Comparison of all published analyses of commercial Pb ores, excluding Mississippi Valley deposits, shows that the ratio of Th-derived (208) to U-derived Pb (206+207) has remained constant at 1.13 to 1.14 for approximately 2 b.y. Mississippi Valley ores (1.10) generally are contaminated by about 1.5% uranogenic Pb which is chiefly 206 and, therefore, constitutes about 7% of this component. By the use of this stable ratio (1.13) one may reconstruct any simple anomalous Pb and with the aid of 206/204 and 208/204 ratios appropriate to the indicated percentage of Pb 204 more complex contamination likewise can be traced to its probable origins. Use of this method suggests that many minor veinlets of contaminated Pb in the Canadian and Baltic shields and Australia are often of Mississippi Valley type and probably represent minor downward leakages from Paleozoic mineralizations formerly present in overlying strata. At Laisvall, Sweden, the original type is still preserved in a major deposit. Segregation of ore leads into post-Precambrian and Precambrian groups reveals a remarkable hiatus in isotopic values implying a long period when no Pb ores were generated. This varies somewhat in position geographically, but generally corresponds to the erosion interval marking the end of Precambrian time. With respect to ore genesis the data seem to demand that both geologists and geochemists revise drastically their present interpretations. Original ore genesis is related closely to long cycles of sedimentation but is not strictly syngenetic and may long antedate sedimentation, as in Mississippi Valley ores. Igneous activity and metamorphism often have redistributed ores and changed their thermal characteristics but without isotopic modification beyond simple homogenization.

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