Geochemical significance for the composition and depositional environments of the Campanian carbonate-rich phosphorite, Abu-Tartur plateau, Western Desert, Egypt

Abstract

Twenty-six samples involved CFA (carbonate fluor-apatite) and intercalated marl were collected from four outcrops of the Campanian phosphorites horizon extending along 9 km of a minable NE-SW side wall of the Abu Tartur Plateau. Samples were analyzed chemically using XRF (x-ray fluorescence) and ICP-OES (Inductively coupled plasma optical emission spectrometry) techniques for measuring oxides, trace elements, and REE (Rare earth elements) for deducing their implication for the paleo-depositional environment. The CFA samples show an average compositional richness in major oxides of CaO (42.23 wt%) and P2O5 (27.22 wt%) with a considerable amount of F (2.75%). Whilst the main average composition of the intercalated marl is SiO2 (54.17 wt%) and CaO (10.58 wt%). The geochemical classification of this marl shows a type of Fe-sand (Ferro-magnesium potassic sandstone). The P2O5 in the marl samples does not exceed 3.46 wt%. Calculation of the CaO/MgO molar ratio of studied samples reveals evidence of the dolomitization process took place, making a conversion of approximately 65.73% of the entire CFA to be dolomitic- and highly dolomitic-francolite may occur over a long time post of the Campanian age. CFA samples have an amount of Fe2O3 ranging from 3.15 wt% to 5.84 wt% (average 4.31 wt%) whereas Fe2O3 in the marl samples range from 3.81 wt% to 6.03 wt% (average 4.41 wt%). The origin of iron is attributed to being delivered by an ancient river system laden from hot, humid chemically weathered (laterite) soils into the basin simultaneously with the dolomitization process. Less saline mixed water is the suggested medium where dolomitization processes occur based on Rb/Sr and Sr/Ba ratios. The normalized REE pattern of the base, middle and top parts of the CFA samples show a similarity of fingerprint patterns of near-shore and margin-marine of either bottom seawater and shallow-to deep-pore-water below the SWI (sediment-water interface). Dysoxic to oxic depositional environments of the marginal shelf are established based on measured trace elements ratios of V/Cr, Ni/Co and V/(V + Ni). The accumulation rate of P shows a progressively increasing trend toward the NE direction of favourable P depositional conditions. The results of the Ce/Ce* and Pr/Pr* anomalies show no-to + ve Ce/Ce* lie between 0.93 and 1.45 (average 1.04) and the Y/Ho ratio lie between 23.43 and 87.75 (average 51.63) which is attributed to depositional sets within the shelf margin of suboxic to anoxic pore-water chemistry conditions at bottom seawater below the sediment-water interface. The fingerprint shape of the REE patterns and the richest of HREE (Heavy rare earth elements) of the studied CFA samples reflect the chemistry of the shallow to the deep pore-water flux of suboxic-anoxic sulfidic characters.