RARE-EARTH ELEMENT GEOCHEMISTRY OF DOLOMITES IN THE MIDDLE DEVONIAN PRESQUILE BARRIER, WESTERN CANADA SEDIMENTARY BASIN - IMPLICATIONS FOR FLUID-ROCK RATIOS DURING DOLOMITIZATION

Rare earth elements (REE) were determined in fine, medium and coarse c rystalline replacement dolomites, and for saddle dolomite cements from the Middle Devonian Presqu'ile barrier from Pine Point and the subsur face of the Northwest Territories and north-eastern British Columbia. REE patterns of the fine crystalline dolomite are similar to those of Middle Devonian limestones from the Presqu'ile barrier. Fine crystalli ne dolomite occurs in the back-barrier facies and may represent peneco ntemporaneous dolomitization at, or just below, the sea floor. Medium crystalline dolomite is widespread in the lower southern and lower cen tral barrier. Medium crystalline dolomite is slightly depleted in heav y REE compared with Devonian marine limestones and fine crystalline do lomite, and has negative Ce and Eu anomalies. Medium crystalline dolom ites replaced pre-existing limestones or were recrystallized from earl ier fine crystalline dolomites. During these processes, the REE patter ns of their precursors were modified. Late stage, coarse crystalline r eplacement dolomite and saddle dolomite cements occur together in the upper barrier and have similar geochemical signatures. Coarse crystall ine dolomites have negative Eu anomalies, and those from the Pine Poin t area also have positive La anomalies. Saddle dolomites are enriched in light REE and have positive La anomalies. The REE patterns of coars e crystalline dolomite and saddle dolomite differ from those of marine limestones and fine and medium crystalline dolomites, suggesting that different diagenetic fluids were responsible for these later dolomite s. Although massive dolomitization requires relatively large volumes o f fluids in order to provide the necessary amounts of Mg2+, dolomitiza tion and subsequent recrystallization may not necessarily modify the R EE signatures of the precursor limestones because of the low concentra tions of REE in most natural fluids. Thus, relative fluid-rock ratios during diagenesis may be estimated from REE patterns in the diagenetic and precursor minerals. Fine crystalline dolomites retain the REE pat terns of their limestone precursors. In the medium and coarse crystall ine dolomites the precursor REE patterns were apparently altered by la rge volumes of fluids involved during dolomitization. This study sugge sts that REE compositions of dolomites and their limestone precursors may provide important information about the relative amounts of fluids involved during diagenetic processes, such as dolomitization.