Geochemical constraints on the origin and timing of palaeofluid flow in the Presqu’ile barrier reef, Western Canada Sedimentary Basin

Abstract

Abstract The Mississippi Valley-type ore deposits at Pine Point are spatially and genetically associated with saddle dolomite cements. The origin and timing of fluid flow that produced these ore deposits are, therefore, constrained by paragenesis and geochemistry of the saddle dolomite cements. Because saddle dolomites occur continuously across the sub-Watt Mountain unconformity, dolomitization and associated mineralization must have occurred after the sub-Watt Mountain exposure during burial. The lateral continuity of saddle dolomite along the barrier for 400 km suggests that dolomitization and mineralization were probably associated with the lateral fluid migration along the barrier reef. From northeastern British Columbia to Pine Point, over a lateral distance of 400 km, saddle dolomites display remarkable trends of decreasing 87 Sr/ 86 Sr ratios (0.7106 to 0.7081) and homogenization temperatures of fluid inclusions (178°C to 92°C) with a corresponding increase in δ 18 O values (−16‰ to −7‰ PDB). These regional trends suggest that hot, radiogenic basinal fluids moved eastward up-dip along the Presqu’ile barrier reef. The movements of basinal fluids were probably related to tectonic compression and sedimentary loading on the western margin of the Western Canada Sedimentary Basin during either the late Devonian-early Carboniferous (Antler Orogeny) or the Jurassic-early Tertiary (Columbia-Laramide Orogenies). The δD values of aqueous fluid inclusions from Pine Point dolomite are very low (−80‰ SMOW to −100‰ SMOW) compared with that of Devonian seawater (−10‰ SMOW) suggesting an input of some Columbia-Laramide meteoric waters. If the low δD values are caused by a mixture of fluids from primary inclusions containing Devonian seawater with secondary inclusions that formed later during the Columbia-Laramide Orogenies, the fluid migration and associated dolomitization and mineralization could be interpreted as late-Devonian events. However, if the measured fluids were mostly from the primary inclusions, the low δD values indicate an entrapment of some Columbia-Laramide meteoric waters with Devonian formation waters at the time of dolomitization. This would suggest a Jurassic to early Tertiary age for dolomitization, which is supported by similar low δD values of present-day Devonian formation waters that consist of a mixture of Laramide meteoric waters and original connate brines. The light δD values of Pine Point dolomite inclusions could also occur as a result of reaction of fluids with organic matter associated with generation of oil and gas, which occurred at the maximum burial during the Laramide Orogeny.