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Abstract

Within the Presqu’ile barrier of northeastern British Columbia, a Middle Devonian carbonate complex extends over a lateral distance of 400 km in the subsurface of the Western Canada Sedimentary Basin (WCSB). Dolomite cements have decreasing 87Sr/86Sr ratios (0.7106 to0. 7081), decreasing fluid inclusion homogenization temperatures (178° to 92°C) and increasing δ18O values (— 16‰ to — 7‰PDB), northeastward. These trends suggest that hot, radiogenic basinal fluids moved updip (northeast) along the Presqu’ile Barrier and mixed with cooler ambient formation waters. This barrier behaved as a deeply buried regional conduit system in focussing and channeling basinal fluids.

The geochemistry of the Rimbey-Meadowbrook reef trend, another extensive subsurface carbonate system in the WCSB, lacks the distinctive regional trends exhibited by the Presqu’ile barrier. With increasing depth southward along the Rimbey-Meadowbrook reef trend, dolomite and calcite cements have a slight decrease in δ180 values (4 to — 7‰ PDB) and slightly more radiogenic Sr/Sr ratios, although no systematic relation between Sr and O isotopic compositions is observed. Minimum fluid inclusion homogenization temperatures (uncorrected for depth and pressure) in most dolomite and calcite cements from the Rimbey-Meadowbrook reef trend are 30° to 40°C higher than is estimated to have been reached during maximum burial in Early Tertiary time. The minimum homogenization temperatures (Th) of inclusions in intermediate-diagenetic dolomite cements do not vary with depth suggesting that they may have formed from hydrothermal fluids during shallow burial. Late-diagenetic pre-thermal sulphate reduction (TSR) calcite cements (deep burial) have minimum Th that occur along a 40°C/km geothermal gradient or higher and may also indicate the involvement of hydrothermal fluids. However when corrected for depth and pressure they plot close to a 30°C/km gradient. Latediagenetic calcites associated with TSR exhibit a more variable pattern and follow geothermal gradients between 20° and 25°C/km.

Replacement dolomites and dolomite cements from Frasnian Leduc and Famennian Wabamun strata near the southeast Peace River Arch formed from saline hydrothermal fluids (between 100° and 200°C) that moved upwards along an extensive fault and fracture conduit system (in part formed by extensive subsurface solution), possibly as early as Early Carboniferous time.

Fluid inclusion homogenization temperatures are best explained in two cases by means of the net lateral flow of hydrothermal fluids along dolomitized conduits, or in one case by upward flow along a fault-fracture conduit system. Data from intermediate-diagenetic dolomite cements from the Peace River and the Rimbey-Meadowbrook reef trend suggest that fluid movements probably occurred during Late Paleozoic shallow burial, possibly related to the Antler orogeny to the west. This requires the movement of hot fluids at shallow depths both across parts of the basin and vertically. Temperatures of later calcite cements suggest that hydrothermal fluid movement took place prior to and after hydrocarbon maturation, probably close to maximum burial during the Laramide orogeny. The movements of hydrothermal fluids in the WCSB, in part focussed through subsurface conduit systems, appear to be mainly related to orogenic compression and sedimentary loading.

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