The Ag–, Co–Ni–sulpharsenide deposits of the Cobalt–Gowganda district are characterized by relatively uniform light-stable-isotope systematics, where δ18O in quartz spans 11.1–16.0‰; in K-feldspar, 10.1–12.3‰; in albite, 8.1–14.4‰; in actinolite, 6.0–7.6‰; in chlorite, 3.2–5.6‰; and δD in chlorite = −42 to −35‰. The temperature of hydrothermal silicate deposition was 150–230 °C, as calculated from Δquartz–chlorite, and triple to quadruple isotopic concordancy is locally preserved amongst quartz, chlorite, actinolite, and K-feldspar or albite. Filling temperature modes at 230 and 330 °C exist for primary inclusions in quartz and carbonates. Ore-forming hydrothermal fluids were isotopically characterized by δ18O = −2.5 to + 5‰, δD = −40 to + 5‰, interpreted to reflect isotopically and chemically evolved formation brines from Huronian aquifers, ultimately derived from marine pore fluids, and Proterozoic meteoric water recharge of the sedimentary basin. The restricted range of δ18Oquartz, Δquartz−chlorite, and δDchlorite from a large population of veins implies that the ore-forming fluids were tapped from a large reservoir, or reservoirs, relatively uniform with respect to temperature, δ18O, and δD.Quartzes in silicate selvages, wall rocks, and carbonate-dominated gangue are isotopically comparable, signifying fluid-dominated conditions and the initial precipitation of carbonates from fluids isotopically similar to those involved in the silicate stage and at comparable temperatures. Vein dolomites (δ18O = 21 to 23.1‰) continued to exchange down to temperatures of 110–140 °C in the presence of fluids where δ18O = 3 ± 2‰, during thermal attenuation of the ore-forming reservoir. Vein calcites (δ18O = 1.7 to 15.7‰) record late incursion of meteoric waters where δ18O = −8 to −22‰ at temperatures of ~50 °C. The population of vein carbonates clusters at δ13C = −3.1 to −5.3‰, and this is probably also close to the carbon-isotope signature of the hydrothermal fluid. The source of carbon is uncertain.Actinolites possess age-corrected 87Sr/86Sr = 0.715 to 0.729, for 2200 Ma, close to estimates for the contemporaneous Huronian ratio (0.7053–0.714) but more radiogenic than contemporaneous Archean volcanics (0.7017–0.7021) or the Nipissing diabase (0.7060–0.7061). On this basis, Sr is interpreted to have been derived principally from the Huronian sedimentary reservoir.Fluid inclusions in quartz and calcite of both mineralized and barren veins in the Cobalt and Gowganda mining camps and environs show five different types type I (L), type II (L + halite), type III (L + V), type IV (L + V + H), and type V (V), with types III and IV being most abundant. A histogram of all mine data shows modes around 100, 230, and 330 °C, with a range from > 560 to < 100 °C. No carbon dioxide was observed in the inclusions, although the dominance of calcite and dolomite in the veins attests to its presence during mineralization. Several samples show evidence of aqueous boiling, allowing a direct pressure determination of about 600 bar (60 MPa). The fluids were highly saline NaCl–CaCl2 brines, with up to 54 wt.% NaCl equivalent and highly variable Na/Ca ratios. Fluid inclusions in samples of barren veins from Lundy Township, outside the areas of known mineralization, do not appear to be significantly different from those of the mineralized veins, indicating that the hydrothermal fluids were active throughout a large area of the Huronian basin; this is corroborated by the disturbance of Pb- and Sr-isotope systems in the Nipissing, Huronian, and Archean.The Nipissing diabase likely served as a heat source to mobilize metals and advect formation brines, which may have derived the metals from either or all of the Huronian sediments or the Archean volcanics Nipissing diabase and sedimentary rocks. We suggest a genetic scheme for the veins involving CO2 effervescence and aqueous boiling inducing pH increase and thereby mediating rapid precipitation of ore minerals coeval with and followed by carbonates. This process explains most of the presently known major and minor characteristics of the vein systems and their host rocks, including the chloritic and sodic metasomatism of the Archean and Huronian rocks, abundant calcite, the compositional and mineralogical variability of the ore minerals, the textural variability of both the carbonates and ore minerals, the paragenetic sequence of alteration and mineralization, the distribution of ore minerals with respect to the diabase and other rocks, the relatively narrow vertical extent of mineralization, variations in ore grade and tonnage, and the distribution of economic deposits on the periphery of the Huronian basin.