurred at adepth of about 2 km at temperatures between 80 and 250 °C. A shallow magmatic heat source was the cause of these thermalfluctuations.

The geological study of Miocene clays from Gram area, North West of Tunisia shows an important series of clay materials to use them in the faience ceramic. Selected samples were studied with the objective of analyzing their chemical and mineralogical composition, morphology, particle size, plasticity, thermal analysis and their ceramic aptitude to be used in the faience ceramic. Raw materials are mainly composed of illite and kaolinite are the dominant clay minerals with minor quartz and dolomite. The plasticity indexes are lower than 15.40%, suggesting that these clays are not plastic. Technical characterization was carried out on one representative mixture of Miocene clay samples. The firing characteristics (shrinkage and water absorption) were measured. The optimum firing temperature of clay mixture (M) has been established. These clays could be used in the manufacture of ceramic pieces.

The Triassic salt diapirs in central Tunisia show splendid large multicoloured crystals of quartz. The study of these crystals and their host rocks sheds light on their origin and, more specifically, the relationships between silica authigenesis and its host rocks. Petrographic analyses of quartz crystals show strongly undulose euhedral to subeuhedral crystals (generally rhombohedral crystals) with concentration of many solid inclusions including sulphates and dolomite. Voids, shelter pores and negative crystals of gypsum, anhydrite and dolomite rhombs are found near the central parts of the crystals. Preserved anhydrite inclusions and dissolved evaporite are also found in these crystals. XRD analyses of white and black quartz crystals show crystalline alpha-quartz as the only silica phase present in most samples with a crystallinity index of 5. Accessory minerals include small relict patches of unreplaced calcite, gypsum, dolomite and anhydrite that escaped replacement. Fourier transform infrared (FTIR) spectra reveal the presence of abundant sulphate and organic compound included in the silica masses. The quartz crystals occurring in the allochthonous Triassic salt bodies are typically authigenic owing to the euhedral shape, the absence of any siliciclastic grains in the host rocks of quartz crystals, and also, to the absence of any sedimentary, wind or water-induced controls on the crystal distribution in their hosting rocks. Quartz growth in Triassic salt diapirs is a complex multistage and possibly continuous mechanism. During the progressive uplift of Triassic evaporites and dolomites, the dissolution of evaporites is enhanced and creates pores for the precipitation of the silica with development of larger quartz crystals. The colour variability in the quartz is ascribed to different ambient materials or fluids during crystallization. Remnant organic matter, before being altered and oxidised, could likely be included in the silica masses of the quartz and may serve not only to catalyse the precipitation and the growth but also to darken the quartz as well. Silica necessary for the formation of the quartz is mainly derived from two potential sources: (1) the presence of green and red clays in the Triassic salts and (2) silica-rich diagenetic fluids percolating from adjacent Lower Cretaceous sandy aquifers.

The Nefza region (Nappe Zone, northern Tunisia) is known both as a Late Miocene magmatic province and a base-metal district. In this region, small, post-nappe, continental extensional basins (Sidi Driss and Douahria) host syndiagenetic Pb–Zn ore deposits that have been classified as Sediment-Hosted Massive Sulphide (SHMS)–Sedimentary-exhalative (Sedex). In addition to this mineralization, the Nappe zone contains other Pb–Zn and polymetallic deposits that show similarities both to the SHMS-Sedex and MVT deposits of the Dome and Graben zones, namely the “peri-diapiric” deposits of Jebel (Jb) el Hamra, Jalta, and Jebel Ghozlane. All of the Nappe Zone deposits share common characteristics, including: (1) age—the deposits formed between late collisional events (Late Tortonian) and inception of the Early Messinian extensional regime, in the context of the Alpine Maghrebide belt formation; (2) presence of pre-existing sulphates—these were likely the main source of sulphur for the sulphides; and (3) hydrothermal systems leading to their genesis—these testify to alternating influx of cold and warm fluids. The involvement of high temperature fluids are deduced from fluid inclusion studies (Th values of 140–240 °C), whereas the presence of distinctive sphalerite textures (microspherules and colloform textures) and sulphur isotopic compositions demonstrate in situ bacterial sulphate reduction, and consequently deposition at temperatures below 80 °C. These data constitute unifying characteristics for these mineralizations that are in contrast to the features of Pb–Zn(–fluorite) MVT deposits occurring in the Dome and Graben zones of Tunisia. The geodynamic context prevailing during the Late Miocene in the Nappe Zone is the underlying common factor that explains the diversity of ore deposits formed at that time in this region. Most deposits are located in the vicinity of shear zones and associated lineaments inherited from the Variscan orogeny. These deep structures were reactivated by transtensional processes during the Alpine orogeny and controlled post-collisional magmatism, circulation of hydrothermal fluids, and locations of related ore deposits and showings in the Nappe Zone.

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

The Ain Allega Pb–Zn–Sr–Ba ore deposit is located in the flysch zone on the Eastern edge of the Triassic diapir of Jebel Hamra. It is part of the extrusive Triassic evaporate formation along the Ghardimaou–Cape Serrat faults. The ore body consists of argilic–dolomite breccias surrounded by argilo–gypsum Triassic formation, which forms the hanging wall of the deposit, and rimmed by the Paleocene marls. The ore minerals show a cap-rock type mineralization with 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 and pyrite. Principal gangue minerals are composed of barite, celestite, calcite, dolomite and quartz. The ore minerals are hosted by the Triassic carbonate rocks which show hydrothermal alteration, dissolution and brecciation. X-ray – crystallographic study of barite–celestite mineral series shows that pure barite and celestite are the abundant species, whereas strontianiferous barite (85–96.5% BaSO4) and barian–celestite (95% SrSO4) are minor. Primary and secondary mono-phase (liquid only) fluid inclusions are common in celestite. Microthermometric analyses in two-phases (liquid and vapour) fluid inclusions suggest that gangue and ore minerals were precipitated by a low-temperature (180 °C) saline (16.37 wt.% NaCl equivalent) solution originated possibly from a basinal brine with some input from magmatic or metamorphic fluid. Based on geology, mineralogy, texture and fluid characteristics, the Ain Allega deposit is classified as a carbonate-hosted Mississippi valley-type deposit.

The Ain Allega ore deposit represents one of the most significant Pb-Zn Sr-Ba deposits of the flysch zone in northern Tunisia. The total quantity of sulphide produced is 22,700 metric tons, with 13,300 t of galena and 9,400 t of sphalerite. It is located on the eastern edge of the Triassic diapir of the Jebel Hamra. The main ore minerals include sphalerite, galena, pyrite and marcasite, and gangue minerals including dolomite, calcite, celestite, quartz and barite. The mineralization hosted by carbonate rocks, shows some evidence of dissolution and brecciation characteristic of cap-rock formation. The fluid inclusion study from the celestite suggests that the primary and secondary monophase liquid (L) inclusions are most abundant compared to the other types. The microthermometric analyses of two-phase primary inclusions display homogenization temperatures ranging from 136 to 208°C (average of 180°C) and salinities ranging from 11.34 to 24.38 equivalent weight % NaCl (average of 16.4). The calculated density, pressure and depth are 0.965 g/cm3, 18 bars and 80 m, respectively. The hydrothermal fluid resulted from mixing of basinal brines and magmatic-meteoric solutions. Geological, mineralogical and thermometric data indicate that the Ain Allega ore deposit results of a mixing of basinal fluids (Mississippi valley type deposit) and hydrothermal fluids related to a shallow intrusion.