Petrography, geochemistry and primary origin of spheroidal dolomite from the Upper Cretaceous/Lower Tertiary Maghra El-Bahari Formation at Gabal Ataqa, Northwest Gulf of Suez, Egypt

Abstract

Abstract Field, petrographical, and geochemical studies of the Maghra El-Bahari Formation at Gabal Ataqa in Egypt indicate an occurrence of spheroidal dolomite as a primary precipitate. Petrographically, this dolomite is microcrystalline (10–40 μm in diameter) and lacks any direct or indirect evidence of a precursor mineral. Each dolomite spherule is composed of a cluster of coalesced incomplete rhombohedral sub-units exhibiting a globular body with a hollow core. This dolomite is commonly associated with authigenic palygorskite and gypsum. Based on its associations, the spheroidal dolomite occurs in two modes: (i) siliciclastic-disseminated dolomite and (ii) evaporitic-associated dolomite. On the basis of its elemental and stable isotopic signature, the spheroidal dolomite is subdivided into two types: Type A and Type B. Type A is Ca-rich (with average CaCO3 and MgCO3 equal to 58.20 and 39.91 mol%, respectively) and with average Sr=108 ppm, average Na=656 ppm, average Mn=72 ppm, and δ18O ranges from −0.4 to +0.9‰ PDB. Type B, compared with Type A, is nearly stoichiometric (with average CaCO3 and MgCO3 equal to 49.32 and 43.75 mol%, respectively) with higher average Sr (508 ppm), average Na (3008 ppm), average Mn (171 ppm), and δ18O (+0.98 to +1.8‰ PDB). Slightly positive values of δ13C (+0.18 to +2.10‰ PDB) are recorded for the two types of dolomite. Type A belongs to the siliciclastic-disseminated dolomite, whereas Type B is of the evaporitic-associated dolomite. It is interpreted that the spheroidal dolomite directly precipitated from a solution that was supersaturated with respect to dolomite components and did not develop by dolomitization of precursor carbonates. Two main sources are suggested for dolomite precipitation: hypersaline water and mixed saline–fresh water.