Highly brecciated carbonate-rich serpentinites (or ophicalcites) of Early Ordovician age in the Dunnage Zone of Quebec are host to fracture-fill, high-temperature (80–230 °C) carbonate cements. Away from, or crosscut by the fractures, centimetre- to decimetre-thick crusts made up of massive to laminated micrite, peloidal layers and threads are associated with low-temperature botryoidal calcite cements. The peloidal masses are characterized by a clotted texture that is reminiscent of interpreted fossilized Ordovician microbial communities. The ophicalcite contains carbonate botryoids, morphology commonly found at shallow reefal margins but also at modern cold CH4 seeps and recently documented at hot CH4 vents from serpentinite, such as the modern Lost City hydrothermal field in the Atlantic Ocean. The δ18OVPDB ratios of the calcite botryoids and peloidal layers indicate formation and (or) precipitation out of cold Ordovician deep-marine waters. To the contrary of botryoids at cold methane seeps, the botryoids associated with the modern and Ordovician hot vents do not show the negative δ13CVPDB ratios indicative of microbial isotopic fractionation. Therefore, any microbial-derived HCO3– has been overwhelmed by the isotopically heavier marine-derived carbon during the open-system diagenesis. Carbonate-rich serpentinites should be carefully revisited in the search for evidence of microbial life in the Precambrian, as the negative δ13CVPDB ratios used as fingerprints of biological activity are not always reliable.