Abstract

Of the anomalous (J-type) leads analyzed, those from the Upper Mississippi Valley are most radiogenic, and show greatest variation; a lead from southeast Missouri is intermediate, and leads from the Illinois-Kentucky district are least radiogenic. Four galenas closely associated with igneous intrusions are also J-type. The radiogenic component shows a progressive increase toward the northeast in the Upper Mississippi Valley district, and toward the southeast in the Illinois-Kentucky district; whether this implies variable ratios from a single source or mixing of leads from multiple sources is unresolved. Samples from the Appalachian Valley districts and galena, presumably diagenetic, from shale and clay, have ordinary lead.

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