Factors controlling the compositional variations among the marine and non-marine black shales from Egypt

Abstract

Non-marine (Jurassic) and marine (Cretaceous) black shales from Egypt were subjected to mineralogical and geochemical analyses to examine the controlling factors of their compositional variations. Non-marine black shales are composed of kaolinite and quartz with traces of gypsum, illite, calcite, feldspars, and dolomite, while marine black shales from the Red Sea area are composed of smectite, kaolinite, quartz, calcite, and dolomite with traces of feldspars. Abu Tartur marine black shales are composed of smectite and quartz with traces of feldspars and gypsum. Non-marine black shales show considerably higher Nb, Ta, Hf, and Zr contents and Th/Yb ratios compared to the marine black shales. On the other hand, marine black shales show considerably higher Cr, V, and Zn contents with positive correlations between these elements and organic carbon (Corg.). Red Sea black shales have higher Ni/Co, V/Cr, and U/Al ratios. Chondrite normalized values of the medium and heavy rare earth elements (MREEs and HREEs, respectively) are higher in the non-marine black shales compared to the marine black shales. Pyrite from non-marine black shales is characterized by high positive δ34S isotope values (average of + 9.3‰). Pyrite from Red Sea black shales has low negative δ34S values (average of −16.7‰), pyrite from black shales of the lower member of the Duwi Formation has positive δ34S values (average of 5.8‰), while pyrite from marine black shales of the middle member has negative δ34S values (average of −0.83‰). Source area composition, weathering conditions, depositional environments, and type of organic matter are considered to be the probable controlling factors of these variations. The more felsic constituents in the source area of non-marine black shales is responsible for the relatively high Nb, Ta, Hf, and Zr contents and Th/Yb ratio. Relatively high kaolinite contents and Chemical Index of Alteration (CIA) values in the non-marine black shales compared to the marine black shales suggest a prevailing of more intensive weathering conditions during the Jurassic time in Egypt compared to the Cretaceous. Higher weathering of non-marine shales is compatible with a higher zircon and TiO2 contents. The higher zircon may account for some of the differences on the contents and mode of occurrence of elements found between the marine and non-marine shales, such as the higher REEs, Nb, Y, and Hf. The higher Corg and Ni/Co, V/Cr, and U/Al ratios in the Red Sea black shales are attributed to more reducing depositional environment of these shales. Marine organic matter seems to be responsible for the selective fixation of some trace elements such as Cr, V, and Zn. Sulfur isotopes from pyrite suggest a marine origin of this pyrite in all shales. However, variations in the δ34S values reflect variations in the depositional settings. δ34S data suggest a sulphate-replete (open) system for the pyrite in the Red Sea and lower member (Duwi Formation) of Abu Tartur black shales, sulphate-limited (closed) system in a brackish setting for the Maghara black shales, and coastal plain setting in the middle member of Abu Tartur area. The similarities among marine and non-marine black shales such as positive correlations between Zr and both Nb and Hf as well as between Fe2O3 and Co might reflect the geochemical behaviors of such elements rather than variations in the provenances and/or depositional environments.