The Riviere des Plantes ophiolitic Melange (Middle Ordovician) in southern Quebec contains carbonatized ophiolites known as ophicalcites. Pervasive carbonatization of ultramafic material followed serpentinization and shearing. Carbonates are also found as discrete void-filling phases in the ophicalcites. The first phase to develop is a laminated and graded micrite (delta 18 O, -5.3 per thousand ; delta 13 C, -0.1 per thousand ) that is dull under cathodoluminescence. Major fracturing followed and was synchronous with calcite cementation. Initial isopachous crusts of nonluminescent palisade calcite (delta 18 O, - 13.0 per thousand ; delta 13 C, 0.8 per thousand ) are followed by blocky, bright (delta 18 O, - 14.4 per thousand ; delta 13 C, -0.6 per thousand ) and then dull (delta 18 O, -13.1 per thousand ; delta 13 C, -0.9 per thousand ) luminescent calcites. The isotopic values of the micrite fall in the range of recently published values for Early to Middle Ordovician seawater (delta 18 O, -4 per thousand to -6.5 per thousand ; delta 13 C, 0 per thousand to -1.5 per thousand ). Crosscutting relationships between internal sediments, cements, and clasts of cemented ophicalcite breccias provide evidence for complex early seafloor fracturing, cementation, and fluid circulation. Seawater-driven serpentinization of ultramafics supplied the Ca (super +2) needed for extensive calcite cementation. The close to normal marine delta 13 C values for cements (-1.4 per thousand to + 1.0 per thousand ) suggest significant involvement of marine waters with some volcanic-derived CO 2 . The oxygen isotopic composition of the carbonate cements argues for precipitation from heated seawater; temperature of precipitation probably reached a maximum of 80 degrees C. We propose a seafloor hydrothermal vent system to explain the synchroneity of micrite sedimentation, fracturing and cementation.