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Previous lead isotopic investigations in the McArthur area have concentrated on the HYC and Coxco deposits. Detailed knowledge of lead isotopic variations for other deposits in the area should assist in elucidating genetic problems and the relationship between deposit size and lead isotopic composition. Deposits in the McArthur area have been subdivided into two main types: concordant mineralization (HYC, and the concordant part of the Ridge II deposit) and discordant mineralization (Ridge I, discordant part of Ridge II, and Cooley I, II and III). The latter group closely resembles the Mississippi Valley-type deposits. Rye and Williams (1981) and Williams (1978) presented geological, Cu/(Pb'Zn) geochemical zoning, and stable isotope evidence suggesting that all the deposits formed from a common fluid that flowed westwards away from the Emu Fault, with the axis of flow being through the Cooley II and Ridge deposits (Fig. 1). To test this hypothesis, galena-rich samples were handpicked from drill core and analysed for their lead isotopic compositions.

The lead isotopic data preclude a common source for the different deposits and this applies particularly to the concordant examples at Ridge II and HYC It is feasible to propose a common source for the HYC and Cooley I deposits but there may be up to three sources of lead in the others even though some lead in Ridge I, Cooley II and Cooley III could be from the same source as that in the HYC The lead data are inconsistent with the simple single-stage model involving the westward migration of ore mineralizing fluid away from the Emu Fault. One explanation for the isotopic variation in the …