Geochemistry, mineralogy and depositional environment of black shales of the Duwi Formation, Qusseir area, Red Sea coast, Egypt

Abstract

This study focuses on the geochemistry of redox-sensitive elements of marine black shales at the Qusseir area of the Duwi Formation to infer their depositional environment. The black shales from the uppermost member of the Duwi Formation in the Qusseir area is conformably overlain by the Qusseir variegated shale and underlain by the Dakhla shale. Detailed mineralogical and geochemical characteristics were examined using several techniques, including X-ray diffractometry, scanning electron microscopy and X-ray fluorescence. Mineralogically, black shales in the Duwi Formation are composed mainly of montmorillonite, kaolinite, calcite, gypsum, quartz and pyrite. They are detrital and authigenic in origin. They are most probably derived from basic volcanic rocks by intensive chemical weathering under the prevalence of semiarid conditions. The redox-sensitive trace metals suggest anoxic sulfidic conditions that favoured the accumulation of these metals in the Qusseir area of the Duwi Formation.

Geochemical and Mineralogical Evaluation of Black Shale and its Hydrocarbon Potentiality,Southwest Sinai, Egypt

“…Framboidal pyrites are comprehensively pseudomorphosed and formed as authigenic, which conformed by X-ray diffractions. The framboids (4 μm) are pointed out the deposition of pyrite in reducing environments; which confirms with Abou El-Anwar and El-Sayed [23], Abou El-Anwar [24] and Abou El-Anwar et al [25,26]. Consequently, the vastly pyritic shale is deposited in euxinic environment.…”

“…Anomalous values may be resulted from the presence of iron oxides as cement materials in these deposits [12]. The positive relation between Fe2O3 and total sulfur (r = 0.39) revealed that the organic sulfur prevails beside with pyrite, which confirmed by XRD and SEM examinations [24,26].The measured sulfur (S wt%) of the studied samples is low with an average value 0.75% (Table 1), that is argument for a terrestrial sedimentation [29] with organic and inorganic (pyrite, Figs. 2 and 3) sources of S.…”

“…The CIA values in the investegated shalevary from 76.4 to 94.2% and averaging 87.8% (Table 1). This value indicate that the source of the principal minerals was feldspars and/or mafic rocks and, commonly, represents highly intensive chemical weathering [26,36].…”

“…Percentage of redox-sensitive elements (Cd, Cr, Cu, Ni, Mo, Re, Sb, U and V) can give good indication about paleo-environments [26,31,32]. These elements are generally enriched in anoxic environment as a result of oxygen depletion and vice versa for oxic environment [31].…”

Organic Petrographic and Geochemical Evaluation of the Black Shale of the Duwi Formation, El Sebaiya, Nile Valley, Egypt

“…Immobile elements such as Al, Fe, Ti, Cr, Th, Pd, Sc, Co, Zr, Nb, Y, Ga and Se, are important indicators of provenance, such as weathering, transportation, and sorting [30]. Strong and moderate positive correlations between Al 2 O 3 and Fe, Ti, Co, Cu, Zr and Pb (r = 0.62, 0.71, 0.80, 0.79 and 0.36; respectively) (Table 3) revealed that these elements were accumulated by weathering processes [31][32][33][34]. The Al/Si ratio of the studied black shale samples is low (0.2-0.28), which indicates that Si has another source in addition to clay minerals.…”