Origin of bentonite deposits in the Eastern Rif Belt (Morocco): Constraints from new mineralogical, geochemical and stable isotopic data

Abstract

The eastern Rif Belt in Morocco represents an important economic region in North Africa due to its extensive bentonite deposits. This study aimed to elucidate the formation processes and mechanisms of Providencia bentonite deposits, classified as yellow and white bentonites, surrounding the Upper Miocene rhyolitic Tidiennit Massif. Multidisciplinary approaches, integrating structural, lithostratigraphic, mineralogical, petrographical and geochemical analyses have been utilized to examine the properties of the bentonites and precursor (host) rocks. The X-ray diffraction analyses of the tuffs, perlites and rhyolites show that the presence of smectite, feldspar, amorphous silica (Opal-A and Opal-CT) and biotite in tuffs, feldspar, quartz, cristobalite and pyroxene in rhyolites, and amorphous silica, pyroxene, feldspar in perlites. The clay fraction patterns of yellow and white bentonites indicate the bentonites are composed of pure or nearly pure dioctahedral (Al-rich) smectite minerals. The characteristic bands observed in Fourier transform infrared spectroscopic analyses were also confirmed the presence of Al-rich smectites in the bentonites. The thermogravimetric and differential thermal analysis data show a total mass loss of 15.7% and 18.13%, and endothermic peaks of 698 °C and 678 °C representing the smectite from white and yellow bentonite, respectively. The major element concentrations indicate the sub-alkaline series, and the composition of dacite to rhyolite for tuffs, whereas rhyolite for perlites and rhyolites. The alteration index (AI) and the chlorite-carbonate-pyrite index (CCPI) data suggest a clear trend towards argillization of the volcanic tuffs surrounding the Tidiennit Massif. The chemical compositions of bentonites and precursor (host) rocks show an increase in Fe₂O₃, MgO, Al₂O₃, TiO₂, P₂O₅, Nb, Y, Zn, Zr, total REE from rhyolites-perlites via tuffs to bentonites. The immobile element (Zr, Ti, Nb, Y in ppm) compositions of the bentonites represent a subduction zone (syn-collision and volcanic arc granites) environment. Bentonites and host rocks exhibit a negative Eu anomaly with respect to the chondrite and upper continental crust composition indicating the chemical fractionation from the rhyolitic/rhyodacitic source rocks through removal of Eu by plagioclase fractionation in a reducing environment. The structural formulas of smectites in yellow and white bentonites indicate the montmorillonite-beidellite series of smectite composition. The δ¹⁸O and δD isotope data of the smectites show that bentonitization was formed as a result of hydrothermal alteration (hypogene origin) of tuffs interacting with hydrothermal fluids at temperature conditions of 127–165 °C for yellow bentonites and 153–174 °C for white bentonites. Oxygen and hydrogen isotope compositions of smectite-forming fluids indicated that yellow bentonites formed from fluids with a relatively high magmatic water component, and subsequent dilution of the hydrothermal fluids, primarily composed of meteoric waters, caused the formation of white bentonites (re-bentonitization). The obtained data indicate that the bentonite deposits are the result of heat flux transmission to the host by conduction and triggering of a convective geothermal circuit facilitated by extensive tectonics favoring hydrothermal fluid circulation. The hydrothermal fluids, that migrated across the N-NE oriented normal faults, were mainly controlled the formation of the bentonites.