Dolomitization of the Lower Ordovician Watts Bight Formation of the St. George Group, western Newfoundland: evidence of hydrothermal fluid alteration

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 delta(18)O values of the earliest (D1) dolomitizing fluids (-6.4 parts per thousand to -9.5 parts per thousand SMOW, Vienna Standard Mean Ocean Water) fall between the estimated delta(18)O values of Tremadocian seawater and meteoric waters and suggest mixing-zone dolomitization. A second phase of coarsely crystalline (up to 400 mu m) 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 degrees C) saline (10 to 25 eq. wt.% NaCl + CaCl(2)) hydrothermal fluids. The calculated delta(18)O(fluid) of D2 ranges from -4.5 parts per thousand to 3.6 parts per thousand VSMOW, and for D3 dolomites, calculated delta(18)O(fluid) ranges from 1.4 parts per thousand to 8.4 parts per thousand 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.