Sedex Cu–Pb–Zn deposits in the Sudbury Basin were deposited on the sea floor from fluids in which δ18O = −1‰ and at temperatures around 170 °C. Distal Mn-bearing sediments were deposited in seawater in which δ18O ≈ −3‰ and at temperatures around 125 °C. The fluids at the vent site (δ18O ≈ −1‰) probably represent a mixture of normal seawater and isotopically positive hydrothermal fluid generated in the substrate by seawater–rock reactions. The heat source responsible for convection and venting onto the sea floor and into the water column was the underlying Sudbury Irruptive, which was emplaced immediately following deposition of the Onaping Formation, directly above which the sedex deposits lie.

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