Strontium isotopic composition of Devonian dolomites, Western Canada Sedimentary Basin: significance of sources of dolomitizing fluids

Abstract

Dolomites from the Middle and Upper Devonian of five regions in the Western Canada Sedimentary Basin wwere studied: Middle Devonian Winnipegosis Formation (outcrop), Keg River Formation reefs in the Rainbow field and Presqu'ile barrier, and the Upper Devonian Miette buildup and the Wabamun Group of northern Alberta. Data from the Upper-Middle Devonian Swan Hills Formation in the Rosevear field and Nisku Formation reefs were also included. In these areas of western Canada there are at least three dolomitizing events: 1. Early dolomites in Winnipegosis reefs, and the subsurface part of the Presqu'ile barrier have 87Sr/ 86Sr values near 0.7080, Wabamun sabkha dolomites have 87Sr/ 86Sr values near 0.7083, and all fall on their respective parts of the seawater curve; thus the sources of dolomitizing fluids were Devonian seawaters; 2. Secondary matrix dolomites from the Presqu'ile barrier, Rainbow, Rosevear, Miette, Nisku and Wabamun all have slightly higher 87Sr/ 86Sr values (0.7081–0.7094) than the correspnding Devonian seawater. Dolomitizing fluids were probably modified from Devonian seawater with small amounts of radiogenic 87Sr being added during early compaction from adjacent or underlying clastics, or older carbonates; 3. Late-stage saddle dolomites from Presqu'ile, Rainbow, Rosevear, Miette, Nisku and Wabamum are all highly radiogenic ( > 0.7100 up to 0.7151), except for most Pine Point saddle dolomites, indicating significant input of radiogenic 87Sr, with likely sources from the underlying Cambrian and Precambrian clastics and/or the Precambrian basement. At Pine Point, some Presqu' ile saddle dolomites have Sr isotopic values similar to matrix dolomites, suggesting a different fluid source, possibly from subsurface brines that may have evolved from pressure solution of Devonian strata. Alternatively, formation waters derived from the deeper part of the basin may have been progressively diluted with low 87Sr/ 86Sr shallow formation waters as they moved updip along the Presu'ile barrier. The 87Sr/ 86Sr values support and refine earlier conclusions based on textual evidence, C and O isotopes, geochemistry and fluid inclusions. They considerably limit the sources of the dolomitizing fluids in open diagenetic systems. However, the timing of the widespread secondary matrix dolomites can only be broadly constrained to sometime during the Late Paleozoic. The similarity of the O and Sr isotopes in the replacement dolomites from widely separated locations geographically and stratigraphically tentatively suggests that these dolomites may have precipitated from similar formation waters on a basin-wide scale. The late-stage saddle dolomites, and probably the coarsely crystalline dolomites, could only have formed near maximum burial conditions in the Late Cretaceous to Early Tertiary from warm hydrothermal brines that moved updip from more deeply buried portions of the basin. Later coarse-crystalline calcites precipitated from highly saline brines and are generally more radiogenic than saddle dolomites. They apparently formed from more radiogenic formation waters than have curretly been reported from the basin.