Dolomite reservoirs are increasingly recognized as an important petroleum exploration target, although the application of a hydrothermal dolomite exploration model to these reservoirs remains controversial. The St. George Group of western Newfoundland consists of a sequence of dolomitised carbonates, with significant porosity development (up to 30%) and petroleum accumulations. Fluid inclusion microthermometry and bulk fluid leach analyses indicated that fluids responsible for matrix dolomitization (associated with intercrystalline porosity) and later saddle dolomitization are CaCl 2 ± MgCl 2 rich, high salinity (up to 26 eq. wt% NaCl) brines. Integration of fluid inclusion data with thermal maturation histories from the St. George Group show that these dolomites formed at temperatures higher than the ambient rock temperature, and are therefore hydrothermal in origin. Bulk leach analyses show that dolomitization is associated with influxes of postevaporitic brines (±Cl enriched magmatic fluids) late in the diagenetic history of these carbonates. This dolomitization is possibly Devonian in age, during a period of significant magmatic activity, extensional tectonics and development of hypersaline basins. Petrographic and geochemical similarities between Paleozoic hosted hydrothermal dolomitization in western Newfoundland, eastern Canada and the northeastern United States are consistent with a regional-scale hydrothermal dolomitization event late in the diagenetic history of these carbonates.

Re-Os geochronometry is combined with fluid inclusion microthermometry to investigate the granite molybdenite system associated with the late-Caledonian Omey Granite, Connemara, western Ireland. Molybdenite in the Omey pluton is hosted by thin vertical quartz veins (,5 cm wide) that trend in a NE-SW direction. The 187 Re and 187 Os systematics yield a model age of 422?5¡1?7 Ma for the vein molybdenite. Three fluid inclusion types occur in the molybdenite bearing quartz veins: Type 1(aqueous-carbonic fluid), Type 2 (carbonic fluid) and Type 3 (aqueous fluid), Type 1 and Type 3 also occur in granite quartz. The Type 1 and 2 fluid inclusions are interpreted as representing the molybdenite mineralising fluid as is the case elsewhere in the Connemara granites. The 422?5¡1?7 Ma age for molybdenite mineralisation is the oldest reported from this region and implies that the Omey Granite was emplaced before the main Galway Batholith and during activity on the major orogen parallel lineaments like the Great Glen and Southern Uplands Faults.

The Watts Bight Formation constitutes the lowermost part of the St. George Group in western Newfoundland. On the Northern Peninsula, it consists of Tremadocian (lower Ordovician) shallow marine platform carbonates (approximately 50 m thick). Dolomitization is extensive throughout the formation except for its topmost part. Petrographic examinations suggest that the succession was affected by at least three successive phases of dolomitization, which influenced secondary porosity. These phases have crystal-size ranges of approximately 4 to 50 µm (earliest sub-to euhedral dolomite D1), 50 to 200 µm (eu-to subhedral D2), and 300 µm to 3mm (anhedral saddle dolomite D3), respectively. They occur as replacement and/or pore-filling cements and exhibit dull (D1 and D3) to zoned (D2) luminescence under the cold cathodoluminoscope. The D2 phase is the dominant dolomite while the other two phases are rare.Microthermometric measurements of the primary two-phase fluid inclusions in D2 (homogenization temperatures up to 157 o C and salinity estimates up to 24.3 eq. wt% NaCl) suggest that it formed under relatively deep burial conditions. This is supported by the petrographic character (eu-to subhedral relatively big crystals) and geochemical composition (depleted δ 18 O mean value of -8.7±1.2‰ VPDB and low Sr contents of 68±30ppm) of the D2 phase. Thin-section examination suggests that porosity is dominantly intercrystalline and associated with the D2 phase. Visual estimates suggest that porosity varies from <1% in most of the formation to approximately 10% in two horizons, each approximately 2m thick, at approximately 10 m and 20 m from formation base. Correlations with the equivalent Watts Bight Formation section in the Isthmus Bay (300 km to South) reveal porous intervals at comparable stratigraphic levels. Also the geochemical results and microthermometric measurements suggest that D2 in the Northern Peninsula section was formed from hotter fluids under relatively more reducing conditions relative to their Isthmus Bay counterparts. The porous zones seem to be associated with fluctuations in sea-level marked by negative shifts in the δ 13 C profile. RÉSUMÉLa Formation de Watts Bight constitue l'unité la plus basse du Groupe de St.George dans l'ouest de Terre-Neuve. Dans la péninsule Great Northern, elle est composée de carbonates de plateaux marins (d'environ 50 m d'épaisseur) peu profonds du Trémadocien (Ordovicien inférieur). La dolomitisation est considérable dans toute la formation, à l'exception de sa partie la plus élevée. Les analyses pétrographiques donnent à penser qu'au moins trois épisodes de dolomitisation ont affecté la succession, ce qui a influencé la porosité secondaire. La taille des cristaux de ces épisodes de dolomitisation varie de 4 µm à 50 µm (dolomite D1 précoce de subautomorphe à automorphe), de 50 µm à 200 µm (D2 automorphe à subautomorphe) et de 300 µm à 3 mm (dolomite D3 en selle anédrique), respectivement. Ils se présentent sous forme de remplacement et/ou de cimentation d'espaces intergra...

The Watts Bight Formation in western Newfoundland consists of a Lower Ordovician succession of shallow-water carbonates and has been extensively dolomitized. These dolomites occur as both replacements and cements and are associated with complex changes in the rock porosity and permeability. Early replacement micritic dolomites (D1) are finely crystalline and indicate that dolomitization began during early stages of diagenesis. The calculated δ18O values of the earliest (D1) dolomitizing fluids (–6.4‰ to –9.5‰ VSMOW, Vienna Standard Mean Ocean Water) fall between the estimated δ18O values of Tremadocian seawater and meteoric waters and suggest mixing-zone dolomitization. A second phase of coarsely crystalline (up to 400 μm) dolomite (D2) replaces D1 dolomite and early calcite and is associated with enhancement in porosity and permeability through the development of intercrystalline pores. A late-stage saddle dolomite (D3) and late burial calcite cements significantly occluded the pores in some horizons. Petrography, fluid inclusions, and geochemistry show that D2 and D3 dolomites formed from warm (65–125 °C) saline (10 to 25 eq. wt.% NaCl + CaCl2) hydrothermal fluids. The calculated δ18Ofluid of D2 ranges from –4.5‰ to 3.6‰ VSMOW, and for D3 dolomites, calculated δ18Ofluid ranges from 1.4‰ to 8.4‰ VSMOW, suggesting an influx of basinal brines. The occurrence of high porosity associated with D2, combined with the laterally sealing tight limestone beds, presence of favourable source rocks, and thermal maturation, may suggest that the Watts Bight carbonates are possible potential hydrocarbon reservoirs and suitable targets for future hydrocarbon exploration in western Newfoundland.