Lead isotope data and textural observations reveal several genetically significant differences between the stratiform Pb-Zn- Ag and Cu orebodies of the Mt. Isa deposit in NW Queensland. Such differences include variations in Pb-isotope ratios, trace-element content and sulphide mineralization. Extensive isotopic data are presented. These results are incompatible with a hypothesis that the Pb-Zn-Ag and Cu orebodies are cogenetic and were derived from the same solution(s) or from a single source of base metals. Pb-Zn orebody formation appears to have been controlled by a suitable sedimentary facies, whereas Cu orebody formation apparently resulted from deposition in separate and structurally favourable sites at some later period of time. Differences evident among the various Cu orebodies themselves suggest differences in Cu sources and/or solution chemistries. Details of a model for the formation of the Mt. Isa deposit are presented.-J.E.S.