Abstract

Re-examination of ore lead isotope systematics in the light of the more precise data now available shows that the simple single-stage development model fails in detail. Other evidence added to this suggests abandonment of the concept of a uniform upper mantle source, along with its presumed constant proportion of U, Th, and Pb. In its place should be substituted some form of multistage model, either episodic or continuous, with more or less complete mixing at each stage. An essential feature of this viewpoint is that material in the crust is the major contributor to the isotopic pattern. As a consequence, growth curve parameters such as "age of the earth" or "model age" of a deposit can have at best only approximate relevance to the real state of affairs.Evidence now available confirms that the immediate source of the ore lead may sometimes be sediments in the near vicinity, from which the lead is leached by aqueous solutions. The hot magma may supply the energy needed for mobilization of such a solution but need bear no isotopic relationship to the solution.Isotopic patterns mostly reflect genetic relationships between source rocks, solutions, deposits, and derivative igneous magmas (which may sometimes be intrusive, sometimes volcanic, in their expression). They show only minor effects of transportation to the site of mineralization, and contribute practically nothing to discussions on the mode of formation of the deposit. In this sense, the observable form of the deposit and the terms "syngenesis" and "epigenesis" are irrelevant to the discussion of the so-called "primary" leads.

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