Abstract

Pb-isotope ratios from galenas and Ni-arsenides of the Midwest deposit indicate that there are three classes of Pb present. The first, with 206Pb/204Pb ratios of about 22.5 and 208Pb/204Pb ratios of about 43.5, occurs only in galenas and is restricted to the basal Athabasca Formation. The second occurs from about 35 m above the unconformity to the deepest samples available to us (60 m into the basement rocks). Characteristically the 206Pb/204Pb ratios are 30–35 and the 208Pb/204Pb ratios are about 50, and this Pb occurs in both galenas and Ni-arsenides. The third class has elevated 206Pb/204Pb ratios ranging up to 350 and occurs within about 10 m of the unconformity in close association with U minerals.A model is developed in which radiogenic additions to galena have occurred during at least two different events. The first added U and Th daughters to "sandstone-type" Pb at 265 ± 50 Ma, and the second added U daughter only to the "basement-type" Pb in the past 50 Ma or less. These events are consistent with Pb loss recorded in the U minerals and indicate that the lead-loss mechanism from these U minerals is catastrophic rather than continuous diffusion.Some trace Pb isotopic compositions in Ni-arsenides suggest to us the occurrence of disequilibrium migration of 238U daughter products, but there is no clear evidence that this occurred on a macroscopic scale in the galena.The model implies that some galena predates the U mineralization, and thus that there is a potential for Pb–(Zn?) mineralization in the basal Athabasca Formation.

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