The origin of sulfate mineralization and the nature of the BaSO4–SrSO4 solid-solution series in the Ain Allega and El Aguiba ore deposits, Northern Tunisia

Abstract

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 minerals show a cap-rock type mineralization with various forms and types, in particular impregnation in dolomite, breccia cement, replacement of carbonate cap-rock 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 the 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 346.97Å3 in barite (100% BaSO4) to 308.29Å3 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 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; 16.37wt.% NaCl equivalent in Ain Allega and 8.2wt.% NaCl equivalent in El Aguiba). Consequently, the precipitation could be from mixing of basinal brines with magmatic–meteoric fluid.