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Dolomitization of the Leduc Formation (Upper Devonian), Southern Rimbey-Meadowbrook Reef Trend, Alberta
Eva Drivet (1), Eric W. Mountjoy (2)
Two generations of dolomite cements postdate replacement dolomites: a coarse and planar-e(s) cement (Dolomite-C1), and a later coarser nonplanar cement (Dolomite-C2). Dolomite-C1 has oxygen (mean d18O = -4.73 PDB) and strontium (mean 87Sr/86Sr = 0.70855) isotopic compositions similar to replacement dolomites. These similarities, combined with the local association of Dolomite-C1 with stylolites, suggest that some Dolomite-C1 precipitated from fluids whose composition reflects pressure solution of the preexisting replacement dolomites. The composition of fluid inclusions in Dolomite-C1 (mean Th = 117°C; mean salinity = 19 wt % aCl equivalent) indicates that most Dolomite-C1 precipitated from warm hydrothermal brines. Little variation in fluid-inclusion homogenization temperatures occur with burial depth, suggesting that Dolomite-C1 precipitated prior to basin tilting, and may be related to Antler orogenic events. Dolomite-C2 postdates Dolomite-C1, is relatively more depleted in 18O (mean d18O = -9.06 PDB), and has a more radiogenic 87Sr/86Sr ratio (0.71087). Preliminary fluid-inclusion data from the Crimson Field suggest precipitation temperatures of about 150°C. Thus, Dolomite-C2 originated from fluids of higher temperatures and different composition than Do omite-C1.
The petrographic, paragenetic, and geochemical characteristics of replacement
dolomites in this study are similar to those of other Leduc buildups along
the reef trend and of other Devonian dolostones of western Canada, supporting
a model of large-scale basinwide fluid flow. The pervasive distribution
of replacement dolomites along the Rimbey-Meadowbrook reef trend, and the
similar petrographic and geochemical character of dolomites from both the
Leduc and Cooking Lake Formations, is consistent with the hypothesis that
the underlying Cooking Lake platform acted as a subsurface conduit system
for the dolomitizing fluids. A potential regional fluid source may have
been formation fluids (including residual evaporite brines) expelled tectonically
during the Antler orogeny.
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