Resumen de Dolomitization of the Miocene carbonates in Gebel Abu Shaar El Qiblie and Salum area, Egypt: a petrographical and geochemical comparative study

Ahmed S. Mansour, Mahmoud T. Abd-Ellatif

• The shallow marine Miocene carbonates of Gebel Abu Shaar El Qiblie and Salum escarpment, Egypt have been extensively dolomitized. Different dolomite textures are identified in the two areas: mimic replacive dolomite with uni- to polymodal size distribution and high preservation of the grain fabrics, non-mimic replacive crystalline dolomite with relatively uniform crystal size distribution, and partial dolomitization with polymodal size distribution and planar crystal boundaries. Minor cavity-filling dolomite cement is also observed. The development of non-planar crystal boundaries, stoichiometric characteristic, Sr content (92-245 ppm), negative d18O values (-9.34 to 1.59 %o) of Abu Shaar dolomites and their association with Fe-Mn ore deposits suggest that these dolomites formed from partial recrystallization and modification of early formed normal marine dolomites by the influence of hydrothermal fluid evolved during rifting of Red Sea. However, recrystallization by freshwater-dominated fluid during meteoric water diagenesis can be supposed. Alternatively, the non-stoichiometric composition, d18O values (2.15-3.19 %o) of marine signature and Sr content (73-204 ppm) of Salum dolomites indicate that dolomitization was probably mediated by seawater or slightly modified seawater within the subtidal zone during early stage of diagenesis. The d13C values (-1.03 to 0.7 %o for Abu Shaar, and -0.23 to 1.69 %o for Salum dolomites) reflect normal marine-derived carbon source and are probably inherited from the precursor marine limestones. The negative values of some Abu Shaar dolomites may be attributed to minimal contribution of carbon derived from soil-derived CO2 or from oxidation of methane derived from deep hydrocarbon reservoir via fractures during rifting of Red Sea.