New measurements of S, C, and O isotopic ratios in the Cooley, Ridge, and H.Y.C. deposits at McArthur River place important constraints on the formation of the deposits. S isotope ratios in the discordant Cooley and Ridge mineralization and concordant Ridge mineralization have absolute values and distributions similar to those of the H.Y.C mineralization, with galena and sphalerite having relatively constant values and pyrite having widely varying values. Galena-sphalerite isotopic fractionations indicate higher equilibrium isotopic temperatures for the Cooley deposits (290 degrees to 275 degrees C) than for the Ridge and H.Y.C. deposits (260 degrees to 100 degrees C), correlating with a systematic temperature decrease away from the Emu fault.C and O isotope ratios for carbonates in the discordant and concordant deposits are related, describing a single linear trend on a delta 13 C vs delta 18 O plot. Ratios for carbonates from unmineralized sediments do not plot on the line, suggesting that the line does not reflect mixing of a hydrothermal and a sedimentary carbonate but rather reflects equilibrium between carbonates and a hydrothermal fluid over a temperature range similar to that indicated by the S isotopic fractionations. In the discordant mineralization the isotopic equilibration occurred during the precipitation of ore-stage carbonates whereas in the concordant mineralization the equilibration occurred during chemical exchange between preexisting carbonates and the mineralizing fluid and possibly also during the precipitation of some ore-stage carbonates.The new data provide additional support for the hypothesis that the discordant and concordant deposits are related, and that both types of deposits formed from a common mineralizing fluid which flowed from the Emu fault zone. The data do not place unequivocal constraints on whether the concordant deposits formed syngenetically or epigenetically, but they do indicate that in the concordant deposits the mineralizing process was operating, at least in part, below the sediment-water interface.