Within the Bear Province, the Echo Bay Group of sediments and volcanics (~1800 m.y.) are intruded by Hudsonian granites and later diabases. Northeasterly trending fracture zones contain vein type U–Ag–Bi–Co–Ni–As–Cu deposits. Samples from the Terra, Norex, Silver Bay, and El Bonanza mines give a general paragenetic sequence of uraninite + hematite; Co–Ni arsenide minerals and silver; dolomite and sulfides; and sulfosalts with bismuth. The vein mineralogy is consistent with an ore fluid of high oxidation potential, low sulfur content (10−3 m) and low pH (~4).Dolomite δ 13C and δ 18O values of around −4‰ (PDB) and +15‰ (SMOW) respectively from all stages reflect a relatively constant temperature of deposition, which from mineralogical and other data is estimated at 200 °C. A δ 18O value of +2‰ (SMOW) is calculated for the ore fluid. The bulk of the sulfide δ 34S values are around +1‰ (CDT), which suggests a magmatic sulfur source. However, an equilibrium model is presented whereby the sulfur is derived from Precambrian sea-water sulfate and the carbon from limestones. The embryo ore fluid (connate sea water?) probably circulated both through the Echo Bay Group and the intrusives, and underwent chemical changes before moving into dilatant zones of mineral deposition.