Abstract

Hydrothermal dolomites are potential hydrocarbon reservoirs in the Late Ordovician–Middle Devonian Gaspé Belt in eastern Canada. These dolomites are recognized in the Lower Silurian Sayabec Formation, Upper Silurian–lowermost Devonian West Point Formation and Lower Devonian Upper Gaspé Limestones and are also recognized in the Lower Silurian La Vieille Formation in northeastern New Brunswick. Hydrothermal dolomitization is expressed in local pervasive dolomitization fronts originating from fracture feeders and by saddle dolomite and high temperature late calcite cements filling fractures and associated dissolution voids. Field evidence such as dolomite clasts in a post-Salinic (early Ludlovian) conglomerate indicates a dolomitization event early in the burial history. Petrographic study documents a complex history of multiple fracture generation and saddle dolomite and/or calcite cement precipitation. Saddle dolomite is locally brecciated and cemented by a later phase of dull luminescent, mm-sized calcite crystals (calcite 1), the latter, in some cases, also completely filling fractures. Saddle dolomite is locally dedolomitized which is expressed as bright luminescent sub-millimetre-sized calcitic zones. Dedolomitization is associated with calcite-cemented (calcite 2), late stage fractures. δ18OVPDB values of the saddle dolomite (−17.3 to −10‰) and calcite 1 (−13.8 to −8.8‰) cements indicate precipitation from an 18O-depleted fluid and/or at relatively high temperatures. Fluid inclusion homogenization temperatures (86 to 212°C, average of 132°C) support a high temperature origin for calcite 1 from very saline fluids (from 12 to 23 wt% NaClequiv.). Calcite 2 is characterized by similar δ18OVPDB values (−9.5 to −8.7‰) with notably different δ13CVPDB values (−4.1 to −3.1‰ and 0.9 to 3.0‰ for calcite 2 and 1, respectively). Moreover, calcite 2 is characterized by fluid inclusions with very low salinities (about 0 wt% NaClequiv.) and is interpreted as late meteoric calcite cement likely precipitated during the Late Silurian global sea level lowstand (Salinic Unconformity) or following the Middle Devonian (Acadian Orogeny) subaerial exposure of the Lower Silurian ramp.

The Early Silurian tectono-magmatic setting of the Gaspé Belt basin played a significant role in the regional hydrothermal alteration of the carbonates. The occurrence of hydrothermal dolomite suggests previously unrecognized reservoir potential of Silurian carbonates that should be considered along with the presence of good Middle and Upper Ordovician source rocks and favourable thermal maturation.

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