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Abstract

The Upper Cambrian Bonneterre formation (400 feet) is widely mineralized, predominantly with galena. The Bonneterre, dolomite or limestone, is underlain generally by highly porous Lamotte sandstone (0–500+ feet) resting on a Precambrian basement that rises in many places into the carbonate series. Above the Bonneterre is the Davis shaly limestone (150 feet), succeeded, where not eroded, by several hundred feet of dolomites which contain extensive barite deposits with accessory galena.

Isotopic studies show conclusively that all ore lead in the Paleozoic beds is anomalous, of J-type, varying considerably within deposits. In general, any vertical section of Bonneterre ore lead, as in a drill hole, is distinctly more radiogenic at the base of the formation, less so upward. Furthermore, any large mineralized area is generally more radiogenic centrally, along guiding faults or controlling basement highs (knobs), and less toward perimeters. Finally, in all clear-cut cases noted, the earlier deposited galena is least radiogenic.

Note, however, that the trace galena in overlying barite deposits is notably more radiogenic than in the Bonneterre ore bodies, apparently due to association with large deep-seated faults.

Traces of post-Precambrian lead in the basement are excessively radiogenic. However, minor veinlets of typically Precambrian galena also are known.

Sulfur isotopes are virtually identical with sea water sulfate (21.80) in the less radiogenic lead but in more radiogenic galena are lighter (22.00). Sulfur variations also show definite stratigraphic relations to horizons of high fossil content (reef rock) in the Bonneterre.

The conclusion is that radiogenic lead from the Precambrian basement mingled with normal lead in the connate fluids of the sediments to yield the anomalous hybrids. The relative proportion of the basement contribution possibly was on the order of one-third of total lead.

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