ABSTRACT

Preliminary petrographic and geochemical data of replacement dolomites from several Leduc buildups indicate a selective replacement of the original components. Three petrographic types of replacement dolomite are present, suggesting different phases of dolomitization. The most abundant is first phase medium to coarse crystalline, planar matrix dolomite. It consists of dense mosaics of subhedral crystals, or forms porous zones characterized by intercrystalline porosity. Luminescence is orange-red mottled or blotchy. Isotope values cluster around +2.0 to +3.8°/¹³C and -6.2 to -5.0°/¹⁸O PDB.

Second phase coarse crystalline, planar, void-filling dolomite occurs mainly in reef-margin wells. Clear crystals are zoned, and have more negative ¹⁸O values (-8.4 to -6.5°/).

The third dolomite is nonplanar and nonmimically replaces fossil fragments. Cathodoluminescence and stable isotopes are similar, or commonly identical, to first phase planar replacement dolomites. Minor nonplanar dolomite cements (saddle dolomite) line vugs and fractures.

Geochemical bulk analyses of replacement dolomites have low concentrations of Sr (<150 ppm) and Mn (250 ppm), and Fe ranges from 860 to 8000 ppm.

The most pervasive are the first phase, planar matrix dolomites. They are similar texturally and geochemically, and also in terms of timing and probable origins, to early pervasive dolomites observed elsewhere in Devonian strata in widely separated parts of the Alberta basin.

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