Depositional environment and stability of the porcelanite within the Ypresian phosphatic series of the Gafsa-Metlaoui basin, southwestern Tunisia

Ypresian porcelanite levels outcrop from the central to the western part of Gafsa-Metlaoui basin are valuable for the geological environment reconstitutions and the industrial applications, as a siliceous industrial filtration aids like diatomite. The present work aims to determine the mineralogical and geochemical characteristics of porcelanites levels and provides some data about their depositional environment. Thus, 29 specimens are sampled from these levels. The XRD data show that all samples are composed mainly of opal CT, clay minerals (smectite, sepiolite, palygorskite), and carbonates (calcite and dolomite), which contain also trace amount of clinoptilolite, feldspar, francolite, quartz, and pyrite. The variations of opal CT and carbonates percentages are in the opposite trends. These results agree with chemical composition of major elements determined by XRF and inductively coupled plasma mass spectrometry, which indicates that the selected samples are characterized by high SiO2 (64 %) and relatively low CaO (11 %) contents. Plot of SiO2 vs. CaO and of SiO2 vs. MgO show negative linear correlations indicating that the depositional conditions favor carbonate dissolution but encourage the formation of opal CT and vice versa. Therefore, carbonate dissolution increases Mg activity of the depositional environment, privileged then the opal CT agglomeration as lepispheres coated by microcalcite. Since, increasing Mg content is required for the autogenesis of Mg-rich fibrous clays and encourages the stability of opal CT, although the high Mg content avoids chert formation.