The lower member of the Cape Phillips Formation (Late Ordovician - Early Silurian) on northeastern Cornwallis Island is a sequence of interbedded carbonates and shales that preserves a continuous record of diagenesis that started with the formation of early diagenetic, nonferroan calcite concretions and ended with precipitation of late stage nonferroan calcite in vugs and fractures. delta(13)C values of concretion calcite range from -9.3 to + 5.8 parts per thousand (PDB), indicating formation from pore waters influenced by bacterial sulphate reduction and methane generation. delta(18)O values of concretionary calcite range from -11.2 parts per thousand to -4.3 parts per thousand (PDB) but the reasons for the variable and negative delta(18)O values remain unclear. Dolomite replaced concretions and bedded limestones during later burial. Nonferroan saddle dolomite cement also precipitated during burial, prior to hydrocarbon migration. After hydrocarbon migration, late stage calcites, characterized by delta(18)O values ranging from -22.2 parts per thousand to -6.1 parts per thousand (PDB), occur as dedolomite and as cement in vugs and cross-cutting fractures. Their negative and variable delta(18)O values can be best explained by a variable hydrothermal input into calcite-precipitating pore waters. delta(13)C values in late stage calcites range from -32.2 to + 4.8 parts per thousand (PDB), with the more negative values likely related to hydrocarbon-oxidizing reactions, a conclusion supported by the presence of minor secondary, fluorescent hydrocarbon-bearing inclusions. The petrographic and geochemical evidence from the carbonates of this study is consistent with regional organic maturation data indicating that these rocks reached temperatures within the oil window. Hydrothermal effects, if real, must have been short-lived and were probably restricted to fracture zones. Elsewhere in the region, probably coeval hydrothermal fluids produced economically significant Mississippi Valley-type Pb-Zn mineralization.