Abstract Lower Ordovician Ogdensburg Formation dolomites (Beekmantown Group) of the St. Lawrence-Ottawa lowlands of New York and Ontario host mineral deposits similar to MVT deposits: Mineralization (sphalerite, pyrite, Cu-sulfides, barite, quartz, calcite and saddle dolomite) is minor and occurs as late-stage vug-, fractureand breccia-filling cements. Primary fluid inclusions in quartz, calcite and saddle dolomite indicate that mineralizing fluids were Na-Ca-Cl brines, but fluid temperatures and salinities were greater than North American MVT deposits. Fluid inclusion homogenization temperatures are quite variable at individual outcrops (160°-325°C) and at the scale of a single crystal (up to 68°C in quartz), whereas salinity (ionic strength = 8-9.4) and major solute ratios are less variable. The data are not compatible with recent interpretations of MVT mineralization involving topographyor compaction-driven regional flow systems. We propose the Ogdensburg mineral deposits formed over a Paleozoic thermal anomaly, a conclusion supported by previous studies of conodont alteration and maturation of organic matter.