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 mu m 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 CaCO (sub 3) and MgCO (sub 3) equal to 58.20 and 39.91 mol%, respectively) and with average Sr = 108 ppm, average Na = 656 ppm, average Mn = 72 ppm, and delta (super 18) O ranges from -0.4 to +0.9 per mil PDB. Type B, compared with Type A, is nearly stoichiometric (with average CaCO (sub 3) and MgCO (sub 3) equal to 49.32 and 43.75 mol%, respectively) with higher average Sr (508 ppm), average Na (3008 ppm), average Mn (171 ppm), and delta (super 18) O (+0.98 to +1.8 per mil PDB). Slightly positive values of delta (super 13) C (+0.18 to +2.10 per mil 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. Abstract Copyright (2002) Elsevier, B.V.