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[en] A linear relation is derived for the secular evolution of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios, that permits tests to be made for open system evolution on each system independently. Application of the method to conformable ore bodies of various geological age indicates that the available data do not demand an open system evolution for the last 3.8 b.y. ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb of 9.66 +- 0.15 and 37.65 +- 1.14 respectively fit best the data for this time interval. A single stage evolution since 4.5 b.y. is unlikely, however, and the major events of continent formation postdate the Earth accretion by at least 400 m.y. The larger scatter of ²⁰⁷Pb/²⁰⁴Pb data about the evolution line relative to the other isotopic ratios is also interpreted as resulting from a series of continental differentiation events taking place at 3.85 +- 0.15 b.y. (author)

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[en] Galena samples from the Tynagh deposit yield a lead isotope model age for mineralization (348 +- 22 m.y.) that agrees with the stratigraphic dating of the deposit. In contrast, lead in galenas from the Silvermines deposits is anomalously radiogenic. It is considered that lead at Tynagh has been derived from multi-cycled sediments that approximate evolution in the orogen. The Silvermines data may be explained if the metalliferous fluids had preferentially leached radiogenic lead from the source rocks. At Navan the ore fluids may have evolved with time so that lead in galenas in the higher orebodies approximates orogene lead, implying that an increasingly larger volume of source rock was encountered by the hydrothermal system with time. It is suggested that the different μ values yielded by Tynagh and Navan galenas reflect differing ²³⁸U/²⁰⁴Pb ratios in their respective source regions. (author)

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