Stable isotopic composition, stoicometry and origin of Siliceous dolomite of naqb siwa member Bahariya oasis, Egypt
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The Lower Campanian Naqb Siwa member exposed near Bahariya Oasis, Egypt, is a shoal ing-upward dolomite sequence that hosts chert nodules and lenses and cavity fillings of quartz and calcite. Dolomite is dominantly replacive, but also includes pore-filling cement. Slightly negative 818O compositions (average -2.2 %o PDB), and oxygen covariation with Na and Sr contents, reflect dolomitization in the presence of variable meteoric-marine fluid mixtures. However, isotopic and trace element values of dolomite rhombs in chert nodules reflect the signature of marine conditions. MgCO3 contents correlate positively with 818O and negatively with Sr contents. A sequence of five diagenetic events is recorded by relations between various authigenic phases. These are: 1) partial dolomitization by marine fluids; 2) early silicification and replacement of original carbonate sediments; 3) complete dolomitization of the carbonate sediments in mixed marine-meteoric fluids; 4) silicification and dolomite cementation; and 5) precipitation of void-filling meteoric calcite (mean 618O = -9.6 %o PDB).