Large euhedral quartz crystals in the Triassic dolomites and evaporites of central Tunisia: implications for silica diagenesis in sulphate-rich and high-Mg environments

Abstract

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.